Thermal energy storage: Recent developments and practical ...

Thermal energy storage: Recent developments and practical aspectsHuili Zhang a, Jan Baeyens b, Gustavo Cáceres c, Jan Degrève a, Yongqin Lv d,*a Bio- & Chemical Reactor Engineering and Safety Section, Department of Chemical Engineering, KU Leuven, 3001 Leuven, Belgiumb European Powder and Process Technology, Park Tremeland 9, 3120 Tremelo, Belgiumc Facultad de Ingeniería y Ciencias, Universidad Adolfo Ibáñez, Diagonal Las Torres, 2640, 7941169 Peñanolén, Santiago, Chiled Beijing Key Lab of Bioprocess, College of Life Science and Technology, Beijing University of Chemical Technology, Beijing 100029, China

A R T I C L E I N F O

Article history:Received 31 July 2015Accepted 24 October 2015Available online 15 December 2015

Keywords:Thermal energy storageSensibleLatentThermo-chemicalEncapsulationMaterial propertiesImprovementsFuture R&D

A B S T R A C T

Thermal energy storage (TES) transfers heat to storage media during the charging period, and releasesit at a later stage during the discharging step. It can be usefully applied in solar plants, or in industrialprocesses, such as metallurgical transformations. Sensible, latent and thermo-chemical media store heatin materials which change temperature, phase or chemical composition, respectively. Sensible heat storageis well-documented. Latent heat storage, using phase change materials (PCMs), mainly using liquid–solid transition to store latent heat, allows a more compact, efficient and therefore economical systemto operate. Thermo-chemical heat storage (TCS) is still at an early stage of laboratory and pilot researchdespite its attractive application for long term energy storage.

The present review will assess previous research, while also adding novel treatments of the subject.TES systems are of growing importance within the energy awareness: TES can reduce the LCOE (levelizedcost of electricity) of renewable energy processes, with the temperature of the storage medium beingthe most important parameter. Sensible heat storage is well-documented in literature and applied at largescale, hence limited in the content of the present review paper. Latent heat storage using PCMs is dealtwith, specifically towards high temperature applications, where inorganic substances offer a high po-tential. Finally, the use of energy storage through reversible chemical reactions (thermo-chemical storage,TCS) is assessed. Since PCM and TCS storage media need to be contained in a capsule (sphere, tube, sand-wich plates) of appropriate materials, potential containment materials are examined. A heat transfer fluid(HTF) is required to convey the heat from capture, to storage and ultimate re-use. Particle suspensionsoffer a valid alternative to common HTF, and a preliminary assessment confirms the advantages of theupflow bubbling fluidized bed and demonstrates that particulate suspensions enable major savings ininvestment and operating costs.

Novel treatments of the TES subject in the review involve the required encapsulation of the latentand chemical storage media, the novel development of powder circulation loops as heat transfer media,the conductivity enhancement of PCMs, the use of lithium salts, among others.

© 2015 Elsevier Ltd. All rights reserved.

Contents

1. Introduction ............................................................................................................................................................................................................................................................. 21.1. Global energy and the required CO2 reduction .............................................................................................................................................................................. 21.2. Maturity of different energy storage systems and cost effects ................................................................................................................................................. 31.3. Energy storage in general, and thermal energy storage specifically ...................................................................................................................................... 51.4. Heat balances in thermal energy storage ......................................................................................................................................................................................... 51.5. Classifications of thermal energy storage ........................................................................................................................................................................................ 6

1.5.1. Classification according to temperature range and associated re-use technology ........................................................................................... 61.5.2. Classification according to energy storage mechanism ............................................................................................................................................. 81.5.3. Classification according to the storage concept ............................................................................................................................................................ 8

1.6. Objectives and structure of the paper ............................................................................................................................................................................................... 8

* Corresponding author. Beijing Key Lab of Bioprocess, College of Life Science and Technology, Beijing University of Chemical Technology, Beijing 100029, China. Tel.: +8610 64454356; Fax: +86 10 6471 5443.

E-mail address: [email protected] (Y.Q. Lv).

http://dx.doi.org/10.1016/j.pecs.2015.10.0030360-1285/© 2015 Elsevier Ltd. All rights reserved.

Progress in Energy and Combustion Science 53 (2016) 1–40

Contents lists available at ScienceDirect

Progress in Energy and Combustion Science

journal homepage: www.elsevier.com/ locate /pecs

mailto:[email protected]

http://www.sciencedirect.com/science/journal/03601285

http://www.elsevier.com/locate/pecs

http://crossmark.crossref.org/dialog/?doi=10.1016/j.pecs.2015.10.003&domain=pdf

2. Sensible heat storage .......................................................................................................................................................................................................................................... 103. Latent heat storage ............................................................................................................................................................................................................................................. 10

3.1. Main characteristics .............................................................................................................................................................................................................................. 103.2. PCMs classification ................................................................................................................................................................................................................................ 103.3. Moderate or high temperature PCMs ............................................................................................................................................................................................. 113.4. Specific PCM applications ................................................................................................................................................................................................................... 13

3.4.1. Latent heat storage with cryogenics .............................................................................................................................................................................. 133.4.2. Latent heat storage with steam accumulators ........................................................................................................................................................... 143.4.3. Moderate to high temperature PCMs ............................................................................................................................................................................ 163.4.4. The growing importance of lithium salts ..................................................................................................................................................................... 16

3.5. Mechanisms to improve phase change material applications ............................................................................................................................................... 183.5.1. General principles ................................................................................................................................................................................................................. 183.5.2. Review of applied enhancement methods .................................................................................................................................................................. 19

3.6. Nanoparticle-encapsulated PCMs .................................................................................................................................................................................................... 213.7. Cascades of PCM systems ................................................................................................................................................................................................................... 22

4. Thermo-chemical heat storage ....................................................................................................................................................................................................................... 234.1. Principles .................................................................................................................................................................................................................................................. 234.2. Thermodynamic assessment ............................................................................................................................................................................................................. 23

4.2.1. Gibbs’ free energy and work recovery ........................................................................................................................................................................... 234.2.2. Calculation results ................................................................................................................................................................................................................ 24

4.3. Kinetics from thermogravimetric analysis .................................................................................................................................................................................... 245. Containment of LHS and TCS materials ....................................................................................................................................................................................................... 26

5.1. Preliminary selection concerns and criteria ................................................................................................................................................................................. 265.1.1. Pressure upon phase change or chemical reaction .................................................................................................................................................... 265.1.2. Design of the encapsulation .............................................................................................................................................................................................. 275.1.3. Preliminary selection criteria ............................................................................................................................................................................................ 28

5.2. Relevant properties of E-materials .................................................................................................................................................................................................. 285.2.1. INCONEL alloy 600 [187] .................................................................................................................................................................................................... 285.2.2. Incoloy 825 [191] ................................................................................................................................................................................................................. 285.2.3. Extruded tubular ceramics [192–196] .......................................................................................................................................................................... 29

5.3. Experimental design data ................................................................................................................................................................................................................... 296. PCM/TCS integration in TES requires heat carriers .................................................................................................................................................................................. 30

6.1. Introduction ............................................................................................................................................................................................................................................. 306.2. Powders as novel heat carriers in renewable energy systems ............................................................................................................................................... 30

6.2.1. Dense up-flow of particles ................................................................................................................................................................................................ 326.2.2. Particle loops in SPT and CSP applications .................................................................................................................................................................. 326.2.3. Recent experimental results ............................................................................................................................................................................................. 32

7. Conclusion and recommendation for priority research ......................................................................................................................................................................... 35Acknowledgements ............................................................................................................................................................................................................................................. 36References .............................................................................................................................................................................................................................................................. 36

1. Introduction

1.1. Global energy and the required CO2 reduction

Energy supply is a vital issue, with special concerns of the publicregarding the emission of greenhouse gases and the need to reducethe use of fossil fuels [1]. The worldwide economic crisis since 2008added additional challenges [2], leading worldwide governmentsto enact new policies and financial incentives in support of renew-able energies, enhancing their implementation and development,while simultaneously creating valuable new business opportuni-ties for companies involved in this energy sector [3,4]. One of thehot topics in the energy strategy is the capture and storage of thermalenergy as applicable to renewable energy concepts and in waste heatrecovery: these advanced energy utilization schemes call for the de-velopment and new usage of existing and/or new materials. Basedupon the current statistics and predictions of energy consump-tion, the U.S. Energy Information Administration predicated anincrease in the total world energy use from 0.15 × 1012 MWh in 2008to 0.18 × 1012 MWh in 2020, and to 0.23 × 1012 MWh in 2035 [5]. Sincecrude oil fuels remain an important source of energy, albeit withdepleting traditional reserves but with significant current frackingoperations, their prices are expected to remain around 60 US$/barrel for the near future, while 20–40 US$/barrel were valid from1985 till 2005, but prices of 120–140 US$/barrel are predicted from2020 onward [5,6].

Because of the use of fossil fuels, current global CO2 emissionsare at 30.6 × 109 tpa of CO2 (against 28.2 × 109 tpa in 2005). Withouteffective measures, it is however expected that world energy-related emissions of CO2 will further increase to 33.5 × 109 tpa in2015 and 43.2 × 109 tpa in 2035 [5].

The imposed reduction in CO2 emissions will require a combi-nation of detailed strategies and tactics, including (i) a mix of energygeneration technologies; (ii) a reduction in energy usage throughthe use of incentives, technologies, taxes and quotas; (iii) maxi-mizing CO2 absorption, through carbon sequestration by both naturalmeans and by technical developments; and (iv) the developmentof highly-efficient energy capture, storage and re-use methods [3,7].There is indeed still a considerable scope for improving the energyefficiency. In the short or medium term, waste heat recovery andhigh temperature thermal energy storage are crucial concepts to im-plement such solutions, and even current power plants can use hightemperature thermal energy storage to improve the energy balanceof their operations, since they increase the flexibility and availabil-ity of heat and electricity in traditional or sustainable power plants.

The re-use “of low grade heat”, typically between ambient tem-perature and 200 °C, is not widespread since it is a technical andeconomic challenge to obtain useful exergy and energy from lowgrade heat. A large amount of low grade heat is available in theprocess industry, e.g. water from cooling towers with exhaust gastemperatures between 35 °C and 55 °C, and stack exhausts with abroader temperature range, between 30 °C and ~180 °C [8,9]. Highly

2 H. Zhang et al./Progress in Energy and Combustion Science 53 (2016) 1–40

efficient heat exchangers or heat pumps might help in recycling thislow-grade heat. The barriers associated with low grade heat utili-zation [7] include (i) the need to match the sources with potentialend-users around the process plant; (ii) the difficulty in harness-ing the potential of the waste water streams available at lowertemperatures (between 35 and 55 °C), while achieving economicbenefits; (iii) the economic and geographic limits towards dis-tances of pumping recovered heat (steam or hot water); (iv) the heatlosses; and (v) the total cost for pipeline installation and pumpingcost.

In “high grade heat” applications, electricity generation and solarenergy are the key topics. We need energy – electrical or thermal– but in most cases where and when it is not available. Energystorage technologies are a strategic and necessary component forthe efficient utilization of renewable energy sources and energy con-servation, since the addition of short and long term energy storagewill enable an extensive and more efficient use of the fluctuatingrenewable energy sources by matching the energy supply withdemand [9]. “High grade waste heat” is already commonly re-used in chemical and mineral processes such as using the hot exhaustgas of cement kilns to preheat the raw clinker meal, or through steamgeneration in chemical reactors, such as FCC cracking regenera-tors, production of acrylonitrile, and other synthesis reactions.

Low cost, fossil fuel-based electricity has always served as a majorcost competitor for electrical power generation and traditional powerplants store their energy resource on site in the form of a stock ofe.g. coal, oil, nuclear fuel or even water behind a dam. The alter-nating current electricity grid requires that supply and demand isalways strictly matched, and electricity network and transmissionsystem operators have arrangements in place to ensure that this re-quirement is always met. The traditional electricity market istherefore largely based on fuels sold and traded as commodities,and used to generate electricity to instantaneously match supplywith demand. A future electricity system predominantly or solelysupplied by renewable energy sources will therefore find it diffi-cult to meet the fundamental stability requirement of alternatingcurrent networks to constantly match supply and demand unlessstorage facilities are integrated to balance both [9].

1.2. Maturity of different energy storage systems and cost effects

The LCOE is calculated as the system’s expected lifetime costs(including construction, financing, fuel, maintenance, taxes, insur-ance and incentives), divided by the system’s lifetime expected poweroutput (kWh). All cost and benefit estimates are adjusted for in-flation and discounted to account for the time-value of money. TheLCOE is very useful to compare various generation options, and arelatively low LCOE means that electricity is produced at a low cost,with higher returns for the investor.

LCOEsystem s expected lifetime costs

system s lifetime expec= ( )€

tted power output kWh( )

For a conventional plant, the effects of inflation on future plantmaintenance must be considered and the price of fuel for the plantlifetime into the future needs to be estimated. A renewable energyplant is initially more expensive to build, but has very low main-tenance costs, and no fuel cost over its 20–30 year life. The coststructure of solar power plants will vary depending on the tech-nology they use. Fig. 1 shows the capital cost structures for solarpower tower and parabolic trough collector, respectively. Clearly,major cost items are the solar field and the receiver, together withthe TES component and the power block. To reduce the capital costand associated LCOE, these are the major items to be considered.Both improved TES-systems (reduction of the storage volume) andan increased operation temperature of the HTF (reduction of solar

receiver capacity, heliostat field size and storage volumes) will enablethis reduction to be achieved. Novel HTF systems using powders andlatent/thermo-chemical TES will be important in this respect, andare subsequently dealt with in the paper. LCOE estimates for con-ventional sources of power depend on very volatile fuel costestimates, as shown in Fig. 2. These uncertainties must be fac-tored into LCOE comparisons between different technologies. LCOEestimates may or may not include the environmental costs and/orbenefits associated with energy production. Governments haveworldwide begun to quantify these costs.

Fig. 2 shows some LCOEs for different renewable energies andfor different MWel produced. The shaded area between both hori-zontal lines at 0.05 and 0.14 $/kWh indicates the range for electricitygeneration based upon fossil fuel. In general the LCOE decreaseswhen more MW is produced and over the years, due mostly to tech-nology improvements. This decrease in LCOE with time is very clearfor wind energy, for solar PV and for CSP (concentrated solar power).Since the energy storage part of the CSP represents 9–18% of therequired investment in PTC or SPT, respectively, reductions in TEScosts will favourably impact the LCOE.

In order to achieve the European climate energy objectives asdefined in the European Union’s “20–20–20” targets and in the Eu-ropean Commission’s Energy Roadmap 2050, energy storagetechnologies are necessary. The energy objectives contain impor-tant assumptions on long term views for the energy policy basedon different energy mix assumptions. The text moreover does notclaim to determine the future.

The percentage of renewable energy sources in the gross finalenergy consumption should attain at least 55% in 2050, and this inall scenarios of the Roadmap. This shift towards renewable energysources will surely lead to a situation in which, from time to time,generation will considerably exceed demand or vice versa, with ex-plicit concern to transmission and distribution networks. To respondto this challenge and to provide a continued security of energy supplyat any time, energy storage is notably well suited. This is why thecrucial role of energy storage technologies for an increasingly de-carbonized European energy system is recognized by the EuropeanCommission Energy Roadmap 2050. A growing use of renewableenergy sources, especially considering e.g. wind and photovoltaic(PV) generation, will boost the demand for flexibility in the energysystem.

Energy storage is indeed very appropriate to respond to this chal-lenge and to assure a continued security of energy supply at anytime.

Fig. 1. Capital cost structure of solar power plants (adapted from Ref. 10).

3H. Zhang et al./Progress in Energy and Combustion Science 53 (2016) 1–40

As mentioned before, there are four main goals met by energystorage.

Firstly, energy storage will ensure the balance between demandand supply, especially in view of the expanding variability at thegeneration side. Since energy storage allows a timely and geograph-ical displacement between consumption and generation sites, itpromotes the integration of renewable energy sources generation.

Secondly, it is possible to manage transmission and distribu-tion grids with energy storage, confirmed to become a fundamentalelement of the future electricity infrastructure to stabilize the grid,even with ideal expansion of transmission capacities. Moreover, somestorage technologies could be realized much faster than grid up-grades. With the current trend of electricity production andconsumption being decentralized and fluctuating, storage can beused to improve generation, transportation and distribution: energystorage will allow to size grids closer to average energy flows insteadof peak power requirements, whilst also resulting in reduced trans-mission losses, and improving the stability and reliability of gridswith the use of storage.

Thirdly, energy storage technologies will have a major role in thetransition to a more efficient and sustainable energy use. This willbe mainly seen in the transportation sector with the use of (hybrid)electric vehicles, but also in the emergence of intelligent build-ings and smart grids. With energy storage, it is also possible tomanage electricity/heat generation and consumption on a local level,in an optimal way for the whole power system.

Finally, energy storage will also contribute to a competitive andsecure electricity supply. Since energy storage can provide an eco-nomically interesting alternative to grid expansion and load shedding,it will be very important in new market designs. Market mecha-

nisms for flexibility and security of supply and specific storageregulation will help to establish a competitive energy storage market.

Within the different types of energy storage such as electrical,thermal, mechanical, electrochemical and chemical storage, the tech-nologies recommended by the European Association for Storage ofEnergy (EASE) are listed in Table 1 with their actual state-of-the-art repartition: thermal, electrochemical and chemical energy storageseem most suitable or possible in different technology sections.

Sensible heat storage is one of the most developed technolo-gies for thermal storage and has been used for many years in boththe domestic and the industrial sector, e.g. in the form of hot waterand ice storage systems, or using thermal fluid or molten salts inconcentrated solar tower technology. Sensible heat storage is indeedmost commonly used for CSP plants using a two-tank storage ofmolten salts. For low temperature applications (<40 °C), under-ground thermal energy storage is recently gaining popularity in theEuropean markets. For high temperature applications very few ex-amples are currently used in the process industry, despite numerouspublications and research.

Latent heat storage, using PCMs, is in full development. By 2015,the specific investment costs of latent heat storage, storage of in-dustrial waste heat, and improved thermal management need tobe reduced below 100 €/kWh. By 2020 the specific investment costfor compact latent heat storage should be below 50 €/kWh. Towards2030 the intention is to have industrial process heat applicationswith thermal energy storage [12].

Thermal–chemical energy storage applies both thermal andchemical storages, using the sensible heat of reactants and the re-action enthalpy of reversible thermo-chemical reactions. Accordingto EASE [12], the niche applications for thermo-chemical storage

Fig. 2. LCOE for different renewable energies and MW electricity produced (adapted from Ref. 11).

Table 1EASE recommended technologies [12].

Technologies in focus Conventional generation Renewable generation Transmission Distribution Customers services

Pumped hydro energy storage Suitable Possible Suitable Possible UnsuitableCompressed air energy storage Suitable Possible Suitable Possible UnsuitableElectrochemical Possible Possible Suitable Suitable SuitableChemical Possible Possible Possible Unsuitable PossibleElectromagnetic energy storage, flywheels Unsuitable Possible Suitable Suitable UnsuitableThermal energy storage Suitable Possible Possible Possible Suitable


still need to be identified. By 2020, the goal is to have a specific in-vestment cost for thermo-chemical storage below 50 €/kWh. Towards2030 the intention is to have thermo-chemical storage tanks for solarthermal power plants and industrial process heat applications. Totake advantage of the high energy storage density, the operating tem-perature should exceed 400 °C [12].

1.3. Energy storage in general, and thermal energy storagespecifically

Energy storage, as suggested by its name, is to store a certainform of energy, which can thus be used later when necessary: thestorage device is generally called an accumulator. Various forms ofenergy, commonly including kinetic energy, potential energy (gravi-tational), chemical energy, electrical energy or thermal energy, canbe stored by using appropriate methods, as illustrated in Fig. 3.

The different systems can be tentatively assessed in view ofenergy density and prices per unit energy-stored, as illustrated inTable 2.

High temperature thermal energy storage is the main objectiveof this paper, although cryogenics and moderate temperature storagewill also be briefly dealt with.

TES works by heating storage media during a charging periodand then releasing the heat when the energy is needed [14], thusincreasing the energy efficiency of a system and being commonlyused to compensate the mismatch between energy supply anddemand [15]. Current applications include thermal building pro-cesses [16]; solar applications such as cooking, solar water boilers,air heating systems, greenhouses and concentrated solar powerplants [17–21]; mining and metallurgy [22,23]; and solar towerpower plants [3,17].

1.4. Heat balances in thermal energy storage

Sensible heat storage (SHS) stores thermal energy by raising thetemperature of a solid or liquid without the occurrence of a phasechange. The following equation expresses the SHS potential:

Fig. 3. Energy storage systems according to form of energy stored according to power rating and discharge period (adapted from Ref. 13).

Table 2Characteristics of different energy storage systems (adapted from Ref. 14).

Storage mechanism Storage period Energy density(kWh/m3)

Price per unit energy-stored (€/kWh)

Pumped hydro energy storage (PHES) Mechanical Day–month 0.5–1.5 10–70Compressed air energy storage (CAES) Mechanical Day 3–6 2–140Flywheel energy storage system Mechanical Hour 20–80 105–4 × 105

Superconducting energy storage system (SCESS) Electrical Hour N/A 7 × 104–2.3 × 105

Superconducting magnetic energy storage (SMES) Electrical Hour 0.2–2.5 6.3 × 104–7.5 × 105

Lead-acid battery Conventional battery Day–month 50–80 100–830Nickel–cadmium (Ni—Cd) Conventional battery Day 60–150 450–1800Lithium-ion (Li-ion) Conventional battery Day–month 200–500 500–2000Sodium–sulphur (NaS) battery Molten salt battery Day–month 156–255 280–700Sodium nickel chloride batteries (ZEBRA) Molten salt battery Day–month 150–290 75-150Zinc–bromine flow battery (ZBB) Flow battery Day–month 30–60 110–750Polysulphide–bromine flow battery (PSB) Flow battery Day–month 16–60 120–1000Vanadium redox battery (VRB) Flow battery Day–month 16–33 110–750Hydrogen-based energy storage system (HESS) Chemical Day–month 2.7–160 at 1–700 bar 2–15Combined powder loop + PCM Thermal Hour–week <150 1–20Thermo-chemical storage (TCS) Thermal Hour–week 1120–250 8–100


Q m when C constantp= = −( )∫ C dT mC T Tp

T

T

p H L

L

H

~ (1)

where m is the mass of storage material, Cp is specific heat at con-stant pressure of the SHS materials, and TL and TH are the low andhigh temperatures of SHS use. Cp is assumed independent of tem-perature within the narrow temperature range of the SHS application.In applications below 100 °C, water is the best SHS liquid, becauseof its wide availability, low cost and relatively high specific heat.Above 100 °C, synthetic oils, molten salts, liquid metals, or powderscan be used. For air heating applications, rock bed type storage ma-terials are more suitable. Besides physical properties, such as densityand specific heat of the storage materials, there are other proper-ties that are important for the efficiency of SHS, including operationaltemperatures, thermal conductivity and diffusivity, vapour pres-sure, compatibility among materials and their stability, heat lossesas a function of the surface area to volume ratio, and cost of thematerials and systems. A SHS system usually consists of a storagemedium, a container tank, and inlet/outlet connections. To preventany potential loss of thermal energy, thermal insulation is re-quired. The storage medium can be solid or liquid. Solid media, suchas concrete and cast ceramics, are commonly used in the form ofpacked beds, which require a fluid for effective heat exchanging andas heat carrier. If the fluid is a liquid, the system is called a dualstorage system, because the heat capacity of the solid in the packedbed cannot be neglected. One of the most significant advantagesof the dual system is the use of inexpensive solids, such as rock, sand,or concrete as storage materials. Liquid media mainly include moltensalts or synthetic oils. They are naturally stratified in the storagevessel due to the difference in density between hot and cold layers:the hot fluid is supplied to the upper part of storage during charg-ing, whereas the cold fluid is extracted from the bottom part duringdischarging.

Latent heat storage (LHS) makes use of the heat absorbed or re-leased by the storage material when it experiences a phase changebetween solid and liquid or liquid and gas. The storage capacity ofa LHS system with phase change materials is given by:

Q m= + +∫ ∫C dT m H m C dTps

T

T

m pl

T

T

L

m

m

H

Δ (2)

where Tm is the melting point of the PCM, Cps and Cpl are the spe-cific heat of the PCM in solid and liquid state respectively, and ΔHm

is the phase change enthalpy. If Cps and Cpl are not a function of tem-perature, the heat storage capability can be calculated by integrationinto the following equation:

Q = −( ) + + −( )[ ]m C T T H C T Tps m L m pl H mΔ (3)

LHS with PCMs is at present very promising, because it exhib-its high energy storage density and can store heat at constanttemperatures, i.e. the phase transition temperatures of the mate-rials [22–26]. Phase transition may occur in forms of solid–solid,solid–liquid, solid–gas, and liquid–gas. Among these phase changes,only the solid–liquid phase change has been used for LHS. Liquid–gas phase changes are not practical for use as thermal storage dueto the large volumes involved or high pressures required to storethe energy when the compounds are in their gaseous or vapourphase, although liquid–gas transitions have a higher latent heat oftransformation than solid–liquid transitions. Solid–solid phasechanges are usually too slow and have a very low value of latentheat of phase change. Although PCMs are expected to have a po-tential advantage towards energy storage in comparison with solesensible heat storage, Fig. 4 illustrates the temperature range wherethe efficacy of sensible heat storage of the heat carrier (e.g. sand)can exceed the latent heat storage capacity, as illustrated for e.g.

Sb2O3–PCM: the total heat stored is lower than the sensible heatstored in SiC and/or SiO2. Molten salts KNO3—NaNO3 andNaOH—NaCl, on the contrary, offer a significant advantage. Due tothe latent heat of fusion, there is a steep jump in energy stored atthe temperature of the PCM melting point.

Thermo-chemical storage (TCS) uses the heat absorbed and re-leased by a reversible chemical reaction. The amount of heat storedis proportional to the amount of storage material (m), the endo-thermic heat of the reaction (ΔHr) and the conversion (α), given by:

Q m Hr= ≤α αΔ with 1 (4)

It should be remembered that the external heat supply will alsoneed to cater for the sensible heat of the initial reactant betweenthe starting (ambient) and the reaction temperature.

For an operation between 750 °C and 250 °C, with specific heatvalue around 1 kJ/kg, the heat storage capacity of sensible heatstorage is ~500 kJ/kg, between 700 and 1000 kJ/kg in latent heatstorage due to the contribution of the heat of phase transition, andin excess of 1000 kJ/kg for thermo-chemical storage, respectively.

1.5. Classifications of thermal energy storage

TES systems can be classified according to different param-eters, being the temperature range of application, the mechanismof energy storage, and the integration within the energy storageconcept.

1.5.1. Classification according to temperature range and associatedre-use technology

Essential to be able to capture, store and re-use thermal energyis the use of a heat transfer fluid (HTF): the ranges of applicablethermal energy storage are hence a function of the HTF selected.According to the temperature range at which the system works, TEScan be classified as low temperature thermal energy storage andhigh temperature thermal energy storage. In this study the divi-sion temperature is considered at about 200 °C. The classificationaccording to the working temperatures of the HTF, TES and thermalblock is illustrated in Fig. 5.

The increased technological risk reflects the impact of safety ofoperations at extremely high temperature and the use of high valueconstruction materials. Traditional HTF include gas, water/steam,thermal fluids and molten salts. The use of particle suspension as

Fig. 4. Total heat stored vs. temperature for sensible and latent heat storage mate-rials. 1: KNO3=NaNO3, 2: NaOH=NaCl, 3: SiO2, 4: SiC, 5: Sb2O3 [27]. HTF: Heat transferfluid.


HTF is considered as a novelty and offers major benefits to enhancethe solar power tower commercial potential [7,21,28]. The use ofparticle suspensions as heat carrier to transfer solar heat from thereceiver to the energy conversion process offers major advantagesin comparison with water/steam, thermal fluids or molten salts. Sincethe particle suspension has a heat capacity similar to that of moltensalts, without temperature limitation except for the maximum al-

lowable wall temperature of the receiver tube, suspensiontemperatures of up to 800 °C can be tolerated for refractory steeltubes (even higher when using ceramic or glass tubes), thus offer-ing new opportunities for highly efficient thermodynamic cycles suchas obtained when using supercritical steam or CO2. With higher tem-perature HTFs, a cascade of effects is noteworthy, with additionalhigh efficiency thermodynamic systems being viable, as illus-trated in Fig. 6 and assessed by e.g. Dunham and Iverson [29]. Ahigher temperature operation will increase the power cycle effi-ciency, whilst increasing the temperature range over which thestorage operates, thus enhancing the storage density. The in-creased efficiency of the power cycle also reduces the thermal powerdemand for a constant receiver efficiency, which allows more elec-tricity to be generated per unit of stored thermal energy. Theincreased capacity of the storage moreover increases the power plantcapacity factor, thus reducing investments, despite the use of moreexpensive construction materials. If a new high-temperature HTFis to improve the economics of e.g. SPT applications, the cost re-duction of the solar field and storage must more than outweigh costincreases of the receiver and power block. A high temperaturepowder circulation loop allows advanced power cycle configura-tions to be used, albeit not yet exploited in current solar power towerapplications. Such suspension receivers with outlet temperatureabove 700 °C are being developed in the Concentrated Solar Powerin Particles European project (CSP2) [30].

Supercritical Rankine cycles and associated steam turbines arecommonly designed for large power outputs (~800 MWel) [31], andwill need to be scaled-down for smaller plants by a redesign in orderto cope with the low volumetric flow rates and the resulting small

Fig. 5. Classification of energy storage systems according to the working temperature [28].

Fig. 6. Evolution of standard reheat Rankine-cycle configurations (numbers refer toTable 3) [28].

Table 3Typical operating conditions reported by Spelling et al. [33].

Nbr. Power cycle Steam conditions Cycle efficiency (%)

°C Bar

1 High-tech. PTC plants 375 100 ~352 High-tech. molten salt SPT plants 535 115 ~403 Old subcritical fossil fuel plants 535 165 ~424 High-tech. subcritical fossil fuel plants 565 165 ~435 Old supercritical fossil fuel plants 565 255 ~446 High-tech. supercritical fossil fuel plants 600/610 270 ~457 Advanced supercritical fossil fuel plants 600/620 285 ~468 Ultra-supercritical fossil fuel plants 700/720 350 ~48


blade sizes, although solutions using radial turbines or operatinga high pressure turbine at higher speeds have been suggested [32].In order to use solar power tower plants in arid locations, the powerblock will use dry, indirect cooling for lack of sufficient cooling water.

1.5.2. Classification according to energy storage mechanismIf the material is not subjected to a phase change or a chemical

reaction, due to temperature variations, it is referred to as SHS. LHSrequires the material to absorb or release enough energy to trans-form its phase structure during melting/solidification. Finally, a TESsystem is classified as TCS, if the storage media undergo revers-ible endothermic/exothermic chemical reactions for heat storage.

1.5.3. Classification according to the storage conceptThe different TES concepts are also classified regarding the lo-

cation of the storage media within the TES and on how the energyreaches it. This classification involves active, passive and hybridstorage, as illustrated in Fig. 7 and dealt with in detail by Kuravi et al.[34].

In an active storage system, the storage medium circulatesthrough a heat exchanger (which can also be a solar receiver or asteam generator) and it is charged or discharged by forced convec-

tion heat transfer. Active storage systems can be subdivided intodirect and indirect systems, where the former uses the same ma-terial as heat transfer fluid and storage medium, whereas the latteruses a secondary storage medium.

Contrary to an active storage, the passive storage medium doesnot circulate through the system, but always stays in the same lo-cation. The heat transfer fluid is responsible of carrying energythrough the system: in the charging process the HTF carries the re-ceived energy from the energy source to the storage medium andreceives energy from the storage medium when discharging. Thistype of system mainly uses a solid heat storage medium such asconcrete, casts, and contained-PCMs.

The hybrid storage combines active and passive concepts in tryingto improve the characteristics of the storage systems.

1.6. Objectives and structure of the paper

Energy capture and storage are “hot” research topics, as illus-trated by the literature review of Fig. 8: both PCMs (and nanoparticle-PCM) and TCS are within the focus of increasing current research,whereas the use of powders as sensible HTF has only recently at-tracted a lot of interest.

Fig. 7. Classification of average storage systems according to applied concepts (adapted from Kuravi et al. [34]).

Fig. 8. Literature (1970–2015), as total number of papers per 5 years, related to energy capture and storage in SCOPUS with keywords: ■ NP-PCM; PCM; ◆ particulateHTFs; ● thermo-chemical storage.


Prior to delineating the objectives of the present review paper,two previous review papers are important to cite, since both alreadydeal with specific aspects of TES.

The assessment of Yu et al. [35] evaluates TES by sorption processusing silica gels, zeolites, alumino-phosphates, silico-alumino-phosphates and metal organic frameworks. These systems arerecognized for their high thermal energy storage densities and longterm applications. Intensive studies are however additionally re-quired to demonstrate the technical feasibility of the concept.Sorption energy storage is however not further discussed in thecurrent review.

A more comprehensive review of TES systems is provided byKuravi et al. [31]. This review discusses the thermal energy storagedesign and practise for application and integration into concen-trating solar power plants. This integration is studied at plant,component and system level. Storage systems using active two-tank, steam accumulation, extended/embedded heat transferstructures, and packed bed systems were assessed. Energy and exergyefficiencies of the TES-systems were calculated to define the eco-nomic optimization. A life cycle assessment of TES systems and acase study conclude the research. The present review will exten-sively refer to Kuravi et al. [34] as far as these specific topics areconcerned. It will however add additional and up-to-date findingsin various fundamental and practical aspects of TES, all integratedinto a design logic as illustrated in Fig. 9.

The structure comprises 7 sections. Section 1 positioned TESsystems and their growing importance within the energy aware-ness, with aspects of TES classification and its main impacts.

Section 2 will deal with sensible heat storage, well-documentedin literature and hence limited in the content of this review.

Section 3 will deal with latent heat storage, specifically towardshigh temperature application, where inorganic substances offer a

high potential. Experimental and modelling data will be com-pared. Since inorganic solid/liquid phase change materials suffer froma low thermal conductivity, conductive inserts offer a high poten-tial, reducing the charging/discharging time by a factor of 2.

Specific applications of latent heat storage with cryogenics andwith steam accumulations will be discussed, both however withinvery special ranges of application. Finally, recent developments inenhanced high temperature PCMs and the important impact of usinglithium salts will be assessed: enhancement techniques appear tobe moderately to highly effective.

Section 4 will review the fundamentals of using reversible chem-ical reactions to store heat for short to long term periods. Withinthe selected reaction pairs, Ca(OH)2, CaCO3, Co3O4 and Mn2O3 offera high potential, both towards reaction temperature and revers-ibility. All reactions can be treated as first order chemical reactions,with activation energy between 50 and 250 kJ/mol and reaction ratesof 1 to 2 × 10−3 s−1 at reaction equilibrium temperature.

Since latent and thermo-chemical storage medium needs to becontained in a capsule (sphere, tube, sandwich plates) of appro-priate materials, Section 5 assesses the mechanical strength of thecontainment from both a theoretical and experimental evalua-tion. Alloy-capsules of 1–2 mm thickness meet the requirements oflong term stability, even after multiple charging/discharging cycles.

Section 6 deals with the current development of a novel heattransfer fluid, involving particulate suspensions. Literature and ex-perimental data confirm the high heat transfer coefficient achievedover 400 W/m2 in circulating fluidized bed, to 1100 W/m2 K in adense upflow fluidized bed. A tentative assessment selects to a po-tential design and demonstrates that particulate suspensions leadto major savings in investment and operating costs. Finally, Section7 will summarizes the findings and highlight required priorityresearch.

Fig. 9. Layout of the review paper.


2. Sensible heat storage

Sensible heat storage materials undergo no phase change withinthe temperature range required for the storage application [14].Table 4 summarizes the main characteristics of the most commonsolid and liquid sensible heat storage materials.

Within the indicated solids, concrete and cast ceramics have beenextensively studied due to their low costs and good thermal con-ductivities, despite their moderate specific heats. In terms of liquids,molten salts [17] and mineral oils [38] are widely used in solar towersand parabolic trough collectors, respectively, being liquid at ambientpressure, providing an efficient and low cost medium, and havingtheir operating temperatures compatible with current high-pressure and high-temperature turbines (temperature range over120–600 °C). From Table 4, it is clear that the main candidates forliquid high temperature sensible heat storage are either a solar salt,i.e. a binary salt consisting of 60% of NaNO3 and 40% of KNO3, thatmelts at 221 °C and is kept liquid at 288 °C in an insulated storagetank; or HitecXL, a ternary salt consisting of 48% Ca(NO3)2, 7% NaNO3,and 45% of KNO3 operating beyond its melting point of 130 °C.

Common mineral particles, such as silicon carbide, crystobalitesilica, exhibit high values of Cp, and are hence subject of increas-ing research as SHS media, as described in Section 6 of this paper.

Sensible liquid heat materials have been widely studied and arecurrently applied in solar thermal plant applications, despite im-portant disadvantages that can affect the storage system design andstability. The low solidification point may be a problem for solarpower plants because of the required heat tracing during non-functioning periods. Furthermore, the sensible fraction of thermalenergy is seldom fully recovered due to the required temperaturedifference for the heat transfer driving force. Another important dis-advantage consists in the low energy storage density of sensible heatmaterials, leading to the requirement of large volumes or quanti-ties in order to deliver the amount of energy storage necessary forhigh temperature thermal energy storage applications. The abovementioned problems can imply significant increments in the costsof sensible heat storage systems. For solid SHS media, the numberof available materials is extensive and current handbooks and da-tabases give no comparison or possible relationships betweenproperties of materials that enable their selection [39]. Materialswith the highest Cp are natural and polymeric materials, such asnatural rubber or the thermoplastic copolymers (acrylonitrile bu-tadiene styrene) with a Cp value of around 2 kJ/kg K. Other compositematerials such as glass fibre reinforced epoxy concretes have Cp

values close to 1 kJ/kg K. The specific heat capacity should be con-

sidered in conjunction with the price of the SHS material. Materialsfor sensible thermal energy storage in the range of 15–200 °C wereconsidered and presented by Fernandez et al. [39]. Commercial pricesvary from 0.02 to 0.08 €/kg for standard concrete, from 0.15 to 0.25 €/kg for high density concrete, from 0.1 to 0.3 €/kg for common mineralpowders, and from 0.5 to 3 €/kg for steel and alloys.

3. Latent heat storage

3.1. Main characteristics

Latent storage systems based on PCMs with solid–liquid tran-sition are considered to be very efficient in comparison to liquid–vapour and solid–solid transitions [40]. Liquid–gas transition requiresa large volume recipient for the PCM and the solid–solid transi-tion presents a low value of latent heat, making both alternativesnot considered as appropriate choices. A large number of materi-als are known to melt with a moderate to high heat of fusion withindifferent ranges of temperature. No material yet studied has all theoptimum characteristics required for a PCM, and the selection of aPCM for a given application requires careful consideration of theproperties of the various substances and/or mixtures [34,41]. Themain disadvantage of a PCM is its low range of thermal conductiv-ity between 0.2 and 0.8 W/mK, whereas sensible heat materials scorebetter. Therefore, improving the PCM thermal conductivity willenhance the TES efficiency by improving its charging/dischargingprocesses as dealt with below.

3.2. PCMs classification

PCMs are generally classified in different categories consider-ing their melting temperature and their material composition. Themelting temperature can be divided in two main groups: low tem-perature (<200 °C) and high temperature (>200 °C). The categoriesfor material composition are mainly threefold, being organic, in-organic and eutectic compounds [34], as illustrated in Fig. 10.

Organic materials can repeatedly freeze and melt without phasesegregations and are usually non-corrosive. Both “paraffin” and “non-paraffin” compounds are limited in use to low and moderatetemperatures, hence outside the scope of the present review. Someof the features of this group are their high heat of fusion, low thermalconductivity and instability at high temperatures. Properties are re-viewed in literature by e.g. Zalba et al. [36], Kuravi et al. [34],Jegadheeswaran et al. [37] and other authors.

Table 4Main characteristics of sensible heat storage solid and liquid materials (adapted from Refs. 34, 36, and 37).

Storage medium Temperature Average density(kg/m3)

Average heatconductivity (W/mK)

Average heatcapacity (kJ/kg K)

Cold (°C) Hot (°C)

Sand-rock-mineral oil 200 300 1700 1 1.3Reinforced concrete 200 400 2200 1.5 0.85NaCl (solid) 200 500 2160 7 0.85Cast iron 200 400 7200 37 0.56Silica fire bricks 200 700 1820 1.5 1Magnesia fire bricks 200 1200 3000 1 1.15HITEC solar salt 120 133 1990 0.60 –Mineral oil 200 300 770 0.12 2.6Synthetic oil 250 350 900 0.11 2.3Silicon oil 300 400 900 0.1 2.1Nitrite salts 250 450 1825 0.57 1.5Nitrate salts 265 565 1870 0.52 1.6Carbonate salts 450 850 2100 2 1.8Liquid sodium 270 53 850 71 1.3Silicon carbide 200 1400 3210 3.6 1.06SiO2 (crystobalite) 200 1200 2350 0.92 1.13


Inorganics are divided in two main groups called “salt hy-drates” and “metallics”.

• Salt hydrates are mixtures of inorganic salts and water forminga typical crystalline solid of general formula AB·nH2O. At themelting point the hydrate crystals decompose into anhydroussalt and water, or into a lower hydrate and water. This type ofmaterial is used as PCM due to its high latent heat of fusion perunit volume, relatively high thermal conductivity (almost twicethe thermal conductivity of paraffin) and small volume changesat melting. The major problem of salt hydrates is that most ofthem have incongruent melting, which means that the salt is notentirely soluble in its water of hydration at the melting point.This results in an irreversible melting–freezing and a reductionof the available salt hydrate with every charge–discharge cycle.

• Metallics include low melting metals and eutectic alloys. Despitetheir advantages, they have not yet been considered as a PCMdue to weight penalties. They are used when volume is a con-sideration because of their high heat of fusion per unit volume.Other features of these materials are their low heat of fusion perunit weight, high heat of fusion per unit volume, high thermalconductivity, low specific heat and relatively low vapour pressure.

Eutectics are melting compositions of two or more compo-nents, which melt and freeze congruently forming a mixture of thecomponent crystals during crystallization. Due to this crystalliza-tion, the materials have little chance of separation. Depending onthe nature of the material composition, they can be divided intoorganic–organic, inorganic–inorganic and inorganic–organic. TowardsCSP application, the most used heat storage medium is a eutecticsalt known as “solar salt”. This is a combination of 60% NaNO3 and40% KNO3.

The low-temperature PCMs (organic, inorganic or eutectic) aremainly used in waste heat recovery systems and buildings, whilehigh temperature PCMs (inorganic or eutectic) can be used in solarpower plants and other high temperature applications [25].

When developing a TES system, the selection of a PCM is an im-portant task, and the selected PCM must fit the requirements of thesystem. Abhat [26], Jegadheeswaran et al. [37] and Zalba et al. [36]described the main criteria that govern this selection, which canbe classified in thermal, physical, chemical and economicalproperties.

Thermal properties should include a melting point in the desiredoperating temperature range (temperature range of application);a high latent heat of fusion per unit mass, so that a smaller amount

of material stores a given amount of energy; a high specific heatto provide additional significant sensible heat storage effects; anda high thermal conductivity, so that the temperature gradients forcharging and discharging the storage material are small.

Physical properties should account for small volume changesduring phase transition, so that a simple container and heat ex-changer geometry can be used; a high density; exhibiting little orno sub-cooling during freezing; and a low vapour pressure in orderto avoid stresses and problems with the container and the neededheat exchangers.

Chemical properties of the PCMs are focused upon their chem-ical stability without decomposition; whilst avoiding corrosion ofconstruction materials. They must moreover contain non-poisonous,non-flammable and non-explosive elements/compounds. Phase seg-regation should be avoided.

Towards process economy, PCM materials should be available inlarge quantities at low cost.

Since organic PCMs have applications at low to moderate tem-perature (110–260 °C) only, Table 5 only lists physical properties ofnon-organic PCMs.

To the extent of our current knowledge, there is no commer-cial high temperature thermal energy storage module using PCMstorage, and only experimental modules have been constructed andassessed. According to a recent study of the International Renew-able Energy Agency (IRENA) [43], the status of the market for hightemperature thermal energy storage modules is still low. All the in-vestment in this area has been focused on research and development.One PCM test module recorded in literature was developed withinthe DISTOR project, developed by the Institute of Technical Ther-modynamics of the German Aerospace Center (DLR), which focuseson the development of thermal storage systems using PCM in thetemperature range from 230 °C to 330 °C for systems using steambetween 30 and 100 bar. In this context, this project has devel-oped innovative PCMs and heat transfer concepts to overcome PCMdisadvantages. From their research, they could conclude that thefinned tube design is the most promising and technically feasibleconcept, therefore, it was decided to build a test 200 kW PCM storagemodule based on the finned tube module design [44].

3.3. Moderate or high temperature PCMs

Although different PCMs can be used within a temperature rangeof 300–700/800 °C, as illustrated in Table 5, their application at thesetemperatures is still subject of widespread investigation. PCMusage in high temperature thermal energy storage has been

Fig. 10. PCM classification considering material composition [24,36,37].


experimented for solar plant applications, but it has not yet beencommercially used [45,46]. The development of high temperaturethermal energy storage using PCMs is of increasing interest sincethey are fairly cheap, have a high energy density, can be availablein large quantities, and are able to store and release thermal energyat a constant temperature. In addition to adapting PCM meltingpoints to a high temperature range, their heat conduction charac-teristics need to be improved in order to increase the efficiency ofthe charging and discharging processes. Although PCMs are ex-pected to have a potential advantage towards energy storage incomparison with sole sensible heat storage, Fig. 4 illustrated thatthe efficiency of sensible heat storage can in some cases exceed thecontained sensible and latent heat of the PCM. Molten salts gen-erally offer significant advantages. To counteract the slow charging/discharging rates of PCMs, the integration of high conductivitystructures has been investigated. Figs. 11 and 12 illustrate some ofthe experimental results [27].

The experimental results illustrate the effect of different param-eters. The temperature of phase change should be a constant 230 °C,

as obtained when a pure substance solidifies. With mixtures, thesolidification takes place over a range of temperatures, sometimesreferred to as the “mushy region” between the solid and liquid zones[36]. The effect of inserted metallic foam or metallic sponge isevident, with a significantly shorter discharge rate as a result. Thecurve with forced water cooling (hence with a far higher externalheat transfer coefficient) is very steep, and the temperature de-creases from 270 °C to 160 °C in less than 700 s (against 2500–3700 s when air cooling is applied). This external heat transfercoefficient is an important parameter of the Biot-number, the di-mensionless group that rates convection and conduction.

The date treatment can be performed by different methods:

(i) Using an analytical, unsteady-state conduction approach, in-volving a material temperature to be dependent on both timeand spacial co-ordinates, with solutions provided by Carslawand Jaeger [47] and Smith [48]. The solution of the equa-tions results in the prediction of the temperature at a givenpoint in the body at time t measured from the start of thecooling or heating operation, for given values of

Table 5Inorganic substances/compounds for potential use as PCM [34,36,41,42].

Inorganic compounds Meltingpoint (°C)

Heat of fusion(kJ/kg)

Density(kg/m3)

Specific heat at themelting point (kJ/kg K)

Thermalconductivity (W/mK)

Mg(NO3)2.2H2O 130 275 NA NA NAHitec XL: 48% Ca(NO3)2—45%KNO3—7%NaNO3 140 NA 1992 1.44 0.519Hitec: KNO3—NaNO2—NaNO3 142 84 1990 1.34 0.6LiNO3—NaNO3 195 NA NA NA NAKNO3/NaNO3 eutetic 223 105 NA NA NANaNO3 307/308 74 2260/2257 NA 0.565.2%NaOH—20%NaCl—14.8%Na2CO3 318 290 2000 1.85 1.0KNO3 333/336 266 2110 NA 0.522.9% KCl—60.6% MnCl2—16.5% NaCl 350 215 2250 0.96 0.95KOH 380 150 2044 NA 0.5MgCl2/KCl/NaCl 380 400 1800 0.96 NANa2CO3—BaCO3/MgO 500–850 NA 2600 NA 5Li2CO3 618 NA 2091 2.07 NASb2O3 652 387 5670 0.43 NAMgCl2 714 542 2140 NA NA80.5% LiF—19.5% CaF2 eutetic 767 790 2100/2670 1.97/1.84 1.7/5.9LiF 850 811 NA NA NANa2CO3 854 276 2533 NA 2K2CO3 897 236 2290 NA 2

NA, not available.

Fig. 11. Temperature vs. time cooling of the E-PCM (nitrates) [27]. Air-cooling of liquidPCM (1) no inserts; (2) metallic sponge; (3) metallic foam. Air-cooling of solid PCM(4) no inserts; (5) metallic sponge; (6) metallic foam; water-cooling of (7) liquidPCM + foam; and (8) solid PCM + foam. Fig. 12. T vs. cooling time for E-PCMs [27].


τ = = ( )ktC R

tRpρ

α2 2

Fourier-number (5)

1Bi

khR

= ( )inverse Biot-number (6)

with h = external heat transfer coefficient (W/m2 K) and n = r/R di-mensionless distance in the direction of the heat conduction.

(ii) By using Comsol-Multiphysics, as previously used by Lopezet al. [49] and later extended by Pitié et al. [50] and Parradoet al. [51], presenting the confined melting in composite ma-terials made of graphite-nitrates, ceramic-nitrates, and copper-nitrates, respectively. Comsol-Multiphysics offers additionalcalculation procedures such as internal pressure build-up dueto the thermal expansion of the PCM. The procedure “ThermalStress contains Heat Transfer and Solid Mechanics phenom-ena” needs to be used. In order to provide results with aminimum error range, it uses an extra-fine mesh. The shellwall is considered to be homogeneous, isotropic and presentat a known and uniform temperature. At the interfaces, anequality of phase temperature and heat flux continuity at themelting front, and an equality of temperature and pressureat the shell/salt interface are accepted.

To predict the effective thermal conductivities of the PCM withinserts (foam, sponge), a series of empirical correlations was used,as listed in Table 6. The symbols keff, kPCM and kINS represent respec-tively the effective thermal conductivities of the composite mix, thethermal conductivity of the PCM itself, and the thermal conduc-tivity of the insert. The porosity, ε, represents the void space (PCM-filled) of the composites. The sphericity, Ψ, is estimated at 0.95 forthe composites. The predicted values for the composite systems aregiven in Table 7, with ±10% deviation noticed between the differ-ent predicted and average values. The inserts clearly increase the

thermal conductivity of the PCM, with sponge (porosity 0.9) havinga slightly higher effect that the metallic foam.

Fig. 13 illustrates the comparison of calculated and experimen-tal discharging rates of both liquid and solid PCM [27,49]: withinthe temperature range under consideration, cooling rates can be fairlywell represented by a linear relationship of temperature and time.This results in respective cooling rates, ΔT/Δt (°C/s), which providea tentative comparison as presented in Fig. 12.

The theoretical calculation of the cooling rate of the liquid PCMunderestimates the experimental temperature gradient, due to thefact that only conduction is taken into consideration, whilst naturalconvection within the liquid phase also contributes to the heat trans-fer. When calculating the Rayleigh-number within the liquid salt,values exceeding 107 are obtained, significantly above the thresh-old 1700 normally given for the onset of natural convection. Sinceit is difficult to predict the contribution of natural convection, Zalbaet al. [36] propose to characterize the melt by an effective thermalconductivity, exceeding the thermal conductivity as such. An ex-pression of this effective thermal conductivity could not be obtainedyet, but it should indeed be expressed as: k k f Ratotal melt= ( )× withRa being the Rayleigh number.

To fit the experimental results, the value of the f(Ra) ranges from1.2 to 1.4, but further refining is required. The Comsol predictionsof the temperature evolution are very promising, again with an un-derestimation of the cooling rate in the liquid phase, but a closeprediction of the periods of phase transformation and solid’s cooling.

3.4. Specific PCM applications

3.4.1. Latent heat storage with cryogenicsCryogenic fuels, such as liquefied natural gas, have been deemed

to be promising alternative fuels due to their high energy density[57]. They are stored at extremely low temperatures to maintainthe liquid state. At the consumers end, the cryogenic liquids needto be re-gasified to about ambient temperature, and plenty of cryo-genic cooling capacity would be released during this vapourizationprocess. It is therefore quite important and imperative to maxi-mize the recovery of the cold energy either through the use of othercryogens or PCMs [55,56]. Cryogens normally refer to as liquid mediaat a temperature below approximately −150 °C and are physicalenergy carriers [58]. The use of cryogen as an energy carrier is dif-ferent from normal heat storage in that the energy is stored through

Table 6The effective conductivity of the PCM and insert-composites [27,52–56].

Ref. keff

52 k kPCM INSε ε∗ −( )1

53 kk k k kk k k kPCM

INS PCM PCM INS

INS PCM PCM INS

+ − −( ) −( )+ + −( ) −( )

2 12 1

εε

⎡⎡⎣⎢

⎤⎦⎥

54 kF

kkFPCM

PCM

INS

1 1 1

1 1 1

+ −( ) × −⎛⎝⎜

⎞⎠⎟

− −( ) × −( )

⎡

⎣

⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥

ε

εwith F = + −⎛

⎝⎜⎞⎠⎟

⎡⎣⎢

⎤⎦⎥

−

=∑1

31 1

1

1

3 kk

fPCM

INSi

i

fi are the semi-principal axes of ellipsoids, proposed asf1 = f2 = 0.125 f3 = 0.75

55 k

kkkk

PCM

PCM

INS

PCM

I

1 1 1 1 1

1 1

3 3

3

− −( ) + −( ) + −( ) − −( )⎡⎣ ⎤⎦

− −( ) +

ε ε ε ε

εNNS

13 −( )

⎡

⎣

⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥ε

56 kk k k k

kPCM

INS INS PCM INS

INS

+ −⎛⎝⎜

⎞⎠⎟

− −⎛⎝⎜

⎞⎠⎟

−( ) −( )

+ −

31

31 1

3ψ ψ

ε

ψ11 1⎛

⎝⎜⎞⎠⎟

+ −( ) −( )

⎡

⎣

⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥k k kINS PCM INSε

Table 7Calculated effective thermal conductivity of the PCM-insert composite.

System Reference of Equation: keff (W/mK)

52 53 54 55 56 Average

Nitrate melt + foam 0.565 0.496 0.435 0.485 0.522 0.501Nitrate solid + foam 0.965 0.869 0.763 0.851 0.914 0.872Nitrate melt + sponge 0.710 0.547 0.492 0.532 0.602 0.567Nitrate solid + sponge 1.178 0.955 0.766 0.932 1.050 0.978

Fig. 13. Comparison of experimental temperature vs. time curves and predictionsby solving the unsteady-state conduction equation and as predicted by ComsolMultiphysics [27].


decreasing its internal energy while increasing its exergy. The useof liquid air/nitrogen as an energy carrier is not commonly knownuntil fairly recently [59–61]. Cryogenic energy storages can have arelatively high energy density (100–200 W h/kg), low capital costper unit energy, are benign to the environment and have a rela-tively long storage period. However, it has a relatively low efficiency(40–50%) according to the current energy consumption for air liq-uefaction [61]. Cryogenic energy storage is still under developmentby Mitsubishi (Japan) [62], the University of Leeds (UK) and theChinese Academy of Sciences [63]. Specific topics of research involvehydrogen releases [64], the phase change itself [65,66], and inte-grated applications [67,68]. Table 8 and Table 9 illustrate thatalthough the specific heat and phase change heat of the cryogensare of a similar order of magnitude than those of common heatstorage materials, the exergy density of cryogens is much higher.

Once produced and stored in insulated containers, the cryogenis ready to be delivered. No extra energy is required except for thepumping power consumption which is negligible. The only energyloss of the cryogen is the heat dissipation of the cryogenic tank,which can be less than 1% per day using conventional insulationtechnologies [70–72].

The exergy efficiency for a cryogenic engine is about 40% as aconsiderable portion of the exergy is in the form of cold, which iswasted during the heating process. In order to improve the effi-ciency, the recovery of the waste cold energy can be integrated withthe direct expansion of the cryogen using cycles such as the Rankinecycle as shown in Fig. 15. Many power cycles, based on Rankine andBrayton cycles, were proposed to recover the cooling capacity of cryo-

genic liquids [74–77]. Cryogenic liquids vapourizers are commonlyused as low temperature thermal sinks for the bottom Rankine cycle(see Fig. 14), with waste heat or low-grade thermal energy (toenhance exergy efficiency) as thermal sources. Its alternative heatsource could also be using the atmosphere [60], unlike a conven-tional heat engine that uses the atmosphere as a cold reservoir orheat sink. The combined methods for cryogenic energy extractionhave been discussed in detail by Li et al. [61,68]. It has been con-cluded that the combined cycle cryogenic heat engines could havean exergy efficiency of 65–78% under ideal conditions [78].

Chen et al. [79] claimed that the exergy efficiency could be im-proved from ~78 to 117% if the working fluid were superheated to~100 °C, and the exergy efficiency could be doubled if the waste heattemperature is as high as 300 °C.

3.4.2. Latent heat storage with steam accumulatorsThe accumulator is an insulated steel pressure tank containing

hot water and steam under pressure, thus storing energy in the formof latent heat stored in the liquid phase, i.e. water. It releases steamby extracting latent heat when needed and to accept steam whenthe demand is low. The volume specific thermal energy densitiesare typically in the range of 20–30 kW h/m3. The released steampowers the steam turbine for electricity production. Storing steamunder pressure in saturated or superheated conditions in pressurevessels is generally not economic due to the low volumetric energydensity [73]; it is nevertheless applied in specific cases as bufferbetween e.g. a boiler and the steam-driven process of a paper mills[80].

Table 8Comparison of specific heat, latent heat and exergy density of cryogens and some commonly used heat storage materials [69].

Media Storage methoda Specific heat (kJ/kg K) Phase change temperature (°C) Fusion/latent heat (kJ/kg) Exergy density (kJ/kg)

N2 (liquid cryogens) S + L 1.0–1.1 −196 199 762CH4 (liquid cryogens) S + L 2.2 −161 511 1081H2 (liquid cryogens) S + L 11.3–14.3 −253 449 11987

a In this description ‘S’ indicates that thermal energy is stored in the form of sensible heat while ‘L’ stands for latent heat.

Table 9Comparison of physical and chemical exergies of the cryogens [61].

Cryogen Thermal exergy (kJ/kg) Chemical exergy (kJ/kg) Gas density (kg/m3) Liquid density (kg/m3)

Liquid H2 11,897 116,528 0.0824 70.85Liquid N2 762 0 1.1452 806.08Liquid CH4 1081 51,759 0.6569 422.36

Fig. 14. Direct expansion-Rankine hybrid cycle [78].


There are three types of steam accumulators, as described andillustrated in Steinmann and Eck [73] and DOE [81]:

• Water filled accumulator:1. Constant pressure accumulator: usually arranged in a verti-

cal position to allow for both hot and cold water to be storedin the same vessel via thermal stratification, where the hotwater is in the top part and the cold water in the bottom part.

2. Pressure drop accumulator: also called feed water accumu-lators, where pressure and temperature are allowed to vary(see Fig. 15). The accumulator is usually arranged horizon-tally and when fully charged, it is usually at 90% full of water.

• Supercritical steam accumulator: this requires a storage condi-tion of a minimum temperature of 647 K at pressure of 220 bar.

During the thermal storage process the temperature of the liquidwater can be increased by condensation of superheated steam, withlittle variation of the liquid storage mass; or the mass in the volumeis increased by feeding saturated liquid water into the system, wherethe pressure remains constant, charging indirectly where a heat ex-changer is integrated into the liquid volume. The medium flowingin the heat exchanger needs not be water; hence heat from a heatsource at lower pressure can be used.

Steam is charged beneath the surface of the water by a distri-bution manifold, which is fitted with a series of steam injectors, untilthe entire water content is at the required pressure and tempera-ture. If the steam accumulator is charged using saturated (or wet)steam, there may be a small gain in water due to the radiation lossesfrom the vessel.

To avoid a pressure drop in a constant pressure water filled ac-cumulator, the application of a separate flash evaporator is required,where the saturated liquid water is taken from the steam accumu-lator, which is later depressurized externally. Cold water is fed intothe bottom of the storage vessel to keep the water level constant,where mixing of hot and cold water must be minimized. Thermalstress resulting from filling the pressure vessel with cold water mustbe considered [73]. An alternative option is to maintain constantpressure by integrating PCM into the storage vessel partly replac-ing the liquid water (Fig. 16). PCMs usually exhibit a low thermalconductivity so layers of this material must be thin to ensure a suf-ficient heat transfer rate or via the encapsulation of PCM in smallcontainers placed inside the liquid volume. PCM is not only attrac-tive regarding the avoidance of thermo-mechanical stress, the

characteristic volume-specific storage capacity of PCMs is also de-sirable, which is in the range of about 100 kW h/m3, significantlyexceeding water (20–30 kW h/m3) [73].

The design of accumulators should fulfill important criteria, aslisted by DOE [81] and summarized below:

• Provide sufficient time from the end of one overload period tothe beginning of the next, to fully recharge the accumulator.

• Sufficient surplus boiler capacity should be available to re-charge the water stored in the accumulator during off-peak times.

• The accumulator volume should be large enough to containenough water to provide the required amount of flash steamduring the discharge period.

Fig. 15. Pressure drop accumulator [73].

Fig. 16. Constant pressure accumulator with PCM [73].


• Higher steam release rates will produce wet steam, preventedby ensuring a large enough water surface area, and hence againdetermining the accumulator size.

• The evaporation capacity must be sufficient. This is a functionof the pressure at which the water is stored when fully charged(the boiler pressure) and the minimum pressure at which theaccumulator will operate at the end of the discharge period (theaccumulator design pressure). More flash steam is produced whenthe differential between these two pressures increases.

• The accumulator design pressure must be higher than the down-stream distribution pressure, to create a pressure differential andallow for the required flow.

• The steam inlet pipes must feed well below the water surfacelevel to ensure the maximum possible liquid head above them.The steam injectors should be installed at a slight angle to avoiderosion of the vessel, and discharge at a very high velocity toprovide turbulence and mixing of the stored water mass.

• Both the design of the inlet pipe and the manifold system, to-gether with the location of the steam injectors, must provide eveninjection of steam throughout the length of the accumulator re-gardless of the actual steam flow-rate.

The steam injector capacity will reduce as the pressure in thevessel increases because the differential pressure between the in-jected steam and the vessel pressure is reduced. At very low flowrates, the steam feed will not be uniformly distributed and will tendto issue from the injectors closest to the steam inlet pipe.

Supercritical water is water that exists above its thermody-namic critical point (374 °C, 22.1 MPa). Using water as the workingfluid at supercritical conditions, the respective efficiencies can beraised from about 39% for subcritical operation to about 45% usingcurrent technology. The supercritical fluid is a single non-condensablephase which has physical properties that are intermediary betweenits liquid phase and gaseous phase, with a density typically beingcloser to that of water but with transport properties being gas-like. Supercritical water at low density behaves like a non-polarsolvent unlike its liquid counterpart due to loss of hydrogen bonding[82]. It has high thermal efficiency, as it combines high tempera-ture, high pressure and its existence as a single phase which benefitsfrom relative simplicity in system design due to not having to handlemulti-phase flow.

The problems associated with supercritical steam is that the pres-ence of ionic species can induce corrosion. At low ionic density (lowpressure), corrosion is typically more severe in subcritical thansupercritical water, due to the fact that ionic species are insolublein supercritical water at low density. This is the reason that com-ponents and/or piping used to preheat or cool fluids in a supercritical

water system are more susceptible to corrosion than the reactor itself[83–85]. At high ionic density (high pressure), most salts will besoluble. Therefore, pure distilled water should be used in supercriticalwater/steam accumulator to eliminate salt-induced corrosion.

Pressure vessels are designed to have a thickness proportionalto the radius and operating pressure of the tank, and inversely pro-portional to the maximum allowed normal stress of the particularmaterial used in the walls of the container. Theoretically, any ma-terial with good tensile properties that is chemically stable in thechosen application could be employed. No matter the shape, theminimum mass of a pressure vessel scales with the pressure andvolume it contains and is inversely proportional to the strength toweight ratio of the construction material (minimum mass de-creases as strength increases). Godall [86] has suggested that thebest ratio of diameter to total length of a steam accumulator shouldbe between 1:4 and 1:6 in order to obtain optimal condition in termsof thermal storage capacity and material cost.

3.4.3. Moderate to high temperature PCMsWithin the high temperature thermal energy storage option using

PCMs, various alternatives have been investigated, as illustrated inFig. 17. Each of the parallel topics is subsequently dealt with. To il-lustrate the practical use of the results reported in previous sections,design approaches for thermal energy storage installations are furtherelaborated. This will be demonstrated by describing potential ap-plications of high temperature thermal energy storage in three typicalcase studies:

• The encapsulated PCM is mixed within a high temperature con-crete for passive heat energy storage.

• The encapsulated PCM is mixed in a liquid HTF.• The encapsulated PCM is used in a solid–gas conveying system

as heat carrier.

In these case studies, described in Table 10, the coated PCM in-teracts with a different heat carrier, being solid, liquid or gas.

3.4.4. The growing importance of lithium salts3.4.4.1. Lithium for sensible heat storage. Lithium has been used toimprove the properties of molten salts used in CSP by adding it totheir composition. Fig. 18 shows new compositions with lithium andin comparison with the commercial solar salt (34 wt% KNO3 and66 wt% NaNO3): the salts have nearly similar heat capacities, there-fore they have similar energy density. The benefits of adding lithiumto solar salts are noticed in the melting temperature of the com-position, which is lowered. The range of working temperature isextended, whilst improving the thermal stability [91]. Due to the

Fig. 17. Potential applications for TES using E-PCMs.


lower temperatures, it is expected that operational and mainte-nance costs of CSPs are reduced, therefore savings should be obtained[91]. In terms of costs, the U.S. Department of Energy performed acomparison between salts used for CSP, including solar salt (40%KNO3 and 60% NaNO3), and the cost of storing energy with each ofthem in a two tank system [92].

Results are shown in Table 11: despite the higher price of thesalts with lithium nitrate, a lower cost per stored kWth is achieved,

which can be attributed to the higher energy densities and thesavings in heat tracing to keep the salts molten.

3.4.4.2. Lithium compounds as phase change materials. PCMs in-cluding lithium have been a potential for building applications andfor high temperature TES. There are different lithium compoundsto be used in building applications which are compared to the well-known octadecane and Glauber salt in Fig. 19. It can be seen that

Table 10Case studies of high temperature thermal storage applications.

Case study Description

High temperatureconcrete as passivestorage media

A three component thermal storage system, combining sensible and latent heat storage was tested by Laing et al. [44] for use in solarpower plants. Investigations and experiments supported the hypothesis that high temperature concrete could serve as sensible heatstorage up to 500 °C. Within the limitations of the solid concrete structure, the incorporation of phase-change materials is beneficial sincecapable of absorbing a high quantity of energy (sensible and latent heat), while keeping the temperature of the matrix close to themelting point of the phase change.This use of encapsulated PCM mixed with concrete was further demonstrated for applications at ambient temperatures [16,24,87],achieving high energy savings in cooling power.

Micro-encapsulatedPCM in slurry

A micro-encapsulated PCM slurry is a suspension where the PCM is dispersed at 10–20 wt % without significantly altering the physicalproperties of the liquid (density, viscosity) [88]: PCM is microencapsulated using a polymeric capsule and dispersed in water. A slurrywith a 10 wt % concentration of paraffin was conducted by Delgado et al. [89] to investigate the effectiveness of its properties as a thermalstorage material and as a heat transfer fluid. Results demonstrated an increase of approximately 25% in the convective heat transfercoefficient when compared to water, as the result of improving the overall heat capacity of the material through applying the latent heatproperties of the PCM.The feasibility of using ionic liquids as liquid thermal storage media and heat transfer fluids in a solar thermal power plant has been thesubject of additional investigations. According to Bridges et al. [90], nanoparticle enhanced ionic liquids have been shown to increase theheat capacity of the ionic liquid without adverse secondary effects on the ionic liquids’ thermal stability. The ionic liquid physicalproperties were shown to be only affected in cases of high nanoparticle loading. Those similarities between high and low temperatureslurries’ properties tend to indicate that a high temperature equivalent of an encapsulated PCM slurry is possible.

Encapsulated PCM inTES heat transferfluids

Heat storage units of new concentrated solar plants could benefit from a heat transfer fluid which incorporates encapsulated particles ofPCM. The heat transfer capacities would be improved by the high conductivity and the high energy density of the PCM. In comparisonwith a conventional thermal fluid, e.g. Santotherm 350, the specific heat will be increased by a fraction of 3 whilst the energy density willincrease by about 5.The main issues for developing salt-PCM modifications would be the loss of fluidity of the high temperature slurry. Further studies haveto prove if the addition of particles increases the thermal properties of the HTF without affecting the viscosity and associated pumpingpower needed. Existing CSPs would then see their global efficiency improved by the higher energy density of the HTF.

Fluidized bed Storageconcept

High heat transfer coefficients can be achieved between a fluidized bed of coated PCM particles and a heat exchanging surface [7]: heatcan be captured by the particulate gas–solid flowing suspension. Further research is required, since it was impossible to coat the PCMparticles by a non-porous metallic layer (chemical vapour deposition or electrolysis)

Fig. 18. Heat capacity and melting temperature comparison of lithium compositions used in solar power plants (adapted from Ref. 91).


the melting temperatures of some lithium compounds are around10 °C, 30–35 °C and 70–80 °C which are useful for cooling, compa-rable with Glauber salt and useful for domestic hot water respectively[91]. Regarding their heat of fusion, lithium composites at 100 kJ/

kg are considered the lowest to be commercially feasible and thosewith a latent heat in excess of 300 kJ/kg are considered of high po-tential towards different applications [91–94]. Lithium compositionsfor high temperature TES are illustrated in Fig. 20.

The advantage of using Li-based PCMs is illustrated in Table 12,where important properties are compared with a common NaNO3/KNO3 molten salt.

Table 12 shows that a lithium salt mixture has the advantage ofa lower point of fusion and twice the sensible heat compared withthe current solar salt. The density values are quite similar, but theprice of the Li mixture is more than two times that of solar salt. Re-garding physical properties as point of fusion, sensible heat anddensity, a reduction in the size of the molten salt tanks, a reduc-tion of heat tracing parasitics and a reduced amount of material canbe obtained. These improvements will reduce the final prices of in-vestment and cost of energy.

3.4.4.3. Lithium market. As far as the availability of the metal is con-cerned, the worldwide reserves of lithium are estimated in 36.72million tonnes, with Bolivia, Chile, Argentina and China having es-timated reserves of 8.9, 8.04, 7.09 and 5.15 million tonnesrespectively. Other countries in Europe, Australia, Africa, Russia andCanada represent reserves of 0.7–1.5 million tonnes each. As illus-trated in Table 12, the cost of Li salts is about 2–3 times the costof common alkali-nitrates.

3.5. Mechanisms to improve phase change material applications

3.5.1. General principlesTo improve the efficiency of the charging and discharging pro-

cesses of PCMs, the most relevant parameter to be studied is theirthermal conductivity. In order to increase the thermal conductiv-ity of PCMs, several heat transfer enhancement techniques have beenstudied [14,27,34], such as the use of metal carrier structures madeof steel or stainless steel; a dispersion of high conductivity mate-rial i.e. copper, silver or aluminium particles, within the PCM; theimpregnation of high conductivity porous materials, either as a metalfoam (copper, steel or aluminium) or as porous material like graph-ite; the use of high conductivity, low density materials such as carbonfibres and paraffin composites; and the micro-encapsulation of PCMsusing graphite [48], polymers, or the nickel film coating of PCMcopper spheres [95]. The general review of these techniques wasundertaken by Agyenim et al. [96], and the encapsulation of PCMin shells was determined as being one of the most promising andsuitable approaches at the present stage of the investigations. Thereasons behind this selection are the result of the detailed assess-ment, which summarizes as given below.

Table 11Costs of candidate salts for CSP [92].

Composition (wt %) Meltingpoint (°C)

Heat capacity(kJ/kg K)

Energy density(MJ/m3)

Salt price(US$/kg)

Two tank system cost/storedenergy (US$/kWhth)

40% KNO3 + 60% NaNO3 222 1.539 756 1.080 31.21KNO3 + LiNO3 + NaNO3 117 2.32 1524 2.206 14.66KNO3 + NaNO2 + NaNO3 99 1.462 1080 1.266 15.87KNO3 + NaNO2 + LiNO2 + NaNO3 79 1.505 1073 1.928 19.11KNO3 + LiNO3 + NaNO3 + MgK 101 1.579 1181 1.537 16.15LiNO3 + NaNO2 + NaNO3 + KNO3 99 1.557 1114 1.809 18.27LiNO3 + NaNO2 + NaNO3 + KNO2 + KNO3 95.7 1.546 1110 1.797 18.23

Fig. 19. Lithium compositions used in building applications (adapted from Ref. 91).

Fig. 20. Potential lithium compositions for solar energy applications (adapted fromRefs. 91–94).

Table 12Comparison of Li-based PCM and solar salt [93,94].

Mixture Point of fusion (°C) Thermal stability (°C) Sensible heat (kJ/kg K) Density (kg/m3) Price (2014) (US$/tonne)

Solar salt (60% NaNO3 + 40%KNO3) 222 588.51 1.54 2192 716.925.92%LiNO3 + 20.01%NaNO3 + 54.07%KNO3 117 Not defined 2.32 1720 1684.4


3.5.2. Review of applied enhancement methods3.5.2.1. Fins. Fins are being used in order to increase the heat trans-fer surface area with the storage media [97], therefore enhancingthe rate at which thermal energy is distributed within it. They canbe axial or radial and are usually attached to tubes [98]. Depend-ing on their shape and their configuration inside the storage tankwith the storage media, this mechanism has different results. Forexample, according to Zhang and Faghri [99], for internally finnedtubes, the melting fraction volume can be significantly increasedby adding more fins and increasing their thickness and height.

Castell et al. [98] experimented adding longitudinal graphite finsto PCMs (sodium acetate trihydrate whose melting point is 58 °C)and tested if these fins could decrease the solidification time of PCMs.They concluded that despite fins altered natural convection withinthe PCM and they did not enhance the heat transfer coefficient ofthe material, it was possible to decrease the solidification time ofthe PCM. Two cases where analysed: PCMs with small fins and PCMwith longer fins. Both were compared with a base case module whichhad no fins. In the first case, a lower temperature difference wasnecessary to achieve the same heat transfer coefficient as with nofins. Therefore, the needed time to solidify the PCM decreased. Inthe second case, a lower heat transfer coefficient was obtained incomparison to the PCM with no fins. This was attributed to the widthof the fins that should be interfering with natural convection of thePCM. Despite that the heat transfer coefficient decreased, it is im-portant to notice that the solidification time of the PCM was reduced.

Lamberg [100] analysed the two phase solidification problem ina low temperature finned PCM storage system (it used aluminiumfins and paraffin as PCM) by using an approximate analytical model.It is important to note that the geometry of the storage module hada big influence in the accuracy of the analytical model. Hence,through this analysis, it was concluded that the solidification wasdominated by conduction while natural convection existed onlyduring the beginning of the solidification process. In terms of com-parison, the natural convection is negligible compared to conduction.

Erek et al. [97] numerically and experimentally investigated latentheat thermal energy storage with phase change around a radiallyfinned tube. The results showed that the stored energy increaseswith increasing fin radius and decreasing fin space.

Velraj et al. [101] investigated the solidification of PCM in a cy-lindrical vertical tube with radial internal fins arrangement. It wasconcluded that this configuration, which forms a V-shaped enclo-sure for the PCM, gives maximum benefit to the fin arrangement.

With an imposed constant heat flux as a boundary condition,Talati et al. [102], and Mostaffa et al. [103] studied solidification ofa finned rectangular PCM storage analytically and numerically, withthe salt hydrate ClimSel C23 as the PCM and aluminium for the fins.Results showed that when the length of fin to the height of storageratio is smaller than unity, the PCM solidified more quickly in thestorage.

Within the framework of the DISTOR project [104], a prototypeof high temperature thermal energy storage module with graph-ite fins was tested: to increase the transfer area, expanded graphitewas adhered to tubes through which flowed a bi-phase medium.It was concluded that for the same amount of PCM, area and metalmaterial, heat transfer was enhanced by using fins [104].

3.5.2.2. Fluidized bed. The term “fluidized bed” is unavoidably con-nected to the term “particulate solid material” [105–110]. Thegeometrical, physical and aerodynamical properties of the par-ticles affect the onset of fluidization, the characteristics, behaviourand the main parameters of fluidized beds. The most important solidproperties are particle density, diameter, and porosity.

Recently, this technology has been used to store energy throughPCM. Izquierdo-Barrientos et al. [111] studied the performance ofan air-fluidized bed of micro-encapsulated PCM as a thermal storage

system and compared it with a fluidized bed of sand and a fixedbed with sand and PCM. They concluded that PCM is an alterna-tive material that can be used to increase the efficiency of storingthermal energy in the form of latent heat in fluidized bed systems.In fact, under the experimental conditions tested in this work, greatercharging efficiencies are observed for the PCM than with just sandin fixed and fluidized beds. Furthermore, when the charging effi-ciency is stabilized, the PCM shows similar results for both beds.The PCM also presents a better recovery efficiency for both beds afterthe solidification time. Moreover, the influence of the bed heightwas tested. This study showed that it takes longer to achieve a certaintemperature when more mass is used, but higher efficiency valuesare obtained. The analysis of the influence of the flow rate showedthat a higher flow rate allows a faster achievement of the set tem-perature. At the end of the charging process, similar efficiencies aremeasured at the different studied flow rates. The cycling study re-vealed that the PCM suffers attrition during the fluidization process,although no loss of PCM is observed under the experimental con-ditions tested in this work (75 h of continuous operation with 15charging/discharging cycles).

Pitié et al. [7] introduced PCMs and their potential applicationin high temperature energy capture and storage, using a circulat-ing fluidized bed (CFB) as transfer/storage mode. Thermalconsiderations determined the optimum size range for the appliedparticles (<400 μm) when convection heat transfer dominates. Inthe design of a CFB heat collector, the heat transfer coefficientbetween the riser wall and the flowing suspension is an impor-tant design parameter to determine the required heat exchangesurface area.

Peng et al. [112] predicted the thermal behaviour of the systemwith a concentric-dispersion model for a packed bed latent heatthermal energy storage using PCM capsules and a molten salt HTF.The radial heat transfer and wall heat losses were considered. Theeffects of PCM capsule diameter, fluid inlet velocity and storage tankheight on the temperature profiles and efficiency of a packed bedwere investigated for the charging process. The heat transfer betweenPCM capsules and molten salt was shown to be significantly influ-enced by the capsule diameter, with finer sizes shortening theeffective charging time, thus increases the charging efficiency. Thenumerical concentric-dispersion model established by the authorsenables a fair prediction of the thermal behaviour in a packed bedlatent heat thermal energy storage. The fluid inlet velocity has asmaller influence on the charging process, whereas a higher thermalstorage tank increases the charging efficiency.

3.5.2.3. Microencapsulation. Microencapsulation is defined as aprocess in which tiny particles or droplets are surrounded by acoating, or embedded in a homogeneous or heterogeneous matrix,to produce small capsules with many useful properties [113–116].Unless the matrix encapsulating the PCM has a high thermal con-ductivity, the microencapsulation system suffers from low heattransfer rate. The rigidity of the matrix prevents convection cur-rents and all heat transfer needs to occur by conduction. This canreduce seriously the heat transfer rates, especially in the chargingprocess. The most important parameters in microencapsulation arethe thickness of the shell; the geometry of the encapsulation; andthe encapsulation size [117].

Microencapsulation of phase change materials refers to a tech-nique in which a large number of small PCM particles are containedwithin a sealed, continuous matrix which ranges in size from lessthan 1 mm to more than 300 mm [118]. Nowadays the cost of themicroencapsulation is high compared to other storage methods, andit is used only in thermal control applications [116]. Microencap-sulation of PCMs is an effective way of enhancing their thermalconductivity and preventing possible interaction with the surround-ings and leakage during the melting process. This method is classified


into three categories: physical, physico-chemical, and chemicalmethods.

Li [119] studied the improvement in thermal conductivity of PCMs(paraffin) with nano-graphite. It was demonstrated that nano-graphite can be prepared easily from exfoliated graphite andexpanded graphite. Moreover, the price of nano-graphite is low, whilethe improvement effect of nano-graphite on the thermal conduc-tivity is significant. Results showed that the thermal conductivityof the composite PCMs with 1% and 10% nano-graphite is 2.89 timesand 7.41 times of that of paraffin, respectively. The phase changetemperature of the nano-graphite/paraffin composite PCM wasslightly less than that of pure paraffin. As the nano-graphite contentincreased, the thermal conductivity of the nano-graphite/paraffincomposite increased while the latent heat gradually decreased.

Parrado et al. [51] conducted a numerical study of a deform-able spherical copper shell containing solar salt at high temperatures.The thermo-mechanical analysis by Comsol Multiphysics was per-formed during charging and discharging, and showed that thevolume expansion and pressure rise are below the acceptable me-chanical properties of copper and are not expected to crack the shell.

3.5.2.4. Macroencapsulation. A significant quantity of heat storagemedia is encapsulated, where the mass per unit may range fromgrams to kilograms [40]: by careful selection of the capsule geom-etry and the capsule material, the macroencapsulation can be usedfor a wide variety of energy storage needs. The shape ofmacrocapsules varies from rectangular panels to spheres or poucheswithout a defined shape. The macrocapsule shape selection willdepend on the needs and design of each intended application. Ac-cording to Regin et al. [40] the most cost-effective containers areplastic bottles (high density and low density polyethylene bottles,polypropylene bottles), tin-plated metal cans and mild steel cans.In the DISTOR project using PCM and expanded graphite, amacroencapsulation of phase change materials was tested [104]. Theextra volume required in this process, based on the thermal ex-pansion of PCM due to temperature changes, drastically reduced thevolumetric storage capacity [101]. This design was compared withthe graphite fin concept, mentioned above. Results showed that forthe same amount of PCM, the same area and material for heat ex-change, heat transfer is more effective using fins.

Cascades of macroencapsulated PCMs, as will be further de-tailed in Section 3.7, can store 50% more energy per unit volumethan the conventional system using two tanks.

3.5.2.5. Metal matrix. A metal matrix, such as a porous matrix, hasbeen used to enhance the heat transfer rate in TES. A porous matrixis characterized by two parameters, i.e. the porosity and the cell sizewhich refer respectively to the percentage of volume that will beoccupied by PCM and the size of the pore expressed in pores perinch (PPI). For the first parameter, Mesalhy et al. [120] indicated thatthe performance improvement depends on both the porosity of thematrix and its conductivity. Low values of porosity lead to a highereffective thermal conductivity, resulting in an increased perfor-mance enhancement. However, a decrease in the porosity has theopposite effect because the low porosity of the matrix hampers themovement of the liquid PCM and the natural convection. For thesecond parameter, Elgafy et al. [121] showed how the mean poresize of the additives could be of critical importance on the systemperformance. If it is too small, the PCM molecular motion will behindered, thereby it will be very difficult to impregnate the porousmedia with the PCM, which will adversely affect the latent heatstorage capacity.

Metals with the highest thermal conductivities are silver, copper,gold and aluminium [122], which turns them into a desirable optionfor metal matrix production to latent heat thermal energy storagesystems. Their characteristics are given in Table 13. Due to their lower

prices in comparison with gold and silver, copper and aluminiumare most commonly used. Also, nickel has been studied, as re-ported in Table 14.

3.5.2.6. Graphite composites. Another alternative to improve thermalenergy storage materials is producing a new compound with thestorage media. To improve the PCM thermal conductivity, the useof graphite to enhance the heat transfer rate has been investi-gated by numerous researchers.

Mehling et al. [130] demonstrated that PCM/graphite compos-ites show a significant improvement in the heat transfer within latentheat storage materials. They studied two PCMs: the eutectic saltMgNO3/LiNO3·6H2O and paraffin RT50. In fact, the thermal conduc-tivity in the composite material is up to 100 times higher than inthe pure PCM, leading to the phase front moving between 10 and30 times faster.

Py et al. [131] experimented a new supported PCM made of par-affin impregnated by capillary forces in compressed expandednatural graphite (CENG) matrix. High loads of paraffin were ob-tained: from 65% to 95% weight depending upon the bulk graphitematrix density. Composite PCM/CENG thermal conductivities werefound to be equivalent to those of the sole graphite matrix from 4to 70 W/mK instead of the 0.24 W/mK of the pure paraffin. Thermalpower and capacity of the composite are theoretically compared tothose of conventional systems in the case of two usual external ge-ometries: tubes and spherical hollow nodules. The compressedexpanded natural graphite induced a decrease in overall solidifi-cation time and a stabilization of the thermal storage power. Anoptimization procedure of the composite composition was pro-posed according to the antagonistic behaviours of the thermal powerand the thermal capacity in respect to the compressed expandednatural graphite content. Within the usual external heat transfercoefficient range, the estimated compressed expanded natural graph-ite matrix optimized densities fell within the practicable range.

Cabeza et al. [132] investigated three heat enhancement methodsto improve the low conductivity of the PCM water/ice. These werethe addition of stainless steel pieces, copper pieces, and a graph-ite matrix impregnated with the PCM. The use of graphite compositeallows an even larger increase in the heat transfer than with copper.The heat flux is about four times larger on heating and three timeslarger on cooling as compared to using pure ice.

Lopez et al. [49] studied the behaviour of graphite with closedand open pores, made with salt as a PCM. A pore-thermo-elasticanalysis was performed from which was concluded that varia-tions in expansion volumes of the salt due to fusion were limitedby the matrix of graphite, which causes an increase of pressure inthe pores. Higher pressure causes a progressive increase in themelting temperature of the salt and a gradual decrease of the latentheat. This implies that a significant part of the energy supplied tothe material is used to heat it (sensible heat instead of latent heat).

Zhao et al. [133] studied the addition of the highly thermal con-ductive ENG-TSA (expanded natural graphite treated with sulphuricacid) to solar salt (60% sodium nitrate and 40% potassium nitrate),in order to enhance the thermal conductivity of nitrates when used

Table 13Metals with high thermal conductivities [42].

Metal Thermalconductivity(W/mK)

Density(kg/m3)

Specificheat(J/kg·K)

Meltingpoint (°C)

Silver 429 10490 237 961.9Copper 387.6 8978 381 1084.6 °CGold 401 19300 129 1064.4Aluminium 237 2800 910 660.2 °CNickel 90.3 8910 440 1450 °C


as PCM. Different samples of this composite were evaluated. Theresults showed that the highest effective thermal conductivity ofcomposite PCM was 50.78 W/mK, which was 110 times higher thanthat of salt powder. Also, it was concluded that the addition of ENG-TSA causes a decrease in latent heat, but there is no remarkablevariation in phase change temperature.

3.6. Nanoparticle-encapsulated PCMs

As illustrated in Table 15, most of the research work in the lit-erature was conducted on improving the thermal properties oforganic PCMs such as paraffins and fatty acids. Paraffin draws a greatattraction to researchers for its desirable characteristics i.e. good heatstorage density, melting or solidification compatibly with little orno sub-cooling, being inert to most common chemical reagents andof low cost. The main drawback of these PCMs is their low thermalconductivity, hence the growing interest in dispersing high con-ductive nanoparticles within the PCMs [134]. Wu et al. [146] testeddifferent metal nanoparticles in order to enhance the thermal con-ductivity of paraffin wax. The results concluded that Cu/paraffincomposite had a better cooling/heating rate i.e., better thermal con-

ductivity than aluminium, carbon/copper nanoparticle dispersedparaffin composite in the same condition showed in Fig. 21.

Additional recent work on nanoparticle PCMs is summarized inTable 15. The review by Kibria et al. [134] illustrates how dispers-ing nanoparticles in PCMs enhance the overall heat capacity, whilstalso demonstrating the effects upon the PCM viscosity andsubcooling.

Major areas of concern remain to be clarified.

• Although the thermal conductivity of the PCM increases withincrease in concentration of nanoparticles, surface modifica-tion of the nanoparticles might be required. The diameter of thenanoparticles was shown to have a limited effect only. Mostlycarbon nano-tubes and fibres, graphite and graphene have beeninvestigated.

• Metal and metal oxide nanoparticles show an enhancement inthermal conductivity that varies with their shape, size and con-centrations. The enhancement is also temperature dependent.

• The presence of nanoparticles reduces the solidification andmelting time of the composition. Too high a concentration ofnanoparticles could negatively modify other properties of PCM.

Table 14Summary of metal matrix studies.

Author Studied material Main conclusions

Copper studies Zhao et al. [123] Copper foam embedded in paraffinRT58 (low temperature storage)

The addition of metal foams with a cell size of 10 PPI and 95% porosity can increasethe overall heat transfer rate by 3–10 times. Also, smaller porosities and pore densitiesproduced better heat transfer performance, depending on the metal foam parameters.

Vadwala [124] Paraffin wax and open cell copperfoam(low temperature storage)

The equivalent thermal conductivity of a foam-wax composite was found to be 3.8 W/mK, which was 18 times higher than that of pure paraffin wax, i.e. 0.21 W/mK.Moreover, copper foam reduced the time required to melt approximately the sameamount of wax to 36% of that without the use of metal foam.

Li et al. [125] Copper matrix and sodium nitrate(high temperature storage)

Heat transfer coefficient of the thermal energy storage with copper matrix can besignificantly increased up to 28.1 times by heat conduction when the PCM is in thesolid phase, and up to 3.1 times by the combination of natural convection and heatconduction when the PCM is in the liquid phase. Hence, both the melting andsolidification times are substantially shortened. In addition, it was concluded thatporosity plays a more important role to enhancing the heat transfer efficiency inthermal energy storage system than pore density.

Aluminiumstudies

Bauer and Wirtz[126]

Composite developed by plate-likethermally conductive aluminiumcontaining pentaglycerine as PCM

An increase in aluminium foam metal fraction results in an increase in the effectivethermal conductivity of the composite because only about 2% of the heat flow isthrough the PCM, and the interfacial bond resistance decreases due to increasedcontact area. The trade-off is a reduced charging time versus a reduced volumetriclatent heat storage due to the increasing aluminium foam metal fraction.

Tong et al. [127] Water/aluminium foam composite(low temperature storage)

An increase in the heat transfer rate during melting and freezing of the PCM (water).

Nickel studies Xiao et al. [128] Paraffin/nickel foam composite(low temperature storage)

It was demonstrated that thermal conductivity of the paraffin/nickel foam compositewas nearly three times larger than that of pure paraffin, 0.4 and 1.2 W/mKrespectively.

Fiedler et al. [129] compared a copper and aluminium metal matrix and found that copper matrices achieve approximately an increase of 80% in the effective thermal con-ductivity compared to aluminium matrices. It was found that a further increase is possible using diamond-coated copper matrices. Xiao et al. [128] and Fiedler et al. [129]concluded that copper is an acceptable matrix material.

Table 15Recent relevant publications concerning nanoparticle-PCM (>2011).

Authors Topics

Kibria et al. [134] Review of thermophysical properties of nanoparticle-dispersed PCMWarzoha et al. [135] Temperature-dependent thermal properties of a paraffin PCM embedded with herringbone style graphite nanofibresSahan et al. [136] Improving thermal conductivity PCM: a study of paraffin nanomagnetite compositesYu et al. 2014 [137] Influence of nanoparticles and graphite foam on the supercooling of acetamideJourabian et al. [138] The expedited melting of PCM through dispersion of nanoparticles in the thermal storage unitMotahar et al. [139] A novel PCM containing mesoporous silica nanoparticles for thermal storage: thermal conductivity and viscosityCingarapu et al. [140] Nanofluids with encapsulated tin nanoparticles for advanced heat transfer and thermal energy storageSciacovelli et al. [141] Melting of PCM in a thermal energy storage unit: Numerical investigation and effect of nanoparticle enhancementKashani et al. [142] Numerical study of solidification of a nano-enhanced PCM in a thermal storage systemKhodadadi et al. [143] Thermal conductivity enhancement of nanostructure-based colloidal suspension utilized as PCM for thermal energy storageAbolghasemi et al. [144] Heat transfer enhancement of a thermal storage unit consisting of a PCM and nanoparticlesGuo and Wang [145] Numerical investigation of nanoparticles-enhanced high temperature PCM for solar energy storage


• Nanoparticles generally reduce the latent heat capacity of PCM.The determination of the optimum addition of nanoparticles hasto be explored.

• Predicting the thermal conductivity and latent heat capacity ofa nanoparticle-PCM is very difficult.

• Nanoparticles present affect the nucleation and decrease thesuper-cooling of the PCM.

• The presence of nanoparticles will moreover significantly in-crease the viscosity of the PCM-melt.

As far as now, mostly organic nanoparticles have been used. Itis however not excluded to use mineral or metal nanoparticles insuspension with the HTF. The segregation of these denser par-ticles from the lighter HTF needs however to be cautiouslyinvestigated.

3.7. Cascades of PCM systems

The use of multiple component PCMs in PCM-TES systems hasbeen shown as an effective way for the enhancement in their per-formances [134]. It means that in a system more than one PCM areused. These PCMs have different melting points. The differencesbetween the temperatures of HTF and the melting points of the PCMshave significant effects on the heat transfer rate of the unit and thus

enhance the performance of the system during the charging(melting) and discharging (solidification) process. In the case of asingle PCM, this temperature difference decreases in the flow di-rection of HTF, thus leading to a decrease in heat transfer rate andpoor performance of the unit. If multiple PCMs with different meltingtemperatures are packed in the unit, in decreasing order of theirmelting points, nearly a constant temperature difference can bemaintained during the melting process, even though the temper-ature of HTF decreases in the flow direction. This leads to an almostconstant heat flux to the PCMs. During discharging, if the flow di-rection of the HTF is reversed, the PCMs remain in increasing orderof their melting points. A nearly constant heat flux from the PCMto HTF is maintained [34,147–149].

An example of experimental multiple PCM is shown by Kuraviet al. [34] the shell side is loaded with different PCMs and synthet-ic oil is allowed to flow through the inner tube, all PCMs beingarranged in the flow direction (axial direction) in contact with theHTF.

In solar thermal applications, the PCMs are mostly kept in thestorage tank, which contains the HTF, used as the heat carrier[150,151]. Since the PCM is surrounded by the HTF, there is no axialtemperature variation within the HTF. In such configurations, mul-tiple PCMs should be arranged in the radial direction instead of inthe axial direction. With multiple PCM arrangement, it is possibleto have lowest melting point PCM at the centre and other PCMs ar-ranged in the increasing order of their melting points from the centreof the unit to the outer wall. This may lead to more or less simul-taneous melting at all points, because a nearly constant temperaturedifference can be maintained in the radial direction.

A similar work, with three coaxial cylindrical tubes to store threePCMs and the entire unit being kept in water bath, has been re-ported by Wang et al. [152,153]. The PCMs are arranged in a similarway as discussed above. The experiments for the charging processwith different HTF temperatures indicate that the melting time inthree PCMs system is decreased by 37–42%, as compared with thatof the single PCM unit, which has been attributed to the fact thatthe three PCM unit terminates the melting process earlier by 15–25%. With multiple phase PCMs, it is possible to produce variabledistribution of phase change temperature [151,152]. In the systemswith constant temperature boundary conditions, the distributionof a linear phase change temperature can be realized by properlyarranging those PCMs with different melting points. Simulation andexperimental results indicated that a 25% reduction in phase changetime can be achieved by using multiple phase PCMs. It is also foundthat the efficiency in the enhancement is dependent on the numberof the PCMs. For practical applications, the number of the PCMsshould be in the range of 5–10 [147,154].

Peiró et al. [155] performed an experimental evaluation of theadvantages of using the multiple PCM configuration instead of thesingle PCM configuration in thermal energy storage (TES) systemsat pilot plant to fill the gap of experimental and high scale studieson this concept in the literature. For the evaluation were selectedtwo PCM with melting temperatures in the range of 150–200 °C dueto their high value of heat of fusion and compared: d-mannitol andhydroquinone. Three configurations were evaluated: single PCMwith hydroquinone; single PCM with d-mannitol; and multiplePCM with hydroquinone and d-mannitol. Results showed that themultiple PCM configuration introduced an effectiveness enhance-ment of 19.36% if compared with single PCM configuration as wellas a higher uniformity on the HTF temperature difference betweenthe inlet and outlet.

In order to improve cascade thermal energy storage, Tian et al.[156] experimented adding metal foam to the PCM. Exergy effi-ciency of STES cannot be significantly improved by CTES (15% to ca.30%), nor by MF-CTES; exergy transfer rate of STES is increased byCTES (up to 23%), and is further increased by MF-CTES (by 2–7 times).

Fig. 21. Heating (a) and cooling (b) curves of PCMs with different metal nanoparticles[146]. Charging cycle (left); discharging cycle (right).


4. Thermo-chemical heat storage

4.1. Principles

One of the most novel approaches to the problem of storingthermal energy from a solar collection field, or from transformingexcess renewable power (wind turbines, photovoltaics) into heat[14], is the use of reversible chemical reactions. Unlike sensible orlatent thermal energy storage, mostly limited in time due to heatlosses, chemical energy storage enables to bridge long durationperiods between supply and demand, hence making it particular-ly suitable for a large scale electricity generation. In such a system,as outlined in Fig. 22, excess energy is converted into high gradeheat at the absorber containing the reactant during the chargingcycle: the high grade heat is converted into chemical energy by pro-moting an endothermic reaction whereby reaction products areeither stored for re-use in the reverse reaction, or exhausted. At thetime of energy demand by the central power generation plant orother users, the high grade heat is regenerated through the reverseexothermic reaction cycle and is converted into electricity and lowgrade heat (CHP concept) through e.g. a standard steam-driven turbo-alternator or through a heat carrier powered Stirling engine.

A large number of reversible chemical reactions can be used forthermo-chemical energy storage, each within a given range of equi-librium temperatures and heats of reaction [157–163]. Table 16selects some of the possible reaction pairs, within the target tem-perature range of 300 °C to ±1000 °C. Numerous low temperaturereversible reactions are not included. Clearly, the Mn2O3—MnO pairdoes not match the proposed temperature range, but can similar-ly equivalent to oxides, be used in the reaction of MnO and H2O toproduce H2 [164–168].

The selection of the optimum system for high temperature ap-plications is governed by both the overall thermodynamicperformance, and by chemical engineering criteria (kinetics, designfactors, etc.) which determine practicability and cost of operation.In a complete analysis, these factors are normally combined in termsof global cost-effectiveness, expressed as the levelized cost of elec-tricity (LCOE), and previously discussed in Section 1. At the presentstage of development, there is a lack of comparative data for such

a complete analysis. It is however important that an early guide asto the viability of each system is obtained. The present section henceexamines thermodynamic and engineering criteria to provide a sci-entific comparison among the systems listed before [169,170]. Theanalysis will moreover provide guidance as to the important as-sessment criteria of new candidate reaction systems.

4.2. Thermodynamic assessment

4.2.1. Gibbs’ free energy and work recoveryA reversible reaction system suitable for high temperature energy

storage, and subsequent steam turbine or Stirling engine use for workrecovery, should absorb heat at temperatures below 1000 °C in orderto lessen problems of material limitation and heat losses, and shouldfurthermore be capable of delivering heat to the turbine inlet at typ-ically 500–600 °C [168]. As initially presented by Williams and Carden[171], the thermodynamic criteria for the preliminary screening ofcandidate reactions are assessed from available thermodynamic equi-librium data. Williams and Carden [171] proposed to use twoparameters for the initial screening of reversible reactions. The firstis the “equilibrium” temperature, T*, i.e the temperature at whichneither forward nor reverse reactiions are thermodynamically fa-voured. T* is derived by applying the definition of Gibbs’ free energyto the condition of T*.

ΔG T* P,( ) = 0 (7)

where ΔG is Gibbs’ free energy change for the reaction, i.e.ΔG = ΔH − TΔS, in this case

TH T PS T P

***

= ( )( )

ΔΔ

,,

(8)

with ΔH and ΔS respectively the reaction enthalpy and entropy atpressure P and temperature T*.

The second screening parameter is the maximum work recov-ery efficiency, ηw

max, defined as the ratio of the maximum availableor ideal work to the actual work, and obtained from standard chem-ical thermodynamics [42].

The “lost work” is only zero, when a process is completely re-versible. For irreversible process, the energy that becomes unavailablefor work is positive, and ~TsΔS. The engineering significance is clear:the greater the irreversibility of a process, with a greater the rateof entropy generation corresponding with a greater amount of energythat becomes unavailable for work.

The maximum work recovery efficiency is hence:

ηwmax s

ideal

s

s

WW

G T PH T P

= = ( )( )

ΔΔ

,,

(9)

where Ts represents the ambient or the sink temperature. The at-tainment of the maximum work implies that the endothermic heatexchange operation must be reversible and complete, whilst alsothe exothermic reaction must proceed reversibly. The reaction must

Fig. 22. Schematics of the thermal energy storage by reversible chemical reactions.

Table 16Possible reaction pairs [157–163].

Reaction Teq (P = 1 atm) (°C) ΔHr at Teq (kJ/kg)

Mg(OH)2 ↔MgO + H2O 259 1396MgCO3 ↔MgO + CO2 303 1126Ca(OH)2 ↔CaO + H2O 479 1288CaMg(CO3)2 ↔MgO + CaO + 2CO2 490 868CaCO3 + H2O ↔

Ca(OH)2 + CO2

573 137

CaCO3 ↔ CaO + CO2 839 17032Co3O4 ↔ 6CoO + O2 870 8445Mn2O3 ↔ 5Mn3O4 + O2 90 6 185Mn2O3 ↔ 2MnO + 1/2 O2 1586 1237


hence proceed so that chemical equilibrium is maintained at all tem-peratures. All the reaction heat and all heat imbalances in the heatexchanger must be converted into maximum work by use of a Carnotcycle available throughout the temperature range. The advantageof both T* and ηw

max is that they may be estimated to a fair approx-imation for a given reaction by use of standard reference data alone.

4.2.2. Calculation resultsFrom the calculations, Figs. 23 and 24 illustrate both screening

parameters, T* and ηwmax, for the selected reaction pairs. Fig. 23 plots

the equilibrium temperature versus the equilibrium pressure for thedifferent reactions in Table 17. The maximum work efficiency versusthe sink temperature is plotted in Fig. 24.

To make a selection between the reactions, different aspects needto be taken into consideration. Firstly, the temperature should pref-erably be as close as possible to 500–900 °C. Secondly a highermaximum work recovery efficiency is preferred. But also the prac-tical aspects need to be considered, because adding water vapouris easier than collecting, storing and injecting carbon dioxide, whilstO2 reactions are most favourable since air can be the reactant of thereverse reaction. Oxides and hydroxides are thus favoured abovecarbonates.

Fig. 24 shows that the temperature is not very dependent on thepressure for most of the systems selected except at low partial pres-sures of the gas/vapour phase, as commonly encountered when a

separate gas carrier is used to exhaust the gas/vapour reaction prod-ucts. For Mn2O3 the temperature is a more outspoken function ofpressure for all pressures. The work efficiency is rather low for car-bonates and hydroxides. Oxides provide the higher value of workefficiency in the selected range of sink temperatures.

4.3. Kinetics from thermogravimetric analysis

Thermogravimetry (TGA) measures the weight of the sampleevery second, recording M0, Mt and M∞, at the initial measure-ment time t0, time t and the time for completing the reaction t∞.The conversion is calculated as

α = −− ∞

M MM M

t0

0

(10)

The reaction rate constant can be derived within the completetime and/or temperature scale, as described in detail for other chem-ical process by Brems et al. [172,173].

For example, typical TGA plots for Mg(OH)2 are illustrated inFig. 25. The decomposition rate is slightly dependent on the tem-perature ramp, β. Similar weight loss curves were obtained for allreaction pairs tested.

Additional TGA plots are illustrated in Figs. 26 and 27 for the re-versible reactions of Mn2O3 ↔ Mn3O4 and Co3O4 ↔ CoO, respectively.

Clearly, the oxidation reaction is slower than the reduction andincomplete, assumed to be the results of compaction of the reducedreaction compound, hence hampering diffusion of O2 during thereverse reaction [170,174]. This is also illustrated in the BET ad-sorption curves of Mn2O3 and Mn3O4 (Fig. 28) [170].

A fair approximation of the reaction kinetics is obtained by trans-forming the TGA data, using the Arrhenius equation of the reactionrate constant, k:

k Aexp= −⎛⎝⎜

⎞⎠⎟

ERT

A (11)

Plotting ln(k) versus 1/T provides –EA/R as the slope of the linearfit, and ln(A) as ln(k) at 1/T = 0.

In this equation k is the reaction rate constant; A the pre-exponential factor with same units as k; Ea the activation energy,J/mol; R the universal gas constant, 8.314 J/mol K; and T the tem-perature, K.

The rate of reaction, dα/dt, is a function of the rate constant k(dependent on temperature) and a function of conversion f(α) (in-dependent on temperature). This results in:

ddt

k T fα α= ( ) ( ) (12)

Since k is described by the Arrhenius expression, the overall ex-pression of the reaction rate is:

Fig. 23. Equilibrium temperature versus pressure for selected reaction pairs.

Fig. 24. Maximum work efficiency vs. sink temperature for the selected reaction pairs.

Table 17Screening parameters for a selection of reaction pairs.

Reaction Teq (P = 1 atm) (°C) ηwmax (30–120 °C) –)

Mg(OH)2 ↔ MgO + H2O 259 0.28–0.43MgCO3 ↔ MgO + CO2 303 0.34–0.48Ca(OH)2 ↔ CaO + H2O 479 0.47–0.60CaMg(CO3)2 ↔ MgO + CaO + 2CO2 490 0.49–0.61CaCO3 + H2O ↔

Ca(OH)2 + CO2

573 0.55–0.64

CaCO3 ↔ CaO + CO2 839 0.64–0.732Co3O4 ↔ 6CoO + O2 870 0.73–0.826Mn2O3 ↔ 4Mn3O4 + O2 906 0.64–0.74


ddt

k T fE

RTfAα α α= ( ) ( ) = −⎛

⎝⎜⎞⎠⎟ ( )Aexp (13)

Different methods were tested to fit the experimental data, i.e.(i) iso-conversional methods (Friedman, Flynn–Wall–Ozawa andKissinger–Akahira–Susone); and (ii) the mechanistic methods basedupon the Coats–Redfern approach, using different expressions forf(α). The iso-conversional methods do not provide the expected

linearity of the conversion rates expression versus 1/T for variousβ-values, resulting in widely different values of the activationenergy with degree of conversion. Within the different f(α) ap-proach (nucleation, diffusion, reaction order), the expression as afirst order reaction [f(α) = −ln(1 − α)] provides a very fair fit for allof the reaction pairs tested. The activation energies and pre-exponential factors (A) were hence determined by solvingEq. 13 for the experimental values of α, and using a first order

Fig. 25. TGA curves of Mg(OH)2 at different heating rates [168].

Fig. 26. Weight vs. time graph for charging and discharging cycles of Mn2O3 ↔ Mn3O4 at β = 5 K/min [170].

Fig. 27. Weight vs. time graph for charging and discharging cycles of Co3O4 ↔ CoO at β = 5 K/min [170].


expression. Values are given in Table 18, with A determined forT > Teq + 100 K:

For the given reversible reactions, the maximum conversion isgiven by Eq. 13 above and the reaction stoichiometry. To achieve99% of the maximum conversion in a first order reaction, Eq. 11 de-termines the required reaction time above Teq below 300 s for allof the reactions studied. This is however tentative only, pilot/largescale experimentation is certainly required.

Recently, the German Aerospace Center (DLR) had been alsotesting TCS heat storage materials at laboratory scale. In order toinvestigate TCS at a larger scale, a test bench as well as a reactorcontaining around 20 kg of reaction material was built and broughtinto operation. Due to the good availability at low cost and itsfavourable temperature range for CSP plants, previous work at DLRfocused on the reversible dissociation reaction of calcium hydrox-ide [175–177]:

CaO H O Ca OH Hs g s reaction( ) ( ) ( )+ ↔ ( ) +2 2 Δ

The dehydration was performed at 450 °C and the rehydrationat about 550 °C. Reversible conversion of 77% of the material withno degradation effects after 10 cycles was determined.

Within their results they evidenced losses of chemically storedthermal energy, but they concluded that these could be estimatedby implementing a loss term into their simulation and accountedfor around 2 kW during the first minutes of the reaction. Addition-ally, the good agreement between experimental and simulated resultsshowed that the reaction kinetics determined in micro-scale mea-surements as well as the known bed properties offered a sufficientrepresentation of the effective conditions within the bed. Anyhow,research is still needed in order to develop TCS at an industrial level.

The selection of the optimum reaction system for high temper-ature applications is governed by both the overall thermodynamicperformance, and by chemical engineering criteria (kinetics, designfactors, etc.) which determine practicability and cost of operation.In a complete analysis, these factors are normally combined in termsof global cost-effectiveness, expressed as the levelized cost of elec-tricity (LCOE), as explained in Section 1. At the present stage ofdevelopment, there is a lack of comparative data for such a com-plete analysis. It is however important that an early guide as to theviability of each system is obtained, and this is provided by ther-modynamic calculations and kinetic measurements.

From the previous and preliminary assessment, TCS certainlyoffers a potential for medium to high temperature heat storage. Thereactions are fast and generally reversible (except Mn2O3 and Co3O4).The reaction rate itself can be considered as moderate (≤300 s), andis thereafter not a limiting factor. The heat penetration inside thechemicals’ packed bed is governed by conduction, where the con-duction approach of PCM storage, as developed in Section 3.3 (withor without inserts), will determine the rate of heat penetration, andhence the overall charging/discharging rate of the TCS system.

Additional pilot-scale experiments with Co3O4 and doped Co3O4

were examined by Tescari et al. [178] and the reaction concepts werenumerically modelled by commercial CFD software. Adding up to10 wt% of non-reactive oxides to the Co3O4 is shown to prevent con-solidation and enhance the reversibility of the Co3O4/CoO redoxreaction.

5. Containment of LHS and TCS materials

5.1. Preliminary selection concerns and criteria

5.1.1. Pressure upon phase change or chemical reactionPhase change materials and thermochemical storage (TCS)

systems certainly offer a significant potential towards thermal energystorage. Of course, the materials used need to be properly con-tained in e.g. a spherical capsule, heat exchanger tube or sandwichplates, or in any other container that meets the required need ofbeing closed and able to withstand the high temperatures envis-aged, to resist possible chemical attacks by the contained materials[179–184], and to resist the pressure build up during the phasechange (expansion for solid to liquid phase) or during the releaseof gases and vapours during the thermochemical transformations.This containment is hereafter referred to as “encapsulation”.

Fig. 28. The BET isotherm linear plots of Mn2O3 (a) and Mn3O4 (b), respectively.

Table 18First-order reaction results for different reactions.

Reaction EA

(kJ/mol)Teq (P = 1atm) (K)

A (s−1)

Mg(OH)2 ↔ MgO + H2O 198 532 4.26 × 1016

MgCO3 ↔ MgO + CO2 52 576 86.3Ca(OH)2 ↔ CaO + H2O 131 752 2.09 × 106

CaMg(CO3)2 ↔ MgO + CaO + 2CO2 219 763 1.15 × 1012

CaCO3 + H2O ↔ Ca(OH)2 + CO2 185 846 2.89 × 108

CaCO3 ↔ CaO + CO2 228 1112 7.03 × 107

2Co3O4 ↔ 6CoO + O2 189 1115 1.41 × 106

6Mn2O3 ↔ 5Mn3O4 + O2 254 1179 2.60 × 108


In most PCM systems, the density of the liquid is 3–5% lower thanthe density of the corresponding solid. In thermochemical storage,gas (CO2, O2) or H2O-vapour are formed, again with a correspond-ing pressure increase in the encapsulation. This pressure increasecan be significantly higher than in PCMs, and only special mea-sures such as a pressure relief valve on the encapsulation will beable to cope with the ΔP.

For PCM or TCS encapsulation, mostly tubular containment willbe applied. A spherical encapsulation, applied by e.g. vapour de-position or electrolysis is also possible, but with additional problemsof very high pressure build-up on the capsule [51].

For PCM encapsulations, the pressure to be considered can becalculated as e.g. for a capsule of volume V, filled for ~95% withmolten PCM. After cooling and solidification, the filling volume willreduce (through contraction) by e.g. 3.1% for Sb2O3 and 4.4% for KNO3

respectively. The tube is then closed by welding a lid onto the cyl-inder with material-appropriate welding rods, or by a ceramic lidin the event of using ceramic tubes. The enclosed air will expandupon heating and re-melting of the PCM.

Since the starting pressure is 1 bar, the internal pressure uponmelting will increase (reducing free volume and increasing tem-perature) to P2.

For Sb2O3

PV

VP P2 1 1

0 05 1 0 050 05

273 15 750273 15 20

5 6= + −( ) ×[ ] × +( )+( )

≈. . %.

..

.

For KNO3

P P P2 1 10 05 0 95 0 044

0 05273 15 400273 15 20

4 2= + × × +( )+( )

≈. . ..

..

.

The internal pressure on the encapsulation will hence increaseto approximately 5.3 bar for Sb2O3 and 4.2 bar for KNO3. These valuesare slightly pessimistic since the expansion of the capsule shouldbe taken into account (V increases as T increases). In TCS-systems,such pressure increases are not expected, since a tubular encapsu-lation will be vented to the atmosphere at 1 bar during theendothermic reaction, whilst pressures will be set at 1–3 bar duringthe exothermic reverse reaction. The mechanical analysis of E-PCMsis hence also valid for the reversible reaction TCS case.

In the event of a complete encapsulation such as in coated PCMspheres [51], no air buffer is present, and the capsule material willneed to fully withstand the expansion of the phase change.

5.1.2. Design of the encapsulationCalculations should focus upon normal design parameters for

straight pipes under internal pressure, to determine:

• minimum required wall thickness• comparison with Young’s modulus (limit of plastic deforma-

tion) [51]• pressure to rupture [185]

These calculations are available on Internet calculators, provid-ed by ASME and engineering companies.

5.1.2.1. Wall thickness. When the wall thickness to withstand an in-ternal pressure is used as a design parameter, it requires that therelief-valve setting may not be higher than the design pressure. Forpiping, the chosen design pressure and temperature are themaximum intended operating pressure and the coincident tem-perature combination. This results in the maximum wall thickness.The allowable stresses for possible capsule materials will be givenin Section 5.2.

The formula for the minimum required wall thickness tm is givenin Eq. 14. For straight metal pipes under internal pressure, thisformula is valid for D0/t ratios larger than 6. The factor Y in thisformula varies with material and temperature. It is used to take theredistribution of circumferential stress into account: this redistri-bution occurs under steady-state creep at high temperatures andpermits a slightly lesser thickness at this range.

tPD

SE PYCm =

+( )+0

2(14)

where

tm = minimum required wall thickness, m;P = design pressure, Pa;D0 = outside diameter of pipe, m;C = sum of allowances for erosion, corrosion and any thread orgroove depth, m;SE = allowable stress, Pa;S = basic allowable stress for materials, excluding casting, joints,or structural-grade quality factors, Pa;E = quality factor as the product of several parameters. Typicalquality factors are: casting quality factor E0, joint quality factorEj, and structural-grade quality factor Es.Y = coefficient: 0.4 for ductile nonferrous materials, 0 for brittlematerials such as cast iron.

5.1.2.2. Pressure to deformation and to burst (rupture). The Barlowformula [183] for the pressure to deformation or to burst is givenin Eq. 15:

PSt

Dw= 2

0ϕ(15)

with:

P = pressure, Pa;S = material strength, Pa, where the time-dependent creepstrength at temperature is commonly usedtw = wall thickness, m;D0 = outside pipe diameter, m;φ = safety factor: calculations commonly use 1 as safety factor

5.1.2.3. Young’s modulus. Young’s modulus (tensile modulus or elasticmodulus) characterizes the material and is the stiffness of an elasticmaterial. The definition of Young’s modulus is the ratio betweenthe stress along an axis, and the strain along that axis in the rangeof stress in which Hooke’s law is valid. The strain is defined as theratio of deformation over the initial length. Eq. 16 shows the def-inition of Young’s modulus.

Etensile stress

extensional strainF A

LFL

A L= = = =σ

ε0

0

0

0Δ ΔL(16)

When a material has high Young’s modulus, it is rigid. It is im-portant to not confuse rigidity with strength, stiffness, hardness ortoughness. The difference between rigidity and stiffness is that ri-gidity characterizes the material (intensive property), the stiffnesslooks to products and construction (extensive property). The hard-ness of a material is the relative resistance of its surface to thepenetration of a harder body. Toughness is the amount of energythat can be absorbed by the material before fracturing when sub-jected to strain. Young’s modulus indicates e.g. how much a sampleextends under tension or shortens under compression. Young’smodulus is directly applicable to uni-axial stresses, these are tensile


or compressive stresses in one direction without stress in the otherdirections.

5.1.3. Preliminary selection criteriaIn view of the concerns of operating temperature, thermal cycling

between charging and discharging, possible oxidation by air or chem-ical attacks by the PCM or TCS reactants, the selection is ratherlimited and illustrated in Table 19.

5.2. Relevant properties of E-materials

Since extensive properties are provided in standard handbooksor websites of suppliers [187–189], or by the American Iron and SteelCorporation [190], only essential properties are summarized here-after; with an example more detailed description for INCONEL® alloy600.

5.2.1. INCONEL alloy 600 [187]INCONEL® alloy 600 is an engineering material with excellent

mechanical properties, high strength and good workability. It is usedfor applications which require resistance to corrosion and heat. Thehigh nickel content makes INCONEL alloy 600 resistant to corro-sion by organic and inorganic compounds and to chloride-ion stress-corrosion cracking. Chromium makes the alloy resistant to sulphurcompounds and to oxidizing conditions at high temperatures or incorrosive solutions. INCONEL alloy 600 is used in a variety of ap-plications with temperatures from cryogenic to above 1095 °C.Mechanical properties are listed in Table 20 for tubes. The yieldstrength is between 170 and 345 MPa, the yield strength togetherwith elongations of 35–55% [187] makes the alloy easy to fabricate.

Fig. 29 shows the high-temperature tensile properties of an an-nealed (870 °C/1 hr) hot-rolled plate. Data for other materials arealso included.

Although both INCONEL alloy 600 and Incoloy 825 are applica-ble at temperatures beyond ~700 °C, the mechanical propertiesdecrease significantly with increasing temperature.

Fig. 30 presents the rupture life of hot-rolled material at tem-peratures of 538–1093 °C, and demonstrates that the applicable stressdecreases with temperature and duration of use.

INCONEL alloy 600 with a minimum nickel content of 72% is vir-tually immune to chloride-ion stress-corrosion cracking. Stress-corrosion problems occur in INCONEL alloy 600 in high-temperature,high-strength caustic alkalis. In this case the material should bestress-relieved at 900 °C/1 hr or 790 °C/4 hr prior to use and oper-ating stresses should be kept to a minimum. Fig. 31 shows the alloy’sresistance to oxidation and scaling at 980 °C, with other potentialalloys included. This figure was obtained from weight-loss deter-minations which indicate the ability of a material to retain aprotective oxide coating under conditions of cyclic exposure to thetemperature. INCONEL alloy 600 offers the best long term oxida-tion resistance.

When the machinability of INCONEL alloy 600 is compared withstainless steel, it is slightly more machinable than Type 304 andslightly less machinable than Type 303. The joining of INCONEL alloy600 is done by conventional welding processes, using specific elec-trodes and either shielded metal-arc, gas tungsten-arc and gas metal-arc, or submerged-arc welding.

5.2.2. Incoloy 825 [191]Similar to Inconel alloy 600, Incoloy® alloy 825 is a Ni—Fe—Cr

alloy with excellent thermal properties and corrosion resistance. Thepresence of molybdenum and copper enhances the resistance to areducing atmosphere and to pitting and crevice corrosion.

Table 19Selection of encapsulating materials (− poor; + acceptable; ++ excellent) [186].

Required property AISI 316 AISI 304 AISI 310 AISI 321 Incoloy 825 Inconel 600 Ceramics

Resistance to corrosive environments + + + ++ ++ ++ ++Resistance to Cl− stress corrosion − − − + ++ + ++Resistance to reducing environments + + + ++ ++ ++ ++Resistance to oxidizing environments ++ ++ ++ ++ ++ ++ ++Resistance to NO3

− − − − + ++ + ++Resistance to localized corrosion − − − + ++ + ++Application 500–700 °C + + ++ ++ ++ ++ ++Application 700–900 °C − − + ++ ++ ++ ++Application <900 °C − − − − ++ ++ ++Possible lid-welding ++ ++ ++ ++ ++ ++ −

It is clear that only AISI 310 (to 700 °C), AISI 321, Incoloy 825, Inconel 600 and ceramic tubes (to 900 °C) meet the required general properties at temperatures in excess of600 °C. In addition to these properties, several thermal and mechanical properties will determine the final selection, together with the mechanical calculations of the ge-ometry of the tubular encapsulation.

Table 20Typical mechanical properties for tubular INCOLOY alloy 600 [187].

Tensilestrength(MPa)

Yield strength(0.2% offset)(MPa)

Hardness,Rockwell (−)

Hot-finished 520–690 170–345 −Annealed hot-finished 520–690 170–345 −Cold drawn, cold-finished 990 915 34 Rockwell CCold drawn, annealed 550–690 170–345 88 max. Rockwell B

Fig. 29. High-temperature tensile properties of annealed, 870 °C/1 hr, hot-rolled plateof various alloys (adapted from Refs. 186 and 187).


The addition of chromium confers resistance to different oxi-dizing substances such as nitric acid, nitrates and oxidizing salt. Theaddition of titanium, in combination with an appropriate heat treat-ment, stabilizes the alloy against sensitization to intergranularcorrosion.

Physical and mechanical properties are discussed in literature[190,191]. INCOLOY alloy 825 has good mechanical properties fromcryogenic temperatures to moderately high temperatures. When thetemperature exceeds 540 °C, microstructural changes (phase for-mation) can occur which lower ductility and impact strength. Soat temperatures where creep-rupture properties are design factors,the alloy is not used. Some tensile properties of INCOLOY alloy 825are given in Table 21: the alloy can be strengthened significantlyby cold work.

Fig. 29 showed the high-temperature tensile properties and in-dicated the typical usage range. The ultimate tensile strength of thematerial was found to be 720 MPa. The alloy has good impactstrength at room temperature and retains its strength at hightemperatures.

5.2.3. Extruded tubular ceramics [192–196]Ceramic materials are far less documented than their steel coun-

terparts. In tubular ceramics, mostly alumina and its composites,zirconia, SiC and SiN are used. The alumina ceramics are recog-nized for their ease of production and moderate costs, although stillbetween 4 and 5 times the cost of their steel equivalents. Some es-sential parameters are given in Table 22.

To seal the ceramic heat exchanger tube after having been filledwith PCM or TCS materials, different options are offered by ceramicspecialists, including tungsten carbide, or silicon carbide or graph-ite seals [196–198]. For less demanding applications, steel hardware(clamps, clips, etc.) are proposed, e.g. Ref. 199.

5.3. Experimental design data

Some initial experiments were carried out by the authors,using a welded AISI 321 tube filled with PCM. The capsules were

Fig. 30. Rupture life of plate in the as-hot-rolled condition (538 °C–704 °C data) andin the hot-rolled and solution-annealed (1121 °C/2 h condition (732 °C–1093 °C data))(adapted from Ref. 187).

Fig. 31. Results of cyclic oxidation tests at 980 °C (cycles were 15 minutes of heatingand 5 minutes of cooling in air) (adapted from Refs. 186 and 187).

Table 21Typical room-temperature tensile properties of INCOLOY alloy 825 [188].

Form and condition Tensilestrength(MPa)

Yield strength(0.2% offset)(MPa)

Elongation(%)

Tubing, annealed 772 441 36Tubing, cold drawn 1000 889 15Plate, annealed 662 338 45Sheet, annealed 758 421 39

Table 22Mechanical–thermal properties of ceramics (adapted from Refs. 192–196).

Material properties Unit Alumina Zirconia Silicon carbide Silicon nitride (10 bar)

Sintered Infiltrated

Purity >99% ZrO2 (TZP) SSiC SiSiC ND-SN

Density β kg/m3 3900 6000 3120 3100 3200

Four-point bending strength σ4PB MPa 350 900 400 360 800Comparison stress σOV MPa 400 1060 500 470 925Young’s modulus E GPa 380 210 400 380 320Fracture toughness KIC MPa/m2 4 8 3 3.5 6.5Hardness 10 HV GPa 20 12 25 20 15Thermal expansion α 10−6/K 8 11 4.5 4 3Thermal conductivity λ W/mK 25 2 100 110 30Max. working temperature Tmax K 1773 1273 1773 1653 1523Chemical resistance Rated Good Good Very good Good GoodRaw material costs €/kg 5 75 15 7 60


subjected to pull and press experiments, and pressures to defor-mation and to rupture were measured and given in Table 23. Clearly,the pressure required to deform or burst the tested encapsula-tions significantly exceeds the pressure generated during the phasechange (~5 bar = 0.5 MPa). Moreover, the KNO3 tube of larger di-ameter and thickness performs better, with a lower reduction inpressure after multiple cycling. These results are however in linewith the theoretical predictions, as shown below.

The minimum required wall thickness at the maximum oper-ating temperature is calculated according to Eq. 14. The pressureto deformation and rupture is calculated according to the Barlowformula, Eq. 15. The maximum intended operating pressure P is3 bar; the outside diameter for cylinder 1 and 2 is respectively 39 mmand 27.6 mm. The allowable stress SE (yield strength) at 650 °C forAISI 321 is 250 MPa. The coefficient Y is 0.4 because the materialis nonferrous and the factor C is taken as 0.01 mm, which is a pes-simistic estimation. The minimum required wall thickness for thetwo cylinders and the selected materials is summarized in Table 24.A commercial wall thickness of 1.5 or 2 mm provides a large excesswall thickness.

For the pressure to deformation, S is the yield strength at 650 °C,this is the same as SE in the calculation for the minimum requiredwall thickness, and the safety factor φ is 1.5. To calculate the pres-sure to rupture, S is the tensile strength and the safety factor is 1.The results of the calculations for the pressure to deformation andrupture are given in Table 25. Clearly, experimental values are about10–20% below the theoretical predictions, so the pressure equa-tions can hence be used. For high numbers of cycles, the designpressure should however be increased by 10 to conservatively 20%.

Form the above data and experimental treatment, it emerges thatthe proper properties of the encapsulation materials as given in spe-cialized references provide a very fair basis for design calculations.Yield strength, tensile strength, and time-dependent creep strengthare the most important properties that enable to assess requiredwall thickness and allowable pressures to deformation and rupture.Thermal cycling of E-PCM sample tubes has demonstrated that the

calculated required wall thickness is far below the standard thick-nesses of commercial tubes. Predicted and experimental pressuresto deformation and rupture are in very fair agreement. Further ex-perimental confirmation is however required.

6. PCM/TCS integration in TES requires heat carriers

6.1. Introduction

TES generally requires the use of a heat transfer fluid during thecharging and discharging processes. Although water/steam, gas, syn-thetic oils and others can be used, current HTFs are commonlymolten salts, when large-scale heat storage is included, however withdrawbacks of temperature limitation required heat tracing of themolten salt circuits, and high pumping power requirements. Themaximum temperature for the application of thermal fluids isbetween 350 and 400 °C, whilst molten salt applications are limitedto ~560 °C.

Molten salts are mostly nitrates, such as solar salt (NaNO3/KNO3) or Hitec® (NaNO2/NaNO3/KNO3), with an upper temperaturelimit of ~600 °C for solar salt, and a lower temperature limit throughsolidification at 230 and 140 °C respectively. The molten salts havea low vapour pressure, but their moderate viscosity as a functionof temperature leads to considerable pumping energy. The insta-bility of the molten salts should be carefully considered, with e.g.a slow conversion at ~538 °C (Hitec® HTS) from NO2

− → NO3−, or a

decomposition of NO3− salts to NO2

− and O-radicals with possibleformation of CO3

2− plugs [200], according to:

NO NO O NO O* O N

and CO O* CO3 2 2 2 2 2

2 32

1 2 2 3 2− − −

−

→ + → + ++ →

; ;

As a result of these instabilities, physical properties (μ, ρ, etc.)will change and environmental issues should be taken into con-sideration (e.g. emissions through pressure venting of the formedgases), whilst plugging problems might occur.

Recently, the use of particle suspensions as HTF has been ad-vocated. Since the particle suspension has a heat capacity similarto that of molten salts, without temperature limitation except forthe maximum allowable wall temperature of the receiver tube, sus-pension temperatures of up to 800 °C can be tolerated for refractorysteel tubes (even higher when using ceramic or glass tubes), thusoffering new opportunities for highly efficient thermodynamic cyclessuch as obtained when using supercritical steam or CO2, as previ-ously illustrated in Fig. 6. Despite the pre-cited advantages of usingcirculating particles as HTF, multiple questions need to be investi-gated concerning powder stability (attrition); equipment erosion;or the overall economy of the system when the receiver tempera-ture is increased, leading to increased power cycle efficiency,increased storage density, reduced thermal power requirements, andincreased plant capacity factor.

6.2. Powders as novel heat carriers in renewable energy systems

A novel application of powders in renewable energy relies ontheir use as heat transfer medium for heat capture, conveying andstorage. Gas, mostly air, but possibly inert gas such as N2, is usedas powder carrying medium. According to the gas velocity applied,different gas–solid concepts are possible. With increasing gas flowrate, the operation moves from a fixed bed to a bubbling fluidizedor ultimately circulating fluidized bed and pneumatic conveying[201]. Without, or with a limited gas flow, falling particle reactorsor mechanical conveying could be used. Whereas powder reactorsin chemical applications mostly use Geldart A or B powders as bedmaterial [202], HTF applications will mostly use A-type powdersin order to limit the required gas flow rate and associated sensible

Table 23Test results for AISI 321, Pdeform (pressure to initial deformation) and Prupt (pressureto rupture).

Number ofcharging/dischargingcycles

KNO3 E-PCM, 39 mm O.D.2.0 mm wall thickness

Sb2O3 E-PCM, 27.6 mm O.D.1.5 mm wall thickness

Pdeform

(MPa)Prupt

(MPa)Pdeform

(MPa)Prupt

(MPa)

0 (reference) 14.8 41.6 16.8 46.2250 14.6 41.5 16.2 45.0500 14.5 40.9 16.0 44.6750 14.4 40.6 15.7 43.91000 14.4 39.8 15.9 43.51250 14.1 39.4 14.8 42.2

Table 24Minimum required wall thickness (mm) for the selected materials.

AISI 321

Test 1 0.033Test 2 0.027

Table 25Pressure to deformation and rupture (MPa) for AISI 321.

Pdeform (MPa) Prupt (MPa)

Test 1 17 45Test 2 18 48


heat losses. Fig. 32 illustrates the different operation modes of solid–gas systems, with representative values of the major characteristictransition velocities given in Table 26.

Compared to the moving bed concept, introducing relative par-ticle movement improves the heat transfer and gas/solid mixing,but abrasion, erosion and particle elutriation increase with increas-ing gas velocity. Bubbling fluidized beds (BFB) and BFB with inducedsolids circulation (UBFB), operating with A-powders and at low U,provide an excellent balance between heat transfer rates and par-ticle stability.

Falling particle reactors have poor heat transfer and particle sta-bility could be critical. The heat capturing capacity of the solids islow per unit reactor length. Circulating fluidized bed (CFB) systemsoffer the advantage of operating at high solid circulation fluxes,(>>500 kg/m2 s), thus being capable of conveying significant amountsof heat. The somewhat lower heat transfer coefficient than in BFBand UBFB, and higher risk of abrasion and attrition are drawbacks.The application of a higher value of U will moreover necessitate

Fig. 32. Operation modes of powder-gas systems [201].

Table 26Transition velocities associated with Fig. 34.

Utrans Equations Ref.

Umb Ud

Fmbp g

g

= ×( )2 070 716

0 06

0 347 45.

exp ..

.

ρμ

, F45: fine fraction lessthan 45 μm

203

Umf Ar = 1823Remf1.07 + 21.27Remf

2 110Utf Retf = 1.24Ar0.45, 2 < Ar < 108 204UTR ReTR = 3.23 + 0.23Ar 201

UCh2 1

0 008724 7

0 77gD sU U

Ch

Ch tg

− −( )−

=.

.. ρ 205

Usalt U dsaltp g

gp=

−4 43.

ρ ρρ

206

Ums U U gDms mf= + 0 07. , D: diameter of the bubblingfluidized bed

207


appropriate de-dusting equipment and heat recovery on the exhaustgas. The application of CFB seems hence limited to very large storageand re-use capacities. The rotary drum and screw conveyer can offeradvantages for heat storage, but can be excluded as heat captureoption due to the low heat transfer rate achieved. Among the dif-ferent concepts, the UBFB (low to moderate CSP capacities) and CFB(large capacities) are recommended.

The BFB, CFB and moving bed concepts can however be appliedin the TES and thermal block of the plant, with some tentative il-lustrations are given in Fig. 33 and Fig. 34 as example applications.The excellent heat transfer characteristics of powder beds more-over facilitate the integration of E-PCM, as temporary heat storagemedium. The loop of Fig. 33 is generally composed of a hot storagesilo fed from the discharge of the heat or energy receiver, and feedinga fluid bed heat exchanger, where the particles transmit their energyto submerged tubes inside whose a working fluid (for examplesteam) is generated and further expanded in a turbine: the fluid bedheat exchanger is a common device in the electrical power indus-try (mostly implemented for coal and biomass combustion influidized beds).

The cooled particles exit the boiler (continuous circulation) andare returned towards the cold storage tank by mechanical convey-ing. Finally, a conveying system raises the particles from the coldsilo to the heat receiver inlet. Particles are used as heat transfer fluidand heat storage medium, possibly complemented by using anE-PCM.

The loop of Fig. 34 includes the same unit components. Heat issupplied to the hot-side of the Stirling engine in a BFB of the par-ticle heat carrier.

6.2.1. Dense up-flow of particlesLiterature concerning dense up-flow systems, with forced up-

flow and bubble-induced mixing is scarce. Initial research mostlycovered moving packed beds. Fluidized bed up-flow reactors weredescribed by several researchers. The dense up-flow column is fedat its bottom by a circulation flow of solids at an appropriate pres-sure to overcome the pressure drop of the upwards-moving bed ofsolids. The column itself is either aerated from the windbox [3,7]or by a combined windbox aeration and additional aeration at thebottom of the riser column itself [21,28].

6.2.2. Particle loops in SPT and CSP applicationsSince the particle solar receiver is the key component, previ-

ous investigations in the field of solar receivers using particles asHTF are dealt with in Table 27. Tubular absorbers are generally pro-posed for use in current solar thermal receivers. The first studieson direct absorption solar receivers started in the early 1980s withthree concepts, the fluidized bed receiver [208], the free falling par-ticle receiver [209] and the rotary kiln receiver [210,211].

In the free falling particles concept, the solids are dropped di-rectly into the concentrated solar beam from the top of the receiverand are heated during their pass through the concentrated solar ra-diation. The principle was validated by on-sun experiments at pilotscale [212]. A receiver prototype was tested at the National SolarThermal Test Facility (NSTTF) in Albuquerque NM, USA. Short batchruns were performed for a total particle inventory of about 1800 kg.The receiver efficiency generally increased with the particle flowrate and varied from about 35% to 52%, thus in good agreement withsimulated data. A review of the falling particle receiver was pro-posed by Tan and Chen [213]. CNRS (FR) developed a “sand heaterloop” using sand particles as HTF [211]: it combined a solar rotarykiln that delivered hot sand to a heat storage/heat recovery sub-system consisting of a hot and a cold heat storage bin and amultistage fluidized heat exchanger.

The air jet flow concept of Röger et al. [214] used falling par-ticles that line the inner wall of a cylinder closed at its top, whilst

the bottom part was facing the concentrated solar beam. Heat re-covery from the hot storage is then possible using fluidized bed heatexchangers as described by Warerkar et al. [215], or by moving bedparticle-air heat exchangers tested by Al-Ansary et al. [216]. In theformer study, storage bins are integrated at the top of the tower.

Direct and indirect absorption particulate receivers were dis-cussed in detail by Zhang et al. [28]. Indirect receivers operate atlower flux density in the range 200–400 kW/m2, but offer a bettercontrol of particle circulation within the receiver and a possible man-agement of operating pressure and heat losses.

The particle mass flow rate inside the solar receiver is one of themain issues for a high power solar concentrating system using par-ticles as HTF. In industry, BFB and CFB are widely used at large scalein oil refineries and in combustors [217,218]: the dense phase flu-idized bed can be used in standpipes to provide an important andsteady downward flow of solid as shown by Bodin et al. [219], Smol-ders et al. [220], and Chan et al. [221]. In this regime, the suspensionis uniform, it has a low voidage, and it circulates slowly (a few cm/s), thus limiting both the energy consumption and its use as HTF.

Novel concepts use a dense suspension of small size solid par-ticles, patented by Flamant and Hemati [222], and are currentlydeveloped in the framework of both a French National and a Eu-ropean project [30]: they involve a dense particle upflow suspensionreceiver (solid fraction in the range 30–40%) in vertical absorbingtubes submitted to concentrated solar energy.

6.2.3. Recent experimental resultsRecent experimental investigations of using a particulate HTF

were presented by Flamant et al. [21] and Zhang et al. [28], wherethe concept of using UBFB and BFB particle suspension carriers wastested at the solar receiver of Font Romeu (F). The selected par-ticles were 64 μm silicon carbide (3120 kg/m3), conveyed in a singletube at air velocities between 0.03 and 0.22 m/s, achieving powdercirculation fluxes between 8 and 50 kg/m2s. The tube wall temper-ature reached up to ~800 °C.

The experimental results of the heat transfer coefficient, HTC,against solid flux and for all values of operating gas velocity are il-lustrated in Fig. 35. The heat transfer coefficient is a clear functionof G. Values of HTC range from >430 W/m2 K to 1120 W/m2 K. Thesemeasured values include both the contribution of particle convec-tion and radiation at higher temperatures. The radiation contributioncan be separately assessed from the Stefan–Boltzmann equation andranges from ~30 W/m2 K at low wall and bed temperature, to 200 W/m2 K for high temperatures [28].

CFB experiments were also carried out and were partly re-ported by Zhang et al. [231]. Solids circulation was achieved via adowncomer and L-valve. Heat supply was by hot water or thermalfluid (Santotherm 350). The downcomer was water-cooled througha 0.2 m long concentric cooler.

Temperatures were measured and recorded (wall, different lo-cations in the CFB, feeding and overflow lines of the fluid). The HTCwas calculated from the known exposed surface area, Aex, the mea-sured temperature difference, ΔT, and heat input, Q. The bed materialused was rounded sand of 75 μm, 2260 kg/m3, and Cps = 1.05 kJ/kg K. Various combined (U,G) values were tested to scan the differentriser hydrodynamic regimes (dilute flow, core/annulus flow, denseflow): G was varied from 8 to ~500 kg/m2 s and U – UTR from 1 to18 m/s (with U, the superficial gas velocity, and UTR, the requiredonset superficial gas velocity for CFB riser flow).

The HTC varied from ~20 W/m2 K (G = 0) to ~400 W/m2 K(G = 500 kg/m2 s). If a higher temperature would have been used (walltemperature >600 °C), the radiation contribution should have in-creased the measured HTC. The required superficial gas velocity ina CFB (>>3 m/s) significantly exceeds the low value in the UBFB(~0.1 m/s).


Fig. 33. Possible layout with heat capture/storage and powder heat carrier system [28].


Fig. 34. Possible layout with BFB heating of the hot side Stirling heat exchanger.

Table 27Literature review on solar receiver developments using powders as HTF.

Year Concept and Use Ref.

1980 Concept of a fluidized bed receiver, no solids circulation, batch operation Flamant et al. [211]1982 Fluidized bed solar receiver Flamant [208]1982 Free falling particles receiver, model of particle selection and radiative heat transfer were proposed [220] Martin and Vitko [209]1985 The use of small refractory pebbles or sand as the working media in a high temperature solar central receiver system

was proposed and examined. Particle selection and radiative heat transfer model of free fall receiverFalcone et al. [223]

1989 A two-part system, where a rotary kiln receiver connects to hot storage/heat recovery sub-system (storage bins andmultistage fluidized heat exchanger)

Bataille et al. [210]

2004 Two high-porosity materials, a double-layer silicon carbide foam and a screen-printed porous silicon carbide materialwere proposed to be used as volumetric receivers for concentrated solar radiation.

Fend et al. [224]

2010 Pilot scale prototype validation of high temperature solar thermal concentrating plant at National Solar Thermal TestFacility (NSTTF). Cavity receiver with falling particle films

Siegel et al. [212]

2010 Effect of wind speed and parasitic air flows inside the cavity on particle aerodynamics Tan and Chen [213]2010 An internally circulating fluidized (draft tube) using fluidized particles as water splitting reactor with solar simulated

light irradiation (non-CSP application)Gokon et al. [225]

2011 Face-down solid particle receiver concept and modelling Röger et al. [214]2011 Fluidized bed heat exchanger model Warerkar et al. [215]2011 A simulation analysis was applied to a porous media solar tower receiver. The established working diagram provides a

reference for receiver design and reconstruction.Xu et al. [226]

2011 Combined conveyor and heat transfer device which consist in a rotatable conveyor drum and heat transfer fluidconduits within the drum

Jeter and Stephens [227]

2012 Particle–air heat exchangers integrated with storage bins at the top of the tower was proposed Al-Ansary et al. [216]2012 CFB particulate receiver Brems et al. [106].2013 CFB with encapsulated PCM particles Pitié et al. [7]2013 Concept of screw conveyor heat exchanger, whereby PCMs can be transported within the shaft and height. Decouples

the heat transfer and storage and develops an economically viable latent heat storage.Zipf et al. [228]

2013 A dense particle suspension receiver (DPSR) was proposed and tested at pilot scale. Upward circulation of a densesuspension of particles (SiC) as HTF to extend the working temperature limit to the bearing capacity of the receivertube.

Flamant et al. [18,21]

2014 An inner-circulating fluidized bed spouted by concentric gas streams with high and low velocities in the centre andouter annulus, respectively, with an irradiation source at the top of the receiver container.

Matsubara et al. [229]

2014 A thermal energy storage (TES) system was developed by NREL using solid particles as the storage medium for CSPplants. Based on their performance analysis, particle TES systems using low-cost, high T withstand able and stablematerial can reach 10$/kWhth, half the cost of the current molten-salt based TES

Ma et al. [230]

2014 A heat transfer model to predict the heat transfer coefficient for different operating conditions in CFB receiver wasproposed, with air flow rate and solids circulation flux as dominant parameters.

Zhang et al. [231]

2014 Optimized optical particle properties for a high temperature solar receiver Ordóñez et al. [232]2015 Design optimization of a small heat exchange receiver for solar tower power plants Fernández and Miller [233]2015 Design of a packed bed TES for high temperature Zanganeh et al. [234]


The studies confirmed that upflowing particles can be used inthe novel design of a solar receiver, both in UBFB and CFB mode.UBFB heat transfer coefficients up to 1100 W/m2 K were obtainedfor operation at low superficial gas velocities, thus limiting sensi-ble heat losses by the exhaust air. Although CFBs could be interestingfor large scale operations, due to the higher solid flux achieved, thecomparison of Table 28 tentatively illustrates the main advan-tages of using the UBFB principle, even for high receiver capacities.This selection accounts for sizes, complexity and operating costs.

The high heat transfer coefficient obtained within a given rangeof superficial gas velocities, U and solid flux, G, and the possible op-eration of the powder upflow bed at high temperatures (>750 °C)moreover offers new opportunities for high efficiency thermody-namic cycles, with a conversion increase from the current 40% toover 46% (leading to a reduction of the receiver and heliostatrequirements).

At equivalent values of the specific heat of molten salts andpowders, and with a lower powder temperature after heat recov-ery (~150 °C against ~300 °C for molten salts), the higher receivertemperatures of the powders reduce the storage requirement on amass basis by ~50%, however only about 30% on a volumetric basis,due to the lower bulk density of the powder bed. Parasitic powerconsumption is also significantly reduced due to the reducedpumping energy, and lack of heat tracing of the circuits. The com-bination of these technological advantages will considerably reducethe LCOE of the SPT concept. The dense particle suspension is apromising option, justifying additional testing (e.g. using multiplereceiver tubes), and allowing a more correct analysis of economicadvantages in both operating and investment costs, as tentativelyproposed in Table 28. A multi-tube receiver (16 parallel tubes) hasbeen studied within the CSP2 project [27] at the CNRS 1MW solarfurnace, and results are presented in a separate paper.

7. Conclusion and recommendation for priority research

This work reviewed some recent developments and design rec-ommendations for the high temperature heat storage, wheresensible/latent/reaction heat each offers a contribution with re-versible reactions being the most effective (heat storage >>1000 kJ/kg), followed by latent heat storage (500–1000 kJ/kg) and sensibleheat storage materials (<500 kJ/kg). This is dealt with in Section 1of the paper.

Section 2 of the paper specifically dealt with sensible heat storage.Sensible heat storage materials are well-documented, have the lowestcost materials but at the same time the lowest storage capacity com-pared to PCM and TCS. Most commonly used minerals’ costs rangefrom 0.05 €/kg to 0.1 €/kg, hence cheaper than latent heat storagecosts sensible heat materials have excellent thermal conductivi-ties: 1.0 W/mK–7.0 W/mK for sand-rock minerals, concrete andfirebricks, 37.0 W/mK–40.0 W/mK for ferroalloys. Their disadvan-tage is their heat capacity, ranging from 0.56 kJ/kg K to 1.3 kJ/kg K,making storage unit sizes to be bigger than when using latent heat.The energy stored by a SHS material towards variations of 1 °C inits temperature can be 100 times lower compared to the energystorage capacity when it has a phase change, as can be noticed in

Fig. 35. Experimental values of the heat transfer coefficient, HTC, vs. G, for all ex-perimental superficial gas velocities.

Table 28Comparison of UBFB and CFB solar receiver (10 MWth) with SiC powders (50 μm, 3100 kg/m3 and specific heat at 500 °C of 1.2 kJ/kg K) [203].

Parameter Equation or reference UBFB CFB

Voidage, ε 0.6–0.7 [21,28] 0.95–0.99 [235,236]Pressure drop, ΔP/L (mbar/m) ρp(1 − ε) 124 6.2Heat transfer coefficient, HTC (W/m2 K) 950 600 for G > 400 kg/m2 s [231]Riser diameter, D (m) 0.04 [21,28] 0.08 [231]Solids circulation flux, G (kg/m2 s) 50 [21,28] >400 [231,235]Solids flow in riser tube, Qs (kg/s) (πD2/4) × G 0.063 2.0Temperature increase in riser passage (K) Inlet ~250 °C

Outlet ~750 °C500 500

Average temperature in riser, Tr (K) (Tin + Tout)/2 500 500Temperature of riser wall, Tw (K) 800–850 800–850Driving ΔT (K) ΔT ≈ (Tw − Tb) 300 300Required heat exchanging surface, S (m2) (10 MWth × 106)/(HTC × ΔT) 30 55.5Height of riser, H (m) – e.g. 3 × 2 m each Commonly ≥ 6 mNumber of parallel riser tubes (–) S/(πHD) 3 × 40 36Pressure drop, ΔP (mbar) ΔP/(LH) +disperser + separator (2 × 124) + 50 ≈ 300 (6 × 6.2) + 50 ≈ 90Air flow to riser, Qa (N m3/h) UBFB: 0.05 Nm/s (i.e. 0.14 m/s at 750 °C)

CFB: 2 Nm/s (i.e. 5.5 m/s at 750 °C)~30 1300

Heat loss by air flow (90% recycle), ΔHloss (kWth) [QaCpa(Tout − 20)] ×0.1 14564 kJ/h0.8 kWth

123,370 kJ/h35 kWth

Air pumping power, P (kWel) UBFB: blowerCFB: fan

<1 kWel <4 kWel

The UBFB application results in lower heat losses, but a slightly higher pressure drop. Towards the number of parallel receiver tubes, the CFB is more advantageous. Theconveying of the powders from the cold storage to the feeding system of the receiver is supposed to be operated by mechanical conveying (as a series of bucket elevators).Further scale-up work is required to confirm the tentative economic and design assessment.


Table 29, where the difference of energy storage capacity of threedifferent materials is illustrated when used as SHS (in their liquidstate) and LHS (when they change from solid to liquid).

Section 3 dealt with latent heat storage, specifically towards hightemperature application, where inorganic substances offer a highpotential. Experimental and modeling data will be compared. Sinceinorganic solid/liquid phase change materials suffer from a lowthermal conductivity, conductive inserts offer a high potential, re-ducing the charging/discharging time by a factor of ~2.

Section 4 reviewed the fundamentals of using reversible chem-ical reactions to store heat for short to long term storage. Withinthe selected reaction pairs, Ca(OH)2, CaCO3, Co3O4 and Mn2O3 offera high potential, both towards achieved reaction temperature andreversibility. All reactions can be treated as first order chemical re-actions, with activation energies between 50 and 250 kJ/mol andreaction rate constants of 1 to 2 × 10−3 s−1 at reaction equilibriumtemperature.

Since latent and thermal chemical storage medium needs to becontained in a capsule (sphere, tube, sandwich plates) of appro-priate materials, Section 5 assessed the mechanical strength of thecontainment from both a theoretical and experimental evalua-tion. Alloy-capsules of 1–2 mm thickness meet the requirements oflong term stability, even after charging/discharging cycles.

Section 6 specifically dealt with a novel particulate HTF, con-sidered for both heat capture and storage. Since particulatesuspensions deal with sensible heat storage, the paper reviews pre-vious investigation and recent experimental results. Literature andexperimental data confirm the high heat transfer coefficient achieved(>400 W/m2 K in circulating fluidized beds, to >1100 W/m2 K in adense upflow fluidized bed).

A tentative technical and economic assessment demonstrates thatparticulate suspensions lead to major savings in investment and op-erating costs.

Although sufficient fundamental and practical information onSHS/LHS and TCS is available in literature, the achievement of a highefficiency TES needs to be proven, preferably on pilot or large scale.These requirements, subjects of additionally required research,include (i) a high energy density in the storage material (storagecapacity); (ii) a high heat transfer between the HTF and the con-tained storage media; (iii) a chemical and mechanical stability ofthe storage materials used during extensive cycles; (iv) the com-patibility between HTF, storage medium and heat exchanger; (v) thecomplete reversibility of the charging/discharging cycles; (vi) workingat low thermal losses; (vi) ease of control, operation, and integra-tion into the proposed heat storage concept; and (vii) operation atas high a temperature as possible.

According to the analysis, specific priority research targets shouldinclude (i) the further development and testing of PCMs and theircharging/discharging enhancement techniques; (ii) the proof ofconcept of using nanoparticle-encapsulated PCMs; (iii) the furtherdevelopment and large-scale confirmation of TCS storage tech-niques, with special emphasis on the complete reversibility of theendothermic/exothermic cycling; (iv) the large scale testing of ap-propriate PCM/TCS containments; (v) the further development ofparticle suspension HTFs, possibly even using TCS powders in a cyclebetween a reducing (charging) and oxidation (discharging) atmo-

sphere; and (vi) finally, exergy and energy analyses should beconducted to optimize a storage module: entropy generation anal-ysis is specifically important in optimizing design and operationparameters of the systems.

Acknowledgements

Yongqin Lv gratefully acknowledges the financial support fromthe special assistance of the National Natural Science Foundationof China (21306006, and 21436002), 973 programs (2014CB745103,2013CB733603), the Xiamen Scientific and Technological project(3502Z20142012), the Public Hatching Platform for Recruited Talentsof Beijing University of Chemical Technology, the High-Level FacultyPrograms of Beijing University of Chemical Technology (20130805),and the Fundamental Research Funds for the Central Universities(YS1407). This work was also supported in Chile by the projectsCONICYT/FONDAP/15110019 (SERC-CHILE), CONICYT/FONDECYT/1151061, by Center UAI Earth and the R&D project engineerMacarena Montané. Huili Zhang would like to express her sinceregratitude to the China Scholarship Council for sponsoring her Ph.D.study at KU Leuven in Belgium (File No. 201206880024). Jan Baeyensand Huili Zhang acknowledge EU funding under the CSP2- program(Project No. 282932).

References

[1] Asif M, Muneer T. Energy supply, its demand and security issues for developedand emerging economies. Renew Sustain Energy Rev 2007;11:1388–413.doi:10.1016/j.rser.2005.12.004.

[2] International Energy Agency. World energy outlook. 2011.[3] Zhang HL, Baeyens J, Degrève J, Cacères G. Concentrated solar power plants:

review and design methodology. Renew Sustain Energy Rev 2013;22:466–81.doi:10.1016/j.rser.2013.01.032.

[4] Zhang HL, Van Gerven T, Baeyens J, Degrève J. Photovoltaics: reviewing theEuropean Feed-in-Tariffs and changing PV efficiencies and costs. Sci World J2014;2014:404913. doi:10.1155/2014/404913.

[5] Energy Information Administration. International energy outlook. Washington,DC: 2011.

[6] International Energy Agency. World energy outlook 2012. Paris, France: 2012.[7] Pitié F, Zhao CY, Baeyens J, Degrève J, Zhang HL. Circulating fluidized bed heat

recovery/storage and its potential to use coated phase-change-material (PCM)particles. Appl Energy 2013;109:505–13. doi:10.1016/j.apenergy.2012.12.048.

[8] Ammar Y, Joyce S, Norman R, Wang Y, Roskilly AP. Low grade thermal energysources and uses from the process industry in the UK. Appl Energy 2012;89:3–20. doi:10.1016/j.apenergy.2011.06.003.

[9] Naish C, McCubbin I, Edberg O, Harfoot M. Outlook of energy storagetechnologies (Study No. IP/A/ITRE/FWC/2006-087/Lot 4/C1/SC2). 2008.

[10] Astrom Technical Advisors, S&L. CSP today. CSP benchmarking int., 2013.[11] Bilgili M, Ozbek A, Sahin B, Kahraman A. Concentrated solar power plants:

review and design methodology. Renew Sustain Energy Rev 2013;22:466–81.doi:10.1016/j.rser.2013.01.032.

[12] Teller O, Nicolai J-P, Lafoz M, Laing D, Tamme R, Pedersen AS, et al. JointEASE/EERA recommendations for a European energy storage technologydevelopment roadmap towards 2030. 2013.

[13] The Liquid Air Energy Network. Energy storage systems according to form ofenergy stored. <http://www.liquidair.org.uk>; 2014. [accessed 05.05.15].

[14] Fernandes D, Pitié F, Cáceres G, Baeyens J. Thermal energy storage: “Howprevious findings determine current research priorities.”. Energy2012;39:246–57. doi:10.1016/j.energy.2012.01.024.

[15] Medrano M, Gil A, Martorell I, Potau X, Cabeza LF. State of the art on high-temperature thermal energy storage for power generation. Part 2 – casestudies. Renew Sustain Energy Rev 2010;14:56–72. doi:10.1016/j.rser.2009.07.036.

Table 29Energy densities from SHS and LHS materials.

SHS LHS

Material Specific heat (kJ/kg K) Energy density (kJ/m3 K) Melting temperature (°C) Latent heat (kJ/kg) Energy density (GJ/m3)

Water (liquid) 4.2 4200 0 335 30760% NaNO3 – 40% KNO3 1.5 3144 223 105 230Paraffin wax 1.92 1593.6 32 251 208


http://refhub.elsevier.com/S0360-1285(15)30014-9/sr0010





























http://www.liquidair.org.uk








[16] Castellón C, Medrano M, Roca J, Nogués M, Castell A, Cabeza LF. Use ofmicroencapsulated phase change materials in building applications. Build X2007.

[17] Torresol Energy. Gemasolar. <http://www.torresolenergy.com/TORRESOL/gemasolar-plant/en>; 2015. [accessed 10.05.15].

[18] Flamant G, Gauthier D, Benoit H, Sans J-L, Boissière B, Ansart R, et al. A newheat transfer fluid for concentrating solar systems: particle flow in tubes.Energy Procedia 2014;49:617–26. doi:10.1016/j.egypro.2014.03.067.

[19] Zhang HL, Baeyens J, Degrève J, Pitié F. Latent heat storage with phase changematerials (PCMs). J Technol Innov Renew Energy 2013;1–12.

[20] Benoit H, Perez I, Gauthier D, Sans J-L, Flamant G. On-sun demonstration ofa 750°C heat transfer fluid for concentrating solar systems: dense particlesuspension in tube. Sol Energy 2015;118:622–33.

[21] Flamant G, Gauthier D, Benoit H, Sans J-L, Garcia R, Boissière B, et al. Densesuspension of solid particles as a new heat transfer fluid for concentrated solarthermal plants: on-sun proof of concept. Chem Eng Sci 2013;102:567–76.doi:10.1016/j.ces.2013.08.051.

[22] Maruoka N, Akiyama T. Development of OCM for high temperature applicationin the steelmaking industry. In: Annex 17, advanced thermal energy storagetechniques – feasibility studies and demonstration projects. 3rd Expert. meet.work. annex, vol. 17. Tokyo, Japan: 2002.

[23] Maruoka N, Sato K, Yagi J, Akiyama T. Development of PCM for recoveringhigh temperature waste heat and utilization for producing hydrogen byreforming reaction of methane. ISIJ Int 2002;42:215–19. doi:10.2355/isijinternational.42.215.

[24] Sharma A, Tyagi VV, Chen CR, Buddhi D. Review on thermal energy storagewith phase change materials and applications. Renew Sustain Energy Rev2009;13:318–45. doi:10.1016/j.rser.2007.10.005.

[25] Kearney D, Herrmann U, Nava P, Kelly B, Mahoney R, Pacheco J, et al.Assessment of a molten salt heat transfer fluid in a parabolic trough solar field.J Sol Energy Eng 2003;125:170. doi:10.1115/1.1565087.

[26] Abhat A. Low temperature latent heat thermal energy storage: heatstorage materials. Sol Energy 1983;30:313–32. doi:10.1016/0038-092X(83)90186-X.

[27] Zhang HL, Baeyens J, Degrève J, Cáceres G, Segal R, Pitié F. Latent heat storagewith tubular-encapsulated phase change materials (PCMs). Energy2014;76:66–72. doi:10.1016/j.energy.2014.03.067.

[28] Zhang HL, Benoit H, Gauthier D, Degrève J, Baeyens J, Pérez López I, et al.Particle circulation loops in solar energy capture and storage: gas-solid flowand heat transfer considerations. Appl Energy 2016(161C):206–24.doi:10.1016/j.apenergy.2015.10.005.

[29] Dunham MT, Iverson BD. High-efficiency thermodynamic power cycles forconcentrated solar power systems. Renew Sustain Energy Rev 2014;30:758–70.doi:10.1016/j.rser.2013.11.010.

[30] CSP2. CSP2 project. <http://www.csp2-project.eu/>; 2015 [accessed 28.01.15].[31] Cziesla F, Kremer H, Much U, Riemschneider J-E, Quinkertz R. Advanced 800+

MW steam power plants and future CCS options. Siemens Ind Turbomach2009;1–21.

[32] Singer C, Buck R, Pitz-Paal R, Müller-Steinhagen H. Assessment of solar powertower driven ultrasupercritical steam cycles applying tubular central receiverswith varied heat transfer media. J Sol Energy Eng 2010;132:041010.doi:10.1115/1.4002137.

[33] Spelling J, Gallo A, Romero M, Gonzalez-Aguilar J. A high efficiency solarthermal power plant using a dense particle suspension as heat transfer fluid.Int. Conf. Conc. Sol. Power Chem. Energy Syst., Beijing, China 2014.

[34] Kuravi S, Trahan J, Goswami DY, Rahman MM, Stefanakos EK. Thermal energystorage technologies and systems for concentrating solar power plants. ProgEnergy Combust Sci 2013;39:285–319. doi:10.1016/j.pecs.2013.02.001.

[35] Yu N, Wang RZ, Wang LW. Sorption thermal storage for solar energy. ProgEnergy Combust Sci 2013;39:489–514. doi:10.1016/j.pecs.2013.05.004.

[36] Zalba B, Marin JM, Cabeza LF, Mehling H. Review on thermal energy storagewith phase change: materials, heat transfer analysis and applications. ApplTherm Eng 2003;23:251–83. doi:10.1016/S1359-4311(02)00192-8.

[37] Jegadheeswaran S, Pohekar SD. Performance enhancement in latent heatthermal storage system: a review. Renew Sustain Energy Rev 2009;13:2225–44. doi:10.1016/j.rser.2009.06.024.

[38] Abengoa. Innovative technology solutions for sustainability. <http://www.abengoa.es/web/es/> 2015 [accessed 01.28.15].

[39] Fernandez AI, Martínez M, Segarra M, Martorell I, Cabeza LF. Selection ofmaterials with potential in sensible thermal energy storage. Sol Energy MaterSol Cells 2010;94:1723–9. doi:10.1016/j.solmat.2010.05.035.

[40] Regin AF, Solanki SC, Saini JS. Heat transfer characteristics of thermal energystorage system using PCM capsules: a review. Renew Sustain Energy Rev2008;12:2438–58. doi:10.1016/j.rser.2007.06.009.

[41] Kenisarin MM. High-temperature phase change materials for thermal energystorage. Renew Sustain Energy Rev 2010;14:955–70. doi:10.1016/j.rser.2009.11.011.

[42] Perry RH, Green DW, editors. Perry’s chemical engineers’ handbook. 8th ed.New York: McGraw-Hill; 2008.

[43] International Renewable Energy Agency. Concentrating solar power technologybrief. 2013.

[44] Laing D. Solar thermal energy storage technologies. Hannover; 2008.[45] Gil A, Medrano M, Martorell I, Lázaro A, Dolado P, Zalba B, et al. State of the

art on high temperature thermal energy storage for power generation. Part1 – concepts, materials and modellization. Renew Sustain Energy Rev2010;14:31–55. doi:10.1016/j.rser.2009.07.035.

[46] Laing D, Bahl C, Bauer T, Lehmann D, Steinmann W-D. Thermal energy storagefor direct steam generation. Sol Energy 2011;85:627–33. doi:10.1016/j.solener.2010.08.015.

[47] Carslaw HS, Jaeger JC. Conduction of heat in solids. Oxford University Press;1986.

[48] Smith GD. Numerical solution of partial differential equations. Finite differencemethods. Oxford, UK: Oxford University Press; 1985.

[49] Lopez J, Caceres G, Palomo Del Barrio E, Jomaa W. Confined melting indeformable porous media: a first attempt to explain the graphite/saltcomposites behaviour. Int J Heat Mass Transf 2010;53:1195–207. doi:10.1016/j.ijheatmasstransfer.2009.10.025.

[50] Pitié F, Zhao CY, Cáceres G. Thermo-mechanical analysis of ceramicencapsulated phase-change-material (PCM) particles. Energy Environ Sci2011;4:2117. doi:10.1039/c0ee00672f.

[51] Parrado C, Cáceres G, Bize F, Bubnovich V, Baeyens J, Degrève J, et al. Thermo-mechanical analysis of copper-encapsulated NaNO3–KNO3. Chem Eng Res Des2015;93:224–31. doi:10.1016/j.cherd.2014.07.007.

[52] Zakri T, Laurent J-P, Vauclin M. Theoretical evidence for Lichtenecker’s mixtureformulae’ based on the effective medium theory. J Phys D Appl Phys1998;31:1589–94. doi:10.1088/0022-3727/31/13/013.

[53] Maxwell JC. A treatise on electricity and magnetism. Oxford University Press;1954.

[54] Lal K, Parshad R. Test and utilization of the Fricke and Pearce equations fordielectric correlation between powder and bulk. J Phys D Appl Phys1974;7:455–61. doi:10.1088/0022-3727/7/3/313.

[55] Geldart D, Abrahamsen AR. Fluidization of fine porous powders. Chem EngProg Symp Ser 1981;77:160.

[56] Hamilton RL, Crosser OK. Thermal conductivity of heterogeneous two-component systems. Ind Eng Chem Fundam 1962;1:187–91. doi:10.1021/i160003a005.

[57] Tan H, Li C, Li Y. Simulation research on PCM freezing process to recover andstore the cold energy of cryogenic gas. Int J Therm Sci 2011;50:2220–7.doi:10.1016/j.ijthermalsci.2011.04.017.

[58] Ahern JE. Applications of the second law of thermodynamics to cryogenics –a review. Energy 1980;5:891–7. doi:10.1016/0360-5442(80)90104-8.

[59] Ordonez C. Liquid nitrogen fueled, closed Brayton cycle cryogenic heatengine. Energy Convers Manag 2000;41:331–41. doi:10.1016/S0196-8904(99)00117-X.

[60] Ordonez CA, Plummer MC. Cold thermal storage and cryogenic heat enginesfor energy storage applications. Energy Sources 1997;19:389–96. doi:10.1080/00908319708908858.

[61] Li Y, Chen H, Ding Y. Fundamentals and applications of cryogen as a thermalenergy carrier: a critical assessment. Int J Therm Sci 2010;49:941–9.doi:10.1016/j.ijthermalsci.2009.12.012.

[62] Kishimoto K, Hasegawa K, Asano T. Development of generator of liquid airstorage energy system. 1998.

[63] Wen DS, Chen HS, Ding YL, Dearman P. Liquid nitrogen injection into water:pressure build-up and heat transfer. Cryogenics (Guildf) 2006;46:740–8.doi:10.1016/j.cryogenics.2006.06.007.

[64] Markert F, Melideo D, Baraldi D. Numerical analysis of accidental hydrogenreleases from high pressure storage at low temperatures. Int J Hydrogen Energy2014;39:7356–64. doi:10.1016/j.ijhydene.2014.02.166.

[65] Byon C. Numerical study on the phase change heat transfer of LNG in glasswool based on the VOF method. Int J Heat Mass Transf 2015;88:20–7.doi:10.1016/j.ijheatmasstransfer.2015.03.092.

[66] Konopka M, Behruzi P, Schmitt S, Dreyer EM. Phase change in cryogenic upperstage tanks. 50th AIAA/ASME/SAE/ASEE Jt. Propuls. Conf., Cleveland, USA: 2014,p. 16. doi:10.2514/6.2014-3994.

[67] Pacheco KA, Li Y, Wang M. Study of integration of cryogenic air energy storageand coal oxy-fuel combustion through modelling and simulation, vol. 33.Elsevier; 2014 doi:10.1016/B978-0-444-63455-9.50091-X.

[68] Li Y, Wang X, Ding Y. A cryogen-based peak-shaving technology: systematicapproach and techno-economic analysis. Int J Energy Res 2013;37:547–57.doi:10.1002/er.1942.

[69] Donatini F, Zamparelli C, Maccari A, Vignolini M. High efficency integrationof thermodynamic solar plant with natural gas combined cycle. 2007 Int ConfClean Electr Power, IEEE 2007;770–6. doi:10.1109/ICCEP.2007.384301.

[70] Fu J, Sunden B, Chen X, Huang Y. Influence of phase change on self-pressurization in cryogenic tanks under microgravity. Appl Therm Eng2015;87:225–33. doi:10.1016/j.applthermaleng.2015.05.020.

[71] Yang JH, Yang GS. The temperature field research for large LNG cryogenicstorage tank wall. Appl. Mech. Mater. 2014;668–669:733–6.

[72] Kotek M, Suma J, Chrz V. Holding time of horizontal cryogenic vacuuminsulated tanks. Refrig Sci Technol 2014;131–9.

[73] Steinmann W-D, Eck M. Buffer storage for direct steam generation. Sol Energy2006;80:1277–82. doi:10.1016/j.solener.2005.05.013.

[74] Zhang N, Lior N. A novel Brayton cycle with the integration of liquid hydrogencryogenic exergy utilization. Int J Hydrogen Energy 2008;214–24.

[75] Wang Q, Li Y, Wang J. Analysis of power cycle based on cold energy of liquefiednatural gas and low-grade heat source. Appl Therm Eng 2004;24:539–48.doi:10.1016/j.applthermaleng.2003.09.010.

[76] Wang Q, Li Y, Chen X. Exergy analysis of liquefied natural gas cold energyrecovering cycles. Int J Energy Res 2005;29:65–78. doi:10.1002/er.1040.

[77] Zhang N, Lior N. A novel near-zero CO2 emission thermal cycle with LNGcryogenic exergy utilization. Energy 2006;31:1666–79. doi:10.1016/j.energy.2005.05.006.





http://www.torresolenergy.com/TORRESOL/gemasolar-plant/en

http://www.torresolenergy.com/TORRESOL/gemasolar-plant/en








































http://www.csp2-project.eu/






















http://www.abengoa.es/web/es/

http://www.abengoa.es/web/es/









































































































[78] Szargut J, Szczygiel I. Utilization of the cryogenic exergy of liquid natural gas(LNG) for the production of electricity. Energy 2009;34:827–37. doi:10.1016/j.energy.2009.02.015.

[79] Chen H, Chen Y, Hsieh H-T, Siegel N. Computational fluid dynamics modelingof gas-particle flow within a solid-particle solar receiver. J Sol Energy Eng2007;129:160. doi:10.1115/1.2716418.

[80] Van de Velden M, Baeyens J, Dougan B, Mc Murdo A. Investigation ofoperational parameters for an industrial CFB combustor of coal, biomass andsludge. China Particuol 2007;5:247–54. doi:10.1016/j.cpart.2007.05.001.

[81] Department of Energy. Improving steam system performance, a sourcebookfor industry (108 pages). DOE/GO-12004-1868. 2012.

[82] Tassaing T, Danten Y, Besnard M. Infrared spectroscopic study of hydrogen-bonding in water at high temperature and pressure. J Mol Liq 2002;101:149–58. doi:10.1016/S0167-7322(02)00089-2.

[83] Gupta G, Ampornrat P, Ren X, Sridharan K, Allen TR, Was GS. Role of grainboundary engineering in the SCC behavior of ferritic–martensitic alloy HT-9.J Nucl Mater 2007;361:160–73. doi:10.1016/j.jnucmat.2006.12.006.

[84] Hirose T, Shiba K, Enoeda M, Akiba M. Corrosion and stress corrosion crackingof ferritic/martensitic steel in super critical pressurized water. J Nucl Mater2007;367–370:1185–9. doi:10.1016/j.jnucmat.2007.03.212.

[85] Mitton DB, Zhang SH, Han EH, Hautanen KE, Latanision RM. Assessment ofcorrosion and failure mechanisms in supercritical water oxidation systems.Proc. 13th ICC, Melbourne, Australia: 1996, p. 25–9.

[86] Godall PM. The efficient use of steam. Surrey, England: IPC Science andTechnology Press Limited; 1980.

[87] Cabeza LF, Castell A, Barreneche C, de Gracia A, Fernández AI. Materials usedas PCM in thermal energy storage in buildings: a review. Renew Sustain EnergyRev 2011;15:1675–95. doi:10.1016/j.rser.2010.11.018.

[88] Chen B, Wang X, Zeng R, Zhang Y, Wang X, Niu J, et al. An experimental studyof convective heat transfer with microencapsulated phase change materialsuspension: laminar flow in a circular tube under constant heat flux. ExpTherm Fluid Sci 2008;32:1638–46. doi:10.1016/j.expthermflusci.2008.05.008.

[89] Delgado M, Lázaro A, Mazo J, Marín JM, Zalba B. Experimental analysis of amicroencapsulated PCM slurry as thermal storage system and as heat transferfluid in laminar flow. Appl Therm Eng 2012;36:370–7. doi:10.1016/j.applthermaleng.2011.10.050.

[90] Bridges NJ, Visser AE, Fox EB. Potential of nanoparticle-enhanced ionic liquids(NEILs) as advanced heat-transfer fluids. Energy Fuels 2011;25:4862–4.doi:10.1021/ef2012084.

[91] Cabeza LF, Gutierrez A, Barreneche C, Ushak S, Fernández ÁG, Inés FernádezA, et al. Lithium in thermal energy storage: a state-of-the-art review. RenewSustain Energy Rev 2015;42:1106–12. doi:10.1016/j.rser.2014.10.096.

[92] U.S. Department of Energy. Solar energy technologies program. 2010.[93] The University of Alabama. Novel molten salts thermal energy storage for

concentrating solar power generation 2010.[94] Fernández AG, Ushak S, Galleguillos H, Pérez FJ. Development of new molten

salts with LiNO3 and Ca(NO3)2 for energy storage in CSP plants. Appl Energy2014;119:131–40. doi:10.1016/j.apenergy.2013.12.061.

[95] Maruoka N, Akiyama T. Thermal stress analysis of PCM encapsulation for heatrecovery of high temperature waste heat. J Chem Eng Japan 2003;36:794–8.doi:10.1252/jcej.36.794.

[96] Agyenim F, Hewitt N, Eames P, Smyth M. A review of materials, heat transferand phase change problem formulation for latent heat thermal energy storagesystems (LHTESS). Renew Sustain Energy Rev 2010;14:615–28. doi:10.1016/j.rser.2009.10.015.

[97] Erek A, Ylken Z, Acar MA. Experimental and numerical investigation of thermalenergy storage with a finned tube. Int J Energy Res 2005;29:283–301.doi:10.1002/er.1057.

[98] Castell A, Solé C, Medrano M, Roca J, Cabeza LF, García D. Natural convectionheat transfer coefficients in phase change material (PCM) modules withexternal vertical fins. Appl Therm Eng 2008;28:1676–86. doi:10.1016/j.applthermaleng.2007.11.004.

[99] Zhang Y, Faghri A. Heat transfer enhancement in latent heat thermal energystorage system by using an external radial finned tube. J Enhanc Heat Transf1996;3:119–27. doi:10.1615/JEnhHeatTransf.v3.i2.50.

[100] Lamberg P. Approximate analytical model for two-phase solidification problemin a finned phase-change material storage. Appl Energy 2004;77:131–52.doi:10.1016/S0306-2619(03)00106-5.

[101] Velraj R, Seeniraj RV, Hafner B, Faber C, Schwarzer K. Experimental analysisand numerical modelling of inward solidification on a finned vertical tubefor a latent heat storage unit. Sol Energy 1997;60:281–90. doi:10.1016/S0038-092X(96)00167-3.

[102] Talati F, Mosaffa AH, Rosen MA. Analytical approximation for solidificationprocesses in PCM storage with internal fins: imposed heat flux. Heat MassTransf 2010;47:369–76. doi:10.1007/s00231-010-0729-9.

[103] Mosaffa A, Talati F, Rosen MA, Basirat TH. Phase change material solidificationin a finned cylindrical shell thermal energy storage: an approximate analyticalapproach. Therm Sci 2013;17:407–18.

[104] DISTOR project: D2.5 Report on material & geometry definition ofmacroencapsulation 2005.

[105] Van Caneghem J, Brems A, Lievens P, Block C, Billen P, Vermeulen I, et al.Fluidized bed waste incinerators: design, operational and environmental issues.Prog Energy Combust Sci 2012;38:551–82. doi:10.1016/j.pecs.2012.03.001.

[106] Brems A, Cáceres G, Dewil R, Baeyens J, Pitié F. Heat transfer to the riser-wallof a circulating fluidised bed (CFB). Energy 2013;50:493–500. doi:10.1016/j.energy.2012.10.037.

[107] Van de Velden M, Baeyens J, Seville JPK, Fan X. The solids flow in the riser ofa circulating fluidised bed (CFB) viewed by positron emission particletracking (PEPT). Powder Technol 2008;183:290–6. doi:10.1016/j.powtec.2007.07.027.

[108] Smolders K, Baeyens J. Thermal degradation of PMMA in fluidised beds. WasteManag 2004;24:849–57. doi:10.1016/j.wasman.2004.06.002.

[109] Smolders K, Baeyens J. Overall solids movement and solids residence timedistribution in a CFB-riser. Chem Eng Sci 2000;55:4101–16. doi:10.1016/S0009-2509(00)00084-1.

[110] Wu SY, Baeyens J. Effect of operating temperature on minimumfluidization velocity. Powder Technol 1991;67:217–20. doi:10.1016/0032-5910(91)80158-F.

[111] Izquierdo-Barrientos MA, Sobrino C, Almendros-Ibáñez JA. Energy storage withPCM in fluidized beds: modeling and experiments. Chem Eng J 2015;264:497–505. doi:10.1016/j.cej.2014.11.107.

[112] Peng H, Dong H, Ling X. Thermal investigation of PCM-based high temperaturethermal energy storage in packed bed. Energy Convers Manag 2014;81:420–7.doi:10.1016/j.enconman.2014.02.052.

[113] Zhang HZ, Wang XD. Synthesis and properties of microencapsulatedn-octadecane with polyurea shells containing different soft segments for heatenergy storage and thermal regulation. Sol Energy Mater Sol Cells2009;93:1366–76. doi:10.1016/j.solmat.2009.02.021.

[114] Salunkhe PB, Shembekar PS. A review on effect of phase change materialencapsulation on the thermal performance of a system. Renew Sustain EnergyRev 2012;16:5603–16. doi:10.1016/j.rser.2012.05.037.

[115] Hawlader MNA, Uddin MS, Khin MM. Microencapsulated PCM thermal-energystorage system. Appl Energy 2003;74:195–202. doi:10.1016/S0306-2619(02)00146-0.

[116] Hawlader MNA, Uddin MS, Zhu HJ. Preparation and evaluation of a novel solarstorage material: microencapsulated paraffin. Int J Sol Energy 2000;20:227–38.doi:10.1080/01425910008914357.

[117] Zhao CY, Zhang GH. Review on microencapsulated phase change materials(MEPCMs): fabrication, characterization and applications. Renew SustainEnergy Rev 2011;15:3813–32. doi:10.1016/j.rser.2011.07.019.

[118] Jamekhorshid A, Sadrameli SM, Farid M. A review of microencapsulationmethods of phase change materials (PCMs) as a thermal energy storage (TES)medium. Renew Sustain Energy Rev 2014;31:531–42. doi:10.1016/j.rser.2013.12.033.

[119] Li M. A nano-graphite/paraffin phase change material with highthermal conductivity. Appl Energy 2013;106:25–30. doi:10.1016/j.apenergy.2013.01.031.

[120] Mesalhy O, Lafdi K, Elgafy A, Bowman K. Numerical study for enhancing thethermal conductivity of phase change material (PCM) storage using highthermal conductivity porous matrix. Energy Convers Manag 2005;46:847–67.doi:10.1016/j.enconman.2004.06.010.

[121] Elgafy A, Lafdi K. Effect of carbon nanofiber additives on thermal behavior ofphase change materials. Carbon N Y 2005;43:3067–74. doi:10.1016/j.carbon.2005.06.042.

[122] Goodfellow. Metal – thermal catalogue 2015.[123] Zhao CY, Lu W, Tian Y. Heat transfer enhancement for thermal energy storage

using metal foams embedded within phase change materials (PCMs). SolEnergy 2010;84:1402–12. doi:10.1016/j.solener.2010.04.022.

[124] Vadwala P. Thermal energy storage in metal foams filled with paraffin wax2011.

[125] Li Z, Wu Z-G. Numerical study on the thermal behavior of phase changematerials (PCMs) embedded in porous metal matrix. Sol Energy 2014;99:172–84. doi:10.1016/j.solener.2013.11.017.

[126] Bauer CA, Wirtz RA. Thermal characteristics of a compact, passive thermalenergy storage device. Proc. 2000 ASME IMECE, Orlando, Florida, USA: 2000,p. 1–7.

[127] Tong X, Khan JA, RuhulAmin M. Enhancement of heat transfer by inserting ametal matrix into a phase change material. Numer Heat Transf Part A Appl1996;30:125–41. doi:10.1080/10407789608913832.

[128] Xiao X, Zhang P, Li M. Preparation and thermal characterization of paraffin/metal foam composite phase change material. Appl Energy 2013;112:1357–66.doi:10.1016/j.apenergy.2013.04.050.

[129] Fiedler T, Öchsner A, Belova IV, Murch GE. Thermal conductivity enhancementof compact heat sinks using cellular metals. Defect Diffus. Forum 2008;273-276:222–6.

[130] Mehling H, Hiebler S, Ziegler F. Latent heat storage using a PCM-graphitecomposite material. Proc. Terrastock 2000-8th int. conf. therm. energy storage,Stuttgart, Germany: 2000, p. 375–80.

[131] Py X, Olives R, Mauran S. Paraffin/porous-graphite-matrix composite as a highand constant power thermal storage material. Int J Heat Mass Transf2001;44:2727–37. doi:10.1016/S0017-9310(00)00309-4.

[132] Cabeza LF, Mehling H, Hiebler S, Ziegler F. Heat transfer enhancement in waterwhen used as PCM in thermal energy storage. Appl Therm Eng 2002;22:1141–51. doi:10.1016/S1359-4311(02)00035-2.

[133] Zhao YJ, Wang RZ, Wang LW, Yu N. Development of highly conductiveKNO3/NaNO3 composite for TES (thermal energy storage). Energy2014;70:272–7. doi:10.1016/j.energy.2014.03.127.

[134] Kibria MA, Anisur MR, Mahfuz MH, Saidur R, Metselaar IHSC. A review onthermophysical properties of nanoparticle dispersed phase change materials.Energy Convers Manag 2015;95:69–89. doi:10.1016/j.enconman.2015.02.028.

[135] Warzoha RJ, Weigand RM, Fleischer AS. Temperature-dependent thermalproperties of a paraffin phase change material embedded with herringbone














































































































































































style graphite nanofibers. Appl Energy 2015;137:716–25. doi:10.1016/j.apenergy.2014.03.091.

[136] Şahan N, Fois M, Paksoy H. Improving thermal conductivity phase changematerials – a study of paraffin nanomagnetite composites. Sol Energy MaterSol Cells 2015;137:61–7. doi:10.1016/j.solmat.2015.01.027.

[137] Yu J, Chen X, Ma XL, Song QF, Zhao YK, Cao JH. Influence of nanoparticles andgraphite foam on the supercooling of acetamide. J Nanomater 2014;2014:1–10.doi:10.1155/2014/313674.

[138] Jourabian M, Farhadi M, Sedighi K. On the expedited melting of phase changematerial (PCM) through dispersion of nanoparticles in the thermal storageunit. Comput Math Appl 2014;67:1358–72. doi:10.1016/j.camwa.2014.02.004.

[139] Motahar S, Nikkam N, Alemrajabi AA, Khodabandeh R, Toprak MS, MuhammedM. A novel phase change material containing mesoporous silica nanoparticlesfor thermal storage: a study on thermal conductivity and viscosity.Int Commun Heat Mass Transf 2014;56:114–20. doi:10.1016/j.icheatmasstransfer.2014.06.005.

[140] Cingarapu S, Singh D, Timofeeva EV, Moravek MR. Nanofluids withencapsulated tin nanoparticles for advanced heat transfer and thermal energystorage. Int J Energy Res 2014;38:51–9. doi:10.1002/er.3041.

[141] Sciacovelli A, Colella F, Verda V. Melting of PCM in a thermal energy storageunit: numerical investigation and effect of nanoparticle enhancement. Int JEnergy Res 2013;37:1610–23. doi:10.1002/er.2974.

[142] Kashani S, Ranjbar AA, Madani MM, Mastiani M, Jalaly H. Numerical studyof solidification of a nano-enhanced phase change material (NEPCM) in athermal storage system. J Appl Mech Tech Phys 2013;54:702–12. doi:10.1134/S0021894413050027.

[143] Khodadadi JM, Fan L, Babaei H. Thermal conductivity enhancement ofnanostructure-based colloidal suspensions utilized as phase change materialsfor thermal energy storage: a review. Renew Sustain Energy Rev 2013;24:418–44. doi:10.1016/j.rser.2013.03.031.

[144] Abolghasemi M, Keshavarz A, Mehrabian MA. Thermodynamic analysis of athermal storage unit under the influence of nano-particles added to the phasechange material and/or the working fluid. Heat Mass Transf 2012;48:1961–70.doi:10.1007/s00231-012-1039-1.

[145] Guo CX, Wang YH. Numerical investigation of nanoparticle-enhanced hightemperature phase change material for solar energy storage. Adv. Mater. Res.2012;512-515:961–4.

[146] Wu S, Zhu D, Zhang X, Huang J. Preparation and melting/freezing characteristicsof Cu/paraffin nanofluid as phase-change material (PCM). Energy and Fuels2010;24:1894–8. doi:10.1021/ef9013967.

[147] Gong Z-X, Mujumdar AS. Cyclic heat transfer in a novel storage unit of multiplephase change materials. Appl Therm Eng 1996;16:807–15. doi:10.1016/1359-4311(95)00088-7.

[148] Gong Z-X, Mujumdar AS. A new solar receiver thermal store for space-basedactivities using multiple composite phase-change materials. J Sol Energy Eng1995;117:215. doi:10.1115/1.2847798.

[149] Michels H, Pitz-Paal R. Cascaded latent heat storage for parabolic trough solarpower plants. Sol Energy 2007;81:829–37. doi:10.1016/j.solener.2006.09.008.

[150] Seeniraj RV, Velraj R, Lakshmi Narasimhan N. Heat transfer enhancement studyof a LHTS unit containing dispersed high conductivity particles. J Sol EnergyEng 2002;124:243. doi:10.1115/1.1488669.

[151] Seeniraj RV, Lakshmi Narasimhan N. Performance enhancement of a solardynamic LHTS module having both fins and multiple PCMs. Sol Energy2008;82:535–42. doi:10.1016/j.solener.2007.11.001.

[152] Wang J, Ouyang Y, Chen G. Experimental study on charging processes of acylindrical heat storage capsule employing multiple-phase-change materials.Int J Energy Res 2001;25:439–47. doi:10.1002/er.695.

[153] Wang J, Chen G, Zheng F. Study on phase change temperature distributionsof composite PCMs in thermal energy storage systems. Int J Energy Res1999;23:277–85.

[154] Cui H, Yuan X, Hou X. Thermal performance analysis for a heat receiver usingmultiple phase change materials. Appl Therm Eng 2003;23:2353–61.doi:10.1016/S1359-4311(03)00210-2.

[155] Peiró G, Gasia J, Miró L, Cabeza LF. Experimental evaluation at pilot plantscale of multiple PCMs (cascaded) vs. single PCM configuration forthermal energy storage. Renew Energy 2015;83:729–36. doi:10.1016/j.renene.2015.05.029.

[156] Tian Y, Zhao CY. Thermal and exergetic analysis of metal foam-enhancedcascaded thermal energy storage (MF-CTES). Int J Heat Mass Transf2013;58:86–96. doi:10.1016/j.ijheatmasstransfer.2012.11.034.

[157] Agrafiotis C, Roeb M, Sattler C. Cobalt oxide-based structured thermochemicalreactors/heat exchangers for solar thermal energy storage in Concentrated SolarPower plants. Proc ASME 2014 8th int conf energy sustain collocated withASME 2014 12th int conf fuel cell sci eng technol 2014. doi:10.1115/ES2014-6336. ISBN 978-0-7918-4586-8.

[158] Aydin D, Casey SP, Riffat S. The latest advancements on thermochemical heatstorage systems. Renew Sustain Energy Rev 2015;41:356–67. doi:10.1016/j.rser.2014.08.054.

[159] Carrillo AJ, Serrano DP, Pizarro P, Coronado JM. Thermochemical heat storagebased on the Mn2O3/Mn3O4 redox couple: influence of the initial particlesize on the morphological evolution and cyclability. J Mater Chem A2014;2:19435–43. doi:10.1039/C4TA03409K.

[160] Carrillo AJ, Moya J, Bayón A, Jana P, de la Peña O’Shea VA, Romero M, et al.Thermochemical energy storage at high temperature via redox cycles of Mnand Co oxides: pure oxides versus mixed ones. Sol Energy Mater Sol Cells2014;123:47–57. doi:10.1016/j.solmat.2013.12.018.

[161] Michel B, Mazet N, Neveu P. Experimental investigation of an innovativethermochemical process operating with a hydrate salt and moist air for thermalstorage of solar energy: global performance. Appl Energy 2014;129:177–86.doi:10.1016/j.apenergy.2014.04.073.

[162] Neveu P, Tescari S, Aussel D, Mazet N. Combined constructal and exergyoptimization of thermochemical reactors for high temperature heatstorage. Energy Convers Manag 2013;71:186–98. doi:10.1016/j.enconman.2013.03.035.

[163] Yan T, Wang RZ, Li TX, Wang LW, Fred IT. A review of promising candidatereactions for chemical heat storage. Renew Sustain Energy Rev 2015;43:13–31.doi:10.1016/j.rser.2014.11.015.

[164] Meng Q-L, Lee C, Kaneko H, Tamaura Y. Solar thermochemical process forhydrogen production via two-step water splitting cycle based on Ce1−xPrxO2−δredox reaction. Thermochim Acta 2012;532:134–8. doi:10.1016/j.tca.2011.01.028.

[165] Le Gal A, Abanades S. Catalytic investigation of ceria-zirconia solid solutionsfor solar hydrogen production. Int J Hydrogen Energy 2011;36:4739–48.doi:10.1016/j.ijhydene.2011.01.078.

[166] Fresno F, Fernández-Saavedra R, Belén Gómez-Mancebo M, Vidal A, SánchezM, Isabel Rucandio M, et al. Solar hydrogen production by two-stepthermochemical cycles: evaluation of the activity of commercial ferrites. IntJ Hydrogen Energy 2009;34:2918–24. doi:10.1016/j.ijhydene.2009.02.020.

[167] Meier A, Steinfeld A. Solar thermochemical production of fuels. Adv Sci Technol2010;74:303–12. doi:10.4028/www.scientific.net/AST.74.303.

[168] Charvin P, Stéphane A, Florent L, Gilles F. Analysis of solar chemical processesfor hydrogen production from water splitting thermochemical cycles. EnergyConvers Manag 2008;49:1547–56. doi:10.1016/j.enconman.2007.12.011.

[169] Solé A, Fontanet X, Barreneche C, Fernández AI, Martorell I, Cabeza LF.Requirements to consider when choosing a thermochemical materialfor solar energy storage. Sol Energy 2013;97:398–404. doi:10.1016/j.solener.2013.08.038.

[170] Zhang HL, Baeyens J, Degrève J, Huys K, Kong WB, Lv YQ. Thermo-chemicalheat storage for dispatching power generation on demand. Proceeding 13thint. conf. energy storage, Beijing, China: 2015, p. 62.

[171] Williams OM, Carden PO. Energy storage efficiency for the ammonia/hydrogen-nitrogen thermochemical energy transfer. Int J Energy Res1979;3(1):29–40. doi:10.1002/er.4440030105.

[172] Brems A, Baeyens J, Beerlandt J, Dewil R. Thermogravimetric pyrolysis of wastepolyethylene-terephthalate and polystyrene: a critical assessment of kineticsmodelling. Resour Conserv Recycl 2011;55:772–81. doi:10.1016/j.resconrec.2011.03.003.

[173] Brems A, Baeyens J, Vandecasteele C, Dewil R. Polymeric cracking of wastepolyethylene terephthalate to chemicals and energy. J Air Waste Manage Assoc2011;61:721–31. doi:10.3155/1047-3289.61.7.721.

[174] Alonso E, Hutter C, Romero M, Steinfeld A, Gonzalez-Aguilar J. Kinetics ofMn2O3-Mn3O4 and Mn3O4-MnO redox reactions performed underconcentrated thermal radiative flux. Energy and Fuels 2013;27:4884–90.doi:10.1021/ef400892j.

[175] Pardo P, Anxionnaz-Minvielle Z, Rougé S, Cognet P, Cabassud M. Ca(OH)2/CaOreversible reaction in a fluidized bed reactor for thermochemical heat storage.Sol Energy 2014;107:605–16. doi:10.1016/j.solener.2014.06.010.

[176] Schmidt M, Szczukowski C, Roßkopf C, Linder M, Wörner A. Experimentalresults of a 10 kW high temperature thermochemical storage reactor basedon calcium hydroxide. Appl Therm Eng 2014;62:553–9. doi:10.1016/j.applthermaleng.2013.09.020.

[177] Linder M, Roßkopf C, Schmidt M, Wörner A. Thermochemical energy storagein kW-scale based on CaO/Ca(OH)2. Energy Procedia 2014;49:888–97.doi:10.1016/j.egypro.2014.03.096.

[178] Tescari S, Agrafiotis C, Breuer S, de Oliveira L, Puttkamer MN, Roeb M, et al.Thermochemical solar energy storage via redox oxides: materials and reactor/heat exchanger concepts. Energy Procedia 2014;49:1034–43. doi:10.1016/j.egypro.2014.03.111.

[179] Porisini FC. Salt hydrates used for latent heat storage: corrosion of metals andreliability of thermal performance. Sol Energy 1988;41:193–7. doi:10.1016/0038-092X(88)90136-3.

[180] Singh IB, Sen U. Influence of temperature and sulphate ion on corrosion ofmild steel in molten NaNo3. Br Corros J 1992;27:299–304. doi:10.1179/bcj.1992.27.4.299.

[181] Bradshaw RW, Goods SH. Corrosion of alloys and metals by molten nitrates,SANDIA report. 2000.

[182] Baraka A, Abdel-Rohman AI, El Hosary AA. Corrosion of mild steel in moltensodium nitrate–potassium nitrate eutectic. Br Corros J 1976;11:44–6.doi:10.1179/000705976798320313.

[183] Cabeza LF, Illa J, Roca J, Badia F, Mehling H, Hiebler S, et al. Immersion corrosiontests on metal-salt hydrate pairs used for latent heat storage in the 32 to 36°Ctemperature range. Mater Corros 2001;52:140–6. doi:10.1002/1521-4176(200102)52:2<140::AID-MACO140>3.0.CO;2-R.

[184] Bradshaw RW, Goods SH. Corrosion resistance of stainless steels during thermalcycling in alkali nitrate molten salts, SANDIA Report. 2001.

[185] Barlow P. Barlow’s formula. <http://en.wikipedia.org/wiki/Barlow%27s_formula>2015 [accessed 05.05.15].

[186] Perry RH, Green DW. Perry’s chemical engineers handbook. [Chapter 23]. 6thed. New York: McGraw-Hill Book Company; 1985.

[187] Specialmetals. INCONEL alloy 600 2008.[188] ATI. Allegheny technologies incorporated. <http://www.atimetals.com>; 2015.[189] Global Metals Pty Ltd. Global metals 2015. <http://www.globalmetals.com.au/>.














































































































http://dx.doi.org/10.4028/www.scientific.net/AST.74.303
























































http://en.wikipedia.org/wiki/Barlow%27s_formula

http://en.wikipedia.org/wiki/Barlow%27s_formula




http://www.atimetals.com

http://www.globalmetals.com.au/

[190] American Iron and Steel Institute. High temperature characteristics of stainlesssteel, vol. 240. 2010 doi:10.1088/1742-6596/240/1/012071.

[191] Special metals. INCOLOY alloy 825 2004.[192] Singh JP, Bansal NP, Goto T, Lamon J, Choi SR, Mahmoud MM, editors.

Processing and properties of advanced ceramics and composites iv: ceramictransactions. Hoboken, New Jersey: John Wiley & Sons; 2012.

[193] Ananthakumar S, Manohar P, Warrier KGK. Microstructural features andmechanical properties of Al2O3–Al2TiO 5 composite processed by gelassisted ceramic extrusion. Br Ceram Trans 2002;101:38–43. doi:10.1179/096797801125000393.

[194] Maddrell ER. Pressureless sintering of silicon carbide. J Mater Sci Lett1987;6:486–8. doi:10.1007/BF01756807.

[195] Guo XM, Yan YJ, Chen J, Huang ZR, Liu XJ. Mechanical properties andmicrostructure of extruded SiC ceramics. J Inorg Mater 2009;24:1155–8.doi:10.3724/SP.J.1077.2009.01155.

[196] RHI AG. RHI. <http://rhi-ag.com/internet_en/>; 2015.[197] Ortech Advanced Ceramics. Ceramic seal. <http://www.ortechceramics.com/

ceramic_seal.htm>; 2015.[198] GraphiteStore.com. Technical ceramics. <https://www.graphitestore.com/

cat.asp/spcat_id/4>; 2015.[199] McDanel Ceramics. Ceramics. <http://www.mcdanelceramics.com/

products.html>; 2015.[200] Bradshaw RW, Siegel NP. Molten nitrate salt development for thermal energy

storage in parabolic trough solar power systems. In: Es2008 Proc 2nd int confenergy sustain, vol. 2. 2009. p. 631–7.

[201] Zhang HL, Degrève J, Dewil R, Baeyens J. Operation diagram of circulatingfluidized beds (CFBs). Procedia Eng 2015;102:1092–103. doi:10.1016/j.proeng.2015.01.232.

[202] Geldart D. Types of gas fluidization. Powder Technol 1973;7:285–92.doi:10.1016/0032-5910(73)80037-3.

[203] Abrahamsen AR, Geldart D. Behaviour of gas-fluidized beds of fine powderspart I. Homogeneous expansion. Powder Technol 1980;26:35–46. doi:10.1016/0032-5910(80)85005-4.

[204] Bi HT, Grace JR. Flow regime diagrams for gas-solid fluidization and upwardtransport. Int J Multiph Flow 1995;21:1229–36. doi:10.1016/0301-9322(95)00037-X.

[205] Punwani DV, Modi MV, Tarman PB. Generalized correlation for estimatingchoking velocity in vertical solids transport. Conf. int. powder bulk solids handl.process. conf., Chicago, USA 1976.

[206] Brook N. Fluid transport of coarse solids. Min Sci Technol 1987;5:197–217.doi:10.1016/S0167-9031(87)90415-4.

[207] Baeyens J, Geldart D. An investigation into slugging fluidized beds. Chem EngSci 1974;29:255–65. doi:10.1016/0009-2509(74)85051-7.

[208] Flamant G. Theoretical and experimental study of radiant heat transfer in asolar fluidized-bed receiver. AIChE J 1982;28:529–35. doi:10.1002/aic.690280402.

[209] Martin J, Vitko J. ASCUAS: a solar central receiver utilizing a solid thermalcarrier. 1982.

[210] Bataillie D, Laguerie C, Royere C, Gauthier D. Echangeurs de chaleur gaz-solideà lits fluidisés multi-étages dans le domaine des moyennes et hautestempératures. Entropie 1989;25:113–26.

[211] Flamant G, Hernandez D, Bonet C, Traverse J-P. Experimental aspects of thethermochemical conversion of solar energy; decarbonation of CaCO3. SolEnergy 1980;24:385–95. doi:10.1016/0038-092X(80)90301-1.

[212] Siegel NP, Ho CK, Khalsa SS, Kolb GJ. Development and evaluation of aprototype solid particle receiver: on-sun testing and model validation. J SolEnergy Eng 2010;132:021008. doi:10.1115/1.4001146.

[213] Tan T, Chen Y. Review of study on solid particle solar receivers. Renew SustainEnergy Rev 2010;14:265–76. doi:10.1016/j.rser.2009.05.012.

[214] Röger M, Amsbeck L, Gobereit B, Buck R. Face-down solid particle receiverusing recirculation. J Sol Energy Eng 2011;133:031009. doi:10.1115/1.4004269.

[215] Warerkar S, Schmitz S, Goettsche J, Hoffschmidt B, Reißel M, Tamme R.Air-sand heat exchanger for high-temperature storage. J Sol Energy Eng2011;133:021010. doi:10.1115/1.4003583.

[216] Al-Ansary H, El-Leathy A, Al-Suhaibani Z, Jeter S, Sadowski D, Alrished A, et al.Experimental study of a sand–air heat exchanger for use with a high-temperature solar gas turbine system. J Sol Energy Eng 2012;134:041017.doi:10.1115/1.4007585.

[217] Mahmoudi S, Chan CW, Brems A, Seville J, Baeyens J. Solids flow diagram ofa CFB riser using Geldart B-type powders. Particuology 2012;10:51–61.doi:10.1016/j.partic.2011.09.002.

[218] Smolders K, Baeyens J. Elutriation of fines from gas fluidized beds: mechanismsof elutriation and effect of freeboard geometry. Powder Technol 1997;92:35–46. doi:10.1016/S0032-5910(97)03214-2.

[219] Bodin S, Briens C, Bergougnou M, Patureaux T. Standpipe flow modeling,experimental validation and design recommendations. Powder Technol2002;124:8–17. doi:10.1016/S0032-5910(01)00473-9.

[220] Smolders K, Geldart D, Baeyens J. The physical models of cyclone diplegs influidized beds. Chinese J Chem Eng 2001;9:337–47.

[221] Chan CW, Seville JPK, Fan X, Baeyens J. Solid particle motion in a standpipeas observed by Positron Emission Particle Tracking. Powder Technol2009;194:58–66.

[222] Flamant G, Hemati M. Dispositif collecteur d’énergie solaire. French PatentNo. 1058565, 2010.

[223] Falcone PK, Noring JE, Hruby JM. Assessment of a solid particle receiver for ahigh temperature solar central receiver system. 1985.

[224] Fend T, Pitz-Paal R, Reutter O, Bauer J, Hoffschmidt B. Two novel high-porositymaterials as volumetric receivers for concentrated solar radiation. Sol EnergyMater Sol Cells 2004;84:291–304. doi:10.1016/j.solmat.2004.01.039.

[225] Gokon N, Yamamoto H, Kondo N, Kodama T. Internally circulating fluidizedbed reactor using m-ZrO[sub 2] supported NiFe[sub 2]O[sub 4] particles forthermochemical two-step water splitting. J Sol Energy Eng 2010;132:021102.doi:10.1115/1.4001154.

[226] Xu C, Song Z, Chen L, Zhen Y. Numerical investigation on porous media heattransfer in a solar tower receiver. Renew Energy 2011;36:1138–44.doi:10.1016/j.renene.2010.09.017.

[227] Jeter SM, Stephens JH. System and method of thermal energy storage. US2012/0132398A1, 2011.

[228] Zipf V, Neuhäuser A, Willert D, Nitz P, Gschwander S, Platzer W.High temperature latent heat storage with a screw heat exchanger: designof prototype. Appl Energy 2013;109:462–9. doi:10.1016/j.apenergy.2012.11.044.

[229] Matsubara K, Kazuma Y, Sakurai A, Suzuki S, Soon-Jae L, Kodama T, et al.High-temperature fluidized receiver for concentrated solar radiation by abeam-down reflector system. Energy Procedia 2014;49:447–56. doi:10.1016/j.egypro.2014.03.048.

[230] Ma Z, Glatzmaier GC, Mehos M. Development of solid particle thermal energystorage for concentrating solar power plants that use fluidized bed technology.Energy Procedia 2014;49:898–907. doi:10.1016/j.egypro.2014.03.097.

[231] Zhang HL, Baeyens J, Degrève J, Brems A, Dewil R. The convection heat transfercoefficient in a circulating fluidized bed (CFB). Adv Powder Technol2014;25:710–15. doi:10.1016/j.apt.2013.10.018.

[232] Ordóñez F, Caliot C, Bataille F, Lauriat G. Optimization of the optical particleproperties for a high temperature solar particle receiver. Sol Energy2014;99:299–311. doi:10.1016/j.solener.2013.11.014.

[233] Fernández P, Miller FJ. Performance analysis and preliminary designoptimization of a small particle heat exchange receiver for solar towerpower plants. Sol Energy 2015;112:458–68. doi:10.1016/j.solener.2014.11.012.

[234] Zanganeh G, Pedretti A, Haselbacher A, Steinfeld A. Design of packed bedthermal energy storage systems for high-temperature industrial process heat.Appl Energy 2015;137:812–22. doi:10.1016/j.apenergy.2014.07.110.

[235] Chan CW, Seville JPK, Parker DJ, Baeyens J. Particle velocities and their residencetime distribution in the riser of a CFB. Powder Technol 2010;203:187–97.doi:10.1016/j.powtec.2010.05.008.

[236] Zhang H, Degrève J, Baeyens J, Dewil R. The voidage in a CFB riser as functionof solids flux and gas velocity. Procedia Eng 2015;102:1112–22. doi:10.1016/j.proeng.2015.01.234.

















http://rhi-ag.com/internet_en/

http://www.ortechceramics.com/ceramic_seal.htm

http://www.ortechceramics.com/ceramic_seal.htm

https://www.graphitestore.com/cat.asp/spcat_id/4

https://www.graphitestore.com/cat.asp/spcat_id/4

http://www.mcdanelceramics.com/products.html

http://www.mcdanelceramics.com/products.html











































































































Thermal energy storage: Recent developments and practical ...

Documents

Transcript of Thermal energy storage: Recent developments and practical ...