Desalination 336 (2014) 121–128

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Desalination

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Concentrating solar power (CSP) system integrated with MED–ROhybrid desalination

G. Iaquaniello a, A. Salladini b,⁎, A. Mari a, A.A. Mabrouk c, H.E.S. Fath d

a KT – Kinetics Technology SpA, Viale Castello della Magliana 75, 00148 Rome, Italyb Processi Innovativi srl, Viale Castello della Magliana 75, 00148 Rome, Italyc Suez University, Suez, Egyptd Egypt–Japan University of Science & Technology, Alexandria, Egypt

H I G H L I G H T S

• An alternative hybrid desalination scheme powered by solar energy is proposed.• Backup system integrated to a CSP system to assure continuous operation mode• Cost-effective produced water by solar assisted desalination• Evaluation of environmental benefit of renewable desalination

⁎ Corresponding author at: Viale Castello della MaTel.: +39 06 60216894; fax: +39 06 65793002.

E-mail address: salladini.a@processiinnovativi.it (A. Sa

0011-9164/$ – see front matter © 2014 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.desal.2013.12.030

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 September 2013Received in revised form 23 December 2013Accepted 26 December 2013Available online 25 January 2014

Keywords:Hybrid desalinationMEDRORenewable energyCSPTechno-economics

Renewable energy technologies, in particular concentrating solar power (CSP), are becomingmore andmore in-teresting for powering water desalination system. Moving from a European Community funded project calledMATS,Multipurpose Applications by Thermodynamics Solar, which is in an advanced phase of detailed engineer-ing, the authors have further developed an alternative scheme by a proper integration of CSP with multi-effectdistillation (MED) and reverse osmoses (RO) desalination processes. According to the proposed scheme MEDis powered by the low temperature exhaust steam delivered from the back pressure steam turbine while theRO is powered by the electricity produced by the same steam turbine in addition to that generated by a conven-tional gas turbine integrated as a thermal backup system. The effectivematch of the alternative solar thermal andelectricity into such hybrid power-desalination scheme is discussed in details. An economical analysis togetherwith a developed comprehensive model is provided where power availability, water production rates and envi-ronmental benefits have been implemented. Desalination using the CSP system through such hybrid integrationallows also for a continuous operation and can be an effective way to lower the total water production costs notonly for large-scale plants.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

The exploitation of renewable energy (RE) sources for heat, electri-cal energy and fresh water production is commonly considered a verypromising way to reduce the impact of humane activities on environ-ment. Concentrating solar power (CSP) plants are today moving fastto become competitive in converting solar radiation into power formul-tipurpose industrial and residential applications, especially in countriesbelonging in the so-called “Sun belt” also to meet the problem of freshwater availability.

The Middle East and North Africa (MENA) region in fact is themost water scarce region of the world. High population growth rate,

gliana 75, Rome 00148, Italy.

lladini).

ghts reserved.

urbanization and industrialization, coupled with natural water scarcityare leading to serious deficits of freshwater availability. In the countriesof theNile Basin for example, the rising populations and rapid economicdevelopment together with pollution and water quality degradation,are decreasingNilewater availability for Egyptwhich is facing an annualwater deficit of around 7 billion cubic meters. As a matter of fact UnitedNations is already warning that Egypt could be in a severe shortage ofwater by the year 2025 [1].

Conventional large-scale desalination plant is cost-prohibitive andenergy-intensive, and not viable for poor countries in the MENA regionespecially with the increasing costs of fossil fuels. However, the relativelyrich oil producing GCC countries have adopted large desalination plantsfor more than 60 years. In fact GCC countries produce more than 40% ofthe world's desalinated water with mainly 75% as thermal technologyand 25% RO. In Egypt large RO plants have been adopted in different tour-istic cities (as Sharm Al-Sheikh and Ghaurgada). Other North African

Table 1Main specifications of the research project MATS.

Electric power production (steam turbine) 1.0 MWeOutlet thermal power 4.0 MWthInlet thermal power 5.7 MWthDesalting unit capacity (MED) 250 m3/day

122 G. Iaquaniello et al. / Desalination 336 (2014) 121–128

countries such as Morocco, Algeria, Tunisia, etc. have adopted RO desali-nation technologies due to its lower specific energy consumption. Inaddition, the environmental impacts of using fossil fuel for desalinationare considered critical on account of CO2. The negative effects of desalina-tion canbeminimized, to someextent, by using renewable energy (RE) topower the plants. RE-powered desalinationmay offer a sustainablemeth-od to increase supply of potable water inMENA countries. The region hastremendous wind and solar energy potential which can be effectivelyutilized in different desalination technologies (thermal, membrane andhybrid systems). The cost of RE desalination is expected to becomemore attractive with technological advancements coupled with risingcosts of freshwater and fossil fuels and the implementation of carbon tax.

The MATS (Multipurpose Applications by Thermodynamics Solar)research project funded by the European Community (EC) [2] is aimingto demonstrate the economical interest in using CSP technology forsmall/middle size multipurpose facilities producing water, power,heating and cooling, which is easy to backup with locally already avail-able renewable or fossil fuels. A process outline of MATS project is re-ported in Fig. 1. More in details project is focused on ENEA (ItalianNational Agency for New Technologies, Energy and Sustainable Eco-nomic Development) developed CSP technology based on molten saltsused as storage medium and as heat transfer fluid [3]. The proposedthermal energy storage system (TES) exploits the sensible heat andthermal stratification properties of a mixture of molten salts (60%NaNO3 and 40% KNO3). Its operation is based on a thermal stratificationcaused by difference in density with temperature already verified byENEA on laboratory scale. The presence of the integrated steam genera-tion becomes the active principle producing stratification during thetime and allowing storage of cold and hot molten salts in the sametank. This architecture may introduce a significant improvement interms of efficiency, reliability and cost reduction with respect to a tradi-tional two-tank based TES system.

The objective of the project is the pilot scale demonstration of suchtechnology in amultipurpose facility located in Egypt. The thermal energyproduced by this unit will supply steam to power and desalination unitsas well as be used for district heating and cooling. The use of a suitableTES systemavoids themismatchof powerproduction fromsolar radiationavailability and allows to extend the operational period. Main specifica-tions of the MATS demonstration pilot plant are listed in Tables 1 and 2.

The implementation of the project will allow to test the CSP technol-ogy in a very advantageous location with regard to the solar radiationrate as an example for the diffusion of this technology inMediterraneancountries. Besides it may represent the start-up for a development ofspecialized local industries.

Regarding desalination package developed in the framework ofMATS project, MED unit is based on two effects only, just to demonstrate

Fig. 1. Process outline of re

the feasibility of multipurpose production. According to the original de-sign, the unit is fed by exhausted steam delivered from a turbine sizedfor a production of 1.0 MWe. Steam exiting 1 MWe sized turbine is fedto the first effect at temperature of 69 °C in order to avoid salts deposi-tion and consequently scaling phenomena.

Due to the pilot plant site location, far away from the sea, unit has tobe fedwithwell brackishwater not so abundant at the selected site. Thefinal condenser, generally fed with seawater, in this case uses air ascoolingmedium in order to reduce the limited saline water withdrawalfrom the site wells. With respect to a cooling water condenser, this ar-rangement requires a higher temperature (60 °C) in the last effect inorder to allow a suitable rate of condensation by air. During night oper-ation, and in order to limit fuel consumption, power production is fullystoppedwhileMEDunit is kept under a turn down of 40% (100 m3/day)considering that it has stable operation over a load range of 30%–120%ofthe designed capacity [4].

Optimization of this schemewas not part of theMATS project whichis mainly focused on the experimental plant operation and testing of itscritical components. It is quite logic to extend the MATS architectureinto an alternative optimized scheme fully or partially dedicated tothe production of desalinated water. This article is devoted to illustratesuch process scheme where two energies, the renewable solar energyand the chemical energy of a fuel, have been combined to produceheat and power in a continuous mode to power an integrated MED/RO plant operating in a hybrid way.

2. System, components and methods

CSP offers an attractive option to power industrial-scale desalinationplants requiring thermal and electrical energy. Several countries such asJordan, Egypt, Saudi Arabia, and UAE are already developing large CSP-based solar power projects that promise to share power and water sup-ply in a new era in the Middle East [5,6]. CSP system for combined pro-duction of power and water may also be an attractive solution towardscontinuous operationmode. The ability to store heat and to be integrat-edwith backup systemallows for higher reliability thatmakes CSP tech-nology the most suitable system for large scale application [7]. In thissense this increased reliability may contribute to significantly reducewater production cost.

search project MATS.

Table 2Main operating specifications of the research project MATS.

Solar radiation rate Hourly peak 1409 W/m2

Yearly total 2308 kWh/m2

Solar collectors Unit length 100 mUnit area 540 m2

Optical efficiency 75 %Thermal efficiency 83 %

Solar field Number collector 18 –

Total collector length 1800 mTotal collector area 9720 m2

Site area 22,000 m2

Annual production Electric energy 3010 MWhThermal energy 8900 MWh

123G. Iaquaniello et al. / Desalination 336 (2014) 121–128

Literature reports a lot of experiences relevant to desalinationpowered by solar energy [8,9]. Themostwidespread integration especial-ly in the MENA region is represented by reverse osmosis (RO) coupledwith the photo voltaic (PV) system even if this technology is still limitedto small or pilot scale applicationmainly in remote areas [10]. Integrationof RO with PV system seems to be cheaper than solar driven thermal de-salination plant even if an economic comparison on a large-scale level,showed that multi-effect distillation (MED) may be more competitiveand cheaper at plant capacity higher than 1000 m3/day [11].

2.1. Alternative hybrid desalination scheme powered by solar energy

The proposed alternative scheme for desalinated water productionby solar energy is shown in Fig. 2. It is based on a CSP system usingmol-ten salts as thermal medium equipped with a thermal storage to in-crease the operating time over direct thermal irradiation period. Abackup system working as a heat recovery device on flue gas deliveredfrom a gas turbine (GT) enables a continuous operation mode. Solarfield and molten salts storage assure continuous operation for 10 and4 h respectively while the residual operating time is covered by a back-up heater.With respect toMATS proposal, the backup thermal system isnomore a simple fired heater but it is directly connectedwith a GT pro-ducing power in continuous mode. By this way fuel required to assurecontinuous operation will allow also to increase the cycle efficiency byincreasing power output together with available heat.

The flue gas heat recovery system (HRS) working also as a thermalbackup, sustains five services: molten salts heater, boiler feed waterpreheater, low pressure steam production, demi-water preheater andMED feed water preheater. A suitable post-firing on exhaust gas allowsto supply during night operation and cloudy days additional heat equalto that normally released by solar field while a quenching air stream isrequired during daily operation to reduce flue gas temperature downto 330 °C. Under normal operation of solar field in fact, only a reduced

SOLARFIELD STORAGE

STEAMGENERATION

BACK UPSYSTEM

POWER Ran

POWERBr

HeatStack

Sun

Fuel

HEATRECOVERY

Fig. 2. Block diagram of the proposed double sch

amount of molten salts is fed to HRS in order to keep the line warm atminimum temperature.

Heat released frommolten salts in the storage system, is used to pro-duce superheated steam to sustain a Rankine cycle. The exhaustedsteam is fed to an MED plant, condensed in the first effect and fedback in closed cycle.

Net power delivered fromRankine andBryton cycles is fully convert-ed into desalinated water through reverse osmosis without any exportpower, although a different sharing is possible depending on the localneeds. Tables 3 and 4 report the technical characteristics of the selectedcommercial gas and steam turbine respectively. Regarding GT system,both performances corresponding to ISO conditions and to an air tem-perature of 30 °C (in brackets) are indicated.

Keeping constant characteristics of solar field as for MATS project(Table 2), a suitable integration between the proposed units has beenselected. A techno-economical analysis is carried out on the proposedflow-sheet in order to estimate water production cost by solar assisteddesalination.

2.2. MED desalination system

The selected MED unit is based on a falling film arrangement due tothehigher heat transfer coefficient realized during evaporation and con-densation. In cogeneration facilities, thermal desalination generally useslow pressure steam delivered from turbine and condensed inside tubesof the first effect. The released heat allows for a partial vaporization ofthe incoming seawater sprayed on the tube bundle while the residualconcentrating saline water (brine) is routed to the next effect's bottomfor additional vapor generation by flashing. The vapor produced in eacheffect is condensed inside the tube of the next one working at reducedtemperature, providing the heat for a further evaporation. Vapor com-ing from the last effect is routed into a final cooling water condenserand the released heat allows for a partial preheating of feed stream.Only a reduced amount of cooling water may be used as MED feedstream while the residual is normally rejected at battery limits. Topbrine temperature is usually kept lower than 70 °C in order to reducescaling phenomena in the first chambers.

Assuming an outlet cooling water temperature in the final condens-er equal to 35 °C, ten effects allow for a gross temperature drop per ef-fect equal to about 3 °C ensuring an overall net temperature differentialin the range of 1.5–2.5 °C. The latter is consistent with values reportedin the state of the art [12]. The gross driving force for heat transfer is re-duced by boiling point elevation, and temperature depression directlyrelated to pressure drop in the demister, in the transmission lines andduring vapor flow and condensation. To further increase efficiency inwater production, in this application MED feed water is preheated byrecovering low grade heat from exhausted gas delivered from GT.

Table 5 reports main data assumed for MED system modeling.

MED

PRODUCTIONkine cycle

PRODUCTIONyton cycle

Heat RO

ELECTRICALENERGY

Plant auxiliary

Fuel

Hybridization

Desalinated water

Sea water

Sea water

Brine

eme for power and fresh water production.

Table 3Technical specification of the selected Kawasaki GT at ISO condition (30 °C).

GT model M1A-13A –

GT generator model GPB15 –

Electrical output 1450 (1252) kWeCompression ratio 9.4 –

Heat rate 15,130 (16,027) kJ/kWheExhaust gas temperature 524 (536) °CExhaust mass flow 28,800 (26,640) kg/hElectrical efficiency 24.0 (22.4) %

Table 5MED design data.

No. of effect, Ne 10 –

Intake saline water temperature at battery limits, Tsw 25 °CFeed saline water salinity at battery limits, Xsw 35,000 ppmConcentration factor, CF 1.43 –

Gross temperature drop per effect, ΔT 3.1 °CHeating steam temperature, Ts 69 °C

124 G. Iaquaniello et al. / Desalination 336 (2014) 121–128

2.3. RO desalination system

Desalination by RO is the most widespread technology coveringabout 60% of the total installed worldwide capacity. The energy con-sumption of RO system is absorbed by feed water high pressure pumprequired to overcome the osmotic pressure and allow for water desali-nation. In the last years important efforts have been adopted in orderto improve RO membrane performance by increasing permeate fluxand reducing scaling and fouling phenomena and to reduce the specificenergy consumption by using high efficiency energy recovery devices(ERD). All these advances along the years brought an important reduc-tion in the water production cost.

The state of the art reports for medium and large capacity seawaterreverse osmosis (SWRO) plant equipped with ERD, energy consump-tion in the range 2.2–2.5 kWhe/m3 [13] even if consumption below2.0 kWhe/m3 may be technically feasible [14]. However, practicalplant's specific power consumption may increase to 3.0–5.0 kWhe/m3

for pretreatment and other auxiliary systems.RO systemmay be designed in a single ormultiple pass arrangement

depending on the feed water characteristic, recovery ratio and requiredstandards on permeate stream. In a two pass arrangement, the secondseparation step is fed by permeate coming from the first pass and highrecovery ratio is generally adopted up to 85%–90%. A multi-pass ar-rangement produces permeate with a lower TDS content and with aconsequent lower global recovery ratio; if the required permeate con-centration is much lower than 300–400 mg/l at least two passes arenecessary to achieve the required standard [15]. In general strict con-strains on boron and chloride content in the permeate stream may re-quire multiple passes working at different levels of pH. Boronrejection is improved if this element is present in the borate formachievable at higher pH thus the first pass generally works at neutralpH while the second pass works at higher pH [16]. A four pass configu-ration with different pH values is adopted for example in the Ashkelonplant in order to match boron and chloride content lower than0.4 mg/l and 20 mg/l respectively [17].

Table 6 reports the main data assumed for the RO desalination sys-tem modeling.

2.4. Hybrid desalination system

Advantages in adopting hybrid configurations of combined thermaland membrane desalination technologies have been extensively inves-tigated in the literature [19,20]. This kind of solution is increasinglybeing adopted in new and revamped facilities allowing for higher ener-gy efficiencies, which result in lowering the cost of water, when

Table 4Technical specification of the selected Siemens steam turbine.

Steam turbine generator model TWIN CA 36 –

Net electrical output 1000 kWePressure inlet 55 baraPressure outlet 0.3 baraTemperature inlet 460 °CSteam flow rate 5570 kg/hElectrical efficiency 19.5 %

compared with “stand-alone” desalination processes. When applied ina conventional dual purpose power plant, the integration of two tech-nologies allows for a higher flexibility in the production of power andwater through a better management of seasonal and daily variation inboth power and water demand [21].

The simplest level of integration may involve the blending of prod-uct coming from two technologies. As well known, a distillate producedin a thermal process is characterized by a higher purity than a permeatecoming fromRO. Improving the overall product quality by blending per-meate and distillatemay avoid installing the second and third RO stagesgenerally required to solve the critical boron and chloride issue. Togeth-er with a reduction in the investment cost related to the possibility touse a single RO stage, the reduction of strict boron requirements allowsfor milder operating conditions with a consequent extension of mem-brane lifetime. The blending of two products allows also to operatewith a higher TDS level on output stream and consequently to lowerthe replacement rate of membranes. Combining thermal MED and ROmembrane technologies in the same site allows also to use a commonintake and outfall facilities with a consequent decrease in the cost ofcivil works and in a reduction of pumping energy.

A fully integrated systemmay bemore suitable for new desalting fa-cilities. A higher level of hybridization may involve for example feedingRO with preheated stream leaving the heat rejection of the MSF systemor final condenser of MED unit exploiting beneficial effect of tempera-ture on RO membrane performance. An increase of feed water temper-ature of 1 °C may increase permeate flux by 3%. El-Sayed et al. [22], forexample, conducted pilot study of theMSF/ROhybrid systems inKuwaitand observed a significant increase in RO productwater flow rate. It wasdemonstrated on the basis of experimental data that 42%–48% gain inthe product water flow could be achieved for a feed temperature of33 °C, over that of an isolated RO plant operating at 15 °C during thewinter season.

Hybridization level adopted in the proposed scheme will bediscussed in paragraph 3.

3. Results and discussion

3.1. Integrated system performance

Heat and material balance around the proposed scheme were per-formed by using Simsci-Esscor's PRO/II process simulation softwarewhile RO unit has been simulated with Hydranautics SWRO System De-sign (IMSD) [23]. Process parameters reported in previous paragraphwere used to simulate power blocks (Tables 3 and 4), MED unit(Table 5) and RO unit (Table 6).

Considering for GT systemperformance corresponding to an inlet airtemperature of 30 °C and assuming that 10% of electrical energy

Table 6RO design data.

No. of pass, Np 1 –

Intake saline water temperature at battery limits, Tsw 25 °CIntake saline water salinity at battery limits, Xsw 35,000 ppmRecovery ratio, RR 50 %Membrane element type, – SWC4B [18] –

Energy recovery device, ERD Pressure exchanger –

Fig. 3. Process scheme of solar assisted hybrid desalination scheme.

125G. Iaquaniello et al. / Desalination 336 (2014) 121–128

delivered from GT (1250 Mwe) and steam turbine (1000 Mwe) has tobe used for plant auxiliary, net power available for RO system is about2.0 MWe. The latter is used as power input in the simulation of mem-brane process. The low energy consumption of RO system equippedwith energy recovery devices, allows for a permeate production ofabout 800 m3/h against a distillate production of 50 m3/h consistentwith a 10 effect MED configuration fed by 5.5 t/h of exhausted steam.The amount of desalinated water produced by the membrane processis much higher than that produced by distillation in case the total netpower coming from the twopower cycles is fully diverted to ROwithoutexport. A different ratio could be used and alternative delivery, for ex-ample, 1.0 MWe of available power to the electric grid and the remain-ing to RO.

To take advantages of the presence of both thermal and membranedesalination processes, the two technologies may be hybridized inorder to reduce intake and outfall capacity and improve RO perfor-mance. The selected arrangement, characterized by an elevated ratiobetween permeate and distillate, reduces the benefit related to theblending of products justifying the selection of a membrane elementwith high salts and boron rejection (Hydranautics-SWC4B). A detailedprocess scheme relevant to the desalination section is reported inFig. 3 with the indication of the selected level of hybridization.

Performance of the proposed scheme, with and without hybridiza-tion among two desalination processes, has been compared with astandalone RO process fully powered by a conventional combined

Table 7Results of the technical analysis.

Solar assisted desalination(hybrid-base case)

Solar field, kWt 5700Steam production, kg/h 5570Power from Rankine cycle, kWe 1000Power from Bryton cycle, kWe 1252Auxiliary consumption, % 10Power from equivalent combined cycle, kWe –

Equivalent combined cycle efficiency, % –

Specific RO energy consumption, kWe/m3 2.52Desalinated water production RO, m3/h 807Desalinated water production MED, m3/h 51Intake seawater capacity, m3/h 1665Outfall capacity, m3/h 807Fuel input, kg/m3 water 0.558CO2 emission, kg/m3 water 1.53

n.s., not specified.

cycle and delivering the same amount of desalinated water. The mainresults are reported in Table 7.

Superheated steamproduced by heat delivered from solarfield is fedto steam turbine to produce a net power of 1.0 MWe. Exhausted steam(5570 kg/h) at pressure of 0.24 bar corresponding to a saturation tem-perature of 69 °C is routed to the first effect of MED unit to be con-densed and to allow for seawater evaporation. MED unit is organizedin 10 effects working in parallel with respect to feed flow. The lastvapor is routed to the final condenser where seawater heated from25 °C to 35 °C is used as cooling medium. About 42% of cooling wateris used as feed stream for MED effects while the residual normallyrejected to the environment may be mixed with intake seawater andfed to RO. To increase temperature of mixed stream, the latter may befurther blended with brine discharged from the last effect at 38 °C and50,000 ppm. RO feed is thus composed of 79% of intake cold seawater(25 °C, 35,000 ppm), 14% of warmed condenser seawater (35 °C,35,000 ppm) and residual 7% of warmed MED brine reject (38 °C,50,000 ppm) which results in a final mixed temperature and salinityof 27 °C and36,000 ppmrespectively. The reduced brine fraction causesa little increase in salinity without any substantial influence on ROpower absorption. This particular assessment results in an MED unitwith zero liquid discharge and in a hybrid RO–MED scheme character-ized by seawater withdrawal and brine outfall reduced by 17% and30% respectively compared to the same scheme with two processesworking without any integration.

Solar assisted desalination Conventional desalination(non-hybrid base case) (base case)

5700 –

5570 n.s.1000 n.s.1252 n.s.10 10– 2378– 322.51 2.51804 85851 –

2003 17161148 8580.560 0.6201.54 1.705

Table 8Input data of the economic analysis.

Economic plant lifetime, years 20Interest rate, % 8Plant factor, % 90Fuel cost, €/kg 0.15Maintenance and chemicals relevant to solar + ST + GT + MED system, % 2Maintenance and chemicals RO, % 3.6General expense, % 0.8

126 G. Iaquaniello et al. / Desalination 336 (2014) 121–128

Environmental benefit in terms of reduced greenhouse gas emis-sions associated to fossil fuel consumption has been also evaluated. Inthe solar assisted scheme, fuel input is located in the combustion cham-ber of GT system and in the post firing section. The latter is kept in op-eration only during the night and cloudy days and account for anincrease of 37% of fuel gas consumed by GT. Thus an average fuel con-sumption has been estimated for process and economic evaluation.

Fuel consumption in the conventional combined cycle is supposed tobe relevant only in the GT system. Assuming for the conventionalscheme the same RO specific consumption as calculated for the solarassisted assessment (2.52 kWhe/m3), the gross power of the equivalentcycle may be calculated from Eq. (1):

Neq ¼ ERO � F � 1þ Auxiliaryð Þ ð1Þ

where ERO is the specific energy consumption associated to RO system, Fis the desalination water flow rate, and Neq is the gross power of theequivalent cycle. Assuming a value for electrical efficiency consistentwith the delivered power, fuel consumption and CO2 emissions associ-ated to the production of the same amount of water, may be estimatedfrom Eqs. (2), (3) and (4):

qfuelcc ¼Neq

ηelcc � LHVð2Þ

qfuels ¼NGT

ηGT � LHVþ qfuelpf ð3Þ

qCO2 ¼ qfuel �PMCO2

PMCH4ð4Þ

where ηel_cc is the electrical efficiency of combined cycle, LHV is the fuellow calorific value, qfuel is fuel mass flow rate, subscript cc and s are rel-evant to the combined cycle and solar assisted plant respectively, PM is

Table 9Results of the economic analysis.

Solar assist(base case)

MED plant capacity, m3/day 1224RO plant capacity, m3/day 19,372Total annual water production, m3/year 6,864,000

Capital cost,Total capital cost solar + ST, € 8,500,000Total capital cost GT, € 2,000,000Total capital cost MED, € 2,918,769Total capital cost RO, € 26,822,924Total capital cost, € 40,241,693Total capital recovery cost, €/year 4,098,705

Operating costFuel cost, €/year 574,200Operating cost RO, €/y 1,180,209Operating cost solar + MED + TG, €/year 375,726Labor, €/year 400,000Total operating cost, €/year 2,530,134

Water cost, €/m3 0

molecular weight and qCO2 is CO2 mass flow rate. Efficiency of 32% hasbeen adopted for a combined cycle delivering a power of 2.4 MWe [24].

Comparing the obtained results, the solar assisted scheme enables areduction of specific CO2 emission of about 10% with respect to a tradi-tional scheme.

Heat and material balance have been also performed around a ca-pacity scaled up by a factor of three in order to take into account the ef-fect of the increased cycle efficiency. Moving from the base to the three-time larger case, a gain of about 5% on the GT electrical efficiency isachievable [25] while for the equivalent combined cycle, efficiencymay raise from 32% to 37% when delivered power moves from2.4 MWe to 7.1 MWe [24]. Applying equations for fuel consumption(Eq. (2)) and CO2 emission (Eq. (4)), the improved efficiency resultsin a production for the scaled solar assisted scheme of 1.24 kgCO2/m3

(0.45 kgfuel/m3) against 1.47 kgCO2/m3 (0.54 kgfuel/m3) of conventionalscheme with a reduction of about 16%. Table 7 reports the main resultsof the technical analysis carried out on two schemes.

3.2. Water production cost evaluation

Capital and operating cost have to be calculated in order to estimatethe specific production cost of water. Regarding desalination facilities,capital cost has been evaluated by using average specific values set to3100 [26] and 1800 US$/(m3/day) [27] for MED and RO respectively.These values are derived from literature and include pre-treatment,post-treatment, electrical, instrument and control equipment as wellas civil structures relevant to intake and outfall. The capital cost of theGT system delivering a rated power of 1.45 MWemay be reasonably es-timated to be about 2 M€ [28]. Solar system, including solar field andthermal storage together with ST have been estimated to be about8.5 M€ according to data collected in the framework of research projectMATS. Table 8 reports basic input data adopted for the economicanalysis.

To study the effect of scale economy, cost associated to solution de-livering a three times capacity has been also estimated through thewell-known power law rule (Eq. (5)):

Capital costplant1Capital costplant2

!¼ Plant capacityplant1

Plant capacityplant2

!m

ð5Þ

Exponent m is set equal to 0.83 and 0.81 for MED and RO plant re-spectively as determined by a linear regression carried out on severalexisting plants [26]. Correlation has been applied also on solar system

ed hybrid desalination Solar assisted desalination(three-time scaled case)

367258,116

20,592,000

17,359,9214,084,6877,264,578

65,309,19594,018,3819,575,980

1,393,1442,873,605803,857400,000

5,470,606

.97 0.73

0%

5%

10%

15%

20%

25%

30%

35%

40%

0,5

0,55

0,6

0,65

0,7

0,75

0,8

0,85

0,9

0,95

0 5 10 15 20

Wat

er p

rod

uct

ion

co

st, €

/m3

Natural gas price, USD/MMBtu

Inci

den

ce o

n w

ater

p

rod

uct

ion

co

st, %

Fig. 4. Influence of natural gas price on water production cost.

127G. Iaquaniello et al. / Desalination 336 (2014) 121–128

and GT setting exponentm equal to 0.65. Table 9 reports results obtain-ed for economic evaluation on two plant capacities. Comparison hasbeen carried out between the base case developed around the sizedCSP system as for theMATS project with two desalination processes hy-bridized and the same system three times scaled. Results show that theincreased capacity allows for a cost reduction of about 25%. In bothcases, a water production cost lower than 1 €/m3 seems to be attractiveand very competitive with values reported in the literature related tosolar assisted desalination application.

Around the three times scaled case, a sensitivity analysis has beencarried out in order to investigate the effect of natural gas price onwater production cost. Reference values for different locations havebeen considered such as 3 USD/MMBtu, 12 USD/MMBtu and 16 USD/MMBtu relevant to US, Europe and Japan respectively [29]. Consideringa fuel gas price typical of the European area, the water production costincreases about 16% with respect to the base case (4.3 USD/MMBtu)typical of US and Middle East.

As shown in Fig. 4, even with a natural gas price as high as 16 USD/MMBtu,water production cost remains less than 1 €/m3. The latter is anattractive value considering that traditional RO plant with capacityaround 50,000 m3/day allows for a water production cost of 0.60–0.65 €/m3 [10].

Plant lifetime also plays an important role in the evaluation of finalwater production cost allowing for longer amortization period. Aroundthe base case (natural gas price of 4.3 USD/MMBtu), plant lifetime hasbeen increased considering that technical lifetime for the CSP plant isassumed in the literature up to 25–30 years [30,31] as well as for desa-lination plant due to last improvements in the adopted technologies[32]. By increasing plant lifetime up to 25 and 30 years, the water pro-duction cost decreases about 5.5% and 8% while its relative incidenceon production cost decreases from 64% to 60% (Fig. 5).

60%

61%

62%

63%

64%

65%

0,6

0,62

0,64

0,66

0,68

0,7

0,72

0,74

15 20 25 30 35

Wat

er p

rod

uct

ion

co

st, €

/m3

Plant lifetime, -

Inci

den

ce o

n w

ater

p

rod

uct

ion

co

st, %

Fig. 5. Influence of plant lifetime on water production cost.

With respect to a cost distribution of a traditional desalination plant,capital cost has a higher incidence due to contribution of solar sectionwhile energy cost associated to fuel gas consumption is lower thanksto the amount of energy derived from solar source.

4. Conclusions

Desalination powered by solar energy in particular by the concen-trating solar plant (CSP) equipped with thermal storage is becomingmore and more attractive especially for the Middle East and NorthAfrica region characterized by severe water shortage.

In the proposed architecture, a suitable thermal storage coupledwith a backup system based on a gas turbine, allows to efficientlysolve one of themain issues of solar system related to the intermittenceof the energy source. The developed configuration allows in fact toworkunder continuous operation mode increasing plant reliability and be-coming a valid scheme in reducing water production cost with respectto the traditional solar assisted schemes.

Combination of the MED and RO process in the same facility offersmany advantages. As well known the concept of hybridization allowsto reduce intake and outfall capacity aswell as to improve performance.A proper integration of two systems, allowed to increase RO feed tem-peraturewith a resulting increased permeate flow rate without any sig-nificant worsening in permeate quality. A further benefit derives fromflexibility of RO system which allows for a better matching of dailyand seasonal variation in power and water demand. Therefore this ar-chitecture will be of interest in remote areas along the sea side to pro-vide water and power in a very flexible way. In case of a high powerdemand for example, the base case would allow to produce about1000 m3/day from MED unit and up to 2.0 MWe without any waterproduction by RO.

The double hybridization proposed in this scheme based on a suit-able integration of solar energy and the chemical energy of a fuel, ledto a water production cost less than 1 €/m3. The latter is a very compet-itive value if compared to a traditional solar assisted scheme and it isalso quite close to water production derived from the conventional RObased scheme. This result is encouraging taking into account that asolar system benefits from a reduction of associated greenhouse gasemissions. Moreover the expected cost reduction of the future CSP sys-tem will allow to increase the thermal storage capacity with a conse-quent further reduction of water production cost.

Acknowledgments

The research leading to these results has received funding from theEuropean Union Seventh Framework Programme (FP7/2007-2013)under Grant Agreement n°268219.

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