Review of water-heating systems: General selection approach based on energy and environmental...

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Building and Environment 72 (2014) 259e286

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Building and Environment

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Review of water-heating systems: General selection approach basedon energy and environmental aspects

Oussama Ibrahim a,c, Farouk Fardoun a, Rafic Younes b,*, Hasna Louahlia-Gualous c

aUniversity Institute of Technology, Department GIM, Lebanese University, Saida, Lebanonb Faculty of Engineering, Lebanese University, Beirut, LebanoncUniversité de Caen Base Normandie, LUSAC, IUT de Cherbourg Manche, 50000 Saint Lô, France

a r t i c l e i n f o

Article history:Received 29 June 2013Received in revised form20 September 2013Accepted 21 September 2013

Keywords:Water-heating systemsEnergy performanceEnvironmentHybrid systemsSelection approach

* Corresponding author. Tel.: þ961 3316864.E-mail addresses: [email protected] (O.

(F. Fardoun), [email protected], [email protected] (H. Louahlia-Gualous).

0360-1323/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.buildenv.2013.09.006

a b s t r a c t

Water heating contributes an important proportion of residential energy consumption all around theworld. Different kinds of domestic hot-water production systems exist. The operational cost, environ-mental effect and performance of these systems differ according to various energy sources, climates,system types and system designs. Hence, the proper choice of a domestic hot-water system could saveenergy, protect nature and reduce operational costs, significantly. This paper illustrates, to the best of theauthor’s knowledge, the existing water-heating systems all along with the principle, advantages, dis-advantages and state-of-the-art for each. Six different categories were presented, namely wood, oil/gas,electric, heat pump, solar and instantaneous systems. The heat-pump systems were further classifiedinto several groups, namely air source, ground source, solar assisted, ground source-solar assisted,photovoltaicethermal and gas-engine driven systems. In addition, concerning solar water heating,different types of systems and collectors were presented and reviewed. Principal conclusions from thereview are outlined and a general approach to recommend the appropriate water-heating system isproposed.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Energy demand is continuously increasing due to global popu-lation growth and improved living standards. However, fossil fuels,the current primary energy source, are being consumed in arandom increasing manner, even though they are non-renewableand their global quantity is limited. Consequently, environmentalpollution and global warming are fearfully increasing. Based on thisfact, worldwide governments are working hard to raise the share ofrenewable energy sources, reduce energy consumption andaccordingly reduce environmental pollution. Different energy ap-proaches have been proposed for various cases such as energyconservation building codes, low energy buildings, ultra-low en-ergy buildings, zero energy buildings and energy-plus buildings.

Water heating is a major energy consumer all around the world.For instance, its share of the total residential energy consumption isabout 11% in USA [1], 14% in Europe [2], 22% in Canada [3], 25% inAustralia [4], 29% inMexico [5], 27% in China [6], 32% in South Africa[7], etc. Different kinds of domestic hot-water production systems

Ibrahim), [email protected](R. Younes), hasna.gualous@

All rights reserved.

exist. The operating cost, environmental effect and performance ofthese systems differ according to energy source, climate, systemtype, and system design. Hence, the proper choice of a domestichot-water system could significantly save energy, protect theenvironment and reduce operating costs. Thus, a comprehensivereview of various water-heating systems is a prerequisite torecommend the proper choice among the existing systems orsuggest new designs and approaches that would enhance re-ductions in energy consumption, environmental pollution andoperating cost.

Few studies have been carried out on the review of domestic-scale water-heating systems. Hepbasli and Kalinci [8] presented areview on heat-pump water-heating systems in terms of energeticand exergetic aspects. Jaisankar et al. [9] made a comprehensivereview on solar water heaters. This study reviewed various tech-niques able to enhance the thermal efficiency in a solar waterheater and conducted a detailed discussion on the limitations ofexisting research. Furthermore, it suggested possible modificationsthat would improve the overall efficiency of the system. Shuklaet al. [10] conducted a review on solar water heaters with phasechange materials used as thermal energy storage media. Theirstudy demonstrated that for better thermal performance of a solarwater heater, a phase change material with high latent heat and

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Nomenclature

A area [m2]Cb synthetical conductance [W/m K]COP coefficient of performance [e]COPp/t comprehensive coefficient of thermal and electrical

performance [e]ES engine speed (rpm)F heat-exchange area per unit of cooling capacity [m2/

kW]f solar fraction [%]F0 collector efficiency factor [e]FR collector heat removal factor [e]h heat transfer coefficient [W/m2 K]I intensity of solar radiation [W/m2 or kJ/m2]k thermal conductivity [W/m k]l length [m]_m mass flow rate [kg/s or L/min]n compressor speed [rpm]Nu Nusselt number [e]PBP payback period [years]Q capacity [W or kJ]T temperature [K or �C]UL collector overall heat loss coefficient [W/m2 �C]~V volume flow rate [m3/h]h efficiency [%]hf energy-saving efficiency [%]h*R characteristic heat removal efficiency [%] [132]h*s system characteristic efficiency [%] [132]h0 modified efficiency ð¼ h*R$h

*s Þ [%]

sa transmittanceeabsorptance product [e]F latitude of the studied location (�)b collector slope (�)z PV cell covering factor [e]s transmissivity [e]ε exergy efficiency [%]εc thermal storage efficiency [e]

AbbreviationsAMDAT Average minimum daily ambient air temperature in

the cold seasonANN artificial neural networksASHP air-source heat pumpASHPWHair-source heat-pump water heatera-Si amorphous siliconbp balance pointc-Si crystalline siliconDHW domestic hot waterDSF direct solar floorDX direct expansionE energy/electric back-upEEV electronic expansion valveEHP electric driven heat pumpEWH electric water heaterETC evacuated-tube collectorEVA ethylene vinyl acetateFPC flat-plate collectorG gas back-upGEHP gas-engine driven heat pumpGHC gas heat consumptionGHG greenhouse gasGIS geographic information systemGSHP ground source heat pumpGSHPWHground source heat-pump water heater

GWH gas water heaterHFO heavy fuel oilHGCHP hybrid ground-coupled heat-pump systemHP heat pumpHPWH heat-pump water heaterHPGHP hybrid-power gas-engine driven heat-pump systemHTF heat transfer fluidHVAC heating, ventilation and air conditioningHVACSIMþ heating, ventilation and air-conditioning simulation

plus other systemsHR heat recoveryICSSWH integrated collector storage solar water heaterISAHP integral-type solar-assisted heat-pump water heaterLPG liquefied petroleum gasMDATR mean daily ambient temperature rangeMFGSHP multi-function ground source heat pumpmin/maxminimum/maximumNiAL nickelealuminumPCM phase change materialPEC primary energy consumptionPER primary energy ratioPP paraffin and palmitic acidPS paraffin and stearic acidPTC parabolic-trough collectorPV photovoltaicPV/T photovoltaicethermalRUWCT reversibly used water cooling towerSA solar assistedS-AV surface area availabilitySAHP solar-assisted heat pumpSAHPWHsolar-assisted heat-pump water heaterSAS solar-air sourceSDHW solar domestic hot-water systemSE mean daily solar energySM stearic acid and myristic acidSMAV shape memory alloy valveSPF seasonal performance factorSWHS solar water-heating systemTMY typical meteorological yearTRNSYS transient system simulation programV volumeWH water heaterWLHPS water-loop heat-pump systemw.r.t. with respect toZNEH zero-net-energy homes

Subscripts Explanationa airavg averagec space coolingcw space cooling and water heatingcl collectordhn daily hot-water needdp(sp)-stSWH double (single) pass sheet-and tube-solar water

heater with external recycleE enginee evaporator, electrich heatingHP heat pumphp heat pipehs heat storeHTF heat transfer fluidi inletk condenser

O. Ibrahim et al. / Building and Environment 72 (2014) 259e286260

L loadm reflectoro outletPV photovoltaicR reservoirref refrigerantSP solar pump

sph single phasest storage tanksys systemt totalth thermalw waterwh water heating

O. Ibrahim et al. / Building and Environment 72 (2014) 259e286 261

large heat transfer surface area is required. Chow [11] presented areview on photovoltaic/thermal hybrid solar technology. In thisstudy, some sections were dedicated to the review of water-typePV/T systems, which are used for water heating and electricitygeneration. Other sections reviewed PV/T integrated heat pumps,where one of their applications is water heating.

The main objective of this study is to carry out a comprehensivereview of different domestic-scale water-heating systems, wheresix different categories are presented; namely wood, oil/gas, elec-tric, heat pump, solar and instantaneous systems. The principle,advantages and disadvantages of these systems are illustrated.Moreover, the state-of-the-art of the studied systems is presentedin detail, where categorization according to the type of study(electric water heaters), energy source (heat pumps) or system type(solar systems) is adopted when necessary. In addition, for somecases, summary tables of results for the state-of-the-art aredeveloped to reduce the content of the paper and illustrateimportant results of different studies in an explicit, abbreviatedform. Furthermore, the paper discusses a general methodology toselect the appropriate water-heating system according to differentparameters. Noting that the review sections present studies thatare mainly dedicated for domestic water heating and others thatillustrate multi-function systems where domestic water heating isone of their applications.

2. Review of water-heating systems

2.1. Wood water heater

2.1.1. PrincipleAwood-firedwater heater is a heating appliance that uses wood

as an energy source. Wood stoves are less used nowadays for waterheating. They are usually used for cooking and space heating.

2.1.2. TypesTwo basic types of wood-fired domestic water-heating systems

exist: direct and indirect. The direct type consists of a water tankthat is heated directly by the fire produced through burning wood.The indirect type uses heat exchangers to transfer heat from thewood stove to the water in a storage tank. Exchangers can bemounted inside the stove, in the stovepipe, or around the stove-pipe. Water circulates through the exchanger whenever a fire isburning, either naturally using the thermosyphon principle or bymeans of a small circulating pump [13].

2.1.3. Advantages

� The operating cost is much lower than that of conventional gas,oil or electric water heaters as wood is significantly lessexpensive than competing fossil fuels

� Wood is considered a renewable energy source because it can becontinually replenished, which leads to a sustainable anddependable supply [14]

� It is a clean energy source, since the net greenhouse gas emis-sions are almost null, as the CO2 generated during combustion of

wood nearly equals CO2 consumed during the lifecycle of thetree [14]

� Using wood as an energy source helps reduce the dependenceon non-renewable fossil fuel energy sources as well as increasesthe energy security of countries that depend on fuel imports tomeet their energy needs.

2.1.4. Disadvantages

� Locally, it produces dangerous fumes� It is difficult to control the water temperature� The energy content of wood is lower than that of traditionalfuels (19 MJ/kg) [16]

� The problem of back drafting exists in wood stoves that use anatural draft to get rid of the combustion gases [15]

� Thermosyphon, indirect wood stove water heating induces theproblem of air pockets in the piping which blocks the watercirculation and imposes the presence of a special design to getrid of the air particles [12].

Johansson et al. [16] compared the emissions related to resi-dential old-type wood boilers, modern-type wood boilers andpellet boilers. Their results revealed that old-type wood boilerscause substantially high emissions of particles and un-oxidizedgaseous compounds, as well as a high climate change effectcompared to the other types. Hence, the substitution of the old typewith a modern wood boiler attached to a storage tank or with apellet boiler would significantly reduce emissions as well as in-crease efficiency. It is worth noting that modern wood boilers areusually designed for down-draught combustion instead of up-draught combustion used in old types. Dias et al. [17] examinedthe performance of awood-pellet boiler with four different types ofpellets that mainly differ in the nitrogen-content composition. Allpellet types resulted in similar boiler thermal performance.Maximum CO emissions were observed during the boiler start-updue to maximum combustion intensity. Under steady-state condi-tions, the minimum CO emissions is achieved with about 13% ox-ygen concentration in the flue-gases and NOx emissions correlatedwell with both excess air and pellets nitrogen content. Persson et al.[18] developed a mathematical model for wood-pellet boilers andstoves implemented in the dynamic simulation program “TRNSYS”.However, in spite of good agreement with measured data, furthermodeling improvements were recommended. Galbally et al. [19]studied the effect of leakage from the heater and flue, related to awood water heater in Launceston, Australia, on the indoor airconcentrations of the BTEX pollutants: benzene, toluene, ethylbenzene and xylene. No direct influence on the concentrations ofBTEX within the house was detected. This was attributed to theleaky house envelope [19]; however, the indoor concentration ofBTEX in all houses in the area increased as a result of releasingemissions to the ambient atmosphere. Verma et al. [20] studiedexperimentally the emissions and combustion efficiencies of wood-pellet boilers equipped with bottom feed, top feed and horizontalfeed burners. Results revealed that bottom and horizontal feed


boilers are preferred against top feed boilers as they have loweremissions and higher efficiencies. Roy and Corscadden [21] inves-tigated the potential use of hay and grass briquettes in domesticwood stoves and concluded that these briquettes have similarperformance and emissions as other woody briquettes. Collazoet al. [22] presented and validated a numerical model describing adomestic pellet boiler. Analyzing the boiler, they showed that thehigh emissions resulting from the system operation are principallyaffected by the position of water tubes, the distribution of air inletsand air infiltrations. Kinsey et al. [23] conducted an experimentalstudy to investigate the performance and emissions of residentialwood-fired hydronic heaters, where four basic appliance designswere investigated, namely: up-draft combustion, down-draftcombustion, bottom fed pellet burner and combustion and gasifi-cation appliances. The first three were fired with split-log cord-wood while hardwood pellets fired the fourth unit, which resultedin the highest operating efficiency and lowest emissions. Rabaçalet al. [24] compared the use of industrial wood wastes and peachstones with the use of pellets of pine in pellet-fired boilers. Thestudy revealed that the boiler thermal efficiency is marginallyaffected while polluting emissions are significantly superior usingthe former type of pellets. However, these become attractive sus-tainable alternative fuels in domestic pellet-fired boilers throughthe optimization of the boiler operating conditions, particularly, theexcess air.

2.2. Storage-type gas/oil water heater

2.2.1. PrincipleIn storage-type gas/oil water heater, cold water is supplied to

the tank and injected at its bottom through a dip tube. The densercold water is heated by a gas/oil burner. As the water heats up, itnaturally rises and is drawn off by a hot-water discharge pipe.

2.2.2. Advantages

� Less contribution to environmental pollution compared withEWHs, due to direct conversion of thermal energy to heat

� Reliability [25]� Generally, operating costs of both oil and gas WHs are cheaperthan those of EWHs [3]

� Less primary energy consumption compared with EWHs.


� Gas/oil leaks from pipes� Produces local pollution� The burner area must be kept free of dust and dirt [25]� Installation cost of oil water heaters is greater than those ofEWHs [3]

� Oil water heaters have less life time (8 years) than EWHs (13years) [3].

Vogt et al. [26] conducted an experimental research on a pulsecombustion gas-fired water heater, where two different combus-tion chambers were examined, a single chamber and a splitchamber. Although the split chamber configuration generally yiel-ded higher thermal efficiencies, it emitted higher levels of noise.Moreover, results showed a substantial heat transfer enhancementof the studied system as compared with the laminar flow non-pulsating case, and a lesser enhancement as compared with theturbulent flow non-pulsating case. Offhaus and Goldschmidt [27]studied the effect of various flueway inserts on the pressure dropin the flueway and the corresponding total heat transfer to the

water side of a gas-fired water heater using a cold-flow set-up.Tests proved that the performance of the system improves as thepressure drop increased up to the onset of incomplete combustion.This result was further demonstrated using a submerged combus-tion chamber water heater. Hence, optimum flueway inserts of agas-fired water heater could be selected from cold-flow tests. Off-haus et al. [28] studied the effect of emissivity of a baffled flue onthe thermal efficiency of a residential gas-fired water heater. Re-sults revealed that the magnitudes of the radiative and convectiveheat transfer rates from the flueway gases to the tube wall changegreatly with the emissivity of the baffled flue, while the effect onthe total heat transfer rate to the flue wall is small. Clark and Gir[29] presented an analysis of low, medium and high efficiencymodels of electric, natural gas and heat-pump water heaters. Theirresults showed that gas water heaters are the most cost effectivefollowed by heat pumps. Moreover, within each class of waterheaters, applying a penalty on the extra pollution produced by theinefficient water heaters may encourage the use of high efficiencymodels. Johansson et al. [16] used oil boilers as basis for comparingthe performance and efficiencies of different types of biofuelboilers. Test results showed that the lowest emissions of particlesand un-oxidized compounds were measured from oil burnerswhich over performed wood boilers. Chang and Cheng [30] per-formed a numerical study to examine the CO-concentration insidea typical room of a residential building, with a natural gas waterheater installed in the adjacent balcony. It was found that theconcentration of CO in the room is significantly decreased when theair flow rate is above 0.0003m/s, due to the entrainment of fresh airinto the bedroom from the inside door.

McDonald [31] measured the emissions from gas, oil and wood-fired burning units and found that emissions from wood-pelletstoves are approximately15 times greater than those of oil-firedunits and approximately 1800 times greater than those of gas-fired units. Jeong et al. [32] developed and validated an analyticalmodel of a flue gas condensing heat exchanger. This model wasused to assess the condensation efficiency, defined as the percentweight ratio of the total condensation rate to the inlet water vaporflow rate. It was predicted that as this ratio varies from 0.5 to 1, thecondensation efficiency ranges from 10 to 30%. Tajwar et al. [33]investigated, experimentally, the introduction of various types ofbaffles in the flue pipe of a gas water heater (flat, conical, finned andbarbed razor wire). It was observed that these design improve-ments resulted in a significant performance enhancement. Thebarbed razor wire bafflewas proved to be the best by improving thecombustion and thermal efficiencies from 68 to 88% and 35 to67.4%, respectively. Li et al. [34] combined the fuzzy and PID con-trols in order to effectively describe the characteristics of gas waterheaters using mathematical models. It was proved that the newcontroller can effectively reduce the overshoot and steady-stateerror as well as it can shorten the adjusting time. Pinto and Viegas[35] experimentally showed that the combination of natural ormechanical ventilation in spaces with gas water heaters may leadto dangerous situations such as stopping the gas appliance safely orreversing the combustion products in the respective exhaust duct.As a result, they recommended either the implementation ofappropriately sized natural ventilation systemswith preventing theinstallation of mechanical ventilators or the installation of gaswater heaters in an exterior location to the considered space.However, if there are constraints in terms of physical space or in thecase of renovation, an exterior air inlet is recommended to ensureproper exhaust of combustion products. Sedeh and Khodadadi [36]designed a helical-shaped baffle to enhance the thermal efficiencyof natural gas water heaters. Numerical and experimental exami-nation of this design showed an enhancement of the water-heaterperformance, where the consumption of natural gas was decreased


by about 5% under steady-state operation. The authors believe thatthe amount of gas savings is less under real usage patterns.

2.3. Storage-type electric water heater

2.3.1. PrincipleA storage-type electric water heater is an electric appliance that

converts electrical energy into heat. The heating element of anelectric water heater is simply an electrical resistor that works onJoule’s law of heating.

2.3.2. Advantages

� An EWH is safer than fuel-fired water heater because it avoidsthe hazards and problems associated with using a combustionprocess

� Locally, it operates cleanly without any emissions� It has lower standby-losses than fuel-fired units because it ismuch better insulated

� In general, electric service availability is higher than any otherenergy source

� It can be located almost anywhere within a building, especiallyat or near the locations where hot water is required, whereasfuel-fired water heaters should be located where combustion airis available and flues can be installed

� It has a low maintenance cost� It usually has longer life time than fuel-fired units.


� In the case of thermal power plants, EWHs� Pollute the environment indirectly as fossil fuels are beingburned to produce electricity

� Participate in fossil fuels depletion� Convert high grade energy to low grade energy� The overall efficiency of converting fossil fuels to electricalenergy and then to thermal energy is quite low

� There can be a possibility of a short circuit with the cables andcords involved, which can be dangerous for the user

� EWHs consume excessive electrical energy to ensure adequatehot-water temperature at the fixtures. The reason is mainly dueto mixing between the incoming cold water and the storage hotwater, which results in a progressive decrease in the tempera-ture of the water out from the EWH [37].

Fig. 1. (a) Conventional EWH; (b) modified design EWH [43].

2.3.4. State-of-the-art2.3.4.1. Control and demand-side management studies. Dolan et al.[38] developed amodel for simulating aggregate residential electricwater heater loads using a rejection type Monte Carlo simulationtechnique. This model can be used in evaluating any water-heaterdemand-side management strategy without conducting a full-scalepilot program. It leads to considerable savings in cost and time forutilities considering implementing a water-heater load manage-ment program. Lacroix [39] examined the performance of threeEWH designs for electric load management and control of bacterialcontamination. The first design was a standard EWH with minormodifications which proved to be beneficial for large capacitywater heaters. The second design was a high temperature waterheater equipped with a heat exchanger or a mixing valve. Thisdesignwas ineffective as it consumes high electrical energy and haslimited life time. The third design was a dual 175 L or 270 L waterheaters connected in series. This design proved to be energeticallyvery effective although it is less aesthetically attractive and less

compact than a single-tank heater. LaMeres et al. [40] studied afuzzy logic-based control strategy for shifting the average powerdemand of residential electric water heaters. Simulation resultsshowed that the proposed strategy can shift the stated power de-mand from periods of high demand for electricity to off-peak pe-riods and thus improve the load factor (average demand/peakdemand) of residential load profile. Nehrir and LaMeres [41] stud-ied a multiple-block fuzzy logic-based demand-side managementstrategy. This aims to shift the peaks of residential electric water-heater power demand profile from periods of high demand to off-peak hours for a certain distribution area. The distinction of thisstudy is represented by dividing the electric water heaters fed by adistribution feeder into several blocks. The peak power demand ofeach block is shifted to a different time period throughout the day,where the electricity demand remains low. This strategy will avoidthe possibility of shifting the overall water-heater peak demand, ofa distribution area, to one point in time which will result in a morelevel utility demand profile and therefore higher load factor.

2.3.4.2. Design and performance studies. Kar and A1-Dossary [42]studied, deeply, the idea of series dual-tank EWH where the po-wer rating and volume capacity of each tank were varied to find thebest combination in terms of energy performance and hot-wateroutput. The simulation results revealed that a dual-tank waterheater, where the second tank has 70e80% of the total power ratingand 10e30% of the total volume, provides about 10% more hotwater while requiring about 4.5% less energy, compared to a single-tank EWH of the same total volume and power rating. Moreover,they examined single-tank EWHs of various tank sizes and powerratings. They found that the amount of hot water provided bysingle-tank water heaters does not vary with tank size but doesvary with power rating. Hegazy and Diab [43] suggested a newstorage-type EWH design (Fig. 1(b)) that has a better performancecompared to a conventional one (Fig. 1(a)). The proposed designdiffered by two aspects. First, the vertical inlet positioned from thetank bottom was replaced by a side-bottom positioned, wedged-pipe inlet that could achieve a good natural stratification insidethe storage tank and overcome the problem of mixing between theincoming cold water and the storage hot water. Second, the longpipe vertical output positioned from the tank bottomwas replacedby an upper-side positioned short tube that could overcome theproblem of heat exchange between the hot outflow and the coldwater along the long pipe outlet. It was found that the suggesteddesign has higher discharge efficiency and thus provides more hotwater at almost constant temperature. Moreover, the thermalperformance was enhanced with increasing tank aspect ratio anddecreasing draw-rate.

Fig. 2. Tested tank with three inlets (i1ei3) and two outlets (o1 and o2) [46].


Sezai et al. [44] investigated, experimentally, the performance ofemploying a secondary heating element near the top part of a 120 Lstorage tank. The vertical bottom and lateral horizontal locations ofthe heating element were examined. It was found that, with theheater located on the lateral surface of the tank, only the waterabove the heater can be heated while the water below the heaterremains almost unaffected. As a result, they suggested a new designof large capacity EWH composed of dual heater elements, with onepositioned vertically at the bottom and the other positioned hori-zontally below the uppermost 50 L volume. The recommendeddesign would give users the chance to switch between the ele-ments depending on the amount of hot water required andconsequently save energy. Fernandez-Seara et al. [45] studied,experimentally, the performance of a 150 L domestic EWH storagetank for the static heating and cooling periods. Their resultsrevealed that the water temperature profiles are affected by theheating power and the pressure in the tank during the heatingprocess and the heating efficiency was over 85%. Moreover, thecooling process was affected by the ambient temperature as well asthe pressure in the tank. In addition, an unexpected phenomenonof water heating at the bottom of the tank at the beginning of thecooling process was observed. Fernandez-Seara et al. [46] per-formed an experimental analysis of the dynamic mode of operationof a 150 L domestic electric hot-water storage tank equipped withthree different inlets (i1ei3) and two different outlets (o1 and o2)whose designs are shown in Fig. 2. It was found that the i2eo1inleteoutlet port configuration provides the best performance.

Fig. 3. (a) Schematic of the studied domestic electrical wa

Hegazy [37] experimentally investigated the effect of inletdesign on the performance of a storage-type domestic electricwater heater (Fig. 3(a)). Three different side-inlet geometries,namely wedged, perforated, and slotted-pipe inlets (Fig. 3(b)),were tested using two 50 L capacity tanks of aspect ratios 1 and 2and two discharge rates 5 and 10 L/min. The study proved that thethree tested inlet designs promote good thermal stratificationinside the storage tank and that the slotted inlet provides the bestperformance. Moreover, small size EWHs perform more effectivelyusing tanks of higher aspect ratios and low draw-rates. De Graciaet al. [47] analyzed the improvement in thermal performance dueto the inclusion of encapsulated phase change materials inside anelectrical domestic hot-water cylinder. The use of PCM increasesthe thermal energy storage capacity of the cylinder and allows theuse of low-cost electricity during low peak periods. It wasconcluded that PCM increases the demand coverage from 40 to55%. In addition, the PCM distribution inside the tank must bedefined depending on the timing and quantity of hot-waterdemand.

2.4. Heat-pump water heater

2.4.1. PrincipleThe heat-pump water heater, based on the principle of Carnot

cycle, absorbs heat from a renewable energy source, such asambient air (ASHPWH), geothermal energy (GSHPWH), solar en-ergy (SAHPWH), or waste heat, at lower temperature and transfersit into a water tank e the higher temperature heat sink. The me-chanical energy consumed to drive the system is a small proportion(w30%) of the transferred energy.

2.4.2. Advantages

� HPWH produces low-pollutant heating energy using renewableenergy (solar energy, ambient air, geothermal energy or wasteheat)

� Supplies much more heat just with the same amount of elec-tricity used for electric water heaters

� Has low operating costs since, depending on its efficiency, up tothree-quarters of the required heating energy is drawn from theenvironment (without cost) [48]

� Heat pumps represent a kind of security for the future usingrenewable energy sources and being able to replace wood, coal,oil, and even natural gas which are non-renewable sources ofenergy and are in a continuously decreasing quantities [48]

� Depending on latitude, ground temperatures range from 7 to21 �C at a few feet depth below the earth’s surface [49], and thus,GSHPWHs are always energy efficient and applicable irre-spective of climate.

ter heater; (b) Schematics of inlet designs tested [37].

Fig. 4. Fast response heat-pump water heater [58].



� HPWHs typically have higher initial costs than conventionalwater heaters [50]

� Fans and compressors used by the HPWH make noise� ASHPWH should be preferably installed in locations withambient temperature above 4.4 �C year round [51].

Fig. 5. Cascade air-source heat-pump water h

2.4.4. State-of-the-art2.4.4.1. ASHPWH. Neksa et al. [52] investigated the CO2eHPWH. Itwas proved that using CO2 as a refrigerant may produce hot waterwith temperatures up to 90 �C without operational problems andthus the area of application is much larger than for the traditionalheat-pump systems, often restricted to hot-water temperatureslower than 55 �C. Modified air-conditioning systems that can ach-ieve multi-functions, such as space heating, space cooling andwater heating, with improved energy performance were intro-duced and studied [50,53,55]. Results showed that this systemprovides amuch better energy performancewhen comparedwith aconventional HPWH or domestic air conditioner. Furthermore,different condenser types; natural convection immersed condenser(bayonet style and U-tube style), wrap-around condenser coil onthe tank and separate condenser, were studied [54,56]. Zhang et al.[57] investigated the system optimization of ASHPWH which con-sisted of a heat pump and a water tank with natural convectionimmersed condenser coil. From the testing results, it was shownthat the system COP could be improved obviously by matching theoptimal tank volume with the compressor electrical power rate.Huang et al. [58] illustrated a design and an experimental testing ofa fast response heat-pumpwater heater with dual-tank (supply andholding tank) inter-connected by a shape memory alloy valve(Fig. 4). The SMAV is a mechanical heat-sensitive device made fromshape memory alloy which keeps the valve closed when the watertemperature is not high enough and actuates the valve to openwhen it is heated up to a certain level. This will isolate the tanks andlet the vapor compression cycle heat up the supply tank only andincrease the temperature response speed. Guo et al. [59] conductedan experimental and simulation research and operation optimiza-tion of ASHPWH. Gang et al. [60] showed that the instantaneousheating mode has better performance, superior physical parame-ters and higher exergy efficiency than cyclic heating modes in anASHPWH. Jiang et al. [61] proved that both stability and efficiencyof the ASHPWH can be improved significantly by using the EEV.

Huchtemann and Müller [62] evaluated the efficiency of elec-trically driven heat pumps used for space heating and water

eater with thermal storage system [63].


heating in existing dwellings using data from a field-test conductedthrough the Fraunhofer Institute for Solar Energy Systems in Frei-burg, Germany. Analyzed data from 21 air-to-water heat pumpsshowed that the majority of investigated systems used more pri-mary energy and produced higher CO2 emissions, when comparedwith a gas-condenser boiler with a 0.96-annual fuel utilization ef-ficiency. Wu et al. [63] constructed and tested a cascade air-sourceheat-pump water heater with thermal storage system (Fig. 5). Dy-namic performance of the heat-pump water heater in single stagemode and cascade mode was compared and discussed. Based onthe transient heating COP values, critical switching curve fromsingle stage mode to cascade mode was founded for the code of thesystem controller. Furthermore, energy performance between wa-ter tanks with and without PCM was compared where the resultsrevealed that a water tank with PCM has a lower energetic effi-ciency mainly due to additional heat transfer process. Moreover,the water tank with PCM supplied 2.3% more energy than thatwithout PCM during the thermal release process, which is consid-ered a very small percentage. This is because PCM is not vastlysuperior to water in the energy storage capacity and has low heat

Table 1Summary of studies conducted on ASHPWH.

Classification Type of study Place/climatic data Application othe

Modeling/simulation Experimental Space heating S

ASHPWH @ Oslo, Norway

@ Hon Kong @ @

@ @ China @ @

@

@ @ Sydney, Australia

@

@ China @ @

@ @ Hefei, China @ @

@ China

@ @ Thailand @ @

@ Taipei, Taiwan

@ @

@ @ Shanghai, China

@ Hefei, China@ China@ Freiburg, Germany @

@ China

transfer rate. Thus, PCMs with larger energy storage capacity andhigher heat transfer rate are desirable.

Concerning modeling, a dynamic model for an air-to-waterdual-mode HP with screw compressor having four-step capac-ities, a water-heating system driven by a HP, an air-source heat-pump system using an immersed water condenser were presentedby Fu et al. [64], Kim et al. [65] and Ji et al. [66], respectively. Themodels were found to be accurate in predicting important systemparameters. Moreover, Techarungpaisan et al. [67] presented asteady-state simulation model to predict the performance of asmall split type air conditioner with integrated water heater andvalidated it. Table 1 illustrates the most important results of thereviewed studies about ASHPWH.

2.4.4.2. GSHPWH. Yang et al. [68] reported that Fanney andDougherty [69] first discussed the performance of a residentialearth-coupled heat pump with an integral desuperheater water-heating circuit. The recorded data showed that the desuperheatercontributed to an average of 27% of the total energy supplied forheating water through the 24-month monitoring period. In

r than WH Results

pace cooling

COP ¼ 4.3 (for heating water from 9 to 60 �C at Te ¼ 0 �C);SPF ¼ 4;PEC reduction compared to EWH or GWH: 75%; Twup to 90�C can be produced [52]For three function model:Mode 1: space-cooling and water heating: COPcw-avg ¼ 4.02,COPc-avg ¼ 2.91 (Ta ¼ 35 �C)Mode 2: water heating only: COPwh,avg ¼ 3.42, 3.25, 2.52, 2.00(for Ta ¼ 31, 25, 15, 4.5 �C).Mode 3: space heating only: COPh,avg ¼ 2.72 (Ta ¼ 7 �C) [53]Heating mode: Tw ¼ 45 �C; Cooling mode: Tw ¼ 7 �C; Simulationmodel is validated [64]Qk & COP are greater for U-tube style than bayonet condensersQk & COP increased as a function of the number of circuits in theU-tube style condensers (COP7-circuit ¼ 2.7 & Qk,7-circuit ¼ 3.662 Wat Ta ¼ 27 �C & Tw-avg ¼ 54 �C) [54]For 40 MJ/day peak winter load: COPintegral-k ¼ 2.3 and annualenergy saving ¼ 56%; COPexternal-k ¼ 1.8 and annual energysaving ¼ 44% [56]As VR [, heat loss Y; as VR [, performance Y [65]In summer, COPcw-avg ¼ 2.77 (Ta ¼ 35�C);In winter, COPwh-avg ¼ 2.5 (Ta ¼ 7�C) [55]Mode 1: water heating only, COPwh ¼ 2e2.5 (Ta ¼ 2.8 �C);Mode 2: space-cooling & water-heating; COPcw ¼ 2.8e5(Ta ¼ 35�C)[50]750 W-HP with 150 L and 1125 W-HP with 200 L are moresuitable for residential uses; For system (150 L, 1125 W):COPwinter ¼ 2.61 (Ta ¼ 0 �C); COPsummer ¼ 5.66 (Ta ¼ 35 �C);COPspring/autumn ¼ 4.817 (Ta ¼ 25 �C) [57]The steady state simulation model is quite accurate in predictingimportant system parameters [67]Fast response heat pump water heater does not need auxiliaryelectric heater;For 6 < Ta < 38 �C: Temperature response speed of the supply tank,before SMAV opened, changes from 0.714 to 1.234 �C/min and thatof the holding tank after SMAV opened, changes from 0.483 to1.1 �C/minEnergy consumption lies in the range 0.008e0.016 kWh/l [58]COP ¼ 2.32e4.41 [66]COPsummer-avg ¼ 5.51 (Ta ¼ 35 �C); COPwinter-avg ¼ 2.82(Ta ¼ 5 �C) [59]COPinstantaneous > by 24% than COPcyclic [60]Stability and efficiency of ASHPWH are enhanced by using EEV [61]Mean SPFASHPs ¼ 2.3; max SPFASHPs ¼ 3 [62]Single stage: COP ¼ 1.5e3.05Cascade mode: COP ¼ 1.74e2.55 [63]

Table 2Summary of studies conducted on GSHPWH.

Classification Type of study Place/climatic data Application other than WH Reference

Modeling/simulation

Experimental Space heating Space cooling

GSHPWH @ Hong Kong @ @ Energy savings w.r.t. EWH: 70%; HGCHP can offer 95% of DHW [71]@ Montreal & Los Angeles @ @ Montreal: compared to EWH, GSHP with desuperheater &

HPHW-GSHP save 36% & 53% of electricity consumption,respectively [72]Los Angeles: compared to EWH, GSHP with desuperheater &HPHW-GSHP save 29% & 53% of electricity consumption,respectively [72]

@ Nanjing, China @ @ COP of MFGSHP > COP of GSHP [73]@ Freiburg, Germany @ Mean SPFGSHPs ¼ 2.9; max SPFGSHPs ¼ 4 [62]


addition, Kavanaugh [70] conducted a similar project andconcluded that the cost savings were considerably based on theutility bill. Cui et al. [71] developed a simulation model of a HGCHPwith domestic hot-water supply system for space residentialcooling/heating and DHW supply in hot-climate areas. A desu-perheater was integrated between the compressor discharge andthe condenser inlet using the HVACSIMþ environment which is asoftware developed by the National Institute of Standards andTechnology (NIST). Results showed that the studied system hassignificantly better performance relative to the conventional sys-tems. Biaoua and Bernier [72] performed a TRNSYS simulationstudy of four alternative means for DHW production in zero-net-energy homes for two climates (Montreal and Los Angeles). Thestudied alternative systems were: (i) a regular electric hot-watertank; (ii) the desuperheater of a GSHP with electric back-up; (iii)thermal solar collectors with electric back-up; and (iv) a HPWHindirectly coupled to a space conditioning GSHP. Results revealedthat the third alternative provided the best performance and thatthe fourth alternative is slightly better than the second from anenergy point of view; however, one must not forget that the fourthalternative uses two HPs which may be considered costly non-effective due to the high initial cost. Li et al. [73] modeled andsimulated the long-term performance of a new multi-functionground source heat-pump system that operates in three modes:air conditioning (space heating/cooling), water heating only andsimultaneous water heating and space cooling in a climate with acold winter and a warm summer. Results indicated that MFGSHPcould alleviate the imbalance of the emitted/extracted heat causedby conventional GSHPs and therefore improve the efficiency of thesystem. Table 2 presents a summary of the reviewed studies con-cerning GSHPWH. In addition to their investigations on ASHPs,Huchtemann and Müller [62] evaluated the efficiency of

Fig. 6. SA-ASHP

geothermal heat pumps used for space heating and water heatingin existing dwellings. Analyzed data from 22 GSHPs showed thatthe investigated systems achieved an average primary energysaving of 18.8% and that half of the systems could achieve savings inCO2 emissions, when compared with a gas-condenser boiler having0.96-annual fuel utilization efficiency.

2.4.4.3. SAHPWH. Kuang and Wang [74] conducted an experi-mental study about a multi-functional direct expansion solar-assisted heat-pump systemwhich can offer space heating inwinter,space cooling in summer and hot-water year round. The studiedsystem could guarantee a long-term operation under very differentweather conditions and relatively low running cost during thewhole year. Hawlader et al. [75] designed and built a solar-assistedheat-pump dryer and water heater. The DX-SAHP system with animmersed type water condenser was studied and proved to have ahigher performance than that of the conventional heat-pump sys-tem. Both COP and collector efficiency (hcl) increase as the ambienttemperature increases; while the increase of solar radiation in-tensity improves COP and decreases collector efficiency. Differentstudies have suggested utilizing the electronic expansion valve andvariable frequency compressor for the DX-SAHPWH [76e78].Furthermore, it was proved that the collector efficiency of DX-SAHPWH system can be more than 1 when the evaporating tem-perature is lower than the ambient temperature [76]. A researchgroup from Taiwan presented and studied an integral-typeSAHPWH [79e81]. In this design, the storage tank and theRankine cycle units were integrated together to make a morecompact size and a thermosyphon loop was used to transfer heatfrom the condenser to the water storage tank. Furthermore, thecondenser was of tube-in-sheet, unglazed type and the solar col-lector was itself the evaporator. The ISAHP absorbs energy from

system [85].

Fig. 7. Heat-pipe enhanced solar-assisted heat-pump water-heater system [86].


solar radiation and ambient air simultaneously. It was found thatthe thermal performance of an ISAHP varies with weather condi-tions. It is worth to mention that Chyng et al. [80] suggested thatthe expansion device does not need to be controlled online,because the rate of valve opening regulation is small and approxi-mately constant, contrary to the recommendation of Huang andChyng [79].

Guoying et al. [82] performed a simulation study of solar-air-source heat-pump water heater (SAS-HPWH) with a speciallydesigned flat-plate heat collector/evaporator with spiral-finnedtubes. It was proved that this system could produce 55 �C-hotwater all around the year, overcoming the problem of DX-SAHP thatfails in rainy days. Anderson and Morrison [83] studied a solar-boosted HPWH with flat unglazed aluminum solar evaporatorpanels to absorb solar and ambient energy. A wrap-aroundcondenser coil on the outside of the water tank was adopted.

Table 3Summary of studies conducted on SAHPWH.

Classification Type of study Place/climatic data

Modeling/simulation Experimental

SAHPWH @ @ Taipei, Taiwan@ @ Taipei, Taiwan@ @ Singapore

@ Taipei, Taiwan@ Shanghai, China

@ Nanjing, China@ Sydney, Australia

@ @ Taipei, Taiwan

@ Shanghai, China@ @ Thailand

@ Hong Kong@ Hong Kong.

@ @ Shanghai, China

They proved that the system performance could be improved byconcentrating the condenser coils in the lower portion of the tank.Nuntaphan et al. [84] studied the performance of an indirectSAHPWH using the refrigerant mixture R22/R124/R152a. It wasnoticed that the hot-water temperature is greater by 40% than thatobtained from conventional solar water-heating systems. Li andYang [85] developed a simplified mathematical model of SA-ASHPsystemwhich mainly consists of two loops: the solar collector loopand the airewater heat-pump unit (Fig. 6). The effect of variousparameters, including circulation flow rate, solar collector area,solar collector tilt angle and initial water temperature in the pre-heating solar tank was investigated.

The results show that the system performance is governedstrongly by the change of circulation flow rate, solar collector areaand initial water temperature in the pre-heating solar tank. Huanget al. [86] designed, built and tested a heat-pipe enhanced solar-

Results

COP ¼ 2.5e3.7 (at 61 < Tw < 25 �C) [79]COPdaily-total ¼ 1.7e2.5 (year around) [80]Experiment: max COP ¼ 7; max f ¼ 61%;Simulation: max COP ¼ 5; max f ¼ 65%;39% < he,cl < 80% [75]Max COPHP-mode ¼ 2.58; max COPhybrid-mode ¼ 3.32 [86]Space heating only mode (winter), 2.6 < COPHP-avg < 3.3, 2.1 < COPsys-avg < 2.7;water heating only mode (spring): 2.1 < COPsys < 3.5 [74]Tw ¼ 55 �C (year round); monthly COPavg ¼ 3.98e4.32 [82]Clear daytime: COP ¼ 5e7; Nighttime: COP ¼ 3e5 [83]Same performance correlation holds for ISAHP operatingwith single or dual energy source [81]Spring: COPseasonal-avg ¼ 5.25; hcl-avg ¼ 108%; εsys ¼ 21% [76]COPsys ¼ 2.5e5; PBP ¼ 2.3 years; Tw obtained by system > by 40% ofthat obtained by solar system;suitable mass of hot water in the storage tank is 400 kg [84]COPyearly-avg ¼ 6.46 [77]15th July: highest COPsys ¼ 3.86 at Ṽw,k ¼ 10.8;15th November: highest COPsys ¼ 3.5 at Ṽw,k ¼ 11.5 [85]Experiment: COPsys ¼ 5.21e6.61; hcl ¼ 88e105%;Simulation: COPsys ¼ 5.23e6.12; hcl ¼ 90.1e107.6% [78]

Table 4Summary of studies conducted on SA-GSHPWH.

Classification Type of study Place/climatic data Application other than WH Results

Modeling/simulation Experimental Space heating Space cooling

SA-GSHPWH @ France @ @ Heating mode: COPHP-avg ¼ 3.75 [87]@ Stockholm, Sweden @ Studied designs (depth ¼ 160 m): COPHP,design 1 ¼ 4;

COPHP,design 2 ¼ 4.15; COPHP,design 3 ¼ 3.95; COPHP,design 4 ¼ 4.05[89]

@ Erzurum, Turkey @ COPHP ¼ 3e3.4; COPsys ¼ 2.7e3; 43 < Tw,k,o < 73 �C [90]


assisted heat-pump water heater. This system is a heat pump withdual heat sources that combines the performance of conventionalSAHP and solar heat-pipe collector (Fig. 7). The designed systemcan operate in two modes: the heat-pump mode when solar radi-ation is low and the heat-pipe mode, without electricity con-sumption, when solar radiation is high and can thus achieve highenergy efficiency. Table 3 presents a summary of the reviewedstudies concerning SAHPWH.

2.4.4.4. SA-GSHPWH. The energetic and exergetic studies of aground-coupled heat pump combined with thermal solar collectorswhich can meet domestic hot water and heatingecooling buildingenergy needs were presented by Trillat-Berdal et al. [87] andHepbasli [88], respectively. Solar heat is used as a priority for do-mestic hot-water heating and when the preset water temperatureis reached, excess solar energy is injected into the ground viaboreholes. Kjellsson et al. [89] studied the combination of solarcollectors with GSHP systems for heating and domestic hot water,using TRNSYS. The study focused on the comparison among fourdifferent system designs: 1) a base system with no solar heating(conventional GSHP); 2) a systemwhere all solar heat recharges theborehole; 3) all solar heat is used for domestic hot water; and 4) allsolar heat recharges the borehole in NovembereFebruary, and isused for domestic hot water during the rest of the year. It wasrecommended that the optimal design is the fourth one. Bakirciet al. [90] constructed an experimental set-up of a SA-GSHP system.From the presented results, it is obvious that the system could beused for instantaneous or storage domestic hot-water production.Table 4 presents a summary of the reviewed studies concerning SA-GSHPWH.

2.4.4.5. PV/T-SAHPWH. A PV/T-SAHP system combines the two-phase solar collector with the PV module to form a hybridcomponent known as the PV/T evaporator. The evaporatingrefrigerant of SAHP is used as the cooling medium of the PV cells;hence, lower operating temperature of PV cell and higher

Fig. 8. (a) Schematic diagram of the experiment rig [9

photovoltaic efficiency will be easily achieved due to the relativelylow operating steady temperature of the evaporating refrigerant. Inaddition, this system has a superior coefficient of performance thanthe conventional heat-pump system [91].

A group of Chinese researchers [91,92] reported different studiesof a photovoltaic solar-assisted heat-pump system (Fig. 8). Thesystem consisted of two evaporators (PV and air evaporators)connected in parallel and two condensers (air and water con-densers) also connected in parallel. It is able to provide multi-functional services such as space cooling, space heating, elec-tricity production and domestic water-heating. These studies dealtonly with the PV evaporator and water cooled condenser. Ji et al.[93] developed and studied a PV-SAHP system which is similar tothat illustrated before but without both air evaporator and aircondenser. The condenser supply water temperature was kept at30 �C. Xu et al. [94] studied a PV/T-heat-pump system having amodified collector/evaporator which consisted of multi-port flatextruded aluminum tubes instead of round copper tubes used inconventional designs. Results revealed that using this collector/evaporator, a better operating performance could be achieved.

Chow et al. [95] presented a photovoltaic-integrated solar heatpump where an immersed condenser was used. Fang et al. [96]conducted an experimental study on the operation performanceof photovoltaicethermal solar heat-pump air-conditioning systemin Nanjing, China. The system can perform five operation modes,namely: refrigeration circulation, PV/T circulation, refrigerationcirculation and PV/T circulation working together, heat-pump cir-culation, and heating water circulation modes. In this study, onlythe PV/T circulation mode was experimentally studied and dis-cussed. Mastrullo and Renno [97] presented energy, exergy andeconomic studies of a PV-SAHP system similar to that presented inRef. [91] but without air condenser; however, this study also dealtonly with the PV evaporator and water cooled condenser. Xu et al.[98] developed a novel low-concentrating solar photovoltaic/ther-mal integrated heat-pump system which can produce electricityand heat at the same time. The solar collector has fixed truncated

1]; (b) Cross sectional view of PV evaporator [92].

Table 5Summary of studies conducted on PV/T-SAHPWH.


Modeling/simulation Experimental Space heating Space cooling Electricity

PV/T-SAHPWH @ Hefei, China @ @ @ Max COPHP ¼ 10.4; max COPp/t ¼ 16.1;COPHP,avg ¼ 5.4; COPp/t,avg ¼ 8.3;hPV ¼ 13.4% [91]

@ @ Hefei, China @ @ Max COPp/t ¼ 8.4; COPp/t,avg ¼ 6.5;hPV ¼ 13.4% [93]

@ Tibet @ Typical sunny winter day: COPp/t ¼ 6.01;hPV,avg ¼ 13.5%; hth,avg ¼ 47.9%;hsys,avg ¼ 62.5% [92]

@ Nanjing & Hong Kong, China @ @ Compared to PV/T-HP system withconventional collector/evaporator,COPP/t [, hth [ & hPV [ by 7, 6 & 2%,respectively using the modifiedcollector/evaporator [94]

@ Hong Kong @ Yearly: COPavg ¼ 5.93 & hPV,avg ¼ 12% [95]@ Nanjing, China @ @ @ PV/T mode: hPV,avg ¼ 10.4%; COPsys,avg ¼ 2.88;

Tw ¼ 42 �C [96]@ Southern Italy @ @ hPV ¼ 13.7e14.2%; hth ¼ 52e84.3% [97]

@ Nanjing, China @ Sunny summer day: COPsys,avg ¼ 4.8 (30< Tw < 70 �C); he ¼ 17.5% [98]

@ @ For 200 < I < 800 W/m2, COPsys ¼ 2.9e4.6(constant ṁwk & Twki);For 25 < Twki < 45 �C, COPsys ¼ 5.2e3.2(constant I & ṁwk);For 1 < ṁwk < 5 L/min, COPsys ¼ 6.7e2.8(constant Twki & I) [99]


parabolic concentrators reflecting the incident sunlight onto thesurface of PV cells. Chen et al. [99] presented an experimental studyon the energy performance of a small hybrid glass vacuum tubetype PV/heat-pump system with a water cooled condenser and

Table 6Summary of studies conducted on GEHP-WH.



Gas engine-driven heat pump @

@

@ Xian, China

@ @

@ @

@

@

@ Vicenza, Italy

small capacity compressor. The condenser water supply tempera-ture and water-flow rate had little effect on the electrical perfor-mance. Table 5 presents a summary of the reviewed studiesconcerning PV/T-SAHPWH.

Application other than WH Results

Space heating Space cooling

@ @ COPoverall,c ¼ 0.8; COPoverall,h ¼ 1.2;COPoverall,wh ¼ 1.4 [104]If Ft ¼ 0.9, PER ¼ 2.08; If Ft ¼ 1.5,PER ¼ 2.43 [105]

@ @ PBP of GEHP compared to [106]:GEHP-WLHPS: 2 yearsEHP-WLHPS: 2.6 years

@ @ PER ¼ 1.67 (Ta ¼ 7�C, ES ¼ 1000)PER ¼ 1.13 (Ta ¼ 7�C, ES ¼ 3500)hE ¼ 83%, hHP ¼ 5 (Ta ¼ 9 �C);hE ¼ 29.7% (Ta ¼ 5�C, at max. power) [107]

@ @ Conventional GEHP: max hth ¼ 33%; minhth ¼ 22%;HPGHP: max hth ¼ 37%; minhth ¼ 27% [108]

@ @ 6.2 �C < Tw,chilled,avg < 15.9 �C46.4 �C < Tw,o < 60.8 �CAs Tewi changes from 12.2 to 23 �C, Qk, HR &PER [ by 18%, 31% and 22%, respectivelyAs Ṽe changes from 1.99 to 3.6 m3/h, Qk

& PER [ by 3.9 & 3%, respectivelyPER [ by 12% as Ta varies from 24.1 to 34.8 �C.Qk, HR & GHC [ by 35, 28 & 44%, respectively,while PER Y by 15% as ES changes from 1200 to1750 rpm [101]

@ @ 35 �C < Two < 70 �CQh & HR Y by 9.3% & 27.7% respectively &GHC [ by 17.5% as Twki changes from 33 to 49 �CQh & GHC [ by 17.3% & 39.4%, respectively & PER Y

by 15.3% as ES changes from 1300 to 1750rpm [102]

@ @ After half life cycle, same plant equipped with anew gas engine driven system is expected to bereliable [103]


2.4.4.6. Gas-engine driven HPWH. A GEHP usually consists of areversible vapor compression heat pump with an open compressordriven by a gas-fueled internal combustion engine. Although theefficiency of a gas engine is not very high (about 30e45%), thewaste heat of fuel combustion can be recovered by approximately80% and this is the main advantage of GEHPs. The heat recovery isdone by utilizing the energy of exhaust gas and by utilizing thewaste heat released by the engine cylinder jacket [100].

Previously, Hepbasli [100] conducted a review on GEHP, themost important results of investigations related to our study areshown in Table 6. Recently, more studies have been carried out;Elgendy et al. [101,102] studied the performance of a GEHP for air-conditioning and hot-water supply over a wide range of enginespeeds, ambient air temperatures, evaporator water-flow rates,evaporator water inlet temperatures and condenser water inlettemperatures. Busato et al. [103] reported the energetic, economicand maintenance evaluation of a 10-year study of ‘‘S. Nicola’’ HVACplant built in Vicenza, Italy. The core of the HVAC system is a gas-engine driven heat pump, integrated with two condensing boilers.The study reported that once the initial problems with the controland management strategies were solved, this system led to sig-nificant primary energy savings. Comparing the studied systemwith that of district heating network, it was proved that it is betterfrom both energetic and economic terms. Table 6 presents a sum-mary of studies conducted on GEHP-WH.

2.5. Solar water-heating systems

2.5.1. PrincipleThemain component of a solar water-heating system is the solar

collector that absorbs solar radiation and transfers it into a heattransfer fluid which in turn transfers the heat gained into water in astorage tank. Note that the water, itself, may be the HTF.

2.5.2. TypesSWHSs are divided into two categories: active and passive. Active

systems use a mechanical system (circulating pump) to circulate theHTFwhile passive systems use density gradients (gravitational forces)to circulate the HTF. There are two types of passive systems: i) Inte-grated collector storage or batch systemswhich use a tank that acts asboth the hot-water storage and the solar collector and ii) thermosy-phon systems in which the storage tank and collector are physicallyseparated and transfer between the two is driven by natural convec-tion. Active and thermosyphon SWHSs are further classified into twotypes: i) direct or open loop systems inwhich the water in the tank isitself the HTF and circulates through the collectors, this type is notappropriate in climates where freezing temperatures occur, and ii)indirect or closed loop systems in which a pump circulates the HTFthrough the collectors and a heat exchanger which transfers heat to

Fig. 9. Parabolic-trough collector [112].

thewater. There are two common types of solar thermal collectors forwater heating: flat plate and evacuated tubes. Glazed flat-plate col-lectors are equipped with insulation and weather proofed boxes thatcontain adarkabsorberplateunderoneormoreglass orplastic covers.Unglazed flat-plate collectors having a dark absorber plate made ofmetal or polymer without a cover or enclosure are typically used forsolar pool heating [109]. Awater-in-glass evacuated tube consists of atwo-layered glass tube and a central feeder tube. A vacuum space iscreated between the cover (outer) glass layer and the absorber (inner)glass layer.With a lowemissivity absorber surface and the presence ofthe vacuum space, the collector allows a better capture of beam ra-diation and avoids the cold winter water-freezing problem [110].Evacuated-tube collectors are more likely to maintain their efficiencyover awide range of ambient temperatures andheating requirements.On the other hand, in constantly sunny climates, flat-plate collectorsare more efficient whereas, in more cloudy conditions, their energyoutput drops off rapidly in comparison with evacuated tubes [111].Another type of collector is the parabolic-trough collector. It usesmirrored surfaces curved in a linearly extended parabolic shape tofocus sunlight on a dark-surfaced absorber tube running the length ofthe trough. A mixture of water and antifreeze or other heat transferfluid is pumped through the absorber tube to pick up the solar heat,and then through heat exchangers to heat water. Because the troughmirrors will reflect only direct-beam sunlight, parabolic-trough sys-tems use single-axis tracking systems to keep them facing the sun[112]. Fig. 9 shows a typical PTC. PTC applications can be divided intotwo main groups. The first and most important is Concentrated SolarPower plants. In this case, temperatures are from 300 to 400 �C. Theother group of applications requires temperatures between 100 and250 �C. These applications are mainly industrial process heat, low-temperature heat demand with high consumption rates (DHW,space heating and swimming pool heating) and heat-driven refrig-eration and cooling [113].

Fernandez-Garcia et al. [113] stated that when a large amount ofhot water is demanded, a large collection area, which sometimesbecomes excessive, must be installed. In this case, PTCs might be ofinterest, because they supply thermal energy at higher tempera-tures than those required by the load and, therefore, higher de-mands can be covered by mixing the hot solar fluid with anothercooler. The advantages of PTCs over the traditional solar collectorsfor water-heating facilities are: i) lower thermal losses and, there-fore, higher efficiency at the higher working temperatures, ii)smaller collecting surface for a given power requirement, and iii) norisk of reaching dangerous stagnation temperatures, since in thatcase, a control system sends the collectors into off-focus position.The disadvantages of PTCs are: i) their solar-tracking system in-creases installation and maintenance costs, ii) the need to regularlyclean their components increases maintenance costs, iii) PTCs canonly use beam solar radiation, and thus their installation isgeographically limited, and iv) at very high wind speeds, operationmust be interrupted and the collectors sent into off-focus position.

2.5.3. Advantages

� SWHS reduces pollution and GHG emissions as it uses cleanrenewable solar energy

� It saves energy and conserves non-renewable energy sourceslike oil, coal and natural gas

� It has very low operating costs.


� High initial cost� For direct systems:


� They should be avoided in climates that experience freezingtemperatures for long periods

� They should be avoided with relatively hard water� Active systems will not function during power outages (devel-oping countries).

2.5.5. State-of-the-artThe literature review of SWHSs is handled in eight groups,1)

general studies which review SWHSs as a whole; 2) ICSSWH; 3)thermosyphon systems; 4) active systems; 5) solar collectors; 6)novel systems; 7) PV/T systems; and 8) PCM integrated systems

2.5.5.1. General studies. Mills [114] noted that current proceduresfor evaluating SWHS performance require detailed assessments oftemperature, solar radiation and hot-water loads for each month ofthe year for any particular location to use as inputs in a modelingprogram. Even though, this sort of analysis is easily conducted foran individual system check, it is not practical for assessing how thesystem would perform anywhere within a very large region, as thenumber of times the program would have to be rerun would beprohibitive. Rather, such a program can be run several times andthen the results are compared against input variables so as todevelop a formula for applying to the input variables. This formulacan be applied within GIS to produce a map of SWHS performancethroughout a large region. Thus, the use of GIS along with theexisting modeling systems is able to generate much more detailedassessments of SWHS performance. Crawford and Treloar [115]showed that the embodied energy component of the net energyrequirement of solar and conventional hot-water systems wasinsignificant. Kaldellisa et al. [116] presented an integrated cost-benefit method, analyzing the economic viability and attractive-ness of domestic SWHSs. Lee and Sharma [117] studied the yearround thermal performance evaluation of active and passive indi-rect SWHSs for rural/urban areas in South Korea. Experimentalresults suggested that active SWHSs must be used in Korea due tothe cold climate. In addition, they proved the feasibility of ethyleneglycol as HTF for both active and passive systems. Garcia-Valladareset al. [118] carried out a simple and inexpensive test method todetermine the thermal behavior of different types of domesticSWHSs. This test can give information to the manufacturer aboutthermal efficiency, thermal stratification, night thermal losses in

Table 7Summary of general studies conducted on SWHSs.

Classification Type of study Place/climatic data Resu


General studies @ Australia GIS casses

@ Melbourne, Australia EnergSWHFor E

Greece (economical study) PBP ¼@ South Korea. Activ

settinPassi

@ Temixco, Mexico Day tNigh

@ Montreal & Los Angeles Los AMont

@ @ Texas, USA 1st yAfter

USA (economical study) SWH@ Portugal Comp

cost o66.5%

the storage tank, the relationship between storage tank volume andthe collection effective area. Biaoua and Bernier [72], in the firstpart of their study, proved that thermal solar collectors representthe best alternative for DHW production in a ZNEH. In the secondpart, a simple economic analysis was presented to determine theoptimum areas of thermal solar collectors and PV array to achieve azero-net-energy hot-water production system i.e. total DHW pro-duction with solar energy. Results revealed that zero-net-energyhot-water production is not cost effective for Montreal.

Degelman [119] reported the performance of a residential solarwater-heating system over a period of 22 years. Results indicatedthat the collector performance has severely deteriorated over thestudied period. Cassard et al. [120] conducted an economical studyon SWHSs and concluded that 50e58% of water-heating energy issaved by using these systems. Jaisankar et al. [9] presented acomprehensive review on solar water heaters and among others;they concluded that the performance of parallel flow solar collec-tors is better than the series flow collectors. Moreover, they rec-ommended that the variation in flow velocity of the working fluidin the riser tubes can be made uniform using variable headers andthe convective heat loss from the glass cover may be reduced usinga suitable aero profile design that will prevent the movement of airover the glass surface. Coimbra and Almeida [121] compared twocooperative housings; one with traditional construction and theother including sustainable building features. Their investigationshowed that the energy consumption for domestic water heatingwas significantly reduced in the sustainable case due to the exis-tence of solar collectors and efficient gas heaters. A summary of thepresented studies is illustrated in Table 7.

2.5.5.2. ICSSWH. Nieuwoudt and Mathews [122] designed, manu-factured and tested prototypes of a solar heat barrow. This device isa low-cost mobile solar water heater composed of an ICSSWH thatpossesses the mobility of a wheelbarrow. The main objective ofsuch design was to find an affordable device in rural areas thatcould assist with fetching domestic water from communal sourcesand heating it, simultaneously. It was demonstrated that the devicehas the ability to store domestic hot water until the evening. Smythet al. [123] conducted a detailed research review of ICSSWHand concluded that the performance of ICSSWHs can also bedemonstrated to be in some cases on a par with distributed SWHSs,if not better when cost versus useful energy gain is considered.

lts

an be joined with existing SWHS models to generate much more detailedsments of SWHS performance [114]y PBP of storage type electric-boosted and instantaneous gas-boostedSs ¼ 0.5 & 2 years for respective conventional EWH & GWH-SWHS: fannular ¼ 62%; For G-SWHS: fannular ¼ 50% [115]8e10 years (No subsidization); PBP ¼ 5e6 years (30% subsidization) [116]

e system: 40.54 < hsys < 46.79%; FR.(sa) ¼ 0.69; FRUL ¼ 3.52; best thermostatg is 8/2�Cve system: 38.69 < hsys < 48.44%; FR.(sa) ¼ 0.61; FRUL ¼ 2.3 [117]est: temperature increment ¼ 18.9e26.3 �Ct test: Tw-homogeneous ¼ 39.8e46.3 �C [118]ngeles: Acl,optimum ¼ 4.5 m2; APV,optimum ¼ 2.06 m2; PBP ¼ 11 years;real: Acl,optimum ¼ 12 m2; APV,optimum ¼ 5.2 m2; PBP ¼ 29 years [72]ear of operation: fannular ¼ 84%;22 years of operation: fannular ¼ 56%, scover Y by 63% [119]S saves 50e58% of water heating energy [120]ared with old GWHs, using solar collectors with efficient GWHs: the operationalf DHW Y about 46% to 70%; annual energy demand/capita for DHW Y by[121]

Fig. 10. Schematic of (a) S1-; (b) S2-; (c) P-tank interconnections [124].


Madhlopa et al. [124] studied the effect of tank-interconnectiongeometry on temperature stratification in an ICSSWH with twohorizontal cylindrical tanks. Three tank interconnections werestudied: parallel (P), series (S1) and series (S2) configurations(Fig. 10). Results revealed that S2-tank configuration was the mosteffective in promoting practical temperature stratification in bothtanks during solar collection and hot-water draw-offs.

Hassan and Beliveau [125] presented a simulation study of anintegrated solar thermal roof collection system. The system consistsof a low-temperature flat-plate collector integrated within a con-crete building envelope linked to a PCM storage tank. Resultsrevealed that the proposed system could cover all domestic hotwater needs year round. Furthermore, a minimum of 88% of thecombined space heating and water-heating requirementsthroughout the year could be supplied, saving the homeowner61.5% of his annual heating bill. Garnier et al. [126] carried out ananalysis of the temperature stratification inside an ICSSWH. Using apreviously developed macro model, which was able to generatecorresponding water bulk temperature in the collector with a givenhourly incident solar radiation, ambient temperature and inletwater temperature and therefore able to predict ICSSWH perfor-mance, a number of improvements was made on the model and anew one was developed. This model was able to compute the bulkwater temperature variation in different solar water-heater col-lectors for a given aspect ratio and water temperature along theheight of the collector (temperature stratification). Computedlongitudinal temperature stratification results were found to be inclose agreement with the experimental data. Kumar and Rosenhave conducted two studies on the performance of ICSSWH with acorrugated absorber surface [127] and with extended storage unit[128]. Both systems were compared to conventional rectangularICSSWH and the obtained results showed that the performance is

Table 8Summary of studies conducted on ICSSWH.

Classification Type of study Place/climatic data Results


ICSSWH @ South Africa Tw,peak,avg ¼ 60disinfect wateICSSWH is onconsidered [12

@ Malawi S2-tank config@ Blacksburg, VA, USA Max THTF,o ¼ 1

In summer: ṼH@ @ Scotland The presented@ Toronto With corrugat

As corrugatedAs ṁw [; max

@ Toronto For the studieICSSWH [128]

enhanced in both cases. A summary of the presented studies isillustrated in Table 8.

2.5.5.3. Thermosyphon systems. Artificial neural networks wereused to predict the performance of both open and closed thermo-syphon SWHSs with horizontal and vertical storage tanks [129].Moreover, Kalogirou and Papamarcou [130] developed a TRNSYSmodel of a direct thermosyphon SWHS with flat-plate collectorusing a TMY data and specific daily hot-water demand profile andvalidated it experimentally. Hussein [131] presented a transientmodel of a two-phase closed thermosyphon flat-plate SWHS andvalidated it experimentally. Chang et al. [132] analyzed, experi-mentally, the overall performance rating of direct and indirectthermosyphon SWHSs with flat-plate and evacuated-tube collec-tors. They considered: i) the thermal performance of the systemduring the energy-collecting phase, ii) the system cooling lossduring the cooling phase and iii) the heat removal efficiency of thesystem during the system application phase. Lima et al. [133]developed an optimization model of a thermosyphon SWHS withflat plat collector for domestic application, using TRNSYS. Theoptimized design gives the slope and area of the flat-plate collector,which results in the minimum cost over the equipment lifecycle. Itcan be noted that the introduction of the timer led to a significantdecrease in the electric energy consumption. Chen et al. [134]investigated experimentally the long-term thermal performanceof a two-phase thermosyphon solar water heater and compared theresults with conventional systems. Budihardjo and Morrison [135]studied the performance of water-in-glass evacuated tube, ther-mosyphon direct solar water heaters and compared themwith flat-plate solar collectors in a range of locations. The performance of atypical 30 tube evacuated-tube array was found to be lower than atypical 2 panelflat-plate array for domesticwater heating in Sydney.

�C; Tw z 40 �C at 20:00; Tw > 50 �C for more than 4 h which is enough tor from Vibrio cholerae [122]a par with distributed SWHSs when cost versus useful energy gain is3]uration is the best for temperature stratification [124]11 �C; THTF,o [ as ṁHTF Y & vice versa; In winter: ṼHTF,optimum ¼ 0.005e0.05 m3/h;TF,optimum ¼ 0.1 m3/h [125]analysis and modeling procedure is valid for all ICSSWH [126]ed surface, Tw [ by 5e10 �Cdepth [, Tw [ & h Y

Tw Y & h [ [127]d system: h [ by 5% & Tw [ by 5e6 �C, Compared with conventional


Kalogirou [136] presented a thermal performance, economicand environmental lifecycle analysis of thermosyphon direct flat-plate solar water heaters and concluded that thermosyphon SWHSsoffer significant protection to the environment and should beemployed whenever possible in order to achieve a sustainablefuture. Huang et al. [137] evaluated the thermal performance ofthermosyphon flat-plate solar water heater with a mantle heatexchanger and theoretically presented an energy equationincluding a “heat exchanger penalty factor”. Results revealed thatthe efficiency of the studied system is lower than that of thermo-syphon flat-plate solar water heaters without heat exchanger, buthigher than that of all-glass evacuated tubular solar water heaters.Zeghib and Chaker [138] modeled a thermosyphon direct SWHSwith flat-plate collector, where three phases of heating-up periodswere observed. Chien et al. [139] investigated “theoretically”, usingthe thermal resistance capacitor method, and “experimentally” atwo-phase thermosyphon solar water heater and proposed the useof double fin tubes and nano-particle fluids to improve the per-formance. Naspolini et al. [140] presented a detailed study on thebehavior of low-cost thermosyphon direct SWHSs consisting offlat-plate collectors, with auxiliary heating provided by electricalshowerheads and not installed inside the hot-water tank. Resultsshowed that the economies obtained are considerable, both interms of energy consumption (kWh) and peak demand (kW)reduction. A summary of the presented studies is illustrated inTable 9.

2.5.5.4. Active systems. Cardinale et al. [141] developed a TRNSYSmodel of an active, indirect SWHS with two PV circulating pumpsand a single-glazed flat-plate collector with selective absorbentsurface. The model was used to calculate the system’s annual en-ergy performance in terms of solar fraction for three Italian local-ities. In addition, the system was economically compared toconventionalWHs that use electricity, gas oil and LPG. Yohanis et al.[142] presented an analysis of direct active SWHSs that determinesthe number of days in each month when solar heated water meetsthe whole demand above a certain set temperature. Three types ofsolar collectors were studied: i) single-cover, non-selective, ii)Double-cover, non-selective and iii) single-cover, selective. Resultsproved that:

Table 9Summary of studies conducted on thermosyphon systems.



Thermosyphon @ Athens, Greece ANN mo@ @ Nicosia, Cyprus fannular ¼@ @ Cairo, Egypt The stud

system [@ Taiwan h0 provi

FPC: hs*,aETC: hs*,a

@ Sao Paulo, Brazil WithoutWith tim(F � 5�)

@ Australia For a 22@ Nicosia, Cyprus Annual s

years; Fo@ Taipei, Taiwan Two pha

(Economical study) Florianópol, Brazil 200 kWPower d

@ @ Kunming, China SWHS w@ Constantine, Algeria 3 phases@ @ Taiwan Two pha

� The use of a second transparent cover as well as selectivecoatings for the absorber does not significantly increase thenumber of days when a specified demand temperature is met bySWHS

� The optimum collector slope is equal to approximately the locallatitude

� The size of the storage tank volume should not be larger thanthe daily hot-water consumption

Chow et al. [110] developed a numerical study on the potentialapplication of a centralized active, indirect SWHS in a high-rise resi-dence, where flat-plate solar thermal collectors occupied top two-third of the south and west facades of a hypothetical high-rise resi-dence. Hobbi and Siddiqui [143] modeled, using TRNSYS, an indirectforced-circulation SWHS with a flat-plate collector for domestic hot-water load of a single-family residential unit in a cold climate. Themodelwas used to optimize the designparameters of both the systemand the collector. Kulkarni et al. [144] performed a numerical opti-mization of the SWHS configuration with varying water replenish-ment profile. The studied system was of active, direct type with flat-plate collector. The annual performance of the system was studiedbased on the optimal proposed water replenishment profile. It wasobserved that the annualized system cost can be reduced by 13.7%.

Ayompe et al. [145] presented a validated TRNSYS model forforced circulation, indirect SWHSs with flat plate and heat-pipeevacuated-tube collectors used in temperate climates. The vali-dated model can be used to predict long-term performance of theSWHSs in different locations and operating conditions as well asoptimize SWHS sizes to match different load profiles. Note that theheat-pipe evacuated-tube solar collectors consist of a heat pipeinside a vacuum-sealed tube, as shown in Fig. 11. The heat istransferred as latent heat energy by evaporating a small amount ofworking fluid in a heating zone (heat-pipe evaporator) andcondensing the vapor in a cooling zone (heat-pipe condenser).Michaelides and Eleftheriou [146] presented an experimental studyabout the performance characteristics of an active indirect SWHSwith two flat-plate solar collectors. They found that knowing theperformance boundaries during the lifecycle, one could use them asa tool to assess the performance degradation of the solar collectorwith time and make a qualitative comparison of the system

del was validated using actual results [129]79%; PBP ¼ 8 years [130]ied model is an efficient tool for the design & optimization of the investigated131]des a more representative measure of the performancevg ¼ 51%; hR* ,avg ¼ 81%; h0,avg ¼ 41%;vg ¼ 59%; hR* ,avg ¼ 79%; h0,avg ¼ 47% [132]timer: 4.29 < Acl,optimum < 6.15 m2;er: 2.14 < Acl,optimum < 3.62 m2;< boptimum < (F + 5�) [133]0L, 30-ETC system: monthly savings ¼ 29e90% [135]olar contribution: 79%; Reduction of GHG ¼ 70%; For Electric back-up: PBP ¼ 2.7r Diesel back-up: PBP ¼ 4.5 years [136]se-SWHS: h [ by 18% compared to conventional SWHS [134]h/residence can be reduced annually;emand at on-peak hours can be reduced at over 2.6 kW/unit on average [140]ith mantle heat exchanger: havg ¼ 50% [137]of heating-up periods can be observed; max hcl ¼ 95% [138]se-SWHS: h ¼ 82% [139]

Fig. 11. Schematic diagram of a heat-pipe evacuated-tube collector [145].


performance. A summary of the presented studies is illustrated inTable 10.

2.5.5.5. Solar collector. Al-Madani [147] designed, manufacturedand evaluated the performance of a cylindrical solar water heater.The system consisted of a cylindrical glass tube (air evacuated) thatworks as the receiver of solar energy and a copper coil throughwhich the water flows (Fig. 12). This coil is placed inside the glasstube and acts as the collector. Results revealed a good capability of

Table 10Summary of studies conducted on active systems.



Active @ Milan, Rome & Palermo, Italy@ Belflast, Irland & London, UK

@ Hong Kong@ Montreal, Canada

@ Pune, India@ Nicosia, Cyprus

@ @ Dublin, Ireland

the system to convert the solar energy to heat and use it for waterheating. This system does not need to be directed to the sunbecause of its circular shape. Furthermore, it has lower heat lossand cost than those of a flat-plate collector-SWHS. Gunerhan andHepbasli [148] studied the optimum tilt angle for solar collectors inIzmir, Turkey. It is found that the solar collector should be orien-tated due south and that it should be mounted at the monthlyaverage tilt angle and the slope adjusted once a month. Varol andOztop [149] numerically investigated the natural convection heattransfer and flow field inside inclined flat-absorber and wavy-absorber collectors. It is observed that the studied parameters arestrongly affected by the shape and inclination angle of the collectorand enhanced heat transfer rate is obtained in the case of a wavy-absorber. Furthermore, for the same aspect ratio, the mean Nusseltnumber increases with the decreasing wave length.

Jaisankar et al. [150e152] found that the heat transferenhancement in a twisted tape collector is higher than that in aplain tube collector and it gradually decreases with increase intwist ratio. In addition, the heat enhancement in full length twistedtape is better than the twist fitted with rod which in turn is betterthan twist fitted with spacer and the decrease in friction factor ismaximum for twist fittedwith spacer compared to twist fittedwithrod. Ma et al. [153] investigated the thermal performance of anindividual glass evacuated-tube solar collector by one-dimensionalanalytical method. The studied collector is a two-layered glassevacuated U-tube, where the absorber film is deposited in the outersurface of the absorber tube-the inner glass tube. Furthermore, theU-tube is welded inside a circular copper fin. Results showed animportant influence of the thermal resistance of the air layer be-tween the absorber tube and the copper fin on the heat efficiency.In addition, efficiency increases with the increase of solar radiationintensity, but it reaches gradually to a constant. Kishor et al. [154]presented a fuzzy modeling approach to represent the solar col-lector in a thermosyphon SWHS. The proposed fuzzy model pro-vided satisfactory prediction as compared to ANN technique.

Li et al. [155] established a heat transfer model for an all-glassvacuum tube collector used in a forced-circulation SWHS andvalidated it by experimental studies. AlShamaileh [156] studied anew solar coating, comprising NiAl alloy particles, for SWHS ap-plications. Results showed that the studied coating collects thermalenergy more efficiently than ordinary commercial black paints.Taherian et al. [157] studied the dynamic simulation of a flat-platecollector in a closed loop thermosyphon SWHS and validated itexperimentally. Tanaka [158] performed numerical analysis of asolar thermal collector with a flat-plate top reflector, which extends

Results

Economically, SWHS is better than EWH but worse than GO & LPG-WHs [141]For all year: if f < 30%, N ¼ 0;For warm half year (AprileSeptember): if f < 50%, N ¼ 0;if Vst ¼ Vdhn, performance [ as Acl [ [142]hcl-avg ¼ 38.4%; fannular ¼ 53.4%; PBP ¼ 9.2 years [110]The designed optimal system could provide 83e97% and 30e62% of the hotwater demands in summer and winter, respectively with fannular ¼ 68% [143]For the cost-optimal system configuration: Acl Y by 12.7% & Vst Y by 10.2% [144]Tank-Tw, Energy collected & hcl-avg are predictable; SWHS is relativelyinsensitive to solar radiation fluctuations and hot water flows [146]Maximum measure in 2/6/2009:FPC: Tcl,o ¼ 67 �C; Qcl ¼ 2500 kJ;hp-ETC: Tcl,o ¼ 68 �C; Qcl ¼ 2700 kJ;Maximum measure in 2/1/2010:FPC: Tcl,o ¼ 20 �C; Qcl ¼ 500 kJ;hp-ETC: Tcl,o ¼ 15 �C; Qcl ¼ 300 kJ [145]

Fig. 12. Schematic of the cylindrical solar water heater [147].


from the upper edge of the collector. He concluded that solar ra-diation absorbed by the absorbing plate of the collector can beincreased using a flat-plate top reflector, which is inclined forwardin winter and backward in summer, and setting the inclinationangle of the reflector at less than 30� throughout the year. Asummary of the presented studies is illustrated in Table 11.

2.5.5.6. Novel systems. Tan and Deng [159] presented a simulationstudy of a water chiller with a desuperheater and a reversibly usedwater cooling tower for service hot-water generation. Using thissystem, part of chilled water is pumped into a RUWCT where it isheated by warmer ambient moist air. The operating characteristicsof the refrigeration system with a desuperheater and a RUWCTwere studied. The required flow rate of chilled water to be pumpedinto the RUWCT was calculated in order to satisfy a certain heatload. Also, the maximum heating capacity of the system underdifferent operating conditions was evaluated. In addition, resultsindicated that the use of a RUWCT would achieve higher energyefficiency than the use of electric heating as back-up heat pro-visions when building space cooling load is reduced. Raab et al.[160] validated the TRNSYS XST-model for calculating the thermalbehavior of ground buried hot-water heat stores. It is worth notingthat the XST-model is an additional type (non-standard type) forTRNSYS. The validated system was composed of a closed activesolar circuit with a flat-plate collector connected to a ground buriedand thermally insulated seasonal hot-water heat store and a districtheating network as shown in Fig. 13.

Furbo et al. [161] presented the investigation of small solardomestic hot-water systems based on smart solar tanks. These

Table 11Summary of studies conducted on solar collectors.



Collector @ Bahrain Max (Two � Twi)@ Izmir, Turkey For March & Sep

of the year [148]@ w.r.t. variations

for flat collectorsFor flat collectorFor wavy collect

@ India As twist ratio Y,twisted tape > h

@ For I ¼ 950 W/mat THTF � Ta ¼ 11at Cb ¼ 29 W/mKAs THTF,i [ fromfor ṁHTF ¼ 0.001at ṁHTF ¼ 0.001 kTcoating [ linearly

@ The developed m@ @ Beijing, China UL has little imp

collector is smal@ Amman, Jordan The new studied

average of 5 �C [@ @ North Iran FPC-Thermosyph@ Japan Compared with

solar radiation a

tanks are small vertical mantle tanks designed in such a way thatthe auxiliary energy supply system heats up the tank from the top.Therefore thermal stratification in the tank is built up in a goodwaywhen the auxiliary energy supply system is used. The study rec-ommended starting the development of smart solar tank units withan oil-fired boiler or a natural gas burner as auxiliary energy supplysystems. Ho and Chen [162] presented a theoretical prediction ofthe performance of a double-pass sheet-and-tube solar waterheater with external recycle and showed that the recycle effect caneffectively enhance the collector efficiency compared with that in asingle-pass devicewith the same flow rate. Roonprasang et al. [163]conducted an experimental study of a new SWHS using a solarwater pump and showed that it is economically comparable to aconventional one. Sutthivirode et al. [164] conducted an experi-mental study of SWHS coupled with a built-in solar water pump.This system adds less weight to a building roof and saves electricalenergy of a circulation pump. It has lower cost compared to a do-mestic SWHS. Zhao et al. [165] developed a computer model andused it to analyze performance and operating characteristics of anovel loop heat-pipe SWHS. The system was composed of wickedheat absorbing pipe arrays, vapor and liquid headers, vapor andliquid transporting lines, as well as a flat-plate heat exchangercoupled with a water storage tank by water tubing as shown inFig. 14(a). In operation, the received solar heat converts the liquidadhered on the wick of the pipes into vapor, which flows upwardsalong the inner space of the pipes and enters the top-side vaporheader, owing to the buoyancy of vapor. The vapor is furthertransported to the heat exchanger inside the building via the vaportransporting line. Within the exchanger, the vapor is condensedinto liquid of the same temperature, transferring heat to the waterflowing across the channels adjacent to the vapor channels. Theliquid then enters the liquid header located right below the vaporheader, due to the gravity caused by the height difference betweenthe exchanger and the header. This amount of liquid is then evenlydistributed to individual pipes through a dedicated liquid feederfitted at the upper part of the pipes, as shown in Fig. 14(b).

Chow et al. [166] reported the integrated thermal performance ofa water-flow absorbing window compared to conventional single

¼ 27.8 �C; max h ¼ 41.8% [147]tember: boptimum ¼ latitude (38.46�); boptimum [ towards the beginning and end

of b, wave length, Rayleigh no. & aspect ratio, variation of Nulocal is almost linear& wavy for wavy collectors

s: highest Rayleigh number is obtained at the highest bors: Numean Y with the increase of b [149]the heat transfer rate [ & need of Acl Y by 8e24%; performance of LefteRightelical twisted tape [150,151,152]2 & Ta ¼ 283 K:0 K, h [ 10% & Tcoating Y 30 �C as Cb [ from 5 to 40 W/mK;, FʹY slightly & Tcoating [ 110 K as (THTF � Ta) [ from 0 to 150 K;

30 to 90 �C, (THTF,o � THTF,i) Y 0.65 �C for ṁHTF ¼ 0.003 kg/s & [ 1.96 �Ckg/s;g/s, h [ 10% & THTF,o [ 16% as Cb [ from 5 to 30 W/mK;as THTF,i [; Tcoating [ slightly as ṁHTF Y; for Cb < 40 W/mK, Fʹ Y largely [153]odel can be used for the estimation of the SWHS performance [154]

act on the calculated results because the heat loss coefficient of vacuum tubel and thus it can be ignored in mathematical models [155]coating shows better performance compared to the untreated black paint by an156]on SWHS: hcl,avg ¼ 68% [157]a conventional one, using a solar collector with a flat plate top reflector increasesbsorption by 19, 26 & 33% for lm/lcl ¼ 0.5, 1 & 2, respectively [158]

Fig. 13. The solar-assisted district heating system in Hannover [160].


and double pane absorptive glazing. The investigated system is adouble pane window whose cavity is connected to a water-flowcircuit. The water passage in this way can effectively lower theglass pane temperature, reduce room heat gain and therefore, theair-conditioning electricity consumption. Furthermore, the water-flow window system can function as a hot-water pre-heating de-vice. Simulation results showed that the integrated performance ofthe water-flow window in terms of reduction in air-conditioning aswell aswater-heating loads is very attractive. DebMondol et al. [167]investigated the performance of a novel heat-exchange unit (‘Sol-asyphon’), shown in Fig. 15. The ‘Solasyphon’ delivered solar heatedwater directly to the top of the storage producing a stratified supplyat a useable temperature. Results showed that the ‘Solasyphon’system is more effective compared to a traditional twin-coil systemfor a domestic application where intermittent hot-water demand is

Fig. 14. (a) The loop heat-pipe SWHS; (b) Schematic of the connec

predominant and under a transient solar input, particularly on in-termediate or poor solar days. However, the twin-coil system wasfound to be more efficient than the ‘Solasyphon’ system under aprolonged heating period. Note that a twin-coil system consists oftwo heat exchangers installed one above the other within a hot-water storage cylinder. The upper and lowerheat-exchanger coils areconnected to the boiler and solar circuit respectively. A summary ofthe presented studies is illustrated in Table 12.

2.5.5.7. PV/T systems. A photovoltaic/thermal hybrid solar water-heating system is a combination of photovoltaic and solar thermalcomponents/systems which simultaneously produce both elec-tricity and heat from one integrated component or system usingwater as heat removal fluid. Fig. 16 shows the main features of aflat-plate PV/T collector.

tion of the headers and heat pipes via the water feeder [165].

Fig. 15. Schematic diagram of a ‘Solasyphon’ unit [167].


Although a water-based PV/T system is able to achieve a higheroverall energy output per unit area when compared to side-by-sidePV and solar water-heating systems [168], the individual thermaland electrical efficiencies are lower.

Chow et al. [169] presented a photovoltaicethermosyphoncollector with rectangular flow channels and discussed its energyperformance. The overall performance of the system was foundpromising in providing an alternative energy source for the do-mestic sector in China. He et al. [170] constructed and tested awater-type hybrid PV/T collector with polycrystalline PV moduleplaced on a flat-box type thermal absorber made of aluminum-alloy. They found that the energy-saving efficiency (hf ¼ hth þ he/hpower) is above that of a conventional system. Ji et al. [171] illus-trated the thermal and electrical behavior of a wall-mounted solarphotovoltaic/thermal collector system through a numerical model.They carried out an optimization on the appropriate water-flowrate and packing factor which is defined as the ratio of area coveredby PV cells to the total area of the solar thermal collector. Ji et al.[172] conducted sensitivity analysis of a flat-box aluminum-alloyPV and water-heating system designed for natural circulation andindicated that the higher the PV cell covering factor and the glazingtransmissivity, the better the overall system performance. Chowet al. [173] described an experimental study of a centralizedphotovoltaic and hot-water collector wall system that can serve as awater pre-heating system, where collectors were mounted at

vertical facades. Results indicated that natural water circulationwas found more preferable than forced circulation. Moreover, withthe PV/T wall, space thermal loads can be much reduced both insummer and winter; leading to substantial energy savings. Chowet al. [174] presented a study on the performance evaluation ofphotovoltaicethermosyphon system for subtropical climate appli-cation. They found that the PBP for this system is equivalent to thatof side-by-side configuration. Fraisse et al. [175] studied the energyperformance of water hybrid PV/T collectors, wherewater was usedas the heating element for a Direct Solar Floor space heating systemand a hot-water supply to the house. Results indicated that in thecase of a glazed collector with a conventional control system forDSF, the maximum temperature reached at the PV modules ishigher than 100 �C which does not allow the use of EVA resin in PVmodules due to high risks of degradation. The use of either a-Si cells(to avoid the use EVA and improve solar absorption) or uncoveredcollector (for more effective cooling) was suggested. Da Silva andFernandes [168] conducted parametric studies and annual tran-sient simulations of PV/T systems using Simulink/Matlab. Theyproposed that the use of vacuum, or a noble gas at low-pressureincreases the thermal efficiency and fluid working temperatures,reduces thermal losses and negligibly decreases the electrical per-formance. Daghigh et al. [176] conducted a simulation study topredict the performance of amorphous and crystalline silicon-based PV/T solar collectors. From their results, it could be noticedthat if electrical production is a priority, the c-Si PV/T based solarcollector is preferred.

Dupeyrat et al. [177] studied and evaluated the thermal andelectrical performances of several PV/T collector concepts using asimple 2-dimentional thermal model. They indicated that thedirect lamination of single c-Si PV cells on an optimized metal heatexchanger leads to the best results. After that, an experimental PV/T collector was built using the single package lamination method,focusing on the enhancement in heat transfer between PV cellsand cooling fluid as well as the improvement of optical perfor-mance (anti-reflective coating on the glass cover). Their experi-mental results indicated a significant improvement of boththermal and electrical performance in comparison to previouswork on PV/T collector concepts. Additionally, the thermal effi-ciency at zero reduced temperature (when the fluid inlet tem-perature is equal to the ambient temperature) of the laminated PV/T collector seems to be very close to the efficiency of a solarthermal-only-collector with or without a selectively coatedabsorber. However, the heat loss coefficients of this collector aresignificantly higher than those of high efficient flat-plate collectors(with selective coating), but still in the same range as some lessefficient solar thermal-only-collectors (with non-selectivecoating). Furthermore, the electric efficiency of the investigatedPV/T collector is significantly lower than that of good single c-Si PVmodules using the same solar cell technology; however, this ismainly due to a low packing factor. A summary of the presentedstudies is illustrated in Table 13.

2.5.5.8. PCM integrated systems. Phase change materials are latentheat storage materials. The thermal energy transfer occurs when amaterial changes from solid to liquid or liquid to solid. The mainadvantages of PCMs are their high storage density and isothermaloperation. PCMs can be incorporated in SWHSs to enhance thethermal energy storage.

Kurklu et al. [178] developed and studied a new type of solarcollectors which consisted of two adjoining sections, one filled withwater and the other with paraffin wax PCM (Fig. 17). The PCMfunctioned both as an energy storage material for the stabilizationof the water temperature and as an insulation material due to itslow thermal conductivity value. It was concluded that this type of

Table 12Summary of novel SWHS studies.



Novel @ Southern China In winter, more chilled water is pumped to RUWCT;In summer, RUWCT operates only at high heatingloads; Max Qwh is 3690, 4010, 4310 & 4620 W for 25%,50%, 75% & 100% of full cooling load respectively;As Qwh [, ṁref [; max ṁref ¼ 0.708 kg/s is at max Qwh for anycooling load; Chiller with desuperheater system:COPsystem+RUWCT > COPsystem+e-backup; Viable use of RUWCTwhen Ta > 15 �C [159]

@ @ Hannover, Germany Vhs ¼ 2795 m3; hhs ¼ 71.2%; f ¼ 28% [160]@ @ Denmark Using small SDHW with smart tanks: Performance [ by

5e35% & performance/cost ratio [ by 25% [161]@ e hdp-stSWH > hsp-stSWH; hdp-stSWH [ as recycle rato, Twi, ṁ & I [ [162]

@ Thailand Solar pump works at 70 < Tcl < 90 �C; hsys-avg ¼ 7e13%;46 < Tw < 61 �C [163]

@ Thailand Solar pump works at 70 < Tcl < 90 �C; as discharge head [, hSP &hsys Y; max Tw ¼ 59 �C; max hsys ¼ 21% [164]

@ @ Beijing, China Optimum heat pipe operating Temperature is w72 �C;Ṽw across heat exchanger should be w5.1 l/min [165]

@ Hong Kong Compared with double and single pane windows: annual roomheat gain z 32% & 52%; annual electricity savings ¼ 111 &140 kWh/m2, respectively [166]

@ Northern Ireland For 6-h heating period at I ¼ 800 W/m2:Solasyphon : hheat gain ¼ 21.3%;Tw ¼ 45 �C (Upper 50 L); hheatgain ¼ 59.5% (Total, 200 L); Tw ¼ 20 �C(Lower 150 L);Coil: hheat gain ¼ 15.6%;Tw ¼ 40 �C (Upper 50 L); hheat gain ¼ 64.5%(Total, 200 L); Tw ¼ 37 �C (Lower 150 L)[167]


collector might pave the way to the use of PCMewater solar panelson the south faces of buildings for energy storage or saving appli-cations. Mehling et al. [179] proved that adding a PCM module atthe top of thewater tank is a technique that gives the system higherenergy storage density, allowing re-heating of the transition layerafter partial unloading and compensation of heat loss in the toplayer for a considerable time. Cabeza et al. [180] studied the addi-tion of several cylinders of PCM at the top of the water tank. Agranular sodium acetate trihydrateegraphite was chosen as thePCM. The experiments were classified as cooling down process, re-heating process and solar operation. The energy density was

Fig. 16. Main features of a flat-plate PVT collector [11].

evaluated by comparing the sensible heat stored in the tank whenonly water was used, with the sensible and latent heat stored in thetank when PCM modules were present. Experimental resultsshowed that the inclusion of a PCM module in water tanks fordomestic hot-water supply would allow either to have hot waterfor longer periods of time evenwithout exterior energy supply or touse smaller tanks for the same purpose.

Ibanez et al. [181] developed a new TRNSYS component, basedon the already existing TYPE 60, called TYPE 60 PCM to simulateand study the behavior of a single-family SDHW system with PCMmodules inserted inside the water tank. They proved that thedeveloped model is a powerful tool to evaluate the performance ofPCM modules in water tanks. Mettaweea and Assassa [182] carriedout outdoor experiments to investigate the heat transfer charac-teristics of a compact PCM (paraffin wax) solar collector during thecharging and discharging processes of PCM. Nallusamya et al. [183]presented a combined sensible and latent heat thermal energystorage system. This system unit contained paraffin as PCM filled inspherical capsules and packed in an insulated cylindrical storagetank. The water, used as HTF to transfer heat from the constanttemperature bath/solar collector to the thermal energy storagetank, also acts as sensible heat storage material. The effects of inletfluid temperature and flow rate of HTF on the performance of thestorage unit during the charging process was studied and dis-cussed. Moreover, experiments were conducted for continuousdischarging and batch wise discharging for both sensible heatstorage system and combined storage system. It was concluded thatthe combined storage system gives better performance than theconventional sensible heat storage system. Talmatsky and Kribus[184] constructed a physical model to describe a heat storage tankwith and without PCM. Their results indicated that contrary toexpectations, the use of PCM in the storage tank provides no en-ergetic advantage to the end-user, and in some conditions, it mayactually be detrimental because re-heating of the water by the PCM

Table 13Summary of studies conducted on PV/T systems.


Modeling/simulation Experimental Electricity

PV/T @ @ China @ hth ¼ 37.6e48.6%; he ¼ 10.3e12.3% [169]@ Hefei, China @ Max hth ¼ 40% if Twi ¼ Ta; he-avg ¼ 9.87% [170]@ Hefei, China @ Optimum value of ṁw improves he & hth [171]@ @ Hefei, China @ Primary energy savings ¼ 65% (z ¼ 0.63, sglazing ¼ 0.83);

he ¼ 10.15%; hth ¼ 45%; hsys ¼ 52% [172]@ Hong Kong. @ he ¼ 8.56%; hth ¼ 38.9% [173]

@ @ Hong Kong @ PBP ¼ 12 years ¼ PBP of side-by-side systems [174]@ @ Macon, France @ Without glass cover: annual he ¼ 10%

With glass cover: annual he ¼ 6.8% [175]@ Lisbon, Portugal @ favg ¼ 67%; he ¼ 9%; hth ¼ 15%; hsys ¼ 24% [168]@ Malaysia @ (a-Si): he ¼ 4.9%; hth ¼ 72%; hsys ¼ 77%

(c-Si): he ¼ 11.6%; hth ¼ 51%; hsys ¼ 63% [176]@ @ e @ he ¼ 8.8%; hth ¼ 79%; hsys ¼ 88% [177]


during nighttime is responsible for increased losses to the envi-ronment, in an amount that is sufficient to cancel gains madeduring the day.

Mazmanet al. [185] investigated the effect of using PCMmodulesin a stratified SDHW tank where three PCM mixtures (PP, PS, SM)were tested for this purpose in cooling and re-heating experiments.In the cooling experiments, the average tank water temperaturedropped below the PCM melting temperature range in about 6e12 h. During re-heating experiments, the PCM could increase thetemperature of 14e36 L ofwater at the upper part of the SDHW tankby 3e4 �C. This effect took place in 10e15 min. Furthermore, it wasconcluded that PS gave the best results for thermal performanceenhancementof the SDHWtank. El Qarnia [186] studied the thermalbehavior and performance of a solar latent heat storage unit withthree different kinds of PCMs as storagemediums, n-octane, paraffinwax and stearic acid. This unit consisted of a series of identical tubesembedded in the PCM. During charging mode, a heat transfer fluid(hot water) from the solar collector passes through the tubes andtransfers the collecting heat of solar radiation to the PCM. The heatstored in the liquid PCM is next transferred to water during the

Fig. 17. Schematic view of the studied solar collector [178].

discharging mode to produce heating water. Results showed thatthe use of n-octane as PCM is not beneficial, whereas using paraffinwax, part of the PCM remained liquid during discharging processand stearic acid offered an acceptable range of outlet temperature ofhot water and full discharge of the storage unit for an optimumwater mass flow rate of 0.005 kg/s. Therefore, stearic acid is bene-ficial for heatingwater application. Al-Hinti et al. [187] presented anexperimental investigation of the performance of water-PCM stor-age for use with conventional SWHS. Paraffinwax was used as PCMwhich was packed in the hot-water storage tank on two levels(upper and lower). Results indicated that the system succeeded inkeeping thewater temperature over 45 �C under all operational andclimatic conditions. Kousksou et al. [188] confirmed the findings ofRef. [184] regarding theuseof PCM in SDHWsystems thatmayprovenot to be substantially beneficial; however, they suggested that theproper choice of PCMmelting temperature may open the prospectsof successfully designing a PCM based DHW system that may bemore efficient. A summary of the presented studies is illustrated inTable 14.

2.6. Instantaneous water heater

2.6.1. PrincipleInstantaneous water heaters are also known as on-demand or

continuous water heaters because they instantly heat water as itflows through the device (upon demand), and because the water isconstantly heated whilst the tap is left on. Electricity, gas or oil isusually used as the energy source in instantaneous water heaters.

2.6.2. Advantages

� No hot-water storage, and thus standby heat loss is eliminated� Higher energy factor than conventional storage units [189],noting that the energy factor is defined as a measure of theportion of input energy that is transferred to the hot water

� Lower operating cost compared to conventional storage units� Unlimited hot-water supply as water is heated while passingthrough the system [189]

� The temperature of a certain hot-water flow is constant� Their compact size use less physical space� Longer life expectancy than conventional storage units [189].


� Start-up delay as water is heated upon demand� Minimum flow rate threshold, below which the unit will notactivate, is required [189]

Table 14Summary of studies conducted on PCM integrated systems.



PCM @ Turkey Day: Tw > 55 �C; Night: Tw > 30 �C; hth ¼ 20e80% [178]@ @ e Energy density [ by 20e45%; top water heat loss delayed in time

by 50e200%; top water was reheated from the PCM module in only20 min [179]

@ Lleida, Spain Increase in tank energy density at DTPCM-water ¼ 1 K & DTPCM-water ¼ 8 K is:For 2 PCM modules: 40% & 6%, respectively;For 4 PCM modules: 57.2% & 12%, respectively;For 6 PCM modules: 66.7% & 16.4%, respectively [180]

@ @ Lleida, Spain f [ by 4 & 8% at TPCM ¼ 45 & 55 �C, respectively, while hcl is notinfluenced [181]

@ Egypt PCM discharge phase: havg [ as molten layer thickness [;PCM charge phase: Qh,u [ as ṁw [; havg starts small & [ with timedue to [ in natural convection [182]

@ India During charging: if THTF,i ¼ cst, ṁHTF has a small effect; heat transfer [as THTF,i [ [183]

@ Tel Aviv, Israel & Munich, Germany In January:with PCM: Heat loss ¼ 13.09 MJ; hcl ¼ 42.14%;without PCM: Heat loss ¼ 13.12 MJ; hcl ¼ 41.51%;In April:with PCM: Heat loss ¼ 15.73 MJ; hcl ¼ 40.36%;without PCM: Heat loss ¼ 15.69 MJ; hcl ¼ 39.85% [184]

@ Lleida, Spain PCM mixture type PS: hheat recovery ¼ 74%; PCM mixture type PP:hheat recovery ¼ 63%; PCM mixture type SM: hheat recovery ¼ 36% [185]

@ @ Marrakech, Morocco n-octane: εc,optimum ¼ 0.889; paraffin: εc,optimum ¼ 0.559; stearic:εc,optimum ¼ 0.439 [186]

@ Jordan Tw > 45 �C all time [187]@ Tel Aviv, Israel & Pau, France Results confirm that of ([184]) [188]


� May not be able to serve simultaneous multiple draws of hotwater [189]

� For electrical type, high instantaneous power is needed� Yearly maintenance is needed to prevent water-flow restrictiondue to calcium build up.

Idem et al. [190] presented a mathematical model of aninstantaneous, condensing, gas-fired water heater that couldaccurately predict the water temperature rise and flue loss for awide range of operating conditions. Thomas et al. [191] presentedboth laboratory and field-test results related to the performance ofgas-fired instantaneous water heaters for residential and com-mercial applications. They concluded that the residential-use effi-ciency of the studied system is approximately 17% higher than thatof a typical power-vented, gas-fired storage water heater. Con-cerning the commercial use, the real use efficiency of the system isclose to its steady-state efficiency in a once through configuration,while it is reduced significantly in large volume circulation loopconfiguration because of the high pipe work heat losses. Moreover,the outlet temperature of an instantaneous gas-fired water heateris recommended to be set between 54 and 60 �C in order to avoidcold water problems with mixing valves. The impact of draw vol-ume size, flow rate, and time intervals between draws on theoverall efficiency of instantaneous gas water heaters was investi-gated at the Davis Energy Group [192]. Experimental tests revealeda small impact of the flow rate and significant efficiency degrada-tion at draw volumes less than four gallons. In addition, small timeintervals between draws yielded higher efficiencies.

Hwang et al. [193] experimentally investigated the heat andmasstransfer characteristics in a titanium heat exchanger with excellentcorrosion resistance used for waste heat recovery with the conden-sation arranged in an instantaneous gas-firedwater heater. Differentarrangements of the tubes of the heat exchanger including in-lineand staggered configurations were also investigated. Results indi-cated that the thermal efficiency of the gas-firedwater heater with alatent heat recovery heat exchanger was enhanced by about 10%

compared with conventional instantaneous water heaters. Also theheat andmass transfer performance of the staggered tube bank typewere approximately 50% and 10% higher than that of the in-line tubebank type, respectively. Czerski et al. [194] studied the possibility ofinstalling gas-fired instantaneous water heaters with combustionchamber sealed with respect to the room in multi-story residentialbuildings. This water heater is equipped with air-supply andcombustion-product ducts systems. The tests conducted in two pilotplants in Poland showed that the use of such systems in multi-storybuildings significantly reduces the danger of CO emissions, improvessanitary conditions and ensures higher energy efficiency (w92%)than that in the case of traditional instantaneous gas water heaters.The main reason, leading to efficiency enhancement, is the possi-bility of reducing the combustion gases temperature at the outlet ofthe heaters, which is not possible in traditional system types.

3. Conclusions

Water heating contributes to a large amount of residential en-ergy consumption all around the world, and thus, the proper choiceof a domestic hot-water system can significantly save energy,protect nature and reduce operational costs.

In this study, a detailed review of different water-heating sys-tems existing worldwide was conducted. Six different categorieswere introduced, namely wood, oil/gas, electric, heat pump, solarand instantaneous systems. The heat pumps were further classifiedin several groups, namely air source, ground source, solar assisted,ground source-solar assisted, photovoltaicethermal and gas-engine driven systems. In addition, concerning solar water heat-ing, different system types and collectors were presented andreviewed.

The main conclusions that are extracted from this study are asfollows:

� Instantaneous water heaters work on a demand principleand thus need to use their energy source (gas/oil/electricity)

Fig. 18. General flow chart of water heater choice.


when hot water is required without hot-water storage.High instantaneous heating energy is needed to provide therequired hot-water temperature. Using gas or oil as the energysource causes local pollution and obliges the use of a continuousfuel supplying system. Continuous electricity supply is neededfor electric-based heaters. In the case of frequent power outagesfor grid-connected homes or off-grid homes, storing electricalenergy in sufficient capacity batteries is required. Accordingly,in the case of non-continuous electrical power supply, theinstantaneous electrical system should be disregarded

� Wood, oil and gas domestic storage-type water heaters arepotential alternatives in the absence of sufficient electricalenergy supply. However, they are not preferred due to theproduction of local pollution which may be harmful to theresidents’ health

� Concerning the use of PCM in SDHW systems, contradictoryresults have been reported by different researchers; hence, it isrecommended to await further technical and research devel-opment before judging this technology

� Concerning the design of the storage hot-water reservoir, thefollowing recommendations could be suggested:� The inlet pipe: side-bottom positioned, slotted-pipe or 90�

downwards-bended pipe� The outlet pipe: upper-side positioned short tube or stainlesssteel coupling positioned vertically at the center of the uppercap of the tank

� High aspect ratio (height/diameter) in order to enhance hot-water stratification

� Use dual-tank reservoir, where the second tank constitute70e80% of the total power rating and 10e30% of the totalvolume

� For large capacity EWHs, use dual heater elements, with onepositioned vertically at the bottom and the other positionedhorizontally below the uppermost 50 L volume

� The size of the storage tank volume should not be larger thanthe daily hot-water consumption

� For SWHSs, smart solar storage tanks (small, vertical, mantleand those that develop good stratification) are preferred

� Concerning SWHS:� The performance of parallel flow solar collectors is preferredmore than series flow collectors for domestic use

� Twisted tape collector tubes could augment the heat transfer,as well as lefteright twisted tape with small twist ratio hasthe best performance

� PTCs installation is geographically limited� Evacuated-tube collectors are more likely to maintain theirefficiency over a wide range of ambient temperatures andheating requirements. On the other hand, in constantly sunnyclimates, flat-plate collectors are more efficient whereas, inmore cloudy conditions, their energy output drops off rapidlyin comparison with evacuated tubes

� Solar radiation absorbed by the absorbing plate of the col-lector can be increased by using a flat-plate top reflector

� Corrugated absorber surface increases the performance ofICSSWH

� The optimum collector slope is approximately equal to thelocal latitude

� From the utilization point of view, thermosyphon solar waterheater occupies a good position in domestic applications dueto its ease of operation without the aid of any external energy

� The energetic performance of two-phase thermosyphonSWHSs is better than that of single-phase systems

� With a proper design of the PV/T hybrid solar system,competitive thermal and electrical efficiencies could be ach-ieved compared to that of conventional solar thermal-onlysystems and PV modules, respectively

� Payback period for PV/T hybrid solar system is equivalent tothat of side-by-side configuration


� PV/T solar systems improve the energy performance per unitarea which is very important for urban areas

� SWHSs with electrical back-up have no local pollution impact� Concerning HPWH:� HPWHs are energy efficient systems� Conventional ASHPWH should be preferably installed in lo-cations with ambient temperature above 4.4 �C year round

� Energetically, the wrap-around condenser coil on the tank isbetter than separate condenser

� Concerning the design of natural convection immersedcondenser, the U-tube style results in a better performancethan bayonet style

� In an ASHPWH, instantaneous heating mode has better en-ergy performance than cyclic heating mode

� Both stability and efficiency of the ASHPWH can be signifi-cantly improved using EEV

� GSHPWH is always energy efficient and applicable irre-spective of climate

� A GSHP with an integral desuperheater water-heating circuitcontributes to an average of 27% of the total energy suppliedfor heating water

� The collector efficiency of the DX-SAHPWH system can begreater than 1 when the evaporating temperature is lowerthan the ambient temperature

� EEV and variable frequency compressor are recommended tobe utilized for the DX-SAHPWH

� SAS-HPWH overcomes the problem of DX-SAHPWH that failsin rainy days

� PV/T HPWH systems improve the energy performance perunit area, the electrical efficiency of PV modules and COP ofthe heat pump

� GEHP systems are novel, very efficient systems and theybecome more efficient when used for both water and spaceheating

� Except for GEHP, HPWHs produce no local pollution

In an attempt to suggest a general scheme that recommends thechoice of a water-heating system, five variables are introduced:

1) Surface area availability, which controls the possibility of usingsolar collectors or geothermal energy

2) Mean daily solar energy3) Average minimum daily ambient air temperature in the cold

season4) Mean daily ambient temperature range (MDATR¼ annual-mean

[max Ta,daily e min Ta,daily])5) Balance point, which is the outdoor temperature at which the

heat-pump capacity equals the building load [195]

According to the previous conclusions, the flow chart, shown inFig. 18, suggests a general road map that controls the choice of awater-heating system for domestic use from energetic and envi-ronmental points of view.

Acknowledgements

The authors would like to thank the Azm research center at E-DST Lebanese University as well as the Lebanese National Councilfor Scientific Research for their support.

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