Applied Energy 91 (2012) 466–474

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Applied Energy

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Low temperature desalination using solar collectors augmented by thermalenergy storage

Veera Gnaneswar Gude a,⇑, Nagamany Nirmalakhandan b, Shuguang Deng c, Anand Maganti d

a Civil Engineering Department, Oregon Institute of Technology, 3201 Campus Drive, Klamath Falls, OR 97601, USAb Civil Engineering Department, New Mexico State University, Las Cruces, NM 88003, USAc Chemical Engineering Department, New Mexico State University, Las Cruces, NM 88003, USAd Department of Transportation, 703 B Street, Marysville, CA, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 April 2011Received in revised form 15 September2011Accepted 12 October 2011Available online 9 November 2011

Keywords:Low temperature desalinationRenewable energySolar collectorsThermal energy storageEnergy and carbon dioxide emissions

0306-2619/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.apenergy.2011.10.018

⇑ Corresponding author. Tel.: +1 541 885 1903.E-mail address: gudevg@gmail.com (V.G. Gude).

A low temperature desalination process capable of producing 100 L/d freshwater was designed to utilizesolar energy harvested from flat plate solar collectors. Since solar insolation is intermittent, a thermalenergy storage system was incorporated to run the desalination process round the clock. The require-ments for solar collector area as well as thermal energy storage volume were estimated based on the vari-ations in solar insolation. Results from this theoretical study confirm that thermal energy storage is auseful component of the system for conserving thermal energy to meet the energy demand when directsolar energy resource is not available. Thermodynamic advantages of the low temperature desalinationusing thermal energy storage, as well as energy and environmental emissions payback period of the sys-tem powered by flat plate solar collectors are presented. It has been determined that a solar collector areaof 18 m2 with a thermal energy storage volume of 3 m3 is adequate to produce 100 L/d of freshwaterround the clock considering fluctuations in the weather conditions. An economic analysis on the desali-nation system with thermal energy storage is also presented.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Desalination using solar energy is an ancient method of produc-ing freshwater. Evidence of Greek sailors, in the 15th century, usingsolar distillation equipment to convert the seawater into freshwa-ter has been reported [1]. Solar energy is a self-renewable resourcewhich can be easily captured and harvested as thermal energy formany beneficial uses. However, the concern with the solar energyis that it is intermittent in nature and its intensity depends on thehour of the day and local weather conditions. One of the solutionsto utilize fluctuating solar energy on a continuous basis is to incor-porate thermal energy storage (TES) system [2].

Energy accumulation, storage and supply are the key elementsof thermal energy storage concept which result in better econom-ics, resource management and lower environmental emissions of avariable energy source powered desalination system. TES helpsmanage the energy resource when supply and demand are mis-matched. Three types of TES systems are in commercial use; (1)sensible heat storage, (2) latent heat storage, and (3) thermo chem-ical storage systems [3]. The most widely used TES is the sensibleheat storage system which stores energy by changing the temper-ature of the storage medium which is either water, air, rock beds,

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or sand [4]. Water is cheap, chemically stable and has a higher heatcapacity compared to many other fluids such as oils, air or rockbeds. The amount of energy stored by a sensible heat storage sys-tem is proportional to the difference between the storage inputand output temperatures, the mass of the storage medium andthe medium’s heat capacity [5]. The energy stored in the TES sys-tem can be used for variety of applications depending on the tem-perature of the medium; for example, TES temperatures in therange of 60–80 �C are suitable energy sources for low temperaturedesalination and TES temperatures in the range of 100–400 �C aresuitable for power generation, as well as cooling and other indus-trial process applications [6–9]. The energy input can be providedby solar collectors, parabolic trough collectors and process wasteheat releases.

In this research, a new low temperature desalination processhas been developed to utilize low grade heat sources such as wasteheat releases, solar or geothermal energy sources. Since, the pro-cess operates at lower temperatures, energy losses and hence en-ergy requirements for desalination are reduced. As this processutilizes renewable energy and waste heat releases, it does not di-rectly contribute to any greenhouse gas emissions, and can be con-sidered a sustainable process. The objective of the present work isto design a continuous flow low temperature desalination systemoperated by a thermal energy storage tank powered by a solar col-lector system. In a previous study, we have reported the use of a

Nomenclature

A area (m2)C concentration (%)c specific heat (kJ/kg K)E amount of energy supplied (kJ)Ex exergy (kJ/h)h latent heat, heat source (kJ/kg)F heat removal factor (dimensionless) (%)I solar insolation (kJ/h m2)m mass of water (kg)M cumulative freshwater (kg)Q energy or useful energy (kJ/h)P pressure (Pa)S entropy (kJ/K)U heat loss/transfer coefficient (kJ/h m2 K)v volume of the saline water or storage tank (m3)q evaporation energy (kJ/h)t time (h, s)T temperature (K)

Special charactersam experimental coefficient (kg K0.5/m2 Pa s)f concentration factor (–)

q density (kg/m3)g energy efficiency (%)W exergy efficiency (%)

Subscripts1 TES temperature2 evaporation chamber temperaturea ambientc collectorComb combinedDES desalinationEC evaporation chamberi,in inlet, supplye evaporation, electricall,ls,L latent, losses, loadr,R recycle, ratios, sc saline water, supply, surface, solar, collectors, TESt totalTES thermal energy storageth thermal

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thermal energy storage system to run the low temperature desali-nation system round the clock by storing the reject heat from thecondenser of a domestic air-conditioning system [10,11]. Thermo-dynamic advantages of low temperature desalination and experi-mental validation of utilizing direct solar energy, low grade heatsource and photovoltaic energy have also been reported previously[12–15]. Here, we present thermodynamic advantages of low tem-perature thermal storage (see Appendix A) and evaluate the needof a TES for the solar collector system considering variations in dai-ly solar insolation patterns and passing cloud effects. Energy andemissions payback time periods for the solar collectors are also dis-cussed. In addition, an economic analysis of the solar powereddesalination system augmented by thermal energy storage ispresented.

Fig. 1. Vapor pressures of fresh and saline waters at low temperatures.

2. Description of the low temperature desalination system

The premise of the proposed approach can be explained by con-sidering two barometric columns at ambient temperature, onewith freshwater and another with feed water. The barometric col-umns contain the head equivalent to the local atmospheric pres-sure creating a vacuum in the headspace. Due to the naturalvacuum generated by this process, the head space of these two col-umns will be occupied by the vapors of the respective fluids attheir respective vapor pressures. If the two head spaces are con-nected to each other, water vapor will distill spontaneously fromthe freshwater column into the feed water column because, the va-por pressure of freshwater is slightly higher than that of feed waterat ambient temperature. However, if the temperature of the feedwater column is maintained slightly higher than that of the freshwater column to raise the vapor pressure on the feed water sideabove that of the fresh water side, water vapor from the feed watercolumn will distill into the fresh water column (see Fig. 1). A tem-perature differential of about 10–20 �C is adequate to overcomethe vapor pressure differential to drive the desalination process(Fig. 1). Such low temperature differentials can be achieved usinglow grade heat sources such as solar energy, waste process heatand thermal energy storage systems.

A schematic arrangement of the low temperature desalinationsystem based on the above principles is shown in Fig. 2. Compo-nents of this unit include an evaporation chamber (EC), a naturaldraft condenser; two heat exchangers, and three barometric col-umns. These three columns serve as the saline water column; thebrine withdrawal column; and the desalinated water column; eachwith its own holding tank, SWT, BT, and DWT, respectively.

The EC is installed atop the three columns at a height of about10 m above the ground level to create a vacuum naturally in theheadspaces of the feed, withdrawal, and desalinated water col-umns. This configuration drives the desalination process withoutany mechanical pumping once in operation except for the initialstart-up of the process [16,17]. The saline water enters the evapo-ration unit through a tube-in-tube heat exchanger (1,2). The tem-perature of the head space of the feed water column is maintainedslightly higher than that of the desalinated water column. Since thehead spaces are at near-vacuum level pressures, a temperature dif-ferential of 10 �C is adequate to evaporate water from the feedwater side and condense in the distilled water side (3–5). In this

Fig. 2. Low temperature desalination system driven by solar collectors.

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manner, saline water can be desalinated at about 40–50 �C, whichis in contrast to the 60–80 �C range in traditional solar stills andother distillation processes. This configuration enables brine tobe withdrawn continuously from the EC through HE1 preheatingthe saline water feed entering the EC (6,7). Further, by maintainingconstant levels of inflow and outflow rates in SWT, BT and DWT,the system can function without any energy input for fluid transferwithin the desalination system. The heat input to EC is provided bythermal energy storage (TES) tank through a heat exchanger HE2which in turn is fed by a solar collector system. Mechanical pump-ing is necessary to transfer the fluids between the solar collectorand TES tank and between the TES tank and desalination system.

2.1. Modeling of the low temperature desalination system

Mass and heat balances around the evaporation chamber (EC)yield the following coupled differential equations, where the sub-scripts refer to the state points shown in Fig. 2. The variables aredefined in the Nomenclature. Thermodynamic benefits of low tem-perature desalination with thermal energy storage are presented inAppendix A.

Mass balance on volume of water in EC:

ddtðqVÞ ¼ m2 �m6 �m3 ð1Þ

Mass balance on solute in EC:

ddtðqVCÞEC ¼ m2C2 �m6C6 ð2Þ

Energy balance for volume of water in EC:

ddtðqVCpTÞEC ¼ Q EC þ ðmcpTÞ2 � ðmcpTÞ6 �m3hLðTÞ � Q l ð3Þ

where QEC is the rate of energy input (load on the TES) to the EC andQl is the rate of energy loss from the EC. m1, m2, m3 refer to salinewater flow streams, m4 and m5 refer to water vapor and freshwater,and m6 and m7 refer to flow rates of brine solution in the system atthe process points respectively. The energy input, QEC, to the evap-oration chamber can be supplied by solar collectors, photovoltaicthermal collectors, geothermal water sources and process waste

heat sources to the TES as discussed in the previous section and iswritten as:

QEC ¼ mscsðTs � TECÞ ð4Þ

where ms, cs, and Ts, are the mass flow rate, specific heat and tem-perature of the water from the TES and TEC is temperature of the sal-ine water in the evaporation chamber.

Desalination efficiency is defined as g ¼ MhLðTÞ

RðQ ECDtÞ ð5Þ

where hLðTÞ ¼ 3:146� 2:36ðT þ 273Þ ð6Þ

Evaporation rate as a function of pressure and temperature[18]:

m3 ¼ AECk fðCEC ÞpðTEC Þ

ðTEC þ 273Þ0:5�

pðT5Þ

ðT5 þ 273Þ0:5

" #ð7Þ

where pðTÞ ¼ ½expð63:02� 7139:6=ðT þ 273Þ � 6:2558 InðT

þ 273Þ� � 102 Pa ð8Þ

The above equations are solved using Extend (ImagineThat Inc.)simulation software. Details of heat transfer relations for evapora-tion chamber and condensation surface, and heat losses by convec-tion and radiation are presented elsewhere [11,19].

2.2. Solar collectors and thermal energy storage (TES)

Flat plate solar collectors supplying low grade heat in the rangeof 60–80 �C can be used to drive the proposed desalination systemduring sunlight hours (Fig. 2). The sensible heat stored in the TEScan provide the heat source to the evaporation chamber duringnon-sunlight hours.

Heat balance across the solar panel can be written as [20]:

dðmcTÞSC

dt¼ FRAC ½ðsaÞIS � ULðTSC � TaÞ� � Q s ð9Þ

where Qs is the solar energy harvested by the solar collectors andstored in the TES tank and is given as:

Qs ¼ mRcRðTSC � TSÞ ð10Þ

where mR and cR are the mass flow rate of the collector fluid and TSC

is the temperature of the water exiting the solar collector and TS isthe temperature of TES tank.

The energy balance on the TES can be written as [21]:

ddtðmcTÞS ¼ Q S � Q EC � Qls ð11Þ

where ms is the total mass of water in the TES, cs and Ts are as de-fined earlier; QS is the solar energy supplied by solar collectorsand Qls energy losses from the TES.

3. Results and discussion

Results obtained from the modeling studies are presented inthis section. A discussion is presented on the need of thermal en-ergy storage based on the theoretical results followed by the dis-cussion of the energy and emissions pay back periods fordomestic solar collector system.

3.1. Performance of the low temperature desalination system

Temperature profiles of the desalination system driven by solarcollectors are shown in Fig. 3a for both evaporation and solar col-lector areas of 1 m2.The TES temperatures reach a maximum value

Fig. 3. Low temperature desalination system driven by solar collector; (a) temperature profiles, (b) energy utilization, (c) daily production rates, and (d) desalinationefficiency.

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of 57 �C during sunlight hours and evaporation temperatures reacha maximum of 45 �C. The maximum ambient temperature duringthis test period was 34 �C. The energy supplied from the TES andthe energy utilized for evaporation are shown in Fig. 3b. Fromthe hourly freshwater production rates and cumulative productshown in Fig. 3c, it can be deduced that a daily product of 6.5–7 L can be produced for 1 m2 evaporator and 1 m2 solar collectorareas. These results are comparable to a previous study by Al-Kharabsheh and Goswami [17]. This output is more than two foldof a typical solar still [22]. In another study, a solar still was incor-porated with PV/T hybrid collectors which produced about 7.9 L/dfor 4 m2 collector area. The system, however, suffers from varia-tions in daily solar insolation as thermal energy storage unit isnot provided [23]. The evaporation efficiency of this proposed pro-cess ranged between 60% and 90%, most of the time in the range of75–90% as shown in Fig. 3d while the exergy efficiency of the desa-lination ranged between 50% and 70%. The TES volume used in thissimulation was 0.1 m3. Excluding thermal energy storage system, asolar collector area of 15 m2 is sufficient on a typical summer dayto produce about 105 L/d of freshwater. For the same desalinationcapacity, the hourly and daily freshwater production rates with

Fig. 4. Low temperature desalination system driven by solar collectors (TESvolume: 3 m3, solar collector area: 18 m2).

inclusion of a thermal energy storage unit are shown in Fig. 4. Solarcollector area and TES volumes required for this application are18 m2 and 3 m3 respectively. As it can be seen from Fig. 4, freshwa-ter production rates for this case approach steady state only after72 h of operation. The freshwater production rate was lower dur-ing initial hours as some of the energy supplied from the solar col-lectors is utilized to increase the sensible heat of the total volumeof the water medium in TES. Stabilized freshwater production ratefor this configuration was 104 L/d.

3.2. Use of thermal energy storage system

The need for thermal energy storage tank was evaluatedthrough simulations considering different TES volumes. Fig. 5shows TES performance for two different volumes (1 m3 and6 m3) over 168 h (7 days). TES temperatures fluctuate in responseto the daily solar insolation and ambient temperatures in bothcases. TES temperatures for a volume of 1 m3 respond quickly tochanges in the solar insolation and ambient temperatures withconsiderably low sensible heat available for desalination duringnon-sunlight hours. Temperatures during sunlight hours reach as

Fig. 5. TES performance over 7 days (168 h).

Fig. 7. TES volume effect on solar collector area and freshwater production rate.

Fig. 8. TES volume effect on daily production rates and cumulative product over21 days.

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high as 65–68 �C and fall down to as low as 30–35 �C during non-sunlight hours. TES temperatures for a TES volume of 6 m3 respondslowly to the daily solar insolation and ambient temperaturesincreasing the sensible heat of the bulk of the water in the tank.For this tank volume, the TES water temperature during non-sun-light hours was around 45–48 �C which is still an adequate heatsource for evaporation during non-sunlight hours enabling 24 hoperation. The maximum and minimum TES temperatures areshown in Fig. 6 for different TES volumes in the range 1–6 m3. FromFig. 6, it can be observed that as the TES volume increases the heatsource available for non-sunlight hour operation increases. Rela-tionship between solar collector areas in connection with the TESvolumes is presented in Fig. 7. It is obvious that, for fixed freshwa-ter production rate of 100 L/d, the solar collector area requirementis higher for higher TES volumes because sensible heat losses to theambient from the TES tank are to be provided by the same solarcollector area. Therefore, the collector area required for 1 m3 TESvolume is 15 m2 which is increased by 25% and 40% for TES vol-umes of 3 m3 and 6 m3 respectively. Hourly and daily freshwaterproduction rates for a fixed solar collector area (15 m2) over21 days of operation are shown in Fig. 8. At the end of 7 days ofoperation, the average daily freshwater production for TES volumeof 1 m3 remained at 100 L/d and for TES volume 6 m3 at 68 L/d.

It should be noted that the average freshwater productionrates continue to increase for the TES volume of 6 m3 and reach86 L/d at the end of 21 days of operation. From these simula-tions, the maximum TES temperatures for TES volume 1 m3 re-mained at 68 �C whereas for TES volume 6 m3 thetemperatures increased from 53.8 �C to 54.5 �C at the end of21 days of operation. This indicates that the energy stored inthe TES is available for longer periods of time enabling continu-ous and stable freshwater production rates. Therefore, to deter-mine the optimum solar collector areas, long term performanceof the TES needs to be considered. The configuration with noor small TES volumes will suffer from the variations in daily so-lar insolation and ambient temperatures. High TES volume mayinitially result in lower freshwater production rates but withcontinued operation, productivity will be increased. For smallTES volumes freshwater production rates cease during non-sun-light hours leaving the unit idle for 33–50% of the day. If highstorage volume of TES is a constraint, freshwater has to bestored in water tank for rest of the day, cloudy and rainy dayneeds as well. Continuous operation allows for downsizing thedesalination unit and reduces the equipment cost. Batch opera-tion requires large evaporation area and equipment with highercosts. Continuous process mode is easily adaptable to other lowgrade waste heat sources and can be scaled to large applicationsto provide freshwater for small rural communities.

Fig. 6. Effect of TES volume on TES and EC temperature profiles.

3.3. Effect of solar energy variation on thermal energy storage anddesalination system

The effect of passing clouds on different TES volumes was inves-tigated. One cloudy day per week was considered over 3 weeks.The freshwater production rate is lower than 50% for the systemwithout thermal energy storage unit. As can be seen from Fig. 9;the average daily production over 21 days of operation decreasesfrom 100 L/d to 92 L/d (a reduction of 8%) for TES volume of 1 m3

with a solar collector area of 15 m2. For the same collector area,the reduction in average daily production over 21 days is from 93to 87 (a reduction of 6.5%) and 86–81 L/d (a reduction of 5.8%)for TES volumes of 3 m3 and 6 m3 respectively. Considering theperformance on the cloudy days alone, the reduction in the dailyproduct amount was 100–63, 93–67, and 86–75 L/d which are37%, 28% and 13% reductions as shown in Fig. 9. The hourly fresh-water production rates decreased from 7.5 to 4.6, 5.1–3.5, and 4.5–3.5 L/h for TES volumes 1, 3 and 6 m3 respectively, the smallestvariation being observed for TES tank with volume of 6 m3. Similarcomparison for different TES volumes with a solar collector area of

Fig. 9. TES volume effect on daily production rates and cumulative product over21 days with cloudy day effects.

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18 m2 was done for 21 days of operation. Based on this analysis,cloudy day production increased for TES volume of 6 m3 from75 L/d (15 m2 solar collector area) to 88 L/d (18 m2 solar collectorarea). The hourly production rates also higher for this configura-tion. As shown in Fig. 10, the solar collector area requirementsbased on 21 days operation are 6% and 13% only compared to7 days operational performance analysis, 25% and 40% for TES vol-umes of 3 and 6 m3 respectively.

3.4. Summary

The characteristic feature of thermal energy storage is to storethe energy for longer periods and supply it when the heat source islimited or not available. The TES with volume 1 m3 was not able toaccumulate the energy as the mass of the medium was smaller com-pared to the TES with volume 3 m3 or 6 m3. This condition could beeven worse for a system without a thermal energy storage unit. Tem-peratures in the TES with lower volume will be higher with higherheat losses to the ambient. With large TES volumes, the available en-ergy is higher during non-sunlight hours to continue the desalina-tion round the clock. The economics of incorporating thermalenergy storage has to be evaluated case-by-case basis. For largeapplications, for example, small rural communities or hospitals,

Fig. 10. TES volume effect on solar collector area and freshwater production rateover 21 days.

where freshwater supply is required round the clock, advantage ofhaving a TES can be very significant in energy conservation, energy,cost savings and environmental emission reductions.

3.5. Energy and environmental emissions payback period of solarcollectors

The prime energy consumption to produce a solar collector ofarea 1 m2 and the supporting frame is about 7 GJ excluding thestorage tank which needs to be determined separately for eachapplication [24,25]. The solar collector area in this study is18 m2. The total energy consumption for the solar collectors’ pro-duction is 126 GJ. If the annual useful thermal energy harvestedby the total solar collector area is 99 GJ (basis: 5.5 GJ/m2 year)which is used for desalination in this application. Considering thepumping energy required in the active solar collector system (be-tween the solar collector system-TES and TES-Desalination sys-tem), the prime energy payback period of the solar collectorsystem is under 2 years [24,25].

The global warming potential (GWP) of a solar collector of1 m2 area is estimated to be around 721 kg eq. CO2. AnnualCO2 eq. emission savings due to thermal energy generated bysolar collectors is estimated as 407 kg eq. CO2 (basis: specificglobal warming factor of 0.0657 kg eq. CO2 per MJ of useful ther-mal energy). The CO2 emissions for the solar collectors produc-tion is around 13 tons. Therefore, the global warming potentialof the solar collectors for the proposed system will be recoveredin less than 2 years with the annual CO2 eq. emission savings of7.326 tons [24]. This data supports the clean idea of utilizingrenewable energy harvested by solar collectors for low tempera-ture desalination.

3.6. Economic analysis of combined system with and without TES

Economic analysis of the low temperature desalination systempowered by a solar collector system was performed for two cases;(1) desalination unit with a TES tank, and (2) desalination unitwithout a TES tank. Cost requirements of the desalination systemfor the two cases are compared in Table 1 [26,27]. Desalination sys-tem costs are determined assuming that the investment is financedat an annual interest rate of 5% over the lifetime of 20 years for thedesalination system [28,29]. The following equations can be usedto calculate the unit cost of the desalinated water produced fromthe desalination system with and without thermal energy storagetank.

Amortizationfactor

Annual capital and operatingcosts

Unit productcost

a ¼ ið1þiÞnð1þiÞn�1

Atotal ¼ Afixed þ Areplacement þ AOM

Aunit ¼ Atotalðf�MÞ�365

where a is the amortization factor, n is the life time of the plant, i isthe annual interest rate (%), f is the plant availability and M is thequantity of produced water (kg).

Based on the above analysis (interest rate: 5%, life time: 20 yearsand plant availability of 90%), the costs for the desalinated water aredetermined to be 1.4 ¢/L or 14 $/m3 and 1.32 ¢/L or 13.2 $/m3 respec-tively for desalination system with and without thermal energystorage tank system. When lifetime of the system is considered as25 years, the costs for the desalinated water are determined as1.17 ¢/L or 11.7 $/m3 and 1.1 ¢/L or 11.0 $/m3 respectively whichare well accepted values for a small scale desalination system.Although, the desalinated water cost is slightly higher for the systemwith a TES, advantages of continuous, uninterrupted water supplyregardless of weather conditions is a major benefit for the systemwith TES tank.

Table 1Economic analysis of the low temperature desalination system with and without a TES tank.

Item Description Estimated cost ($US)with TES

Description Estimated cost ($US)w/o TES

Evaporator 1 m2 cross sectional area 400 2 m2 cross sectional area 800Evaporator heat

exchanger1.27 cm copper tube, 10 m long 30 1.27 cm copper tube, 20 m long 60

Condenser 15 cm copper tube with fins 200 20 cm copper tube with fins 500Tube-in-tube heat

exchanger1.27 cm inside copper tube, 2.54 cmoutside PVC tube

25 2.54 cm inside copper tube, 5.08 cmoutside PVC tube

50

PVC pipes PVC tube of 1.27 cm diameter and 30 mlong

30 PVC tube of 2.54 cm diameter and 30 mlong

50

Pipe fittings 20 40Storage tanks Four, 40 L capacity 100 Four, 40 L capacity 100Supporting structure 32 ft high structurea 32 ft high structurea

Labor 500 300Solar collectors $100/m2 1800 1500

TES tank volumeb 800 0Water storage 0 100Pumps 300 200

5105 4450

a Existing support structure.b Installed storage cost is included.

Fig. A1. Low temperature sensible heat storage (TES) systems: (a) TES at 90 �C; (b) TES at 60 �C.

Table A1Thermodynamic performance of low temperature desalination using thermal energystorage.

Unit Component Energy (%) Exergy (%) Entropy (kJ/K)

TES tank A (90 �C) TES 78.3 68.7 6DES 87.5 83 0.7Combined 68.5 57.1 6.7

TES tank B (60 �C) TES 87.6 69.1 2.1DES 93.8 83 0.2Combined 82.1 57.3 2.3

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4. Conclusions

Feasibility of round the clock operation of a low temperaturedesalination system powered by solar collectors with the use of

thermal energy storage has been studied theoretically. Thermody-namic advantage of a low temperature desalination system usingthermal energy storage tank has been evaluated.

1. The study confirmed that thermal energy storage plays an impor-tant role in managing the variable energy resource to maintainthe performance of the desalination system during non-sunlighthours and cloudy days.

2. Results show that a low temperature desalination systemdesigned to produce 100 L/d of freshwater would require a solarcollector area of 15 m2 with 1 m3 of TES volume or 18 m2 with3 m3 of TES volume. This additional area of the solar collectorshelps the TES to accumulate the excess energy which can bestored and supplied on a cloudy day or low solar insolation day.

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3. The energy and environmental emissions payback period of thedesalination system powered by solar collectors can be lessthan 2 years which very well supports the idea of utilizing thesolar collectors for low temperature desalination.

4. Thermal energy storage can improve the economics of the com-bined system and reduce the environmental emissions. How-ever, the economics of incorporating TES has to be evaluatedcase-by-case basis and the benefits of having the TES can bevery significant in applications where freshwater productionis required on a continuous basis.

Appendix A

A1. Thermodynamics of the low temperature desalination using TES

The advantage of low temperature desalination can be illus-trated using the First law efficiency and the Second law efficiency.The First law (energy) efficiency of the TES can be expressed as:

gTES ¼Q out

Qin¼ mscsðT1 � T2Þ

mhchðTh � T1ÞðA1:1Þ

where Qin is the energy supplied to the TES by a hot stream of massflow rate mh, specific heat ch and inlet and outlet temperatures of Th

and T1 respectively. Qout is the energy supplied by the TES to thedesalination system with a mass flow rate ms, specific heat cs andinlet and outlet temperatures of T2 (evaporation temperature TEC)and T1 respectively.

First law (energy) efficiency of the desalination system (DES)can be found from:

gDES ¼mkðTEC Þ

mscs½ðT1 � T2Þ�ðA1:2Þ

where m is the mass of the freshwater produced, k is the latent heatof the freshwater at the evaporation temperature TEC which is sameas T2.

First law (energy) efficiency of the combined system can befound from:

gComb ¼mkðTEC Þ

mhch½ðTh � T1Þ�ðA1:3Þ

The Second law (exergy) efficiency of the TES can be found from[30]:

wTES ¼Exout

ExinðA1:4Þ

where

Exin ¼ mhch½ðTh � T1Þ � T0ðlnðTh=T1Þ� ðA1:5Þ

[Thermal exergy associated with the heat source flow streamcan be written as: mh{(hh � h1)–To(sh � s1)}. Substituting the fol-lowing relations for hh and sh; the above expression can be obtai-ned.hh = ho + ch (Th–To) and sh = so + ch ln(Th/To)] [30–33] and

Exout ¼ mscs½ðT1 � T2Þ � T0ðlnðT1=T2Þ� ðA1:6Þ

where To is the ambient temperature which is taken as 293.15 K(20 �C). Exin is the exergy supplied to the TES from the variable en-ergy source, and Exout is the exergy delivered from the TES.

Second law (exergy) efficiency of the DES can be found from[30]:

wDES ¼Exout

ExinðA1:7Þ

where,

Exin ¼ mscs½ðT1—T2Þ � T0ðlnðT1=T2Þ� ðA1:8Þ

Exout ¼ mk½1� ðT0=TECÞ�Þ ðA1:9Þ

Second law (exergy) efficiency of the combined system can befound from:

wComb ¼Exout

ExinðA1:10Þ

Exin ¼ mhch½ðTh—T1Þ � T0ðlnðTh=T1Þ� ðA1:11ÞExout ¼ mk½1� ðT0=TECÞ�Þ ðA1:12Þ

The application of the above equations in demonstrating theadvantage of using low temperature thermal energy storage sys-tem (TES) for desalination is illustrated by considering two identi-cal sensible heat storage tanks, A and B each of volume 1 m3 and atambient temperature of 20 �C. Assume that these TES tanks oper-ate through a cycle of charging, storing, and discharging steps asshown in Fig. A1, each tank receiving 41,750 kJ of thermal energyover a cycle from an external heat source. Tank A is maintainedat 90 �C (by an external heat source flowing at mh, and Th of1000 kg/h and 100 �C for 1 h) and B is maintained at 60 �C (by anexternal heat source flowing at mh, and Th of 1000 kg/h and 70 �Cfor 1 h). Consider a case where this heat energy is stored for 12 hbefore it is put to use. Assuming a heat loss coefficient of 0.5 W/m2 K and a surface area of 6 m2; the heat loss from the tank A is9072 kJ and the heat loss from tank B is 5184 kJ (note: losses dur-ing charging and discharging are ignored). Due to the heat lossfrom the tanks, the TES temperatures at the end of 12 h storageare 87.8 �C and 58.8 �C for tanks A and B respectively.

Now, after storage for12 h, the available energy in the two TEStanks A and B are 32,678 kJ and 36,566 kJ respectively; supposethis energy is used to heat the evaporation chambers ECs, in thelow temperature desalination system to maintain the two evapora-tion chambers, X and Y at evaporation temperatures 80 �C and50 �C respectively to produce freshwater (with flow rates of 335and 332 kg/h respectively). Assuming that the available energywas supplied in a time period of 3 h, the heat loss from the ECs Xand Y are 972 kJ and 486 kJ respectively, assuming a heat loss coef-ficient of 0.5 W/m2 K and a surface area of 3 m2. Freshwater pro-duced from X and Y are 12.4 kg and 14.4 kg respectively(considering the minimum amount of energy required to increasethe sensible heat from 20 �C and the latent heat to evaporate fresh-water in the ECs X and Y at 80 �C and 50 �C as 2563 and 2508 kJ/kgrespectively).

From the above analysis, it can be noted that the First law effi-ciency of the combined TES-Desalination system is higher for thetank B (TES: 87.6%, DES: 93.8%, Combined: 82.1%) than for tank A(TES: 78.3%, DES: 87.5%, Combined: 68.5%; Table A1). However,the Second law efficiency of the tank A (TES: 68.7%, DES: 83%, Com-bined: 57.1%) and tank B (TES: 69.1%, DES: 83%, Combined: 57.3%)are comparable because the availability in the TES tank A is higherdue to higher storage temperature. It should be noted that the sec-ond law efficiency for the desalination system is comparable inboth cases because of negligible exergy destruction in the system.As a result, it can also be noted that the combined system exergyefficiency is comparable or slightly higher for tank B than tank A.

This analysis is based on once-through desalination processwhere the heat source is passed through the desalination processonly once. If the exergetic value of the condensation energy is con-sidered for further work, to cause evaporation in the next evapora-tion chamber as in the case of multi-effect configuration, theavailable energy (exergy) in the water vapor at higher condensa-tion temperatures (TES tank A) will be higher and can be reusedmany times compared to the available energy (exergy) at lowercondensation temperatures (TES tank B). This analysis has beenperformed only to provide an understanding that the low grade

474 V.G. Gude et al. / Applied Energy 91 (2012) 466–474

heat sources can also be both energy and exergy efficient if utilizedin a thermodynamically stable process, in this case, low tempera-ture desalination process. This analysis also reveals that even whenthe quality of the heat source is low (low temperature and henceavailability) and the quantity is limited, low temperature desalina-tion process can be both energy and exergy efficient resulting inlower energy and exergy losses and higher freshwater productionrates.

A2. Entropy generation

To further analyze the operation of low temperature desalina-tion system, entropy generation of the individual systems can beevaluated. In the above example, both the TES tanks involve heatlosses to the environment during charging, storing and dischargingperiods and through the desalination process. To calculate the irre-versibility of both the tanks A and B in the form of entropy gener-ated, the following expression can be used [34].

DS ¼ Q=T ðA1:13Þ

where DS is the entropy change for each TES tank, Q is the amountof heat lost to the environment and T is the absolute temperature ofthe TES tank.

The above expression can be written as DSTES = QTES/TTES (for TES:Source) and DSamb = Qamb/Tamb (for environment: Sink) respectively.Tank A involves heat loss of 9072 kJ from the source temperature of90 �C to the ambient at 20 �C and Tank B involves heat loss of 5184 kJfrom the source temperature of 60 �C to the ambient at 20 �C (ignor-ing the heat loss during charging and discharging and assumingaverage temperatures of the TES tanks during the storage period).

The entropy generated in this process is:Tank A:

Source: DSTES = QTES/TTES = �9072 kJ/363.15 K = �25 kJ/K.Sink: DSamb = Qamb/Tamb = 9072 kJ/293.15 K = +31 kJ/K.SGen = DStotal = DSTES + DSamb = (�25 + 31) kJ/K = +6 kJ/K.

Tank B:

Source: DSTES = QTES/TTES = �5184 kJ/333.15 K = �15.6 kJ/K.Sink: DSamb = Qamb/Tamb = 5184 kJ/293.15 K = +17.7 kJ/K.SGen = DStotal = DSTES + DSamb = (�15.6 + 17.7) kJ/K = +2.1 kJ/K.

The total entropy change for the heat transfer in the TES tank Bis smaller than the TES tank A and it can be said that TES tank B isless irreversible. This is because the heat losses from Tank B arelower than from tank A and tank A is at higher temperature thantank B. Thus irreversibility in tank B is smaller than tank A. Simi-larly, the entropy generation for the desalination system and thecombined system can be calculated (Table A1).

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