Sustainable low temperature desalination: A case for renewable energy

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Sustainable low temperature desalination: A case for renewable energyVeera Gnaneswar Gude, Nagamany Nirmalakhandan, and Shuguang Deng Citation: Journal of Renewable and Sustainable Energy 3, 043108 (2011); doi: 10.1063/1.3608910 View online: http://dx.doi.org/10.1063/1.3608910 View Table of Contents: http://scitation.aip.org/content/aip/journal/jrse/3/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A sustainable renewable energy mix option for the secluded society J. Renewable Sustainable Energy 6, 023124 (2014); 10.1063/1.4873129 Exceptional ion rejection ability of directional solvent for non-membrane desalination Appl. Phys. Lett. 104, 024102 (2014); 10.1063/1.4861835 Potential role of renewable energy in water desalination in Australia J. Renewable Sustainable Energy 4, 013108 (2012); 10.1063/1.3682060 Industrial effluent treatment: Theoretical and experimental analysis J. Renewable Sustainable Energy 3, 013107 (2011); 10.1063/1.3558862 SolarPowered Desalination: A Modelling and Experimental Study AIP Conf. Proc. 941, 249 (2007); 10.1063/1.2806091 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21Sustainable low temperature desalination: A case forrenewable energyVeera Gnaneswar Gude,1,a) Nagamany Nirmalakhandan,2 andShuguang Deng11Chemical Engineering Department, New Mexico State University,Las Cruces, New Mexico 88001, USA2Civil Engineering Department, New Mexico State University, Las Cruces,New Mexico 88001, USA(Received 9 August 2010; accepted 10 June 2011; published online 27 July 2011)In this paper, different configurations for running a low temperature desalinationprocess at a production capacity of 100 liters=day are presented. Renewable energysources such as solar and geothermal energy sources are evaluated as renewable,reliable, and suitable energy sources for driving the low temperature desalinationprocess round the clock. A case study is presented to evaluate the feasibility ofsustainable recovery of potable water from the effluent streams of wastewatertreatment plant. Results obtained from theoretical and experimental studiesdemonstrate that the low temperature desalination unit has the potential for largescale applications using renewable energy sources to produce freshwater in asustainable manner. The following renewable energy=waste heat recoveryconfigurations may produce around 100 liters=day of desalinated water: (1) solarcollector area of 18 m2 with a thermal energy storage (TES) volume of 3 m3; (2)photovoltaic thermal collector area of 30 m2 to provide 1418 kW electricity and120 liters=day freshwater with an optimum mass flow rate of the circulating fluidaround 4050 kg=h m2; (3) A geothermal source at 60 C with a flow rate of 320kg=h; and (4) waste heat rejected from the condenser of an absorption refrigerationsystem rated at 3.25 kW (0.95 tons refrigeration), supported by 25 m2 solarcollector area and 10 m3 TES volume. Additionally, the secondary effluent of localwastewater treatment plant was processed to recover potable quality water.Experimental results showed that >95% of all the water contaminants such asbiological oxygen demand (BOD), total dissolved solids (TDS), total suspendedsolids (TSS), ammonia, chlorides, nitrates, and coliform bacteria can be removedto provide clean water for many beneficial uses. VC 2011 American Institute ofPhysics. [doi:10.1063/1.3608910]I. INTRODUCTIONIn many parts of the world, desalination has become an imperative and inevitable solutionto overcome the shortage of potable water. Current desalination technologies are based on ther-mal evaporation or membrane separation principles. Thermal desalination technologies requirelarge quantities of energy and fossil fuels have traditionally been used to provide the energyrequirements for desalination of seawater or brackish waters. The idea of utilizing the fossilfuels to produce freshwater through desalination processes is not a sustainable approach anymore due to the rapid decline in these resources and resultant high fuel costs and negative envi-ronmental impacts. In an effort to conserve the depleting natural fossil fuel resources, desalina-tion industry has been adopting several energy-saving measures in recent years. Examplesa)Author to whom correspondence should be addressed. Present address: Civil Engineering Department, Oregon Instituteof Technology, 3201 Campus Drive, Klamath Falls, Oregon 97601, USA. Electronic mail: gudevg@gmail.com. Tel.:1(530) 751 6061. FAX: 1(575) 646 7706.1941-7012/2011/3(4)/043108/25/$30.00 VC 2011 American Institute of Physics3, 043108-1JOURNAL OF RENEWABLE AND SUSTAINABLE ENERGY 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21include recovery and recycling of energy as in the case of staging, low temperature desalina-tion, and utilization of waste heat or renewable energy.Various renewable energy resources are available that suit the energy needs of differentdesalination processes. Currently, the resources which are well explored and exploited fordesalination applications include solar energy (harvested by solar collectors or photovoltaicmodules), wind energy, geothermal energy, and wave energy.1 Sustainable use of these resour-ces depends on the conversion technology employed and the end-user process configuration.Sustainability, in this context, can be interpreted as how the available energy resource is beingutilized. Conserving, recycling and increasing the efficiency of the conversion technologies aresome approaches which result in a sustainable use of an energy resource. For instance, daily so-lar energy available on the surface of the earth is roughly 15 000 times greater than the dailyenergy consumption of the world population,2 which means that only much less than 1% of thedaily solar energy captured in energy devices could solve the energy problems of the world.However, the maximum energy conversion rate of the current photovoltaic modules in the mar-ket has not exceeded 15% to date. This indicates that the energy harvested through theseresources, though freely available, still have a very high value and need to be utilized effi-ciently or utilized in a sustainable manner.One way to utilize the renewable energy sources more efficiently is by coupling with anenergy-efficient desalination process. Thermal desalination processes operating at higher tem-peratures such as multi-stage flash distillation (MSF), multi-effect distillation (MED) technologyrequire high quality heat sources at higher temperatures and result in higher fugitive losses andconsumption of prime non-renewable energy sources. On the other hand, low temperaturedesalination processes have lower specific energy requirements and a higher thermodynamic ef-ficiency. Apart from the above, other advantages include lower corrosion rates, low-cost materi-als of construction with a longer plant life, lower scaling, lower heat losses, and shorter start-upperiods. The motive energy for driving the low temperature processes can be provided by lowgrade heat sources (renewable energy) or process waste heat rejections, so that better economiesof the overall processes can be achieved.35In this research, a new low temperature desalination process has been developed which canutilize low grade heat sources such as waste heat releases, solar, photovoltaic=thermal(PV=Thermal), and geothermal energy sources. Since the process operates at lower tempera-tures, energy losses and, hence, the energy requirements for desalination are reduced. As thisprocess utilizes renewable energy and waste heat releases, it does not directly contribute to anygreenhouse gas emissions and can be considered a sustainable process. Results obtained fromtheoretical modelling studies and experimental studies are presented in this paper to demon-strate the viability of the proposed desalination process. Different configurations in which theproposed process can be driven using different energy sources at a desalination productioncapacity of 100 liters=day and the energy requirements are discussed. This paper focuses ontheoretical development of the low temperature desalination system using different renewableenergy sources with a limited analysis of experimental results.A. Description of the desalination systemThe premise of the proposed system can be explained by considering two barometric col-umns at ambient temperature, one filled with freshwater and the other with saline water asshown in Fig. 1. The barometric columns contain the head equivalent to local atmospheric pres-sure and when closed, a vacuum will be created in the headspace by the amount of the fluidvolume displaced by gravity. Due to the natural vacuum generated by this process, the headspace of these two columns will be occupied by the vapors of the respective fluids at their re-spective vapor pressures. If the two head spaces are connected to one another, water vapor willdistill spontaneously from the freshwater column into the saline water column, because thevapor pressure of freshwater is slightly higher than that of saline water at ambient temperature.However, if the temperature of the saline water column is maintained slightly higher than thatof the fresh water column to raise the vapor pressure of the feed water side above that of the043108-2 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21fresh water side, water vapor from the saline water column will distill into the fresh water col-umn. A temperature differential of about 1015 C is adequate to overcome the vapor pressuredifferential to drive this desalination process. Such low temperature differentials can beachieved using low grade heat sources such as solar energy, process waste heat, thermal energystorage (TES) systems, etc.A schematic arrangement of the low temperature desalination system based on the aboveprinciples is shown in Fig. 2. Components of this unit include an evaporation chamber (EC), anatural draft condenser, two heat exchangers, and three barometric columns. These three col-umns serve as the saline water column, the brine withdrawal column, and the desalinated watercolumn, each with its own holding tank, SWT (seawater tank), BT (brine tank), and DWT(desalinated water tank), respectively. The brine tank holds the concentrate removed from theevaporation chamber to maintain the salt concentration in the evaporation chamber. TheFIG. 1. Physical principle of the low temperature desalination system.FIG. 2. Low temperature desalination system powered by renewable and waste heat sources.043108-3 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21evaporation chamber can be designed to use direct solar energy (with glass top exposed to solarradiation) and waste heat sources (as shown in Fig. 2).The EC is installed atop the three columns at a height of about 10 m above ground levelto create vacuum naturally in the headspaces of the feed, withdrawal, and desalinated watercolumns. This configuration drives the desalination process without any mechanical pumping.6The saline water enters the evaporation unit through a tube-in-tube heat exchanger.1,2 Thetemperature of the head space of the saline water column is maintained slightly higher thanthat of the desalinated water column. Since the head spaces are at near-vacuum level pres-sures, a temperature differential of 10 C is adequate to evaporate water from the saline waterside and condense in the distilled water side.35 In this manner, saline water can be desali-nated at about 4050 C, which is in contrast to the 60100 C range in traditional solar stills(SSs) and other distillation processes. This configuration enables the brine to be withdrawncontinuously from the EC through heat exchanger 1 (HE1), preheating the saline water feedentering the EC.6,7 Further, by maintaining constant levels of inflow and outflow rates inSWT, BT, and DWT, the system can function without any energy input for fluid transfer inthe desalination system. The heat input to EC is provided by TES tank through a heatexchanger 2 (HE2) which in turn is fed by a low grade waste heat or renewable energysource. Different heat sources evaluated in this study are solar collectors, photovoltaic thermalcollectors, geothermal energy sources, and process waste heat. Experimental results obtainedfor a configuration using glass top evaporation chamber to utilize direct solar energy werecompared with theoretical results obtained in Sec. III. A closed top evaporation chamber wasalso tested for recovering potable quality water from the secondary effluent of the local waste-water treatment plant.B. Theoretical analysis of the desalination systemMass and energy balances around the EC yield the following coupled differential equations,where the subscripts refer to the state points shown in Fig. 2. The variables are defined in theAppendix.Mass balance on volume of water in EC,ddtqV m2 m6 m3: (1)Mass balance on solute in EC,ddtqVCEC m2C2 m6C6: (2)Energy balance for volume of water in EC,ddtqVcpTEC QEC mcpT2 mcpT6 m3hLT Ql; (3)where QEC is the rate of energy input (load on the TES) to the EC and Ql is the rate of energyloss from the EC. The energy input, QEC, to the evaporation chamber can be supplied by thesolar collectors, photovoltaic thermal collectors, geothermal water sources, and process wasteheat sources to the TES as discussed in Sec. I A and is written asQEC mscsTs TEC; (4)where, ms, cs, and Ts, are the mass flow rate, specific heat, and temperature, respectively, of thewater from the TES and TEC is the temperature of the saline water in the evaporation chamber.Theoretical expressions for the different heat sources are discussed next.Desalination efficiency is defined as043108-4 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21g MhLTRQECDt ; (5)wherehLT 3; 146 2:36T 273: (6)Evaporation rate as a function of pressure and temperature,8m3 AECk fCECpTECTEC 2730:5 pT5T5 2730:5" #; (7)wherepT exp63:02 7139:6=T 273 6:2558 lnT 273 102Pa: (8)The above coupled equations are solved using Extend (Imagine That Inc.) and EngineeringEquation Solver (EES) simulation software. Details of heat transfer relations for evaporationchamber and condensation surface and heat losses by convection and radiation are presentedelsewhere.7,8 Model parameters for the low temperature desalination process are presented inTable I.II. THEORETICAL STUDIESIn this section, theoretical analyses for different energy sources and the results from themodeling studies are presented. The expressions for different energy sources can be substitutedin the overall energy balance (Eq. (3)) to generate the simulations. Schematics for different con-figurations are shown in Fig. 3.A. Solar collectorsFlat plate solar collectors supplying low grade heat in the range of 5070 C can be usedto drive the proposed desalination system during sunlight hours (Fig. 3(a)). The sensible heatstored in the TES will provide the heat source to the evaporation chamber during non sunlighthours.Energy balance across the solar panel can be written asdmcTscdt FRACsaIs ULTSC Ta QS; (9)where Qs is the solar energy harvested by the solar collectors and stored in the TES tank and isgiven asQs mRcRTSC Ts; (10)TABLE I. Model parameters for the low temperature desalination system.Parameter Value Parameter ValueEC area m2 15 Solar insolation W=m2 200-1000Condenser area m2 15 Seawater concentration % 3.5Water depth in the EC m 0.05-0.1 Seawater density kg=m3 1020Height of EC m 0.5 Seawater, TES reference Temperature C 25TES volume m3 1 Ambient temperature C 3 to 35043108-5 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21where mR and cR are the mass flow rate of the collector fluid, Tsc is the temperature of thewater exiting the solar collector, and Ts is the temperature of TES tank. The energy balance onthe TES can be written as8ddtMcTs Qs QEC Qls; (11)FIG. 3. Energy balance on different heat sources: (a) solar collectors, (b) photovoltaic thermal collectors, (c) geothermalsource, and (d) process waste heat.043108-6 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21where Ms (q) is the total mass of water in the TES, cs, and Ts are as defined earlier, and Qs isthe thermal energy supplied by the solar collectors. Actual energy supplied from the TES tankto the evaporation chamber, QEC can be calculated using Eq. (4). Qls are the energy losses fromthe TES.1. Performance of the low temperature desalination systemTemperature profiles of the desalination system driven by the solar collectors are shown inFig. 4(a) for both evaporation and solar collector areas of 1 m2. The TES temperatures reach amaximum value of 57 C during sunlight hours and evaporation temperatures reach a maximumvalue of 45 C. The maximum ambient temperature is 34 C. The energy supplied from theTES and the energy utilized for evaporation are shown in Fig. 4(b). Hourly freshwater produc-tion rates and cumulative product are shown in Fig. 4(c). A daily product of 6.5-7 liters can beproduced for 1 m2 evaporator and 1 m2 solar collector areas. These results are compared tothose reported previously.6 The evaporation efficiency of the process ranged between 60% and90%, most of the time in the range 75%-90% as shown in Fig. 4(d). The TES volume used inthis simulation was 0.1 m3. Hourly and daily freshwater production rates are shown for a solarcollector area of 18 m2 and a TES volume of 3 m3 in Fig. 5. As it can be seen from Fig. 5, thefreshwater production rates for this case start to stabilize after 72 h of operation. The freshwaterproduction rate was lower during initial hours as some of the energy supplied from the solarcollectors is utilized to increase the sensible heat of the total volume of the water in TES. Thestabilized freshwater production rate for this configuration is 104 liters=day.2. Use of Thermal Energy Storage SystemIn this configuration, the need for thermal energy storage tank was evaluated through simu-lations. Fig. 6 shows the TES performance for two different volumes (1 m3 and 6 m3) over 168h (7 days). The TES temperatures fluctuate in relation with the daily solar insolation and ambi-ent temperatures in both cases. TES temperatures for a TES volume of 1 m3 respond quickly tothe changes in the solar insolation and ambient temperatures with very little sensible heat avail-able for desalination during nonsunlight hours. Temperatures during sunlight hours reach ashigh as 6568 C and fall down to as low as 3035 C during nonsunlight hours. TESFIG. 4. Low temperature desalination system driven by solar energy; (a) temperature profiles, (b) energy utilization, (c)daily production rates, and (d) desalination efficiency.043108-7 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21temperatures for a TES volume of 6 m3 respond slowly to the daily solar insolation, and ambi-ent temperatures increasing sensible heat of the bulk of the water in the tank. For this tank vol-ume, the TES water temperature during nonsunlight hours was around 4548 C, which is stilla good heat source for evaporation during nonsunlight hours enabling 24 h operation. The max-imum and minimum TES temperatures are shown in Fig. 7 for different TES volumes in therange 16 m3. From Fig. 7, it can be observed that as the TES volume increases the heat sourceavailable for nonsunlight hour operation increases. Relation between the solar collector areas inFIG. 5. Low temperature desalination system driven by solar collectors round the clock (TES volume: 3 m3, solar collectorarea: 18 m2).FIG. 6. TES performance over 7 days (168 h).043108-8 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21connection with the TES volumes is presented in Fig. 8. For higher TES volumes, it is obviousthat the solar collector area requirement is high for the freshwater production rate of around100 liters=day. It is because sensible heat losses to the ambient from the TES tank are to beprovided by the same collector area. Therefore, the collector area required for 1 m3 TES vol-ume is 15 m2, which is increased by 25% and 40% for TES volumes of 3 m3 and 6 m3, respec-tively. Hourly and daily freshwater production rates for a fixed solar collector area of 15 m2over 21 days of operation are shown in Fig. 9. At the end of 7 days of operation, the averagedaily freshwater production for TES volume of 1 m3 remained at 100 litres=day and for TESvolume of 6 m3 at 68 liters=day. It should be noted that the average freshwater production ratescontinue to increase for the TES volume of 6 m3 and reach 86 liters=day at the end of 21 daysof operation. From these simulations, the maximum TES temperatures for TES volume of 1 m3remained at 68 C, whereas for TES volume of 6 m3, the temperatures increased from 53.8 Cto 54.5 C at the end of 21 days of operation. This indicates that the energy stored in the TESis available for longer periods of time enabling continuous and stable freshwater productionFIG. 7. Effect of TES volume on TES and EC temperature profiles.FIG. 8. TES volume effect on solar collector area and freshwater production rate.043108-9 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21rates. Therefore, to determine the optimum solar collector areas, long term performance of theTES needs to be considered. The configuration with small TES volumes will suffer from thechanges in daily solar insolation and ambient temperatures. High TES volume may initiallyresult in lower freshwater production rates but with continued operation the productivity will beincreased. For small TES volumes, the freshwater production rates cease during nonsunlighthours leaving the unit idle for 33%50% of the day. If the high storage volume of TES is aconstraint, freshwater has to be stored in water tank for rest of the day, cloudy, and rainy dayneeds as well. Continuous operation allows for downsizing the desalination unit and reducesthe equipment cost. Batch operation requires a large evaporation area and equipment withhigher costs. Continuous process mode is easily adaptable to other low grade waste heat sourcesand can be scaled to large applications to provide freshwater for small rural communities.3. Cloudy Day Effect on Thermal Energy Storage SystemThe effect of cloudy days on different TES volumes was investigated. One cloudy day perweek was considered over 3 weeks (21 days). As can be seen from Fig. 10, the average dailyproduction over 21 days of operation decreases from 100 liters=day to 92 liters=day (by 8 liter-s=day) for TES volume of 1 m3 with a solar collector area of 15 m2. For the same collectorarea, the average daily production over 21 days decreased from 93 to 87 (by 6 liters=day) and86 to 81 liters=day (by 5 liters=day), respectively, for TES of 3 m3 and 6 m3 volumes. Consid-ering the performance on the cloudy days alone, the reduction in the daily product amount was100 to 63, 93 to 67, and 86 to 75 liters=day, which are 37%, 28%, and 13% reductions asshown in Fig. 10. The hourly freshwater production rates decreased from 7.5 to 4.6, 5.1 to 3.5,and 4.5 to 3.5 liters=h for TES volumes of 1, 3, and 6 m3, respectively, the smallest variationbeing observed for 6 m3. Similar comparison for different TES volumes with a solar collectorarea of 18 m2 was done over 21 days of operation. Based on this analysis, the daily productionincreased for TES volume of 6 m3 from 75 liters=day (15 m2 solar collector area) to 88 liter-s=day (18 m2 solar collector area). The hourly production rates also stabilized with this configu-ration. As shown in Fig. 11, the solar collector area requirements based on 21 days operationare 6% and 13% only compared to 7 days operational performance analysis, 25% and 40% forTES volumes of 3 and 6 m3, respectively. Therefore, based on the above analysis, it is clearthat the TES performance has to be evaluated on a long term performance basis and theFIG. 9. TES volume effect on daily production rates and cumulative product over 21 days.043108-10 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21advantage of the TES is recognized when long term operations are considered. The processconditions are more stable with variations in energy supply and demand trends. The need forthe TES, however, depends on the type, scale, and economics of a particular application.B. PV=Thermal collectorsPhotovoltaic thermal collectors have the ability to produce both electrical and thermalenergy from solar energy. The overall efficiency of the PV=Thermal collectors is higher thanthe sum of the efficiencies of separate photovoltaic modules and solar thermal collectors.9 Ther-mal energy produced from PV=Thermal collectors is suitable for low temperature desalinationby the proposed process while electricity produced can be used for domestic uses (Fig. 3(b)).Theoretical analysis for the desalination system utilizing photovoltaic thermal energy is asfollows.The energy balance on the PV=Thermal collector and the absorber plate can be written asfollows10:FIG. 11. TES volume effect on solar collector area and freshwater production rate based on 21 days performance.FIG. 10. TES volume effect on daily production rates and cumulative product over 21 days with cloudy day effects.043108-11 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21MpCpdTPVTdt Qsp Qlp Pe Qu; (12)where Mp and Cp are the mass and specific heat capacity of the absorber plate, respectively,Qsp is the incident solar energy, Qlp are the losses from the PV=Thermal collector, Pe is theelectrical energy derived from the module, and Qu is the useful energy (thermal) extracted bythe collector fluid.Solar energy absorbed by the PV=Thermal panel material is given asQsp Issgap: (13)Heat losses through radiation and convection from the PV=Thermal absorber plate to the glasscover can be written as10Qlp eaegrfT4p T4gg hpgTp Tg: (14)Electrical energy generated by PV=Thermal collector system is given as11Pe IssgFcgstdf1 0:005Tp 298:15g: (15)Useful thermal energy derived from PV=Thermal collectors can be expressed asQu mCpf Tf Ti; (16)where Tf is the collector fluid exit temperature and Ti is the collector fluid inlet temperature. mand Cpf are the mass flow rate and specific heat capacity, respectively, of the collector fluidwhich is water in this case.The circulating fluid exit temperature, Tf, can be calculated as follows:10Tf TPVT Ti 1 exp 4 x=dNuRe: Pr Ti; (17)where TPVT is the absorber plate temperature, x and d are the length and diameter of the circu-lating fluid tube, respectively.The heat input to the EC is the useful heat extracted from the PV=Thermal collectors andstored in thermal energy storage tank and is given bydvqcTsdt Qu QEC Qlo; (18)where v is the volume of the storage tank, Cps is the specific heat of the water in the storagetank, and Ts is the storage tank temperature. QEC is the heat supplied to the EC and Qlo is theenergy losses from the storage tank. Actual amount of heat supplied to the EC from TES canbe obtained by using Eq. (4).Thermal and electrical efficiencies of the PV=Thermal collector at a given time are asfollows:12Thermal efficiency of the PV=Thermal collector gPVT;th mCpf Tf TiIAc; (19)Total PV=Thermal collector efficiency gPVT mCpf Tf Ti PeIAc; (20)where Ac is the collector area.043108-12 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:211. Integrated PV=Thermal-desalination systemNumerical simulations have been performed for a site in southern New Mexico. Parametersused in the numerical simulations are presented in Table II for the PV=Thermal collector sys-tem. The solar insolation and ambient temperatures on a summer day in June varied between2251000 W=m2 and 1633 C, respectively. The temperature profiles for the photovoltaic ther-mal system are shown in Fig. 12. The absorber plate temperatures reached up to 68.7 C, whilethe collector fluid temperature reached a maximum value of 64.5 C in the middle of the day.The glass cover was in contact with the ambient air and reached a maximum temperature of43.1 C, while the maximum ambient temperature was 33 C. In this simulation, the mass flowrate of the circulating fluid was 40 kg=h m2. The volume of the TES was 1 m3 with a volumeto collector area ratio of around 40 litres=m2.Thermal and electrical energy production rates and their efficiencies during sunlight hoursare shown in Fig. 13. Thermal energy is the useful energy extracted by the circulating fluid inthe PV=Thermal collector. Total PV=Thermal system efficiency of 59.4% and thermal effi-ciency of nearly 50.5% are predicted. These results are comparable with the previous stud-ies.13,14 The primary energy saving efficiency of PV=Thermal system is predicted as 72.7%,which considers the total energy that will otherwise be required to generate the electricity by aconventional power plant. A photovoltaic collector (Sharp NT-S5E1U, cell efficiency 17.5%,and module efficiency 14.2%) was used in these simulations. It is well known that the photo-voltaic cell efficiency would decrease with increasing absorber plate temperature. The electricalefficiency of the PV=Thermal system varied between 12.5% and 8.5% during the sunlighthours.2. Low Temperature Desalination System Driven by PV=Thermal SystemUseful energy extracted from the PV=Thermal system was used as the heat source to drivethe desalination system. The mass flow rate of the circulating fluid between the TES and theevaporation chamber was fixed at 60 kg=h m2 in these simulations. The resulting evaporationtemperatures in the desalination system and freshwater and ambient temperatures are shown inFig. 14. The maximum saline water temperature in the evaporation chamber is predicted as51.2 C at the maximum TES and ambient temperatures 57.2 C and 33 C, respectively. Thesaline water temperature decreased with the collector fluid temperature and eventually reachedambient temperatures during non-sunlight hours. The useful energy supplied to the evaporationchamber is utilized for evaporation, with energy losses ranging from 10% to 20% of the totaluseful energy supplied, resulting in 80%90% evaporation efficiencies. The hourly freshwaterproduction rate is shown in Fig. 15. As expected, the evaporation rate increased with increasein the heat source temperature and a maximum evaporation rate of 15 liters=h is obtained. Thecumulative amount of freshwater produced from the desalination system under the specific con-ditions was 120 liters. The optimum PV=Thermal collector area for this application was foundto be 30 m2 which can also provide 1418 kW h of electricity needs for a household.15 Themechanical energy required to circulate the collector fluid is calculated as 4 kJ=kg of freshwaterproduced.TABLE II. Model parameters for photovoltaic thermal collector system.Parameter Value Parameter ValueCell material Mono-Si Material of absorber aluminumPV=T module area (m2) variable Absorption factor, PV cell 0.9Total PV=T module area (m2) 2030 Radiation factors, cover glass-eg, and absorber ea 0.05, 0.5Cell efficiency (%)a) 17.5 Absorption factor, cover glass-eg, and absorber ea 0.05, 0.16Coefficient of temperature inversion (%=C) 0.5 Collector fluid (kg=h=m2) Water, 180a)At Ta 25 C, Is 1000 W=m2.043108-13 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21FIG. 12. Temperature profiles of the PV=Thermal collector system.FIG. 13. Energy and efficiency profiles of the PV=Thermal collector system.FIG. 14. Temperature profiles of the integrated PV=Thermal-low temperature desalination system.043108-14 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21C. Geothermal energyGeothermal energy sources deliver an energy quantity of 160200 kW h=m2 annuallywhich is much higher than photovoltaic (50100 kW h=m2), biomass (1545 kW h=m2), andwind energy resources (1118 kW h=m2).16 Low grade geothermal source with temperatureof about 60 C can be used directly to heat the saline water or to maintain a thermal energystorage tank which can then provide the energy needs to the evaporation chamber(Fig. 3(c)). The amount of thermal energy supplied by the geothermal source, QG, can bequantified asQG mGcpgTgi Tgo; (21)where mG and cpg are the mass flow rate of the geothermal water and Tgi and Tgo are the inletand outlet temperatures of the geothermal water, respectively. Saline geothermal energy sourcescan be used both as feed (saline water) and heat source.1. Geothermal Energy RequirementsGeothermal source flow rate for a known freshwater production rate can be estimated usingthe following energy balance:QG mGcpgTgi Tgo mefcpeTw Ti hLTwg; (22)mGmeg R; (23)where, me is the desired evaporation rate (freshwater production rate, litres=day), cpe is the spe-cific heat of the water, Tw is the evaporation temperature of the brackish water, Ti is the inlettemperature of the brackish water, hL is the latent heat of the brackish water at evaporationtemperature, and g is thermal efficiency of the desalination system. R is the ratio of geothermalsource to the mass of freshwater to be evaporated. Based on Eq. (23), for a fixed evaporationrate of 100 liters=day, the energy requirements and geothermal water flow rates were calculated(Fig. 16). As expected, the energy and flow requirements increase with the geothermal watertemperatures. This also indicates that when the heat resource is limited, low temperature opera-tion can provide the benefits of higher thermodynamic efficiency and higher freshwater produc-tion rates.4FIG. 15. Evaporation rate and cumulative product of the low temperature desalination system.043108-15 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:212. Performance of the Low Temperature Desalination SystemTemperature profiles for the low temperature desalination system driven by a geothermalsource at 60 C are shown in Fig. 17. The saline water temperatures in the evaporation chamberare slightly higher during the sunlight hours due to higher ambient temperatures, and the tem-perature gradient available between the evaporator and condenser is lower during the daytimeresulting in lower evaporation rates. Temperature gradients between the evaporator and con-denser are higher during the nonsunlight hours which favor the convection and condensation ofwater vapors from the evaporator side to the condenser. From the simulations, it was observedthat the freshwater production rate was around 3.9 liters=h during sunlight hours whereas it wasaround 4.8 liters=h during nonsunlight hours. Fig. 17 also shows the temperature profiles forfluids flowing in and out of the HE1. Saline water enters (cold in) the tube-in-tube heatexchanger at 25 C and exits at 36 C before entering the EC. The brine in (saline water fromEC) temperature is same as the saline water temperature in the EC and is about 51 C and exitsHE1 at 30 C. Thus, the heat exchanger operates in the range 75%85% of thermal efficiencypreheating the saline water entering the EC.A geothermal water flow rate of 320 kg=h has been considered for numerical simulationsin Fig. 18. Theoretical simulations show that this system can produce up to 100 liters=day ofdesalinated water with an average evaporation rate of 4.5 liters=h at a geothermal water temper-ature of 60 C. The evaporation rates for geothermal temperatures at 50 C, 70 C, and 80 Care 3 liters=h, 6.5 liters=h, and 8.6 liters=h, respectively. Geothermal sources have potential forlarge scale application of the low temperature desalination system. High temperature geother-mal waters (80100 C) are suitable for multi-effect low temperature desalination process toprovide freshwater for small rural communities. They provide continuous source of water andqualify as a new source of water as the feed itself can be the geothermal water or depending onthe availability of brackish water=seawater sources.D. Waste heat sourcesA general scheme for low temperature desalination system utilizing the process waste heatis shown in Fig. 3(d). Examples of process waste heat include reject heat from the condenser ofdomestic air-conditioning system, exhaust gases from diesel generators, and circulating coolingwater jacket type reactors. The feasibility of the proposed system using a TES system storingthe waste heat rejected by a Li-Br absorption refrigeration system (ARS) has been simulated.8,17A schematic of this configuration is shown in Fig. 19. In this configuration, the EC area was 5m2 and the TES was sized to maintain its temperature at 50 C. An ARS system rated at 3.25kW (0.975 tons of refrigeration) along with an additional energy input of 208 kJ=kg of desali-nated water was adequate to produce desalinated water at an average rate of 4.5 kg=h. On aFIG. 16. Energy and flow rate requirements for the low temperature desalination by geothermal energy043108-16 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21typical day, the heat demand by the EC (including heat loss) on the TES varied from 8700 to14 200 kJ=h over a 24-h period; a TES tank volume of 10 m3 was found to be adequate tomaintain its temperature at 50 C to provide the energy needs of the EC. The operating charac-teristics of the ARS system were as follows: absorber temperature 28 C, condensertemperature 55 C, evaporator temperature 12 C, generator temperature 100 C,FIG. 17. Low temperature desalination system driven by geothermal energy; (a) EC temperature profiles and (b) HE1 tem-perature profiles.FIG. 18. Low temperature desalination system driven by geothermal energy; (a) daily and (b) hourly production rates.043108-17 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21condenser=generator pressure 15.75 kPa, absorber=evaporator pressure 1.403 kPa, and coef-ficient of performance 0.72.In this configuration, the ARS was powered by the solar collectors and augmented by anauxiliary electric heater. The solar collector system was sized to maintain the generator of theARS at 100 C by maintaining the storage tank of the solar panel at 110 C. The energy pro-vided by the auxiliary heater is equal to the difference between the energy required by thegenerator and that can be collected from the solar insolation. The energy contributed by solarcollectors from the solar energy is called the solar fraction. For this design, the solar fractionwas 0.4 (40%). The optimum area of the solar collectors for this application was found to be25 m2. Thermal energy supplied or transferred between the condenser of ARS system and theTES tank and the heat transfer between TES tank and desalination system depend on theavailable temperature gradients and ambient temperature conditions. The TES volume wasdetermined to be 10 m3 to maintain temperature of the TES water medium within 60.01 Cduring winter conditions. This unit can be designed to drive the absorption refrigeration unitround the clock with the solar energy harvested by solar collectors (by increasing the solarfraction).Results from the modeling studies discussed above show that the proposed process has thepotential to be driven solely by renewable energy sources or waste heat releases and can beoperated on a continuous basis with moderate yields. More details of the theoretical modelingresults are presented elsewhere.7E. Specific energy requirementsSpecific energy required to produce 1 kg of freshwater for the four energy sources areshown in Table III. Specific energy requirements include the heat energy used for evaporationand mechanical energy for pumping the heat source. This table suggests that the above configu-rations have the potential to produce freshwater either on batch or continuous modes of opera-tion. The specific energy requirement is also dependent on the mode of operation. The ARSconfiguration is the most suitable for domestic applications, since free energy is available fromthe ARS condenser and the specific energy requirements are much lower than the remainingconfigurations.7FIG. 19. Low temperature desalination system driven by waste heat rejected by ARS system.043108-18 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21III. EXPERIMENTALA. Direct solar energyIn this section, the theoretical modeling results are compared with experimental results forthe system using direct solar energy. The top of the evaporation chamber was exposed to theincident solar energy to cause evaporation in the evaporation chamber, thus, the system can becalled solar still under vacuum (SSV) as shown in Fig. 20. Experimental results obtained forother two configurations (solar still under vacuum with a reflector (SSR) and external photovol-taic energy (SSPV)) are also presented. All of the results are compared with the traditional solarstill configuration to illustrate the benefits of low temperature desalination unit. The details oftheoretical modeling and experimental results are presented elsewhere.71. Temperature and Freshwater Production ProfilesThe experimental studies were conducted in summer at engineering research facility in LasCruces, USA. The solar insolation varied between 4001100 W=m2 while the ambient tempera-tures ranged 1535 C during summer. The maximum ambient temperature recorded was 35 C,and the maximum temperature of the brackish water in the EC was 52.75 C. The predictedFIG. 20. Photo of experimental desalination system driven by direct solar=photovoltaic energy.TABLE III. Specific energy requirements for the low temperature desalination process using different sources of energy.Energy source Mode of operationThermal energyrequired (kJ=kg)Mechanical energyrequired (kJ=kg)Total Energyrequired (kJ=kg)Solar collectors Batch=continuous 3118 4.1 3122.1PV=Thermal collectors Batch=continuous 3118 4 3122Geothermal source Continuous 2934 144 3078ARS configuration Continuous 194 14 208043108-19 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21maximum temperature was 52 C as shown in Fig. 21(a). The correlation between the predictedand measured EC temperature was satisfactory with r2 0.943, F 2358.2, and p< 0.001. As acomparison, the maximum saline water temperatures measured for different configurations wereas follows: low temperature desalination process using direct solar energy (solar still configura-tion, SSV)50 C, low temperature desalination process using direct solar energy fitted with anexternal reflector (solar still configuration, SSVR)53 C, and low temperature desalinationprocess using solar energy as well as photovoltaic energy (SSPV)55 C, respectively, asshown in Fig. 21(b). These temperatures are lower than those commonly reported for solar stillswhich are in the range of 6075 C.18,19The predicted distillate volume during the above test is compared against the measured dis-tillate volume in Fig. 21(c). Cumulative volume predicted by the model for a 24-h period was5.25 liters=day m2 while the measured value was 4.95 liters=day m2. The difference (of 5.5%)in the cumulative distillate volume is mainly due to the assumption that the entire volume ofthe vapor distilled on the freshwater side, whereas during the test it was observed that some ofthe vapor condensed on the roof of the evaporator and trickled back to the saline water. Corre-lation between the predicted and measured distillate volume as a function of time was strongwith r2 0.988, F 11,839.4, and p< 0.001.Daily freshwater production rates for different configurations are shown in Fig. 21(d). Thelow temperature desalination process as a SSV produces freshwater of about 5 liters=d m2,nearly 1.5 to 2 times that of typical solar stills.18,19 This improvement can be attributed to thereduction in energy losses by the low temperature desalination process. The near-vacuum pres-sures created by natural means of gravity and barometric head allow for the evaporation offreshwater to occur at low temperatures resulting in higher energy efficiency. This configura-tion, when fitted with a reflector (SSVR), produced about 7.58 liters=day of distillate which isthree times that of typical solar still. As the solar insolation incident on the solar still was inten-sified by the reflector, the saline water temperatures rose quickly resulting in evaporation offreshwater as shown in Fig. 21(d).Low temperature desalination process powered by photovoltaic energy (SSPV) producedover 12 l=day when fitted with a reflector. Photovoltaic area required for this configuration was6 m2. Photovoltaic energy generated during the day is sufficient to produce freshwater of 45liters=day during the night time. The efficiency of the PV modules is 14.2%. The process canFIG. 21. Saline water temperature and distillate production profiles for different configurations.043108-20 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21be designed to operate round the clock with addition of an external heat source (during non-sunlight hours) as in the case of solar collectors. Although, this configuration (SSPV) may notprove economical, it could be beneficial in arid areas where the need for freshwater and energyare highly pronounced.2. Thermal Efficiency and Specific Energy ConsumptionTraditional solar stills have a thermal efficiency around 30% and rarely exceed 45%.18,19Normal solar still (SS) operating with an efficiency of 45%, requires 5040 kJ of thermal energyper kg of freshwater produced. The proposed process (SSV) operates at higher thermal efficien-cies with a specific energy consumption of 3900 kJ=kg of freshwater. While the measured ther-mal efficiency of SSV configuration was 61%, that of the configuration with a reflector (SSVR)was 75% with a specific energy consumption of 3200 kJ=kg. The specific energy required forthe configuration with photovoltaic energy (SSPV) is only 28002900 kJ=kg of freshwater withthermal efficiencies ranging between 80% and 90%. In the case of traditional solar stills andSSV, major energy losses occur through the glass cover during sunlight hours. However, forSSPV, the glass cover can be covered with insulation during non-sunlight hours to reduce theenergy losses through the glass cover. Additionally, lower ambient temperatures during non-sunlight hours favor the convection and condensation of freshwater vapors from the evaporationchamber to the condenser side.4 Specific energy consumption for different operational modesare summarized in Table IV.5B. Recovery of potable water from secondary effluent using low grade waste heat1. Using Low Grade Heat SourceFeasibility of running the low temperature desalination process using a low grade thermalsource was demonstrated. A hot water tank was used as heat source in this configuration.20 Thedesign of the unit was slightly modified in this configuration to integrate the evaporation andcondensation sides of the desalination unit. The condenser plate was arranged at the top of theevaporation chamber to dissipate the heat of condensation to the ambient, thus reducing thefootprint of the unit. During these tests, the circulation rate of the hot water was maintained at9 kg=h, while the temperature of the source was varied between 50 C70 C. Typical tempera-ture profiles recorded over 24 h operation with continuous operation mode are presented inFig. 22. Results from these tests showed that the low temperature desalination system operatedwith higher efficiency at lower evaporation temperatures since the losses to the ambient arereduced. At higher evaporation temperatures, the heat dissipation rate depends on the condensersurface area available and, thus mass of water evaporated. Hourly freshwater production ratesvaried between 7791 ml=h. Thermal energy supplied through the hot water source for tempera-ture range 5070 C varied between 355 and 395 W, while thermal efficiency declined from53% to 47%. The effect of cooling the condenser surface was tested with a small flow rate ofcooling water of 500 ml=h which was collected at the bottom of the condenser. An increase inTABLE IV. Specific energy requirements for the low temperature desalination process by different configurations usingsolar energy.Experiment DescriptionMode ofoperationSpecific energyrequirement (kJ=kg)Case 1 Direct solar energy Batch 3900Case 2 Direct solar energy with a reflector Batch 3118Case 3 Solar energy during sunlight hours, photovoltaicenergy during non-sunlight hoursContinuous 2926Case 4 Solar and photovoltaic energy together Batch 3325043108-21 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21thermal efficiency of 1015% was observed with cooling. Thermal efficiency increased from amaximum value of 53%67% for heat source temperature at 50 C.The energy efficiency can be improved further with adequate insulation between the evapo-rator basin and condenser top, addition of fins on the condenser plate for more efficient heatdissipation and external cooling by recycling the product water. This configuration can bescaled to utilize low grade heat sources for large scale applications. As an illustration, a casestudy for the City of Las Cruces wastewater treatment plant is presented in the next sub-section.The source water contained biochemical oxygen demand (BOD), total dissolved solids (TDS),total suspended solids (TSS), nitrates, nitrites, chlorides, and coliform bacteria. However, theprocess was able to achieve more than 90% reductions for each of the above contaminants asshown in Table V. The process produces high quality distillate with TDS< 50 ppm which issuitable for many non-potable uses.2. Case StudyThe City of Las Cruces treats an average of 10 million gallons per day (MGD) of waste-water. The wastewater treatment plant has anaerobic sludge digester in place to process theFIG. 22. Temperature profiles of the low temperature desalination system driven by low grade heat source.TABLE V. Characteristics of secondary effluent and product water.Low temperature desalinationusing low grade heatParameter Secondary effluent Recovered water % ReductionUSEPA drinkingwater standardBOD (mg=l) 12.7 TDS (mg=l) 783 16 98 500TSS (mg=l) 8 0.3 96.2 Nitrate (as N mg=l) 2.6 biomass. The anaerobic digester produces biogas which can generate up to 350 kW of energyon a daily basis.21 A multi-effect low temperature unit demonstrated in this study with a gainto output ratio (GOR) 5 would require a specific energy consumption of 470 kJ=kg of pota-ble-quality water produced. A total volume of 17 000 gal=day of freshwater can be producedutilizing the energy generated by the biogas. This freshwater can be used for process coolingoperations, plant maintenance, or cooling and heating applications saving the water and heatingbills for the wastewater treatment plant or can be sold to other industrial or irrigationapplications.IV. CONCLUSIONSModeling and experimental results are presented to show that the proposed low temperaturedesalination system has potential to utilize renewable energy sources for both small and largescale applications. Sustainable use of these energy sources has been studied both by theoreticaland experimental studies. Conclusions from the studies are as follows:1. Solar collectors can be used with and without thermal energy storage tank. A solar collectorarea of 15 m2 is sufficient to produce 100 liters=day of freshwater from the low temperaturedesalination system. Inclusion of TES requires additional solar collector area; however, theyare beneficial to compensate the temporary effects of clouding and to ensure continuous supplyof freshwater. The configuration without TES requires a water storage tank because if energycannot be stored, water has to be produced when energy is available and stored for future uses.The configuration without TES is more susceptible to changes in the daily solar insolation andambient temperatures.2. Photovoltaic thermal collectors can be successfully employed to provide both the electri-cal and thermal energy needs of a household. The electrical efficiency of the PV=T col-lectors increases with circulating fluid which again provides the heat source to the lowtemperature desalination system. 30 m2 PVT collector area can provide a household with120 liters=day of freshwater and 1418 kW h of electricity for daily needs. This configu-ration is suitable for remote and rural applications where scarcity of both energy andfreshwater is high.3. Geothermal water sources can be utilized to drive the low temperature desalination systemround the clock. Freshwater production rate and required geothermal source flow rate vary withthe available geothermal source temperature. Geothermal sources can be easily applied to largescale applications. A flow rate of 320 kg=h at 60 C was found to be sufficient to drive the lowtemperature desalination system round the clock to produce 100 liters=day. Provision of TES isoptional and needs to be evaluated in connection with the pumping costs and capital costs ofthe TES tank.4. Utilizing low grade waste heat sources is a green approach to desalination. An absorp-tion refrigeration system has been illustrated as an example of potential low grade heatsource. Theoretical simulations show that an ARS system rated at 3.25 kW (0.975 tonsof refrigeration) along with an additional energy input of 208 kJ=kg of desalinated wateris adequate to produce desalinated water at an average rate of 4.5 liters=h. The solar col-lector and TES tank volumes required for this application are 25 m2 and 10 m3,respectively.Experimental results obtained by different test configurations complement the theoreticalanalysis and prove the operational feasibility of the low temperature desalination system. Thelow temperature desalination process was configured in different ways to capture and utilizethe solar energy source to the best during the day. Experimental results prove that the renew-able energy sources are best utilized when they are coupled with energy-efficient desalinationprocesses. Additionally, a sustainable use of waste heat release to drive the low temperaturedesalination system to recover potable quality water from the secondary effluent of localwaste water treatment facility was evaluated with a case study. Theoretical and experimentalresults confirm that the low temperature desalination system is suitable for remote applications043108-23 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21where there is no electrical grid. However, the benefit of utilizing natural vacuum principle tosave mechanical energy needs has to be studied on a large scale basis to validate the processfeasibility.NOMENCLATUREA Area (m2)c Specific heat (kJ=kg K)C Concentration (%)d Diameter of the circulating fluid tube (m)h Latent heat, heat transfer coefficient (kJ=kg)f Concentration factor (-)F Heat removal factor (dimensionless), packing factor (%)I Solar insolation (kJ=h m2)m,M Mass (kg)Nu Nusselt number (-)P Pressure (-)Pr Prandtl number (-)q Evaporation energy (kJ=h)Q Energy or useful energy (kJ=h)Re Reynolds number (-)t Time (h,s)T Temperature (K)U Heat loss=transfer coefficient (kJ=h m2 K)v Volume of the saline water or storage tank (m3)x Length of the circulating fluid route (m)Greeka Absorptivity (-)am Experimental coefficient (107106) (kg K0.5=m2 Pa s)e Radiation factor (-)g Efficiency (%)s Transmitivity of glass (-)q Density (kg=m3)r Stefan-Boltzmann constant (5.7 108) (W=m2 K4)Subscriptsa Ambientc Collector, cell, specific heate Evaporation, electricalEC Evaporation chamberf Fluidgi Geothermal ingo Geothermal outG,g Geothermal, glassi,in Inlet, supplyl,ls,lo,lp,L Latent heat, lossesp,PVT Absorber plate, photovoltaic thermal collectorpg Glass, geothermalpower Efficiency of thermal power plantr,R Recycle, ratio043108-24 Gude, Nirmalakhandan, and Deng J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21s,sc Saline water, supply, surface, solarth Thermalu Useful energy1S. A. Kalogirou, Prog. Energy Combust. Sci. 31, 242 (2005).2T. Szacsvay, P. Hofer-Noser, and M. Posnansky, Desalination 122, 185 (1999).3G. Kronenberg and F. Lokiec, Desalination 136, 189 (2001).4V. G. Gude and N. Nirmalakhandan, Desalination 244, 239 (2009).5V. G. Gude, N. N. Khandan, and S. Deng, Renewable Sustainable Energy Rev. 14, 2641 (2010).6S. Al-Kharabsheh and D. Y. Goswami, Desalination 156, 323 (2003).7V. G. Gude, Desalination using low grade heat sources, Ph.D. dissertation, New Mexico State University, Las Cruces,NM, 2007.8V. G. Gude and N. Nirmalakhandan, J. Energy Eng. 134, 95 (2008).9G. J. H. Wim, J. Z. Ronald, and A. Z. Herbert, Prog. Photovoltaics 12, 415 (2004).10Y. Morita, T. Fujisawa, and T. Tani, Electr. Eng. Jpn. 133, 43 (2000).11M. M. Jong and H. A. Zondag, Proceedings of the 9th International Conference on Solar Energy in High Latitudes, North-sun, Leiden, The Netherlands, 2001.12H. P. Garg and R. K. Agarwal, Energy Convers. Manage. 36, 87 (1995).13T. Fujisawa and T. Tani, Proceedings of ISES Solar World Congress, Taejon, Korea 1997.14T. Bergene and O. Lovik, Sol. Energy 55(6), 453 (1998).15G. A. Mertz, G. S. Raffio, K. Kissock, and K. P. Hallinan, Proceedings of the ISEC Conference, Florida, 2005.16S. R. Bull, Proc. IEEE. 89, 8 (2001).17V. G. Gude and N. Nirmalakhandan, Energy Convers. Manage. 49, 3326 (2008).18S. A. Zeinab and A. Lasheen, Desalination 217, 52 (2007).19A. E. Kabeel, Energy 34, 1504 (2009).20V. G. Gude, N. N. Khandan, and S. Deng, Desalination Water Treat. 20, 281 (2010).21Biomass and Alternative Methane Fuels (BAMF) Super ESPC Program Fact sheet, Wastewater Treatment Gas to Energyfor Federal Facilities, U.S. Department of Energy, ORNL 2004-02594/abh, July 2004.043108-25 Desalination using renewable energy J. Renewable Sustainable Energy 3, 043108 (2011) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:85.179.202.206 On: Tue, 29 Apr 2014 10:38:21

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