An Overview Of Solar Thermal Desalination Technologies
Latest Trends in Renewable Energy and Environmental InformaticsAn overview of solar thermal desalination technologiesI. ULLAH, M.G. RASUL* AND M.M.K. KHANSchool of Engineering and Technology, Central Queensland UniversityRockhampton, Queensland 4702, Australia*Email: m.rasul@cqu.edu.auAbstract: - Solar energy has a very promising role to play in addressing water scarcity problems throughthermal desalination processes around the world. Direct solar desalination uses solar energy to producedistillate directly in the solar collector whereas indirect solar desalination combines conventional desalinationtechniques, such as multistage flash desalination (MSF), vapor compression (VC), reverse osmosis (RO),membrane distillation (MD) and electrodialysis with solar collectors for heat generation. This paper describesseveral desalination technologies which could be integrated into solar thermal energy systems.Keywords: Solar energy, solar thermal desalination, solar stillsDesalinated water is most abundantly used inMiddle Eastern Arab countries, namely, SaudiArabia, Kuwait, United Arab Emirates, Qatar,Oman, and Bahrain [4]. The technology fordesalination can be classified into three majorcategories, as follows: Process based on a physical change in state ofthe water – i.e. distillation or freezing Process using membranes – i.e. reverse osmosisor electrodialysis, and Process acting on chemical bonds – i.e. ionexchange.1 IntroductionDesalination is defined as a process of removingdissolved minerals from feedwater sources such asseawater, brackish water or treated wastewater inorder to improve the taste or properties of the water.The lack of drinking water is an acute problem thatexists in arid regions of the world where fresh wateris becoming very scarce and costly. Potable water isone of the most important international health issuestoday and large quantities of fresh water arerequired in many parts of the world for agricultural,industrial and domestic uses. Even today, one fourthof mankind is suffering from inadequate fresh watersupply [1]. Globally, the total installed capacity ofdesalination plants was 71.9 million m3 per day in2012 [2]. Seawater desalination accounts for 67% ofproduction, followed by brackish water at 19%,river water at 8%, and wastewater at 6%. Figure 1shows the worldwide feed-water percentage used indesalination [3].Of the above processes, those based on chemicalbonds, such as ion exchange, are mainly used toproduce extremely high quality water for industrialpurposes and are not suitable for treating seawateror brackish water. The other two processes areregularly used to treat seawater and brackish waterand have been developed over many years in largescale commercial applications. The major processesof desalination are therefore membrane anddistillation processes.Figure 2 shows a simplified diagram of adesalination system. Raw feedwater is pre-treatedprior to entering one or more parallel desalinationprocess trains. There are 3 process trains in thisfigure. Desalted product water undergoes posttreatment, as required for the application, and ispumped to the distribution system. The salts andother residuals separated out of the feedwater in thedesalination process are continuously discharged towaste as concentrate.A seawater desalination process separatessaline seawater into two streams: a fresh waterstream containing a low concentration of dissolvedFigure 1: Worldwide feed-water percentage used indesalinationISBN: 978-1-61804-175-3335
Latest Trends in Renewable Energy and Environmental Informaticssteam. The simple solar still of the basin type is theoldest method and improvements in its design havebeen made to increase its efficiency [9]. A singleeffect solar still is a simple device which can beused to convert saline, brackish water into drinkingwater. In Solar stills a transparent cover encloses apan of saline water which traps solar energy withinthe enclosure. This heats up the water causingevaporation and condensation on the inner face ofthe sloping transparent cover. This distilled water isgenerally potable; the quality of the distillate is veryhigh because all the salts, inorganic and organiccomponents and microbes are left behind in thebath.Under reasonable conditions of sunlight thetemperature of the water will rise sufficiently to killall pathogenic bacteria anyway. A film or layer ofsludge is likely to develop in the bottom of the tankand this should be flushed out as often as necessary.Solar still has an average daily yield of 4-5 L/m2/dayafter losses. Currently state-of-the-art single-effectsolar stills have an efficiency of about 30–40% [10].One of the main drawbacks of this type ofdesalination plant is the low thermal efficiency andproductivity. This could be improved by variouspassive and active methods. These modifications aredescribed briefly, as follows.salts and a concentrated brine stream. This processrequires some form of energy to desalinate, andutilizes several different technologies for separation.A variety of desalination technologies has beendeveloped over the years on the basis of thermaldistillation, membrane separation, freezing,electrodialysis, etc [5, 6].Figure 2: Schematic of overall process ofdesalinationSolar energy can directly or indirectly be used fordesalination. There are two types of collectionsystems: Direct desalination systems are collectionsystems that use solar energy to produce distillatedirectly in the solar collector whereas indirectsystems combine solar energy collection systemswith conventional desalination systems. In indirectsystems, solar energy is used either to generate theheat required for desalination and/or to generateelectricity that is used to provide the requiredelectric power for conventional desalination plantssuch as multi-effect (ME), multi-stage flash (MSF)or reverse osmosis (RO) systems [7]. The recentdevelopments in solar thermal desalinationtechnologies are discussed in this paper.2.2 Modifications using passive methods2.2.1 Basin stillsThe operating performance of a simple basin typepassive still can be increased by the followingtechniques, Still with black dye: Injecting black dye inthe seawater increases the distillate yield[11]. Still with additional condenser: Fath [11]found that adding a passive condenser in theshaded region of a single slopped stillincreases the still efficiency by 45%. Single slope versus double slope basin stills:Single slope still gave better performancethan a double slope still under cold climaticconditions while the opposite is true undersummer climatic conditions [12]. Still with cover cooling: Increasing thetemperature difference between the basin(heat source) and the cover (heat sink) leadto increase the water evaporation rate [13].In stills with cover cooling, cooling water orsaline solution is fed in the gap of a doubleglass cover to maximize the temperaturedifference. The cost, as such, is increased.2 Solar thermal desalinationtechnologiesThe existing solar thermal desalination technologiesare described and discussed below.2.1 Direct solar desalinationThe method of direct solar desalination is suitablefor small production systems, such as solar stills, inregions where the freshwater demand is less than200 m3/day [8]. This low production rate is due tothe low operating temperature and pressure of theISBN: 978-1-61804-175-3336
Latest Trends in Renewable Energy and Environmental Informaticsof the greenhouse air was lowered, resulting in abetter climate for the crops and less ventilationrequirement. Which eventually lead to a decrease inthe water consumption of the crops [17].2.2.2 Wick stillsIn a wick still, the feed water flows slowly throughwick (a porous, radiation-absorbing pad). Wickstills have two distinct advantages over basin stills.Firstly, the wick can be tilted so that the feed waterpresents a better angle to the sun reducing reflectionand presenting a large effective area. Secondly,there is less feed water in the still at any time andtherefore water is heated more quickly and to ahigher temperature. Tanaka et al. have provensuperiority of the tilted wick type solar still andconfirmed an increase in productivity by 20–50%[14].2.2.5. Multiple-effect basin stillsMultiple-effect basin stills consist of two or morecompartments. The condensing surface of the lowercompartment is the floor of the upper compartment.The condensing vapor provides heat energy tovaporize the feed water. Multiple-effect solardesalination systems are more productive thansingle effect systems due to the fact that it reuses thelatent heat of condensation. The increase inefficiency, though, must be counterpoised againstthe increase in capital and operating costs. Typicalefficiency of these stills is 35% or more, which isgreater than a single basin still but the cost andcomplexity are correspondingly higher. Thedesalination unit consists of a solar collector and adesalination tower made of six stages with a watercirculation system to avoid salt accumulation in thetower. The production rate of the unit can reach 25L/m2/day for a value of 4.8 kW h/m2/day of solarradiation [16].2.2.3 Diffusion stillsDiffusion solar stills consist of two separate units.One is a hot storage tank, coupled to a solarcollector, and the other is the distillation unit, whichproduces the distilled water. Four-effect still is oneof the most recent designs of this type of still [15].The evaporation process in a four-effect still for thedesalination of sea and brackish water wasexperimentally investigated in a test facility underdifferent modes and configurations of heat recovery,and natural or forced convection in the fourdistillation chambers (“effects”). The theoreticaldistillate output from a 4-effect distillation unit was8.7 kg/ m2/ h with an energy input of 2.0 kW/m2under experimental conditions [16].2.2.6 Externally heated (active) solar stillsThe temperature of saline water in the basin can beincreased through additional external heating. Forthis purpose the still can be integrated with a(a) solar concentrator(b) solar heater(c) waste heat recovery system2.2.4 Solar still greenhouse combinationThe Seawater Greenhouse combines a solardesalination system with an environment forcultivating crops in which transpiration isminimized, at the same time producing sufficientwater for its own use through a process of solardistillation. Integrated design of greenhousescombined with solar stills represents an interestingpossibility for the development of small-scalecultivation in places where only saline water orbrackish water is available [12]. Chaibi constructedand analysed this system [17], where the south slopeof the greenhouse roof was built as a solar still.Saline water was pumped from a reservoir to therooftop of the greenhouse, from where it wasdistributed evenly to the evaporation surface in thestill during the day. The top cover of the still was aregular glass sheet, while the bottom of the solarstill composed of an only partly light transparentmaterial, which absorbed a substantial amount of thesolar irradiation, but transmitted wavelengths thatare favourable for photosynthesis of vegetation (thephotosynthetic active radiation (PAR) has thewavelength interval 380–710 nm). As still absorbedmost of the heat radiation, therefore the temperatureISBN: 978-1-61804-175-3Water circulation through the heater or theconcentrator could either be through naturalcirculation (Thermosyphon) or through forcedcirculation using a pump.2.3 Water desalination with humidification–dehumidification (HD)This process utilizes the principle of mass diffusionand uses dry air to evaporate saline water, thushumidifying the air. The HD process is based on thefact that air can be mixed with significant quantitiesof vapour. One of the problems that badly affect thestill performance is the direct contact between thecollector and the saline water; this may causecorrosion and scaling in the still and thereby reducethe thermal efficiency [18]. In HD desalination air isused as a working fluid, which eliminates thisproblem.A temperature increment enhances thevapour carrying capacity of air i.e. 1 kg of dry aircan carry 0.5 kg of vapor and about 670 kcal when337
Latest Trends in Renewable Energy and Environmental Informaticsoff in one vessel can be used to heat the next one,and only the first one (at the highest pressure)requires an external source of heat [19] as shown inFigure3[19].its temperature increases from 30 to 80 C [16].Freshwater is produced by condensing out the watervapor, resulting in dehumidification of the air. Asignificant advantage of this type of technology isthat it provides a means for low pressure, lowtemperature desalination that can operate off wasteheat and is potentially very cost effective. Theprinciple of MEH plants is the distillation underatmospheric conditions by an air loop saturated withwater vapor. The air is circulated by natural orforced convection (fans). The evaporator–condensercombination is termed a “humidification cycle”,because the airflow is humidified in the evaporatorand dehumidified in the condenser.Figure 3: Multiple-effect Distillation process [20]2.4. Multi-stage flash (MSF) processMany multiple-effect distillation (MED) plants ofmedium capacity powered by solar energy havebeen built worldwide. The small-scale MEDdesalination plant is a simple prototype small scalesolar desalination system which consists of a towerwith series of flat trays for effects and used a flatplate solar collector with oil as a heating mediumfor thermal energy [22]. Oil is circulated by naturalconvection between the solar collector and the firsteffect. The vapor from the first stage condenses atthe bottom wall of the second stage, releasing it’slatent heat. The condensed water moves through achannel to be collected outside the unit.MSF process is a water desalination processthat distils sea water by flashing a portion of thewater into steam in multiple stages of what areessentially counter current heat exchangers asshown in Figure 2. MSF produces 85% of alldesalinated water in the world [19].2.6 FreezingFreeze distillation is a process of enriching asolution by partially freezing it and removing frozenmaterial that is poorer in the dissolved material thanis the liquid portion left behind. The concept isappealing in theory because the minimumthermodynamic energy required for freezing is lessthan for evaporation since the latent heat of fusionof water is 6.01 kJ/mole while the latent heat ofvaporization at 100 C is 40.66 kJ/mole. Freezinghas some advantages over distillation i.e. a lowertheoretical energy requirement, minimal potentialfor corrosion and little scaling or precipitation. Thedisadvantage is that it involves handling ice andwater mixtures that are mechanically complex tomove and process. Despite the process advantage,freezing has not established itself as a commercialdesalination technique because of the cost andcomplications of refrigeration systems and the needfor freshwater to wash the crystals prior to melting.There are many designs of freeze separationprocesses as there are methods of refrigeration. Themost commonly used methods are: vacuumfreezing, vapour compression, ejector-absorption,and refrigeration freezing and secondary refrigerant.Figure 2: Multi stage flash process [20]Block found that solar-powered MSF plants canproduce 6–60 L/m2/day, in comparison with the 3–4L/m2/day typical of solar stills [21]. The mostcommonly used type of solar collectors is salinitygradient solar ponds, and the parabolic troughcollector, which is used in i.e. a MSF desalinationplant in Kuwait for a production rate of 100 m3/day[8].2.5. Multiple-effect distillationMultiple-effect distillation (MED), as shown inFigure 3, is the low temperature thermal process ofobtaining fresh water by recovering the vapour ofboiling sea water in a sequence of vessels, (calledeffects) each maintained at a lower temperature thanthe last. Because the boiling point of waterdecreases as pressure decreases, the vapour boiledISBN: 978-1-61804-175-3338
Latest Trends in Renewable Energy and Environmental Informaticsof the methods of disposal or reuse to beadopted.3 Further studySolar energy coupled to desalination offers apromising prospect for covering the fundamentalneeds of power and water in remote regions, whereconnection to the public electric grid is either notcost effective or not feasible, and where the waterscarcity is severe. Further study should be carriedout on the technological advancement of the variousstate-of-art hybrid desalination systems powered bysolar thermal energy. Numerical modelling anddesign of an innovative solar thermal powered smallto medium scale desalination prototype plant couldbe highly efficient and cost effective system forseawater/brackish water desalination. Authors arecurrently undertaking a project on Feasibility ofSolar Desalination Plant at Central QueenslandUniversity (CQUniversity), Australia at theirRockhampton campus. The desalination of wastewater being drained to ponds/lakes, rain watercollected at ponds and salty underground water willbe investigated as a future source of potable watersupply to CQU network in order to assess whetherthe current water costs of CQUniversity can bereduced. The tasks that will be undertaken are to: Detail the quantity and quality of the feedwater and indicate the expected variation infeed water quality parameters. In particularconsideration should be given to salinity(TDS), turbidity, organic content, pH andthe concentration of scale forming salts andnon-ionic fouling species.Relate desalination costs to the quality andvariability of the source water.Thisvariability can have an impact on the pretreatment required (the impact is, to someextent, dependent on the type ofdesalination plant selected) and therefore itwill be necessary to comment in some detailon the quality of the source water and inparticular to quality variations.It isessential that the requirements and costsassociated with pre-treatment and wastedisposal or release be fully assessed. Investigate the need for pre-treatment whererequired, with particular reference tobiological fouling and the need to limitmaintenance costs. Investigate the method of disposal or reuseof saline effluent providing full details ofthe method and costs associated withensuring and maintaining the sustainabilityISBN: 978-1-61804-175-3 Undertake an environmental assessmentidentifying any adverse impacts and theworks or controls required to minimisethese impacts. Undertake a social impact assessmentidentifying any adverse impacts and theworks or measures required to minimisethese impacts. Investigate the maintenance requirementsincluding plant and component life. Reporton the technical expertise required tooperate and manage the various processesconsidered. Confirm the capacity requirements of apipeline to CQU distribution system inaddition to Rockhampton City Council’spipeline system. Consideration shouldinclude the benefits of utilising adesalination plant to provide a based flowwith the existing system meeting demandfluctuations. Identify pipeline sizing and route selectionoptions including pump station anddesalination plant location. Provide a plan of the complete installationrequired showing all components. Prepare capital and operating cost estimates.3 SummaryAn overview of solar thermal desalinationtechnologies is presented. Solar desalinationprocesses can be devised in two categories: directand indirect collection systems. The “direct method”uses solar energy to produce distillate directly in thesolar collector, whereas indirect solar desalinationcombines conventional desalination techniques suchas multistage flash desalination (MSF), vaporcompression (VC), reverse osmosis (RO),membrane distillation (MD) and Electrodialysiswith solar collectors for heat generation. Solarthermal desalination plants utilizing indirectcollection of solar energy can be classified into ehumidification, multi-stage flash(MSF), multi-effect distillation (MED) andmembrane distillation (MD). Authors are currentlyundertaking a project on developing a solar339
Latest Trends in Renewable Energy and Environmental Informaticsdesalination process for waste/drainage water atCentral Queensland University, Australia.[15] F. Graeter, M. Duerrbeck and J. Rheinlaender,Multi-effect still for hybrid solar/fossildesalination of sea and brackish water,Desalination, 138 (2001) 111–119.[16] Hazim Mohameed Qiblawey, Fawzi Banat,Solar thermal desalination technologies,ELSEVIER, Desalination 220 (2008) 633–644[17] M. Chaibi, Greenhouse systems with integratedwater desalination for arid areas based on solarenergy, Doctoral thesis, Swedish University ofAgricultural Sciences, Alnarp, 2003.[18] H. Fath and A. Ghazy, Solar desalination usinghumidification–dehumidification technology,Desalination, 142 (2002) 119–133.[19] Shoaiba Desalination Plant. Water Technology.Retrieved on March 20, 2011.[20] Desalination, CalderTM – A les/Literature/FPD/fpd-10-ea4.pdf, February 2013.[21] D. Block, Solar Desalination of Water,FSECRR- 14-89, Florida Solar Energy Center,Cape Canaveral, February 1989.[22] K. Schwarzer, M. Eugenia, C. Faber and C.Muller, Solar thermal desalination system withheat recovery, Desalination, 137 (2001) 23–29.References[1] Fiorenza G, Sharma VK, Braccio G. Technoeconomic evaluation of a solar powered waterdesalination plant. Energy Conversion andManagement 2003; 44:2217–40.[2] http://www.desalination.biz/news/news story.asp?id 6746&title Installed desalination growth slowed in 2011%26%238209%3B2012,visited January 29, 2013.[3] CORDIS Database; 2006.[4] Ali A. Al-Karaghouli and L.L. Kazmerski .Renewable Energy Opportunities in WaterDesalination. National Renewable EnergyLaboratory Golden, Colorado, 80401, USA[5] E.D. Howe, Fundamentals of WaterDesalination, Marcel Dekker, New York, 1974.[6] B. Van der Bruggen, Desalination bydistillation and by reverse osmosis — trendstowards the future, Membrane Technology,2003 (2) (2003) 6–9[7] L. Garzia-Rodriguez and C. Gomez-Camacho,Perspectives of solar-assisted seawaterdistillation, Desalination, 136 (2000) 213–218.[8] L. Garzia-Rodriguez, Seawater desalinationdriven by renewable energies: a review,Desalination, 143 (2002) 103–113.[9] M. Naim, A. Mervat and Abd El-Kawi,Nonconventional solar stills. Part 1: Nonconventional solar stills with charcoal particlesas absorber medium, Desalination, 153 (2003)55–64.[10] G. Mink, M. Aboabbous and E. Karmazsin,Design parameters, performance testing andanalysis of a double-glazed, air-blown solarstill with heat recycling, Solar Energy, 62(1998) 309–317.[11] H. Fath, Solar distillation: a promisingalternative for water provision with free energy,a simple technology and a clean environment,Desalination, 116 (1998) 45–56.[12] M. Malik, G. Tiwari, A. Kumar and M. Sodha,Solar Distillation: A Practical Study of a WideRange of Stills and their Optimum DesignConstruction and Performance, PergamonPress, 1996.[13] O. Haddad, M. Al-Nimer and A. Maqableh,Enhanced solar still performance using aradiative cooling system, Renewable Energy,21 (2000) 459–469.[14] T. Tanaka, A. Yamashita and K. Watanabe,Proc. International Solar Energy Congress,Brighton, England, Vol. 2, 1981, p. 1087.ISBN: 978-1-61804-175-3340
thermal desalination processes around the world. Direct solar desalination uses solar energy to produce distillate directly in the solar collector whereas indirect solar desalination combines conventional desalination t
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