Renewable energy driven desalination systems modelling

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discussed as an innovative approach to desalinate water economically and in an environmentally friendly manner. The stochasticnature of renewable energy sources (RES) which results in the use of expensive energy storage systems usually limits the penetrationof RES to the power generation system of a region. Desalination systems can utilise in a more economically ecient way the avail-able RES potential. The energy produced is consumed for potable water production which can be stored economically for a largeperiod of time before consumption. An integrated model for the use of renewable energies (wind, solar) in the desalination of sea-water has been developed in the context of REDDES project. In this work, a model is developed where desalination technologies arecoupled with RES power systems to produce potable water at the lower possible cost. The presented model is incorporated in theREDDES software. 2005 Elsevier Ltd. All rights reserved.Keywords: Renewable energy systems; Water desalination systems; Energy eciency; Water security1. IntroductionThe limited market penetration of renewable sourcesof energy (RES) can be attributed to a large number ofconstraints, including problems related to nancing, reg-ulation, technical issues, lack of information, educationand training. The stochastic nature of RES which resultsin the use of expensive energy storage systems usuallylimits the penetration of RES to the power generationsystem of a region. The produced energy varies in timeas wind speed or solar radiance varies and the powerhas to be consumed directly or else it will be lost. RESpenetration in the desalination industry does not facethe same barriers as in the case of RES for electricitypower production. In the case of RESedesalinationcoupling the energy is consumed directly for water pro-duction, the water can be stored cheaply in large quan-tities and for long periods [1].Several desalination processes have been developedbut not all of them are reliable and in commercial use.The most important processes are split into two maincategories [1]: Thermal (or distillation) processes: Multi-StageFlash Distillation (MSF), the Multi-Eect Distilla-tion (MED), the Thermal Vapour Compression(TVC) and the Mechanical Vapour Compression(MVC) processes.Renewable energy driven deC. Koroneos*, A. DLaboratory of Heat Transfer and EnvironmentalP.O. Box 483, 541 2Received 10 January 20Available onlineAbstractRenewable energy sources for powering desalination processewhere the use of conventional energy is costly or unavailable. ReJournal of Cleaner Productio* Corresponding author. Tel.: C30 231 0995968; fax: C30 2310996012.E-mail address: (C. Koroneos).0959-6526/$ - see front matter 2005 Elsevier Ltd. All rights reserveddoi:10.1016/j.jclepro.2005.07.017salination systems modellingompros, G. RoumbasEngineering, Aristotle University of Thessaloniki,4 Thessaloniki, Greece05; accepted 6 July 200519 September 2005s is a very promising option especially in remote and arid regionsnewable energy driven desalination systems have been extensivelyn 15 (2007) Membrane processes: Reverse Osmosis (RO) andElectrodialysis (ED) processes. ED is conned to de-salination of brackish water while RO can be usedfor both, brackish and seawater desalination..desalination processes.f. Geothermal-heateThermal Vapour conversion(TVC)g. Geothermal-heateMulti-Stage Flash Distillation(MSF)RESSolarWind PV system Shaft ElectricityROMVC EDFig. 1. Coupling of RES withGenerally, solar thermal distillation application aresolar assisted rather than stand-alone. These unitsare suitable when low enthalpy energy is also avail-able. These systems are not considered suitable forGeothermalSolar-ThermalHeatMEDMSFTVCThe selection of a process usually is based on severalparameters, such as site conditions, local circumstances,energy availability, etc. The best desalination systemfor a particular application will be the system that reli-ably produces water of the expected quality and quantityat reasonable cost.The feasible RESedesalination technology combina-tions are very clearly demonstrated in Fig. 1. The dier-ent power forms derived from RES are coupled withthe equivalent desalination technology. The RESedesalination coupling schemes under examination couldbe divided in two categories:1. RESedesalination coupling schemes that require theRES unit and the desalination unit to be located inthe same area. Such couplings are:a. Wind-shafteMechanical Vapour Compression(MVC) couplingb. Solar thermal-heateThermal Vapour conversion(TVC)c. Solar thermal-heateMulti-Stage Flash Distilla-tion (MSF)d. Solar thermal-heateMulti-Eect Distillation(MED)e. Solar thermal-heateDistillationh. Geothermal-heateMulti-Eect Distillation (MED)2. RESedesalination coupling schemes that do notrequire the RES unit and the desalination unit tobe located in the same area. Such couplings are:a. Wind-electricityeMechanical Vapour Compres-sion (MVC) couplingb. Wind-electricityeReverse Osmosis (RO)c. Solar PV-electricityeReverse Osmosis (RO)d. Solar PV-electricityeMechanical Vapour Com-pression (MVC) couplinge. Geothermal-electricityeMechanical VapourCom-pression (MVC) couplingf. Geothermal-electricityeReverse Osmosis (RO)A short assessment of each technically feasible appli-cation can be made: Thermal-distillation processes. Thermal distillationplants such as MED, MSF or TVC can utilise solarthermal energy or geothermal energy. Thermal pro-cesses require a high-energy input (due to the energyrequired for change of phase) and also auxiliary elec-tricity for pumping needs. Solar thermal systems areheavily dependent on solar radiation and weatherconditions and they need large heat accumulators.450 C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464small scale and remote areas. Geothermal systemsare ideal for thermal-distillation processes but arelimited to areas where geothermal elds exist. Solar PVeRO. The electricity form PV systems canbe used to drive high-pressure pumps in RO desali-nation plants. The main advantage of PVedesalina-tion systems is their ability to develop small sizedesalination plants. The energy production unit con-sists of a number of photovoltaic modules, whichconvert solar radiation into direct electric current(DC). DC/AC inverters have to be used becauseRO uses alternating current (AC) for the pumps.Energy storage (batteries) is required for PV outputpower smoothing or for sustaining system operationwhen insucient solar energy is available. WindeRO/MVC. Wind energy can be coupled withRO and MVC processes for the desalination of wa-ter. RO and MVC require electrical or mechanicalenergy as primary energy input, which can be pro-vided from a single wind turbine or a wind farm.The selection between the technologies depends onthe feed water quality and the required product waterquality. MVC, as all distillation processes producewater with very low salinity (below 20 ppm totaldissolved solids). Membrane processes (RO) pro-duce water with higher salinity (500 ppm TDS).Because of the variability of the wind speed, it isdicult to predict the energy output. Appropriatepower control and conditioning systems are re-quired in order to match the ratio of the power in-put to the desalination load.Because of the spatial distribution of the RES, RESedesalination coupling schemes that do not require theRES unit and the desalination unit to be located in thesame physical area are of special interest. In the presentpaper, wind energy and solar PV coupled with RO andMVC are examined. The models were developed in theframework of the Renewable Energy Driven DESalina-tion (REDDES) project which was funded by the EU [2].2. Renewable energy sources2.1. Modelling of solar photovoltaic energy (PV)Many types of semiconductor materials can convertlight into electrical power by means of the photovoltaic(PV) eect, but only a few of them have been used assolar cells to date. The commercial market for PV cellstoday is dominated by crystalline silicon (Si). Howeverother materials, notably amorphous silicon (a-Si), cad-mium telluride (CdTe), copper indium diselenide(CuInSe2, CIS), and gallium arsenide (GaAS), are alsoavailable and substantial investments are being madeC. Koroneos et al. / Journal of Cleain their development [3].When light is absorbed by a solar cell, electrons are re-leased and move according to the internal electric poten-tial so that when a load is connected across the contactsan electric current ows. The voltage across a solar cell isprimarily dependent on the design and the materials ofthe cell, whilst the electrical current depends primarilyon the incident solar irradiance and the cell area. Theoutput from a typical solar cell, which is exposed to thesun, increases from zero at sunrise to a maximum at mid-day and then falls again to zero at dusk. The ratio of elec-trical power produced by a solar cell to the incident solarirradiance is known as the PV cell eciency.Groups of cells are mounted together on a glass plateand wired in series to form a PVmodule typically around0.5 m2 in size. Groups of modules can be connected to-gether electrically to form a PV array. PV arrays canbe mounted on a xed structure or on a sun-trackingstructure to maximize the incident solar radiation.The power production capacity of a PV array isexpressed in Watt-peak (Wp) units. A PV cell of 1 Wpproduces 1 W of electrical energy when exposed to solarirradiance of 1000 Wm2 at a cell temperature of 25 C.Most of the PV systems use monocrystalline or poly-crystalline silicon cells. The most advance commercialcrystalline silicon modules have eciencies more than15% but typically eciencies range from 11 to 13%.Other PV system components, such as cabling,batteries, battery charge controllers, inverters, etc. arecommonly known as Balance of System (BOS) com-ponents. These components provide the necessary in-terface with the grid or a specic application such ashousehold loads, telecommunication equipment, etc.The energy produced by solar cells depends on the so-lar radiation of geographical specied region, the area ofthe PV cells and their eciency. The produced energy iscalculated by the equation [3]:PPVZhPVAPVGZhmphT;coeffGAPV 1where hPV is the overall eciency, hmp is the maximumpower point eciency of the PV module, hT,coe is thetotal eciency of dirt (hd), inverter losses (hinv) and los-ses due to connections (hcon) (hT,coeZ hdhinvhcon), APVis the PV area (m2), and G is the average solar radiation(W/m2).The maximum power point eciency of the PV mod-ule hmp decreases with increasing cell temperature andEq. (2) is used to take this into account [3]:hmpZhmpSOCCmP;mpTC TSOC 2where hmpSOC is the module eciency under standardoperating conditions (SOC), mP,mp is the temperature co-ecient of eciency at the maximum power point (it hasnegative value) ( C1), T is the actual cell temperature451ner Production 15 (2007) 449e464C( C) and TSOC is the temperature under standard oper-ating conditions 25 C.The module eciency at standard operating condi-tions is given by Eq. (3) [3]:hmpSOCZImpSOCVmpSOCAPVCGSOC3where ImpSOC is the current at the maximum powerpoint of the module under standard operating condi-tions (A), VmpSOC is the voltage at the maximum powerpoint of the module at standard operating conditions,APVC is the module area (m2) and GSOCZ 1000 Wm2is the solar insolation under standard operating condi-tions. ImpSOC, VmpSOC, APVC in Eq. (3) are the PV mod-ule specic and are provided by the manufacturer.The temperature coecient mP,mp of the eciency atthe maximum power point is approximated as follows:mP;mpZhmpSOCmVOCVmpSOC4where mVOC is the voltage temperature coecient(V C1) and is also provided by the manufacturer.The working temperature of the module/cell TC is es-timated by the method of Due and Beckman, which isbased on the energy balance on the cell surface [3]:TCZTaCGtaUL1 nCta5where G is the incident solar radiation (Wm2), UL isthe heat loss coecient from the cell to the surroundings(including losses by convection and radiation from thetop and bottom of the cell, and losses by conductionthrough the mounting framework) (Wm2 C1), Ta isthe ambient temperature ( C), t is the transmittanceof the cover over the cells, a is the fraction of the radi-ation incident that is absorbed on the surface of thecells, nC is the eciency of the module in converting ra-diation into electrical power.The eciency of the module nC is approximated bytaking it equal to hmpSOC. The product ta is consideredconstant equal to 0.9 (taZ 0.9). The UL is calculated bythe equation [3]:taULZTC;NOST Ta;NOSTGT;NOSTZTC;NOST 208006where TC,NOST is the nominal operating temperature,dened as the cell temperature that is reached whenthe cell is mounted in a normal way at a solar radiationof GT,NOSTZ 800 Wm2, wind speed of 1 m s1 andambient temperature of Ta,NOSTZ 20 C and at no load452 C. Koroneos et al. / Journal of Cleaoperation. For the purposes of the model, TC,NOST istaken to be constant equal to 46 C.If thd is the average hours of daylight in a region (h/day) and G the average incident solar radiation (W/m2),then the PV cell power production is given by theequation:PPVMZhmphT;coeffGAPVMthd!103 7where APVM is the module area and PPVM is the averagedaily PV module energy production (kWh/day). InEq. (7), the PV modules are considered tilted at opti-mum angle for maximum performance.The required energy production PPV (kWh/day) isgiven by the equation:PPVZPDESthd=24C1 thd=24aBCHaBDCH 8where PDES is the energy required by the desalinationunit (kWh/day), aBCH is the charging eciency of thebattery, and aBDCH is the discharging eciency of thebattery.The total number of modules needed (NPVM) and thetotal PV area (APV) are calculated by the followingequations:NPVMZ1thd=24C1 thd=24aBCHaBDCHPDESPPVM9andAPVZNPVMAPVC 10The total hardware cost of a PV plant depends on thesurface area or the peak power of the PV modules andthe storage capacity of the batteries [4]:CPVtotalZCPVMCCSUPCCpcondPPV;peakCCB;storageSBPVCCDC=ACPload 11where:1. CPVM is the cost of PV modules (V/Wp) and is takenequal to 5 V/Wp [4];2. CSUP is the support cost (V/Wp), and is taken equalto 1 V/Wp [4];3. Cpcond is the power conditioning cost (V/Wp), and istaken equal to 0.5 V/Wp;4. CB,storage is the battery storage cost (V/kWh), and istaken equal to 170 V/kWh;5. CDC/AC is the DC/AC converter in (V/W). The costof the inverter is given by the equation [4,5]:CDC=ACZ1099!103!P069load 12ner Production 15 (2007) 449e464CPVO&MZ0:025!CPVtotal 15A simple calculation of the cost of the electricity pro-duced by a PV plant may be obtained by using theexpression:CPV kWhZCPVtotalCCRCCPVO&MCCB;storageSPVnBPPV!36516where CPVkWh is energy production cost (V/year), nB isthe lifetime of batteries, CCR is the capital cost recoveryfactor and is given by the equation:CCRZr1Crn1Crn 1 17where r is the discount rate and n is the number of yearsof useful life of the plant.0.24 RadiaEURO/kWh producedFig. 2. Energy production costs of an AEG:PQ 40/50 PV module as a fnZ 15 achieved with either electrical or hydraulic yaw drives.The horizontal axis wind turbine (or HAWT) dominatesthroughout the world and there is no signicant contri-bution from vertical axis machines (VAWTs) in Europe.This is because the HAWT has proven more cost eec-tive than the VAWT.The system cost eectiveness has improved by a factorof 3 over the last 10 years. Reliability is also very highwith the machines available for generation for upwardsof 96% of the time. This technology is at a stage where itcan deliver large-scale implementation reliably, and ata price approaching that of a conventional generationplants [6,7].The relation between power production and instantwind speed at certain atmospheric conditions is giveby the equation [7]:PWTUZ12rAIRCDUpDWT24U3 1814 19 24tion (kWh/m2day)6 hours/day daylight8 hours/day daylight12 hours/day daylightunction of the average solar radiation and daylight hours, rZ 7% and6. PPV,peak is the peak power output of the PV modules(Wp) and is given by the equation:PPV;peakZhmpAPV!1000 137. Pload is the power consumption of the desalinationunit (kW) and is equal to PDES/24;8. SPV is the storage capacity of the batteries in kWhand is given by the equation:SPVZPPV1 thdaBCH 14Operating and maintenance (O&M) costs are gen-erally low, because there are no moving parts. O&Mcosts represent an annual share going from 1.5 to 2.5%of the total capital cost [3]. In this work the gure of2.5% per year is used, orThe electricity production cost depends (Fig. 2) onthe average solar radiation and the average daylighthours of the geographic site under examination.2.2. Modelling of wind energyWind turbines extract the kinetic energy of the windby transferring the momentum of the air passingthrough the wind turbine rotor, into the rotor blades.The rotor blades are aerofoils that act in a similar wayas the wings act on the aircraft. The modern wind tur-bine is a very ecient device in concentrating the energyof the airow into a single rotating shaft. The power inthe shaft can then be utilised in any way.The wind turbine rotor can be set on either a horizon-tal or a vertical shaft. In a horizontal shafted turbine,the rotor must be orientated towards the wind, and this453C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464pUZC Cexp C19where k is the shape factor and C is the scale parameter.The scale parameter of the Weibull distribution canbe estimated using Eq. (19#).CZUAVGG1C1k 19 0where UAVG is the average wind speed (m/s) and G is theGamma function.Consequently, the only necessary inputs for model-ling the wind speed in a specic geographic region arethe mean annual wind speed and the parameter shape k.Sites with small shape factor k have greater wind var-iability (Fig. 4), and thus the power production froma wind farm would vary accordingly. Thus in sites withlarge k factor, a wind turbine would provide steadierpower.01002003004005006007008000 10WindPower (kW)"Nordex N27/150""Vestas V39 600/39""NEG Micon 750/44 50 Hz""Bonus 300/33.4 Mk III"5Fig. 3. Typical windWTENWTZNWTEWT 22The annual output of a Bonus 300/33.4 Mk III windturbine based on Eq. (21) for various average windspeeds and shape factors is shown in Fig. 5. The greaterthe average wind speed of a site the greater is the energyproduction. At the same time, the greater the shape fac-tor the greater is the energy production from the windturbine.The total capital cost (V) of a wind farm is given bythe following equation.TCCWTZ929:2C2435:6!expPWTR33:4!PWTR1CfWTNWT 23where the PWTR is the rated power of the wind turbine inkW and fWT expresses installation costs. Eq. (23) wasdeveloped for the Greek market [13].15 20 25 30 speed (m/s)where rAIR is the air density at a given temperature (kg/m3), U is the wind speed (m/s), DWT is the blade diam-eter of the wind turbine (m), and CD(U ) is the eciencyof the wind turbine that depends also on wind speed.The produced energy from a wind turbine depends onthe power curve of the wind turbine, which is suppliedby the manufacturer. The power curve relates the windspeed and the power generated by the wind turbine.Typical power curves of commercial turbines are pre-sented in Fig. 3.The time variability of wind speed is usually modelledby the K-Weibull distribution. The probability of occur-rence of a specic wind speed is estimated using Eq. (19)[8e12].kUk1 UkUsually the mean annual wind speed is measured ata height of 10 m. The wind speed at rotor height canbe estimated from the wind speed at 10 m using Eq. (20).UAVGZUrefHHrefr20where Uref is the average wind speed at height Href and ris an empirical factor.The annual energy production of the wind turbineEWT in kWh/year is estimated using Eq. (21).EWTZ8760!Z UCUTOUTUCUTINpUPU dU 21And in a wind farm with N of wind turbines:454 C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464turbine power curves.lCWTkWhZTCCWT!CCRCCWTO&MEWT25 and shape factor for discount rate rZ 7% and of usefullife of the plant nZ 15 years.8000007 8 9 10 11 12100000012000001400000160000018000002000000Average wind speed (m/s)Energy kWh/yeark=1.5 k=1.8 k=2k=2.5 Installation costs vary considerably ( fWTw 30e60%) and depend on the number of wind turbines,and the remoteness of the area. Because the presentstudy is referring to remote areas, a high constant valueis assumed 50%.The operational and maintenance costs are given bythe equation [13]:CWTO&MZ0:03!TCCWT 24A simple calculation of the cost of the electricity pro-duced by a wind farm plant may be obtained by usingthe expression:WindFig. 4. WeibulFig. 5. Energy production of a Bonwhere CWTkWh is energy production cost (V/kWh), CCRis the capital cost recovery factor and is given by theequation:CCRZr1Crn1Crn 1 26where r is discount rate and n is the number of years ofuseful life of the plant.The energy production cost depends strongly on thewind characteristics of the site under examination. Theproduction cost is decreasing as the site is windiest(greater UAVG) and the greater the shape factor k, forthe same wind turbine. Fig. 6 presents the annual costsof a wind farm as a function of average wind speeddistribution.00 15 20 25 30 35 Speed (m/s)Probability density k=1.5 mean U=7.5m/sk=2.5 mean U=7.5m/sk=1.5 mean U=10.5m/sk=2.5 mean U=10.5m/s455C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464us 300/33.4 Mk III wind turbine.isergy sources are estimated by Eq. (28).EAuxDESZEDES EWTDES 28The excess power that is not used by the desalinationunit is sold to the grid (in the case of grid-connectedplants) or dumped in the case of stand-alone plants.The wind energy sold to the grid is given by Eq. (29).EWTSOLDZEWT EWTDES 293. Modelling desalination technologies3.1. Mechanical Vapour Compression (MVC)distillationThe Mechanical Vapour Compression (MVC) distil-the compressor. Operation at low temperatures mini-mizes the formation of scaling and corrosion of materi-als. MVC units are usually built in the 20e2000 m3/day(0.005e0.5 mgd) range [2].Capital and energy costs are signicant factors in thedetermination of the total water production cost. Theenergy demand is mainly required to drive the vapourcompressor motor. The operation and maintenance ofthe vapour compressor motor may be half of the totaloperating cost. The energy requirements of VC plantsare between 8 and 12 kWh/m3 [2].The main equipment for the MVC desalination pro-cess is the evaporator, the heat exchanger and the com-pressor. The feed water is preheated in a heat exchangeror a series of heat exchangers by the hot discharge ofthe brine and the distillate. The distilled water producedby the condensation leaves the plant through the pre-heaters as product water. Plant layout and necessaryDesalination plants operate with constant power in-put PDES but the wind turbine power output varies somaximum annual wind energy that the desalinationplant can absorb is estimated by:EWTDESZ8760!NWTZ UDESUCUTINpUPU dUCEDESZ UCUTOUTUDESpU dU 27where EDES is the annual energy requirements of the de-salination plant (kWh/year) and UDES is the intercept ofthe power curve of the turbine with the power input re-quirements of the desalination plant: PDESZ P(UDES).The auxiliary energy is considered to be provided bythe grid in order to cover the energy demand during lowwind speed. The annual energy ows from auxiliary en-0.037 8 90.0350.040.0450.050.0550.060.0650.07Average wEnergy cost EURO/yearFig. 6. Energy production costs of a Bonus 300/33.4 Mk III wind turbine ayears.456 C. Koroneos et al. / Journal of Cleanelation process is generally used for small and mediumscale seawater desalting units. The heat for evaporationcomes from the compression of vapour rather thanthe direct exchange of heat from steam produced inthe boiler. The mechanical compressor is usually electri-cally driven, allowing the sole use of electrical power toproduce water by distillation (Fig. 7). The compressorcreates a vacuum in the vessel and then compresses thevapor taken from the vessel and condenses it in a tubebundle, also in the same vessel. Seawater is sprayed onthe outside of the heated tube bundle where it boilsand partially evaporates, producing more vapours [2].The plants, that use this process, are generally de-signed to take advantage of the principle of reducingthe boiling point temperature by reducing the pressure.MVC units have been built in a variety of congurationsto promote the exchange of heat to evaporate the seawa-ter. Extra care is required with the control of the brinelevel in the evaporator and the proper maintenance of10 11 12nd speed (m/s)k=1.5k=1.8k=2k=2.5function of average wind speed and shape factor, rZ 7% and nZ 15r Production 15 (2007) 449e464input parameters are presented in Fig. 7 [14e16]. Theter (kg/day), QF is the seawater feed (kg/day), QB is therejected brine (kg/day), and PW is the energy require-ment (kJ/day).The recovery ratio of the process is given by:RZQPWQF33From Eqs. (31) and (33), the energy balance for theevaporator can be written as:Rh2Ch4ZRh1CRh6C1Rh8 34From Eqs. (32) and (33), the energy balance for heat ex-changer can be written as:h5 h4ZRh3 h6C1Rh7 h8 35CMVCO&MZCLABOURCCMAINTCCCHEMCCENRG 39where:CLABOURZ0:2!QPW!3600!365!24 (V/year), thelabour costCMAINTZ0:08!QPW!3600!365!24 (V/year), themaintenance costCCHEMZ0:15!QW!3600!365!24 (V/year), thechemicals costCENRGZ cost of energy (V/year)A simple calculation of the cost of water producedby an MVC plant may be obtained by using theexpression:CMVCZTCCMVC!CCRCCMVCO&MQ40energy requirements of the compressor are estimated byEq. (30):PMVCZQPWh2 h1 30The energy balance for the evaporator is:QPWh2CQFh4ZQPWh1CQPWh6CQBh8 31The energy balance for the heat exchanger is:QFh5 h4ZQPWh3 h6CQBh7 h8 32where hi is the specic enthalpy of the i stream at tem-perature Ti (kJ/kg), QPW is the produced desalinated wa-BrineBrine DischargeBrine RecirculationPreheated FeedSeawater & BrinePreheated FeedFig. 7. Typical ow diagram of VaporC. Koroneos et al. / Journal of CleanUsing Eqs. (34) and (35):h2 h1ZRh6 h5CRh3 h6C1Rh7R36Then Eq. (30) can be written as:PMVCZQPWRh3 h5C1Rh7R37The total capital cost (TCCRO) including the site de-velopment and indirect costs is given by the equation:TCCMVCZ2500!QPW!24!365!3600 38The operational and maintenance costs (O&M, CO&M)are given by the equation:Brine DischargeSeawater FeedProduct WaterVapor CompresorVaporCompressedVaporWork in12PWQPWQPWQPWQPW375QFQBQB8QF 46Compression (VC) distillation plant.457er Production 15 (2007) 449e464PWmaterial) by owing through a membrane. No heatingor phase change is necessary for this separation. Themajor energy required is for pressurizing the feed water.In practice, the saline feed water is pumped intoa closed vessel where it is pressurized against the mem-brane. As a portion of the water passes through themembrane, the remaining feed water increases in saltcontent. At the same time, a portion of this feed wateris discharged without passing through the membrane.The amount of the feed water discharged to waste in thisbrine stream varies from 20 to 70% of the feed ow, de-pending on the salt content of the feed water [2].A typical RO system is made up of the following ba-sic components (Figs. 8 and 11): Pre-treatment: Feed water pre-treatment is impor-tant in RO because the feed water must pass throughvery narrow passages during the process. Therefore,suspended solids must be removed and the waterpre-treated so that salt precipitation or microorgan-ism growth does not occur on the membranes (bio-fouling). Usually, the pre-treatment consists ofsterilization, ne ltration and the addition of acidor other chemicals to inhibit precipitation.BrineSeawater FeedWork inHigh Pressure PumpEnergy Recovery SystemWork inPre-treatmentFig. 8. Typical ow diwater. The concentrated reject stream (brine)emerges from the membrane modules at high pres-sure. In large plants the reject brine pressure energyis recovered by a turbine, recovering from 20 up to40% of the consumed energy. There are two typesof RO membranes that are being used commercially.These are the spiral wound (SW) membranes and thehollow ber (HF) membranes, both are used forbrackish and seawater desalination. The choice be-tween the two is based on factors such as cost, feedwater quality and product water capacity. Post-treatment: It consists of sterilization, stabiliza-tion, mineral enrichment and pH adjustment of theproduct water. Energy recovery system: A system where a portion ofthe pressure energy of the brine is recovered.Due to the operation of the RO process in ambienttemperature, corrosion and scaling problems are dimin-ished in comparison with distillation processes. How-ever, eective pre-treatment of the feed water is requiredto minimize fouling, scaling and membrane degradation.Generally a seawater RO plant has low capital costand signicant maintenance cost due to the high cost DischargeProduct WaterMembranePost-treatmentWaterBrineagram of the RO unit.where CMVC is the water production cost (V/m3), CCR isthe capital cost recovery factor and is given by theequation:CCRZr1Crn1Crn 1 41where r is discount rate and n is the number of years ofuseful life of the plant.3.2. Reverse Osmosis (RO)Reverse Osmosis (RO) is a membrane separation pro-cess in which the water from a pressurized saline solu-tion is separated from the solutes (the dissolved High-pressure pump: The high-pressure pump sup-plies the pressure needed to enable the water to passthrough the membrane and have the salts rejected.This pressure ranges from 17 to 27 bar for brackishwater and from 54 to 80 bar for seawater. Membrane modules: The membrane assembly con-sists of a pressure vessel and a membrane that per-mits the feed water to be pressurized against themembrane. The membrane must be able to with-stand the drop of the entire pressure across it. Thesemi-permeable membranes are fragile and vary intheir ability to pass fresh water and reject the pas-sage of salts. No membrane is perfect in its abilityto reject salts, so a small amount of salts passthrough the membrane and appears in the product458 C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464porosity, and Q is the brine ow rate.of membrane replacement. The major energy require-ment for RO desalination is for pressuring the feedwater. The energy requirements of a seawater SW-ROplant is around 5 kWh/m3 for large units with energyrecovery, while for small units it is around 15 kWh/m3.The application of a pressure larger than the osmoticpressure of a saline solution against a semi-permeablemembrane has as a result the passage of pure waterthrough the membrane. The basic equation that de-scribes the RO process is [17e19]:JWZQWAMZK 0WDPDPDPDROPdZKWDPDPDPDROP 42where JW is the ux of permeate water (m/s), Qw is theow rate of water through the membrane (m3 s1), KWis the specic permeability of water through the mem-brane (m2 h1 Pa1), AM is the surface of the membrane(m2), d is the thickness of membrane (m), DP is the os-motic pressure dierence between feed water on the sur-face of the membrane and product water (Pa), DP is thepressure dierence between feed and product water (Pa),and DPDROP is the pressure drop across the module inthe feed channel (Pa).The permeability of water through the membrane(KW) depends on the material (polymer) that the mem-brane is constructed of, the temperature and the opera-tional time of the membrane. The temperature aectsconsiderably the water ow rate through the membrane.An increase in temperature of 1 C results to an increaseof about 3% of water ow rate. As it can be seen fromEq. (42), in order to achieve the desirable ow rate ofproduct water it is necessary to apply pressure to thefeed water above that of the osmotic pressure of the feedwater. RO systems work with feed compression 2e3times greater than the osmotic pressure value. Seawaterosmotic pressure is calculated using the equation:DPZ0:6955C0:0025!T!108rSWCM CP 43where T is the temperature in C, CM is the salt concen-tration on the membrane surface (kg/m3), CP is the saltconcentration in the produced water (kg/m3), and rSWis the density of the seawater (kg/m3) given by theequation:rSWZ498:4!mTC248;400!mT2C752:4!mT!CSW1=244where mTZ1:0069 2:757!104!T and CSW is theseawater salt concentration (kg/m3).C. Koroneos et al. / Journal of CleanThe average concentration CM on the membranessurface is given by the equation:CM CPCB CPZexpJWk45where JW is the ux of water through the membrane(m/s); CB, CM, CP are the concentrations of salts ofthe brine, on the membrane and product water(kg/m3), respectively; and k is the mass transfer coe-cient through the membrane (m/s).The mass transfer coecient through the membrane,k, for spiral wound modules is given by the relation:ShZkdhDBZ0:065!Re0:875Sc0:25 46where Sh is the Sherwood number, DB is the diusivityof brine entering the feed channel (m2/s), dh is the hy-draulic diameter of the channel (m), Re is the Reynoldsnumber inside the feed channel, and Sc is the Schmidtnumber.The diusivity DB of the brine is given by therelation:DBZ6:725!106!exp0:1546!103!CB 2513273:15CT47The Reynolds number inside the feed channel is givenby the equation:ReZrSWdhUBhSW48where hSW is the viscosity of the seawater (Pa s).hSWZ1:234!106!exp0:0212!CBC1965273:15CT49The velocity UB of the brine in the feed channel ofa spiral wound module is given by the equation:UBZQBACZQBWMdSP3SP50whereWM is the width of the feed channel of the module(m), dSP is the channels spacer thickens, eSP is the spacer459er Production 15 (2007) 449e464BeREC is the eciency of the energy recovery system,The Schmidt number of brine is given by the relation:ScZhSWrSW!DB51The pressure losses in the feed channel DPDROP forspiral wound modules is given by the followingequation:DPDROPZ6:23!Re0:3 rSWU2B2Lmdh52where UB is the velocity of brine entering the feed chan-nel (m/s), Lm is the length of the feed channel (m), dh isthe hydraulic diameter of the channel (m), and Re is theReynolds number inside the feed channel.Due to the fact that no membrane is perfect and theconcentration dierence between the brine and the prod-uct water, some small amount of salts pass through themembrane. This is a mass transport phenomenon andcan be described by the following equation:JSZQSAMZKSCM CP 53where JS is the ux of salts through the membrane (kg/sm2); QS is the ow rate of salts through the membrane(kg/s); CM, CP are the concentrations of salts of themembrane and product water (kg/m3); AM is the areaof the membrane (m2); and KS is the mass transfer coef-cient of salts through the membrane (m2/s).It is clear that the KW must be as large as possible andKS as small as possible in order to achieve the smallestresistance to water permeation through the membraneand the greater resistance to salts permeation.Two important factors for the membrane are the saltpermeation and the salt rejection, which are dened bythe following equation:Salt rejectionZ1 salt permeationZ1CP=CF 54Also the intrinsic salt permeation and the intrinsicsalt rejection (RIN) are important parameters, whichare dened by the following equation:Table 1Rejection of ions from an RO membrane [1]Ion Salt rejection (%) Ion Salt rejection (%)NH4C 92 Nitrates 85NaC 95 Chlorates 95KC 95 Fluorides 95MgC 97 Sulfates 97SrC 97 Phosphates 99Ca2C 98 Acid carbonates 95460 C. Koroneos et al. / Journal of CleaRINZ1 intrinsic salt permeationZ1CP=CM 55The salt rejection is an important characteristic of themembrane and is dierent for dierent ions (Table 1).Intrinsic salt rejection (RIN) is an important operationalcharacteristic of the membrane module and is consid-ered to be independent of the driving pressure in themodule.Another important factor is the recovery ratio (RW),which is dened as the ratio between the ow rates ofproduct and feed water. RO systems in the case of sea-water feed are designed for recovery ratios from 20 to35%.Given a specic membrane spiral wound element thegeometrical characteristics AM, dh, dSP, eSP, WM, LMand the operational constants KW, KS, RIN can be deter-mined by the manufacturer. Then Eqs. (9)e(20) can beused for the calculation of the recovery ratio of the ele-ment as a function of the applied pressure DP and thebrine characteristics (CB, QB, T ) entering the element.In Fig. 9 the eect of the salinity of the feed on mem-brane module performance (driving pressure vs recoveryratio) is shown for a typical membrane module. There isa linear relation between driving pressure and recoveryratio as stated by Eq. (42) and the feed water salinity af-fects the oset of the curve as it changes the osmoticpressure of the feed. The feed ow rate aects the slopeand the oset of the driving pressureerecovery ratiocurve (Fig. 10). The increase in feed ow rate increasesthe slope because the recovery ratio is inversely propor-tional to the feed ow rate. The oset of the curve in-creases with the increase in feed ow rate because thehigher the ow rate the higher is the pressure drop insidethe element (factor DPDROP, Eq. (42)).In an RO plant the membrane assembly consists ofa number of pressure vessels (NPV) and a number ofmembrane elements (NM) in a row inside the pressurevessel that permits the feed water to be pressurizedagainst the membrane (Fig. 11). Inside the pressure ves-sel the brine exiting of one element is the feed for thenext one and so on.The overall driving pressure required DPF, and thusthe energy requirements depend on the production rateQW, the seawater feed (CSW, T ) characteristics, thenumber of pressure vessels NPRV, the number NM ofmembrane modules in a pressure vessel and the recoveryratio (RW).The energy requirements of an RO unit given the DPFcan be calculated by the equation:PROZ8:76QWDPFRWeHPP 1RWRWQWDPBReREC56where eHPP is the eciency of the high-pressure pump,ner Production 15 (2007) 449e4644.00E+064.50E+065.00E+065.50E+066.00E+066.50E+067.00E+067.50E+068.00E+068.50E+060.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13 0.14RO Membrane Module Recovery RatioDriving Pressure (Pa)CFEED=35 kg/m3,Feed=400 m3/ day CFEED=60 kg/m3,Feed=400 m3/ day CFEED=35 kg/m3,Feed=300 m3/ day CFEED=60 kg/m3,Feed=300 m3/ day DPBR is the pressure of the rejected brine (Pa), and PROis the energy requirement (kWh/year).An example of the implementation of Eq. (56) isshown in Fig. 12. As it can be seen that increasing therecovery ratio increases the energy requirements. In-creasing the number of membrane modules per pressurevessel decreases the energy requirements for the same re-covery ratio. However, increasing the membrane mod-ules above 14 the decrease is not signicant.For a given seawater feed (CSW, T ) and a requiredwater production QW the parameters: the number ofpressure vessels NPV, the number NM of membranemodules in a pressure vessel, the driving pressure ofthe feed DPF and the recovery ratio (RW) can be opti-mised in order to minimize water cost.Fig. 10. Driving pressureerecovery ratio (R) of a membrane module atThe total equipment cost (TECRO) of an RO unitconsists of the cost of membranes, the cost of pressurevessels, the cost of high-pressure pumps, the cost of en-ergy recovery system, the cost of pre-treatment andpumping unit:EQCROZCMENMNPRVCCPRVNPRVCCHPPCCRECCCPRETR 57where CME is the cost of membrane element (V), CPRV isthe cost of pressure vessel (V), CHPP is the cost of high-pressure pump HPP (V), CREC is the cost of the energyrecovery system (V), and CPRETR is the cost of pre-treat-ment and intake system (V).4.00E+064.50E+065.00E+065.50E+066.00E+066.50E+067.00E+067.50E+068.00E+068.50E+060.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13 0.14RO Membrane Module Recovery RatioDriving Pressure (Pa)CFEED=35 kg/m3,Feed=300 m3/day CFEED=40 kg/m3,Feed=300 m3/day CFEED=50 kg/m3,Feed=300 m3/day CFEED=60 kg/m3,Feed=300 m3/day CFEED=70 kg/m3,Feed=300 m3/day Fig. 9. Driving pressureerecovery ratio (R) of a membrane module at 300 m3/day saline feed rate at various feed salinities (CFEED).461C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464300 and 400 m3/day saline feed rate at various feed salinities (CFEED).Feed = 1000 m3/day Feef Salinity 35 kg/m32.E+0503.E+054.E+055.E+056.E+057.E+058.E+059.E+051.E+060.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8Recovery RatioEnergy requirements (kWh/yr)Nm = 4 Nm = 6 Nm = 8 Nm = 10Nm = 12Nm = 16Nm = 18Nm = 20Fig. 12. Energy requirements of an RO plant with energy recovery system vs recovery ratio and number of membrane modules per pressure vesselThe cost equations for HPP and energy recovery sys-tems are [20]:CHPPZaHPPQWDPFRWeHPPbHPP58CRECZaREC1RWRWQWDPBReRECbREC59The cost of pre-treatment and intake system given by theequation:CPRETRZaPRETRQW!24!3600RWbPRETR60(NM). Number of pressure vesselsZ 3.The total capital cost (TCCRO) including the site de-velopment and indirect costs is given by equation:TCCROZ1:411!EQCRO 61The operational and maintenance costs (O&M, CO&M)are given by the equation [21e23]:CROO&MZCLABOURCCMAINTCCCHEMCCMEMREPCCENRG 62where:CLABOURZ0:2!QW!3600!365!24 (V/year), thelabour costCMAINTZ0:05!QW!3600!365!24 (V/year), themaintenance costFeed seawater Product WaterBrineHigh PressurePump Membrane ElementNM Membrane Elementsper Pressure VesselNPV Pressure VesselsFig. 11. Typical RO plant, the membrane assembly.462 C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e4641Cr 1 examined the ROeWIND and MVCeWIND optionswhere r is the discount rate and n is the number of yearsof useful life of the plant.4. Results and discussionWhen several alternative RESedesalination schemesare compared for a specic case, the nal decision con-cerning the most prominent combination should bebased, on criteria such as:- commercial maturity of technology (an appropriateway to validate this is by examining the performanceof similar existing applications);- availability of local support (installers, technicians,machine shops, etc.);- simplicity of operation and maintenance of theare comparable while the MVCePV option is the mostexpensive solution.Water production costs of an RESedesalination con-guration depend heavily on the available RES poten-tial. The greater the RES potential the smaller is theenergy production cost from the RES unit and thussmaller water production costs from the desalinationunit.References[1] Desalination guide using renewable energy sources. JOULE-THERMIE; 1998.[2] REDDES project,![3] Development of a logistic model for the design of autonomousdesalination systems with renewable energy sources. Middle EastDesalination Research Centre Report; February 2002.[4] Notton G, Muselli M, Poggi P. Costing of a stand-alone photo-CCHEMZ0:03!QW!3600!365!24 (V/year), thechemicals costCMEMREPZNMNPRVCME=3 (V/year), membrane ele-ment replacement costCENRGZ cost of energy (V/year)A simple calculation of the cost of water produced byan RO plant may be obtained by using the expression:CROZTCCRO!CCRCCROO&MQW63where CRO is the water production cost (V/m3), CCR isthe capital cost recovery factor and is given by theequation:CCRZr1Crnn 641.49 1.5000.511.522.533.54EURO/m3RO-WIND MVC-WINDFig. 13. WaterC. Koroneos et al. / Journal of Cleasystem.The above factors, in conjunction with available tech-nical information (feed water quality, output water re-quirements (quality and quantity) as well as the typeof RES available) provide a starting point for the engi-neer and the decision maker.The equations presented can be used easily to esti-mate the energy requirements, the size of the RES unitrequired (Wind or PV) and water production cost de-pending on the RESedesalination conguration. InFig. 13 the water production costs of RESedesalinationunits with production capacity of 500 m3/day is pre-sented as calculated by the model equations. The inputsrequired are wind characteristics (kZ 1.5 and UAVGZ7.5 m/s), solar radiation characteristics (thdZ 8.3 h/day,TaZ 20 C GZ 5 kWh/day/m2), the RO membranecharacteristics, the wind turbine characteristics (powercurve) and the solar PV characteristics. For the case3.672.77MVC-PV RO-PVproduction cost.463ner Production 15 (2007) 449e464voltaic system. Energy 1998;23(4):289e308.[5] Muselli M, Notton G, Poggi P, Louche A. Computer-aided anal-ysis of the integration of renewable-energy systems in remoteareas using a geographical-information system. Applied Energy1999;63:141e60.[6] ![7] Wind energy e the facts, vols. 1e5. Directorate-General forEnergy, European Commission.[8] Kershman Sultan A, Rheinlander Jurden, Gabler Hansjorg.Seawater reverse osmosis powered from renewable energy sources-hybrid wind/photovoltaic/grid power supply for small scaledesalination in Libya. Desalination 2002;153:17e23.[9] Manolakos D, Papadakis G, Papantonis D, Kyritsis S. A simula-tion e optimisation programme for the designing hybrid energysystems for supplying electricity and fresh water through desalina-tion to remote areas. Case study: the Merssini village, Donousaisland, Agean Sea, Greece. Energy 2001;26:679e704.[10] Voivontas D, Misirilis K, Manoli E, Arampatzis G,Assimakopoulos D, Zervos A. A tool for the design of desalina-tion plants powered by renewable energies. Desalination 2001;133:175e98.[11] Voivontas D, Yannopoulos K, Rados K, Zervos A,Assimacopoulos D. Market potential of renewable energy pow-ered desalination systems in Greece. Desalination 1999;121:159e72.[12] Zervos A, Assimacopoulos D. Estimating the cost of water pro-duced by RES power desalination systems. In: Mediterranean[14] Al-Juwayhel Faisal, El-Dessouky Hisham, Ettouney Hisman.Analysis of single-eect evaporator desalination systems com-bined with vapour compression heat pumps. Desalination 1997;114:253e75.[15] Aybar Hikmet S. Analysis of a mechanical vapor compressiondesalination system. Desalination 2002;142:181e6.[16] Karameldin Aly, Lotfy A, Mekhemar S. The Red Sea wind-drivenmechanical vapor compression desalination system. Desalination2002;153:47e53.[17] Water Treatment Estimation Routine (WATER). User guide.Water Desalination Research and Development Program. ReportNo. 43, U.S. Department of the Interior, Bureau of Reclamation;August 1999.[18] Taniguchi Masahide, Kurihara Masaru, Kimura Shoji. Behaviourof a reverse osmosis plant adopting a brine conversion two-stageprocess and its computer simulation. Journal of MembraneScience 2001;183:249e57.[19] Van der Meer WGJ, Riemersma M, Van Dijk JC. Only twomembrane modules per pressure vessel? Hydraulic optimizationof spiral-wound membrane ltration plants. Desalination 1998;119:57e64.[20] Malek A, Hawlader MNA, Ho JC. Design and economics of ROseawater desalination. Desalination 1996;105:245e61.[21] Wilf Mark, Klinko Kenneth. Optimization of seawater ROsystems design. Desalination 2001;138:299e306.464 C. Koroneos et al. / Journal of Cleaner Production 15 (2007) 449e464conference on renewable energy sources for water production.Santorini, Greece; June 2000.[13] Kaldellis JK, Garvas ThJ. The economic viability of the commer-cial wind plants in Greece. A complete sensitivity analysis. EnergyPolicy 2000;28:509e17.[22] Ali El-Saie MH, Ali El-Saie YahyaMH,Moneer Ahmed Deghedi.Financial, economical and technical aspects of establishing remotedesalination plants. Desalination 2001;135:25e42.[23] Avlonitis SA. Operational water cost and productivity improve-ments for small-size RO desalination plants. Desalination 2002;295e304.Renewable energy driven desalination systems modellingIntroductionRenewable energy sourcesModelling of solar photovoltaic energy (PV)Modelling of wind energyModelling desalination technologiesMechanical Vapour Compression (MVC) distillationReverse Osmosis (RO)Results and discussionReferences


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