Optimization of Renewable Energy Systems: The Case of Desalination

Transcription

1 Optmzaton of Renewable Energy Systems: The Case of Desalnaton 5 Karm Bouroun Ecole Natonale d Ingéneurs de Tuns, Tunsa 1. Introducton The applcaton of renewable energes (RE) for drvng desalnaton unts (DES) s very promsng n solated areas (.e. slands, vllages n the desert, etc.), where the electrcty producton s very expensve, potable water resources are nexstent and the potental of renewable energes (solar and wnd) s very mportant (Koroneos et al., 2007; Kalogrou, 2005). The applcaton of renewable energes n the desalnaton ndustry does not face the same barrers as n the case of RES for electrcty power producton (expensve storage systems to compensate the stochastc characterstcs of the renewable energes). In the case of RES/DES couplng, the energy s consumed drectly for water producton, the water can be stored, cheaply n large quanttes and for long perods (Koroneos et al., 2007). The operatonal performances, the cost and the relablty of RES/DES unts depend on the desgn and calbraton of such systems. Optmal use of RES potental s necessary n order to reduce the cost of produced water. In ths frame consderng hybrd confguratons (PV and Wnd) s the best way to optmze the use of RE. On the other hand, the desgn of such systems s complex because of uncertan renewable energy supples, load demands and the non-lnear characterstcs of some components. Despte the great effort done to mprove the effcency of RES/DES systems the desalnated water cost stll relatvely hgh compared to conventonal desalnaton unts and more effort should be done to optmze ths knd of systems. In ths chapter we present dfferent methods to optmze renewable energy systems drvng desalnaton unt, wth a partcular nterest to the RO drven by hybrd PV/Wnd systems. For ths last confguraton we wll present a new methodology based on Genetc Algorthms. The objectve functon used n ths optmzaton s the unt cost of desalnated water durng the lfe cycle of the plant (20 years). The presented methodology conssts n selectng from avalable components n the market, the optmal number and the type of each unt (PV panels, wnd turbnes, membranes, etc.) n such way that the water needs are satsfed and the producton cost s mnmzed. The total water cost for the lfe cycle of the plant s equal to the sum of the captal and mantenance costs.

2 90 Modelng and Optmzaton of Renewable Energy Systems The present methodology has the advantage to take nto account all the crtcal functonng parameters that have an nfluence on the electrcty and desalnated water productons and the nvestment and operatonal costs. The mnmzaton of the functon total cost was mplemented by usng Genetc algorthms (GA), that have the capacty to reach the soluton correspondng to the global optmum wth a relatve smple calculaton. The beneft of usng Genetc algorthms n the proposed methodology s the calculaton of the optmal soluton n the global space of feasble solutons of desalnaton systems (ndvduals). These later are obtaned by dfferent smulaton durng all over a year. 2. Desalnaton technologes Future water supply s a major concern n developed and developng countres of the world, such as n Mddle East and North Afrca (MENA regon). In these countres conventonal water resources are lmted and cannot sustan the growng demand where populaton growth s ncreasng dramatcally. Hence, snce 1970 other non conventonal methods have been adopted on a large scale to satsfy ths growng demand. One of the most promsng technologes s water desalnaton whom the total world capacty ncreased from m 3 /day n 1970 to m 3 /day n 2007 (GWI, 2010). By lookng at the total desalnaton plants nstalled n the world we smply realze that three major desalnaton technologes are used: Mult stage flash process (MSF) 43.5% of world producton, Reverse osmoss (RO) 43.5% of world producton, and Mult effect dstllaton (MED), whch has ncreased dramatcally n the world durng the last years. MED and MSF are classfed us thermal desalnaton technologes, however RO desalnaton s consdered as membrane technology. Besdes these technologes other technques can be used (.e. Mechancal Vapor Compresson, MVC; Electrodalyss, ED; Humdfcaton and Dehumdfcaton of Ar, HD; etc.). However ther applcaton stll lmted for specfc context. 2.1 MSF desalnaton An MSF dstllaton plant conssts of several consecutve stages (evaporatng chambers) mantaned at decreasng pressures from the frst stage (hot) to the last stage (cold). The vapor condenses to form fresh water. At vacuum condtons the bolng pont of water s low requrng less energy. Before enterng the frst cell, seawater sweeps all cells from the last one to the frst by flowng through the tubes of the heat exchangers where t s warmed by condensaton of the vapor produced n each stage (Fg. 1). Its temperature ncreases from sea temperature to nlet temperature of the brne heater. The seawater then flows through the brne heater where t receves the heat necessary for the process (generally by condensng steam). At the outlet of the brne heater, when enterng the frst cell, seawater s overheated exceedng bolng pont compared to the temperature and pressure of stage 1.

3 Optmzaton of Renewable Energy Systems: The Case of Desalnaton 91 Therefore, t s the abrupt ntroducton of ths sea water nto a lower pressure stage that makes t bol so quckly as to flash nto steam to reach equlbrum wth stage condtons. The produced vapor s condensed nto fresh water on the tubular exchanger at the top of the stage. The process takes place agan once the water s ntroduced nto the followng stage, and so on untl the last and coldest stage. The cumulated fresh water bulds up the dstllate producton whch s extracted from the coldest stage. Seawater slghtly concentrates from stage to stage and bulds up the brne flow whch s extracted from the last stage. Typcally a number of unts are constructed alongsde a combned cycle power plant and utlze low-grade steam (sem waste heat) from the power plant to produce the desalnated water. An MSF plant performance s selected to ensure the overall optmzaton of the plant power and steam cycles. Fg. 1. MSF Desalnaton Process 2.2 MED desalnaton MED, lke MSF, takes place n successve effects and uses the prncple of reducng the ambent pressure n the varous effects. Ths permts the seawater feed to undergo multple bolng wthout supplyng addtonal heat after the frst effect. In a MED plant, the seawater enters the frst effect and s rased to the bolng pont after beng preheated n tubes. The seawater s ether sprayed or dstrbuted onto the surface of evaporator tubes n a thn flm to promote rapd bolng and evaporaton. The tubes are heated by steam from a boler or other source, whch s condensed on the nsde of the tubes. The condensate from the boler steam s recycled to the boler for reuse (Fg. 2). In MED the maxmum temperature s now lmted to 80 C to reduce the scale deposton, whch lmt the gan output rato (GOR) to a maxmum level of 12 kg dstllate/kg of steam. However, wth the ntroducton of a compresson technology plant (hybrd) to the MED process the performance has been radcally mproved to GOR of 15. The compresson s provded by electrc compressors or thermo-compressors, whch utlze motve steam. Thermal desalnaton (MED and MSF) produce very low TDS producton (50 mg/l), and does not depend on feed qualty, as s the case wth the Reverse Osmoss technology.

4 92 Modelng and Optmzaton of Renewable Energy Systems Fg. 2. MED Desalnaton Process 2.3 Reverse osmoss (RO) RO s a pressure-drven process that separates two solutons wth dfferent concentratons across a sem-permeable membrane. The fresh water flow rate through the membrane s proportonal to the pressure dfferental that exceeds the natural osmotc pressure dfferental. The membrane tself represents a major pressure dfferental to the flow of fresh water. For bracksh water desalnaton the operatng pressures range from 15 to 30 bar, and for seawater desalnaton from 55 to 70 bar (Abdallah et al., 2005). The ntal pressurzaton of the feed water represents the major energy requrement. As fresh water permeates across the membrane, the feed water becomes more and more concentrated. There s a lmt to the amount of fresh water that can be recovered from the feed wthout causng foulng. Seawater RO plants have recoveres from 25 to 45%, whle bracksh water RO plants have recovery rates as hgh as 90%. RO system major components nclude membrane modules, hgh-pressure pumps, power plant, and energy recovery devces as needed (Fg 3). Fg. 3. Schematc dagram of one RO Desalnaton process wth two stages

5 Optmzaton of Renewable Energy Systems: The Case of Desalnaton 93 Membrane propertes and feed water salnty are the two major factors controllng the energy requrements of an RO system. Hgher water salnty requres more energy to overcome the osmotc pressure. Pre-treatment of seawater feedng RO membranes s recognzed as a key n desgnng RO desalnaton plants (Gad and Treal, 2007). The use of an adapted pre-treatment mnmzes the foulng problems and can provde good protecton of the membranes and a longer lfetme. 3. Renewable energy systems for desalnaton Solar and Wnd systems can be used to provde heat requred to produce steam for the thermal desalnaton plants and electrcty to drve hgh pressure pumps n RO unts and auxlary components n the dfferent desalnaton technologes. 3.1 Solar technologes Dfferent solar energy collectors may be used n order to convert solar energy to thermal energy. In most of them, a flud s heated by the solar radaton as t crculates along the solar collector through an absorber ppe. Ths heat transfer flud s usually water or synthetc ol. The flud heated at the solar collector feld may be ether stored at an nsulated tank or used to heat another thermal storage medum. The solar collector may be a statc or suntrackng devce. The second ones may have one or two axes of sun trackng. Otherwse, wth respect to solar concentraton, solar collectors are already commercally avalable; nevertheless, many collector mprovements and advanced solar technologes are beng developed. The man solar collectors sutable for seawater dstllaton are as follow Flat-plate collector Flat-plate collectors (FPCs) are used as heat transfer flud, whch crculates through absorber ppes made of ether metal or plastc. The absorber selectve coatngs are used to reduce heat losses and to ncrease radaton absorpton. Thus the thermal effcency ncreases although the collector cost also ncrease. A typcal flat-plate collector s an nsulated metal box wth a glass or plastc cover and a darkcolored absorber plate. The flow tubes can be routed n parallel or n a serpentne pattern. Flat plate collectors have not been found as a useful technology for desalnaton (Belessots and Delyanns, 2001; Graca-Rodrguez, 2002). Although they have been used for relatvely small desalnated water producton volumes, producton of large volumes of water would requre an addtonal energy source Parabolc trough collector A parabolc trough s a lnear collector wth a parabolc cross-secton. Its reflectve surface concentrates sunlght onto a recever tube located along the trough s focal lne, heatng the heat transfer flud n the tube. Parabolc troughs typcally have concentraton ratos of 10 to 100, leadng to operatng temperatures of C. Parabolc trough collectors (PTCs) requre sun trackng along one axs only. In ths way, the recever tube can acheve a much hgher temperature than flat-plate or evacuated-tube

6 94 Modelng and Optmzaton of Renewable Energy Systems collectors. The parabolc trough collector systems usually nclude a mechancal control system that keeps the trough reflector ponted at the sun throughout the day. Parabolctrough concentratng systems can provde hot water and steam, and are generally used n commercal and ndustral applcatons. Due to the hgh temperatures parabolc troughs are capable of producng hgh-grade thermal energy that s generally used for electrcty generaton (Belessots and Delyanns, 2001). Parabolc troughs could be a sutable energy supply for most desalnaton methods, but n practce, they have manly been used for thermal dstllaton as these methods can take advantage of both the heat and electrcty troughs produce. Parabolc Trough Collectors can also drve RO unts by usng Rankne Organc Cycle Photovoltac systems Photovoltac systems consst of a number of PV modules, whch convert solar radaton nto drect-current (DC) electrcty. The voltage and current of the system can be ncreased by connectng multple cells n seres and parallel, respectvely. The other system equpment ncludes a charge controller, batteres, nverter, and other components needed to provde the output electrc power sutable to operate the systems coupled wth the PV system. PV s a rapdly developng technology, wth costs fallng dramatcally wth tme, and ths wll lead to ts broad applcaton n all types of systems. Today, however, t s clear that PV-RO and PV-ED wll ntally be most cost-compettve for small-scale systems where other technologes are less compettve. The electrcty form PV systems can be used to drve hgh-pressure pumps n RO desalnaton plants. The man advantage of PV/desalnaton systems s ther ablty to develop small sze desalnaton plants. The energy producton unt conssts of a number of photovoltac modules, whch convert solar radaton nto drect electrc current (DC). DC/AC nverters have to be used because RO uses alternatng current (AC) for the pumps. Energy storage (batteres) s requred for PV output power smoothng or for sustanng system operaton when nsuffcent solar energy s avalable. 3.2 Wnd systems Wnd energy and desalnaton plants can be coupled n varous ways (Ma and Lu, 2011). Currently, wnd energy can power desalnaton plants drectly or ndrectly through four types of energy meda (Fg. 4): electrcty, thermal energy, gravtatonal potental energy and knematcal power (shaft power). Electrcty s the most commonly used energy form as the nterface between wnd energy and desalnaton process. After havng converted nto electrcty, the energy from wnd plant can be employed to drve desalnaton processes such as RO, ED and MVC (Kalogrou, 2005). The wnd plant can be on or off the grd. Due to the ntermttent characterstc of wnd power, usually backup facltes lke battery, water tank, flywheel system mght be ntegrated nto the system to store or release energy when the wnd speed exceeds or cannot acheve the requred level.

7 Optmzaton of Renewable Energy Systems: The Case of Desalnaton 95 Interface Electrcty Thermal Energy Wnd Energy Gravtatonal Potental Energy Desalnaton Unt Knematcal Power Fg. 4. Exstng nterfaces between wnd energy and desalnaton unt. Drect converson from wnd energy to thermal energy to drve thermal desalnaton unts (dstller) has been studed snce the effcency of drect wnd-thermal converson s hgher than that of wnd-electrcty converson and ther structures are smpler (Nakatake and Tanaka, 2005). The proposed dstller could produce 1.5 kg/d or more when a 6 m/s wnd blew steadly all day on a sunny or cloudy day. To reduce the energy loss caused by the wnd-electrcty converson, gravtatonal energy has also been used as the nterface between wnd energy and desalnaton process. Fadgas and Das (2009) desgned an alternatve confguraton to conventonal RO desalnaton systems by ncorporatng the use of gravtatonal potental energy, wthout usng ether electrcty or fossl fuels. The gravtatonal potental energy, presented by water stored n a reservor above a certan heght, was converted by wnd energy from wndmlls (or wnd turbnes). 4. RE/DES systems Many dfferent renewable energy desalnaton systems are techncally feasble (Kalogrou, 2005). Fg.5 presents the possble combnatons between desalnaton processes and RE technologes. A methodology for selectng the most approprate combnaton between desalnaton technologes and renewable energes for a gven ste based on dfferent crtera was developed (Setawan et al., 2009). Desalnaton systems are energy ntensve, and ther energy consumpton s a drvng factor n determnng ther economc feasblty when they are coupled to RES. Typcal energy consumptons for dfferent desalnaton processes are shown n Table 1.

8 96 Modelng and Optmzaton of Renewable Energy Systems Geotherme Geothermal Renewable Energes (ER) Solare Solar Eolenne Wnd Cha Heat leur Electrcte Electrcty PV Solarre thermque thermal HD MED TVC MD 1. Dstllaton Technques RO MVC ED Electrcte Electrcty HD: Ar Humdfcaton and Dehumdfcaton M ED: M ultple-effect Dstlla ton MSF: Multstage Flash TVC: Thermal Vapor Compresson MVC: Mechancal Vapor Compresson SD: So la r Dstlla to n 2. Membrane Technques RO: Reverse Osmoss ED: Electrodalyss Procede Drect Processes drect HD SD Capteurs Solar Collectors solares Mecanque Shaft Electrcte Electrcty HD MED MSF TVC MD RO MVC RO ED MVC Fg. 5. Technologcal combnatons of the man renewable energes and desalnaton methods Desalnaton Process Thermal Energy (kj/kg) Electrcal Energy (kwh/m 3 ) Seawater Mult-Stage Flash (MSF) Mult-Effect Dstllaton (MED) Vapor Compresson (VC) Reverse Osmoss (RO) wthout Energy Recovery Reverse Osmoss (RO) wth Energy Recovery Bracksh water Reverse Osmoss (RO) wthout Energy Recovery 1-3 Reverse Osmoss (RO) wth Energy Recovery Electrodalyss Table 1. Energy consumpton and electrc power cogeneraton (Blton et al., 2011) In table 1 the energy requrements are separated nto thermal energy whch s used to heat the seawater and electrcal energy whch s used to drve pumps, compressors and auxlary equpment. For seawater desalnaton, reverse osmoss requres the least amount of overall energy. However, f thermal energy s nexpensve (the case of Mddle East), a thermal desalnaton process lke mult-effect dstllaton can be practcal. Fg.6 llustrates the breakdown of renewable energy powered desalnaton system technologes mplemented worldwde n 2007 (Forstmeer, 2007). It shows that the most used RES/Desalnaton systems are RES/RO (51% of the total worldwde nstalled RES/DES plants).

9 Optmzaton of Renewable Energy Systems: The Case of Desalnaton 97 Solar/MED (13%) PV/ ED (6%) Solar/ Wnd/ MSF MVC (6%) (5%) Hybrd (4%) Other (15%) Wnd/RO (19%) PV/RO (32%) Fg. 6. Breakdown of renewable energy powered desalnaton system technologes mplemented worldwde (Forstmeer, 2007). Reverse Osmoss (RO) s the desalnaton process whch can be coupled, n relable and economc way, wth RES. The sutablty of renewable energy technologes, especally wnd turbnes and photovoltacs, for RO desalnaton systems s due to the convenence of RO for desalnatng small quantty of water for remote and solated areas; t has low energy consumpton (Table 1) and lttle need for mantenance (Wener et al., 2001). 4.1 MSF desalnaton plants drven by solar energy Solar-powered MSF plants can produce 6 60 L/m 2 /day, n comparson wth the 3 4 L/m 2 /day typcal of solar stlls (Block, 1989)). The use of solar troughs for MSF desalnaton was tested manly n the USA. In a typcal commercal small plant 48 kw s requred to produce 450 L/day n three stages. In Szacsvay et al. (1999), a Solar/MSF system usng an Atlants autoflash multstage stage desalnaton unt s descrbed. Snce the standard MSF process s not able to operate coupled to any varable heat source, an adapted MSF system called Autoflash was developped. Performance and layout data were obtaned both from computer smulaton and expermental results wth a small-szed Solar/MSF systems n Swtzerland. The system had been n operaton for 9 years. From these studes t was shown that the cost of dstllate could be reduced from 5.48 $/m 3 for small desalnaton system wth a capacty of 15 m 3 /day to 2.39 $/m 3 for desalnaton systems wth a capacty of 300 m 3 /day. 4.2 Multple-effect dstllaton drven by solar energy Several multple-effect dstllaton (MED) plants of medum capacty powered by solar energy were bult worldwde. One MED-plant desgned for a maxmum capacty of 120 m 3 /day wth 18 stack type stages and pre-heaters was analyzed n UAE (El-Nashar and Samad, 1998). Evacuated-tube solar collectors of 1862 m 2 were used wth water as heat carryng medum. It had a heat accumulator of 300 m 3 capacty. Specfc heat consumpton of the plant was 43.8 kcal/kg wth performance rato of Due to heat accumulator the

10 98 Modelng and Optmzaton of Renewable Energy Systems evaporator could run 24 h a day durng sunny days producng freshwater of 85 m 3 /day. The plant was able to desalt seawater of 55,000 ppm. The total seawater requrement was 42.5 m 3 /h. The major problem was the mantenance of the pumps. It was shown that the acd cleanng and slt removal were extremely necessary for better performance of the plant. A practcal scale desalnaton system of three effects usng only solar energy from solar collectors as the heat source and the electrcal power from the PV-cells was nvestgated by Abu-Jabal et al. (2001). The unt was developed and manufactured by the Ebara Corporaton (Tokyo) and tested at the Al Azhar Unversty n Gaza. The average producton rate was n the range of 6 13 L/m 2 /day. Thomas (1997) carred several experments on MED and MSF unts drven by solar energy n Kuwat. He reported several dffcultes operatng under the varable condtons of solar nsulaton. Greater success has been found wth self-regulatng solar MSF plants than solar MED plants. In Forenza et al. (2003) the water producton cost for seawater desalnaton by MED powered by a solar thermal feld has been estmated. The results obtaned for plants of capacty varyng between 500 and 5000 m 3 /d have shown that the cost of water produced can be reduced by ncreasng the plant capacty;.e. 3.2 $/m 3 for the 500 m 3 /d plant capacty and 2 $/m 3 for the 5000 m 3 /d plant capacty. 4.3 Reverse osmoss desalnaton drven by photovoltac Photovoltac (PV) powered RO systems have been mplemented n dfferent regons,.e: remote areas of the Tunsa desert, rural areas of Jordan, remote communtes n Australa, etc. Several nvestgatons were carred to analyze the cost of PV/RO desalnaton systems (Kalogrou, 2001). If PV connected to a RO system s commercal nowadays, the man problem of ths technology s reported to be the hgh cost of the PV cells. The dstance at whch the PV energy s compettve wth conventonal energy depends on the plant capacty, on the dstance to the electrc grd and on the salt concentraton of the feed (Garza- Rodrguez, 2002). In Saud Araba, a PV/RO bracksh water desalnaton plant was nstalled (Hasnan and Alajlan, 1998). It was connected to a solar stll wth 5 m 3 /d producton. The feed water of the water stll was the blowdown of the RO unt (10 m 3 /d). A detaled cost analyss was also reported. Bouroun and Chab (2009) presented a PV/RO desalnaton plant, for supplyng one vllage, n southern Tunsa. It uses solar energy to power a reverse osmoss bracksh water desalnaton unt wth a capacty of 15 m 3 /day. They present an analytcal descrpton of the plant components and reports expermental results for a 6-month operatng perod. Several problems were hghlghted such as brne rejecton, low effcences and hgh cost. Several tools were developed to mprove the desgn of PV/RO systems by usng teratve procedure (Herbert et al., 2007). The sze of the RO unt s computed accordng to the desalnated-water requrements, whle the PV system nomnal power ratng s calculated such that the correspondng energy requrements of the RO unt are satsfed, takng nto account the avalable solar radaton potental of the nstallaton area. The sze of the battery

11 Optmzaton of Renewable Energy Systems: The Case of Desalnaton 99 ncorporated n the system s computed such that the daly varatons of the solar radaton are compensated. On the other hand, up to now the desgn process does not nclude the optmzaton of the components' number and type and the mnmzaton of the total system cost. 4.4 Reverse osmoss drven by wnd energy Snce the coastal areas present a hgh avalablty of wnd power resources, wnd powered desalnaton represents a promsng alternatve of renewable energy desalnaton (Graca- Rodrguez et al., 2002). Wnd-powered RO plants have been mplemented on the slands of the County of Splt and Dalmata (Croata), on the sland Utsra n Norway, and n remote communtes n Australa. A prototype wnd-powered RO desalnaton system was constructed and tested on Coconut Island off the northern coast of Oahu, Hawa, for bracksh water desalnaton (Lu et al., 2002). The system has four major subsystems: multvaned wndmll/pump, flow/pressure stablzer, RO module, and control mechansm. It was shown that the flow rate of 13 l/mn could be processed for an average wnd speed of 5 m/s, and a bracksh feed water at a total dssolved solds concentraton of 3000 mg/l. The average rejecton rate and recovery rato were 97% and 20%, respectvely. Energy effcency equal to 35% was shown to be comparable to the typcal energy effcency of well-operated mult-vaned wndmlls. A prototype of a fully autonomous wnd powered desalnaton system has been nstalled on the sland of Gran Canara n the Canaran Archpelago (Carta et al., 2003). The system conssts of a wnd farm, made up of two wnd turbnes and a flywheel, whch supples the energy needs of a group of eght RO modules throughout the complete desalnaton process (from the pumpng of seawater to the storage of the product water), as well as the energy requrements of the control subsystems. It was hghlghted that ths system can be appled to seawater desalnaton, both on a small and large scale, n coastal regons wth a scarcty of water for domestc and/or agrcultural use. The economc feasblty of a wnd-powered RO plant was evaluated by mathematcal modellng analyss (Forstmeer et al., 2007). It was shown that the costs of a wndpowered RO desalnaton system are n lne wth what s expected for a conventonal desalnaton system, provng to be partcularly cost-compettve n areas wth good wnd resources that have hgh costs of energy. The unt cost of freshwater producton by a conventonal RO plant can be reduced up to 20% for regons wth an average wnd speed of 5 m/s or hgher. In order to optmze the desgn of Wnd/RO systems Kranouds et al. (1997) consdered desalnaton plants power-suppled by one Wnd Generator (W/G). The optmal desgn objectves are the determnaton of the optmum sze and type of the W/G and the optmum structure of the RO desalnaton unt membranes, such that the system total annual cost s mnmzed, wth respect to certan product qualty and quantty demand constrants. Ths procedure s mplemented usng a successve quadratc optmzaton algorthm. The W/Gdesalnaton systems nvestgated do not ncorporate ether electrc energy, or produced water storage unts.

12 100 Modelng and Optmzaton of Renewable Energy Systems 4.5 Reverse osmoss desalnaton drven by hybrd pv/wnd systems RO desalnaton unts drven by hybrd PV/Wnd power systems have been desgned and mplemented n dfferent areas of the world (e.g. Sultanate of Oman, Israel, Mexco, Tunsa, etc.). The performances of these unts were reported by Wener et al. (2001), Peterson et al. (1981), Bouroun and Chab (2009) and Peterson et al. (1979). Two RO desalnaton plants (Germany) usng a plate module system suppled by a 6 kw wnd energy converter and a 2.5 kw solar generator have been desgned for remote areas (Peterson et al., 1979). Two of these prototypes were nstalled n the northern part of Mexco and n a small sland on the German coast of the North Sea (Peterson et al., 1981). The desgn of a stand-alone, hybrd PV/Wnd system, used to power-supply a seawater RO desalnaton unt, based on a technoeconomc analyss, s proposed by Mohamed and Papadaks (2004). The system contans both a battery bank and a storage tank for the produced water, n order to cover the potable water demand durng the days wth neglgble solar and/or wnd energy producton. The RO unt s desgned to be able to cover the maxmum daly water demand, dctatng the correspondng maxmum total power requrements. In the second step of the proposed methodology the number of PV modules s calculated such that the maxmum energy requrements durng the year are covered, takng nto account the avalable solar radaton potental. The battery bank capacty s computed such that the electrc energy requred for two days can be stored. The volume of the water storage tank s calculated n such way that t provdes two summer day autonomy. In order to mnmze the total system cost, the developed software elmnates a part of the PV modules, determnes the correspondng produced daly energy and replaces them by one or more W/G. Ths calculaton s performed for varous combnatons of PV and W/G contrbuton percentages to the hybrd system total energy producton. The combnaton achevng the mnmum water producton cost s selected as the fnal hybrd system confguraton. Manolakos et al. (2001) dscussed the developed and the applcaton a software tool for desgnng hybrd PV/Wnd systems, whch are used to cover the electrcty and water demands of remote areas. The nomnal power ratng of the W/G and the number of the PV modules are determned through several program runs smulatng the system operaton, n order to satsfy the electrc energy and water needs. The battery bank s szed takng nto account several days of energy autonomy of the system, n order to ensure the unnterrupted power-supply durng the tme perods of low solar radaton and/or low wnd speed. The volume of the desalnated-water tank s computed to satsfy the water demand, even durng the tme perods of low RES potental avalablty. Vovontas et al. (2001) developed a computer-aded desgn tool for the prelmnary desgn of desalnaton plants drven by RES and the evaluaton of the correspondng water producton cost. The RES power producton capablty s determned usng an teratve procedure allowng an energy balance between the energy produced by the RES and the auxlary energy sources (e.g. electrc grd, desel generators etc.) and the energy requrements of the desalnaton unt. However, ths desgn method does not nclude the economc optmzaton of the resultng confguratons. The nvestgatons carred on RO desalnaton plants drven by hybrd PV/Wnd systems showed that ths knd of unts s the most effcent compared to the other RES/DES technologes. Moreover, ths technology can be mproved by optmzng

13 Optmzaton of Renewable Energy Systems: The Case of Desalnaton 101 the desgn of the overall plant. For these reasons we focus n ths chapter on the optmzaton of ths knd of systems. 5. New methodology of optmzaton hybrd PV/WIND/RO systems The operatonal performance and the relablty of the desalnaton systems drven by RES depend on ther proper desgn and szng. The optmal explotaton of the avalable RES potental s necessary n order to reduce the cost of the water produced. The objectve of ths chapter s to present a new methodology to optmze RO desalnaton system, whch s power-suppled by hybrd Photovoltac (PV) and Wnd-Generator (W/G) energy sources. Compared to the past-proposed methodologes, whch have been used n order to desgn water desalnaton systems drven by RES, the methodology presented n ths chapter has the advantage to take nto account all the crtcal operatonal parameters that affect both the resultng electrc energy and desalnated-water producton levels and the system captal and mantenance costs. The block dagram of the PV/Wnd/RO system consdered n ths study s llustrated n Fg.7. The Battery bank s charged from the respectve PV and W/G nput power sources by usng battery chargers, connected to a common DC bus. The power sources are usually confgured n multple power generaton blocks accordng to the devces nomnal power ratngs and the redundancy requrements. The battery bank, whch s usually of lead-acd type, s used to store the generated electrc energy surplus and to supply the RO desalnaton unts n case of low solar radaton and/or wnd speed condtons. DC/AC converters are used to nterface the DC battery voltage to the AC requrements of the RO desalnaton unts. A water tank s used to store the produced desalnated water surplus, whch s not drectly consumed. Power Generaton Blocks PV array W/G Battery =/= Charger Battery ~/= Charger... Battery Bank DC/AC =/~ Inverters. DC/AC =/~. Inverters RO Unt Potable water for consumpton Storage Tank Fg. 7. Block dagram of RO drven by Hybrd PV and W/G energy sources. The purpose of the proposed methodology s to derve, among a lst of commercally avalable system devces, the optmal number and type of unts such that the lfe tme round total system cost (CO Tot ) s mnmzed. At the same tme the desalnated-water demand s completely covered.

14 102 Modelng and Optmzaton of Renewable Energy Systems CO Tot s equal to the sum of the respectve components captal and mantenance costs. The decson varables for the optmzaton are: () the number and the type of the membranes, () the number and the type of the PV modules, () the number and the type of the wnd turbnes, (v) the batteres charger, (v) the DC/AC converters, (v) the heght of the turbnes and the volume of the storage tank. The mnmzaton of the system total cost functon has been mplemented usng genetc algorthms (GAs), whch have the ablty to attan the global optmum soluton wth relatve computatonal smplcty. The scope of the GAs n the proposed methodology s the calculaton of the optmum solutons n the overall state space of the desalnaton system szng problem. The block dagram depcted n Fg.8 summarzes the proposed optmzaton methodology. Ths methodology uses a database ncludng: () the techncal characterstcs of commercally avalable system devces, () ther assocated per unt captal and () ther mantenance costs. The nput of the model are the: () feed and the desalnated water qualty specfcatons () water demand profle, () daly solar rradaton on a horzontal plane, (v) the hourly mean values of ambent temperature and wnd speed. Water demand Feedwater characterstcs Desalnated water specfcatons Membranes specfcatons Daly rradaton and hourly meanvalues of temperature and wnd speed Dfferent desgns of RO unts and determnng ther energy requrements Select a combnaton of RO unts, PV modules, battery chargers, W/G and DC/AC nverter types Battery Chargers Specs. PV Modules Specs. W/G Specs. Optmal szng Batteres Specs. No All combnatons optmzed Yes Select the combnaton correspondng to the lowest cost DC/AC Inverters Specs. Fg. 8. The flowchart of the proposed optmzaton methodology.

15 Optmzaton of Renewable Energy Systems: The Case of Desalnaton 103 At the frst step the RO plant s desgned and optmzed based on water demand, feed water characterstcs and desalnated water specfcatons. One mportant outcome of ths step s the determnaton of the energy requred to operate the pumps and other auxlares. In the second step, specal attenton s pad to the desgn of energy systems related to the chosen technology and the arrangements of varous components that can meet the goal of energy demand. In ths step, the structure of the power unt, batteres, water storage and nverters are studed. Several Hybrd PV/Wnd combnatons are possble to power the desgned RO plant. To valdate the RES confguraton, a smulaton of the system operaton s performed durng the year n order to examne whether t fulfls the desalnated-water requrements. In the thrd step, a process employng GAs s executed, n order to dynamcally search for the system confguraton, whch subject to the crteron set n the frst step, results n mnmum total system cost. Durng the applcaton of the proposed methodology, the system operaton s smulated for one year wth a tme step of one hour. The power produced by the PV and W/G sources and the desalnated-water flow rate are assumed to be constant durng that tme step and they are arthmetcally equal to the correspondng energy and water volume, respectvely. 5.1 Modelng of the RO unt The equatons of flow and salt dstrbuton, used n the model, are smlar to those provded by the software for the desgn of RO membrane "FILMTEC-ROSA (DOW, 2006). RO membranes are selected after checkng the feed water characterstcs. Hence, the number of membranes N mb whch s a functon of unt capacty, the stream flow Q p and the membrane surface S mb s calculated as follows. Where f s the pure water transport coeffcent. N mb Qp (1) f.s The number N tp of pressure vessels n the system s calculated the followng equaton: Where N T s the total number of membranes per unt. N tp mb Nmb (2) N Equaton 3 s used to calculate the water flow rate produced by RO membranes. p T Q AS.. TCFF.. P (3) mb A s the membrane pure water permeablty, TCF s the temperature correcton factor, F s the membrane foulng factor (0.8 F 1), ΔP s the appled transmembrane pressure and Δп s the transmembrane osmotc pressure. The osmotc pressure n the dfferent elements of RO unt s gven by equaton 4.

16 104 Modelng and Optmzaton of Renewable Energy Systems T 273. C. C (4) Where C s the salt concentraton. The average pressure drop P between the frst and the last element s gven by equaton 5. P P 1 f P 2 fs (5) ΔP fs represents the pressure drop between feed and dscharge of a sngle element, t s gven as follows: 1.7 fc P Q (6) fs The effcency (Y k ) of the membrane s a functon of the overall performance of the RO system Y and N mb n the system (equaton 7). Yk 1 N mb 1 (1 Y) (7) The product concentraton C P s functon of recovery rate and salt rejecton (equaton 8). The brne concentraton C c of RO element s calculated from the equaton 9. Smb Cp (1 Rmb) Cfcpf TCF (8) Q p Q. C Q. C Q. C (9) f f p p c c By applyng equatons (6), (7) and (8) the: flow rates and concentratons of permeate and concentrated brne n the frst element are determned respectvely. Thus, product water s collected n the central tube and the brne becomes feed to the second element. Ths process s repeated for all elements n seres. To determne the feed pressure of the system, the model starts from last element for whch the appled pressure P a s calculated from equaton (3). The total water quantty Q T produced by RO system s gven by equaton 10. Where Q k s the amount of water produced by the cell k. The desalnated water salnty concentraton s deduced from equaton 11. Q T N mb Q (10) k1 k C T N mb k1 C. Q Q k T k (11)

17 Optmzaton of Renewable Energy Systems: The Case of Desalnaton 105 It should be noted that many other parameters are consdered n the model ncludng estmaton of the water needs, chemcal analyss of the feed water and the choce of the membranes. The optmal system desgn s targetng towards the mnmzaton of the RO energy consumpton. In ths frame energy recovery systems can be consdered. 5.2 Modelng of the photovoltac PV panels Each PV power generaton block shown n Fg. 7, conssts of N P PV modules connected n parallel and N S PV modules connected n seres. On one day (1 365) and at hour t (1 t 24) the maxmum output power of each PV power generaton block s determned. Ths calculaton s based on the specfcatons of the PV module under Standard Test Condtons (STC, cell temperature=25 C and solar rradance=1 kw/m 2 ), provded by the manufacturer, as well as the ambent temperature and solar rradaton condtons. The followng equatons (12-15) are used to desgn the PV modules: P () t N. N. V (). t I t,. FF () t (12) M s p OC SC G t, ISC(, t ) ISC, STC K I Tc () t 25 C VOC () t VOC, STC K v Tc () t 25C (13) (14) M NOCT 20C Tc() t TA() t. G (, t ) (15) 800 Where P ( t) s the maxmum output power of the PV array, I ( t, ) s the PV module short-crcut current (A), SC, STC G t, s the global rradance (W/m 2 ) ncdent on the PV module placed at tlt angle ( ), K I s the short-crcut SC I s the short-crcut current under STC, current temperature coeffcent (A/ C), VOC ( t) s the open-crcut voltage (V), V, open-crcut voltage under STC (V), A OC STC s the K v s the open-crcut voltage temperature coeffcent (V/ C), T () t s the ambent temperature ( C), NOCT s the Nomnal Operatng Cell Temperature ( C), provded by the manufacturer and FF ( t) s the Fll Factor, (Markvart, 1994). The number of PV modules connected n seres n each PV power generaton block, N S, s calculated accordng to the battery charger maxmum nput voltage, V modules maxmum open-crcut voltage level, m V OC : m DC m OC m DC, and the PV V Ns (16) V The values of the daly solar rradaton on the horzontal plane are used to calculate the value of, G t accordng to the methodology analyzed by Lorenzo (1994).

18 106 Modelng and Optmzaton of Renewable Energy Systems The battery charger power converson factor n s s defned as follows: Where, PV M PPV t, ns n1. n (17) 2 P (, t ) P t s the PV power really transferred to the battery bank by each PV power generaton block, n 1 s the battery charger power electronc nterface effcency and n 2 s a converson factor, whch depends on the battery chargng algorthm executed durng the charger operaton and ndcates the devaton of the actual PV power generated from the correspondng maxmum power. In case that the battery charger operates accordng to the Maxmum Power Pont Trackng (MPPT) prncple (Esram and Chapman, 2007), n 2 s approxmately equal to 1, otherwse ts value s much lower. The values of n 1 and n 2 are specfed by the battery charger manufacturer. 5.3 Modelng of wnd generator W/G The varaton of the W/G output power versus the wnd speed s provded by the manufacturer. It usually ndcates the actual power transferred to the battery bank from the W/G source, takng nto account the effects of both the battery charger power electronc nterface effcency and the MPPT operaton, f avalable. Thus, n the proposed methodology, the power transferred to the battery bank at hour t of day, from each W/G WG power generaton block, P ( t, h ), s calculated usng the followng lnear relaton: P P PWG (, t h) P1 v (, t h) v 1. v v (18) where h (m) s the W/G nstallaton heght, v ( t, h) s the wnd speed (m/s) at heght h (h low h h hgh accordng to the lmts h low and h hgh specfed by the W/G manufacturer) and (P 1, v 1 ), (P 2, v 2 ) are the W/G output power and wnd speed pars. If the nput wnd speed data are measured at a dfferent heght than the desred W/G nstallaton heght, h, thus, v (, t h) s corrected usng the followng exponental law: h v (, t h) vref (). t h ref (19) ref where v ( t) s the reference (nput) wnd speed (m/s) measured at heght h ref (m) and the exponent ranges from 1/7 to 1/ Modelng of batteres The number of batteres connected n seres n each of the multple, parallel-connected s battery strngs formng the battery bank, n B, depends on the nomnal DC bus voltage and the nomnal voltage of each ndvdual battery, V B (V):

19 Optmzaton of Renewable Energy Systems: The Case of Desalnaton 107 n s B V BUS (20) VB The value of the battery bank nomnal capacty, C n (Ah), depends on the total number of batteres, N BAT, the number of seres connected batteres and the nomnal capacty of each battery, C B (Ah): C n NBAT. C S B (21) n The maxmum permssble battery depth of dscharge, DOD (%) s specfed by the system desgner at the begnnng of the optmal szng procedure and t dctates the value of the mnmum permssble battery bank capacty durng dschargng, C mn (Ah), whch s calculated as follows: C mn B DOD C (22) Durng the desalnaton system operaton the avalable battery bank capacty s modfed accordng to the PV and W/G energy producton levels and the power requrements of the desalnaton unts. Ths varaton s expressed by the followng equaton:. n PB () t C () t C ( t1) nb.. t (23) V 1 C BUS C (24) (0) (24) where C (t), C (t 1) s the avalable battery capacty (Ah) at hour t and t 1, respectvely, of day, n B =80% s the battery round-trp effcency durng chargng and n B =100% durng dschargng (Borowy and Salameh, 1996), V BUS s the nomnal DC bus voltage (V), P ( t ) s B the battery nput/output power (W) ( P () t <0 durng dschargng and P () t >0 durng chargng) and Δt s the smulaton tme step (Δt=1 h). In order to avod the battery performance degradaton under practcal operatng condtons the maxmum permssble battery bank chargng or dschargng current has been lmted to (C n /5) h. The ntal capacty of the battery bank, C 1 (0), s calculated usng the followng equaton: B B 1 1 DOD C (0). C 2 n (25) 5.5 Modelng of the global RE/DES system The PV panels and W/G must be szed such that the produced energy durng the year allows to completely satsfy the desalnaton system energy requrements. Hence, the remanng battery bank capacty at the end of the smulaton perod must be hgher than ts ntal value: C (24) C (0) (26)

20 108 Modelng and Optmzaton of Renewable Energy Systems When the necessary power for the RO operaton s avalable, then the desalnaton process s performed and desalnated water s produced. Otherwse, the operaton of the RO unts s suspended. In ths case, cleanng of each RO unt membranes should be performed, usng flushng technques. The total power produced by the PV and Wnd Turbnes at hour t of day s calculated as follows: PV RE ch S M WG WG P () t N. n. P (, t ) N. P (, t h) (27) where N WG s the total number of W/G power generaton blocks ncorporated n the desalnaton system. At the hour t of the day the total DC power nput to the DC/AC T nverters, P ( t ) (W), s related wth the total AC power supplyng the desalnaton unts, RO P () t (W), accordng to the followng equaton: PRO() t PT () t (28) n where n (%) s the power converson effcency of the DC/AC nverters. The mnmum permssble amount of water stored n the tank, V mn (m 3 ), should be fxed (generally set equal to 25% to 30% of the tank total volume, V TANK (m 3 )). The volume of the avalable water stored n the tank at hour t of day, V (t) (m 3 ), s modfed durng the desalnaton system operaton, such that: mn () V V t V TANK (29) When the desalnated water demand at hour t of day V ( t ), (m 3 ), s defned then the energy and water flows among the components of the system can be descrbed by the Fg.9. The developed desalnaton system model should be used to smulate the system operaton on a yearly bass to check the feasblty of the proposed soluton. The optmzaton of the whole system s acheved by usng the Genetc Algorthms methods by consderng potental solutons. 5.6 System total cost mnmzaton usng gas The genetc algorthms (GAs) are used for desgnng and szng a, through the calculaton of optmum solutons n the overall state space. The role of the GA s to derve the optmal desalnaton system confguraton by selectng chromosomes from the total state space of potental solutons, whch mnmze the problem's objectve functon and smultaneously lead to a successful system operaton durng the whole year. GAs s an optmum search technque based on the concepts of natural selecton and survval of the fttest ndvduals. It works wth a fxed-sze populaton of possble solutons of a problem, whch are evolvng n tme. A genetc algorthm utlzes three prncpal genetc operators; selecton, crossover andmutaton. Compared to conventonal optmzaton methods, such as dynamc programmng and gradent technques, genetc algorthms are able to: () handle complex D

21 Optmzaton of Renewable Energy Systems: The Case of Desalnaton 109 problems wth lnear or non-lnear cost functons, both accurately and effcently and () attan the global optmum soluton wth relatve computatonal smplcty, wthout beng restrcted by local optma (Mchalewcz, 1994). PT (t) sdetermned PRE( t) PT ( t) yes The RO desalnaton plant s turned off No vtank ( t) vd( t) yes -The Water Tank s dscharged by vd(t) - The battery bank s charged by PRE(t) No The stored energy n the Battery banks adequate yes -The batterybank s dscharged by PT ( t) PRE( t) No vro( t) vd( t) yes The amount of the water stored remans constant No vro( t) vd( t) yes -The water surplus Is stored n the tank vro( t) vd( t) yes -The battery bank s charged by PRE( t) PT ( t) -The water surplus VRO( t) VD( t) Is stored n the tank No vtank( t) vd( t) vro( t) vro( t) vd( t) No yes -The battery bank s charged by PRE( t) PT ( t) - The amount of the water stored remans constant No vtank( t) vd( t) vro( t) yes -The battery bank s charged by PRE( t) PT ( t) - The water demand s covered by the water stored n the tank No -The system s consdered unsuccessful PBAT ( t) PFL ( t) yes Flushng s processed Flushng s postponed -The system s consdered unsuccessful yes -The battery bank s charged by PRE( t) PT ( t) - The water demand s covered by the water stored n the tank -The system s consdered unsuccessful Fg. 9. The flowchart of energy and water flows among the components of the system. The GA chromosomes are n the form of X = [N mb, N PV, N WG, N BAT, h,, W TANK ]. The objectve functon to be mnmzed by the GA s equal to the sum of the captal and mantenance costs evolvng durng the desalnaton system lfetme perod: f ( x) COAq. RO 20. COM. RO COAq. INV ( INV 1) COM. INV.(20 INV 1) N CO CO N CO CO h CO h CO PV Aq PV M PV WG Aq WG M WG Aq h M h N. BAT COAq. BAT BAT. COAq. BAT (20 BAT 1). COM. BAT ) PV PV PV Nch. PV PV COAq. ch ch 1 COM. ch.(20 ch 1) V TANK. COAq. Tank 20. CO M. TANK wth the followng constrants : N mb 1; N PV 0; N WG 0; N / n 1 ;V TANK 0; h low h h hgh ; BAT s B (30)

22 110 Modelng and Optmzaton of Renewable Energy Systems where Aq. RO CO, CO Aq. PV, CO Aq. WG, CO Aq. BAT, CO Aq. INV, CO Aq. ch, CO Aq. Tank and CO. are the captal costs of the RO desalnaton unts, PV modules, W/Gs, batteres, DC/AC nverters, PV battery chargers, water storage tank (per m 3 ), and W/G nstallaton tower (per m), respectvely. CO M. RO, CO M. PV, CO M. WG, CO M. BAT, CO M. INV, CO M. ch, CO M. TANK and CO are the annual mantenance costs of the RO plant, PV modules, W/Gs, batteres, M. h DC/AC nverters, PV battery chargers, water storage tank (per m 3 ) and W/G nstallaton tower (per m), respectvely. BAT s the expected number of battery replacements durng the 20-year system operaton, because of lmted battery lfetme and PV ch and INV are the expected numbers of PV battery chargers and DC/AC nverters replacements durng the system 20-year lfetme perod, whch are equal to the system lfetme perod (20 years) dvded by the Mean Tme Between Falures (MTBF) of power electronc converters. Each of the captal costs ncorporated n Eq. (30) ncorporates the market prce and the nstallaton cost of the respectve devce. Intally, a populaton of chromosomes s generated randomly and the constrants descrbed by the nequaltes (Eq. 30) are evaluated for each chromosome. If any of the ntal populaton chromosomes volates these constrants then t s replaced by a new, randomly generated chromosome, whch fulfls these constrants. The frst step of the GAbased optmal szng algorthm teraton s the ftness functon evaluaton for each chromosome of the extracted populaton. If any of the resultng ftness functon values s lower than the lowest value obtaned at the prevous teratons then ths value s consdered to be the optmal soluton of the mnmzaton problem and the correspondng chromosome's values are consdered to be the desalnaton system's optmal szng and operatonal parameters. Ths optmal soluton s replaced by better solutons, f any, produced n subsequent GA generatons durng the program evoluton. The selecton of the chromosomes whch wll be subject to the crossover andmutaton operatons, thus producng the next generaton populaton, s based on the roulette wheel method (Mchalewcz, 1994). The crossover mechansm uses the Smple Crossover, Smple Arthmetcal Crossover and Whole Arthmetcal Crossover operators. Next, the selected chromosomes are subject to the mutaton mechansm, whch s performed usng the Unform Mutaton, Boundary Mutaton and Non-Unform Mutaton operators. In case that the applcaton of the crossover or mutaton operators results n a chromosome whch does not satsfy the optmzaton problem constrants, then a repar procedure s performed and that chromosome s replaced by the correspondng parent. In case of the Smple Crossover operaton, where each new chromosome s generated by two parents, then the chromosome s replaced by the parent wth the best ftness functon value. The GA optmzaton process descrbed above s repeated untl a predefned number of populaton generatons have been evaluated. PV PV Aq h 6. Smulaton results and dscusson The proposed methodology, has been appled and tested for the desgn and optmal szng of RO desalnaton systems power-suppled by PV and W/G energy sources, located n the area of Ksar Ghlène Vllage (300 nhabtants), southern Tunsa at: