Volume 3 solar thermal systems components and applications 3 16 – solar desalination

Volume 3 solar thermal systems components and applications 3 16 – solar desalination Volume 3 solar thermal systems components and applications 3 16 – solar desalination Volume 3 solar thermal systems components and applications 3 16 – solar desalination Volume 3 solar thermal systems components and applications 3 16 – solar desalination Volume 3 solar thermal systems components and applications 3 16 – solar desalination Volume 3 solar thermal systems components and applications 3 16 – solar desalination

3.16 Solar Desalination E Tzen, Centre for Renewable Energy Sources and Saving (CRES), Pikermi, Attica, Greece G Zaragoza and D-C Alarcón Padilla, Centro de Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT), Plataforma Solar de Almeria, Almeria, Spain © 2012 Elsevier Ltd All rights reserved 3.16.1 3.16.2 3.16.3 3.16.4 3.16.5 3.16.6 3.16.7 3.16.8 3.16.8.1 3.16.8.2 3.16.8.3 3.16.8.4 3.16.8.5 3.16.8.6 3.16.8.7 3.16.9 3.16.10 3.16.11 3.16.12 References Introduction Solar Thermal Desalination Systems Photovoltaics-Driven Desalination Systems Solar Stills Solar Humidification–Dehumidification Solar Membrane Distillation Technologies Selection Guidelines Solar Desalination Applications Solar Thermal MES Plant for Seawater Desalination, Abu Dhabi, UAE Solar Thermal MED Plant for Seawater Desalination, Almeria, Spain PV–RO Plant for Seawater Desalination, Lampedusa Island, Italy PV–RO Plant for Brackish Water Desalination, Ceara, Brazil PV–RO Plant for Seawater Desalination, Pozo Izquierdo, Gran Canaria Island PV Water Pumping RO for Brackish Water Desalination, Saudi Arabia PV–RO Brackish Water Desalination, Aqaba, Jordan Lessons Learned Economics Market Conclusions Glossary Brackish water Saline water with a salt concentration ranging from 1000 mg/l to about 25 000 mg/l total dissolved solids (TDS) Desalination Process of removing salts from water sources Distillation A method of desalting water that uses heat to vaporize water and collect the condensed water Electrodialysis (ED) A process by which ions are transferred through membranes to a more concentrated solution as a result of using a direct current electrical potential Electrodialysis reversal (EDR) A variation of ED in which polarity and cell function change periodically to maintain efficient performance Energy recovery Possible energy saving in reverse osmosis in which the concentrate stream, under pressure, is used to drive a turbine that provides part of the feed requirement kWh Kilowatthours A measure of electrical usage Membrane In desalting, used to describe a semipermeable film Membranes used in electrodialysis 529 530 536 541 542 543 546 547 547 549 553 556 558 558 560 563 564 564 564 565 are permeable to ions of either positive or negative charge Reverse osmosis membranes ideally allow the passage of pure water and block the passage of salts Performance ratio (PR) A performance rating associated with the distillation desalting process It is defined as the number of pounds of distillate produced for each 1000 Btu of heat input, or as kg/MJ in metric Post-treatment The processes, such as pH adjustment and chlorination, that may be employed on the product water from a desalting unit Pretreatment The processes such as chlorination, clarification, coagulation, scale inhibition, acidification, and deaeration that may be employed on the feed water to a desalting unit to minimize algae growth, scaling, and corrosion Recovery ratio The ratio of the product flow rate to the feed water flow rate Reverse osmosis (RO) Method of desalination which uses pressure to move water from a concentrated solution to a dilute solution through a membrane separating two solutions 3.16.1 Introduction Converting seawater or brackish water into freshwater is a promising approach to overcome the insufficiency in water supply caused by population increase, agricultural and irrigation needs, industrial needs, etc Production of freshwater using desalination technologies driven by renewable energy sources (RESs) is thought to be a viable solution to the water scarcity in remote areas characterized by lack of potable water and lack of an electricity grid RES–desalination matching is mainly categorized as distillation Comprehensive Renewable Energy, Volume doi:10.1016/B978-0-08-087872-0.00316-4 529 530 Applications desalination technologies driven by heat produced by RESs, and membrane and distillation desalination technologies driven by electricity or mechanical energy produced by RESs Indirect use of solar energy by means of solar thermal systems and photovoltaics (PVs) in tandem with desalination seems to be the most applicable technology Direct use of solar energy for desalination, such as the use of solar stills, is the oldest, simplest, and most used method Figure presents the possible combinations of solar energy technologies with desalination The selection of the most appropriate combination is mainly site specific Several parameters that affect the final decision are discussed in the following paragraphs Many applications of relatively small scale exist around the world; some of the most known are examined in this chapter The majority of the solar desalination systems involve the use of photovoltaic-driven reverse osmosis (PV–RO) units for brackish and seawater desalination Most of the already existing applications have been built within National or European projects and are pilot demonstration units [1] Some of the installed units cover basic needs of the region where they have been installed, while some are no longer in operation Nevertheless, the lessons learned from their operation have been passed on and are the guidelines for the new installations This chapter focuses on the state of the art of the solar desalination technologies, their current applications, the lessons learned, and the economics and market for these technologies 3.16.2 Solar Thermal Desalination Systems Solar energy refers to applications of solar thermal conversion and PV conversion Solar thermal systems are usually classified according to the temperature level reached by the thermal fluid in the collector (Figures and 3) The thermal effects produced by Electric energy Solar energy Direct use Solar stills Indirect use T h e r m a l e n e r g y Photovoltaics Reverse osmosis Electrodialysis Multieffect distillation Solar thermal collectors Multistage flash Membrane distillation Humidification− Dehumidification Figure Solar energy–desalination matching Figure Parabolic-trough collectors, CIEMAT PSA, Almeria, Spain Solar Desalination 531 Figure Fresnel Technology, CIEMAT PSA, Almeria, Spain solar radiation enables Man to take direct advantage of them by using devices that collect, concentrate, and intensify the heat and transfer the thermal energy to other fluids by heating them Depending on the design of the collector, it can provide heat for domestic applications, industrial processes, and electricity production There are basically two types of solar collectors: Stationary or nonconcentrating Concentrating A nonconcentrating collector has the same area for intercepting and absorbing solar radiation, whereas a sun-tracking, concentrating solar collector usually has concave reflecting surfaces to intercept and focus the sun’s radiation to a smaller receiving area, thereby increasing the radiation flux Table presents the different types of available solar collectors and their main characteristics [2] A solar thermal energy system mainly consists of a solar collector array, a storage tank, and necessary controls (Figure 4) The solar collector system provides the desalination unit with the required hot steam Analytically, solar thermal distillation plants include a field of solar collectors, where a thermal fluid is heated This hot fluid is used, by means of a heat exchanger, to warm up the feedwater circulating through the distillation plant The collectors must be able to heat the thermal fluid up to medium temperatures so that after appropriate heat transfer, the water fed to the evaporator reaches temperatures between 70 and 120 °C Temperature limits protect the distillation plant from scaling and corrosion problems The best known solar thermal distillation combinations are the solar multistage flash (MSF) and solar multieffect distillation (MED) Both processes are classified as phase-change or thermal processes Distillation units routinely use designs that convert as much thermal energy as possible by interchanging the heat of condensation and heat of vaporization within the units The major energy requirement in the distillation process is, thus, providing the heat of vaporization to the feedwater MSF and MED processes consist of a number of stages or effects at successively decreasing temperatures and pressures, and generally operate on the principle of reducing the vapor pressure of water within the unit to permit boiling to occur at lower temperatures, without any extra heat Table Main types of solar collectors Reproduced from: Kalogirou S (2003) The potential of solar industrial process heat applications Applied Energy 76(4): 337–361 Motion Collector type Absorber type Concentration ratio Indicative temperature range (°C) Stationary Flat-plate collector (FPC) Evacuated tube collector (ETC) Flat Flat Single-axis tracking Compound parabolic collector (CPC) Linear Fresnel reflector (LFR) Parabolic-trough collector (PTC) Cylindrical-trough collector (CTC) Parabolic-dish reflector (PDR) Heliostat field collector (HFC) Tubular Tubular Tubular Tubular Point Point 1 1–5 5–15 10–40 15–45 10–50 100–1000 100–1500 30–80 50–200 60–240 60–300 60–250 60–300 60–300 100–500 150–2000 Two-axes tracking Note: Concentration ratio is defined as the aperture area divided by the receiver/absorber area of the collector 532 Applications Three-way valve Solar collector array Conventional boiler Hot water storage tank Desalination unit Solar pump Figure Typical configuration of a solar thermal desalination plant [2] Adapted from Kalogirou S (2003) The potential of solar industrial process heat applications Applied Energy 76(4): 337–361 The MSF process is based on the generation of vapor from seawater or brine caused by a sudden pressure reduction when seawater enters an evacuated chamber [3] The process is repeated stage by stage at successively decreasing pressures This process requires an external steam supply, normally at a temperature between 100 and 110 °C The maximum temperature is limited by the salt concentration to avoid scaling, and this limits the performance of the process There are two process arrangements for the MSF process: once-through and brine recirculation In the brine recirculation MSF, the system is divided into the heat-rejection, the heat-recovery, and the heating sections (see Figure 5) Seawater is fed through the heat-rejection section that rejects thermal energy from the plant and discharges the product and brine at the lowest possible temperature [3] The feed is then mixed with a large mass of water, which is recirculated around the plant This water then passes through a series of heat exchangers to raise its temperature The water then enters the solar collector array or a conventional brine heater to raise its temperature to nearly the saturation temperature at the maximum system pressure The water then enters the first stage through an orifice and in doing so has its pressure reduced Since the water was at the saturation temperature at high pressure, it becomes superheated and flashes into steam The vapor produced passes through a wire mesh (demister) to remove any entrained brine droplets and then into the heat exchanger, where it condenses and drips into a distillate tray Generally, only a small percentage of this water is converted to steam (water vapor), depending on the pressure maintained in this stage, since boiling will continue only until the water cools [5] This process is repeated through the plant as both brine and distillate streams flash as they enter subsequent stages, which are at successively lower pressures To atmosphere Steam jet ejector HP steam supply LP steam supply Cooling water return Concen­ trate heater Sea water supply Distillate Tube bundle section Deaerator Flash chamber section Decarbonator Make-up Stage Heat rejection section Stage Stage Heat recovery section Condensate pump Blowdown pump Concentrate recycle Figure MSF brine recirculation schematic Adapted from Watson IC, Morin OJ, Jr., and Henthorne L (2003) Desalting Handbook for Planners, 3rd edn USA: U.S Department of the Interior Bureau of Reclamation [4] Solar Desalination 533 In MSF, the number of stages is not tied rigidly to the performance ratio (PR), which is the ratio of the distillate produced to the heat input (steam consumed, kilograms per megajoule) required from the plant In practice, the minimum temperature must be slightly greater than the PR, while the maximum is determined by the boiling-point elevation The minimum interstage temperature drop must exceed the boiling-point elevation for flashing to occur at a finite rate This is advantageous because as the number of stages is increased, the terminal temperature difference over the heat exchangers increases and hence a smaller heat transfer area is required with obvious savings in plant capital cost MSF is the most widely used desalination process in terms of capacity MSF plants are generally built-in units of about 4000–55 000 m3 day−1 Current commercial installations are designed with 10–30 stages (2 °C temperature drop per stage) [3] The MED process takes place in a series of vessels (effects) and uses the principles of condensation and evaporation at reduced ambient pressure in the various effects This permits the seawater feed to boil without the need for supply of additional heat beyond the first effect In general, an effect consists of a vessel, a heat exchanger, and devices for transporting the various fluids between the effects As with the MSF plant, the incoming brine in the MED process also passes through a series of heaters, but after passing through the last of these, instead of entering the brine heater, the feed enters the top effect, where the heating steam raises its temperature to the saturation temperature for the effect pressure Further amounts of steam, either from a solar collector system or from a conventional boiler, are used to produce evaporation in this effect The vapor then goes, in part, to heat the incoming feed and, in part, to provide the heat supply for the second effect, which is at a lower pressure and receives its feed from the brine of the first effect (see Figure 6) This process is repeated all the way through (down) the plant The distillate also passes down the plant Unlike the MSF plant, the PR for an MED plant is more rigidly linked to and cannot exceed a limit set by the number of effects in the plant For instance, a plant with 13 effects might typically have a PR of 10 However, an MSF plant with a PR of 10 could have 13–35 stages depending on the design MED plants commonly have PRs as high as 12–14 [3] The choice and optimization of the PR and the number of effects in an MED plant is a matter of balancing the high energy costs associated with a setup with a low PR and a small number of effects against the high capital costs of a setup with a large number of effects, and large transfer surfaces in both the effects and the feed heaters Three main arrangements have evolved for MED processes They are based primarily on the arrangement of the heat exchanger tubing which can be as follows [4]: • Horizontal tube arrangement • Vertical tube arrangement • Vertically stacked tube bundles MED plants are typically built-in units of 2000–20 000 m3 day−1 Some of the more recent plants have been built to operate with a top temperature (in the first effect) of about 70 °C, which reduces the potential for scaling of seawater within the plant [5] Steam jet ejector HP steam supply Feed water supply Main condenser Steam supply Tube bundle section Tube bundle section Tube bundle section Tube bundle section Tube bundle section Condensate return Cooling water return (2) (2) Effect pumps (1) Degassifier deaerator (2) (2) Effect pumps (2) Concentrate chamber (2) (1) Make-up pump Product to storage Blowdown Figure MED schematic [4] Adapted from Watson IC, Morin OJ, Jr., and Henthorne L (2003) Desalting Handbook for Planners, 3rd edn USA: U.S Department of the Interior Bureau of Reclamation 534 Applications As can be concluded from the above, thermal distillation technologies are best fitted to large capacities; however, research has been done in small capacities also Several solar thermal plants have been installed and examined around the world (see Table 2) [1] An example is the MSF plant installed in 1987 in El Paso, Texas The combination was somewhat unusual involving a 3355 m2 solar pond and a cogeneration system, producing electricity in a Rankine cycle and water in a 24-stage MSF evaporator capable of producing 19 m3 day−1 More reference cases can be found at San Luis de la Paz, Mexico, where a double solar field (194 m2 flat-plate collectors plus 160 m2 concentrating collectors) provides heat for a 10 m3 day−1 MSF unit, with 10 stages The plant was commissioned in 1980 One more example is found in Lampedusa Island in Italy The plant was commissioned in 1983 The MSF plant had a capacity of 7.2 m3 day−1 driven by 408 m2 solar collectors Another solar MSF system was installed in Safat, Kuwait, in 1981 The 12-stage MSF plant had a capacity of 10 m3 day−1 driven by 220 m2 parabolic-trough solar collectors Concerning solar MED applications in 1981, a solar MED plant of 10 m3 day−1 capacity was installed in Takeshima Island in Japan, and in the same year a m3 day−1 solar MED plant started its operation at the Black Sea, Bulgaria A very famous solar MED plant is the one in Abu Dhabi, United Arab Emirates The system consists of 1862 m2 evacuated-tube collectors (ETCs) and an MED plant of around 120 m3 day−1 distillate water capacity The plant has many years of satisfactory operation The plant is discussed analytically in the following paragraphs Another well-known example of a solar MED plant of m3 h−1 water capacity and 500 m2 compound parabolic concentrators (CPC) is located at the Plataforma Solar de Almeria (PSA), Spain, as part of the AQUASOL project (Figure 7) The plant is described analytically in the following paragraphs [6] A small solar thermal MED plant was commissioned in 2002 in Muscat in the Sultanate of Oman (owned by the M/S Power System International) and was operational for a year (see Figure 8) The pilot MED plant was designed to produce m3 day−1 in h Table Solar thermal desalination plants Location Desalination unit Solar collector Year of installation Hazeg, Tunisia San Luis de la Paz, Mexico Lampedusa Island, Italy Safat, Kuwait Takami Island, Japan Abu Dhabi, UAE Almeria, Spain Almeria, Spain Almeria, Spain, AQUASOL project 0.1–0.35 m3 h−1 distillation 10 m3 day−1 MSF 7.2 m3 day−1 MSF 10 m3 day−1 MSF 16 m3 day−1 MED 80–120 m3 day−1 MED 72 m3 day−1 MED 24 m3 day−1 MED 30–40 l day−1 1980 1980 1983 1984 – 1984 1993 1988/1990 1998 Al Azhar, PSA Pozo Izquierdo, Gran Canaria, Spain, SODESA project Oman 0.2 m3 day−1 MSF 0.6 m3 day−1 – 352 m2 FPC + PTC 408 m2 low-concentration solar collectors 220 m2 low-concentration solar collectors FPC ETC PCP Parabolic concentrating m2 Vacuum-tube solar collectors FPC + PVs 50 m2 solar collectors + PVs m3 day−1 MED 5.34 m2 VTC 2002 Figure Compound parabolic collectors (CPC), CIEMAT PSA, Almeria, Spain 1998/2000 2000 Solar Desalination 535 Figure View of the solar thermal MED unit in Oman Adapted from Report on the Status of Autonomous Desalination Units Based on Renewable Energy Systems (2005) INCO-CT-2004-509093, Co-ordination Action for Autonomous Desalination Units Based on Renewable Energy Systems, ADU-RES Project www.medrc.org.om of operation during the day, using solar energy The innovative techniques and methods used in the plant included high-temperature tubular solar collectors, scale-preventing coating, and a device for water softening The desalination system operated at a top brine temperature of 100 °C Water recovery reached 80–85% The MED unit included 12 effects and preheaters The thermal energy consumption was 64 kWh m−3, while the electric energy consumption was 1.4 kWh m−3 The plant was tested with water salinity of 30 000–35 000 ppm total dissolved solids (TDS) The salinity of the water produced by the plant was 80–120 ppm TDS The solar thermal system consisted of high-temperature vacuum tubular solar collectors with mirror concentrators, a separator for steam and water separation, and a solar tracking system The solar power plant included 16 collector panels, with an effective collector area of 5.34 m2 The circulation flow rate through the whole collector system was 460 l h−1 and the maximum pressure of operation was 1.05 bar The system’s electricity demand was satisfied by m2 PV cells [6] The project was cofinanced by the Middle East Desalination Research Center (MEDRC) in Oman Also, a number of small distillation plants have been installed in Tunisia: a 40–50 l h−1 single-effect evaporation process for brackish water desalination in Hazeg, Sfax (1986), an MED of 150–200 l day−1 for seawater desalination in Béni Khiar, Nabeul (2003), and a solar multiple condensation evaporation cycle (SMCEC) system of 12–30 l h−1 for brackish water desalination in the University of Sfax, Sfax [7] Finally, a solar MED plant of small scale has been recently installed in Paphos, Cyprus, to cover the water needs of a public swimming pool (see Figures 9–11) The one-effect MED unit desalinates seawater and has a capacity of m3 day−1 The solar thermal plant consists of 110 m2 high-efficiency selective flat-plate collectors The MED plant operates at a temperature around 75 °C The plant was developed within the ADIRA project [8] Heat exchanger Solar collectors (100 m2) Condenser Evaporator Thermal storage tank (5m ) Distillate Posttreatment Potable water Seawater Saline water Figure Schematic diagram of the solar MED plant 536 Applications Figure 10 The MED seawater desalination plant, Paphos, Cyprus Figure 11 The solar collector area, Paphos, Cyprus 3.16.3 Photovoltaics-Driven Desalination Systems PVs are specially designed semiconductor devices that convert sunlight directly into electricity They are modular devices having long life and characterized by low maintenance requirements The basic component of a PV system is the solar cell Groups of cells are mounted on a glass plate and wired in series to form a PV module Groups of modules electrically connected together form a PV array (Figure 12) The nominal voltage and current of an array depend on the number of modules connected in series and in parallel PV arrays can be mounted on fixed or on sun-tracking structures to maximize the incident solar radiation on the solar cell surface The power production capacity of a PV array is expressed in watt peak (Wp) units A solar cell is said to be of Wp power if it produces W of electric power when exposed to ‘peak’ solar irradiance (1000 W m−2) at a solar cell temperature of 25 °C [9] Figure 12 The kWp photovoltaic system, CRES, Greece Solar Desalination 537 There are various types of PV cells The main ones are monocrystalline, polycrystalline, amorphous silicon, and other thin films PVs can be used directly with the load, such as in water pumping and grid connected or stand-alone The energy production unit consists of a number of PV modules, which convert solar into direct current (DC) electricity The most suitable desalination processes for this combination should use electricity in some form Therefore, reverse osmosis (RO) and electrodialysis (ED) appear as the most suitable choices for coupling with PV systems RO is a membrane separation process in which the water from a pressurized saline solution is separated from the solutes (the dissolved material) by making it flow through a membrane The amount of desalinated water that can be obtained (recovery ratio) ranges between 30% and 75% of the volume of the input water, depending on the initial water quality, the quality of the product needed, and the technology and membranes involved No heating or phase change is necessary for this separation The major energy required for desalting is for pressurizing the feedwater (the saline feedwater is pumped into a closed vessel where it is pressurized against the membrane) (see Figure 13) Theoretically, the only energy requirement is to pump the feedwater at a pressure above the osmotic pressure In practice, higher pressures must be used, typically 14–25 bar for brackish water and from 55–80 bar for seawater desalination, in order to have a sufficient amount of water pass through a unit area of membrane Figure 13 shows an illustration of the feed being pressurized by a high-pressure pump and which is made to flow across the membrane surface Part of this feed passes through the membrane, where most of the dissolved solids are removed The remainder, together with the remaining salts, is rejected at high pressure In large plants, it is economically viable to recover the rejected brine energy with a suitable brine turbine [3] Such systems are called energy recovery devices The fraction of power, recovered by the power recovery device, depends on the type and efficiency of the power recovery equipment used In general, two recent developments have helped to reduce the operating cost of RO plants in the past decade: the use of energy recovery devices and the development of more efficient membranes (operational at lower pressures) The main advantages of RO process are the modularity/compactness and the sufficient performance and reliability in all scales ED is an electrochemical process and a low-cost method for the desalination of brackish water Due to the dependency of the energy consumption on the feedwater salt concentration, the ED process is not economically attractive for the desalination of seawater In the ED process, ions are transported through a membrane by an electric field applied across the membrane The process utilizes a DC electric field to remove salt ions in the brackish water [3] Saline feedwater contains dissolved salts separated into positively charged sodium and negatively charged chlorine ions These ions will move toward an oppositely charged electrode immersed in the solution, that is, positive ions (cations) will move toward the negative electrode (cathode) and negative ions (anions) toward the positive electrode (anode) If special membranes, alternatively cation-permeable and anion-permeable, separate the electrodes, the gap between these membranes will be depleted of salts [3] In an actual process, a large number of alternating cation and anion membranes are stacked together, separated by plastic flow spacers that allow the passage of water (Figure 14) The streams through alternating flow-spacers are a sequence of diluted and concentrated water which flow parallel to each other To prevent scaling, inverters are used to reverse the polarity of the electric field in about every 20 As the energy requirements of the system are proportional to the water’s salinity, ED is economically attractive for low concentration brackish water with TDS equal to or less than 3500 ppm A typical stand-alone system consists of the PV modules, the charge controller(s), the battery bank, and the inverters(s) The main advantages in the coupling of PVs with desalination units are the ability to develop small-size desalination plants, the limited maintenance cost of PVs, as well as easy transportation and installation High pressure pump RO plant Steriliser Cartridge filter Seawater Antiscalant Dual media filter Acid Membranes modules Decarbonator Pretreatment Brine to waste Energy recovery turbine (if fitted) Figure 13 RO schematic [2] Posttreatment Lime Product 538 Applications Feed in Concentrate in Electrode feed Top end plate Electrode waste (−) Cathode Cation transfer membrane Demineralized flow spacer Anlon transfer membrane Concentrate flow spacer (+) Anode Electrode feed Bottom end plate Electrode waste Product Concentrate out Figure 14 ED stack assembly Adapted from Watson IC, Morin OJ, Jr., and Henthorne L (2003) Desalting Handbook for Planners, 3rd edn USA: U.S Department of the Interior Bureau of Reclamation RO usually uses alternating current (AC) for the pumps, which means that DC/AC inverters have to be used Energy storage is again a matter of concern, and batteries are used for PV output power smoothing or for sustaining system operation when sufficient solar energy is not available The typical PV–RO applications are of stand-alone type, and there exist some interesting examples Table presents the data on the PV–RO plants that have been installed within the last two decades for seawater and brackish water desalination Several plants have been built during the 1980s A brackish water desalination application was installed in 1982 in Perth, Australia The plant consisted of 1.2 kWp PV to drive a 0.1 m3 h−1 RO unit Another such plant was installed in 1984 in Vancouver, Canada: a seawater RO unit of m3 day−1 product water capacity with a 4.8 kWp PV system In 1984, a 11.2 kWp PV to drive a 5.7 m3 day−1 seawater RO unit was installed in Doha, Qatar Another RO plant, set up in 1986, is located at El Hamrawein, at the edge of Red Sea The PV array is rated at 19.84 kWp, delivering voltage of 104 V for the pumps as main consumption plus a secondary array rated 0.64 kWp at 24 V for instruments and control The battery storage unit has a capacity of 208 kWh and is designed for days of autonomy The RO plant has a capacity of 10 m3 h−1, operating at a pressure of 13 bar The feedwater is brackish water having a salinity of 4400 mg l−1 TDS The unit energy consumption is 0.89 kWh m−3 During 1990–2000, with technical improvements in both technologies, bigger RES desalination plants were installed to cater to water needs [11] For RO, the development of efficient energy recovery devices and the operation of the membranes at lower pressures significantly reduced the energy requirements and obviously the power requirements of the RES power supply plants On the other hand, the cost of PV fell dramatically During this period, a lot of work was done by the Instituto Tecnológico de Canarias (ITC) in Spain [12] Several combinations of RES desalination systems such as photovoltaic–electrodialysis reversal (PV–EDR), wind–mechanical vapor compression (MVC), and PV–RO were installed and examined; one of them is presented in this chapter Furthermore, in order to reduce the cost and maintenance requirements, the direct coupling of PV to RO unit is examined In most cases, the power variability from the solar source reduces the lifetime of the membranes The Centre for Renewable Energy Systems Technology, CREST, UK, installed a 1.54 kWp PV-powered seawater RO unit without batteries The system operates at variable flow, enabling it to make efficient use of the naturally varying solar resource, without the need of batteries The same RO unit has also been coupled and tested with a wind turbine without any battery bank Frequent replacement of the RO membranes is mentioned The electricity from PVs for desalination applications can be used for electromechanical devices such as pumps, or in a DC device for ED ED uses DC for the electrodes at the cell stack, and hence it can use the energy supply from the PV panels without major Solar Desalination Table 551 Technical characteristics of the Almeria plant Nominal distillate production Heat source energy consumption Performance ratio (kilogram distillate/2300 KJ heat input) Output salinity Seawater flow At 10 °C At 25 °C Feedwater flow Brine reject Number of cells Vacuum system m3 h−1 190 kW >9 50 ppm TDS m3 h−1 m3 h−1 m3 h−1 m3 h−1 14 Hydroejectors (seawater at bar) Figure 32 View of the solar MED plant at PSA/CIEMAT start-up and to compensate for the small amounts of air and gases released from feedwater and from small leaks through the gaskets The most outstanding evaluation results obtained during Phase I were the following: • High reliability of the system, as no major problem was observed in the coupling of the solar collector field with the MED plant • Low thermal inertia: it usually took 35 to reach the nominal distillate production • Specific electricity consumption in the range of 3.3–5 kWh m−3 of distillate • The plant showed a PR (e.g., number of kilograms of distillate produced by 2300 kJ heat input) within the range of 9.4–10.4 when operating with low-pressure steam PR increases up to the range of 12–14 if high-pressure steam is used to feed the plant From the results obtained during Phase I, it was possible to identify potential relevant improvements that could be implemented in the MED solar system to increase its efficiency and competitiveness 552 Applications This analysis concluded that • the plant electrical demand could be reduced by replacing the initial hydroejector-based vacuum system with a steam ejector system, and • the plant thermal demand is 50% reduced by incorporating a double-effect absorption heat pump coupled to the MED plant Since these improvements would considerably reduce the specific cost of distillate produced by the optimized solar MED desalination system, it was decided that the Phase II of the STD project be carried out A schematic diagram of the improved desalination system in which an absorption heat pump was coupled to the MED plant has been shown in Figure 33 The heat pump delivers around 200 kW of thermal energy at �65 °C to the MED plant The desalination process in the plant evaporator body uses only 90 kW of the 200 kW, while the remaining 110 kW is recovered by the heat pump evaporator at 35 °C and pumped to usable temperature of 65 °C For this, the heat pump needs 90 kW of thermal power at 180 °C The energy consumption of the desalination system was thus reduced from 200 to 90 kW The improvements implemented in the desalination system (i.e., absorption heat pump and steam-ejector-based vacuum system) reduced the thermal energy consumption of the desalination system by 44%, that is, from 63 to 36 kWh m−3, and electricity consumption by 12%, from 3.3 to 2.9 kWh m−3 A new R&D European project, named AQUASOL, was initiated in 2002, trying to improve the existing system at PSA AQUASOL project objective was the development of a low-cost and more energy-efficient seawater desalination technology based on MED process with zero brine discharge Specific proposed technological developments (new design of CPC solar collectors and a new prototype of absorption heat pump, hybridization with natural gas, and recovering of salt) were implemented to both improve the energy efficiency of the process and for process economy The expected result was an enhanced MED technology with market possibilities and suitable to be applied in the Mediterranean area and similar locations around the world If a fuel cost (i.e., natural gas) of € 4.5 GJ−1 is considered, the needed cost of solar system (considering a solar contribution of 50% to the overall system) for the achievement of the same economic competitiveness as conventional MED plant is equivalent to around € 150 m−2 of solar collector Steam Solar collectors Low pressure boiler 70 °C 0.35 bar Vapor Steam generator 130 °C Vapor 10 bar Condensate Vapor Heat transfer loop Steam electron Thermal storage tank (Thermocline) 13 65 °C 0.25 bar 180 °C 10 bar 35 °C 0.067 bar Double effect absorption heat pump 14 Feed water –1 (8 m h ) Oil circuit Seawater Distillate Brine Brine –1 (3 m h ) Figure 33 Improved solar MED system (Phase II of STD project) Seawater Distilate –1 –1 (8 m h ) (3 m h ) Solar Desalination 3.16.8.3 553 PV–RO Plant for Seawater Desalination, Lampedusa Island, Italy The matching of PVs with RO has a large number of applications due to the modularity, efficiency, and simplicity of the combination The largest and most known stand-alone PV–RO seawater plant was installed in 1990 in Lampedusa Island in Italy (Figure 34) The plant is characterized by successful operation providing freshwater at a reasonable cost The RO unit consists of two units with a total water production capacity of m3 h−1 The power supply system consists of 100 kWp PV arrays, � 2000 Ah at 220 V (DC) – 880 kWh batteries and inverters [9] The system was sized to provide m3 h−1 of desalinated water for days of h operation on three consecutive nonsunny days (Figure 35) The original plant was powered by a 100 kW PV system Having run as a demonstration plant for years and shown that the unit can perform satisfactorily as an autonomous system, it was decided in 1995 to modify the system and incorporate it into the island grid This allows the RO plant to be run continuously at full output, which makes better use of this capital resource The PV system is then used to reduce the consumption of diesel fuel The pretreatment of the desalination system consists of addition of chemicals to prevent colloidal and alkaline scaling and passing of the feedwater through cartridge filters (5, 20 μm) before entering the high-pressure pump (Figure 36) The high-pressure pump is a piston pump, which includes an energy recovery system, recovering 15–20% of the consumed energy The energy requirements of the RO plant (including the energy recovery system) are of the order of 5.5 kWh m−3 of produced water Both units have similar layout, with freshwater flow of m3 h−1 for the first unit (three pumps, three permeators) and m3 h−1 for the second unit (two pumps, two permeators) The module arrangement of each unit is of one stage, operating with seawater Spiral wound permeators are used The salt content of the produced water is