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<\/figure>\n\n\n\nFIGURE 8.12<\/strong> <\/a>Connection of a solar system with a desalination system.<\/p>\n\n\n\n In some desalination systems, such as the MEB, the solar system must be able to provide low-pressure steam. For this purpose, parabolic trough collectors are usually employed, and one of the three methods of solar steam generation outlined in\u00a0Chapter 7\u00a0can be used.<\/p>\n\n\n\n Solar thermal energy is one of the most promising applications of renewable energy for seawater desalination. A solar distillation system may consist of two separated devices: the solar collector and\u00a0the conventional distiller (indirect solar desalination). Indirect solar desalination systems usually consist of a commercial desalination plant connected to commercial or special solar thermal collectors. With respect to special solar thermal collectors,\u00a0Rajvanshi (1980)\u00a0designed such a solar collector to be connected to an MSF distillation plant.\u00a0Hermann et al. (2000)\u00a0reported on the design and testing of a corrosion-free solar collector for driving a multiple-effect humidification process. The pilot plant was installed at Pozo Izquierdo, Gran Canaria, Spain (Rommel et al., 2000).<\/p>\n\n\n\n Details of solar ponds are given in\u00a0Chapter 10. These are used mainly for electric power generation. Another use of the output from salt-gradient solar ponds, however, is to operate low-temperature distillation units to desalt seawater. This concept has applicability in desert areas near the oceans. Solar pond coupled desalination involves using hot brine from the pond as a thermal source to evaporate the water to be desalted at low pressure in a multiple-effect boiling (MEB) evaporator.<\/p>\n\n\n\n Matz and Feist (1967)\u00a0proposed solar ponds as a solution to brine disposal at inland ED plants as well as a source of thermal energy to heat the feed of an ED plant, which can increase its performance.<\/p>\n\n\n\n Details of photovoltaic systems are given in\u00a0Chapter 9. The photovoltaic (PV) technology can be connected directly to an RO system; the main problem, however, is the currently high cost of PV cells. The extent to which the PV energy is competitive with conventional energy depends on the plant capacity, the distance to the electricity grid, and the salt concentration of the feed.\u00a0Kalogirou (2001)\u00a0and\u00a0Tzen et al. (1998)\u00a0analyzed the cost of PV-RO desalination systems.\u00a0Al-Suleimani and Nair (2000)\u00a0presented a detailed cost analysis of a system installed in Oman.\u00a0Thomson and Infield (2003)\u00a0presented the simulation and implementation of a PV-driven RO for Eritrea with variable flow that is able to operate without batteries. The production capacity of the system was 3\u00a0m3<\/sup>\/day with a PV array of 2.4\u00a0kWp. The model was validated with laboratory tests. The Canary Islands Technological Institute (ITC) developed a stand-alone system (DESSOL) with capacity of 1\u20135\u00a0m3<\/sup>\/day of nominal output.<\/p>\n\n\n\n Another way of using PV is in combination with ED. The ED process is more suitable than RO for brackish water desalination in remote areas. Several pilot plants of ED systems connected to photovoltaic cells by means of batteries have been implemented.\u00a0Gomkale (1988)\u00a0analyzed solar desalination for Indian villages and concluded that solar cell-operated ED seems more advantageous for desalting brackish water than conventional solar stills. A PV-driven ED plant was installed at Spencer Valley, New Mexico. It was developed by the Bureau of Reclamation in the United States (Maurel, 1991). Experimental research in PV-ED was also performed at the Laboratory for Water Research at the University of Miami (Kvajic, 1981) as well as at the University of Bahrain (Al-Madani, 2003).<\/p>\n\n\n\n A brief historical introduction to wind energy is given in\u00a0Chapter 1, Section 1.6.1, whereas more details are given in\u00a0Chapter 13. Since RO is the desalination process with the lowest energy requirements (see Section\u00a08.6) and coastal areas present a high availability of wind power resources (Doucet, 2001), wind-powered desalination is one of the most promising alternatives for renewable energy desalination. A preliminary cost evaluation of wind-powered RO is presented by\u00a0Garcia-Rodriguez et al. (2001). In particular, the influence of climatic conditions and plant capacity on product cost is analyzed for seawater RO driven by wind power. Additionally, the possible evolution of product cost due to possible future changes in wind power and RO technologies is evaluated. Finally, the influence on the competitiveness of wind-powered RO with conventional RO plants due to the evolution of financial parameters and cost of conventional energy is pointed out.<\/p>\n\n\n\n Another area of interest is the direct coupling of a wind energy system and an RO unit by means of shaft power. Research in this field has been carried out at the Canary Islands Technological Institute (ITC, 2001). On Coconut Island, off the northern coast of Oahu, Hawaii, a brackish water desalination wind-powered RO plant was installed. The system directly uses the shaft power production of a windmill with the high-pressure pump and RO. In particular, constant freshwater production of 13\u00a0l\/min can be maintained for wind speed of 5\u00a0m\/s (Lui et al., 2002).<\/p>\n\n\n\n Another possibility investigated is the use of wind power directly with an MVC. A detailed analysis of the influence of the main parameters of such systems was performed by\u00a0Karameldin et al. (2003). On Borkum Island, in the North Sea, a pilot plant was erected with a freshwater production of about 0.3\u20132\u00a0m3<\/sup>\/h (Bier et al., 1991). On R\u00fcgen Island, Germany, another pilot plant was installed with a 300\u00a0kW wind energy converter and 120\u2013300\u00a0m3<\/sup>\/day freshwater production (Plantikow, 1999).<\/p>\n\n\n\n Finally, another possibility investigated is the use of wind power with ED. Modeling and experimental test results of one such system with a capacity range of 72\u2013192\u00a0m3<\/sup>\/day, installed at the ITC, Gran Canaria, Spain, are presented by\u00a0Veza et al. (2001).<\/p>\n\n\n\n The complementary features of wind and solar resources make the use of hybrid wind-solar systems to drive a desalination unit a promising alternative, since usually, when there is no sun, the wind is stronger, and vice versa. The Cadarache Centre in France designed a pilot unit that was installed in 1980 at Borj-Cedria, Tunisia (Maurel, 1991). The system consists of a 0.1\u00a0m3<\/sup>\/day compact solar distiller, a 0.25\u00a0m3<\/sup>\/h RO plant, and an ED plant for 4000\u00a0ppm brackish water. The energy supply system consists of a photovoltaic field with a capacity of 4\u00a0kW peak and two wind turbines.<\/p>\n\n\n\n Measurements show that for shallow geothermal systems, the ground temperature below a certain depth remains relatively constant throughout the year. This is because the temperature fluctuations at the surface are diminished as the depth of the ground increases due to the high thermal inertia of the soil.<\/p>\n\n\n\n According to\u00a0Popiel et al. (2001), from the point of view of the temperature distribution, three ground zones can be distinguished:<\/p>\n\n\n\n 1. <\/a>Surface zone<\/em> reaching a depth of about 1 m, in which the ground temperature is very sensitive to short time changes of weather conditions.<\/p>\n\n\n\n8.5.2 Solar ponds<\/h3>\n\n\n\n
8.5.3 Solar photovoltaic technology<\/h3>\n\n\n\n
8.5.4 Wind power<\/h3>\n\n\n\n
8.5.5 Hybrid solar PV-wind power<\/h3>\n\n\n\n
8.5.6 Geothermal energy<\/h3>\n\n\n\n