The only nearly inexhaustible sources of water are the oceans. Their main drawback, however, is their high salinity. Therefore, it would be attractive to tackle the water-shortage problem by desalinizing of this water. Desalinize, in general, means to remove salt from seawater or generally saline water.
According to the World Health Organization (WHO), the permissible limit of salinity in water is 500 parts per million (ppm) and for special cases up to 1000ppm. Most of the water available on earth has salinity up to 10,000ppm, and seawater normally has salinity in the range of 35,000-45,000ppm in the form of total dissolved salts. Excess brackishness causes the problem of bad taste, stomach problems, and laxative effects. The purpose of a desalination system is to clean or purify brackish water or seawater and supply water with total dissolved solids within the permissible limit of 500 ppm or less. This is accomplished by several desalination methods that are analyzed in this chapter.
Desalination processes require significant quantities of energy to achieve separation of salts from seawater. This is highly significant because it is a recurrent cost that few of the water-short areas of the world can afford. Many countries in the Middle East, because of oil income, have enough money to invest and run desalination equipment. However, people in many other areas of the world have neither the cash nor the oil resources to allow them to develop in a similar manner. The installed capacity of desalinated water systems in the year 2000 was about 22 million m3/d, which is expected to increase drastically in the next decades. The dramatic increase of desalinated water supply will create a series of problems, the most significant of which are those related to energy consumption and environmental pollution caused by the use of fossil fuels. It has been estimated that the production of 22 million m3/d requires about 203 million tons of oil per annum (about 8.5 EJ/a or 2.36 X 1012 kWh/a of fuel). Given the current concern about the environmental problems related to the use of fossil fuels, if oil were much more widely available, it is questionable whether we could afford to burn it on the scale needed to provide everyone with freshwater. Given current understanding of the greenhouse effect and the importance of CO2 levels, this use of oil is debatable. Therefore, apart from satisfying the additional energy demand, environmental pollution would be a major concern. If desalination is accomplished by conventional technology, then it will require burning substantial quantities of fossil fuels. Given that conventional sources of energy are polluting, sources of energy that are not polluting must be developed. Fortunately, many parts of the world that are short of water have exploitable renewable sources of energy that could be used to drive desalination processes (Kalogirou, 2005).
Solar desalination is used by nature to produce rain, which is the main source of the freshwater supply. Solar radiation falling on the surface of the sea is absorbed as heat and causes evaporation of the water. The vapor rises above the surface and is moved by winds. When this vapor cools down to its dew point, condensation occurs and freshwater precipitates as rain. All available manmade distillation systems are small-scale duplications of this natural process.
Desalination of brackish water and seawater is one way to meet the water demand. Renewable energy systems produce energy from sources that are freely available in nature. Their main characteristic is that they are friendly to the environment, i.e., they do not produce harmful effluents. Production of freshwater using desalination technologies driven by renewable energy systems is thought to be a viable solution to the water scarcity at remote areas characterized by lack of potable water and conventional energy sources such as a heat and electricity grid. Worldwide, several renewable energy desalination pilot plants have been installed and the majority have been successfully operated for a number of years. Virtually all of them are custom designed for specific locations and utilize solar, wind, or geothermal energy to produce freshwater. Operational data and experience from these plants can be utilized to achieve higher reliability and cost minimization. Although renewable energy-powered desalination systems cannot compete with conventional systems in terms of the cost of water produced, they are applicable in certain areas and are likely to become more widely feasible solutions in the near future.
This chapter presents a description of the various methods used for seawa-ter desalination. Only methods that are industrially mature are included. Other methods, such as freezing and humidification-dehumidification methods, are not included in this chapter, since they were developed at a laboratory scale and have not been used on a large scale for desalination. Special attention is given to the use of renewable energy systems in desalination. Among the various renewable energy systems, the ones that have been used, or can be used, for desalination are described. These include solar thermal collectors, solar ponds, photovoltaics, wind turbines, and geothermal energy.
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Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.