The scarcity of fresh water resources and the need for additional water supplies is already critical in many arid regions of the world and will be increasingly important in the future. It is very likely that the water issue will be considered, like fossil energy resources, to be one of the determining factors of world stability. Many arid areas simply do not have fresh water resources in the form of surface water such as rivers, lakes, etc. and have only limited underground water resources that are becoming more brackish as abstraction of water from the aquifers continues. The world-wide availability of renewable energies and the availability of mature technologies in this field make it possible to consider the coupling of desalination plants with renewable energy production processes in order to ensure the production of water in a sustainable and environmentally friendly scheme for the regions concerned. Solar desalination is used by nature to produce rain which is the main source of fresh water on earth. All available man-made distillation systems are a duplication on a small scale of this natural process. Recently, considerable attention has been given to the use of renewable energy as sources for desalination, especially in remote areas and islands, because of the high costs of fossil fuels, difficulties in obtaining it, attempts to conserve fossil fuels, interest in reducing air pollution, and the lack of electrical power in remote areas.
Desalination processes require considerable amounts of energy to achieve separation of dissolved salts in seawater or brackish water. It has been estimated by Kalogirou (1996) that the production of 1000 m3 per day of fresh water requires 10 000 tons of oil per year. This is highly significant as it involves a recurrent energy expense which few of the water-short areas of the world can afford. The Middle East is unique in that, because of the oil income, it has the financial resources required to invest and run desalination equipment. Many other areas of the world have neither the cash nor the oil resource to allow them to develop in a similar manner. Even if oil were much more widely available, could we afford to burn it in such a manner as to provide everyone with fresh water? Given the current understanding of the greenhouse effect and the importance of carbon dioxide levels in the atmosphere, environmental pollution caused by burning fossil fuel for desalination is a major concern. The thermal energy required for desalination using thermally-driven distillation processes can be achieved by collection of solar energy using flat plate collectors, evacuated tube collectors or solar ponds. Such devices can achieve temperatures of 80-130 C which are quite suitable for such desalination processes. Solar energy can also be converted to electrical power using either photovoltaic panels or high-temperature concentrating collectors associated with a heat engine operating on a thermodynamic cycle. Such electrical power can then be used to operate power-driven desalination processes such as reverse osmosis or vapor compression. Conventional desalination technology is fairly well developed and some of the processes may be considered quite mature, although there is still considerable scope for improvement and innovation. Conventional desalination is energy intensive. Thus, one of the major concerns to developing water production by desalination is the cost of energy. Apart from the cost implications, there are environmental concerns with regard to the burning of fossil fuels. The coupling of renewable energy sources with desalination processes is seen by some as having the potential to offer a sustainable route for increasing the supplies of potable water. Renewable energy processes are less mature but are developing rapidly. Wind energy and photovoltaics (PV) in particular have made enormous advances over the last two decades but still have plenty of scope for improvement. The amount of energy used in the world for desalination is comparable to the total energy requirement of an industrialized country such as Sweden. This gives an idea of the amount of CO2 emitted by this industry. Global phenomena such as the greenhouse effect must receive due attention. Last but not least there are regions where a significant fraction of the total energy consumption is due to desalination and which are deficient in conventional sources. In these cases regional or national economies may benefit from a reduction of oil imports.