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Life on earth depends on water. Europe is relatively ‘prosperous’ in water resources, but the supply is unevenly balanced, with shortages largely affecting the southern member countries. Three quarters of the planet’s surface is covered with water, but 97% of this huge quantity is contained in the oceans, and is therefore salty, while only a tiny 3% is made up of fresh water. This small percentage of the earth’s water, however, supplies most of the needs of humanity, and is found in lakes, rivers and groundwater. It is thus obvious that the only practically inexhaustible source of water is the oceans, though their very high salinity is much above the safe consumption limit. It would therefore be possible to address the water shortage problem faced by many countries and many millions of people with seawater desalination. However, the separation of salts from seawater requires large amounts of energy which, when produced from fossil fuels, can increase environmental pollution and exacerbate the earth’s climate-related problems. There is therefore a need to employ environmentallyfriendly energy sources such as renewables to desalinate seawater. Fortunately, Europe’s southern region, which is dry and thus in more need of desalinated water, enjoys high levels of renewables: mainly solar and wind energy.
Desalination processes require substantial quantities of energy to achieve the removal of salts from seawater. This is a very important parameter as energy is an expensive commodity and a permanent running cost, which varies with fuel prices and which few water-starved regions can afford. A variety of systems used to convert seawater into fresh water suitable for human use exists as well as a variety of systems which can be used to convert renewable energy sources into useful forms of energy. These can be used to power desalination systems and include a variety of solar collectors, photovoltaics and wind turbines. There are two main categories of desalination system: direct and indirect collection systems. Table 1 presents the most important technologies in use today. The motive power in the phase-change or thermal processes is a thermal energy source, whereas in the membrane or singlephase processes, electricity is used. An exception is membrane distillation (MD) which is classified as a phase change process but needs membranes to operate. All processes presented in Table 1 require the chemical pre-treatment of seawater to avoid foaming, fouling, scaling and corrosion as well as chemical post-treatment, mainly for disinfection.
Direct collection systems use one piece of equipment, which both collects solar radiation and uses the energy collected to desalinate seawater. Representative of this type of system is the solar still, available in various designs, as shown in Table 1. It is a simple and cheap system but its water production per collector area is relatively small – in the order of 4-5 litres per square meter at best. Indirect collection systems employ two different subsystems; one for collecting renewable energy and another for desalination. These comprise two broad categories; the phase change processes, which include the multistage flash, multiple effect distillation, vapour compression and membrane distillation; and the membrane processes, which include reverse osmosis and electrodialysis. It should be noted that conventional desalination systems are similar to solar systems, since the same type of equipment is used. The main difference is that in conventional systems, a boiler or mains electricity is used, whereas in the renewable ones, solar radiation or wind energy is employed.
Generally speaking, renewable energy systems produce energy from resources that are ‘freely’ available in nature and do not deplete with consumption. Above all, their collection and use is carried out in a pollution-free way and is friendly to the environment. Therefore, fresh water production using renewable energy-powered desalination technologies is considered to be a viable solution to water scarcity problems. Several renewable energy desalination pilot plants have been installed across the world. While some were experimental, erected simply to prove their suitability and viability, most have operated successfully for years. They are custom designed for specific application of solar or wind energy to produce fresh water. The energy required for various desalination processes, established by a survey of manufacturers’ data, is presented in Table 2. Renewable desalination systems are applicable in areas with abundant renewable energy resources, and are today considered to be economically viable. These viable renewable energy options, available in many southern European countries today, include large PV parks and wind parks to power osmotic-type units and combinations of concentrating solar power plants with thermal desalination units, which can be integrated with the condenser of the thermodynamic cycle employed.