Water is one of the most abundant resources on Earth, covering approximately three-quarters of the planet’s surface. About 97% of the Earth’s water is salt water in the oceans: 3% of all fresh water is in ground water, lakes and rivers, which supply most of that needed by humans and animals.

Water is essential to life. The importance of supplying potable water can hardly be overstressed. Man has been dependent on rivers, lakes and underground water-reservoirs for fresh-water requirements in domestic life, agriculture and industry. However, rapid industrial-growth and the population explosion world-wide have resulted in a large escalation of the demand for fresh water. Added to this is the problem of pollution of rivers and lakes by industrial wastes and the large amounts of sewage discharged. On a global scale, man-made pollution of natural sources of water is becoming the single largest cause for fresh-water shortages. Besides the only inexhaustible sources of water are the oceans. Their main drawback, however, is the high salinity of such water. It would be attractive to tackle the water-shortage problem with desalination of this water, which may be mixed with brackish water to increase the amount of fresh water and reduce the concentration of salts to around 500 ppm.

Solar distillation has been practised for many generations. According to Malik et al., the earliest documented work is that of an Arab alchemist in the 15th century, as reported by Mouchot in 1869. Mouchot stated that the Arab alchemist had used polished Damascus mirrors for solar distillation. The great French chemist Lavoisier (1862) used large glass lenses, mounted on elaborate supporting-structures, to concentrate solar energy on the contents of distillation flasks. The use of silver- or aluminum-coated glass reflectors to concentrate solar energy for distillation was also been described by Mouchot.

Solar stills were the first to be used in large-scale, distilled-water production. The first water-distillation plant constructed was a system built at Las Salinas, Chile, in 1874. The still covered 4700 m² and produced up to 23 000 litres of fresh water per day (4.9 litre/m²) in clear sky conditions. The still was operated for 40 years and only abandoned after a fresh-water pipe was installed to supply water to the area from the mountain region.

The use of solar concentrators in solar distillation was reported by Pasteur (1928), who used a concentrator to focus solar rays onto a copper boiler containing water. The steam generated from the boiler was piped to a conventional water-cooled condenser in which distilled water was accumulated. Renewal of interest in solar distillation occurred soon after the First World War, during which several new devices had been developed, such as the roof-type, tilted-wick, inclined-tray and inflated stills.

Desalination can be achieved by using several techniques. These may be classified into the following categories: (i) phase-change or thermal processes and (ii) membrane or single-phase processes. In the phase-change or thermal processes, the distillation of sea water is achieved by utilising a heat source. The thermal energy may be obtained from a conventional fossil-fuel source, nuclear energy or from a non-conventional solar-energy source. In the membrane processes, electricity is used either for driving high pressure pumps or for ionisation of salts contained in the sea water.

Desalination processes require significant quantities of energy to achieve separation. This is highly significant as it is a recurrent cost which 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. 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. According to Marinos et al. and Morris and Hanbury, the installed capacity of desalinated water systems in 1990 reached 13 million m³/day, which, by the year 2000, is expected to double. The dramatic increase in desalinated water supply will create a series of problems, the most significant of which are those related to energy consumption. It has been estimated that a production of 13 million m³ of portable water per day requires 130 million tons of oil per year. Even if oil were much more widely available, could we afford to burn it on the scale needed to provide everyone with fresh water? Given the current understanding of the greenhouse effect and the importance of CO₂ levels, this use of oil is debatable. Thus, 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 the burning of substantial quantities of fossil fuels. Given that conventional sources of energy are polluting, sources of energy that are not polluting will have to be used. Fortunately, there are many parts of the world that are short of water but have exploitable renewable-energy sources that could be used to drive desalination processes.

Solar desalination is used in nature to produce rain, which is the main source of fresh-water supply. Solar radiation falling on the surface of the sea is absorbed as heat and causes evaporation of the water. The vapour rises above the surface and is moved by winds. When this vapour cools down to its dew point, condensation occurs and fresh water precipitates as rain. All available man-made distillation systems are small scale duplications of this natural process.

Solar energy can be used for sea-water desalination either by producing the thermal energy required to drive the phase-change processes or by generating the electricity required to drive the membrane processes. Solar-desalination systems are thus classified into two categories, i.e. direct and indirect collection-systems. As their name implies, direct-collection systems use solar-energy to produce distillate directly in the solar collector, whereas in indirect collection systems, two sub-systems are employed (one for solar-energy collection and one for desalination). Conventional desalination systems are similar to solar systems because the same type of equipment is applied. The prime difference is that in the former, either a conventional boiler is used to provide the required heat or mains electricity is used to provide the required electric power, whereas in the latter, solar energy is applied.

During the design effect, there is a need to select a process suitable for a particular application. The factors to be considered during such a selection are:

1. Suitability of the process for solar-energy application.

2. The effectiveness of the process with respect to energy consumption.

3. The amount of fresh water required in a particular application in combination with the range of applicability of the various desalination-processes.

4. The sea-water treatment requirements.

5. The capital cost of the equipment.

6. The land area required, or could be made available, for the installation of the equipment.

Solar energy can generally be converted into useful energy either as heat, with solar collectors and solar ponds, or as electricity, via photovoltaic cells. Both methods have been used to power desalination systems. The direct collection systems can only utilise solar energy whenever it is available, and their collection is inefficient. Alternatively, in the indirect collection systems, solar energy can be collected, by more-efficient solar collectors, and be in the form of hot water or steam. It should be noted, however, that solar energy is only available for almost half of the day. This implies that the process operates for only half the time available unless some storage device is used. The latter, which is usually expensive, can be replaced by a back-up boiler or electricity from the grid in order to operate the system during low-insolation periods or during the night. When such a system operates without thermal buffering, the desalination sub-system must be able to follow a variable energy supply, without upset.