General Comments

Generally, the cost of water produced in solar distillation systems depends on the total capital investment to build the plant, the maintenance requirements, and the amount of water produced. No energy is required to operate the solar stills unless pumps are used to transfer the water from the sea. Therefore, the major share of the water cost in solar distillation is that of amortization of the capital cost. The production rate is proportional to the area of the solar still, which means that the cost per unit of water produced is nearly the same regardless of the size of the installation. This is in contrast with conditions for freshwater supplies as well as for most other desalination methods, where the capital cost of equipment per unit of capacity decreases as the capacity increases. This means that solar distillation may be more attractive than other methods for plants of small sizes. Howe and Tleimat (1974) reported that the solar distillation plants having capacity less than 200m3/d are more economical than other plants.

Kudish and Gale (1986) presented the economic analysis of a solar distillation plant in Israel, assuming the maintenance cost of the system to be constant. An economic analysis for basin and multiple-wick solar stills was carried out by various scientists (Delyannis and Delyannis, 1985; Tiwari and Yadav, 1985; Mukherjee and Tiwari, 1986). Their economic analyses incorporated the effects of subsidy, rainfall collection, salvage value, and maintenance cost of the system.

Zein and Al-Dallal (1984) performed chemical analysis to find out its possible use as potable water and results were compared with tap water. They concluded that the condensed water can be mixed with well water to produce potable water and the quality of this water is comparable with that obtained from industrial distillation plants. The tests performed also showed that impurities such as nitrates, chlorides, iron, and dissolved solids in the water are completely removed by the solar still.

Although the yield of solar stills is very low, their use may prove to be economically viable if small water quantities are required and the cost of pipework and other equipment required to supply an arid area with naturally produced freshwater is high.

Solar stills can be used as desalinators for remote settlements where salty water is the only water available, power is scarce, and demand is less than 200 m3/d (Howe and Tleimat, 1974). This is very feasible if setting of water pipelines for such areas is uneconomical and delivery by truck is unreliable or expensive. Since other desalination plants are uneconomical for low-capacity freshwater demand, solar stills are viewed as means for communities to attain self-reliance and ensure a regular supply of freshwater.

In conclusion, solar stills are the cheapest, with respect to their initial cost, of all available desalination systems in use today. They are direct collection systems, which are very easy to construct and operate. The disadvantage of solar stills is the very low yield, which implies that large areas of flat ground are required. It is questionable whether solar stills can be viable unless cheap, desert-like land is available near the sea. However, obtaining freshwater from saline or brackish water with solar stills is useful for arid, remote areas where no other economical means of obtaining water supply is available.

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