Among the non-conventional methods to desalinate brackish water or seawater is solar distillation. This process requires a comparatively simple technology and can be operated by unskilled workers. Also, due to the low maintenance requirement, it can be used anywhere with a smaller number of problems.
A representative example of the direct collection system is the typical solar still, which uses the greenhouse effect to evaporate salty water. It consists of a basin in which a constant amount of seawater is enclosed in an inverted V-shaped glass envelope (see Figure 8.1). The sun's rays pass though the glass roof and are absorbed by the blackened bottom of the basin. As the water is
heated, its vapor pressure is increased. The resultant water vapor is condensed on the underside of the roof and runs down into the troughs, which conduct the distilled water to the reservoir. The still acts as a heat trap because the roof is transparent to the incoming sunlight but opaque to the infrared radiation emitted by the hot water (greenhouse effect). The roof encloses the vapor, prevents losses, and keeps the wind from reaching and cooling the salty water.
Figure 8.1 shows the various components of energy balance and thermal energy loss in a conventional double-slope symmetrical solar distillation unit (also known as a roof-type or greenhouse-type solar still). The still consists of an airtight basin, usually constructed out of concrete, galvanized iron sheet (GI), or fiber-reinforced plastic (FRP), with a top cover of transparent material such as glass or plastic. The inner surface of the base, known as the basin liner, is blackened to efficiently absorb the solar radiation incident on it. There is also a provision to collect distillate output at the lower ends of the top cover. The brackish or saline water is fed inside the basin for purification using solar energy.
The stills require frequent flushing, which is usually done during the night. Flushing is performed to prevent salt precipitation. Design problems encountered with solar stills are brine depth, vapor tightness of the enclosure, distillate leakage, methods of thermal insulation, and cover slope, shape, and material (Eibling et al., 1971). A typical still efficiency, defined as the ratio of the energy utilized in vaporizing the water in the still to the solar energy incident on the glass cover, is 35% (maximum) and daily still production is about 3-4 L/m2 (Daniels, 1974).
Talbert et al. (1970) gave an excellent historical review of solar distillation. Delyannis and Delyannis (1973) reviewed the major solar distillation plants around the world. This review also includes the work of A. Delyannis (1965), Delyannis and Piperoglou (1968), and Delyannis and Delyannis (1970). Malik et al. (1982) reviewed the work on passive solar distillation system until 1982, and this was updated up to 1992 by Tiwari (1992), who also included active solar distillation. Kalogirou (1997a) also reviewed various types of solar stills.
Several attempts have been made to use cheaper materials, such as plastics. These are less breakable, lighter in weight for transportation, and easier to set up and mount. Their main disadvantage is their shorter life. Many variations of the basic shape shown in Figure 8.1 have been developed to increase the production rates of solar stills (Eibling et al., 1971; Tleimat, 1978; Kreider and Kreith, 1981). Some of the most popular are shown in Figure 8.2. Most of these designs also include provisions for rainfall collection.
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