Efficiency of 30 to 35 per cent can be ob tained in a

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efficiency of 30 to 35 per cent can be ob- tained in a well-designed still on a good sunny day. A number of basin-type solar-still plants having areas greater than 100 m 2 are in operation in many parts of Africa and the West Indies. 3.13.6 Drying One of the traditional uses of solar energy has been for drying of agricultural products. The drying process removes moisture and helps in the preservation of the product. Traditionally, drying is done on open ground. The disadvantages associated with this are that the process is slow and that insects and dust get mixed with the product. The use of dryers helps to eliminate these disadvantages. Drying can then be done faster and in a controlled fashion. In addition, a better-quality product is obtained. A cabinet-type solar dryer, suitable for Figure 3.23: Schematic view of Solar Distillation
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33 small- scale use, is shown in Fig. 3.24. The dryer consists of an enclosure with a transparent cover. The material to be dried is placed on perforated trays. Solar radiation entering the enclosure is absorbed in the product itself and the surrounding internal surfaces of the enclosure. As a result, moisture is removed from the product and the air inside is heated. Cabinet dryer Suitable openings at the bottom and top ensure a natural circulation. Temperatures ranging from 50°C to 80°C are usually attained and the drying time ranges from 2 to 4 days. Typical products which can be dried in such devices are dates, apricots, chillies, grapes, etc. For large-scale drying, the passive device of Fig. 3.24 relying on natural circulation is replaced by an active device with forced circulation as shown in Fig.3.25. Systems of this type have been used for drying timber. An indirect type of active device is used when the solar radiation falling directly on the product (as in Figs 3.24 and 3.25) is not adequate, or the temperature of the product needs to be controlled. One such system is shown in Fig.3.26. Here, the air is heated separately in an array of solar air heaters and then ducted to the chamber in which the product to be dried is stored. Such dryers are suitable for food grains, tea, spices, etc. and for products like leather and ceramics. An 11 m 2 solar air heating system installed in Cuddallore, Tamil Nadu for drying fish. 3.13.7 Cooking An important domestic thermal application is that of cooking. Over the past 40 years, a number of designs of solar cookers have been developed, a few of which are described here. Box type : Solar cooker designs generally fall into one of two categories. One category is the box-type cooker, a slow cooking device suitable for domestic purposes. It essentially consists of a rectangular enclosure insulated on the bottom and sides, and having one or two glass covers on the top. Solar radiation enters through the top and heats up the enclosure in which the food to be cooked is placed in shallow vessels. A typical size available has an enclosure about 50 cm square and 12 cm deep. Temperatures around 100°C can be obtained in these cookers on sunny days and pulses, rice, vegetables, etc., can be readily cooked.
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