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Visavale - Principles, Classification and Selection of Solar Dryers quite long and result in post-harvest losses(more than 30%) . Solar drying of agricultural products in enclosed structures by forced convection is an attractive way of reducing post-harvest losses and low quality of dried products associated with traditional open sun-drying methods (Jain and Tiwari, 2003). In many rural locations in most developing countries, grid-connected electricity and supplies of other non-renewable sources of energy are either unavailable, unreliable or, too expensive. In such conditions, solar dryers appear increasingly to be attractive as commercial propositions (Mekhilefa et al., 2011; Xingxing et al., 2012). During the last decades, several developing countries have started to change their energy policies toward further reduction of petroleum import and to alter their energy use toward the utilization of renewable energies. With very few exceptions, the developing countries are situated in climatic zones of the world where the insolation is considerably higher than the world average of 3.82 kWh/m 2 day. In Figure 1.7 daily average horizontal insolation data and sunshine hours of some developing countries are given. An alternative to traditional drying techniques and a contribution toward the solution of the open air drying problems is the use of solar dryers. Accordingly, the availability of solar energy and the operational marketing and economy reasons offer a good opportunity for using solar drying all over the world. Figure 1.7. Total horizontal solar insolation for some developing countries (Visavale, 2009) 1.8.1. Classification of Solar Dryers Drying equipment may be classified in several ways. The two most useful classifications are based on (1) the method of transferring heat to the wet solids or (2) the handling characteristics and physical properties of the wet material. The first method of classification reveals differences in dryer design and operation, while the second method is most useful in the selection of a group of dryers for preliminary consideration in a given drying problem. A classification chart of drying equipment on the basis of heat transfer is shown in Figure 1.8 (Ekechukwu,1999; Sharma et al., 2009). This chart classifies dryers as direct or indirect, with subclasses of continuous or batchwise operation. Solar- 14 Drying of Foods, Vegetables and Fruits
Visavale - Principles, Classification and Selection of Solar Dryers energy drying systems are classified primarily according to their heating modes and the manner in which the solar heat is utilized (Belessiotis et al., 2011). In broad terms, they can be classified into two major groups, namely: • passive solar-energy drying systems (conventionally termed natural-circulation solar drying systems) and, active solar-energy drying systems (most types of which are often termed hybrid solar dryers). SOLAR DRYERS Conventional drying Solar energy drying Bulk or storage (low temperature) dryers Batch and continuous flow (high temperature) dryers Natural dryers Solar energy dryers Crops dried in-situ Active dryers Passive dryers Distributed type dryers Mixed mode dryers Integral type dryers Drying on ground mats concrete and floor Drying on trays Distributed type dryers Mixed mode dryers Integral type dryers Horizontal trays Vertical or inclined racks Cabinet type dryers Green house dryers Figure 1.8. Classifications of dryers and drying modes (Ekechukwu,1999) Three distinct sub-classes of either the active or passive solar drying systems can be identified namely: Drying of Foods, Vegetables and Fruits 15
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