Solar Drying: Fundamentals,Applications and Innovations - National ...

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Visavale - Principles, Classification and Selection of Solar Dryers Figure 1.19 shows the earliest form of passive solar greenhouse dryer by the Brace Research Institute, with slanted glass roof, allowing direct solar radiation over the product. The length-wise north-south alignment of the dryer had black coated internals for improved absorption of solar radiation with the ridge-cap over the roof for exit vent. Doe et al. (1977) later designed the widely reported poly-ethylene tent dryer, illustrated in Figure 1.20 consisting of a ridged bamboo framework clad with a clear polythene sheet over it. A black poly-ethylene sheet was also spread on the floor inside the tent to enhance the absorption of solar radiation. The air flow into the tent was controlled by rolling/ unrolling of the cladding at the bottom edge of front side and the vents at the top served as the exit for the moist exhaust air. Sachithananthan et al. (1983) reported a horticultural greenhouse of clear plastic sheet cladding over a semi-cylindrical metal frame (Figure 1.21). The modification were with a black galvanized iron sheet absorber at floor, inlet vents along the full length of both sides of base and exit with plastic nets at the top to protect from insects and dust. Fleming et al. (1986) reported a typical greenhouse type solar dryer with a transparent semi-cylindrical chamber with a cylindrical solar chimney posted vertically at one end and a door for air inlet and access to the chamber at other end as shown in Figure 1. 22. Rathore et al. (2010) has conducted various experimental studies on a modified design of hemi-cylindrical solar tunnel dryer for drying of grapes also few researchers (Jaijai et al., 2011) have used a polycarbonate cover for its construction. Afriyie et al. (2011) has reported the study of simulation and optimization of a chimney ventilated solar crop dryer. Figure 1.19. Natural-circulation glass-roof solar-energy dryer (Ekechukwu and Norton, 1999) Figure 1.20. Natural-circulation polythene-tent dryer (Ekechukwu and Norton, 1999) Figure 1.21. Natural-circulation solar dome dryer (Ekechukwu and Norton, 1999) Figure 1. 22 A greenhouse type natural-circulation solar–energy dryer (Ekechukwu and Norton, 1999) 24 Drying of Foods, Vegetables and Fruits
Visavale - Principles, Classification and Selection of Solar Dryers Figure 1.23. Features of a typical mixedmode natural-circulation solar-energy dryer (Ekechukwu and Norton, 1999) Figure 1.24. A mixed-mode naturalcirculation solar rice dryer (Ekechukwu and Norton, 1999) Figure 1.25. A mixed-mode naturalcirculation solar-energy dryer with thermal storage (Ekechukwu and Norton, 1999) Figure 1.26. A multi-stacked mixedmode natural circulation solar-energy dryer (Ekechukwu and Norton, 1999) 1.10.4. Hybrid-Type Passive Solar-Energy Dryers A hybrid type passive solar-energy dryer would have the same typical structural features as the indirect-type and direct-type (i.e. a solar air heater, a separate drying chamber and a chimney), and in addition has glazed walls inside the drying chamber so that the solar radiation impinges directly on the product as in the direct-type dryers as shown in Figure 1.23. Exell et. al. (1980), Sodha et al. (1987), at Asian Institute of Technology developed the widely reported solar rice dryer (Figure 1.24). Ayensu and Asiedu (1986) designed the hybrid dryer consisting of an air heater with a pile of granite functioning as an absorber cum heat storage (Figure 1.25). Sauliner first reported a multi stacked hybrid design different from that of Exell, followed by Lawand at Brace Research Institute also designed and tested by Sharma et al. (1986, 1987), as shown in figure 1.26. The multi-stacked design enables the simultaneous drying of a variety of crops. Several other designs of the hybrid-type passive solar energy have also been reported in (Amer et al., 2010; Hughes et al., 2011). Drying of Foods, Vegetables and Fruits 25
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