Enhanced performance of flat-plate solar air heater assisted dryers for agriculture products

Ethiopia is blessed with plenty of solar radiation for the most part of the year and due to the low latitude (5 deg C-15 deg C), the solar radiation is significant on the horizontal surface. Indirect drying, with solar-heated air supplied to a conventional batch dryer, can be used for drying of a variety of agriculture products. However, the reported energy collection efficiency of the conventional batch dryer, can be used for drying of a variety of agriculture products. However, the reported energy collection efficiency of the conventional forced flow solar air heaters is only 25-40%. This paper presents various designs of artificially roughened flat-plate solar air heaters developed in the last decade to enhance their energy collection efficiency. The enhancement in the thermal efficiency has been reported to be 10% - 40% due to a substantial enhancement in the heat transfer coefficient. Extensive experimental studies have shown that the geometry of the roughness (roughness shape, pitch, height, etc.) and their orientation with respect to the flow direction have a marked influence on the heat transfer and friction characteristics of the roughened surfaces of such collectors. The present study shows that the preferred roughness geometry is 15 deg C integral chamfered rib roughness at relative roughness pitch of 4.5-7 arranged in v-down discrete pattern with respect to the flow. At low to medium mass flow rates per unit area of plate, air heaters with roughness height to hydraulic diameter ratio, e/Dh of 0.03 to 0.05 are most attractive while at high mass flow rates (corresponding to low temperature rise), roughened absorber plate with low value of e/Dh (say 0.01) is recommended. Friction factor and heat transfer coefficient correlations have been presented by researchers of the different roughness types, which can be utilized for the design of the enhanced performance forced convection solar air heaters. For the remote areas, where grid supplies are not available, the drying can be assisted by natural convection solar air heaters with corrugated absorber plates.