3. Previous work on solar air heaters
To improve the thermal performance of the indirect solar dryers, the thermal performance of the solar air heater connected to the drying chamber should be improved. The next sections review the methods that were used to improve the thermal performance of the solar air heaters. Several designs for solar air heaters had been proposed and dis cussed in the literature. The designer and potential user of these systems must consider a number of factors when comparing their merits. These can mainly be categorized as: (i) thermal performance, (ii) cost and (iii) lifetime, durability, maintenance and ease of installation. Thermal performance of collectors was compared by using the concept of thermal efficiency. It was generally believed that the thermal efficiency of a solar air heater is the major requirement for the prediction of thermal performance of the complete solar system of which the solar air heater is a part [57] Ekechukwu and Norton [58] classified the solar air heaters broadly into two types: bare plate and cover plate solar air heaters. The bare plate solar air heater consists of a single channel design with single air flow between absorber and bottom plates with insulation. Choudhury et al. [59] studied the thermal performance of this design. The thermal performance of this air heater had been also predicted by Ong [60]. Njomo [61] and Njomo and Daguenet [62] investigated heat transfer in this design.
In the cover plate solar air heaters the heater was covered by a single or double glass covers. The single cover solar air heater in which the air flow in a single channel between the cover and absorber plate was investigated by [60,62-68]. Hegazy [69] investigated the effect of variation in the absorber width on both the thermal and hydraulic performances of the single cover solar air heater. Aboul-Enein et al. [70] analyzed a flat plate solar air heater with and without thermal storage material under the absorber plate. The single duct double glass solar air heaters with air flowing between the lower glass cover and the absorber plate had been studied by Njomo and Daguenet [621, Mohamad I651 and Naphon and Kongtragool [67] Mohamad [65] investigated the heat transfer in this design under the steady state conditions
In the back-pass solar air heater, the absorber plate was placed directly behind the transparent cover with a layer of static air separating it from the cover. The air to be heated flows between the inner surface of the absorber plate and the layer of insulation [58]