ASSESSMENT OF THERMAL PERFORMANCE OF OVERPASS AND DOUBLE-PASS SOLAR AIR COLLECTORS
Fruits have a very short shelf-life because they have a large water content. Therefore, by dehydrating the fruit, they can stay in good condition for a longer time and thus minimize losses. Solar air collectors are used in diverse solar thermal applications. One such system is the indirect passive solar drying of agricultural products, including fruits. We therefore realized the importance in researching to improve the performance of these systems. The purpose of this study is to evaluate the performance of Double-pass and Over-pass solar air collectors. The collectors were fabricated at Ashikaga University, Japan. They are flat plate type equipped with back insulation. When a solar collector is operating, the driving forces are dynamic. Hence, it was deemed important to observe its transient behavior as it absorbed solar irradiation, retaining some and transferring the other to the working fluid (air) and the ambient air. The collector was set to face the sun’s azimuth and tilted to 30 degrees. Air temperature sensors were place at the inlet and outlet of the collector. Thermocouples were placed on the absorber plate and the cover glazing. A stopwatch was used to measure the time beginning at zero when the sun radiation starts falling on the collector. The temperatures of the absorber plate and glazing were recorded against time until they stabilized. This was done for the two types of the air collectors. Within 8 minutes, the absorber plate temperatures had reached 66 oC. The cover temperatures were 50 oC and 47 oC for the double-pass solar collector and overpass solar collector respectively. Pertinent equations and the recorded data were inputted into excel, the obtained data were analyzed. Heat transfer coefficient of energy loss through the top cover was obtained to be 5.2 W/m2◦C for overpass collector and 6.3 W/m2◦C for the double-pass collector. Convection heat transfer coefficient from the absorber plate to the heated air was found to be 20.8 W/m2◦C and 18.8 W/m2◦C for the overpass and double-pass collector respectively. The convection heat transfer coefficient between the plate and the cover was 3 W/m2◦C for overpass collector and 7.5 W/m2◦C for the double-pass collector. The thermal efficiency of the overpass solar collector was 60.1%. It was 55.3% for the double-pass. From these results it is concluded that for the same flow area, the thermal performance of an overpass solar collector is better than that of double-pass.