Efficient Solar Air Heater with Perforated Absorber for Crop Drying

Outdoor experiments on a once-through single glazed solar air heater with perforated metal absorber plate were conducted to determine the practical effect of absorber plate porosity as well as suction air flow rate on the collector thermal efficiency and its total pressure drop. Three aluminum absorber plates were made perforated by drilling circular holes with different diameter/pitch ratios in square layout. A fan was employed at the top of the collector to suck ambient air from the bottom side through absorber plate perforations. The flow channel was designed such that uniform air flow over the entire absorber plate area could be achieved. Five levels of air mass flow rates (0.0065 to 0.0321 kg m-2 s-1) were adopted. Pressure drop across the apparatus was measured. The inlet air was preheated by short wavelength radiation absorbed by the cover as well as the long wavelength emission by the absorber prior to catching the heat from transpired absorber plate. A maximum thermal efficiency of 84% could be achieved for the most part of the porous absorber plate at the highest air mass flow rate. The collector with minimum porosity showed a maximum pressure drop. In some experiments, the glass cover was removed to determine the outdoor effect of glazing. Comparing the performance of the collector with and without glazing showed that the unglazed collector was about 25% less efficient than the glazed one at the same overall operating conditions. This reduction can be attributed to high top radiative and convective heat losses for the unglazed collector at the outdoor conditions. The pressure drop for the uncovered collector showed a lower magnitude in comparison to the covered one. The inlet air passes and heats up (21-59°C above the ambient) through the solar collector, therefore the fresh and clean hot air can be continuously supplied for many purposes such as solar drying system.