Investigations on the performance of solar air heaters

Abstract

The solar air heater (SAH) is a common device used for the conversion of solar energy into thermal energy. The converted thermal energy is used for domestic, agricultural and other specific applications. The challenging issue with respect to the performance of solar air heaters is the low heat transfer coefficient between the flowing air and the absorber plate. Also, due to the formation of laminar sub layer, adjacent to the absorber surface, it results in less efficiency of the system. This could be overcome by the use of heat transfer increasing elements and artificial roughness elements on the absorber plate. The crucial problem in familiarizing the usage of solar devices in rural areas is the cost to be spent in installing the system using solar energy. Hence, a successful research should meet the outcome of the efficiency of the system and at the same time it should also justify its fabrication and operation economically. This dissertation titled Investigations on the performance of solar air heaters presents the performance -study of solar air heaters through three separate experimentations. The economic aspects of the solar air heaters are also discussed. Primarily, experimentation was carried out on a double pass solar air heater (DPAH) with packed bed consisting of wire mesh (DPAHWM) as packed bed conditions. The modifications in the gap between the wire meshes were 10 cm, 12 cm and 14 cm and the collector tilt varied as 9o , 20o and 25o with the mass flow variation as 0.010 kg/s, 0.015 kg/s, 0.020 kg/s and 0.025 kg/s. Before experimentation, a preliminary theoretical analysis was carried out for specified solar intensities and mass flow rates to study and the feasibility of fabricating and conducting experiments on the above mentioned experimental setup. The energy balance equations were solved by matrix method of solutions. Java script codes were used to solve the energy balance equations and the inverse matrix. The effects of air mass flow rate, wire mesh pitch and collector inclination on temperature rise and thermal efficiency had been deliberated. Results showed that efficiency increased with mass flow rate. For the same mass flow rate, the thermal efficiency increased with the decrease in the wire mesh pitch. The maximum daily average efficiency of air heater was 79.8% at 0.025 kg/s mass flow rate with 10 cm wire mesh gap and 9° collector inclination facing south. The highest instantaneous efficiency was 81.62% at 12.30 hours during which solar Intensity was 985 W/m2 . The highest collector efficiency was observed in solar air heater with 10 cm wire mesh gap. The temperature difference and efficiency of DPAHWM were encouraging. Results were presented. Key conclusions out of the research work are also formulated. The theoretical prediction initially worked out and the experimental results obtained were coinciding with tolerable deviations. newline