Analytical and experimental studies on the thermal performance of cross-corrugated and flat-plate solar air heaters

The cross-corrugated heaters consist of a wavelike absorbing plate and a wavelike bottom plate, which are crosswise positioned to form the air flow channel. Two types of these heaters are considered. For the type 1 heater, the wavelike shape of the absorbing plate is along the flow direction and that of the bottom plate is perpendicular to the flow direction, while for the type 2 heater it is the wavelike shape of the bottom plate that is along the flow direction and that of the absorbing plate is perpendicular to the flow direction. The aims of the use of the cross-corrugated absorbing plate and bottom plate is to enhance the turbulence and the heat-transfer rate inside the air flow channel, which are crucial to the improvement of efficiencies of solar air-heaters. To quantify the achievable improvements with the cross-corrugated absorbing and bottom plates, flat-plate solar air-heaters which have both a flat absorbing plate and a flat bottom plate, are also considered. The thermal performance of these three types of solar air-heaters are analyzed, measured and compared under several configurations and operating conditions. All the analytical and experimental results show that, although the thermal performance of the type 2 heater is just slightly superior to that of the type 1 heater, both of these cross-corrugated solar air-heaters have a much superior thermal performances to that of the flat-plate one. It is also found that the use of selected coatings on the absorbing plates of all the heaters considered can substantially enhance the thermal performances of the heaters, whereas such a selected coating on the bottom plates or/and the glass covers does not have such a significant effect on the thermal performances of the heaters.