A Thermal Model for a CPC-Type Double-Pass Solar Air Heater

The double-pass solar air heater consists of a linear CPC with a flat receiver, where the air circulates counterflow through the cavity formed by the absorber plate, the cover, and the reflectors to preheat the fluid and then incorporate it into the receiver duct. This research's objective was to demonstrate that the CPC-type double-pass heater's thermal performance is better than a conventional or single-pass CPC heater. In this sense, two one-dimensional steady-state thermal models were developed, one for each CPC-type heater. The heat transfer equations were solved in the cover, the cavity, the absorber plate, and the receiving duct, using the numerical method of control volumes. Both models were evaluated for a truncated concentrator, with a mass flow range of 0.0013 kg/s to 0.1050 kg/s, setting the global solar irradiance at 950 W/m(2), the wind speed 3 m/s, and the ambient temperature and the inlet temperature of the fluid at 33 degrees C. In all the cases analyzed, the double pass heater provides higher outlet air temperature and instantaneous efficiency than the conventional ones, with improvements until 9.4% and 73.1%, respectively. In the double pass air heater, the air temperature at the concentrator outlet was 88.4 degrees C (with laminar flow in the receiving duct) to 37.8 degrees C (with turbulent flow), instantaneous efficiency was 7.3% to 59.0%. Based on the results obtained, it is concluded that the CPC-type double pass solar air heater allows the profiteering of the heat lost by the receiver from the pre-circulation of air through the cavity.