Analytical Study of the Heat Transfer Process of Flat Solar Collectors in Conditions of Free Air Flow

A mathematical model of the heat-transfer process in flat-plate solar collectors with vertical plates operating in free air flow was developed. The model was validated on collectors of different sizes, and for the prototype the deviation between calculated and experimental data did not exceed 3–8%, confirming its accuracy. The analysis shows that the thermal performance of the pipes decreases with distance from the absorber plate, which makes heat transfer from remote regions more difficult and leads to higher heat loading of the tubes per unit area. Enhanced air circulation through the collector pipes increases the overall heat exchange of the system. Under the studied conditions, the daily useful heat gain increased by about 800 MJ at different ambient temperatures, and the daily efficiency of the system reached up to 48% at low ambient temperatures.