Thermal Analysis Of A Flat Plate Solar Air Collector With Double Channels

Solar air collector is one of the main method of solar heat utilization, as it has advantages of simple structure, moderate costs, convenient maintenance, without antifreezing measures, it is widely used in the drying of agricultural, building heating, desalination, hot water supply, air conditioning and other fields.Based on a flat plate solar collector, the flat plate solar air collector with double channels is proposed. In length direction of the collector, air flow channels are arranged with diagonal form. A set up of the new type air collector is established, thermal performance is tested and analyzed. The results show that the average collector efficiency based on the daylighting area is 51.29%, the maximum of that is 53.93%, when the average solar radiation intensity and environmental temperature are 535.2W/m2 and 22.2℃, the mass flow rate of air is 0.024kg/s, respectively. The maximum temperature rise and temperature difference of the working fluid(air) are 60.4℃ and 40.6℃,respectively, and the heat-removal factor is about 0.55; the relationship between heat transfer and efficiency is linear.The mathematical model of a flat plate solar air collector with double channels is established to analyze the heat transfer process between air and absorber plate theoretically. The mathematical equation of air outlet temperature is also deduced. The maximum relative error between theoretical and experimental results is 4.9%. The effect of air flow rate and air flow channels on the thermal performance is investigated theoretically. The results show that air flow rate has a significant effect on the performance, and it should be controlled in a lower flow rate; the better thermal performance is obtained when the height ratio of top air flow channel to the total is 0.5.The 3D model of the air collector is obtained by Gambit software and Fluent software is used for simulating the thermal performance. The results show that there is a good agreement between the simulation results and the experimental results, and the maximum of relative errors is 8.9%.The air channels with the diagonal form has significantly affected the fluid flow and heat transfer, the local high temperature and flow obstruction are appeared in the absorber plate, when the solar radiation intensity, environmental temperature and inlet temperature are 745.3W/m2, 21.3 ℃and 25.8℃, respectively. In order to improve this problem, a flat plate solar air collector with baffles is regarded as the optimized structure, and is studied by Fluent. The results show that the local high temperature is only 349 K and the air outlet temperature can reach 62.0℃.Finally, Fluent software is used to simulate numerically the effect of several parameters on the thermal performance of air collector. The results show that solar radiation intensity has a significant effect on air outlet temperature, but less influence on collector efficiency; with an increase of air flow rate, air outlet temperature decreases and the collector efficiency increases, when air flow rate exceeds 0.04kg/s, the thermal performance is not improved effectively, and the power consumption increases; As air inlet temperature increases continuously, the change of air outlet temperature increases linearly and the trend of efficiency decreases linearly; air outlet temperature and efficiency both increase linearly with an increase of environmental temperature, but the environmental temperature effect on the thermal efficiency is not significant, and the collector efficiency in summer is higher than that in winter; As the thickness of the absorber plate increases, air outlet temperature and efficiency both increase first and be steady after, when the thickness of the absorber plate is more than 0.35 mm, there is no practical meaning for the performance improvement.The thermal performance of a flat plate solar air collector with double channels is investigated by the methods of experiment, theory and numerical simulation. The research results are helpful to provide the basic theory and optimize the structure of the kind of air collector.