Performance investigation of integrated concentrating solar air heater with curved Fresnel lens as the cover: When combined with seasonal soil heat storage

In this paper, an integrated concentrating solar air heater (ICSAH) was presented. It’s compounded of a curved Fresnel lens as cover, a V-groove secondary reflector as sidewalls, and a receiver placed in the glass tube flow channel without need of evacuated tube. And both ends of ICSAH were encapsulated with cover plates. The cavity between receiver and Fresnel lens acts as a greenhouse, enhancing the insulation of ICSAH. Sunlight is focused on the receiver after redirected by Fresnel lens and secondary reflector. In the glass tube, air flows through the receiver and is heated. The optical properties of ICSAH were simulated using Lighttools. The results showed that it’s capable of working long enough hours during the day without tracking and only seasonal adjustment is needed. In addition, its maximum optical efficiency is 0.77. Two set-ups of different lengths (3m, 41m) were built for experimentation. Basing on the 3m-long ICSAH, the effects of the incidence angle α, γ and the air mass flux on the collection efficiency were studied. This preliminary experimental results indicated that the temperature differential is significantly affected by α, while the change of γ from 0° to 30° has no obvious effect on the collection efficiency. And the larger the air mass flux the higher the collection efficiency. The collection efficiency was 0.63 at air mass flux of 6.54kg/min when α=γ=0°. The 41m-long ICSAH was mounted on the ground in the east-west direction and facing the ground with a tilt angle of 30°. An entire daytime test was conducted on a sunny winter day, and the results showed that the outlet air temperature stayed above 100°C for 4.5h and the maximum temperature differential reached 108°C at air mass flux of 8.1kg/min, indicating the considerably good heating performance of ICSAH in winter. What’s more, with 6.5 working hours per day, it achieved a daily efficiency of 0.59 and supplied a soil heat storage of 287.4MJ. In a word, the ICSAH has the advantages of cost-efficient, low tracking demand, good heating performance and high daily efficiency, which is suitable for use in conjunction with heat storage system for winter heating.