Paper presented to the 3rd Southern African Solar Energy Conference, South Africa, 11-13 May, 2015.
Heated air has many applications including space heating, cooking, absorption refrigeration, curing of industrial products and drying of foodstuffs. Typically air heated using solar irradiance is achieved in a collector which incorporates absorber plates enclosed with a transparent surface to retain heat in the collector. The temperature of air heated in such collectors is limited to some 80 °C. An experimental investigation was carried out in which ambient air was passed through three sequentially connected evacuated tubes, each fitted with parabolic concentrator with a reflectance of 97%. Tests were carried out on 1 September when the maximum elevation of the sun was 56.7° corresponding to a required tilt angle at solar noon of 33.3°. The rig was set at a tilt angle of 30° and an azimuth angle of -12°. The effects of the irradiance angle of incidence on the outlet temperature of the air from the three tubes were evident with the maximum temperatures occurring at solar noon when the outlet temperatures of the air from the first tube was 151.2°C, from the second 221.0 °C, and from the third 250.1 °C. The flow rate of air was 0.00223 kg/s. Using a reflector cross sectional area of 0.51m² per tube the efficiencies of the tubes at solar noon based on the normal component of irradiance into the parabolic reflectors were for the first second and third tubes 42%, 24%, and 12% respectively with an overall efficiency of 28%. The results indicated that heating air to elevated temperatures using concentrated solar power evacuated tubes is feasible but it will be necessary to redesign the system to reduce the excessive energy loss to the atmosphere. This energy loss was found to be directly proportional to the temperature difference between the mean temperature of the air in a tube and atmospheric temperature.
