Development and testing of a compound parabolic collector for large acceptance angle thermal applications

Concentrating the solar energy is a well-known technique for increasing the working temperatures of solar collectors while maintaining high conversion efficiencies. In the industrial and residential sectors medium temperature solar collectors (80-250°C) are gaining more and more interest for power production and absorption cycle applications. In the present paper the attention is focused on the design and testing of a solar collector where a high performance evacuated tube is coupled with a Compound Parabolic Concentrator. The design of the solar mirror is carried out by imposing a series of constraints related to a broad acceptance angle (about 30°), reduced mirror height, possibility to employ a displaced tubular receiver. The mirror surface was tested in laboratory with a ray tracing approach. The complete CPC collector was finally tested under real outdoor operating conditions according to the Standard EN 12975-2. In such a way the instantaneous efficiency of the collector has been evaluated. The main findings are related to the effects of the concentration ratio and truncated height on the collector efficiency; they also highlight some limits and possible corrections to be employed when the results of the Outdoor steady state test method are applied in daily simulations of the collector behavior.