Receiver shape optimization for maximizing medium temperature CPC collector efficiency

Abstract

Low optical-concentration solar thermal CPC collectors for process heat at 150–300°C generally use thermal oil as the collector fluid. Thermal oils have low thermal conductivity and high viscosity, which leads to significant thermal resistance and hence reduced collector thermal efficiency. One way to minimize the thermal resistance is by having turbulent flow of the thermal oil within the receiver. For a given receiver area and mass flow rate of the fluid, this can be achieved by narrowing the flow passage but keeping the receiver area constant by adding external flat fins. In this paper a new receiver design for a compound parabolic concentrator is proposed which is a hybrid of a U-shaped tubular receiver and a bifacially irradiated flat receiver. To keep the receiver area constant, the fins are increased in width as the tube diameter is decreased. Its performance when enclosed in a glass vacuum tube and a CPC has been modelled. The transmission and absorption of solar energy, optical losses due to the receiver–reflector gap, heat transfer within the receiver, and the thermal losses have been modelled. Keeping the receiver area and fluid flow rate constant, the thermal resistance of the thermal oil flow within the receiver reduces when the flow passage is narrowed leading to increased thermal efficiency. On the other hand, the hybrid receiver has lower optical efficiency as compared to a tubular receiver due to its higher gap loss. Overall, the hybrid receiver has similar or better thermal efficiency than the tubular receiver. Thermal efficiency and effective thermal efficiency, which accounts for the pumping power penalty, shows that the performance improvement with thermal oil due to receiver shape optimization depends on the receiver area, concentration ratio, absorptivity and emissivity of the selective surface, the mass flow rate through the receiver and fluid temperature. Highest effective thermal efficiency is generally achieved in the laminar–turbulent transitional regime. For temperatures below 150°C, water has been found to give better performance than thermal oil at all mass flow rates with no significant improvement in collector performance achieved by reducing the tube diameter.