Performance enhancement of cavity receivers with spillage skirts and secondary reflectors in concentrated solar dish and tower systems

Abstract

A cavity receiver geometry may represent a better option than an external receiver geometry for high temperature solar power systems because of reduced thermal losses. However, a limitation is that the small aperture necessary for an efficient (and affordable, practical) cavity receiver may lead to high spillage losses, negating the thermal efficiency gains. In this study, Monte Carlo ray tracing is coupled with heat transfer models to assess the benefit of secondary reflectors and spillage skirts on the performance of cavity receivers in the context of both dish and tower CSP systems. The heat transfer fluid is chosen as liquid sodium with inlet and outlet temperatures of 520 °C and 740 °C respectively. A spillage skirt and three different types of secondary reflectors, including conical, trumpet and compound parabolic concentrator (CPC) reflectors are investigated to decrease the energy loss. The results show that the spillage skirt helps in greatly decreasing the aperture size, while simultaneously reducing the spillage loss. The conical and trumpet reflectors increase the efficiency by reflecting rays outside the cavity inlet into the receiver, but the CPC does not achieve efficiency gains comparable to the other designs in both the dish and tower systems. For dish systems, highest efficiency is obtained with a combination of a spillage skirt and conical or trumpet reflectors, with a 1.0% increment in receiver efficiency compared to the best cylindrical receiver. For the tower system the efficiency gain is approximately 1.3% relative to the optimised cavity receiver alone.