Air-cooled window for a recuperated solar-dish Brayton cycle using a turbocharger and short-term thermal storage

Abstract

A recuperated solar-dish Brayton cycle using an off-the-shelf turbocharger as a micro-turbine and a rectangular cavity receiver with integrated thermal storage was considered for this study. Due to the high temperatures that these solar receivers reach, a considerable amount of heat is lost to the environment through the aperture, decreasing the efficiency of the cycle. In this research, the heat losses from the solar receiver with integrated thermal storage had to be reduced by utilising a glass channel on the inside of the cavity receiver, which ran parallel to the receiver walls and was cooled by the working fluid (air) flowing from the compressor. The objective of this conceptual study was to investigate the impact of the air-cooled window on the performance of the cycle and to provide insight into the feasibility of the implementation of the cooling window. An entropy generation minimisation technique combined with a SolTrace analysis was used to analyse the impact of the cooling window on the performance of the cycle at steady state. Results showed that the maximum solar-to-mechanical efficiencies were on average between 41% and 45% lower than for the cycle without the window. The results also indicated that a smaller cooling channel width increased the pressure drop and cooling effectiveness but further decreased the solar-to-mechanical efficiency of the cycle. However, a smaller cooling channel width also produced lower glass surface temperatures, which was very important for the structural integrity of the glass. Furthermore, it was found that the cooling window increased the exhaust temperature of the optimised cycle. The exhaust temperature of the cycle with the window was higher than the exhaust temperature of the cycle without the window, which led to a higher energy utilisation factor of between 9% and 11% if the exhaust was used for cogeneration such as water heating or thermal energy storage. Therefore, this conceptual study indicated that it might not be feasible to implement the cooling window, except where a higher cycle exhaust temperature was preferred for cogeneration.