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.
