Analysis and comparison of different thermal cycles for power generation in space

This paper presents an analysis of solar-heat driven Brayton and Rankine power cycles operating in space with different working fluids. State of the art literature show that generation of power in space for terrestrial use can represent a great opportunity in the future for many reasons, including the facts that (1) the low-temperature of space, ~3K, which acts as the power system heat sink, allows the attainment of very high efficiency even with low-temperature heat inputs, and (2) the solar energy input is higher than on earth, all this while using traditional cycles. This study is focused on analysis and comparison of performance of advanced Brayton and Rankine cycles operating under space conditions, with a main objective to advance the identification of system configurations, working fluids and conditions leading to the design of space power systems that combine high efficiency and low weight. Starting from previous studies that show the potential high efficiency of the use of diatomic gases (H2, N2) in regenerative Brayton and Rankine cycles we have presented a comparative analysis of different and more advanced Brayton and Rankine configurations to advance the understanding of the optimal trade-off between high efficiency (thermal and exergetic) and the smallest needed heat rejection exchanger area. The effect of the main cycles' operational parameters such as pressure ratio, turbine inlet temperature, working fluid mixture and different plant layouts on thermal and exergy efficiency and power to radiator area ratio have been analyzed. Under the examined conditions the thermal efficiency of regenerative - reheated-intercooled Brayton, that resulted being the best Brayton choice, reaches 71.8% while the efficiency of the reheated-regenerative Rankine cycle reaches 88.9%, both significantly higher than the previously analysed cycles taken as reference. The power/(radiator area) ratio, however, was an order of magnitude higher for the reheated-intercooled Brayton cycle, which may lead to lower costs of the generated power. This ratio was also found to increase with the introduction of reheating for both the Rankine and Brayton cycles, while the intercooling was in all cases disadvantageous.