Thermodynamic analysis of a two-stage cascade refrigeration cycle for application to venus lander payload electronics cooling

This paper presents the results of a thermodynamic analysis of a two-stage, cascaded vapor compression refrigeration cycle developed for high-temperature, high-pressure applications, such as the one encountered in Venus surface lander missions. The bottoming cycle uses Ammonia (NH₃) while the topping cycle uses Fatty Acid Methyl Ester Methyl Linoleate (FAME-MLL) as the working fluid. The working fluid FAME-MLL is selected for its critical point temperature of 526 °C, which is greater than the local Venus atmospheric temperature of 465 °C, thus providing a temperature potential to reject heat to the Venus environment. The FAME-MLL cycle employs an ejector in order to alleviate overloading the compressor. The paper present the thermodynamic model of the system followed by predictions of system performance in terms of ejector flow rate, condenser temperature, evaporator temperature, and compressor efficiency. The results herein show that over the ranges of ejector entrainment ratios of 2 < w < 3, and compressor efficiencies of 70% < η_c < 80% the systems Coefficient of Performance (COP) falls within the range of 0.7 < COP < 0.8. The main objective of this research is to provide an active thermal control system architecture that will maintain a payload on Venus rejecting 100 W at 100 °C for an extended period of days or weeks. Thus the overall longevity of the mission is deemed more important than a system with a large COP value. Thus, the conceptual design provided herein is seen to meet the primary objectives.