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This paper describes the process and analysis used to select a refrigerant and thermodynamic cycle as the basis of a vapor compression heat pump requiring a high temperature lift. Use of a vapor compression heat pump versus other types was based on prior work performed for the Electric Power Research Institute. A high lift heat pump is needed to enable a thermal control system to remove heat down to 275K from a habitable volume when the external thermal environment is severe. For example, a long term habitat will reject heat from a space radiator to a 325K environment. The first step in the selection process was to perform an optimization trade study, quantifying the effect of radiator operating temperature and heat pump efficiency on total system mass; then, select the radiator operating temperature corresponding to the lowest system mass. Total system mass included radiators, all heat pump components and the power supply system.

This paper determines the feasibility of potential Active Thermal Control System (ATCS) growth paths by assessing thermal, integration, and implementation impacts. TRASYS/IRTRAN models were used to evaluate the effects of increased radiator temperature, increased radiator area, and radiator wing addition on Space Station Freedom (SSF) elements, including energy reflected back to the ATCS. SINDA/FLUINT models were used to determine the heat rejection capability of an ATCS loop with an integrated heat pump that operates with Electrical Power System (EPS) peak power. The effects of upgrading the ATCS by advanced technology ORU implementation during maintenance replacements was also evaluated. The study results presented lead to conclusions on which paths are best suited for different growth scenarios.

The “restructured” baseline has eliminated many options for Active Thermal Control System (ATCS) growth for Space Station Freedom (SSF). Modular addition of baseline technology to increase heat rejection will be extremely difficult. The system design and the available real estate no longer accommodate this type of growth. As the station matures during its thirty years of operation, a demand of up to 165 kW of heat rejection can be expected. The baseline configuration will be able to provide 82.5 kW at Eight Manned Crew Capability (EMCC). A promising technology that could increase heat rejection by the necessary 82.5 kW is the heat pump. This paper provides a preliminary feasibility assessment of the application of a vapor compression heat pump to the ATCS.

Extended manned missions to the lunar and martian surfaces pose new challenges for active thermal control systems (ATCS's). Moderate-temperature heat rejection becomes a problem during the lunar day, when the effective sink temperature exceeds that of the heat-rejection system. The martian atmosphere poses unique problems for rejecting moderate-temperature waste heat because of the presence of carbon dioxide and dust. During a recent study, several ATCS options including heat pumps, radiator shading devices, and single-phase flow loops were considered. The ATCS chosen for both lunar and martian habitats consists of a heat pump integral with a nontoxic fluid acquisition and transport loop, and vertically oriented modular reflux-boiler radiators. The heat pump operates only during the lunar day. The lunar and martian transfer vehicles have an internal single-phase water-acquisition loop and an external two-phase ammonia rejection system with rotating inflatable radiators.