Heat pump systems for enhancement of heat rejection from spacecraft

Abstract

Temperature boosting of waste heat from spacecraft by means of heat pumps makes it possible, under some conditions, to achieve considerable savings in radiator area and mass. This study considers several possibilities for employing work-actuated and heat-actuated heat pumps (WAHP and HAHP, respectively) for this purpose. In the former case, the spacecraft power source is required to generate extra power to operate the heat pump; in the latter, use is made of the heat rejected from the power source to energize the heat pump. The mass and area savings are calculated for a range of operating parameters including the temperatures of the waste heat from the power source and payload and that of the effective heat sink. The dimensionless parameters governing the behavior are determined. It is shown that for given operating conditions, a proper choice of the temperature boost and the associated heat pump coefficient of performance leads to an optimum in radiator area and system mass savings. A detailed cycle calculation is presented for an absorption heat pump—a particularly promising HAHP with few moving parts. Design considerations are given for space-based WAHP and HAHP systems. Nomenclature A - radiator area, m2 e = power source efficiency m = system mass, kg mp = specific mass of power source, kg/kW mr = specific mass of radiator, kg/m2 Q = heat rate, kW s = solar power supplied by solar collectors per unit area, kW/m2 T = temperature, K W - electric power, kW ]8 = heat pump coefficient of performance e = emissivity of radiator surface r? = radiator fin efficiency fi = power mass penalty parameter, Eq. (14) a = Stefan-Boltzman coefficient, W/m2K4 0 = portion of Carnot efficiency