Investigation of advanced heat pump augmented spacecraft heat rejection systems

Future military space missions will introduce significant new technological needs for spacecraft energy systems. It is generally accepted that spacecraft heat rejection systems that use heat pumps to boost the radiator temperature will reduce the radiator area. However, these systems must also result In weight savings and high rellabliity. Mainstream Engineering Corporation is involved in the development of advanced, high-temperature, thermallyand electrically-driven heat pumps for spacecraft applicatioits. This paper discusses the development of several heat pump configurations. This presentation also reviews the potential benefits of an innovative, spacecraft heat pump, thermalmanagementlthermal-transport loop for high-power applications. Practical Spacecraft Vapor-Compression Heat Pump Conflguratlons Deslgnlng a heat pump for spacecraft operations would require the use of onboard power to drive the compressor. However, tl high-temperature waste heat Is available (such as waste heat from a nuclear or sdar dynamic power system), then a thermailydriven system, rather than an electrically-driven system, mlght be preferred. Mainstream is pursulng development of both thermallyand 6iectrlcailydrlven heat pump systems. In the area of thermaiiydriven systems, the englneer has the choice of several chemlcal heat pump systems or the use of a heat engine to power a vapor-compresslon heat pump. The chemical heat pumps (absorptlon, metal-hydride, or complex-compound heat pumps) typically have only ilqukl pumps and therefore tend to be falsely represented as less complex than heat-engine vapor-compression (Rankine-Rankine or Stirllng-Rankine) systems. However, the metal-hydride and complexcompound systems are intermment, or batch, systems requiring several systems operating at different intefvals to approximate contlnuous operation. Besides the control and plumbing problems associated with these designs, chemical heat pumps also tend to have an inherently low Coefficlent of Performance (COPc) and are larger and heavler than many systems of equal capacity. The absorption systems are continuous systems but also tend to be both Inherently low in COPC and heavy. A heat engine drivlng a vapor-compression heat pump typically out performs an absorption heat pump system because the heat engine can utilize the high-temperature heat avallabie (from nuclear power source waste heat, for example), and typically the absorptlon system cannot (although Malnstream Is investigating new absorption-refrigerant pairs that could solve that problem). in addltion, the absorption systems tend to be larger and heavler because they require a large iiquld-vapor surface area to achieve the equilibrium concentrations necessary for optimum refrigerant transport. Metal-hydrlde, complexcompound. and other chemlcal heat pump systems also tend to be heavy and large for a given capacity. A lighter, more compact alternative Is t o utilize a heat engine drlvlng a vapor-compression heat pump. This approach typically has a higher COPC. Brayton cycles are sometimes used in place of Rankine cycles for power converslon (and reverse Brayton cycles are sometlmes used In place of reverse Ranklne cycles for heat pumps). Brayton cycles utilize stable, gaseous RuMs over the entlre temperature range of the cycle. In spite of this thermal stablllty. the Brayton cycles have inherent disadvantages when compared to Rankine cycles. The first disadvantage in utilizing j Copyright 0 American Institute of Aeronautics Astronauries. Inc.. 1989. AI1 rights reserved and Brayton cycles is that they typically have lower cycle efficiencies because only the sensible heating of the working fluid Is available to transfer energy in the cycie(t-3) resulting in larger mass flow rates (compared to the Rankine cycle. which utilizes the latent heat of vaporization of the fluid to carry a slgnfflcant amount of the energy) The second disadvantage Is that the weight of the Brayton cycle is typically higher than the weight of a comparable Rankine cycle due to the low heat transfer coefficient of the gaseous working fluid (which results in larger heat exchangers) and higher flow rates (or higher pressures) for the Brayton system. The weight difference between comparable Rankine and Brayton cycles is documented in the iiterature(4) in terms of Stirling-powered heat pumps, Mainstream has investigated the concept in which a free-piston Stirling engine drives a third compression piston within the Stirling enclosure in this design, the Stirling working fluid and the vaporcompression heat pump working fluid would be the same, allowing the system to be hermetically sealed The working fluid is selected to insure superheated conditions in the Stirling system. Although the Stirling-powered vaporcompression heat pump has the potential for higher performance due to the higher theoretical petformance of the Stirling heat engine (compared to the Rankine heat engine), this performance benefit has never been practically demonstrated in any Stirling heat engine in addition, the Stirling heat engine will be heavier than the Rankine heat engine, and long-term reliability of the Stirling heat engine has not been demonstrated. Finally (for the free piston configuration), it Is not clear that the effect of the variable compressor load on the motion and frequency of the Stirling machine dispiacer piston is well