This paper describes the performance benefits and current technology progress of an active heat pump loop (HPL) thermal control bus for spacecraft and planetary thermal control applications. Having initiated this research more than 14 years ago, this paper also briefly highlights the technical developments and obstacles overcome during this 14-year development.
This paper discusses the unique features of the HPL approach that make it an attractive design choice for future manned thermal control applications: the use of an heat pump to reject heat to space at a temperature above the heat acquisition temperature, the use of non-toxic thermally stable working fluids, and the use of high-performance lubrication-free (gravity independent) refrigeration compressors. The HPL approach has the performance benefits of a traditional two-phase pumped loop thermal bus coupled with the simplicity of a single-phase pumped loop. The HPL loop also offers the ability to achieve predetermined and isothermal cold plate operating temperatures independent of radiator temperature, and heat load.
We have demonstrated that the HPL can accommodate multiple evaporative cold plates, multiple evaporation temperatures, variable thermal loads, and variable radiator temperatures. We have identified nontoxic and nonflammable working fluids that are stable under the conditions of the catalytic oxidizer used on board manned spacecraft. Two lubrication-free compressor designs have been developed and demonstrated. For small thermal loads, the HPL can use a small lubrication-free positive-displacement reciprocating compressor that is suitable for use in micro-gravity. This unique lubrication-free compressor design has already been fabricated for use in the International Space Station refrigerated centrifuge. For larger cooling loads, the HPL can use a low-vibration, long-life, compact, magnetic-bearing centrifugal compressor. The magnetic-bearing centrifugal compressor has already been demonstrated for other applications.