Vapour compression refrigeration for space applications
An IJR review paper presents the history of vapour-compression refrigeration systems used in microgravity environments and sets the next steps of research needed to achieve reliable systems.
Frequent flights to the Moon, human missions to Mars and extraterrestrial habitats are primary goals for current space exploration development activities. One of the main challenges in achieving these goals is the development of lighter, more energy efficient, and resilient thermal management systems for spacecraft. To date, most spacecraft have been able to reject heat directly into the cold of space by radiation, without the need of a refrigeration system. However, future missions to warmer environments, such as certain regions of the Moon, will be different. Even today, spacecraft often provide a refrigerated space to preserve biological samples or for other scientific purposes. Human space exploration missions or a trip to Mars will require a refrigerated space for long-term food storage. Current methods allow astronauts’ food to be stored at ambient temperatures where its nutritional value degrades after long periods of time.
Radiating against deep space is the most straightforward way to achieve low temperatures. For instance, the International Space Station’s active thermal control systems pump fluids through closed-loop pipes; a liquid-ammonia coolant loop keeps the station’s electricity-generating solar panels cool. (1) However, depending on the desired cooling temperature, the radiator size and mass for heat rejection may become excessive. Therefore, there is a growing interest in space refrigeration systems that use the cooling loops provided by the radiators as heat sink.
In particular, the demand for cryogenic temperatures, often to enable sensitive sensors to collect critical measurements, has brought almost all types of cryogenic coolers into space. Along with cryogenic coolers using notably Stirling and Brayton cycles, thermoelectric coolers and incubators are used in spacecraft as a reliable technology independent of gravity . The technologies mentioned may claim high technology readiness levels (TRLs), but are outperformed in terms of energy efficiency by a vapor compression cycle (VCC) for the traditional temperature range of refrigerators/freezers.
Considering the decades of research into two-phase flow in microgravity, it is surprising that vapour compression refrigerators are not yet standard in space. An important reason for the low TRL of corresponding two-phase cycles is the lack of accessible microgravity testing.
An IJR paper from US researchers from Purdue University and NASA’s Johnson Space Center summarises existing information on VCC refrigeration under microgravity and poses questions that should be addressed in future research works.
The authors conclude that additional effort is required to enable reliable VCC refrigeration in microgravity. A conceivable risk for VCC systems in microgravity is liquid entering the compression chamber. A modification or design to prevent this potential problem should be developed. Furthermore, research should explore the suitability of oil-free versus oil-lubricated compressors, microgravity dehumidification for air-cooling applications and different expansion devices. Additional investigations on thermal gravitational scaling could yield guidance for the design of microgravity VCC systems. Furthermore, the authors suggest that future space cooling technologies be compared on the basis of their equivalent total mass, accounting for their physical mass, energy consumption and volume used.
(2) Brendel L.P.M. et al. Review of vapor compression refrigeration in microgravity environments, March 2021. Available in FRIDOC (free of charge for IIR members): https://iifiir.org/en/fridoc/review-of-vapor-compression-refrigeration-in-microgravity-environments-143211