Lightweight, Durable PCM Heat Exchanger for Spacecraft Thermal Control
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We have built and demonstrated a proof-of-concept thermal storage system using an innovative design that enables very high thermal storage per unit mass. Thermal storage is a key technology for future spacecraft because it can reduce the size of thermal management systems by smoothing out variations in the heat load and/or heat rejection environment. This approach reduces system mass by enabling designers to size active components based on a spacecraft’s average operating conditions instead of the least favorable conditions. Heat exchangers that contain phase-change material (PCM) enable thermal storage by freezing the PCM when extra heat rejection is available and thawing it when the heat loads are high. Low-mass PCM heat exchangers are difficult to design, however, due to the large amount of additional mass needed to provide a durable heat exchanger structure and to provide low-resistance pathways for heat conduction into or out of the PCM. Our PCM heat exchanger design is based on innovative heat exchanger fabrication techniques that enable essentially direct-contact heat exchange with the PCM through a very lightweight containment structure. We have fabricated a prototypical core sample and measured its energy storage performance using paraffin PCM in thermal cycle tests that simulate operation in low-lunar orbit. The tests demonstrate very high thermal energy storage per unit mass of heat exchanger (130 kJ/kg ) in the heat exchanger core, low pressure losses, and efficient heat transfer. Based on these results, we have produced a conceptual design of a full-size PCM heat exchanger based on requirements for a manned spacecraft in low Lunar orbit. The test results and design calculations show that the PCM heat exchanger can achieve an overall PCM mass fraction greater than 50% in a 3,700 kJ thermal storage unit.