A Self-Regulating Freezable Heat Exchanger for Spacecraft

A spacecraft thermal control system must keep the vehicle, avionics and atmosphere (if crewed) within a defined temperature range. Since water is non-toxic and good for heat transport, it is typically used as the coolant that circulates within the crew cabin boundary. This loop then interfaces with another low freeze point fluid, such as ammonia, for transport of heat to a radiator where the temperatures can be considerably below the freezing point of water. The volumetric expansion during freeze usually prevents its use in external systems since freezing will damage the components. Yet, if the system can accommodate the forces generated by freezing, then selectively allowing parts of a heat exchanger to freeze can be used to passively increase the turn-down of the heat rejection from radiators. TDA Research, Inc. has been developing freezable water/ice phase change heat exchangers for several years that offer several advantages: they can eliminate the need for a separate heavy Freon or ammonia loop; use the buildup of ice to regulate the rate of heat transfer, and the endotherm of melting ice can absorb peak loads from the spacecraft to reduce the size and mass of the radiator. Therefore, TDA Research and the University of Colorado set out to demonstrate a lightweight and freeze tolerant water/ice heat exchanger to passively regulate the heat rejection rate from the water coolant loop of a manned spacecraft to its heat sink systems. The heat exchanger has no actively moving parts and is thus extremely reliable. In Phase I, we designed and built a self-regulating freezable heat exchanger that we put through 191 freeze/thaw cycles without damage and it has the capability to transfer the loads expected in crewed spacecraft. In Phase II, we will design, build and test a large-scale freeze tolerant water/ice heat exchanger that forms the heart of a thermal control system that we will deliver to NASA.

A water coolant loop is usually part of the thermal control system for manned spacecraft. The water loop then interfaces with a Freon or ammonia loop to reject heat to the heat sink systems. A simpler approach would be to design the water coolant heat exchangers to be freeze tolerant and utilize the phase change of water to ice as part of the thermal control system. This would eliminate the need for a second heavier fluid loop using Freon or ammonia (heavier because these fluids are poorer heat transfer media). Further, a water/ice heat exchanger can use the buildup of ice to self-regulate heat transport from the spacecraft to space. This approach to thermal control will result in a safer and more reliable system. In spacesuits, a freeze tolerant heat exchanger/radiator system will dramatically reduce (by roughly 75%) the single largest consumable during EVA. A spacesuit radiator can replace the PLSS covering with very little net increase in weight and yet will cut the amount of water needed to cool the astronaut during an EVA by up to 6 lbs. This will represent a significant cost savings to future missions and especially in Lunar and Mars EVA missions where the reduction in water loss is not merely nice, it is essential.

The largest and nearest term commercial applications are the use of freeze tolerant tubing on earth. These earth-based applications include sprinkler systems and potable water supply in homes and commercial buildings. This market is potentially very large and virtually un-tapped because of the lack of a viable freeze tolerant tube. The Insurance Institute for Property Loss Reduction says frozen pipes have cost the insurance companies in the USA $4 billion in damage to insured homes and buildings over the past decade (i.e., about $400,000,000 per year). The savings in insurance rates alone could more than offset the cost to the user, who would have the added benefit of not having valuables destroyed by water damage and their lives disturbed during repairs of the water damage.

This is early/mid-stage (TRL 6) — the most realistic path in is NASA SBIR/STTR, which funds small businesses and research institutions to develop technology aligned with NASA's needs (equity-free, phased funding). Check whether a current SBIR/STTR solicitation topic overlaps with this project's technology area, or contact the project directly (above) to ask.

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