A Self-Regulating Freezable Heat Exchanger for Spacecraft

A spacecraft thermal control system must keep the cabin (both air and its structure if manned) and electronic equipment within a narrow temperature range even though the environment may vary from very cold to warmer than room temperature. Since water is safe to use and an excellent coolant (other than its high freeze point and volumetric expansion during freeze), a water coolant loop often is used to transport heat to or from the spacecraft via heat exchangers to the heat sink systems that reject heat to space. Some of the heat exchangers would freeze, particularly the ones transporting heat to a flash evaporator or cold radiators exposed to deep space, if not for system controls to prevent it. Yet, the principle of allowing a heat exchanger to freeze can be utilized to increase the turn-down of the heat rejection rate (e.g. to vary the heat rejection from radiators). Unfortunately, the expansion during the phase change of water to ice may damage and ultimately fail the heat exchanger if it is not designed to withstand this event. TDA Research, Inc. has been developing water/ice phase change heat exchangers for several years, since the thermal control system can be simpler (a secondary loop between the coolant water loop and the heat sink systems may no longer be needed) and smaller in size while reducing the use of consumables. Therefore, TDA Research and the University of Colorado propose a lightweight and freeze tolerant water/ice heat exchanger that can passively regulate the heat rejection rate from the water coolant loop to the heat sink systems. The heat exchanger will have no moving parts and thus will be extremely reliable. In Phase I, we will design and build a freeze tolerant water/ice heat exchanger without resorting to a large heavy-walled structure and then subject it to hundreds of freeze/thaw cycles to verify its integrity.

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 should 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 $4.2 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.

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