Aspacecraft thermal control system must keep the vehicle, avionics, and atmosphere (if crewed) within a defined temperature range. Water coolant loops are typically used to transport heat to or from the cabin of a crewed spacecraft via heat exchangers to the heat sink systems that reject the heat to space. Water is non-toxic and good for heat transport, but it has a high freeze point. Thus, there is concern that the water loop can freeze and damage the thermal control system unless a low-freeze-point intermediate fluid loop is included. Incorporating a freeze-tolerant water/ice heat exchanger can eliminate this risk, and offers a novel approach to spacecraft thermal control, since parts of the heat exchanger can be selectively frozen to passively increase the turndown of the heat rejection rate. In addition, it has the potential to simplify the thermal control system and thereby reduce its size and mass.
A self-regulating heat exchanger (SRHX) was designed and built that uses the phase change of water to ice to self-throttle the rate of heat transfer, and provide endothermic heat capacity during thaw. The SRHX has no actively moving parts.
The heat exchanger is a double pipe (or tube in a tube) heat exchanger. The inner tube takes the warm water from the spacecraft, cools it, and returns it to the spacecraft, rejecting its heat. The outer pipe picks up the heat load from the spacecraft and carries it to the radiator, which rejects it to space. The inner tube has fins to help efficiently transfer heat to the wall; however, the space between two of the fins is insulated (from both the wall and the fins) so it stays warm and never freezes. When the heat exchanger is transferring its maximum load to the fluid in the outer tube, it stays fully melted; however, when the load drops, the water in the inner pipe will drop to freezing towards the end of the heat exchanger (the coldest end), and will start to freeze on the walls and the fins. This layer of ice is an insulator, and the heat flux through this section drops dramatically. If the heat load continues to drop, more and more of the inner heat exchanger freezes, and the flux to the heat rejection system drops as well. Finally, at the minimum heat flux, little more than the insulated path remains open.
For this concept to work, the phase change material (PCM) heat exchanger must be able to freeze without structural failure, it must thaw quickly when the heat flux increases again, and the heat rejection capacity needs to respond in proportion to the load. To cope with the expansion that occurs as the water turns to ice, a collapsible bladder is inserted in the center of the inner tube to provide the needed room for expansion. The heat exchanger also contains a thermally insulated flow channel that remains open in nearly all operating scenarios (it will only freeze if there is a total loss of coolant water to the HX). When the water in this channel heats up again as the load increases, the warm water (which is in direct contact with the ice) quickly thaws the ice (both from the left to the right, and from the center out to the wall).
The system can provide passive, load-adaptive heat rejection to space-bused thermal control systems (both manned and unmanned) with no moving parts. The freezable radiator is lightweight and quickly responds to changing environments or metabolic loads, both freezing and thawing as needed.