Most thermal storage systems are designed to store heat by melting a Phase Change Material (PCM). They are then recharged by freezing the PCM. Such liquid/solid systems can operate over a very large number of cycles. When only a few cycles must be handled, Thermal Storage with Venting should be considered, since it can result in lighter and/or more compact systems.
Sublimators have been used for many manned spacecraft and space suits, going back to the Apollo program. Liquid water is pumped through a heat exchanger to remove the waste heat. The heated water is then pumped through the sublimator, which consists of tubes in contact with porous media. The outer walls of the porous tubes are exposed to the vacuum of space. A portion of the water freezes in the porous media, cooling the remaining liquid water. The ice also prevents liquid water from exiting through the porous media. The ice gradually sublimes (changes directly to gas), add additional water freezes, so the cycle can continue.
The one major drawback to sublimators is that they only work when the outside pressure is very low (below the triple point of water, 611 Pascals). To overcome this restriction, ACT has developed Thermal Storage Systems with Venting that can operate even at atmospheric pressure. Examples include:
- Single Use Vapor Venting Systems (One to a Few Cycles) When heated, liquid held in a porous structure boils, vaporizes, and then the vapor is vented.
- Hydride Thermal Storage Systems use metal hydrides, which desorb hydrogen when heated.
Note that “Single Use” vapor venting and hydride venting systems can be recharged, and then be reused.
The mass and/or volume can be reduced considerably for a venting system, when compared with a non-venting PCM thermal storage system; see Table 1. Four systems are compared, all of them designed to handle 1.8 MJ:
- Paraffin Wax Thermal Storage
- Methanol Venting System, with a 45% porosity copper wick
- Methanol Venting System, with a 45% porosity carbon wick
- Hydride Venting System
The methanol with a copper wick weighs more than the PCM system, but takes up only one-quarter of the volume. If a carbon foam wick is used, then the system has one-third of the mass, and one-quarter of the volume. Finally, the hydride system has one-half of the mass of a PCM system, but only 7% of the volume.
Table 1. Comparison of Mass and Volume for PCM, Methanol Venting, and Hydride Venting Systems.
|1 kW for 30 min = 1.8 MJ||Parrafin Wax||20.5 kg||26.6 liter|
|Methanol / Copper Wick||31.4 kg||7.17 liter|
|Methanol / Carbon Wick||6.89 kg||7.17 liter|
|Hydride||10.3 kg||1.69 liter|