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 more compact systems when compared with a non-venting PCM thermal storage system; see Table 2. The first venting system assumes a 45% porosity copper wick (current technology). The mass could be further reduced by developing a lighter, 45% porosity carbon wick. 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.
Table 2. Comparison of Mass and Volume for PCM and Methanol Venting Systems.
|3 kW for 45 sec = .0135 MJ||PCM||1.5 kg||2.0 liter|
|Methanol / Copper Wick||2.36 kg||.54 liter|
|Methanol / Carbon Wick||.52 kg||.54 liter|
|1 kW for 30 min = 1.8 MJ||PCM||20.5 kg||26.6 liter|
|Methanol / Copper Wick||31.4 kg||7.17 liter|
|Methanol / Carbon Wick||6.89 kg||7.17 liter|
The most commonly used vapor venting system is the Sublimator, which uses a liquid-ice-vapor transition to remove heat from manned spacecraft and space suits, going back to the Apollo program. 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 Vapor Venting that can operate even at atmospheric pressure. Thermal energy is captured as latent heat by evaporating a liquid and the vapor is then vented from the system. A schematic of the system is shown in Figure 1. A fluid chamber is attached to the heat source which is to be maintained within the given temperature range. The interior of this chamber contains a wick structure. Prior to use, the chamber will be charged with a specified quantity of the working fluid such that the entire liquid inventory is contained within the wick. A transport line connects the chamber to the ambient and is controlled by a series of valves.
As heat is applied to the chamber, the liquid in the wick will evaporate, raising both the temperature and pressure within the chamber. The vapor will flow through channels in the wick away from the heat source. As the pressure increases within the chamber the pressure control valve will open allowing vapor to be vented off. The pressure will then drop and the valve will close. The valve will continue to pulse open and closed to maintain the pressure for which the valve is set. Since the fluid contained within the chamber is two-phase at saturation conditions, the temperature is directly related to this pressure. This relationship allows the temperature to be set and maintained within a narrow range that is determined by fluid properties, as well as the lift and seat pressure of the control valve.
Several other optional valves can be included to increase the versatility of the system. For long life storage, a rupture disk would be installed downstream from the pressure control valve. This disk would prevent leakage of the working fluid, and minimize corrosion of the pressure control valve. For a system that is designed for venting to vacuum or low pressures found at high altitudes a check valve can be included to ensure that ambient air is not pulled into the chamber before the correct ambient condition is reached. If the system will be exposed to temperatures above the set point during storage but will be pre-cooled to below the set point before the beginning of the mission, then the shut-off valve can be included to prevent venting fluid before it is required. In this case, a second rupture disk may be included on the chamber for pressure relief as a safety precaution to prevent the chamber from bursting if exposed to extreme high temperatures. If either rupture disk is broken, then the system would need to be recharged before use.
Applications where a vapor vented thermal storage system would be used include:
- Several cycles per mission (can be recharged)
- Volume and weight constrained systems
- High heat load and short time durations
- Large lateral and normal g-loading environments
- Long duration storage
- Hydrides can also be used
The vapor-vented thermal storage system is currently at Technology Readiness Level (TRL) 6, System Prototype Demonstration in a Relevant Environment. ACT has already demonstrated a vapor-venting system, using methanol as the working fluid, and shown that it can dissipate 1 kW for 120 seconds:
- Suitable for high g-loading
- Maximum weight: 1.5 lbs.
- Limited Volume: 0.4 L
- Copper wick
- Easy to vent for testing, and then refill