VCHPs for Over-Temperature Protection
Variable Conductance Heat Pipes (VCHPs) are most commonly used for precisely controlling the temperature of electronics. A related application is to provide over-temperature protection for devices with a fixed heat supply, and varying demand.
An example is the Advanced Stirling Radioisotope Generator (ASRG), which is under development at NASA to provide power for deep space missions, as well as missions to Jupiter and beyond, where there is insufficient light for solar arrays to operate efficiently. The ASRG radiator is powered by a General Purpose Heat Source (GPHS), which provides heat from radioisotope decay to power the Stirling engine. Once the GPHS is installed in the radioisotopeStirlingsystem, it must be continually cooled. Normally, the Stirling convertor removes the heat, keeping the GPHS modules cool. There are three basic times when it may be desirable to stop and restart the Stirling convertor:
- During installation of the GPHS
- During some missions when taking scientific measurements to minimize electromagnetic interference and vibration
- Any unexpected stoppage of the convertor during operation on the ground or during a mission.
A dual-condenser VCHP has been developed to provide the cooling when necessary, where the primary condenser supplies heat to the Stirling convertor, and the secondary condenser provides back-up cooling when the Stirling convertor is stopped; see Figure 1. This is in contrast to a normal VCHP where the Non-Condensable Gas (NCG) front blankets a portion of the condenser surface. As the temperature increases slightly, the increased vapor pressure compresses the NCG, exposing more of the condenser surface.
Figure 1. A VCHP with two condensers allows the Stirling convertor to be shut-off,
for example when making sensitive scientific measurements. During normal
operation, heat from the General Purpose Heat Source is supplied to the
Stirling engine, while the non-condensable gas blocks the secondary condenser.
When the Stirling is shut-off, heat is automatically and passivel rejected to the
secondary condenser, preventing the GPHS from overheating.
As shown in the video, a GPHS module supplies heat to the heat collector which, in turn, wraps around the hot end of the Stirling convertor’s heater head, so the normal heat flow path is GPHS –> heat collector –> heater head. The non-condensable gas (NCG) charge in the system is sized so the radiator is blocked during normal operation (see Figure 1(a)). When the Stirling engine is stopped, the temperature of the entire system starts to increase. Since the system is saturated, the working fluid vapor pressure increases as the temperature increases. This compresses the NCG. As shown in Figure 1(b), this opens up the radiator. Once the radiator is fully open, all of the heat is transferred from the radiator to the cold side flange, and the temperature stabilizes. Once the Stirling engine starts operating again, the vapor temperature and pressure start to drop. The non-condensable gas blankets the radiator, and the system returns back to the normal state
ACT and NASA Glenn Research Center have recently demonstrated the ability of a VCHP to provide over-temperature protection to a Stirling convertor with a constant power heat source, using the VCHP shown in Figure 2. (See Alkali Metal Backup Cooling for Stirling Systems – Experimental Results) As shown in Figure 2 (b), heat supplied by electric cartridge heaters is directed to the Stirling convertor when it is turned on; otherwise the heat is rejected from a second condenser to a simple radiator. The integrated system is shown in Figure 3.
Figure 2. VCHP for over-temperature protection integrates with a Stirling convertor. (a) Dual condenser VCHP (the first condenser is the inner surface of the annulus, where the Stirling convertor is inserted. (b) Diagram showing how the VCHP fits over the Stirling convertor.
Figure 3. VCHP Prototype Integrated with a Stirling Engine.
Figure 4. Operation of the dual condenser VCHP for over-temperature protection.
Typical test results are shown in Figure 4 At the start of the graph, electrical power was supplied to the cartridge heaters, and the Stirling engine produced power. The power remained constant during the entire test time (with the exception of a small glitch at 240 minutes). After the first 60 minutes, the Stirling convertor was turned off. The VCHP vapor temperature and pressure increase, moving the NCG gas front past the second condenser (back-up radiator). The back-up radiator temperature gradually increases to ~ 650°C, as the radiator rejects the input heat. At around 150 minutes, the Stirling convertor is turned back on, and starts to produce electric power. The temperature in the VCHP drops, and heat is no longer supplied to the back-up radiator.