Existing technologies [Loop Heat Pipe (LHP) and passive Thermal Control Valve (TCV)] are integrated and made to work together to provide a passive variable thermal link. The result is a novel LHP with passive TCV that was developed to provide variable heat rejection (turn-down) allowing efficient operation during periods of low dissipation and cold environments, as well as periods of peak loads and warm environments. The thermal control valve installed in the vapor line routes the vapor flow to the radiator during normal operation, or directly to the compensation chamber during periods of cold radiator sink temperatures or low power. The vapor bypassing the condenser cancels the circulation to the radiator, thereby minimizing heat transport and rejection.
Future lunar surface missions will require a variable thermal link for thermal management of the Warm Electronics Box (WEB) and batteries. During the long lunar day, the thermal management system must be capable of removing the waste heat from the WEB/batteries to keep the WEB and batteries within the acceptable temperature range. Since the heater power availability is restricted during the long lunar night, the variable thermal link must limit the amount of heat removed from the WEB and radiated to space. At the same time, the variable thermal link should use as little power as possible. Integrating an LHP with a passive TCV would provide a passive variable thermal link. Since the valve is fully passive, no control power is needed.
Conventional LHPs can provide the required variable thermal conductance needed to maintain the WEB and battery temperature. During the lunar day, the LHP will transfer the thermal load to the radiator for heat rejection. During the 14-day-long lunar night, the sink temperature drops, potentially lowering the LHP and the WEB/battery temperatures. Without a variable thermal link, the LHP will continue to remove heat during the lunar night, cooling the electronics and batteries to unacceptably low temperatures.
An ammonia LHP with TCV is shown in Figure 1, where the TCV is marked with “3.” In addition to the TCV, tubing and valves were added so that the behaviors of the LHP with and without the TCV could be compared. The test plan was as follows:
- Set the power input to the evaporator to 100 W and allow the evaporator temperature to reach a steady state.
- Decrease the condenser temperature in stepwise increments until a steady state of –60 °C is achieved.
- Decrease the power input to the evaporator to 0 W and allow the evaporator to reach a steady state condition.
The results for the LHP with TCV are shown in Figure 2(a). The evaporator and compensation chamber were operating near 25 °C, while the condenser was operating near –55 °C. After 2 hours, the evaporator power was reduced to zero. As the evaporator temperature started to drop, the thermal control valve acted to passively shut down the LHP. With no evaporator power, the temperature dropped very slowly over the remainder of the 26-hour test. Figure 2(b) shows the behavior of the LHP without a TCV. In this case, the temperature dropped significantly even when the evaporator power was on. After the power was shut off, the LHP continued to transfer heat, with the evaporator operating near –45 °C.
In a thermal vacuum chamber, this LHP with TCV design demonstrated a heat rejection conductance turn-down capability in excess of 50:1. Current state-of-the-art LHPs can provide variable heat rejection, but at the cost of electrical power required to minimize heat rejection, which makes them unattractive for resource-limited landers and rovers. A major benefit of the LHP with TCV is that it passively provides a variable heat rejection capability, without requiring any electrical power or control to modulate the LHP.