Another application for Variable Conductance Heat Pipes (VCHPs) is as a Variable Thermal Link, which is used to maintain the evaporator (and associated electronics) temperature in a modest temperature range, while subjected to large changes in power and/or large variations in heat sink temperature. The variable thermal link should transmit heat readily during hot sink conditions, but minimize heat transmission during cold sink conditions.
Applications that can benefit from using VCHPs as variable thermal links include:
- Lunar and Martian Landers and Rovers
- Research Balloons
- Lunar and Space Fission Reactors
- Spacecraft Radiators
The above applications can allow relatively wide temperature swings, but need to minimize or eliminate electrical heater power.
Applications that require variable thermal links generally have the following factors:
- Variable system loads resulting from intermittent use
- Desire to power down systems between missions
- Results in large turn down ratios, on the order of 10:1 or higher
- Large changes in environment temperature
- Lunar surface temperature range: -140 °C to 120 °C
- Mars equatorial, near-surface temperature range: -100 °C to 0 °C
- Research balloons: air temperatures ranging from -90 °C to +40 °C
- Limited electrical power
- Lunar applications must survive the 14-day-long Lunar night, and 1 W = 5 kg of energy storage (batteries) and generation
- Research balloons have similar constraints, since they often fly near the North and South Poles in the winter
Since the lowest sink temperature can be below the freezing point of the working fluid, many applications with variable thermal links also need to consider freeze/thaw and start-up from a frozen state. Fortunately, the Non-Condensable Gas (NCG) in the heat pipe also helps when the pipe is frozen, and during start-up
The main difference between a VCHP for Temperature Control and a Variable Thermal Link is the amount of NCG that is added. In a VCHP for Temperature Control, the NCG blocks a portion of the condenser, and the amount of blockage is controlled by a reservoir heater. In a Variable Thermal Link, more NCG is added. During operation at high power/high sink temperature, the NCG is not in the active portion of the condenser. As the power and/or heat sink temperature drops, the NCG blocks the condenser and as well as most of the adiabatic section. In case, the majority of heat transfer occurs by conduction in the adiabatic section wall and wick. If minimal heat transfer is required, the adiabatic section can be fabricated from a lower thermal conductivity material than the evaporator and condenser; see Figure 1.
Figure 2 shows a radiator panel that was fabricated at ACT for a Lunar fission surface power application ( Variable Conductance Heat Pipe Radiator Trade Study for Lunar Fission Power Systems). The radiator has 5 titanium/water VCHPs, which remove heat from a hot water loop (not shown), and radiate the heat to the environment. During the lunar day, the heat must be rejected to a hot environment, roughly 200 to 350 K (-70 to 120°C). During the Lunar night, the sink temperature can drop down to 100 K (-170°C). With a conventional heat pipe radiator, the heat pipe temperature would drop substantially, and the heat pipe would freeze. As shown on the right side of Figure 3, a VCHP radiator adjusts the active length of the condenser, maintaining the water coolant loop and the heat pipe evaporator above freezing.
Figure 4 shows active and inactive temperature profiles for the radiator in Figure 2. When 3 kW is removed from the radiator, the average radiator panel temperature is about 370 K (100°C), and the VCHP condensers are fully open, see the left side of Figure 4. The right side of the figure shows the radiator panel when the water temperature in the coolant loop is reduced to 300 K (25°C). The NCG completely blocks the condensers, allowing the coolant loop and heat pipe evaporator to remain warm, even when the sink temperature is further reduced. Further testing with the radiator.
After initial tests at ACT, the radiator panel was shipped to NASA Glenn Research Center for further testing; see Figure 5 (Heat Rejection from a Variable Conductance Heat Pipe Radiator Panel). The panel is yellow, and the radiator above the five heat pipes is almost white. The VCHP sections are the bent darker purple sections above the panel. The radiator panel was successfully operated over a range of inlet water temperatures and flow rates, as well as a freeze/thaw cycle.