Variable Conductance Heat Pipes (VCHP): When, How, and Why To Use Them in Space Systems

With so many different types of heat pipes, how does one know when to use a VCHP? In the ever-evolving realm of thermal management for space systems, engineers continue to seek enabling technologies that maximize performance in an environment that yields conflicting requirements.

The Building Blocks of Heat Pipes in Thermal Management

Before we explore the intricacies of VCHPs, it’s essential to understand their foundation—the constant conductance heat pipe (CCHP). Picture a vacuum-sealed pressure vessel containing a working fluid and a wick structure operating at saturation. It contains an evaporator section (heat goes in), a condenser section (heat goes out), and an adiabatic section in between.  CCHPs are designed to be capable of operating during the highest power case, efficiently moving heat over significant distances to location where the heat can be dissipated (i.e. radiator panels). CCHPs operate passively (no pumps, moving parts, or external controls required); this cycle can be summarized as follows:

• Energy (heat) put into the evaporator vaporizes the working fluid
• The incoming heat results in a higher temperature (pressure) at the evaporator and a lower temperature (pressure) at the condenser
• Vapor moves from the evaporator to the condenser due to the pressure gradient in the system
• The vapor condenses, depositing stored energy to the condenser
• The wick structure returns the working fluid to the evaporator

The result is a “thermal superconductor” capable of transferring hundreds of Watts of energy with a minimal temperature difference between the electronics and effective sink.

When To Use Variable Conductance Heat Pipes for Thermal Management

CCHPs are appropriate for many applications, but they are not the best choice in all scenarios. CCHPs, as noted above, operate entirely passively. Thus, they have one operating condition: ON. This means that the CCHP will continue to transfer energy whenever a temperature difference is present. While this is what you want to prevent your electronics from overheating during operation, this condition is opposite of what a thermal control system architect wants during a minimum power or survival case. You want each Watt of energy you are putting into the system to go into maintaining a “warm enough” temperature of your electronics to keep them happy – the CCHP can threaten to take the electronics below their allowable temperature unless the system delivers an inhibitive amount of power to your survival heaters to compensate.

So, to what technology should the thermal architect turn that is capable of transferring energy when needed and stifling it when not? One option to evaluate would be VCHPs, an advanced thermal management solution designed to mitigate the risks associated with highly varying power levels and sink temperatures.

How Are Variable conductance heat pipes Different from other types heat pipes?

The brilliance of VCHPs lies in their adaptability to changing power conditions. The main differentiator between VCHPs and CCHPs lies in the introduction of a reservoir containing a non-condensable gas (NCG). The gas acts as a thermal “spring”, expanding and contracting as the pressure inside the heat pipe changes with varying temperatures. Under normal operating power and warm operating temperatures, both pipes function the same; the non-condensable gas is confined to the reservoir and the condenser is fully open and available to receive hot vapor from the evaporator. This results in a high conductance operating mode, and the system can transfer the heat from evaporator to condenser at the same efficiency (temperature difference) as a traditional CCHP.

Constant Conductance Heat Pipe Operation

As power decreases and/or sink temperature drops, the working fluid saturation pressure – responsible for compressing the non-condensable gas into the reservoir – lessens and the NCG is able to expand into the vapor space within the condenser. This reduces the condenser’s efficiency, limiting the length available for two-phase heat dissipation, as the expanded NCG “blocks” incoming vapor. At minimum power input and sink temperatures, the gas expands further and can fully block the condenser. In this “fully closed” condition, losses are primarily limited to conduction through the envelope/wick material of the pipe and survival power budget lost to the sink is minimized.

Variable Conductance Heat Pipe Operation

Why Use variable conductance heat pipes in Thermal Management?

• Adaptability to changing power and sink conditions
• Design flexibility: the reservoir can be on the condenser side or on the evaporator
• Passive and active control options
  • Selection may impact the amount of thermal control possible with the VCHP
• Tight thermal management of the evaporator
• Minimize survival power
• Dampen amplitude of temperature during orbit (reduced solder fatigue)

Cold Reservoir vs. Warm Reservoir Variable Conductance Heat Pipes

There are two primary types of VCHPs: Cold-Reservoir VCHP and Warm-Reservoir VCHP. The temperature (Cold vs. Hot) refers to where the reservoir is located on the heat pipe. The table below summarizes some points of comparison between the two options:

Conclusion

Variable conductance heat pipes represent a groundbreaking solution for thermal control in space systems. By introducing a non-condensable gas reservoir, engineers can fine-tune heat dissipation based on power input and sink temperature, ensuring desired performance in non-nominal conditions. For those eager to explore the applications and feasibility of VCHPs further, Advanced Cooling Technologies, Inc. is the go-to source for expertise, information and ultimately manufacturing.