Development of a Variable Conductance Cold Plate for Spatial and Temporal Isothermality Across Power Scales

For low-earth orbiting satellites, environmental conditions rapidly change throughout the course of a single orbit. These changes affect the radiative heat sink of these satellites, and in turn, rapidly alter the temperatures of sensitive onboard instrumentation. As such, there exists a need for a thermal control system to maintain such instrumentation at a uniform and consistent temperature despite environmental changes. Such a thermal system must also be low in size, weight, and power if it is to be adopted into LEO satellites. Through a NASA Phase II SBIR, ACT has developed such a thermal system, consisting of a variable conductance cold plate (VCCP) to provide spatial and temporal isothermality to mounted electronic components. The technology builds on ACT's experience with passive two-phase devices, specifically variable conductance heat pipes (VCHP) and vapor chambers, which address spatial isothermality. Temporal isothermality is handled by the VCHPs, which are charged with a working fluid at saturation conditions and a non-condensable gas (NCG). During operation, vapor that is generated from the VCHP's evaporator moves to the condenser. As the heat load/sink temperature changes, the working fluid seeks to operate at a higher temperature. This increases pressure as the working fluid exists at saturation conditions. This increase pushes back the NCG and opens more condensing area. As the opposite occurs, NCG pushes forward and blankets a portion of the condensing area of the VCHP. As the condenser surface area changes, the conductance of the heat pipe varies, which allows the VCHP to carry heat at a higher rate and mitigate the effects of heat load or sink temperature changes. ACT has tested two subscale versions for different power levels, charged with either acetone or ammonia, and expanded this technology to a cold plate boasting a heat collection surface area of 0.5 m2.