Graphene loop heat pipe in space

We report a graphene enhanced capillary pump for a Loop Heat Pipe (LHP). LHPs are passive devices for thermal control of equipment and payloads for satellites and probes. Capable to transfer heat from sources to the satellite radiators, LHPs use the waste heat of the devices to be cooled as the driving energy for pumping a fluid through a loop. The core of a LHP is a capillary pump, a phase separator that allows pressure being generated by capillarity across a porous structure, in two-phase conditions for the operating fluid. Traditionally, LHPs capillary pumps are sintered metallic wicks. Various properties of graphene have been considered for enhancing the LHP performance, acting at capillary pump level: porosity, wettability and thermal conductivity. Major issues in using conventional LHPs, i.e. with metallic wicks, in operational space missions are related to their non-deterministic start-up behaviour. Additionally, reliable LHPs are required to ensure operations without interruption for the entire mission duration. Superior start-up capabilities have been achieved with graphene oxide coated Ni sintered wicks, using acetone as a fluid, operated in a vacuum chamber on ground. A LHP capillary pump was tested during a zero-g parabolic flights campaign, to eliminate gravity effects. The measurements show up to 8 times improvement in evaporation rate compared a conventional metal only wick.