Satellite Heat Pipes
Researchers from Graphene Flagship partner institutes Universite libre de Bruxelles (Belgium) University of Cambridge (UK), CNR Bologna (Italy), and Leonardo (Italy) are designing an experiment to test how graphene-based coatings can improve efficiency in loop heat pipes, which are used as cooling systems in satellites. The team travelled to Bordeaux, France between 6-17 November 2017 to test the graphene-coated loop heat pipes in microgravity in a parabolic flight, operated by the European Space Agency (ESA) and Novespace (France).
The parabolic flight is a method of creating almost zero gravity without going to orbit, with up to 25 seconds of weightlessness as the plane follows a parabolic trajectory. Each 3-hour flight will make around 30 parabolic arcs. In addition to the weightlessness, up to double the Earth’s gravitational force is experienced onboard as the plane prepares for the next parabola, similar to going up and down on a roller-coaster.
Thermal management is very important in satellites, as the lack of air requires specific technological solutions to reject the heat toward deep space: high temperatures decrease reliability of electronics and can affect the performance of the electronic equipment and instruments onboard. The temperature difference between the two sides of a satellite facing towards and away from the sun can be extreme, reaching more than 200°C. Using a loop heat pipe, heat can be transferred from the hot spots to the cool parts, radiating the excess heat into space.
Graphene’s thermal and physical properties will be exploited to improve the efficiency of the loop heat pipe. The loop heat pipe cools electronic devices and instruments by using the heat they generate to evaporate a fluid inside a wick, typically made of porous metal. By coating the wick with graphene, the researchers aim to improve the heat transfer between the electronic units to the fluid through the wick.
The porous metal wicks are coated with a graphene-based foam, creating a “foam-within-a-foam” structure. This increases the surface area of interaction with the liquid, so heat can be transferred more smoothly from the wick to the fluid, reducing the temperature differential. The graphene-based foams will also improve the capillary pressure of the wick, allowing the working fluid to travel more quickly through the loop heat pipe. Capillary action draws the working fluid back into the wick, so that the fluid is continuously pumped around the closed loop pipe, giving constant cooling.
The experiment is built inside a specially designed rack, to hold everything in place during the periods of weightlessness. Researchers at the Microgravity Research Centre (Universite libre de Bruxelles) are assembling the experiment rack and testing the wicks before and after coating with graphene. The wicks are coated using different methods at the Cambridge Graphene Centre (University of Cambridge) and CNR. Leonardo, the industrial partner, provides guidelines and specifications to make the graphene-based wicks ready for use in real space applications in orbit, after these simulated space conditions. This experiment is the 67th ESA Parabolic Flight Campaign, and is operated by Novespace in partnership with ESA.