The project THERS is based on the vision to treat thermal energy the same way as electric systems treat electricity. It shall be assembled out of sources causing heat, guidelines for the transfer such as heat pipes into storage systems working like a battery keeping the thermal energy for other purposes. Associating that vision with a mobile system, it is combined with a hard break even related to the weight and space of the system but first of all, these components need to be developed. Heatpipes are state of the art in high performance systems such as computers and brakes or engines are well known as heat sources but the battery system is still an empty spot in that assembly.
Phase Change Material (PCM) was the material of choice to store thermal energy in a material transition from solid to liquid state. For many technical applications the material is reacting to slow. Heating a large tank of PCM the material that is in touch with the outer shell will melt first leaving a liquid zone with very low heat conductivity that turns the high capacity energy storing system into a very slow reacting thermal battery.
The increase of the reaction speed was a major objective of a research project at the Fraunhofer IWU. By using the combination of PCM with a cellular metal matrix that is transferring the heat deeply into the wax structure, the paraffin in direct contact with the heated surface is highly increased. Open cellular structures as well as closed cellular structures realized by the powder-metallurgical-production route have been used. These structures show micro cracks in the cells that allow an infiltration process too.
A comparison of the thermal behavior of all test specimens resulted in a thermal difference inside the test samples strongly related to the components used. That way the simulated functionality of the heat spreader was validated. The closed porous sample shows the lowest difference between hottest and coldest spot inside which stands for a very homogeneous heat distribution realized by the spreader and the Al-foam structure.
The combination of cellular metals with PCM fulfills the requirements of a thermal battery as it reacts relatively fast and is able to store thermal energy. It is a first successful step to finally realize the vision of a thermal circuit and will be followed by a combination with heat pipes in the next project.