Experimental Evaluation of Innovative Thermal Energy Storage Options for a Hypersonic Non-Airbreathing Vehicle's Internal Loads

Managing the thermal loads inside a non-airbreathing hypersonic vehicle is particularly difficult. The heat generated by the power electronics, avionics, etc. must be removed so that the components do not exceed their maximum temperatures. These vehicles cannot dump the waste heat into fuel or ram air because they carry no fuel and do not have provisions for ram air. This means that the thermal energy resulting from the heat generated must be dumped into an onboard heat sink. Existing solutions to this problem have been passive systems based on solid-liquid phase change materials (PCMs), which store thermal energy as they melt. Since space is at a premium, a heat sink must store a lot of energy per unit volume, while keeping components below their maximum temperature. In this project, three heat sink concepts are tested, i.e., one based on PCMs, a second on thermal to chemical (TTC) energy storage, and a third on a hybrid combination of the first two. For the first, three different PCMs are tested and for the second a single endothermic chemical reaction. The hybrid PCM/TTC concept consists of a single PCM which plays the dual role of PCM and reactant in the endothermic chemical reaction of the TTC energy storage. To enhance heat sink performance, the use of thermoelectric generators (TEGs) and a local coolant loop are investigated. The advantage of the former is that they transform waste heat into usable electricity, reducing the amount of thermal energy that needs to be stored by the heat sink. The advantage of the latter is that it results in a more uniform cooling of the heat source and more uniform heating of the heat sink. Prototypes of each of the heat sink concepts and the coolant loop are designed, built, and tested. Experimental results indicate that all the solutions tested in this project outperform widely used paraffin heat sink technologies on an energy per unit volume basis. Our experiments also show that a local coolant loop is indeed advantageous and that current off-the-shelf thermoelectric generators do not generate enough power to offset the power requirements of the coolant loop. Significant improvements in the ZT factor of the thermoelectric materials used by the TEG would be required.