Insulation with controllable heat transfer for thermal energy storages

Kurzfassung

The storage of thermal energy is fundamentally linked to the presence of mass. Conversely, the question comes up, if existing masses could be seen as potential thermal energy storages that can be thermally activated, e.g. by electric heating. In this way, building structures or even batteries in electric vehicles could be used as active thermal components. Besides technical limitations, e.g. maximum temperatures, the prerequisite for thermally activating masses is the possibility to control the heat flux across the storage boundaries. In order to control this heat transfer, an adjustable insulation layer can be realized by using reversible gas-solid reactions in combination with porous insulation structures. The controllable insulation system accordingly consists of two coupled core components: a porous insulation material and a metal hydride reactor. Porous structures show a dependency of the thermal conductivity on their internal gas pressure, known as Knudsen effect. In the presented system, this pressure level in the insulation panel is set by a connected reversible endo-/exothermal metal hydride reaction system. By adjusting the temperature of the reacting metal hydride the pressure in the system can be controlled according to its equilibrium line. Since both components are coupled, a change in pressure is directly linked to a change in thermal conductivity of the porous structure according to its sigmoidal curve. As a result, the actual heat flux from any isolated system could be influenced by changing the temperature of a single, separate component. At the German Aerospace Center in Stuttgart, a test bench was set up that allows for a separate investigation of the individual components as well as of the overall coupled system. The central challenge is the interaction of the key components, the insulating panel and the metal hydride reactor, in terms of matching pore sizes, pressure ratios and metal hydride materials. Therefore, in current work insulating materials with different pore sizes are tested in combination with different metal hydrides to better understand their interaction, to identify potential limitations and to approach optimal combinations that could be used for different applications. The experimentally gained proof of operability of the system will be presented as well as measurement results of different pore-sized vacuum panels and gas pressure ranges in terms of optimized switching factors of heat transfer rates. Our recent experimental results show the general applicability of this kind of controllable heat transfer in the context of thermal energy storage. Consequently, our contribution will address critical properties of the individual components, show experimental results and discuss examples of potential applications.