Dynamic and Efficient Reactor as precise Gas Pressure Control Unit for Thermal Switch Devices

Kurzfassung

Adaptive building envelopes are capable of contributing to significant energy savings, according to a referred study in a comment in Nature Energy, which declares variable conductance building envelopes to be one of the five grand challenges for decarbonization. This would allow solar heat to be gained temporally and partially in winter and transitional seasons as needed. Most importantly, variable conductance building envelopes can reduce the energy consumption of air-conditioning in non-residential buildings by taking advantage of the passive cooling effect by night during hot summer periods – at least in mid-latitudes. Another scenario for controllable insulation in buildings is the utilization of existing high-capacity building components as thermal energy storage and thereby transforming buildings into active players for grid flexibility. Provided the heat flux through an insulation layer can be controlled in terms of time and amplitude, thus thermally activated components, charged by surplus energy, are able to contribute to space heating/cooling and peak shaving. Such a controllable insulation can be realized by combining the gas pressure dependent thermal conductivity of porous structures with reversible gas-solid-reactions. The thermal conductivity of porous media is highly dependent on the prevailing gas pressure, the relation follows an S-shaped curve (Knudsen effect). On the other hand, the gas pressure of thermochemical reaction systems can be controlled thermally by setting the temperature along the equilibrium line. By connecting these two mechanisms, the gas pressure in variable conductance components, such as controllable insulation layers, can be specifically adjusted and thus the heat flux through the insulation can be controlled. At DLR, we employ metal hydrides/hydrogen as thermochemical reaction system for the gas pressure adjustment. Our controllable insulation system is based on two individual core components – connected via valve - that correspond to the respective physical/chemical effects: a controlling thermochemical reactor and an openpored vacuum insulation panel. This means the metal hydride material is not integrated into the panel, but a separate reactor is used, which serves as an independent control unit for the reversible gas pressure adjustment. The thermochemical reactor is a key component in the integral adaptive insulation system. The reactor component governs the controllability of the gas pressure in the porous insulation material and thus the heat flux. The operation range of the gas pressure adjustment range according to the set temperatures directly corresponding to the employed metal hydride material. Important characteristics of the overall insulation system as controlling sensitivity, dynamic behavior as well as energy consumption for varying the heat flux strongly depends on the reactor component. On the one hand, the selection of suitable metal hydride materials and the accurate characterization of them in vacuum is relevant to precisely adjust the gas pressure based on setting the temperature of the thermochemical reaction system. On the other hand, a well-developed reactor concept as container for the metal hydride material is needed to technically provide the required hydrogen pressure in the connected insulation panel fast, reliably and with low energy consumption. In the contribution, we will present a novel design, built-up prototype and experimental characterization of a reactor component that is capable to precisely adjust a gas pressure, especially in vacuum, and being highly dynamic and efficient at the same time. The concept of the reactor component is based on thermal detachment of the active reaction material from the necessary but passive casing and the energy supply for the reaction is provided thermoelectrically by applying Peltier elements.