Coupled Metal Hydride Systems for Energy Storage

Kurzfassung

In present times, huge efforts are made to transform the present fossil based energy system to a more renewable one. Hydrogen technologies have the potential to contribute to that change. Among them, metal hydrides can be used versatilely, but thermal management with the ambient is necessary which can be challenging - especially for high temperature metal hydrides. Thermal coupling of a metal hydride with another thermochemical system acting as thermal energy storage is an approach to face that challenge. In this thesis, it was investigated how two selected couplings of a metal hydride with another thermochemical system can be used for energy storage. First, the thermal coupling of magnesium hydride MgH2 with the Mg(OH)2/MgO system was evaluated for hydrogen storage. Second, a system of two metal hydrides being coupled both thermally and on the gaseous side was analyzed for the ability to store electric energy as a thermochemical battery. Both systems can be operated with minimal external heat management, the hydrogen storage reactor in the idealized case even adiabatically. For the adiabatic hydrogen storage reactor, a model was extended and a numerical analysis was conducted to describe the hydrogen release. In addition, an experimental prototype reactor has been designed and tested. For that purpose a test bench was set up to provide both water vapor at up to 10 bar and hydrogen to the reactor. It was found that the thermodynamic properties of the materials fit to each other. The absorption and desorption rates of hydrogen in the MgH2 have been enhanced by the thermochemical cooling and heating, respectively. The experiments showed that the reaction rate of Mg(OH)2 / MgO system is the limiting step. A water vapor pressure of 10 bar is required to obtain a temperature of 300 °C in the MgO compartment during hydration, which is a higher pressure than the numerical analysis predicted and has never been investigated before. Therefore, material modifications of the Mg(OH)2 / MgO system are required to improve its properties. Nevertheless, the operational principle of the adiabatic storage reactor could be proofed. For the thermochemical battery, an energetic analysis showed that electricity storage and release is feasible with a gas compression- and expansion unit. A storage density of approximately 62.6 Wh Lmat-1 and an electric efficiency of up to 47% are achievable for some configurations. Depending on the pair of metal hydrides and operating conditions, additional generation of heat or cold is possible.