High hydrogen storage capacities at low pressures: the adiabatic storage concept

Kurzfassung

State of the art hydrogen storage for mobile applications is based on 700 bar high pressure tanks. However, due to the energy required for compression as well as due to safety aspects, there is still a need for cheap and safe alternative options. In the past years, metal hydrides have been widely discussed, but due to the low storage capacities or high operation temperatures, so far they are not suitable for the mobile sector. A so called adiabatic hydrogen storage reactor for storage materials with high hydrogen capacities and high operation temperatures can be a solution. The basic operation principle of such a reactor has been published before and in this contribution at WHEC2018, the results of further studies will be presented. In short, the operation principle is based on an adiabatic integration of e.g. MgH2 that exhibits a comparatively high gravimetric storage density of up to 7.7 wt%. To absorb or release the heat of reaction of the metal hydride, a thermochemical heat storage material is incorporated into the reactor, e.g. Mg(OH)2. The reactor-design includes two material-compartments being in thermal contact to enable rapid heat transfer. Since both, the hydrogen- and heat storage are gas - solid reactions, the temperatures and pressures in the compartments can be adjusted independently within the operating window of the reactions. The whole reactor is insulated to the environment and does not need any thermal integration, thus it is adiabatic. Without the necessity of external heat management, new areas of application emerge for metal hydrides with high operation temperatures (>200 °C). Compared to a 700 bar hydrogen pressure tank, this new reactor can achieve double the volumetric hydrogen storage density at a pressure below 20 bar, while the gravimetric storage density is about the same. Therefore, the automobile sector and especially heavy-duty vehicles are promising areas of application. In addition, the considered thermochemical materials are very cheap and operate with water vapour. While for the hydrogen release, sufficient water vapour is produced by the fuel cell, in the reverse process, the water vapour can be released to the ambient. In the present work, the previously published numerical 2D simulations for absorption are extended to the desorption process. Furthermore, other possible material combinations as well as their potential storage capacities are presented.