Non-dimensional numerical analysis of coupled Metal Hydride-Phase Change Material hydrogen storage system

Efficient storage solutions that decouple energy use and production are pivotal for the green energy transition, due to the non-controllable operation of solar and wind power. In this scenario, hydrogen, and in particular metal hydride storage, has shown excellent potential. In this paper we develop a mathematical model to characterize the operation of several cylindrical Metal Hydride-Phase Change Material tank layouts and to determine the improved configuration in terms of charge/discharge time and power. We use non-dimensional parameters to guide the design of a hybrid metal hydride-phase change material hydrogen storage system. We introduce a critical value for the state of charge of the storage system, equal to ϕc∗=0.15, above which heat exchange dominates the process efficiency. Results show that, when varying the canister main aspect ratio between 5 and 100, the equivalent inlet/outlet power increases by a factor ≈10. The ratio of the thermal conductivities is found to have a significant impact in the desorption phase, where the equivalent power increases by a factor ≈4 when raising the ratio from 0.1 to 0.8. Finally, we evaluate three case studies by introducing three different improved configurations and comparing them with the baseline design. A LaNi5/LiNO3−3H2O system for the storage of 1kWh of H2 exhibits 5.65kW and 0.83kW of average power in absorption and desorption, respectively. Such an improved configuration is 93% faster in charge/discharge process with respect to the baseline design. A coupled Mg2NiH4 - NaNO3 exhibits 2.93 kW and 0.30 kW of average power in absorption and desorption, respectively. This configuration is 81% faster than the baseline design. A coupled Mg2NiH4 - KNO3 exhibits 1.66 kW and 0.56 kW of average power in absorption and desorption, while the cycle time is reduced from 1220 min to 147 min (−88%).