Accelerating hydrogen absorption in a metal hydride storage tank by physical mixing

A novel method to improve the hydrogen absorption rate in a metal hydride tank is proposed by introducing physical mixing of the metal hydride powder to promote heat removal and accelerate the kinetics of the hydriding process. Experiments were conducted with and without mixing to demonstrate that the hydrogen absorption rate can be improved significantly by mixing. Mixing was achieved by tilting the cylindrical metal hydride tank back and forth by 90° during charging. A mathematical model was also developed to simulate the effects of physical mixing. The model results indicate that physical mixing enhances heat transfer by redistributing the hydride powder from the hot core to the boundary and facilitates heat removal by convection at the tank walls. After validating the model against experimental results, the effect of physical mixing on accelerating hydrogen storage was explored by changing the mixing rate and the convection coefficient at the tank wall, and by increasing the thermal conductivity of the hydride bed by adding aluminum foam. It was found that while higher mixing rates generally improve the absorption rate, the benefits of mixing are reduced for higher convection coefficients, and for higher weight fractions of Al foam. Simulations were also conducted with and without mixing as a function of tank size. The results show that the benefit of physical mixing increases with tank size.