We experimentally studied hydrogen desorption from MgH2 by supplying heat via a hot gas flow. Porous sheets of MgH2 held in a sponge-like carbon nanotube (CNT) matrix were developed and placed in a coaxial double-tube reactor. Ar gas was heated using a cylindrical heater and then flowed alongside the MgH2-CNT sheets, thereby increasing the temperature of the sheets and enabling the desorption of hydrogen into the gas flow. The total energy efficiency was approximately 6.2% when 64% of hydrogen in MgH2 was desorbed. A numerical simulation was conducted for the heat transfer and hydrogen desorption, and the obtained results were consistent with the experimental results. According to the simulation, the low energy efficiency was attributed to the small heat capacity ratio of MgH2 to the reactor (0.082) and considerable radiative heat loss (53%). The simulation was used to predict energy efficiency improvements, and the efficiency was considered to increase to 12% upon increasing the heat capacity ratio from 0.082 to 1.4, and further to 21% upon doubling the Ar flow rate, which enhanced the convective heat transfer from the heater to MgH2.

We propose using hydrogen as a heat transfer medium to supply waste heat from hydrogen-driven devices to hydrogen storage tanks. In our model, MgH2 is used in the form of porous sheets, set in parallel in the tank, and heat is supplied via hot hydrogen flowed through the interspaces between the porous sheets. Feasibility of the hydrogen desorption reaction in this process was verified numerically. Hydrogen efficiently carried heat to the stack of porous MgH2 sheets via convective heat transfer and then carried heat into the porous MgH2 sheets via conductive heat transfer through the pores owing to its high thermal conductivity. We found that the hydrogen desorption is also fast enough to allow the supplied heat to be used efficiently to drive the endothermic hydrogen desorption reaction. It was understood that the thickness of the MgH2 sheet and hot hydrogen flow speed affected hydrogen desorption. These factors can be evaluated by using the dimensionless number of τs/τh which is the ratio of the space time to the time constant for heat transfer in the MgH2 sheet. Under τs/τh > 0.01 range, both the reaction and heat transfer are fast enough, the hydrogen desorption is limited by heat supply, and hydrogen desorption amount is proportional to the heat supplied to the reactor. The tank structure and operating conditions can be designed by using the dimensionless number of τs/τh.