Porous Metal Hydride (PMH) Compacts for Thermal Energy Applications

Pelletized Porous Metal Hydride (PMH) was investigated in order to assess its thermal capability for energy storage/transfer applications. Metal hydrides have been known as promising materials for hydrogen storage systems, heat storage systems, and thermal devices, thanks to their nearly reversible reaction characteristics during the hydrogen absorbing and desorbing processes. The conventional powder-type metal hydrides however have a relatively low thermal conductivity, which is responsible for low heat generation. In the present study three representative metal hydrides, LaNi₅, Ca_0.6Mm_0.4Ni₅, and LaNi_4.75Al_0.25, metal hydride powders were coated with thin copper and pressed at 3,000 psig with metal additives in order to improve the thermal conductivity. This pelletizing process does not require the use of an organic binder and additional processes such as sintering under high pressure. The pelletized PMH compacts employing the copper coating exhibit higher thermal conductivity compared to raw metal hydride powders. However, pelletizing may deteriorate the permeability of the PMH compacts, lowering mass transfer of hydrogen. Therefore, the permeability must be observed to verify whether it meets the required level for suitable applications. Measurements were performed by varying copper fractions and plotted against the upstream/downstream pressure differential. Darcy’s equation in conjunction with an ideal gas assumption was used to calculate the permeability of a rigid wall design. This investigation reveals that rising copper content is accompanied with decreases in permeability. Permeability values for most samples tested in this study were found to be larger than the desirable level, 5 × 10⁻¹⁵ m². Additionally, the thermal performance of the LaNi₅ PMH compacts was tested by calculating and comparing the heat generation of the PMH pellets and powders filled reactors during the hydrogen absorption process in water bath medium.