Measurement of the effective thermal conductivity of a metal hydride bed with chemical reaction

Abstract

Hydrogen is an important raw material for the chemical industry. At present, pure hydrogen is not used directly to produce energy, but it is believed that hydrogen could be used economically as an indirect source of energy in the future, because the reserves of fossil energy, in particular oil, are limited. Hydrogen can be generated by using primary energy sources such as nuclear and solar energy. Therefore there is a need to transport, utilize, and store hydrogen safely and effectively. Reversible metal hydrides are able to fulfill these needs, and magnesium seems to be one of the most promising metal hydrides. Investigations in this field are concentrated on the optimization of metal hydride stores and machines. This optimization of the reactors requires knowledge of the effective thermal conductivity of the metal hydride bed, because that is the controlling factor for the hydrogen absorption/desorption rate. The present study introduces an experimental transient method for the measurement of the effective thermal conductivity of a metal hydride bed in a high temperature range (573–673 K) and for a hydrogen pressure atmosphere up to 5 MPa. The change of the radial temperature profile is studied in a cylindrical metal hydride bed by applying a constant heat flux at the boundary. The value of the effective thermal conductivity can be calculated using Fourier's law provided that the desorption power for the release of hydrogen is analytically eliminated. The measured values of the effective thermal conductivity are in the range of 4–9 W/(m K). It is shown that the effective thermal conductivity decreases with increasing reacted fraction. The experimental results are in agreement with values determined earlier by Strassburger and coworkers using a steady-state method.