Abstract
Effective thermal conductivity is an important thermophysical property in the design of metal-organic framework-5 (MOF-5)-based hydrogen storage tanks. A modified thermal conductivity model is built by coupling a theoretical model with the grand canonical Monte Carlo simulation (GCMC) to predict the effect of the H2 adsorption process on the effective thermal conductivity of a MOF-5 powder bed at pressures ranging from 0.01 MPa to 50 MPa and temperatures ranging from 273.15 K to 368.15 K. Results show that the mean pore diameter of the MOF-5 crystal decreases with an increase in pressure and increases with an increase in temperature. The thermal conductivity of the adsorbed H2 increases with an increased amount of H2 adsorption. The effective thermal conductivity of the MOF-5 crystal is significantly enhanced by the H2 adsorption at high pressure and low temperature. The effective thermal conductivity of the MOF-5 powder bed increases with an increase in pressure and remains nearly unchanged with an increase in temperature. The thermal conductivity of the MOF-5 powder bed increases linearly with the decreased porosity and increased thermal conductivity of the skeleton of the MOF-5 crystal. The variation in the effective thermal conductivities of the MOF-5 crystals and bed mainly results from the thermal conductivities of the gaseous and adsorption phases.
Highlights
As a type of clean energy, hydrogen is expected to be used in hydrogen vehicles, but hydrogen-based energy cycles require hydrogen storage as the key technology
The mechanical compaction leads the pores in the crystals to collapse slightly, which in turn leads to the decreased porosity of the metal-organic framework-5 (MOF-5) crystal; the porosity in the powder bed is higher than the theoretical calculation [6]
Simulation to predict the effect of the H2 adsorption process on the effective thermal conductivity of a MOF-5 powder bed at pressures ranging from 0.01 MPa to 50 MPa and temperatures ranging from 273.15 K
Summary
As a type of clean energy, hydrogen is expected to be used in hydrogen vehicles, but hydrogen-based energy cycles require hydrogen storage as the key technology. A promising hydrogen storage material is the metal-organic framework-5 (MOF-5) which features high surface area and permanent porosity; this material can absorb up to 7.1 wt% hydrogen at 77 K and 40 bar [1]. MOF-5 is composed of 1,4-benzenedicarboxylate organic linkers and Zn4O tetrahedral clusters treated as secondary building units [2]. The process of H2 adsorption is an exothermic reaction, with the heat of H2 adsorption ranging from 2 kJ/mol to 5 kJ/mol [3], and the amount of H2 adsorption decreases rapidly with an increase in temperature. Effective thermal conductivity is an important criterion to judge how fast the heat of adsorption is dissipated and to determine how to maintain a low temperature in a fuel storage tank during hydrogenation
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