Thermal Management Analysis of On-Board High-Pressure Metal Hydride Systems

Reversible metal hydrides are ideal vehicular hydrogen storage materials for the realization of on-board filling. Systems utilizing metal hydrides with high hydrogen release pressure (> 3 bar at -30 °C) can provide excellent cold-start capability. Although the required hydrogen filling pressure will also be high accordingly (> 100 bar), high-pressure (HP) metal hydride (MH) systems can store 20% to 50% more hydrogen in the void space between hydride particles in addition to the hydrogen absorbed by the metal alloys. To maintain a sufficiently high hydriding driving force during filling, it is very important to keep the MH temperature below a desirable level (85 °C). This issue becomes more important when the systems operate at high pressures, because the stress limits of materials for the container and other components decrease with increasing temperature. Efficient thermal management is needed to dissipate the large amount of heat produced during the initial rapid compression process (< 20 seconds) and the subsequent fast hydriding process (< 5 minutes). In this paper, thermal management design and analysis of a bench-scale rectangular-shaped HPMH module is reported. This module is approximately 1/70 of a vehicle-scale hydrogen storage tank. The modular approach provides flexibility to apply the knowledge obtained in this study to vehicle-scale designs. A typical AB₂ HPMH is used as the hydrogen storage material. During the hydrogen filling process, the time-averaged volumetric heat release rate is approximately 3 MW/m³. Inner coolant passages are adopted to remove the heat. Through a scaling analysis of the energy conservation equation, the results indicate that thermal conduction in the metal hydride bed and convection in the coolant passages are both important factors. For the test module under development, finned tubes in conjunction with two-phase convection have been designed to meet the cooling requirements. Fin designs (material, thickness and spacing) have been evaluated using 3D numerical analysis. The knowledge learned from theoretical and numerical analyses is used to guide the construction of the HPME module, and hydrogen filling tests will be conducted soon.