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This study presents a design analysis for the development of highly efficient heat exchangers within stationary metal hydride heat pumps. The design constraints and selected performance criteria are applied to three representative heat exchangers. The proposed thermal model can be applied to select the most efficient heat exchanger design and provides outcomes generally valid in a pre-design stage. Heat transfer effectiveness is the principal performance parameter guiding the selection analysis, the results of which appear to be mildly (up to 13%) affected by the specific Nusselt correlation used. The thermo-physical properties of the heat transfer medium and geometrical parameters are varied in the sensitivity analysis, suggesting that the length of independent tubes is the physical parameter that influences the performance of the heat exchangers the most. The practical operative regions for each heat exchanger are identified by finding the conditions over which the heat removal from the solid bed enables a complete and continuous hydriding reaction. The most efficient solution is a design example that achieves the target effectiveness of 95%.

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A dynamic fueling model is built to simulate the fueling process of a hydrogen tank with an integrated passive cooling system. The study investigates the possibility of absorbing a part of the heat of compression in the high latent-heat material during melting, with the aim of saving the monetary and energy resources spent at the refueling station to cool the gas prior to tank filling. This is done while respecting the technical constraint of keeping the walls below the critical temperature of 85 °C to ensure the mechanical stability of the storage system even when the gas is fueled at ambient temperature. Results show that a 10-mm-thick layer of paraffin wax can absorb enough heat to reduce the adiabatic temperature by 20 K when compared to a standard Type IV tank, but its influence on the hydrogen peak temperature that occurs at the end of refueling is modest. The heat transfer from the gas to the phase change material, mainly occurs after the fueling is completed, resulting in a hydrogen peak temperature higher than 85 °C and a lower fueled mass than a gas-cooled system. Such a mass reduction accounts for 12% with respect to the case of a standard tank system fueled at -40 °C. A parametric analysis that embraces the main thermal properties of the heat-absorbing material as well as the major design parameters is here carried out to determine possible solutions. It is found that the improvement of a single thermal property does not provide any significant benefit and that the most effective strategy consists in augmenting the heat transfer area by employing extended surfaces or the encapsulation technique.Keywords: phase change material; hydrogen storage; hydrogen fueling; dynamic model; heat transfer.International Journal of Hydrogen Energy, Vol. 41, No. 12, 2016, p. 1060-1073 IntroductionRecent interest has raised for phase change materials (PCMs) as attractive options to store the thermal energy coming from intermittent sources and enable a continuous availability of the stored heat. Their use provides considerable advantages when compared to technologies that are based on sensible cooling/heating processes. Indeed, great convenience arises from the possibility of using their high energy storage densities to store large amounts of heat in a reduced space, as well as the capability of absorbing and releasing the heat via isothermal heat transfer. These aspects have made PCMs promising candidates for a large variety of applications, including solar energy storage, air conditioning of buildings and spacecrafts, passive cooling of electronics, textiles and fabrics [1]- [9]. A considerably less explored area of study is hydrogen storage. In such a field, research has mainly focused on employing PCMs to absorb the heat that originates during hydrogen fueling in metal hydride tanks for stationary applications [10], [11].In this study, our interest is to investigate the potential advantages and drawbacks of integrating an adequate PCM into compressed gaseous hydrogen (CHG) storage systems for light duty veh...