Compact integrated microchannel combustor, recuperator and heat exchanger For hydrogen storage applications Public Deposited


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  • A novel microscale combustor-heat exchanger (μCHX) for hydrogen storage applications is presented in this dissertation. The design of the μCHX is motivated by its application to two particular systems for automotive use- those that utilize metal hydrides (MH) and cryo-adsorbents (CA) to store hydrogen. Thermal energy needs to be supplied to the MH bed to raise its temperature to 170 °C in order to desorb hydrogen for use in a fuel cell. On the other hand, in a CA system, hydrogen gas that exits the storage tank needs to be heated to a minimum temperature of -40 °C prior to entering the fuel cell. During cold start conditions, heat exchange with ambient air or with the fuel cell coolant is insufficient to provide this minimum temperature, thereby requiring an additional source of thermal energy. For both storage systems, the required thermal energy can be provided by oxidizing a small amount of hydrogen and transferring the heat from combustion to the working fluid, which is an oil in a MH system and hydrogen in a CA system. The μCHX presented herein is a compact and highly efficient way of providing the required thermal energy rate. The µCHX is comprised of repeating unit cells that each perform identical unit operations, namely (a) catalytic combustion of a hydrogen-air mixture, (b) transfer of produced thermal energy rate to a heat transfer fluid, and (c) recuperation between the exhaust gas stream and incoming reactant gases. Heterogeneous catalytic combustion occurs on the walls of microchannels in the presence of a platinum catalyst. A multi-kilowatt µCHX device can be achieved by having multiple unit cells with appropriate fluidic distribution headers. The µCHX design is common for both hydrogen storage systems with minor changes. The design of the µCHX is performed using computational fluid dynamics simulations (CFD) at the unit cell and device level. At the unit cell level, the performance is documented using validated CFD simulations for variations in geometric and fluidic parameters. Varied geometric parameters include the length and location of the catalyst bed, length of the device, and the height of the combustion channel. Varied fluidic parameters include the working fluid inlet temperature, flow rates of the working fluid and reactants, and the equivalence ratio of the reactants. Performance is characterized using a global efficiency, heat transfer effectiveness and hydrogen conversion. The parametric variations are captured non-dimensionally as variations in Damkohler and Peclet numbers, which are in turn used to describe the changes in hydrogen conversion and efficiency. Performance maps for hydrogen conversion and pressure drop in the combustion channel are presented based on values of the Damkohler and Peclet numbers and regions of desired operability of the combustor are identified. For the CA µCHX, it is shown that with the help of a novel distributed catalyst arrangement, extinction of the reaction due to the cold gas stream is prevented and hydrogen conversion in excess of 95 percent is achieved for a range of operating conditions. Comparisons between simulations and experiments are not performed since, in the simulations, (a) ideal catalyst activity and surface distribution are assumed, and (b) the effect of substrate conduction is not considered. At the device level, three dimensional simulations of fluid flow are performed to ensure uniformity in flow distribution while maintaining low pressure drop through the device. Fabrication constraints are also incorporated into the device level design and simulations. A multi-watt CA µCHX consisting of 16 unit cells is experimental characterized using nitrogen as a surrogate heat exchange fluid. In experiments, a total catalyst length of 22.5 mm is used instead of 12.5 mm (which was used for the simulations) to account for reduced catalyst activity from the simulated ideal case. Experimental results show that hydrogen residence time and body temperature have significant effects on the overall efficiency of the device. Conversions as high as 94.1% and efficiencies as high as 88.3% are achieved. Highest hydrogen conversion is recorded at an equivalence ratio of 0.6.
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