Optimization of heat and mass transfer in metal hydride systems

The optimization problem for metal hydride heat pumps is defined and approached by carefully distinguishing between issues inherent to reactor engineering (“reactor optimization”) and separate issues that influence the power/efficiency characteristics for a specific cycle. The starting point for reactor optimization is based upon maximizing the cyclical transfer of the theoretical hydrogen inventory for the class of reactors in which heat transfer is the dominant energy transfer process. Necessary and sufficient conditions for reactor optimization are developed using two techniques. First, the fundamental constraints associated with heat transfer to high energy-density reactors in communication with external heat baths are examined. Second, a heat and mass transfer model is formulated and numerically solved in order to examine reactor performance as a function of internal transport parameters. The combined analysis provides a unified basis for a design problem involving many physical parameters and heat and mass transport mechanisms and has been successfully applied to the fabrication of high-power metal hydride reactors.; The scope of the work includes applying the reactor optimization results to further the understanding of the performance characteristics of the heat-driven and compressor-driven cycles. The primary tool is the model formulation in the local thermodynamic equilibrium (LTE) limit. However in view of the high energy densities and rapid intrinsic kinetic rates the general non-local thermodynamic equilibrium (NLTE) model is solved to discern possible effects on heterogeneous reactor dynamics. The issues which affect the performance of an arbitrary cycle are diverse. The difficulty in achieving a general analysis sophisticated enough to elucidate the reactor dynamics is overcome by removing non-reacting subsystems from analysis. The effect of these systems can be introduced at a later time and studied at length using simplified reactor models. The power/efficiency characteristics of a compressor-driven system are investigated for the first time and are found to be more complex due to the direct effect of compressor ratio on efficiency. On the other hand, the absence of a high temperature reactor in which the boundary conditions promote the formation of sharp reaction fronts and the symmetrical placement of the energy input to the cycle are predicted to better utilize the potential of optimized reactors.