Development and optimization of a high-performance thermal reactor for metal hydride-based refrigeration machines

Kurzfassung

Hydrogen-powered vehicles can be a key component of a future decarbonized energy system. In the state-of-the-art, hydrogen is stored in 350 bar- or 700 bar-tanks for this purpose, whereby the energy expense for compression, accounting to approximately 15% (based on the lower heating value of hydrogen), reduces the overall efficiency of hydrogen-powered vehicles. By means of the thermochemical reaction of metal hydrides, parts of this energy can be converted into a heat pump effect in order to support or replace the conventional vehicle air conditioning system and thus increase the vehicle efficiency. The core of this work is the fundamental analysis and technological development of the metal hydride-based cooling system (MHCS) for vehicle applications. In the course of the work, the dominating influences on the performance of the MHCS were identified to be the parameters for thermodynamic and kinetic of the metal hydride, the heat and mass transport properties and thermal mas of the reactor and the boundary conditions in terms of pressure, temperature and mass flow rate. A simplified model was built, which represents these influences, using a 0+1D approach. It enables the determination of the characteristics of the MHCS: On the one hand, a decreasing efficiency with increasing specific desorption mass flow and, and on the other hand, a maximum specific performance at an optimum specific desorption mass flow. Due to the generic modelling approach, the determined characteristics can also be transferred to other quasi-continuous and desorption-controlled thermochemical systems. The validated model was used for the systematic design of a lightweight reactor with increased thermal performance. A comprehensive characterization of the realized reactor verified the increased specific power of 80-300% compared to the state of the art. With an experimentally proven value of 229 W/kgMH at a temperature lift of 40 K, the application-relevant target requirements were met and the basis was set for initial vehicle integration.