Investigation of Long Term Cycle Stability - First Results of Metal Hydride Composites

Abstract

Reversible reactions of solid powders and hydrogen can be used for hydrogen storage as well as for thermal applications like heat storages or heat conversion systems. For all applications, one of the fundamental prerequisites is the cycle stability of the reaction material and - if required - its bulk structure. At the Institute of Technical Thermodynamics at the German Aerospace Center a test bench to investigate the cycle stability of reversible gas/solid-reactions has been developed and brought into operation (Figure 1, left). The main feature is the possibility to investigate larger quantities of the materials and therefore to investigate also complete structures, e.g. matrixes for heat transfer enhancement. Until now Hydralloy C5 pellets have been cycled more than 1000 times. The bench can be operated automatically, provide temperature levels between 50 and 400°C and a pressure up to 100 bar. The temperature is controlled by a thermostatic bath and allows monitoring any shift in the heat transfer during cycling. The pressure can be increased or decreased quickly so highly dynamic tests can be run. The measurement of temperature and pressure is carried out online at different positions and the overall conversion is measured by an included Sievert’s apparatus. In cooperation with the Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM we have recently investigated different compositions of hydralloy C5 and graphite as well as four different pellets of metal hydride with different diameter at the same time. Figure 1, right shows the temperature (T), pressure (p) and pressure difference caused by absorption (DSD) for two cycles of one of these pellets as an example.