High Density Hydrogen Storage in Metal Hydride Composites with Air Cooling

INTRODUCTION In order to combine fluctuating renewable energy sources with the actual demand of electrical energy, storages are essential. The surplus energy can be stored as hydrogen to be used either for mobile use, chemical synthesis or reconversion when needed. One possibility to store the hydrogen gas at high volumetric densities, moderate temperatures and low pressures is based on a chemical reaction with metal hydrides. Such storages must be able to absorb and desorb the hydrogen quickly with a simple and light design and preferably a simple thermal management. One of the biggest challenges of metal hydrides is their low thermal conductivity. For fast absorption and desorption, the heat has to be transferred through the material quickly. Therefore complicated and heavy heat exchanger and complex active thermal management is normally required. In the work presented here we designed a novel hydrogen storage based on cycle-stable metal hydride composites that does not require any active thermal management. EXPERIMENTAL/THEORETICAL STUDY INVESTIGATION OF COMPOSITES Composites fabricated at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials using a mixture of Hydralloy C5 and expanded natural graphite (ENG) compacted at high pressure show superb thermal conductivity1. They were tested at an automated test bench at the Institute of Engineering Thermodynamics of the German Aerospace Center (DLR) for 1000 cycles. The test bench provides temperatures between 50 and 400 °C, pressure of up to 100 bar and can cycle large quantities of reaction material (up to one kilogram). The temperature and conversion rate is measured in situ. The mechanical stability and thermal conductivity is measured afterwards. Throughout all cycles the composites showed excellent thermal performance. After 1000 cycles the composites were still mechanically stabile and showed thermal conductivity more than ten times as high as the powder. DESIGN OF HYDROGEN STORAGE Based on these results a laboratory test storage with composites was designed to store hydrogen, e.g. produced by electrolysis at 30 bar and to supply a fuel cell of 1.2 kW for one hour at a pressure of 4 bar. It is based on a simple heat exchanger design that can be operated at room temperature with simple air cooling with fans. RESULTS AND DISCUSSION The novel hydrogen storage is shown in Fig. 1. The storage contains around 6 kg of composites with a diameter of 20 mm. Due to the high thermal conductivity small temperature gradients within the composites are sufficient for a dynamic absorption and desorption even with simple air cooling. The storage was designed as modular storage that fits into conventional rack systems. Therefore, the required storage capacity can be easily adapted to the specific requirements, e.g. uninterruptable power supply. CONCLUSION Composites of metal hydride and ENG showing good cycle stability and high thermal conductivity were used to design and manufacture a modular storage tank for integration into conventional rack systems. The contribution will show first experimental results of the modular laboratory storage and derive conclusions regarding up-scaling possibilities. REFERENCES 1. C. Pohlmann et. al, Int. J. Hydrogen Energy, 38 (2013), 1685-169