Computational study of hydrogen storage performance in metal hydride reactors

Hydrogen as the most abundant element on Earth is viewed to be a promising energy carrier. For transmission, hydrogen stored as metal hydrides is a potent candidate for its advantages in safe and reliability and being able to offer high energy density compared to the conventional ways such as high pressure gas and liquefaction. Metal hydriding is basically an exothermic process. The heat released will cause an increase in temperature and raise the absorption equilibrium pressure as high as that of the supplied hydrogen gas, which may in turn stop the hydriding process. On the other hand, metal dehydriding is an endothermic process. A temperature decrease can retard desorption and even bring down the dissociation equilibrium pressure as low as the back pressure to stop dehydriding. Therefore, reducing thermal resistance of the storage vessels and enhancing heat transfer of the storage system have become a critical issue for the success of hydrogen storage using metal hydrides. This work models the metal hydriding/dehydriding process in order to assess the vessel design on heat transfer enhancement to improve the performance of hydrogen storage with metal hydrides. First of all, the thermal-fluid behavior of hydrogen storage was modeled including gas flow and energy equations. The vessel is considered to be equipped with an air pipe at the centre line with internal fins. Detailed theoretical models that describe force convection of the heat exchange pipe and natural convection at the lateral wall are constructed. Results from the simulation show that the addition of a concentric heat exchanger pipe with fins can enhance the reaction rates. The work demonstrates how computer aided engineering can be applied to evaluate the performance of hydrogen storage designs, and help reduce experimental efforts in developing the hydrogen storage systems.