Heat transfer techniques in large-scale hydrogen storage using metal hydride materials

Abstract

One of the methods being investigated for storing hydrogen is the use of metal hydride materials. Metal hydrides are able to absorb and release hydrogen, giving them a wide range of potential applications for hydrogen storage. These materials are generally considered to be safe, stable, reusable, and are able to store and release hydrogen at lower pressures and near ambient temperatures. Despite these benefits, there are still existing limitations that hinder their widespread applicability. One of the main issues presented is the necessity for effective heat removal during the absorption process. When hydrogen is being absorbed by the metal hydride material, a large amount of heat is generated, which must be efficiently removed from the metal hydride reactor in order to achieve reasonable charging times. The first section of this thesis investigates the existing methods that have been proposed and studied for heat removal during this process. Some of these methods include the use of embedded cooling tubes, external water jackets, phase change materials, and high thermal conductivity additives. A method to characterize each type of reactor is also introduced in this section, based on certain parameters, which include characteristic length, mass of metal hydride material stored, mass of hydrogen stored, and cooling time. The following section simulates cooling times for two proposed large-scale shell-and-tube metal hydride reactors. The first reactor tested has embedded cooling tubes and metal hydride powder packed into the shell side. It was found that the absorption process could be completed in 1500-2000 s, depending on the tube bundle configuration. Additionally, a large hydrogen supply pressure (30 bar) was needed for reasonable reaction times to be achieved. The other reactor packed embedded tubes with annular metal hydride pellets, with the cooling fluid passing through the shell side of the reactor. This type of reactor showed a promising cooling time of 430 s with a hydrogen supply pressure of 10 bar. This type of reactor was more limited though by the percent volume that could be occupied by metal hydride material.