Development of a Hydrogen Producing Thermal Control for Chemical Hydrogen Storage
St. John, Adam
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This thesis investigated a potential improvement to hydrogen storage for fuel cells using a thermally efficient hydrogen storage method. The efficiency of the storage system was improved using a metal hydride system to act as a thermal control unit for an exothermic chemical hydrogen storage system. A cylindrical shaped “hybrid” reactor was created to allow hydrogen production from a sodium borohydride packed bed reactor and the metal hydride. Additionally, a custom built pressure-composition-temperature apparatus was built to record the amount of hydrogen desorption from the metal hydride while isolating the metal from potential poisons such as oxygen. Before using the chemical hydride packed bed, heat transfer through the reactor was studied using circulating water. The water experiments showed that an increase in heat flux to the reactor led to a faster desorption rate of hydrogen from the metal hydride resulting in a larger temperature drop throughout the reactor. After the operating characteristics of the hybrid reactor were studied, a 10 wt% solution of sodium borohydride was created and pumped through the packed bed to produce enough hydrogen for a 300 W fuel cell. The amount of heat produced from the packed bed portion of the reactor was significant, but temperatures levelled to around 80 °C. As expected, temperature control was directly proportional to the rate of hydrogen release from the metal hydride. On average, approximately 10% of the available heat energy was transferred to the metal hydride, and the hybrid reactor operated with gravimetric and volumetric energy densities of 0.27 kWh·kg-1 and 1.29 kWh·L-1 respectively. If the hybrid reactor is used solely to control peak temperatures, the amount of metal hydride necessary for thermal control could be decreased. Additionally, improvements in heat transfer as well as the hydrogen storage materials themselves would increase the energy density values further. When compared to other energy storage devices, the hybrid reactor without improvements is competitive as a backup power generator due to its silent operation and large volumetric energy density. Since the hybrid reactor can provide quiet and cool energy storage in a relatively small volume, it may become an effective and efficient means for hydrogen storage with limited improvements.