Abstract
In order to mitigate the emission of greenhouse gases in the atmosphere, alternatives to the current energy carriers need to be developed, with a special focus on harnessing the potential of sustainable energy production. In this context, hydrogen storage has the potential not only to provide short- and long-term energy storage solutions but also to enable the decarbonisation of sectors which are either not heavily reliant on electrical energy as its main energy source or depend on gaseous H2 or reducing agents as reactants in their operation. Among the alternatives to store hydrogen, the storage in metal and complex hydrides allows hydrogen storage with comparatively high gravimetric and volumetric storage capacities on the material level and have a wide range of temperatures and pressures under which these materials can ab- and desorb hydrogen, depending on the material. The lithium reactive hydride composite (Li-RHC) presents advantages over other hydride materials due to its high theoretical hydrogen storage capacity of 11.5 wt.%. This material is a mixture of LiBH4 and MgH2 in the specific molar proportions of 2:1 which can reversibly react with hydrogen at temperatures of at least 350 °C under moderate pressure levels. During H2-absorption heat is released. The opposite occurs during H2-desorption. In most of the developed hydrogen storage systems, the released heat is dissipated as waste heat with or without the help of heat exchangers or other heat management systems. During desorption, it is usual that surplus energy has to be provided to the system. In both cases, the energy associated is eventually lost to the environment. The use of thermochemical energy storage systems (TCES) aims to allow the storage of this heat energy in the form of chemical bonds for later use. This work aims to design a system based on Li-RHC as a hydrogen storage material and CaO/Ca(OH)2 as a TCES. To do that, it is first necessary to have not only descriptions of the hydride’s reactions with hydrogen as a function of pressure and temperature, as well as reliable thermodynamic data. In this work, Li-RHC powders have been produced with suitable catalysts in order to acquire data for the kinetic modelling of both absorption and desorption reactions which have been investigated to help understand the material’s behaviour as well as to provide the necessary equations and parameters to design the tank system using finite-element-method (FEM) simulations. This work provides thorough information on the intrinsic kinetics of the Li-RHC system, with particularly significant developments for the description of the desorption reaction. The hydrogen absorption reaction was shown to follow a Johnson-Mehl-Avrami-Erofeyev-Kholmogorov (JMAEK) with n = 1 reaction mechanism, with an apparent activation energy of 146 ± 3 kJ/mol H2 after accounting for the pressure dependency. The dehydrogenation of the Li-RHC occurs in two steps, with the MgH2 desorption reaching its completion in a couple of minutes while the LiBH4 desorption takes considerably longer. The reaction mechanisms were identified to be JMAEK with n = 1 and Prout-Tompkins. After correcting for the pressure dependency, the apparent activation energies were calculated to be 63 ± 3 kJ/mol H2 and 94 ± 13 kJ/mol H2, respectively. Using pellets instead of powder material reduces the macroscopic phase separation in Li-RHC and increases storage density. Because of this, further investigations focused on the production, determination of its kinetic properties and modelling of their behaviour under absorption and desorption conditions using FEM simulation. An empirical model to represent the change in the permeability of the H2 gas in the pellet has been proposed, with an excellent agreement between simulated and experimental results. The validated experimental results were used as the basis for the numerical investigation of a prototype adiabatic tank as a proof-of-concept using CaO/Ca(OH)2 coupled with Li-RHC. The design optimisation was aided by 2-D and 3-D FEM simulations which took into account geometric features and was able to identify performance-limiting aspects for the hydrogen loading and unloading cases. With the chosen design, the optimised tank geometry comprises of a reactor part with 192 mm diameter, with 36 cavities drilled into a cylindric, metallic body. Half of these cavities are filled with Li-RHC and drilled from the top; the other half are drilled from the bottom and contain CaO/Ca(OH)2. These cavities have different diameters (10.3 mm and 15.1 mm respectively) and the reactor is 150 mm high. In the designed prototype, up to 20 g of hydrogen can be stored. The investigation of Li-RHC in powder and pellet forms as well as the development of the adiabatic tank prototype are valuable contributions to the development of energy-efficient hydrogen storage systems and the insights provided here are able to provide new directions for further optimisation of this system and other hydrogen storage materials.
