###### Abstract
The abovementioned metal and complex hydrides were investigated for the purpose of using the stored hydrogen for either internal combustion (IC) engines as a lean burning fuel with available atmospheric oxygen or to be electrochemically combined with O\({}_{2}\) in a fuel cell to provide electrical output for stationary or automotive applications. One such combustion characteristic of hydrogen/air fuel for combustion via a four-point lean direct injection (LDI) system was simulated by [PERSON] et al. [12]. In this research paper, the authors designed and demonstrated a swirl-venturi 2 by 2 array four-point LDI combustor that can mix a hydrogen and air combustible mixture with different equivalence ratios. This simulation via the Reynolds-averaged Navier-Stokes code for steady-state computations enabled the authors to measure various parameters such as the axial velocity, swirl number, velocity angle, effective area, total pressure drop coefficient, total temperature, mass fraction of hydroxide (OH), and emission of the pollutant nitrous oxide (NO). As the equivalence ratios of hydrogen to air increased, the total temperatures and temperature rises of the co-swirling four-point LDI combustors also increased at approximately the same rate. When the equivalence ratio decreased from 1.0 to 0.6, the emission index of NO (EI\({}_{\text{NO}}\)) reduced as well. Overall, this section of hydrogen energy storage, all of the described research papers addresses the deployment of hydrogen as an alternative fuel that can be readily available from solid-state hydride storage beds or hydride canisters. Salt Cavern Energy Storage: In this Special Issue, problems related to the efficient storage of gases other than hydrogen via rock salt, carbonates, synthesis gas, etc. are considered as well. In addition to material characterization and property measurement, design aspects such as the cantilever fluid flow type or slurry bubble column type reactors are also important. [PERSON] et al. [13] determined and successfully demonstrated the optimum permeability and porosity of synthetic and mudstone rock salts for the assessment of natural gas storage facilities. Their study of a storage cavern and mudstone interlayer permeability was based on Jintan, in Jinansu Province of China. Gas slippage or the Klinkenberg effect was widely used by the authors to understand the gas flow in the low-permeability porous medium of the mudstone rock salt and/or layered rock salt. The permeability increased greatly when the porosity exceeded the value of 10%; hence, the permeability depends on the internal pores and microcracks as well as their interconnectivity. In a related work to the previous article on natural gas storage, [PERSON] et al. [14] reported on carbonate gas reservoirs and the effects of water invasion in such reservoirs. An experimental system of water invasion in gas reservoirs with edge and bottom aquifers was established to simulate the process of water invasion. The effects of gas reservoir properties and production parameters such as water invasion energy, aquifer, production rate, permeability, and fracture were investigated systematically. The authors found that because of the physical simulation of water invasion, the elastic expansion of water-gas bubbles has greater influence. Additionally, the size of the aquifer and its production rate affect the water invasion in gas reservoirs with dissolved pores. The gas flow optimization for Fischer-Tropsch (F-T) synthesis was demonstrated by [PERSON] et al. [15] via comprehensive computer modeling of a lab-scale slurry bubble column reactor (SBCR). According to this modeling, a set of partial differential equations in terms of mass transfer and chemical reactions was coupled successfully to predict the behavior of all F-Tcomponents in both gas and liquid phases over the SBCR bed. The authors found that the temperature distribution of the slurry reactor remains constant under a base load and varying load conditions The experimental findings agreed with the computer modeling in this study.