# Green Hydrogen Storage in an Underground Cavern: A Case Study in Salt Diapir of Spain
## 4 Discussion  
Spain has a land area of 50 million hectares and large territories with low population density, and experiences Mediterranean and Atlantic winds; hence, it is among the European countries having the greatest potential for the use of renewable energies. Major technological improvements in recent years have led to the implementation of numerous renewable energy production facilities, especially in the form of photovoltaic cells and wind turbines. Due to the variation in solar radiation and the volatile nature of wind, renewable energy production fluctuates, preventing the supply of a reliable baseload. Renewable energy storage would allow short-term and seasonal imbalances to be compensated for, and the excess production during low demand hours to be used. Hydrogen is a fuel that has a high energy density per unit mass, but very low energy density per unit volume, in both liquid and gaseous states. The availability of an adequate storage system is one of the major obstacles to its large-scale use. It requires large volumes associated with high pressures (700 bar), low liquefaction temperatures (\(-253\;^{\circ}\)C), or its combination with other heavier materials [30]. Hydrogen may be transported through natural gas pipelines and infrastructure, in variable proportions of up to 10-20%, limited by specific regulations of each country, supply pressure, and end use (domestic, industrial, energy, etc.) [31,32]. This aspect enables the coupling of the gas and electricity sectors, thereby allowing for joint demand management. Furthermore, an improvement and optimization of the energy system would result in flexibility of the electricity system, which is essential to achieve a high penetration of non-manageable renewable generation. The development of the chemical process by electrolysis and its underground storage in appropriate geological structures for subsequent consumption in fuel cells, or injection into the local distribution or transport network, is of particular relevance in the implementation of hydrogen technology. The high diffusivity of hydrogen makes it capable of diffusing even through solids, resulting in a loss of the stored fuel and its release into the atmosphere. Another aspect to consider, especially at high pressure, is the possible embrittlement of the metals or geological formations used for its confinement, in addition to the increase in its permeability. In this sense, salt caverns are suitable underground storages for pure H\({}_{2}\)(g), providing a good seal and a very low permeability, which ensures a minimum loss of hydrogen and a very low risk of contamination with impurities from the environment. The structural and design characteristics of the salt cavern will determine the amount of hydrogen that can be stored within the formation. Safety standards and risk analysis tools similar, for example, to those developed by the refining industry should be adopted during use. This will enable the application of best practices in new green hydrogen plants, and the use of optimized maintenance, reliability, and incident response strategies. The study of H\({}_{2}\) storage in porous media such as aquifers generally has the advantage of higher capacity and availability in the subsurface. Although they have similar geological characteristics to depleted hydrocarbon reservoirs, which were originally gas filled, they require more precise geological studies and analyses of porosity and permeability, in combination with the presence of a suitable seal formation to ensure the tightness of storage. Leaching cycles are well developed in evaporite rocks such as rock salt (halite) and potash, which dissolve at a high rate [11]. The irregular shape of the caverns is a consequence of the presence in the salt formations of water-insoluble interlayers, such as anhydrites and shales, reducing their volume and utilization rate [10].