# Green Hydrogen Storage in an Underground Cavern: A Case Study in Salt Diapir of Spain
## 1 Introduction  
Due to concerns about climate change and the harmful effects on the environment caused by the use of fossil fuels as the main source of energy, in addition to those caused by the security of energy supplies, many analysts have considered hydrogen as a potential alternative energy source. To meet the sustainable development goals, the International Energy Agency (SDS) has highlighted the fundamental role of hydrogen within the new energy model. The United Nations Industrial Development Organization [1] has defined hydrogen as the paradigm shift for efficient energy storage. The Ministerio para la Transicion Ecologica y el Reto Demografico of Spain [2] considers that this element is essential to the decarbonization of some sectors, such as transportation, electricity, and heat generation. The main characteristic of hydrogen is its function as an \"energy vector\", since, although it needs an energy input to be generated, it is capable of storing and releasing this energy when required. Hydrogen is a colorless, odorless, tasteless, and non-toxic gas, foundin air at concentrations of 100 ppm [3]. At the Earth's surface, it is found as a compound together with other elements, such as oxygen, carbon, or nitrogen [4]. It has a higher energy density per unit mass than the other hydrocarbons, with a LHV and specific gravity, respectively, of 120 MJ/kg (\(\times\)3 gasoline) and 0.089 (\(\div\)10 natural gas) in NC, in addition to a reduced level of pollutant emissions. However, it requires a larger storage volume to generate the same energy as other fuels (\(\times\)4 gasoline) [5; 6], which makes large-scale storage and long-distance transport more expensive. Depending on the raw material used to generate hydrogen and the CO\({}_{2}\) emissions associated with its production, it is classified in decreasing order of environmental impact as gray, blue, or green. Hydrogen consumption in Spain is around 500,000 ton/year and it is used as a raw material in refineries (70%), in the chemical industry (25%), and in other sectors such as metallurgy [7]. This hydrogen is 99% gray hydrogen, obtained from the particular production processes of each industry, and can be replaced by green or renewable hydrogen. Renewable energies in Spain have a large share in the energy mix, accounting for 54.0% in 2020 (Figure 1). Wind power, which represents almost 50% of the generated renewable energy, is the technology with the highest share (24.8%) in the power structure, producing 27.5 GW. Spain is the second-ranked country in Europe in terms of the installed wind power capacity, well behind Germany, which has just over 60 GW, with France in third place with about 18 GW. In terms of the contribution of this source to total electricity generation, the leader is be Denmark, with nearly 60% of its production coming from wind, and Spain is in seventh place [8]. Furthermore, the great variability of wind power means that the contribution of wind power generation to the hourly coverage of electricity demand varies considerably. Globally, in 2020, onshore wind power capacity was installed in Asia-Pacific (336,286 MW), Europe (194,075 MW), Americas (169,758 MW), and Africa, Middle East (7277 MW); and offshore in Europe (24,837 MW), Asia-Pacific (10,414 MW), Americas (42 MW), and Africa, Middle East (0 MW). Figure 2 shows the total values of installed wind power capacity for onshore (707.4 GW) and offshore (35.3 GW), and the percentage contribution by country. Hydrogen is generated inside alkaline and proton exchange membrane electrolyzers. The passage of electricity between two electrodes (anode and cathode) immersed in water produces its dissociation through a process of electrolysis. The use of solar or wind energy allows a \"power to gas\" hydrogen generation process [9]. Its seasonal storage in pipelines, vessels, or underground formations (depleted gas fields, aquifers, salt caverns, etc. ), enables the subsequent use of fuel cells to produce green electricity during periods of low \"power to power\" generation [10].