## 2 Materials and Methods
### Potential for Replication
#### 2.1.3 Hydrogen Storage  
One of the main challenges of using green hydrogen as an energy source is its storage capacity and the process required to achieve this efficiently. Although hydrogen has many advantages as an alternative to fossil fuels, such as the absence of carbon dioxide emissions, its implementation has been limited by several factors [30]. One of the most important problems is the high cost of production compared to traditional fuels [31]. However, with the rising price of fossil fuels and the improved efficiency of electrolysers, the key equipment for hydrogen production, the economic viability of green hydrogen has increased considerably. In this context, the UNDERGY project, in which the research team is participating, has identified several key points for the adoption of green hydrogen as a sustainable energy source in Spain. Firstly, there are surpluses of renewable energy that need to be harnessed, and hydrogen may be a suitable candidate to be explored [32]. Secondly, it has been shown that it is possible to identify suitable geographical areas for hydrogen storage using existing infrastructure or facilities that also generate renewable energy above ground. Finally, by combining hydrogen generation and storage in these areas, it is possible to move towards energy independence in Europe, reducing dependence on fossil fuels. Figure 3 shows the flowsheet developed by the UNDERGY project team. Despite the key points addressed above, hydrogen presents a major technical challenge: its ability to cause embrittlement of the materials used in storage and transport infrastructures, such as existing pipelines and wells [33]. The project has therefore investigated hydrogen-resistant materials and technologies to ensure their long-term viability. The project has carried out work on resistant pumps, valves, and pipelines and has explored the use of above-ground storage tanks for short-term storage. In addition to meeting long-term energy needs, the project explored the possibility of seasonal hydrogen storage in large underground reservoirs. The case study was a depleted natural gas field converted into a hydrogen storage facility. The storage capacity, together with its geochemical and geomechanical stability, was analysed by means of advanced simulations with the FLAC3D and TOUGH+ programmes, which allow the behaviour of hydrogen under these extreme conditions to be accurately modelled, as shown in the figures in the results section. As for the operating pressures, based on previous experience with the reservoir, it has been established that the reservoir is capable of withstanding pressures of between 18 and 86 bar, respectively. To fill the 130 million cubic metres of reservoir capacity, it has been proposed to take advantage of the months with the greatest surplus of renewable energy, for example, from March to September (\(\mathrm{t_{re}}=210\) days), where photovoltaic and wind energy production is greatest. In this way, during the winter months, when energy demand  
Figure 3: Flowsheet of the UNDERGY project. Prepared by the authors within the framework of the UNDERGY project. is highest and natural gas prices rise, it will be possible to extract the stored hydrogen to meet market needs. Finally, to calculate the volume of hydrogen storage potential, depleted gas deposits, aquifers, and salt caverns were considered. The hydrogen storage capacity for depleted gas deposits was obtained from [34]; for aquifers, it is obtained from the results of the project Hystories [35]; and for hydrogen stored in salt caverns formed in diapirs, data of diapirs in Spain were obtained from Instituto Geologico y Minero de Espana (IGME) [36]. To transform the storage volume into energy capacity, the following conditions are considered:  
1. For depleted hydrocarbon deposits and aquifers, the density of hydrogen is considered at the lithostatic pressure. The lithostatic pressure is dependent on the depth, as 10 MPa/km [37].