# Green Hydrogen Storage in an Underground Cavern: A Case Study in Salt Diapir of Spain
## 3 Methodology and Results
### Energy Balance  
Figure 6 shows the scheme of hydrogen generation with renewable wind energy, and the process of storage (injection) and production (extraction) from inside the cavern in the salt diapir.  
The energy balance associated with the loading and unloading of hydrogen in the storage facility was calculated taking into account the energy consumption of Castilla y Leon in 2020, assuming that it is constant throughout the year. Wind energy production [6] in that year was 12,576 GWh, compared to 22,147 GWh of total renewable energy production.  
In the Autonomous Community of Castilla y Leon during the months of October to June, wind energy production is higher than its consumption, due to more favorable wind conditions in autumn and winter. Conversely, an extra supply of energy is needed in the period from July to September [6].  
The energy study considers, in the first period, the accumulation of excess wind energy in the form of hydrogen injected into the salt cavern, in order to allow this energy to be recovered in the extraction phase.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & & & & & & \\ \cline{2-7}  & **250** & **273.15** & **298.15** & **350** & **400** & **450** & **500** \\ \hline \multirow{8}{*}{Pressure (bar)} & 1 & 1.00070 & 1.00004 & 1.0006 & 1.00055 & 1.00047 & 1.00041 & 1.00041 \\  & 5  
An electrolyzer needs about 48 kWh to produce 1 kg of hydrogen [7] with an efficiency of 70%. The electrolysis process to be able to fill a cavern resulting in a quantity of energy consumed is calculated in Equation (11).  
\[\text{ECE}=48\times 10^{-6}\frac{\text{GWh}}{\text{kg}_{\text{H}_{2}}}\times 5.8 \times 10^{6}\text{kg}_{\text{H}_{2}}=279\text{ GWh} \tag{11}\]  
Considering a hydrogen storage efficiency in salt caverns of 98% (IEA, 2019), 396 GWh would need to be provided for the total amount of gas in the first year of storage.  
The energy content of the working gas can be calculated (Equation (12)):  
\[\text{C}=\text{m}_{\text{work}}\times\text{LHV} \tag{12}\]  
The LHV of hydrogen, 33.3 kWh/kg, represents the energy released as heat due to its combustion, resulting in a cavern capacity of 117 \(\times\) 10\({}^{6}\) kWh or 117 GWh.  
Following the average wind energy cycle in Castilla y Leon, an excess of 4% of this energy was estimated between October and June [29].  
The wind turbines of the WP Paramo de Poza have a total power of 99.75 MW, which allows the calculation of the surplus energy produced during an 8-month period (Equation (13)).  
\[\text{WES}=99.75\text{ MW}\times 8\text{ meses}\times 30\text{ day}/\text{mes}\times 24\text{ h}/\text{day}\times 0.04=23.0\text{ GWh} \tag{13}\]  
As the excess wind energy produced by the WP Paramo de Poza is less than the capacity of the cavern, it is possible to receive and store renewable energy from other wind farms in the form of hydrogen.  
Given the dimensions of the diapir of Poza de la Sal and the fact that, in Castilla y Leon, 57% of the renewable energy generated is of wind origin, the possibility of building two caverns with the same dimensions is being considered. They will allow for simultaneous injection in one of them and hydrogen production in the other, thus providing a total capacity of 396 \(\times\) 2 = 792 GWh; that is, the energy necessary for providing the total amount of gas in the first year of storage. Consequently, 4% of the surplus of the 12,576 GWh of wind energy generated in all of Castilla y Leon in 2020 (503 GWh) could be stored in the two caverns. The distance between the longitudinal axes of both caverns must be four times greater than their diameter [12], i.e., 192 m, to ensure the stability of the storage.