# Green Hydrogen Storage in an Underground Cavern: A Case Study in Salt Diapir of Spain
## 5 Conclusions  
The saline diapirs constitute safe hydrogen gas storages, allowing injection-extraction cycles in the short and long term. The creep phenomenon determines the cavern generation process and well stability in hydrogen storage and production processes.  
The parameters that have the greatest influence are the tensional state and temperature, and it is necessary that the drilling mud of the well exerts a hydrostatic pressure equal to the horizontal tension, and that an aqueous mud saturated in salt is considered in order to avoid the dissolution of the walls of the well. The casing design must be devised according to the rock stresses and cementing of the well using cement having high salt concentrations. Control or monitoring of the design parameters during salt rock leaching will ensure the correct shape and dimensions.  
The shape of the cavity affects the spatial distribution of stresses, the convergence of the salt rock, and the volume loss of the cavern. The geometry of the capsule-shaped cavern, of cylindrical design with a hemispherical shape at the top and bottom, ensures a low stress state and lower convergence. The geomechanical safety of the cavern is conditioned by the gas pressure inside the cavern remaining within a given range. The rate of injection and extraction of hydrogen from the cavern is therefore limited by the maximum and minimum pressure, by the maximum pressure change per unit of time that allows its stability, and by the maximum flow velocity inside the production pipe, which depends on its diameter. The operation of the caverns must be carried out under safe conditions, so as to minimize the risks to the environment and people.  
The Poza de la Sal salt dome meets the geological, technical-economic, and environmental criteria for renewable hydrogen storage. During the filling stage, the injection ratio is higher than the extraction ratio due to the variation in hydrogen density related to the pressure in the cavity, which increases during the filling of the cavern. It is necessary to carry out simulation studies to control the variables that may affect the injection ratio, in order to adjust the injection-extraction flow rate in each case.  
The storage capacity is calculated based on thermodynamic considerations according to specific storage data. Taking into account the surplus of wind energy production in the Autonomous Community of Castilla y Leon during the months of October to June 2020, production of 503 GWh is available for the production of storable hydrogen. During this period, a wind energy surplus of 23 GWh was obtained in the Paramo de Poza WP.  
The construction of two caverns, having a capacity of 396 GWh each, is proposed as an option, which would make it possible to store all the surplus wind energy in Castilla y Leon; one may be used to store surplus energy from the wind park, and the other for hydrogen production. The sale to the chemical industry of the brine obtained in the leaching phase can result in a benefit to the project.  
Conceptualization, L.M.V.-F., C.G.-O., L.F.M.-M., A.H.-B. and S.E.-D.; Supervision, L.M.V.-F. and C.G.-O.; Validation, L.M.V.-F., C.G.-O., L.F.M.-M., A.H.-B., S.E.-D. and R.R.-P.; Writing--original draft, L.M.V.-F. and C.G.-O.; Writing--review & editing, L.M.V.-F., C.G.-O., L.F.M.-M., A.H.-B., S.E.-D. and R.R.-P. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Oil and Gas Engineering Master's degree (mip) of the Universidad Politecnica de Madrid (UPM), which is taught at the Escuela Tecnica Superior de Ingenieros de Minas y Energia (ETSIME).  
Not applicable.  
The authors declare no conflict of interest.