Hydrogen Storage in Salt Caverns. Case Study Poza de la Sal, Burgos (Spain)

Large-scale underground storage of H2 (UHS) involves injecting hydrogen into a geological formation. This technology plays an important role in the low-emission energy system by providing the large storage capacities needed to buffer seasonal energy demand. It allows the safe storage of large volumes of hydrogen, at high pressures, with high energy densities, without the environmental impact caused by surface tanks. Gas can be stored in porous sandstone or carbonate formations, and in caverns excavated or extracted by dissolution in the rock. The Poza de la Sal diapir is a closed circular depression with Cretaceous Mesozoic materials, formed by gypsum, Keuper clays, and a large extension of salt in the center with intercalations of ophite. The low seismic activity of the area, the reduced permeability and porosity of the salt caverns and the proximity to the Páramo de Poza wind park, make it a suitable place for the construction of an underground storage of green hydrogen obtained from of surplus wind power. An adequate design of the well allows to carry out the leaching process to build the cavern and the subsequent injection and extraction of H2. In the design of the cavern, its shape influences the stability and safety, suggesting the capsular shape with a calculated volume of 515,355 m3. The construction of two storage caverns by leaching process at a depth of 1000 m with equal dimensions, taking into account in their design the difference in tensions, temperature and confinement pressure involved in the process of deformation of the salt, allows to carry out simultaneously the injection in one of them and the production of hydrogen in the other. Among the operating parameters, the depth of the base of the cavity and the average temperature of the cavern will determine the maximum and minimum values of hydrogen density, 11.4 kg/m3 and 4.51 kg/m3 respectively. The injection and extraction ratio must be developed in conditions of geomechanical safety of the cavity, so as to minimize the risks to the environment and people, being conditioned to the gas pressure inside the cavity to remain in a maximum and minimum range. Part of the stored hydrogen (base gas or cushion gas volume) will remain in the cavity in order to maintain the minimum operating pressure, leaving the rest of the stored hydrogen (working gas volume) for extraction when required for consumption, being the maximum working pressure when the cavity is full. The maximum total storage capacity will be determined based on the energy that can be supplied by the WP Páramo de Poza, as well as the working hydrogen required to refill the cavity.