Flooded mineshaft compressed air energy storage in the Witwatersrand Goldmine Complex – case study of a proposed novel energy storage method

With renewable energy sources representing a rapidly-growing share of the global energy mix, their intermittent nature has led to growing interest in mechanisms of storing energy during periods of high availability, for use during times of high demand. Several different types of compressed-air energy storage have been considered, including constant-volume approaches such as salt dome or artificial vessel storage and newer constant-pressure approaches such as submerged cavern storage. This manuscript proposes a novel approach where abandoned, flooded mineshafts are pressurized with air, displacing water downward in the shaft but upward in other hydraulically-linked shafts in the same geological complex. This approach is something of a hybrid between compressed-air storage and pumped storage and is thermodynamically distinct in that neither pressure nor volume are constant. An application to the abandoned goldmine shafts of South Africa’s Witwatersrand geological complex is considered, and thermodynamic analysis indicates that this approach offers around 10% greater energy density than conventional compressed air energy storage, and that a single mineshaft could have isothermal storage capacity exceeding 1GWh, with some operational advantages specific to the Witwatersrand context, meriting further research to design processes.