German engineering technology company thyssenkrupp will conduct a pre-feasibility study to store green energy in a repurposed South African gold mine after signing a contract with Pumped Hydro Storage Sweden.

The company said it also would work with uranium producer Wismut on the ‘Renewable Underground Pumped Hydroelectric Energy Storage' (RUPHES) project.

Underground pumped energy storage, also known as pumped hydroelectric storage (PHS), utilises sub-surface water supplies, such as reservoirs in mines, for generating eco-friendly fuels. 

Energy is stored in an underground water source and pumped to higher elevation and released through turbines to produce electric power when needed. 

The company explained that the project would meet the growing demand for energy storage, driven by more renewable energy generation.

Furthermore, the company pointed out that gold mines are ideal for storing green energy because they already have the necessary underground sources of water needed for pumped hydro plants.

It said the "stable hard rock geology" made ultra-deep gold mines ideal for green energy storage and producing "internationally cost-competitive, reliable, green electricity, as well as green hydrogen and green ammonia".

Pumped Hydro Storage Sweden, a subsidiary of Sustainable Energy Solutions, is already developing a 2MW/8MW underground pumped energy storage project in an abandoned iron mine in Aland, Finland; the project demonstrates the costs benefits of repurposing mine tunnels for energy storage.