Along these lines, MIT Prof. Alex Slocum's proposal was to build concrete domes underwater, and evacuate them to store energy / re-fill them to release energy.

Along these lines, MIT Prof. Alex Slocum's proposal was to build concrete domes underwater, and evacuate them to store energy / re-fill them to release energy. The pressure difference is the difference between vacuum and hundreds of atmospheres, and there are no compressible working fluids to cause adiabatic heating losses. https://newatlas.com/mit-offshore-wind-concrete-sphere-energy-storage/27357/ https://newatlas.com/mit-offshore-wind-concrete-sphere-energ... (Edit: better link)