Belowground pumped storage

A reservoir and powerplant about 3000 ft below the earth's surface pumping water during nonpeak hours to the surface and releasing it to flow to the underground reservoir and generate electricity during peak hours have more flexibility for siting than aboveground systems; however, the price is high. A 2500 MW plant with a 4000 ft head would cost $213.86/kW with $78.40/kW going for the reservoir. A 624 MW plant with a 2000 ft head would cost $491.49/kW with $242.79/kW for the reservoir. The optimum depth is estimated at about 3000 ft. Reversible pump turbine equipment has an 1800 to 2000 ft head limit. Multistage reversible pump turbines have been used in Europe to go to 3000 ft, but the price is too high.