You are referring to a hydraulic accumulator[0], such systems were built in the 19th century. There is a modern effort to use the technology in a massively sca

You are referring to a hydraulic accumulator[0], such systems were built in the 19th century. There is a modern effort to use the technology in a massively scaled up fashion by Heindl Energy[1]. Their concept uses pumped water to raise a rock weight 100 meters or more in diameter. [0] https://en.wikipedia.org/wiki/Hydraulic_accumulator https://en.wikipedia.org/wiki/Hydraulic_accumulator [1] http://www.heindl-energy.com/hydraulic-rock-storage/overview.html http://www.heindl-energy.com/hydraulic-rock-storage/overview...