Or compressed air: https://www.torontohydro.com/sites/electricsystem/gridinvestment/powerup/pages/compressedairenergystorageproject.aspx https://www.torontohydr

Or compressed air: https://www.torontohydro.com/sites/electricsystem/gridinvestment/powerup/pages/compressedairenergystorageproject.aspx https://www.torontohydro.com/sites/electricsystem/gridinvest... Not sure why they use those balloons though, seems like a maintenance nightmare. A diving bell would work just as well and be much simpler.