Grid Scale Power Storage Takes Many Forms

The newest class of aluminum-carbon batteries will out-pace all other forms of electrochemical energy storage, as to cost, simplicity, design flexibility, actual flexibility, scale-up, fire safety (no fire hazards known on this one, compare to lithium or sodium ion cells that can catch fire when overheated during charging). Redox storage batteries are good, but cannot match the energy density of the aluminum system. Very extremely fast charging, even compared to Li+ or Na+ cells. Many thousands of charging cycles, long life. No brainer.

Another (almost Roman tech) is the use of coal mine type elevators moving "stones". The stones need only be moved from bottom of hill, canyon, or escarpment to the top in order to store energy, and reverse motor is generator on these during downhill run. Efficiency is up there. Stored heat in the form of hot gravel beds under pressure also works. Compressor set runs either way to either store energy or produce electric power. Not rocket science, not site selective (compared to pumped hydroelectric, or the stone conveyor trick). Even the stone conveyor trick can be made non-site specific by introducing a vertical shaft, but the storage capactiy would be very small compared to the scale projected in the 1000's MWH for large projects on an escarpment, or mountain. No water tied up in projects either.