According to the International   Hydropower Association, or IHA, a  facility with two reservoirs roughly   the size of two Olympic swimming pools, and  a 500-metre height difference between them,   would have an energy capacity of about three and  a half megawatt hours. And they last for decades,

The ANU found more than six   hundred thousand potential sites around the world  where closed loop pumped hydro systems could work,   00:07:29 at least from a geographical and topographical  point of view anyway, representing a potential   of about twenty-three million gigawatt hours of  energy storage, which would be about a hundred   times what we’d need for a one hundred percent  renewable powered global electricity system.   Now, obviously not all of those sites will turn  out to be appropriate, so we need to keep our feet   well and true planted on the ground here. The ANU  itself points out that, apart from discounting any   00:07:56 urban areas and known areas of environmentally  protected zones, no other comprehensive   geological, hydrological, environmental, or  heritage studies were carried out as part of   their research project, so it’s highly likely that  only a relatively small percentage will prove to   be viable. But, even if it’s only one percent,  that still gets us over the line, doesn’t it?

As a rough rule of thumb, based on analysis  in Australia, to achieve a one hundred percent   00:08:21 renewable electricity grid, you need about one  gigawatt of power for every one million people,   plus probably twenty hours of energy storage  to give yourself plenty of leeway from the five   hours minimum number in the recent studies I  mentioned earlier. So that’s twenty gigawatt   hours of energy storage per million people in a  well-connected high-energy-use country like Aus,   with good wind and solar resources. That equates  to a total Australian requirement of about   00:08:46 five hundred gigawatt hours in a country that  has storage potential about three hundred times

the National   Renewable Energy Laboratory, or NREL used well  established Geographic Information Systems,   or GIS, to assess potential new closed-loop  pumped storage hydropower, or PSH,   systems across ALL the states, including  Alaska and Hawaii, as well as Puerto Rico.  That analysis turned up no fewer than  fourteen-thousand-eight-hundred-and-forty-six   00:04:51 potential sites with a combined storage capacity  of three-point five terawatts that could be   discharged over a ten-hour period, providing  some thirty-five terawatt-HOURS of energy into   the various American grid networks.

Piconi says Energy Vault relies on gravity in the same way, but "instead of using water, we're using these composite blocks."

The steel tower is a giant mechanical energy storage system, designed by American-Swiss startup Energy Vault, that relies on gravity and 35-ton bricks to store and release energy.

Churchill Falls hydro-electric project

The Churchill Falls hydro-electric project was inaugurated by Pierre Trudeau, the Canadian Prime Minister, on June 16, 1972. This hydro-electric plant was constructed between 1967 and 1975 and completed one year ahead of the predicted schedule. At the height of its construction, approximately 6,300 workers were present in the summer of 1970. The majority of construction occurred in the summer months, although construction continued year-round despite harsh conditions in Labrador where temperatures dipped to -21°C with a mean annual snowfall of 406 centimeters. The Churchill Falls power station is located in southern Labrador about 1,100 kilometers from an urban area. The Churchill Falls hydro-electric project was the largest hydro-electric project at the time, capable of generating 5,225 mW of electricity. It creates this energy by utilizing the water of the Churchill and Naskaupi Rivers which have a total catchment area of about 67,340 km2 combined. The underground power station is about 305 meters below ground. It uses eleven generators with a combined capacity of 5,225,000 kW. In order to utilize this harvested energy, large power lines capable of handling voltages up to 735 kV were put in place to transmit the energy from Churchill Falls to the Hydro-Quebec transmission system in the Manicouagan-Outardes hydro complex. The distance between these two stations is 606 kilometers. The energy from Churchill Falls was also transmitted via power lines to the Labrador City-Wabush area (Crabb, 1973).