Renewable energy is great in itself, but to be really useful, it needs to be stored. Say you soak up all sorts of solar power during the day. What do you do with that till it is employed electrifying factories or people's homes, providing lighting for highways, or running air-conditioning or heating and appliances for supermarkets, schools, office buildings, military installations, airports, and such? Cost-efficient, large capacity batteries for the storage of renewable energy are as yet a work in progress.

Jointly owned, Dominion Energy's and Allegheny Power System's Bath County (VA) Pumped Storage Station (BCPSS) is one way to meet the challenge. Oddly enough, BCPSS became operational in 1985 and so uses old school energy technology. Yet storage based on this model can take advantage of new techniques for renewable energy generation.

Bath County Virginia Pumped Storage Station Reservoirs (virginiaplaces.org)

The basic idea is that two reservoirs of different elevations are utilized. When energy use in the grid is low, water is pumped to the upper reservoir. When it is high, water is allowed to flow back down to the lower reservoir at a rate of up to 13.5 million gallons a minute, powering six colossal generators that provide the needed electricity. While BCPSS can power all the energy requirements of 750,000 homes, it is actually used more widely than that, balancing the power needs of several million residential and business users across six states.

The recharge stage of this huge battery involves using six colossal motors to pump the water through gigantic tunnels back up to the higher reservoir at a rate of up to 12.7 million gallons per minute. Thanks to gravity, the newly elevated water, held back by a great dam till needed, is stored energy. The pumps double as generators, and when they are turned by the force of the water hurtling back down again from above, now spinning in the opposite direction from when they were in the pump phase, they can produce enough energy that this operation is one of the ten largest power sources in the nation.

The construction of BCPSS was Herculean. Enough concrete and metals were moved and shaped to equal a 1000 foot high mountain. Dams needed to be built to hold in the vast amount of water in both the upper and lower reservoirs. Great tunnels were created. And of course the mammoth pump-turbines had to be manufactured and moved into position, not to mention all the requisite control panels, cables, valves, etc.

Originally, this kind of power generating plant was to use nuclear power. Now, the idea is being recycled for renewable energy. Projects like BCPSS are on the drawing boards to help use and store wind and solar energy, for instance in CA where at times renewable sources are already providing 50% of the state's demand. Dozens of BCPSS ventures around the country could provide most of the energy needs of the U.S., primarily powered by either wind or the sun's rays.

This would not be an easy transition, however. Any plans on such a scale would run into political, environmental, and cost hurdles. Yet if these are overcome, entire cities might be powered almost exclusively via the natural energy potential of the sun, gravity, and the movement of air.

Primary source: A New Look At An Old Way To Store Energy. Dan Charles in Morning Edition - NPR News; July 12, 2018.