In July 1930, the magazine Popular Science ran an article announcing the start of operations at the first U.S. “ten-mile storage battery”—or pumped-hydro energy storage plant—near New Milford, Connecticut. Connecticut Electric Light and Power Company built the plant to meet the region’s peak electricity load and mitigate seasonal water shortages. The article goes on to describe how the plant stores electricity, its economic justification, and efficiency:

Surplus electricity from a steam-power plant at Devon, Connecticut, forty-five miles away, charges this giant storage battery by pumping water . . . from the Housatonic River, beside the power station, to the storage reservoir, 230 feet above. . . .

When Connecticut needs more power, gates at the reservoir are opened. The water rushes downhill . . . through a water turbine that drives a 44,000-horsepower generator.

Thus, at “peak” hours, electricity from the dynamo is fed back into the power network that supplies the state. Not only at certain times of day, but from week to week this “power storage” produces startling economies. . . .

So efficient is this great “storage battery” that it delivers sixty-one horsepower for every hundred horsepower that is used to pump water.

Pumped-hydro energy storage technology has not changed an awful lot since Popular Science announced the first U.S. energy storage plant. Today, pumped-hydro is the most widely used grid storage technology in the United States by far, comprising 22 of the 23 gigawatts of energy storage capacity installed as of 2011. Much of it serves the same purpose that Connecticut Electric Light and Power Company’s plant did in 1930: it pumps water uphill to store electricity when the demand for power is low, and then allows the water to flow downhill and generate electricity when electricity demand peaks. By doing so, pumped-hydro storage reduces the amount of power plants needed to meet peak electricity demand.

Since the New Milford, Connecticut plant first became operational, pumped-hydro storage plants have gotten a lot bigger. The New Milford plant could discharge a maximum of 44,000 horsepower, roughly equivalent to 33 megawatts of electricity, or enough to power 30,000 modern homes and likely many more in 1930. Today, the largest U.S. pumped-hydro energy storage plant can produce a maximum of 3,000 megawatts of power—100 times the New Milford plant’s capacity and roughly equivalent to the combined power output of three nuclear generators.

Pumped-hydro electricity storage has also gotten a lot more efficient since the New Milford plant became operational. It could deliver “sixty-one horsepower for every hundred horsepower . . . used to pump water,” which equates to a round-trip energy storage efficiency of 61 percent. Today, a modern pumped-hydro plant can attain a round-trip efficiency as high as 80 percent. Despite modern advances, 20 percent of the electricity stored is still lost in pumps, turbines, and through water evaporation.

While pumped-hydro is a long-established energy storage technology capable of operating reliably for decades (the New Milford plant is still operational), there are a number of barriers to widespread use of the technology in the United States. The 22 gigawatts of pumped storage installed on the U.S. grid pale in comparison to the 769 gigawatts of electricity demand, so despite the presence of pumped-hydro the grid has very little capability to store energy. While storing more energy in pumped-hydro plants would make the grid more flexible, reliable, and capable of integrating renewable energy, environmental concerns, financial uncertainties, and the lack of technically feasible sites limit how much new pumped-hydro energy storage can be installed. Between 1986 and 2006, six major pumped-hydro projects were initiated and then abandoned, mostly due to market uncertainty. While the New Milford pumped-storage plant produced “startling economies” at the time it was built, the same isn’t true for new pumped-hydro facilities today.

The difficulty associated with building new pumped-hydro energy storage facilities is part of the motivation for interest in grid-scale battery energy storage, which can be deployed anywhere on the grid and doesn’t rely on water—a limited resource—to function. Researchers around the world are working to reduce the cost and improve the performance of battery technologies with the hope of one day storing grid electricity at a price that makes grid storage appealing and enables wider use of intermittent renewable energy.

Perhaps the best takeaway from the story of the New Milford pumped-hydro plant is that we have the technical ability to store electricity on a large scale. However, for the most part it is presently not economically practical to do so in either a new pumped-storage facility or a battery. As the share of renewable energy on the grid increases, the cost of grid battery storage decreases, and the cost of natural gas and other fuels changes over time, the economic reality for grid storage might change, but for now it is still cheaper to make electricity when we need it than it is to store electricity on a large scale.


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