A pumped hydroelectric storage plant
is a variation on a traditional hydropower plant that operates with two reservoirs: a lower and an upper one. Such a plant utilizes gravity to "store" electricity in the form of potential energy. In generating mode, water flows in traditional fashion from the upper reservoir to the lower, driving turbines and generating electricity. When there is a surplus of electricity in the grid, for example, when demand is low or wind/solar are producing more than needed, electricity is drawn from the grid and used to pump water from the lower reservoir back up to the upper reservoir. (See Figure 1.) Thus, the system acts like a giant battery to store electricity until needed.
A pumped hydroelectric storage plant typically uses reversible pump/turbines that can either generate electricity or pump water. And, although such equipment can be very efficient, the plant is still a net user of electricity. I.e., it takes more electricity to pump water from the lower to upper reservoir than is generated by the same amount of water flowing from the upper to lower reservoir. Some estimates put the energy loss at 15-30%; however, this is good for a storage system and comparable to battery storage. And, with the ability to store power for use when needed and take advantage of electricity price differentials between peak and off-peak hourse, these plants can be very cost effective. Essentially, electricity is generated when demand is high for peak price, and electricity is stored when demand is low for reduced price. Managed properly, such price differentials will more than offset losses in efficiency. Figure 2 illustrates electrical generation and demand by such a plant.
Pumped storage plants are classified as either "closed loop" or "open loop". A closed loop system is one in which both reservoirs are independent of any free flowing water source. Open loop systems have one or both reservoirs associated with a free flowing water source. Although virtually all pumped storage plants in the US are of the open loop variety, the closed loop systems are currently preferred for their reduced environmental impact.
As of 2019, there were 40 pumped storage plants
operating in the United States totaling approximately 22.9 gigawatts (GW) of storage capacity, or roughly 2% of US generating capacity (see Figure 3). A good example of such a facility is PG&E's Helms Pumped Storage Facility
located in the Sierra Nevada Mountains approximately 50 miles northeast of Fresno. At 1,212 MW Helms is the largest pumped hydroelectric storage project in California. Helms uses three reversible turbine generators (see Figure 4) and can go from a dead stop to full generation in 6.5 minutes, making it an ideal dispatchable resource for the electrical power grid.
Although there has been increased interest in building new pumped storage plants, no new plants are under construction.
Given the attractiveness of on-demand power, the German firm Max Boegl Wind AG teamed up with GE Renewable Energy to develop the world's first wind farm with an integrated hydropower plant. The four-turbine pilot project, called Gaildorf
(see Figure 5), was connected to the grid and began operating in 2018. The project consists of four 3.4MW turbines supplied by GE, for a total of 13.6MW wind capacity, connected to a 16MW hydroelectric facility. Each turbine tower holds 1.6 million gallons of water in its base and sits in a reservoir that can hold another 6 million gallons. A holding reservoir is located below the power plant. When electricity is needed, water flowing downhill from the turbine reservoirs will power the hydro plant. When the wind is blowing and excess capacity is available, the hydro plant will pump the water back up the hill to the reservoirs, thus creating on-demand electricity from 100% renewable energy.
Although not hydroelectric, there is a new concept being developed that uses the same approach of creating potential energy then converting it into electricity - Advanced Rail Energy Storage (ARES)
. Excess renewable energy is used to power electric trains that pull giant slabs of concrete up a gentle slope. In effect, the trains convert the excess electricity to potential energy. When the grid needs that energy, the same rail cars let gravity pull the slabs downhill, converting the potential energy back into electricity by running its electric motors in reverse. A company called ARES has been testing the concept at a track in the Tehachapi Mountains in California, and broke ground in 2020 for its first commercial-scale project near Pahrump, Nevada. The project, called ARES Nevada, will consist of a 5.5-mile track traveling up an 8-degree slope, with a capacity of 50 MW and capability of generating 12.5 MWh of electricity per year. (See Video.)