How Energy Storage Works

The electricity grid is a complex system in which power supply and demand must be equal at any given moment. Constant adjustments to the supply are needed for predictable changes in demand, such as the daily patterns of human activity, as well as unexpected changes from equipment overloads and storms. Energy storage plays an important role in this balancing act and helps to create a more flexible and reliable grid system. 

For example, when there is more supply than demand, such as during the night when low-cost power plants continue to operate, the excess electricity generation can be used to power storage devices. When demand is greater than supply, storage facilities can discharge their stored energy to the grid.

Pumping water back behind hydroelectric dams has been used for decades as a form of storage that absorbs excess capacity from the grid and returns capacity to the grid later when it is needed. In the future, as more storage technology options emerge and the United States transitions to a cleaner energy economy, energy storage is poised to play an even greater role.

The benefits of energy storage

Because some renewable energy technologies – such as wind and solar – have variable outputs, storage technologies have great potential for smoothing out the electricity supply from these sources and ensuring that the supply of generation matches the demand.

Energy storage is also valued for its rapid response – most storage technologies can begin discharging power to the grid very quickly, while fossil fuel sources tend to take longer to ramp up. This rapid response is important for ensuring stability of the grid when unexpected increases in demand occur.

Energy storage also becomes more important the farther you are from the electrical grid. For example, when you turn on the lights in your home, the power comes from the grid; but when you turn on a flashlight while camping, you must rely on the stored energy in the batteries. Similarly, homes that are farther away from the transmission grid are more vulnerable to disruption than homes in large metropolitan areas. Islands and microgrids that are disconnected from the larger electrical grid system depend on energy storage to ensure power stability, just like you depend on the batteries in your flashlight while camping.

Current U.S. energy storage capacity

The U.S. has about 23 gigawatts (GW) of storage capacity, approximately equal to the capacity of 38 typical coal plants [1].

Pumped hydroelectric storage accounts for about 96 percent of this total storage capacity [2], most of which was built in the 1960s and 1970s to accompany the new fleet of nuclear power plants. Because nuclear power plants are not designed to ramp up or down, their generation is constant at all times of the day. When demand for electricity is low at night, pumped hydro facilities store the energy from nuclear plants for later use during peak demand. These pumped hydro plants have proven valuable for quickly adjusting to small changes in demand or supply.

Emerging storage facilities will allow us to store energy generated from wind and solar resources on shorter time frames to smooth variability, and on longer cycles to replace ever more fossil fuel. By charging storage facilities with energy generated from renewable sources, we can reduce our greenhouse gas emissions and our dependence on fossil fuels.

While the U.S. electric grid does not necessarily need more storage now, storage capacity will become more important as wind, solar, and other variable renewable energy resources expand in the power mix. Studies have shown that the existing grid can accommodate a sizeable increase in variable generation [3], but there are many exciting technologies in development that could help us store energy in the future and support an even greater amount of renewable energy on the grid.

Batteries

Batteries, like those in a flashlight or cell phone, can also be used to store energy on a large scale.

Like flywheels, batteries can be located anywhere so they are often seen as storage for distribution, when a battery facility is located near consumers to provide power stability; or end-use, like batteries in electric vehicles.

There are many different types of batteries that have large-scale energy storage potential, including sodium-sulfur, metal air, lithium ion, and lead-acid batteries. There are several battery installations at wind farms; including the Notrees Wind Storage Demonstration Project in Texas, which uses a 36 MW battery facility to help ensure stability of the power supply even when the wind isn’t blowing [17].

Advancements in battery technologies have been made largely due to the expanding electric vehicle (EV) industry. As more developments are made with EVs, battery cost should continue to decline [18]. Electric vehicles could also have an impact on energy storage through vehicle-to-grid technologies, in which their batteries can be connected to the grid and discharge power for others to use.

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