[HOW IT WORKS]
At any given second of the day, the operators of our nation’s electricity grid must match electricity supply from multiple energy sources with demand from multiple consumers. Both ends of this equation are constantly variable; on the supply side, electricity generation can drop due to planned power plant outages for maintenance purposes and unplanned outages stemming from severe weather, equipment failure, or natural disasters. Fortunately, grid operators have a number of strategies for balancing supply and demand; one of them is energy storage.
Hydropower facilities, concentrating solar power plants, and electric vehicle batteries can store excess electricity and return it to the grid as needed to ensure consistent, sufficient electricity output.
Keeping the Juice Flowing
There are many types of storage technologies, some of which have been used for decades to help integrate coal and nuclear power plants (which are difficult to ramp up or down to meet demand) into the grid, and to compensate for variability and uncertainty in the power system more generally. These include:
Pumped hydroelectric. Some hydroelectric plants have reservoirs at a higher elevation to which they pump water when electricity supply exceeds demand; that water can then run downhill through a turbine to produce electricity when demand is high. With 22 gigawatts (GW) of installed capacity in the United States, pumped hydro is the largest form of energy storage in the power system today, though it constitutes less than 2 percent of total U.S. generating capacity. The potential for more capacity of this type is limited due to the long permitting process and high costs involved in new hydropower facilities, and the amount of land and water needed for reservoirs.
Thermal storage. Concentrating solar power plants can store the sun’s heat in water, molten salts, or other fluids, and use it to generate electricity for hours after sunset. Several such plants are operating in Arizona and Nevada, and another is proposed in California. Also, a pilot program under way in the Northwest connects utility customers’ water heaters to the grid, allowing them to store excess power (in the form of hot water) from a nearby wind farm.
Compressed air. These systems use excess electricity to compress air and store it in underground caverns. When needed, the compressed air is heated and used to generate electricity in a natural gas combustion turbine. One such facility is operating in Alabama, and developers have proposed several new projects in California and Texas.
Batteries. Rechargeable batteries like the ones in cell phones and cameras can be used on a much larger scale to supply electricity to the grid. For example, batteries are used on the Hawaiian islands of Kauai and Lanai to lessen the variability in output from solar power plants, which generate a large portion of the islands’ electricity. Batteries in plug-in electric vehicles that are outfitted with special equipment can also supply electricity to the grid when the vehicles are idle. A current challenge with this technology is that the batteries can wear out sooner with frequent charging cycles, though newer designs may reduce this risk. In addition, the owner must be sure to leave enough power to actually drive the vehicle.
Hydrogen. Excess electricity can be used to produce hydrogen gas from water molecules; the hydrogen is then stored for later use in a fuel cell, engine, or gas turbine. The National Renewable Energy Laboratory (NREL) has also researched the possibility of producing hydrogen from wind power and storing it for use in generating electricity when demand is high and the wind is not blowing. No commercial-scale hydrogen storage systems currently exist.
Part of a Cleaner Energy Future
Energy storage technologies differ in the services they provide to the grid; some can help manage variability over short time frames while others can store larger amounts of electricity for times when demand is high. Most can respond within seconds to meet demand (unlike coal or nuclear plants) and their output can be easily adjusted to the specific needs of the grid. They can also be useful in remote locations such as rural and island communities, where long-distance transmission lines are difficult or expensive to build.
Despite these potential benefits, energy storage systems currently do not play a major role in the growing renewable energy market. While renewable energy facilities have low operating costs (their “fuel”—such as wind and sunlight—is free), transferring electricity to and from storage systems also uses electricity, which reduces their overall efficiency. Energy storage is also expensive: a 2010 NREL study found that few large-scale, commercially available storage systems cost less than $1,000 per kilowatt of capacity—comparable to the cost of building new natural gas generators. Other power management strategies such as reducing energy demand (see the sidebar) are less expensive in most circumstances.
Nevertheless, energy storage does have a role to play as renewable energy development expands; indeed, NREL has found that renewable energy could supply 80 percent of U.S. electricity by 2050. Though that is still a long way from the 17.6 percent we generate today, energy storage—along with other improvements to the grid and strong state and national policies—will ultimately make the goal of clean energy that meets demand at all hours, in every region of the country, achievable.
Heather Tuttle is editor at UCS.
Learn more about how we can build a clean, reliable U.S. electricity system.