The energy content of coal, liquid fuels, or natural gas is easily stored. When the demand increases, we simply take more from stockpiles and burn it. But electricity is different. It is very difficult to store in an efficient and cost-effective manner, so it generally has to be consumed the moment it is produced.
When the demand increases beyond what the main “baseload” power plants can provide, the customary response is to ramp up output in adjustable, intermediate, “load following” generators and/or bring additional “peaker” power plants on line. Like the main plants, both consume fossil fuels and have substantial investment and maintenance costs. And both have characteristic lags in response time.
One prime goal in storage is to provide capacity that can be activated within a few seconds or minutes of a sudden demand spike or abrupt reduction in power.
Conversely, if the production exceeds the demand, there is typically no cost-effective way to save the surplus for later use to satisfy spot shortages and reduce outages. The same problem also prevents the electric grid from taking full advantage of renewable sources such as wind and solar power whose output may not be needed at the time it is produced. So it is not surprising that the U.S. Department of Energy declared that “modernizing the grid will require a substantial deployment of energy storage.”
There are many possible solutions to the storage problem and all are being actively explored. They range from using off-peak electricity to charge banks of batteries similar to those in cars and trucks to harnessing electrical power to spin up giant flywheels. When needed, the flywheel rotation is employed to turn rotors in power generators. Many other technologies are in development, such as using electricity to extract hydrogen from water for fuel cells, charging advanced technology electrochemical batteries, and storing current in superconducting coils where it can circulate without resistance.
Today the predominant method—which in 2013 accounted for fully 95% of electricity storage in more than 200 facilities in the United States—is pumped hydroelectric. Off-peak electricity is used to move water to higher elevations. Whenever the demand increases, the water is released to drive turbines as in a hydroelectric dam. Nationwide, pumped systems total around 25 gigawatts (GW). By comparison, the Hoover Dam produces 2 GW.
In addition, more than 1 GW is stored using one of three technologies: various kinds of batteries, thermal storage, and compressed air. Batteries provided 304 megawatts (MW) in 2013. In thermal systems (431 MW), electrical energy is stored in the form of heat in water, concrete, rocks, or other materials. Another 423 MW is saved by compressing air, usually in underground caverns. When released, the air flow powers turbine generators.
One prime goal in storage is to provide capacity that can be activated within a few seconds or minutes of a sudden demand spike or abrupt reduction in power. This is hard to do by firing up peaking plants, but comparatively easy with battery systems of all kinds. Even pumped water and flywheels can be used for fast-response, short-term action to smooth intermittent generation or stabilize frequency.
Extensive research is under way to make electrical storage systems cost-competitive and more widely accepted by industry. But whatever the pace of adoption, it is clear that electrical storage will play an increasingly important role in the way all Americans receive electricity.
The greatest value of energy storage is realized when it is operated for the benefit of the entire system, and not dedicated to balancing any particular resource on the system. Storage tied to smart transmission and distribution grids would become a valuable component of any power system, and could provide numerous benefits to the system.
- Electricity from Renewable Resources: Status, Prospects, and Impediments (2010)
- Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2012 Symposium (2013)
- Overview and Summary of America’s Energy Future: Technology and Transformation (2010)
- The Resilience of the Electric Power Delivery System in Response to Terrorism and Natural Disasters—Summary of a Workshop (2013)