Energy storage technologies
Different energy storage technologies contribute to electricity stability by working at various stages of the grid, from generation to consumer end-use.
Thermal Storage
Thermal storage is used for electricity generation by using power from the sun, even when the sun is not shining. Concentrating solar plants can capture heat from the sun and store the energy in water, molten salts, or other fluids. This stored energy is later used to generate electricity, enabling the use of solar energy even after sunset.
Plants like these are currently operating or proposed in California, Arizona, and Nevada [4]. For example, the proposed Rice Solar Energy Project in Blythe, California will use a molten salt storage system with a concentrating solar tower to provide power for approximately 68,000 homes each year [5].
Concentrating solar plants focus the sun's heat to store energy in water, molten salts, or other fluids, which can be utilized even after the sun has set. Photo: NASA
Thermal storage technologies also exist for end-use energy storage. One method is freezing water at night using off-peak electricity, then releasing the stored cold energy from the ice to help with air conditioning during the day [6].
For example, Ice Energy’s Ice Bear system creates a block of ice at night, and then uses the ice during the day to condense the air conditioning system’s refrigerant [7]. In this way, the Ice Bear system shifts the building’s electricity consumption from the daytime peak to off-peak times when the electricity is less expensive. Additionally, the Bonneville Power Administration is conducting a pilot program on storing excess wind generation in residential water heaters [8].
Compressed Air
Compressed Air Energy Storage (CAES) also works as a generation storage technology by using the elastic potential energy of compressed air to improve the efficiencies of conventional gas turbines.
CAES systems compress air using electricity during off-peak times, and then store the air in underground caverns. During times of peak demand, the air is drawn from storage and fired with natural gas in a combustion turbine to generate electricity [9]. This method uses only a third of the natural gas used in conventional methods [10]. Because CAES plants require some sort of underground reservoir, they are limited by their locations. Two commercial CAES plants currently operate in Huntorf, Germany and MacIntosh, Alabama, though plants have been proposed in other parts of the United States.
Hydrogen
Hydrogen can be used as a zero-carbon fuel for generation. Excess electricity can be used to create hydrogen, which can be stored and used later in fuel cells, engines, or gas turbines to generate electricity without producing harmful emissions [11]. NREL has studied the potential for creating hydrogen from wind power and storing it in the wind turbine towers for electricity generation when the wind isn’t blowing [12].
The Wind to Hydrogen Project at NREL studies the storage of wind energy as hydrogen. Photo: NREL
Pumped Hydroelectric Storage
Pumped hydroelectric storage offers a way to store energy at the grid’s transmission stage, by storing excess generation for later use.
Many hydroelectric power plants include two reservoirs at different elevations. These plants store energy by pumping water into the upper reservoir when supply exceeds demand. When demand exceeds supply, the water is released into the lower reservoir by running downhill through turbines to generate electricity.
With more than 22 GW of installed capacity in the United States, pumped hydro storage is the largest storage system operating today [13]. However, the long permitting process and high cost of pumped storage makes further projects unlikely.
Flywheels
Flywheels can provide a variety of benefits to the grid at either the transmission or distribution level, by storing electricity in the form of a spinning mass.
The device is shaped liked a cylinder and contains a large rotor inside a vacuum. When the flywheel draws power from the grid, the rotor accelerates to very high speeds, storing the electricity as rotational energy. To discharge the stored energy, the rotor switches to generation mode, slows down, and runs on inertial energy, thus returning electricity to the grid [14].
A flywheel rotor, pictured here, is spun at high speed to store electricity as rotational energy. Photo: Wikimedia Commons
Flywheels typically have long lifetimes and require little maintenance. The devices also have high efficiencies and rapid response times. Because they can be placed almost anywhere, flywheels can be located close to the consumers and store electricity for distribution.
While a single flywheel device has a typical capacity on the order of kilowatts, many flywheels can be connected in a “flywheel farm” to create a storage facility on the order of megawatts [15]. Beacon Power’s Stephentown Flywheel Energy Storage Plant in New York is the largest flywheel facility in the United States, with an operating capacity of 20 MW [16].
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.
Batteries can be located in communities to provide power stability for homes. Photo: Green Energy Futures/Flickr
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.
The future of energy storage
As new energy storage technologies are researched and tested, some barriers are likely to slow the commercialization of these technologies.
Energy storage is expensive, especially without policies that place a monetary value on the unique benefits of storage. Plus there is no current need for additional storage capacity to maintain electricity grid reliability. Without an operational need, it is difficult for storage to be cost-effective in the present [19]. Furthermore, storage lacks a robust track record of large commercial-scale projects (with the exception of pumped hydro), making it difficult to deploy new projects.
Despite these potential barriers, certain programs and policies can help drive the development and deployment of storage technologies. The Department of Energy’s Energy Storage Program researches different storage technologies and works closely with industry on pilot storage programs [20].
Research programs can help advance the deployment and commercialization of energy storage. Photo: Argonne National Laboratory/Flickr
The deployment of storage technologies can also be advanced through renewable electricity standards (RES). Some states recognize storage technologies as acceptable renewable generation in their RES, and other states award Renewable Energy Credits (REC) to energy generation from storage devices that were charged by renewables [21].
The Federal Energy Regulatory Commission (FERC), the agency that regulates the electricity grid, has created a pricing structure that pays storage technologies and other fast-ramping resources a higher price for their services. This pricing structure, called Pay-for-Performance, recognizes the value of rapid response in providing stability to the grid. Pay-for-Performance has the potential to make storage technologies more cost-effective on a commercial-scale. An investment tax credit (ITC) would also help accelerate the deployment of storage technologies.
With the support of government and industry, energy storage technologies can continue to develop and expand, aid in the increasing deployment of variable renewable energy sources, and help store an ever-growing amount of clean, renewable energy in the future.