A Storage Solution Is in the Air
By ERICA GIES
PARIS — Renewable energy sources like wind and solar have a problem: When the wind stops or it is night, they stop generating power. That drawback has focused minds on the question of how to store electricity generated by intermittent sources.
The global market for energy storage services could be worth as much as $31.5 billion in 2017, according to Brian Warshay, an analyst at Lux Research, a consulting firm based in Boston.
The ultimate goal of grid operators is to match the power they deliver to the needs of their customers. Utilities have traditionally increased production from gas-fired power plants when needed to smooth supply and demand, but energy storage systems could, in theory, do the job more quickly and cleanly. Depending on where the storage is situated, it could also reduce the need for new transmission lines.
With increased interest has come investment: Venture capitalists and governments are funding research into batteries, pumped hydroelectricity, flywheels and compressed air.
Battery technologies have received much of this attention, but compressed air is also moving forward.
In an ideal storage system, all the energy put in can be extracted later. When air is compressed, however, the work that goes into compression produces heat as a side effect. In early compression storage systems — technically known as diabatic — this heat has been lost into their surroundings, making them less efficient.
Current experimental systems aim to be more efficient by retaining or recovering the heat. Some, known as isothermal, use a coolant to absorb it, keeping the air at a near constant temperature. The coolant, stored separately, is tapped later to give back the energy through a heat exchange system. Others, known as adiabatic, allow the temperature of the compressed air to rise and fall, using the air, when hot, to warm heat storage units that retain energy within the system.
SustainX, based in Seabrook, New Hampshire, is set to build a pilot isothermal plant capable of storing and releasing up to two megawatts of power, making it the largest demonstration of the technology to date. At the same time, a German consortium is planning a truly grid-scale project — initially up 90 megawatts — based on the adiabatic approach.
Isothermal and adiabatic are terms from thermodynamics that promise perfect efficiency, with no energy loss. In practice, each system must make compromises that lower the efficiency rate to 50 to 70 percent.
SustainX’s facility, a follow-up to a 40-kilowatt demonstration plant, is funded partly by a $5.4 million U.S. government grant under the 2009 American Recovery and Reinvestment Act and partly by venture capital. Richard Brody, the company’s vice president of business development, said it could start test operations by next June, though it would have only an hour or two of storage capacity and would not initially be intended to supply power to the grid. “Its goal will be to validate and demonstrate the technology to our engineering team and to customers,” he said.
Both SustainX and the German consortium are seeking to improve on the diabatic technology used in existing utility-scale facilities in Huntorf, Germany, and McIntosh, Alabama. In these systems, the operators use surplus power from the grid when demand is low to pump air at high pressure into underground salt caverns, expelling the byproduct heat into the atmosphere. When energy is needed on the grid, the air is released and reheated by burning gas before being sent through turbines to generate electricity.
In the SustainX isothermal process, a fine mist of water is used as the coolant. When grid power is needed, the compressed air is passed back through the heated water to recover the stored energy before being fed to the turbines.
A venture capital-funded start-up, LightSail Energy in Berkeley, California, has built two prototypes using similar technology — both LightSail and SustainX use specially built containers rather than underground caverns to store compressed air.
Meanwhile, General Compression in Newton, Massachusetts, is working on a near-isothermal system using salt cavern storage, like the early diabatic systems. General Compression and its partner ConocoPhillips are developing a project in western Texas.
Mr. Warshay, who has written a review of SustainX, said he expected that the project would provide valuable data. “Because it’s D.O.E.-funded, they will have to publish their performance data, which will help the industry as a whole understand the technology’s capabilities,” he said, referring to the U.S. Department of Energy. And because the technology is built with off-the-shelf components, the market will be able to estimate its cost, he added.
The German consortium, meanwhile, is seeking to develop a related adiabatic technology in which the temperature of the compressed air would be allowed to vary within a range of 600 degrees Celsius to 40 degrees Celsius, or 1,112 degrees Fahrenheit to 104 degrees Fahrenheit. Heat generated by the compression process would be stored in large, ceramic-filled concrete pressure vessels. A first planned prototype plant would have a capacity of up to 90 megawatts.
The consortium, called Adele — the German acronym for adiabatic compressed air energy storage — includes the power utility RWE, the construction and engineering contractor Züblin, the German Aerospace Center and the U.S. conglomerate General Electric. It is currently negotiating for German government funding and hopes to have the demonstration plant operational by 2019.
Like the older diabatic plants, Adele would use salt cavern compressed air storage — its planners are focusing on a suitable site near Stassfurt, a city in the state of Sachsen-Anhalt — but the consortium says the addition of the ceramic-concrete heat storage units should raise the efficiency of the whole system to above 60 percent.
An advantage that SustainX and LightSail have over Adele is that their technologies are not site-specific. They can be placed wherever they are needed: next to a wind farm, for example, or a factory that generates its own power.
But they are also more costly: Roland Marquardt, research and development manager for RWE Power on the Adele project, said pressurized air storage cannot be done more cheaply than by using Adele’s large-scale process.
The growing percentage of renewable generation in the electricity system, particularly in Germany, requires thinking big, he said. “Three-digit-megawatt scale is what we are looking for to have significant impact,” Mr. Marquardt said.
Still, he added, the future will likely offer different roles for different technologies.
Mr. Brody of SustainX warned that given the low price of natural gas in the United States currently and policy frameworks that are not creating strong incentives, “it will be very difficult for any storage technology to compete in conventional applications.”
SustainX is therefore eyeing other markets, where fossil fuel prices and policy incentives are more conducive to economic success. The company also plans to target more creative applications.
“We see substantial value, both in the United States and in other markets, where grids are facing constraints that cannot be addressed through traditional generation or T and D assets,” he said, referring to costly investments in transmission and distribution lines.