Grid-scale energy storage is gaining momentum as batteries, flywheels and compressed air systems begin proving they can regulate frequency and ancillary services with the same efficiency of "spinning reserves" from fossil fuel-fired power plants.

“We still hear people say storage isn’t ready for primetime, but that isn’t the case because we already have 20-MW storage plants being built all over the country,” said Brad Roberts, executive director of the Electricity Storage Association (ESA).

As more renewable energy hits the grid, generators and independent system operators are looking to new storage systems to provide emissions-free backup and regulation when intermittency interrupts solar and wind power.

“We are interested in the potential of battery storage to be a game changer in our industry in both regulated utilities and commercial businesses,” said Greg Efthimiou, spokesman for Duke Energy, which operates more than 1,000 MW of wind farms.

Duke Energy is installing the country’s largest battery storage system, a 36-MW unit, near its 153-MW Notrees Windpower Project. The system will regulate frequency and store excess energy for use during peak demand. In Texas, where nearly 11,000 MW of generation comes from wind farms, grid operator Electric Reliability Council of Texas relies on standby gas turbines and steam coal generators to ramp frequency up or down as wind generation changes.

The Notrees battery system is funded by a $22 million grant from the U.S. Department of Energy (DOE) and matching funds from Duke Energy, which will use Austin-based Xtreme Power’s proprietary dry cell technology.

Investment, Policy Gains

ESA’s Roberts said the $158 million in stimulus earmarked by the DOE for storage research generated $780 million in investments for battery, compressed air, flywheels and other systems.

The storage industry has been calling for creation of an ITC to further stimulate growth. With passage of Assembly Bill No. 2514 in September, California began the process of developing a portfolio standard for energy storage.

Storage technology pulled in $150.3 million in venture capital during the second quarter, according to Ernst & Young. General Compression received the largest percentage, with $54.5 million. The company plans to use General Compression Advanced Energy Storage (GCAES), a unique heat transfer technology, to compress air in underground caverns. Investors include ConocoPhillips, US Renewables Group, Duke Energy and Serious Change L.P.

Compressing air underground has the potential for storage in excess of 100 MW but there are only two such projects in the world: A 290-MW facility built in Huntorf, Germany in 1978, and a 110-MW facility completed in 1991 in McIntosh, AL. And in July Iowa officials and the DOE scrapped the Iowa Stored Energy Park, a facility intended to store up to 270 MW of wind energy in a limestone cavern 3,000 feet underground. Studies showed limitations with air permeability in the site’s geology.

CAES Winners, Losers

Compressed Air Energy Storage (CAES) utilizing man-made, above-ground storage tanks has also gained traction with DOE funding. Startup SustainX, Inc. is working with AES Energy Storage to demonstrate a one-hour, 4-MW storage system. SustainX was founded in 2007 by engineers at the Thayer School of Engineering at Dartmouth College. It received $5.39 million DOE grant.

The Arizona Research Institute for Solar Energy (azRISE) at the University of Arizona has been developing a CAES solution for three years. DOE funds will enable azRISE to scale up a 10-kW proof-of-concept prototype (image, below) that will be grid-tied to a 1.6-MW solar power plant.

SOLON Corp., the solar plant’s developer, is working with azRISE and Tucson Electric Power (TEP) to demonstrate a variety of storage projects, including lithium ion batteries.

“As we see more integration of solar, we want to control storage for our utility customers,” said Bill Richardson, SOLON’s director of research and development. “The ultimate goal is to make renewable energy plants look like traditional plants with dispatchable energy.”

Joseph Simmons, azRISE director, said storage prices are high, particularly for battery systems, but he predicts a breakthrough will come with “intermediate size” compressed air systems that use off-the-shelf storage containers and have capacities of up to 100 kW. The institute’s concept, he said, can be scaled up to 1 MW with a coiled natural gas pipeline buried underground.

“I like batteries but they are very expensive,” Simmons said. “We expect to see more of a combination of batteries and compressed air storage.”

Heat transfer is another issue associated with CAES since air cools when it expands and warms during compression.

The azRISE system removes heat from a compressor then stores it in fluid. The system recovers electricity when heat returns to the compressed air before entering an expander. SustainX manages heat by uses an isothermal system to cycle air in hydraulic cylinders.

Price Points Still High

In just three years, the storage industry has grown rapidly from a handful of prototypes to revenue-generating corporations, Roberts said. Current battery technology has a long way to go before renewable energy can be stored and dispatched in meaningful amounts. Meanwhile, revenue is limited to ancillary services, critical observers say. And then there’s the price: At $43.6 million, the 36-MW Notrees system costs $1,211 per kilowatt. Others are down to $400.

“But the price is still way too high for this market,” said Donald R. Sadoway, Professor of Materials Chemistry at MIT.

The leading battery systems are based on lithium ion or sulfur-sodium (NaS) power cells. A multiple-megawatt storage system using these technologies can require thousands of cells.

Sadoway co-founded Liquid Metal Battery Corp., a startup that uses pizza box-sized power cells made with liquid metal and molten salt. Sadoway is banking on his batteries to provide game-changing, cost-effective power storage capacity.

“Storage will have to be below $200 per kilowatt if we’re going to be major players in the long-term storage firming in renewables without government subsidy,” he said.

Pumped-hydro, which accounts for 20 GW of the country’s energy storage, can provide 1,000-MW storage systems for $100 per kilowatt-hour, according to The New York Times. It requires massive reservoirs that cost more than $1 billion and take years to construct with ideal geography and abundant water resources. That pretty much rules out the arid Southwest, so researchers like Simmons and Sadoway look to alternatives. (For a primer on Energy Storage Costs, see Sidebar: Understanding Energy Storage Costs, below.)

Sadoway’s company received a $6.9 million grant from the Advanced Research Projects Agency-Energy (ARPA-E) as well as seed money from Total, an oil company, and Microsoft Co-Founder Bill Gates. Sadoway said the size of his batteries will broaden grid-scale storage capacity since more liquid will be present per cell than conventional cells.

Lithium Ion Still Popular

“We’re starting to see prices come down as we scale up each project,” said John Zahurancik, AES Energy Storage vice president.

AES Energy Storage is installing a 32-MW lithium ion storage system to regulate the 100-MW Laurel Mountain Wind Farm in West Virginia. Since it was founded in 2007, AES Energy Storage has completed more than 32 MW of storage, and it claims to have 500 MW “in the pipeline.”

“We are starting to demonstrate the real commercial competence of storage,” Zahurancik said.

Storage is attractive to generators – parent company AES Corp. operates 132 power plants worldwide – because it provides “fuel-free” power during peak hours, Zahurancik said. Large-scale battery storage is still years away, so revenue streams are limited to ancillary services, which represent a small piece of all sales on the electricity market. The ESA and American Wind Energy Association are lobbying utility regulators who oversee electricity sales to create markets that recognize a premium for emissions-free storage and regulation.

Some systems are growing outside of ancillary services. AES is installing an A123 Systems battery unit capable of providing 20 MW of spinning reserve for 15 minutes in Northern Chile.

“We’re moving out of the lab and into large production facilities,” said Chris Campbell, vice president of marketing for A123 Systems’ Energy Solutions Group.

He said A123 Systems European customers are interested in 100-MWh to 500-MWh storage systems that will help them meet clean air goals. Zahurancik said battery storage devices in the next three years will offer two to four hours of storage for that can transfer nighttime wind energy for peak use.

“We’re already seeing our market grow like the solar and wind industries,” he said.

Sidebar: Understanding Energy Storage Costs

Energy storage systems are typically quantified in terms of capacity (kilowatts or kW) and generation (kilowatt-hours or kWh), but there are some exceptions.

“In the case of energy storage costing, dollars per kilowatt-hour can be very misleading,” said Brad Roberts, executive director of the Energy Storage Association.

Battery storage is growing rapidly, but costs remain high, so the industry is striving for average prices to dip below $500 per kWh within three years.

“Technologies like lithium ion need to see huge price declines in the next few years which may be possible as electric transportation grows,” Roberts said.

The approximate cost of a 1-MW, 6-hour sodium-sulfur (NaS) battery is $3,000 per kW. That translates to a cost of $500 per kWh ($3,000/ 6 hours = $500), Roberts said.

Xtreme Power Chief Development Officer Darrell Hayslip said battery costs are expressed in dollars per kWh when considered “stored energy.” Xtreme Power often quotes prices in terms of dollars-per-kilowatt because it markets its dry cell products as a “generating or supply resource,” Hayslip said.

Xtreme Power’s 36-MW Notrees project is funded by about $22 million U.S. Department of Energy grant and $22 million in matching funds from Duke Energy, which Hayslip said brings the cost to $1,211 per kW ($43.6 million/36,000 kilowatts = $1,200).

Joseph Simmons, director of Arizona Research Institute for Solar Energy (azRISE), estimates a 10-kW compressed air storage system with a $50,000 price tag can generate 30 kilowatt-hours, or three hours of electricity at 33 cents per kilowatt-hour.

Calculations for the University of Arizona system call for use of a 1,000-gallon storage tank. He said the system could be scaled up to 10 MW using coiled pipeline to create 1 million gallons of storage space. Such a system would generate 30 MWh, but price is unknown at this point, he said.

--RC