By Helen Campbell 

Alternative, intermittent energy sources like sun and wind present a major conundrum when it comes to dispatchability. But compressed air energy storage (CAES) could soon resolve this challenge.

Energy produced off-peak from a renewable energy source or standard power plant is used to power air compressors that pump air into either pressurized storage tanks or underground caverns. When electricity is needed, the air is expanded via turbines, to generate energy. CAES saves renewable energy, quite literally, for a rainy day

CAES for Brayton

Southwest Solar Technologies in the US, in partnership with Brayton Energy, has just launched its pilot test for CAES and solar in Phoenix, Arizona, taking a dish-turbine approach and using a Brayton cycle turbine. 

“The test started a couple of months ago with the dish and solar receiver, and we plan to bring some of the first turbine generator alternators onsite here to test within the next few months,” says Herb Hayden, Southwest’s chief technology officer. 

“The received air is being raised to 925° C. That’s pretty high, and is the temperature needed for an efficient Brayton cycle. If you look at solar trough steam projects, they are typically 400° C or less, and that is not a good enough temperature for a Brayton cycle. We plan to use that temperature when we bring the turbine equipment out.”

It’s a simple process, but the real issue is the geographic constraints, says John Crockett of the San Diego State University (SDSU) Research Foundation and part of a California Energy Commission-supported programme that provided funding for the Southwest/Brayton project.  

“All you need is a reservoir structure, a rock formation, such as limestone, fractured shale, or a salt cavern, to provide a porous deposit under pressure, and a capstone such as silt or some sort of finer grade material that caps the reservoir,” Crockett says.

“If someone could identify a good way to make it transportable, to make it arbitrary of geography and geology, either through material advances or identification of under-utilized assets, there is a lot you can do with CAES.”

However, in the US, when you overlay a geological resources map, showing limestone or depleted oilfields, with a solar resource map, there aren’t that many places where they overlap.

The other stumbling block for CAES, with solar or any other energy source, is the market penetration of renewables as a whole. Customers and producers will try to avoid paying for storage until they absolutely have to, and there is nothing irrational about that, says Southwest’s Hayden.

“The market potential of CAES is huge, but in the US there has been a slowdown in demand, so utilities are, at the moment, soft in their demand for any new power.

“The clearest incentives they have are towards some of the PV and wind projects that are already in the pipeline. If renewables such as PV and wind remain a small percentage of the marketplace, then [any intermittency] will be accommodated through existing utility power.

“When you start getting renewables penetration levels of 30% or higher, then those options are no longer sufficient and that is when there is a more easily quantified market for power including storage.”    

Alternative formations

Southwest has plans to explore CAES’ potential of in other applications. Proven commerciality in other areas may impact positively on CAES’ applicability to solar.

“We are also looking at how to do CAES with deepwater bodies like oceans using membrane airbag-type ideas that are submerged a few 100 feet under water and can provide a very convenient compressed air storage method,” says Hayden. “This method of storing air at a few hundred psi would be of great interest for the application of solar with CAES.”

The only two existing power plants globally that already use CAES, one in Alabama and one in Germany, store the compressed air in domal salt caverns. Other projects in the US, in Iowa, California and New York are testing CAES in ‘alternative’ formations, including depleted gas reservoirs and aquifers.

Paul Denholm, who leads analysis efforts on energy storage at the Department of Energy’s National Renewable Energy Laboratory (NREL), says success would mean a lot of potential for CAES.

“The availability of domal salt in the US and globally is fairly limited,” Denholm says. “One of the important things holding CAES back is demonstration of CAES in these alternative formations. If we can demonstrate CAES in those other types of formation, there is a lot of potential for those types of technology.” 

The success of CAES, emanating from any energy source, is also of high interest to the military, and is believed to already being used on a small scale to run such facilities as field communications centres.

“Most military installations are connected to the grid and that is where they get most of their electrical power from, but the grid is not a military asset and the grid is vulnerable,” says SDSU’s Crockett.

“The question these kinds of projects need to answer is; what is the value of firm power, of energy security? Of somewhere like Miramar, our Marine Corps air station, being under attack and being able to continue to land and launch helicopters and other vehicles, without interruption, of having completely dispatchable power available? You could say it’s infinite.” 

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Image credit: Brayton Energy LLC. Taken from the Brayton Energy LLC solar technologies peer review presentation, 2010.