Renewable energy stored as compressed air

Dr. Neil Canter, Contributing Editor | TLT Tech Beat October 2013

Researchers develop a process using compressed air energy storage to capture excess power generated during the late spring.

KEY CONCEPTS
• There are several challenges in storing electricity generated by renewable energy sources during certain times of the year when demand is low and supply is high.
• In the Pacific Northwest, a detailed proposal has been developed that uses underground, compressed air energy storage to capture the excess power generated during the late spring.
• A hybrid storage facility proposed for one of the sites uses geothermal heat to increase temperature of compressed air due to the unavailability of natural gas.

WITH THE INCREASING USE OF RENEWABLE ENERGY SOURCES such as solar and wind, problems can exist in using them because of their intrinsic variability in providing power. The reasons are due to the climate when the sun is not available during daylight hours because cloud cover interferes. Wind speeds vary substantially in most locations throughout the day and seasonally.

A previous TLT article discussed an aqueous, electrolyte battery developed by researchers to store energy from renewable sources. This battery consists of a cathode similar in composition to the dye, Prussian Blue and a hybrid anode combining activated carbon with polypyrrole (1). Potassium ions are used to cycle charge, and the battery displayed good efficiency and durability.

Energy storage challenges will probably be resolved on more of a regional level because climate varies so widely. A case in point is the northwestern part of the U.S. known as the Pacific Northwest.

Dr. Pete McGrail, lab fellow and project leader at the Pacific Northwest National Laboratory in Richland, Wash., says, “The two most widely used renewable energy sources in our area are hydroelectric and wind. We face an excess, power overgeneration issue that has been increasing for a number of years. This problem is particularly prevalent during the late spring.”

As McGrail explains, the snowpack built up over the winter in the mountains reaches a peak water runoff that maximizes the amount of power that the hydroelectric system can generate during the late spring. He adds, “The U.S. government limits the amount of water that can be spilled over the dams due to concerns about impacts on fisheries.”

Late spring also marks the time when the wind in the Pacific Northwest reaches its peak average velocity. The problem is that demand is also low for power, particularly along the highly populated West Coast.

McGrail says, “We face a situation where there is too much power available for the demand at that time. If nothing is done, then wind farm operators are told to shut down to ensure that the electric grid remains stable. This obviously significantly impacts their annual revenue.”

One other factor is that available power from wind turbines is now 3.5 gigawatts and is expected to increase to between 6 and 7 gigawatts by 2017.

The organization responsible for managing the power grid in the Pacific Northwest, the Bonneville Power Administration (BPA), approached McGrail and his associates to see if they could develop a way to store excess power in a cost-effective manner so that it can be readily used when there is heightened demand. A plan has now been developed using compressed air as the storage approach.

HYBRID STORAGE FACILITY
McGrail and his associates, in collaboration with the BPA, developed a detailed proposal to use compressed air energy storage (CAES) to capture the excess power generated and use it when needed by the electric grid. He says, “CAES has been known for over 30 years and is used at two commercial plants in Germany and the U.S.”

The actual technology for using compressed air is pretty simple, according to McGrail. He says, “When excess electrical capacity exists, power is drawn to run compressors that bring air up to high pressures and is then injected into a geological formation for storage. Later, when more power is needed on the grid, the compressed air is released back to the surface, heated up and expanded through a turbine to generate electricity.”

The original temperature of the stored compressed air can range from 32 C to 65 C. McGrail says, “If the air was expanded without heating, then it would go cryogenic. This necessitates a heat source in the CAES plant design. Conventional CAES plants use exhaust heat from a natural gas turbine to heat the compressed air.”

The efficiency of this process can get up to between 70 and 75 percent, which means that for every megawatt of power stored about 0.7 megawatt is eventually recovered.

Finding a suitable geological formation is vital to ensure that compressed air is stored properly. The researchers evaluated several sites in the states of Oregon and Washington.

McGrail says, “Ideally, we were looking for a geological structure called an anticline that looks like a big umbrella underground. The convex upward structure is suitable for keeping compressed air at the top, making it more efficient to move large volumes of air in a short time into the structure and remove the air quickly when needed.”

The pressure of the stored compressed air depends upon the depth of the geological formation. It can range up as high as 6,000 psi.

Two sites in Eastern Washington known as Columbia Hills and Yakima Minerals were identified as having the right geology for storing compressed air. The Columbia Hills site has access to a nearby natural gas pipeline, so a conventional CAES plant with a natural gas turbine was selected for that site. But no natural gas turned out to be accessible at the Yakima Minerals site.

McGrail says, “We looked at a number of ways to find a heat source to reheat the compressed air as it is extracted from underground. A surfacebased, molten salt unit for short duration storage was coupled with geothermal heat extracted from deep underground using water as the heat transfer fluid.” The geothermal source performed double duty in their design, also powering an absorption chiller that provided cooling to the air compressors, making them operate more efficiently.

A schematic showing the design of this hybrid storage facility is shown in Figure 3.


Figure 3. A hybrid facility is proposed for one of the locations identified in the Pacific Northwest suitable for storing excess renewable energy in the form of compressed air. The heat source needed for this facility is geothermal, which is transported to the surface by water. (Courtesy of the Pacific Northwest National Laboratory)

At this point, McGrail indicates that the BPA is in discussions about using CAES with local utilities and a more detailed examination of the hybrid geothermal design is expected to begin this fall.

Additional information may be obtained by contacting McGrail at pete.mcgrail@pnnl.gov or by reading the report, which is found here.

REFERENCES
1. Canter, N. (2013), “New Process for Storing Renewable Energy,” TLT, 69 (1), pp. 12-13.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.