Using Compressed Air To Store Up Electricity

If you think that compressed air engines (CAE) are only about cars, think again! There are a variety of uses for compressed air. Here's a New York Times article that gives you another use for our CAE...

Technology; Using Compressed Air To Store Up Electricity

By MATTHEW L. WALD
Published: September 29, 1991


Electric utilities confront a challenge unique in manufacturing: their product is consumed at the instant it is produced; customers can demand as much as they want whenever they want it, and running short of supply, even for a moment, is catastrophic. Thus, for as long as the industry has existed, engineers have been looking for ways to store electricity.

They have used batteries, which are effective but expensive, and hydroelectric plants that run in both directions, which are less costly but damage the environment.

On Friday, however, the Alabama Electric Cooperative dedicated a plant that uses a new system, one that experts say represents a relatively clean, efficient and cost-effective solution: compressed air.

The $65 million plant, in McIntosh, Ala., 40 miles north of Mobile, draws electricity from a coal-fired power station 20 miles away at night, when demand for power is low. The McIntosh plant uses an electric motor and a compressor to pressurize an underground chamber of 19 million cubic feet -- 220 feet in diameter and 1,000 feet tall -- to 1,100 pounds per square inch. The pressure may sound high, but it is only about one-fifth of what the chamber could withstand.

When the cooperative needs extra power, the air is withdrawn, releasing energy the way a balloon does. But it is not the air itself that provides the power to make electricity; the compressed air is fed into a turbine on the surface above the cavern. Turbines, essentially jet engines chained to the ground, burn natural gas or fuel oil mixed with the compressed air to spin a shaft, which then turns an electric generator to make power.

Using compressed air in a turbine is not new; in fact, virtually all utility turbines use the technique. Usually, some of the turbine's mechanical power is diverted to compress the air. In this case, however, the compression is done the night before, by a different plant.

The electric generator is the motor that is used to compress air. At night, it uses electricity to create a mechanical force, and in the daytime mechanical power is applied to generate electricity.

As a result, the plant produces one kilowatt-hour -- or 1,000 watt-hours -- of electricity for each 870 watts consumed the previous night. In contrast, the most common mode of energy storage is pumped hydro, in which water is pumped uphill at night, and during the day a valve is turned and the water runs back down, with the pumps recapturing the mechanical energy and turning it into electricity. But in that system, each kilowatt-hour put in delivers no more than 700 or 750 watts back out again. Batteries have about the same ratio.

Including the energy of the fuel burned in the turbine, the compressed-air system uses about 13,200 B.T.U.'s to produce one kilowatt-hour of electricity. This would be below par for most power plants, but good for a storage plant. In a pumped-storage system, putting in the same amount of energy would produce about 12 percent less electricity.

Hydroelectric plants often cost $1,000 per kilowatt of capacity, and batteries cost far more. The cost of building the Alabama plant was about $550 per kilowatt of capacity.

The compressed-air concept is not completely new. A similar plant opened in Huntorf, Germany, in 1978 and has run well since then, according to the Electric Power Research Institute, a consortium based in Palo Alto, Calif., that contributed $8 million to build the Alabama plant. (The National Rural Electric Cooperative Association chipped in $660,000.) The American plant has one new twist, however: the exhaust gases from the turbine are used to preheat the compressed air after it is brought up from the cavern. That makes it 25 percent more efficient than its German predecessor, the institute says.

Despite that innovation, there are no new inventions at the plant. "We're integrating proven components," said Dr. Robert B. Schainker, an engineer at the institute. "We're using very low-temperature machinery, standard kinds of machines. A dozen utilities are discussing similar plants, he said, and he would like to try a more advanced turbine operating at higher temperatures, which would be more efficient.

Utilities know nearly nothing about building underground caverns, and Mr. Meyer said this was the element that most worried the co-op. But, he said, "everything went pretty well according to plan."

The technology of mining in salt, the geologic medium in Alabama, has been in wide use in the oil and gas business for decades. In fact, the method chosen -- solution mining, in which water is pumped in and brine is pumped out, leaving a void -- is the same one that was used by the Energy Department to create storage space for the Strategic Petroleum Reserve. The co-op's cavern begins 1,500 feet below the surface and stretches down to 2,500 feet.

The plant's output is 110 megawatts at full capacity, which is fairly typical of power plants now coming on line. It can run for 26 hours from a fully charged cavern and supply the demands of 11,000 homes, the institute says. Typically, however, it would run 10 hours a day or less, when demand is high. Another advantage, Mr. Meyer said, is that it can increase and decrease its power level quickly.

According to the institute, three-quarters of the United States has geologic formations "potentially suitable" for compressed air storage. A dozen utilities are discussing compressed-air storage plants, according to Dr. Schainker. Building caverns in solid rock would be more expensive, he said, but some areas have alternatives cheaper than salt, like abandoned mines or natural gas fields. The natural gas industry already uses depleted fields as storage grounds for its product.

At the North American Electric Reliability Council, a nationwide utility consortium that co-ordinates planning and issues forecasts, Gene Gorzelnick, a spokesman, said compressed air storage was "another technology that you can draw upon that allows you to use your existing facilities more effectively." A utility with insufficient power for peak hours might still have idle generating stations in off-hours; this puts idle plants to work and cuts the need for new plants. The cooperative has high hopes for its new plant, which entered commercial service on May 31, but was shut for modifications from early August until mid-September.

According to Robert C. Meyer, the project manager at the cooperative, 85 percent of the company's customers are residential, and their demand varies sharply by time of day; as a result, a graph of customer demand has a peak every day and a valley every night, the peak about twice as high as the valley.

That is mild, however, compared with the load profile of urban utilities, where the peak can be three times as high as the valley.

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