How the Iowa Stored Energy Park Will Work

The ISEP concept was first proposed in 2003, when members of the Iowa Association of Municipal Utilities (IAMU) formed a study committee to explore how wind energy could be used in creative ways to supplement the state's power production.

Iowa is the third largest producer of wind energy in the United States, just behind California and Texas. To advance its leadership position in wind production even more, the IAMU study committee knew that another large wind farm -- even one containing state-of-the-art, super-efficient turbines -- would not be enough all by itself. They needed a mechanism to store energy. The committee proposed a power plant that would integrate two key elements: a 100-megawatt wind farm and a compressed air energy storage facility.

The idea behind ISEP is based on two successful CAES facilities already in operation -- one in Huntorf, Germany, operated by Nordwest Deutsche Kraftwerke since 1978; and another in McIntosh, Ala., operated by Alabama's Electric Cooperative since 1991. Both of these facilities store compressed air underground. The Huntorf plant uses salt caverns as its storage reservoir. The McIntosh plant uses preexisting mines.

The ISEP planning committee wanted to experiment with storing compressed air in an aquifer. An aquifer is an underground layer of rock that is capable of holding water. The rock can do this because it contains millions of tiny spaces between rock and gravel particles. These little spaces trap water and hold it. As it turns out, this highly permeable rock can also be filled with air. In fact, if you pump air into an aquifer under great pressure, it acts like a giant bubble and displaces the groundwater. A few months after it initiated the study, the committee found an aquifer near Fort Dodge, Iowa, that seemed ideal. The aquifer was close to the electric transmission grid and a gas pipeline. However, the site eventually proved unsuitable for a variety of reasons.

As the team began screening other sites, it also went through some organizational changes. In 2005, the IAMU committee passed responsibility for ISEP to the Iowa Stored Energy Park Agency (ISEPA), an Iowa corporation representing more than 130 municipal utilities in Iowa, Minnesota and the Dakotas.

Two years later, in January 2007, the agency finally completed its screening process and settled on a site just west of Dallas Center in central Iowa. The site is ideal for a few reasons. The aquifer, 3,000 feet (914 meters) below the surface, is deep and wide, which means it can store a large amount of air. Another appealing feature is the site's geology. The aquifer is comprised of layers of sandstone capped by dense shale. Sandstone is very porous and holds both air and water well. Finally, the site is within 30 miles (48.3 km) of downtown Des Moines, which is good for economic development. The agency hopes that ISEP will become a tourist destination, and being close to the state capital could bring in more traffic.

The Dallas Center site is not, however, the best wind area in Iowa. As a result, ISEPA is investigating the possibility of a remote wind farm. The agency could build the farm or it could contract with an existing privately owned wind farm. Either way, wind is a critical component of the project. In fact, using wind as an energy source is one of the main ways in which the ISEP is different from the Huntorf and McIntosh plants. Both Huntorf and McIntosh use off-peak electricity from traditional nuclear or coal-burning power plants to run the compressor that pumps air underground. ISEP will use wind-generated electricity to run the compressor and will direct any excess energy back to the grid. By using CAES and wind power together, ISEP will be able to provide an environmentally friendly, alternative energy source for homes and businesses.

In the next section, we'll look at how exactly ISEP will make electricity.

Although wind energy is an important component of ISEP, it will not completely eliminate fossil fuels from the energy production equation. Instead, it will reduce the amount of fossil fuel used to make electricity. To understand why this is so, let's first consider a conventional turbine power plant, which relies on natural gas as its source of energy.

At the heart of such a facility is a three-section combustion turbine. The first section, the compressor, draws air into the engine and pressurizes it. The second section, the combustion system, burns a mixture of fuel and air, which produces a high-temperature, high-pressure gas stream. As the gas stream expands through the turbine, the third section, it spins rotating blades. The rotating blades perform two functions: They drive the compressor, and they spin a generator to make electricity. In fact, most of the energy used in a combustion turbine goes to running the compressor, not to generating electricity.

CAES improves the operating efficiency of gas turbines because compression takes place separately. Off-peak electricity runs a motor that forces air into an underground reservoir. During times of peak demand, air is released from the storage chamber and piped into the combustion system of a gas turbine. The air is already compressed, so the turbine doesn't have to run a compressor; all of the energy goes to operate the generator. As a result, much less natural gas is used.

ISEP will take this one step further by combining wind -- a clean, sustainable energy source -- with underground storage in an aquifer. The illustration below shows how ISEP will look and work. Let's walk through the steps:

ISEPA is still evaluating the best solution for wind production, but is looking to complete preliminary design work by May 2008. In September, the agency will begin acquiring the necessary permits from the Iowa Utilities Board. The facility should be operational and generating electricity by 2011. When it's up and running, ISEP could account for 20 percent of the energy used in a year at a typical municipal Iowa utility. It could also save cities and their utilities as much as $5 million each year in purchased energy [source: Energy Services Bulletin].

Other utilities around the country are watching ISEP with great interest. Some have even begun their own CAES projects. In West Texas, TXU Energy is working with Shell WindEnergy to build a 3,000-megawatt wind farm connected to a CAES system that will pump air into underground salt domes. Other sites are being explored in New Mexico and the Gulf Coast. Either way -- using underground salt caves or aquifers -- CAES may still provide the best hope of making wind a serious contributor to the total U.S. supply of electricity. The Electric Power Research Institute estimates that more than 85 percent of the U.S. has subterranean features that could support the technique. Perhaps one day, a nationwide network of facilities combining CAES with wind will supply as much as 10 percent of America's electricity [source: BusinessWeek].

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