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How the Iowa Stored Energy Park Will Work


ISEP and Electrical Generation
Combustion turbines burn a mixture of fuel and air to generate electricity. Most of the energy in the process goes to the processor, not to the turbines, which create electricity.
Combustion turbines burn a mixture of fuel and air to generate electricity. Most of the energy in the process goes to the processor, not to the turbines, which create electricity.
2008 HowStuffWorks

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:

  1. Spinning turbines on a wind farm generate electricity as moving air blows through the blades.
  2. Some of that electricity, especially during peak demand, is directed to the power grid.
  3. The excess electricity is directed to a compressor that pumps air through pipes deep into the ground.
  4. The air is stored in porous sandstone. As pressure rises, the air displaces groundwater like a giant bubble. In essence, the sandstone acts like a battery capable of storing about 20 weeks worth of air.
  5. During the day or whenever demand peaks, the utility can draw up compressed air and feed it into the combustion system of a gas turbine. The air mixes with natural gas, and the fuel-air mixture is burned at extremely high temperatures. The turbine uses 50 percent less natural gas because it does not have to run the compressor.
  6. The gas turbine operates a generator, which produces electricity.
  7. Electricity is sent to homes and businesses.

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].

For more on wind power and related topics, take a look at the links on the next page.