Iowa's Compressed Air Energy Storage Project Provides an Elegant Concept for Using Variable Wind Power

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Researched by Industrialinfo.com (Industrial Information Resources, Incorporated; Houston, Texas). 'Hybrid' has become the accepted classification for an energy producing technology which combines the prime benefits of two or more previously defined technologies to produce a cleaner and more efficient energy supply. The hybrid label is commonly applied to auto engines and electrical power generators ranging from hand-held micros to industrial size macros. These hybrid systems require intense application of capital and development effort to become cost effective and commercial.

It might seem that the idea of 'elegant' engineering solutions to produce effective applications is wasted nostalgia in age of big buck burns and should be left to academic mathematicians and the occasional bridge builder. But a proposed project from the Iowa Association of Municipal Utilities (IAMU) to use windpower, compressed air energy storage, and heat turbine generation does have an appealing logic about it. It also answers some problems, such as storage of variable wind energy, which have been knocking around for some time in the area of base load/peak load sustainable energy applications.

In the Iowa Stored Energy Plant (ISEP) project, the existing Compressed Air Energy Storage (CAES) methodology has been developed with the innovations of storage the compressed air in an underground aquifer as opposed to impermeable caverns and the use of wind energy to compress air. The windpower will be used in addition to available off-peak power which is the source for other CAES schemes.

There are a number of operating CAES systems around the world of which the two largest are the most quoted. The Huntorf plant in Germany, which was commissioned in 1978, is rated at 290 MW the plant, supported by two 10 million cubic foot caverns, has demonstrated 90% availability and 99% starting reliability, over a ten year period.

In 1991, PB Energy Storage Services (PB ESS; Houston, Texas), a subsidiary of Parsons Brinckerhoff (New York, New York) constructed the first CAES facility in the US for peak shaving at Alabama's McIntosh salt dome. A 20 million cubic foot cavern supports a 110 MW generation plant. On nights and weekends air is pumped underground and compressed using low-cost electricity at pressures of up to 1,078 psi. Average air pressure at sea level is 14.7 psi.

Other major schemes operate or are at the planning stage in the US, Russia, Italy, and Israel.

The central benefit of CAES systems lies in carrying out the air compression at a different time to when power generation is needed. In a gas turbine 50-60% of the energy produced by the power turbine is consumed by the compressor. In the CAES power generating mode, compressed air is mixed with fuel and expanded through a modified gas turbine driving the generator. In storage mode the generator is used to drive the compressor to feed the underground storage.

In the Iowa project, windpower will provide the compression energy and CAES will be operated to follow loads and fill in where other generation is unavailable or uneconomical. As wind generation can be highly variable CAES combines in the overall system to fill in the compression energy supply gaps and to turn the wind energy into a steadily available resource. In addition compressed air replaces up to 66% of the natural gas used in the turbine. The ISEP project would use a separate section of the underground aquifer to store natural gas. This will allow the facility and other gas utilities to buy natural gas when prices are lower. Storage in an aquifer, as opposed to a dry cavern, also assists in balancing the pressure of on hand air and gas storage with water levels.

IAMU is hoping that the project will expand the role of wind in the region's power generation mix. They envision a combination of natural gas and Iowa's abundant wind resource. for lower carbon emissions in power generation. The replacement of natural gas with shelled corn, another Iowa staple, is also on the horizon.

An underground aquifer near Fort Dodge is seen as an ideal site for the project as it has the dome structure, capacity and porosity needed for storage of both compressed air and natural gas alongside Iowa's wind and corn.

Preliminary cost estimates show that a configuration of 200 MW of CAES generation capacity with a 100 MW windfarm, is the most economical. Income from leases of underground aquifer storage rights and for the placement sites of wind turbines would give local landowners an ongoing revenue stream/ IAMU believes that, with an estimated $215 million construction cost, the ISEP would provide an economic benefit not only for the Fort Dodge area, but for the entire state of Iowa.

Looking out over the prairie with the wind riffling through the corn you can see the rotors rising just as high as an elephant's eye (Oklahoma! could do it too). Refreshingly simple and elegant.