Catching the wind: the search for energy storage

Wind screams across the bone-dry rangeland in north-west Texas, flinging grit into your eyes with every gust. Low to the ground, the sagebrush grows sideways, telling which direction the wind blows. But even here, in one of the windiest areas of North America, the gale sometimes slows to a barely-perceptible breeze. And that’s a problem for companies that hope to harvest the wind and produce electricity.

Wind is one of the cornerstones of the world’s hope for a greener energy future. It’s free. It’s eternally renewable. And it doesn’t produce greenhouse gas. Unfortunately, when the wind stops blowing, the wind turbines stop turning — and electricity stops flowing. To meet this challenge, utility and energy companies are exploring ways to capture and store the wind’s energy until it is needed.

A question of balance

“Wind seems like the ideal energy source, but some utility companies can manage only small amounts of it with the resources they have,” said Houston-based Chris Ziesler, wind analysis manager for Shell WindEnergy, Inc.

Matching intermittent wind power to demand can be tricky. In Texas, the wind tends to blow hardest just before dawn, when nobody is awake to need the extra electricity. And, like a bad joke, winds typically die down in the late afternoon — especially in the hottest part of the summer — during the very peak of energy demand.

In some parts of the world, electricity can be transferred between regions as needed. The Texas transmission grid lacks this capability. Yet utility companies can easily accommodate routine fluctuations when wind is just a small part of the overall energy mix. They simply ramp up or down other generating plants as needed to provide a steady, reliable stream of electricity.

However, if wind contributes a large part of the overall power mix, companies require equally large back-up power plants to compensate when the wind dies. Conversely, when electricity demand declines and wind output is not needed, the only option is to shut down the turbines and stop harvesting energy.

Storing the excess energy until it’s needed could help balance the system and reduce the need for expensive back-up resources. Moreover, if companies succeed in perfecting cost-effective storage technology, wind projects could maximise the use of their turbines. This would make them more attractive financially, accelerating the development of wind as an energy source.

Squeezing air

Companies have experimented with batteries, flywheels, capacitors and hydroelectric systems to capture the wind’s energy for later use. But one solution may see large-scale use on the windy ranch land in Briscoe County, Texas (pictured below) where Shell is considering plans to develop a 1,500- to 3,000-megawatt wind farm. The project may store energy underground in the form of compressed air, which would help generate additional electricity when used in gas-powered turbines.

Approximately 200 landowners have signed leases with Shell to allow wind turbines on their ranches. But the project, which is in the planning stage, still has years of development work ahead. The project team is studying local wind patterns, air compression techniques and local geology. They’re also evaluating the area’s wildlife to be sure the project safely co-exists with the area’s current animal residents.

If the project moves forward, hundreds of white wind turbines would rise on 100,000 sparsely populated acres — an area roughly five times the size of Manhattan. It would be the largest wind farm in the world.

The project’s unique feature could lie deep in the earth. Energy that the turbines generate at night, when consumer demand is low, could be used to compress air into underground caverns that have been mined from salt beds. Later, when the wind blows more gently and the turbines are still, compressed air would mix with natural gas to fuel turbines, producing electricity nearly on demand. This combination uses only one-third as much gas as a typical gas generator and greatly reduces emissions.

Compressed air technology is hardly new: two small facilities already use it. One was built 30 years ago in Germany and the other in 1991 in the southern US state of Alabama. Shell also experimented with compressed air during the 1980s.

Today, companies and governments have renewed their interest in the technology as society searches for new sources of clean, home-grown energy. For instance, the U.S. Department of Energy and Sandia National Laboratories are collaborating with the Iowa Stored Energy Plant Agency to further investigate the potential for compressed air, with plans for a 200-megawatt compressed air power plant and 75-megawatt wind plant. Other small projects have also been announced.

But basic questions about using compressed air storage on a large scale remain unanswered. For instance, engineers must ensure the salt beds or other underground formations can hold sufficient air to make the system function reliably. Studies suggest that a wind farm needs to generate at least two gigawatts of power — the size of a modern coal-fired power plant — before compressed air storage becomes cost effective. But that has yet to be proven on an actual project.

The Briscoe County project, if built, would help answer such questions. It would also prove that it really is possible to catch the wind — even the wild wind of north-west Texas.

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