Cave ‘batteries’ will store wind power

The Sunday Times
March 29, 2009

Special caverns will be used to store energy for when it is needed most
Dominic O’Connell

Promoters of wind-power projects tout them as the feel-good alternative to burning fossil fuels. Energy secretary Ed Miliband seems to agree, saying last week that it should become “socially unacceptable” to oppose new wind-farm developments in your area.
Wind power has an Achilles Heel, however. It is fickle — as fickle as the wind. Energy planners who have to make sure the lights don’t go out cannot guarantee power will arrive, no matter how many turbines are built. Conventional power plants, probably run on coal or gas, have to be ready to pick up the slack if the wind doesn’t blow.
Now two British companies are putting money into a technology that could overcome this problem. Electricity from wind farms will be used to compress air, which will be stored in caverns below ground. When power is needed, the compressed air will be released, driving turbines to generate electricity.
If the technology works, energy planners will be able to rely on power from wind farms when it comes to planning the next day’s output — and the plants that burn fossil fuel can be stood down. Such a development could be critical to Britain, which wants to increase wind power tenfold by 2020 as part of the government’s plan to have 35% of electricity generated from renewable sources.
The compressed-air technique may sound far-fetched, but it is already in operation. At Huntdorf in Germany, a 290MW plant — powerful enough to run 290,000 homes — has been running for 25 years. A 110MW facility at McIntosh, Alabama, opened in 1991. Neither stores wind power, but excess production from conventional stations. They were built to store electricity generated at periods of low demand, with it being sold to the local grid whenever demand surged.
Sirius Exploration, a group quoted on the Alternative Investment Market, London’s junior stock market, has a controlling stake in an ambitious power-storage project in North Dakota. The state is America’s windiest, and President Barack Obama has made the construction of wind farms there one of the planks of his administration’s alternative-energy policy. Power from the turbines is destined for mid-western cities, including Chicago.
Sirius has invested in an American company, Dakota Salts, which has a lease on 5,000 acres of salt pan. It plans to mine potash found in deposits beneath the surface, leaving behind caverns that have the ideal geology to store compressed air.
Richard Poulden, Sirius chairman, said he was attracted to the idea because of his knowledge of cars that ran on compressed air. “I had a misspent youth taking cars apart and putting them back together, so I knew there was this alternative power source that actually worked very well for storing energy, better than batteries,” he said.
Sirius had also been watching the Dakota project because the caverns could be used to store gas or other hydrocarbons — and gas pipes from Alaska and Canada to American markets run straight across Dakota.
Walter Doyle, boss of Dakota Salts, said the technology had the potential to transform the economics of wind farms. “In the midwest, the local grid will buy off-peak power at 6-8 cents per kilowatt-hour. Peak prices are more like 26 cents an hour. With the storage option, you can guarantee your availability for the peak.”
The ultimate size of the Dakota plant is still to be decided. Potash mining has to take place first, a task that will probably be given to a joint venture with a mining group.
The resulting caverns will be about 16m in diameter and 160m tall, and capable of generating some 100MW of electricity. Sirius and Dakota plan up to eight caverns in the project’s first phase, with the units coming on line in about five years. Doyle said that he was look- ing at similar projects in China and Australia.
Closer to home, the Irish power group Gaelectric has begun investigating a storage facility near Larne in County Antrim. Subject to further geological investigation and planning permission, Gaelectric plans
to spend £200m on a cavern with a minimum 136MW capacity — and possibly 300MW.
“We have been looking at this since we started the company in 2004,” said Keith McGrane, the firm’s head of energy storage. “It’s not a new idea, but with the increasing proportion of wind power across Europe it is an idea whose time has come.”
McGrane said the technique allowed wind farms to take on the characteristics of conventional power plants.
“The problem with wind is that it is not reliable. Even if you have a large number of turbines spread across a number of locations, forecasting wind patterns is not particularly accurate — and wind can often blow more strongly at night, meaning you are generating power when there is no demand.
“Storage means that wind power can be reliable, and grid planners won’t need the back-up fossil-fuel capacity. There is a big social benefit in avoiding that pollution.”
The Larne plant is unlikely to be an eyesore. McGrane said the project needed only four acres of land, with a few buildings on the surface and the business end of the plant hidden deep underground. It will store air at up to 68 times atmospheric pressure.
A modern compression plant should be able to have an overall efficiency of about 75%. A crucial part of the process is capturing the heat generated when air is compressed. If that heat is not harnessed, the efficiency can fall to 65% or less.
The caverns are created by “solution” mining — where the substance being extracted is dissolved and pumped out in solution rather than dug out.
McGrane said it would not be necessary to build wind farms alongside the storage plant. “We don’t care where the electrons come from. We just need a connection to a power supply.” McGrane said the plant could be up and running by 2015.
Energy experts say that the biggest obstacle to the storage projects is the cheapness of conventional generation.
“The storage technology is a good technology, but it will be expensive to build and wind-generated power is more expensive than fossil-fuel power,” said Bikash Pal, reader in power systems at Imperial College, London.
“If there was a way of capturing the wider socio-economic benefits from the avoidance of fossil-fuel stations, it would make a big difference to the economics of the schemes,” he added.