Designing out intermittency

Variability is the last refuge to which an opponent of wind energy clings. When all other arguments fail (costs, noise, bird-kills, visual) rest assured that wind foes will point to the intermittent nature of the resource and make the point that it is unpredictable and therefore could never really replace greenhouse gas-emitting fossil fuels.

What about grid-connecting wind farms that span a geographic area wide enough to ensure that the wind would be turning turbines somewhere within the system at any given time? (GW)

A Grid of Wind Turbines to Pick Up the Slack

By Henry Fountain
New York Times
April 12, 2010

Like most other sources of alternative energy, the wind can be intermittent. It does not blow uniformly, so power output from wind turbines rises and falls. And when the wind doesn’t blow at all, output drops to zero.

Intermittency is not much of a problem now in the United States, since there are relatively few wind farms and plenty of interconnected conventional power plants to pick up the slack when wind output falls, keeping the power supply stable. But if the proportion of electricity supplied by wind were to grow to, say, 20 percent or more, it would become increasingly difficult to handle the fluctuations in output.

One proposed solution to the intermittency problem is to tie many wind farms together with a transmission line — making an electric grid, as it were, consisting of wind turbines. Now, Willett Kempton of the Center for Carbon-free Power Integration at the University of Delaware and colleagues have shown how this “all-for-one” approach might work with offshore wind farms along the Eastern Seaboard.

The researchers looked at five years of wind data from 11 sites from Maine to Florida, and calculated potential hourly power output for a typical turbine at each site. As they write in The Proceedings of the National Academy of Sciences, they found that, as expected, fluctuating wind conditions caused power output to rise and fall at each site and sometimes reach zero.

But since wind conditions can differ greatly over the 1,500 miles of coastal ocean, when the researchers simulated connecting all the sites with a high-voltage undersea transmission line, the overall output became much more stable — reaching full or low power less often and never, over the five years studied, reaching zero power.

No offshore wind farms have been built yet, though several are in the planning stages. Operators would then have to agree to build the undersea transmission line, which could cost well over a billion dollars. “But the whole idea is that it would pay off over time,” Dr. Kempton said.