Predicting Volatile Wind, Sun

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Largely due to the unpredictability of the heavens, the thousands of wind turbines across the country collectively produced 1.3% of actual electricity in 2008.

But capacity to produce is not actual production. Largely due to wind's unpredictability, the thousands of wind turbines installed across the country collectively produced only 1.3% of actual U.S. electricity in 2008, the department's figures show.

At any moment, the problem can be either too little wind or too much. The Bonneville Power Administration, a government-owned utility based in Portland, Ore., taps one of the biggest collections of wind farms in the country. Between January and August, average wind-power production accounted for 12% of average electricity consumption in Bonneville's service area.

From hour to hour, though, wind power swings wildly depending on how things blow at the Columbia River Gorge. That stretch, which separates Oregon and Washington, is where most of the wind turbines in Bonneville's service area are located.

This Tuesday was typically erratic. At 1 a.m., wind farms in the Bonneville service area were cranking out about 1,550 megawatts of power. By 7 a.m., that fell to about 800 megawatts, just as people were waking up and turning on their lights and toasters. That night, once most people were asleep, the wind picked up again. By 11:45 p.m., wind power topped 2,000 megawatts.

Most of the electricity in Bonneville's service area comes from hydroelectric power. To compensate for the volatility of wind, Bonneville tweaks the amount of water it lets through the dams. But that doesn't work for the most extreme shifts in wind. Sometimes, when the wind is blowing hard, Bonneville releases extra water over the tops of dams without using it to generate electricity. Otherwise, electrical wires might get overloaded. And when the wind is so strong that Bonneville can't ditch enough water, the utility orders wind turbines shut off.

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"Everything changes with wind," says Bart McManus, a wind expert at Bonneville.

Sudden doldrums can be as troublesome as sudden gusts. That was the problem on Feb. 26, 2008, in Texas, which produces more wind power than any other state.

At 3 p.m. that afternoon, Texas' wind farms, concentrated in the western part of the state, were throwing off about 2,000 megawatts of electricity, enough to serve about one million households. Then a cold front blew in. By 6:30 p.m. -- when electricity demand typically peaks -- wind production in Texas had cratered to about 360 megawatts.

Exacerbating matters, Texans began turning up their heat -- much of which, in rural parts of the state, comes from electricity. So, just as wind power unexpectedly plummeted, demand for power spiked.

The operator of Texas' electrical grid, the Electric Reliability Council of Texas, known as Ercot, scrambled. It cut off power to various industrial customers that, in exchange for payment, had agreed to let Ercot pull their plugs in emergencies.

To avert situations like these, Ercot has hired a company to provide, every hour, a forecast of how the wind will blow at every wind project on the Ercot grid. It requires wind-power producers to install gauges that feed into those forecasts.

The forecasts look not just at temperature, but also at wind speed and direction at the height of wind turbines, an altitude that until now hasn't attracted much interest.

If there were a viable way to store large amounts of renewable energy, Ercot might have been able to tap it on that February afternoon. Investors and the government are backing efforts to develop storage ideas. One hope is to build a better battery. Other ideas include systems that would store water in uphill sites or compress air underground, for later release when electricity is needed.

So far, these options are largely experimental. But Mr. Denholm and other scientists say they are optimistic that renewable energy one day will be a major contributor to the world's power supply. Many recent studies suggest that would require a combination of approaches, all of which are under way to some extent: investment in high-voltage transmission wires to carry renewable electricity from remote areas to cities; policies to encourage energy efficiency; and coordinated construction of renewable facilities so that one form of energy can fill in when others are dormant. In many places, wind is calmest at midday, when solar power is most available.

Until these measures are widely adopted, utilities say better forecasting is their best bet for taking advantage of renewable energy. But the forecasts remain frustratingly inexact. Just after midnight on Christmas morning 2007, an unexpected wind surge hit in Colorado, a state with a lot of wind turbines. It sent power production soaring on the system operated by Xcel Energy, a utility that is trying to improve its wind forecasts.

"We were walloped," says Tom Imbler, vice president of commercial operations for Minneapolis-based Xcel. To compensate, Xcel scrambled to dial down some of its fossil-fuel power plants. Those plants "were never designed to ramp up and ramp down at the level we're asking them to" in the age of renewable energy, he says. "We're learning as we go."

Printed in The Wall Street Journal, page A14