Intermittent Nature of Green Power Is Challenge for Utilities

Herb Swanson for The New York Times

Wind turbines atop the ridge line of Lowell Mountain in Vermont. The wind farm is owned by Green Mountain Power.

The 21 turbines at the Kingdom Community Wind farm in Vermont soar above Lowell Mountain, a testament in steel and fiberglass to the state's growing use of green energy.

Except when they aren't allowed to spin at their fastest. That has been the case several times in the farm's short existence, including during the record July heat wave when it could have produced enough much-needed energy to fuel a small town. Instead, the grid system operator held it at times to just one-third of what it could have produced.

"We were being told to turn on diesel-fired units that are very expensive and dirty and told to ramp down what is renewable, cost-effective energy for our customers," said Mary Powell, chief executive of Green Mountain Power, the utility that owns and operates the wind plant. "We should go with the sources that can have the highest value, especially during peak times."

It is not the first time the grid system operator, ISO New England, which operates in six states, has cut back energy from the farm since it began operating at the end of last year, or from others in the region, including some in Maine and New Hampshire. Other windy states and regions like Texas and the Midwest have experienced similar cutbacks, known as curtailments.

But the recent Vermont episode, which set off a debate among government officials, the New England grid executives and the wind farm producers, highlights a broader struggle taking place across the country as utilities increasingly turn to renewable sources of energy. Because energy produced by wind, for example, is intermittent, its generating capacity is harder to predict than conventional power's. And a lack of widely available, cost-effective ways to store electricity generated by wind only compounds the complex current marketplace.

Moreover, although the wind industry has been growing for decades, it is still relatively new at operating large-scale wind farms, so it is often only once a farm is up and running that some of the problems emerge, developers say.

And those problems are likely to grow. Last year, wind power was the most prevalent source of new energy capacity — 43 percent of overall generation installed — while its price neared an all-time low, according to a recent report for the Department of Energy by Lawrence Berkeley National Laboratory.

A number of factors can trigger curtailments in wind output, including reducing the danger to bats or birds flying around the spinning blades. But more commonly, regional grid managers, who must match demand and supply instantaneously, call for a reduction in wind power when more energy is produced than the system can safely transport, they say.

Indeed, in New England in recent months, the grid system operator has cut back power from wind and hydroelectric plants several times, generally, its representatives say, because they were making too much electricity. New wind farms are frequently located in sparsely populated areas or along mountain ridges where there has not been a need for transmission lines with a robust carrying capacity, officials say.

In addition, it is more difficult to properly synchronize wind's fluctuating power flow with a system built for the steadier electric stream that fossil fuel plants tend to produce.

"As these wind turbines are built and interconnected, they're interconnecting to local transmission lines, and for the most part they can produce electricity and inject it onto the system," said Ellen Foley, a spokeswoman for the grid operator. "But there may be occasions where they may have to be backed down or curtailed because of the limitations of the line — it's literally the size of the line."

The fear, she said, is that a surge in energy could overload and shut down the wires, leading to a drop in voltage on the system that could spread blackouts through the region and to other parts of the country.

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