Throwing Shade: The Great American Eclipse vs. Solar Power

On Monday, Aug. 21, a total eclipse will slice across the United States for the first time since 1918. It will take just 93 minutes for the eclipse to move across the entire country, and it will appear only briefly.

Since the last coast-to-coast eclipse nearly a century ago, solar generators have come to provide a small but growing piece of the nation’s energy needs, and the eclipse will at least partially obscure the sun for approximately 1,900 utility-scale plants. More than 21 GW of solar capacity will be impacted, according to the U.S. Energy Information Administration.

While the impact is minor on a broad scale—utility-scale solar provides less than 1 percent of the United States’ electricity use—utilities in specific regions see the impact as large enough to develop contingency plans.

With less than three minutes of darkness, the moon will block more than 90 percent of the sunshine in the Carolinas, driving Duke Energy to map out step-by-step demand management strategies in anticipation.

According to Randy Wheeless, spokesperson for Duke Energy, the Charlotte-based electric company expects solar capacity in North Carolina to dip from 2,500 megawatts to just 200 on Monday afternoon, affecting roughly 3 percent of electricity generation in the state.

Wheeless said Duke has been planning for the worst-case scenario, where solar output sees a sharp plunge followed by a quick uptake, straining a carefully balanced electricity grid.

“The most extreme example would be a very sunny day with low humidity—the lower the humidity, the better for solar generation,” he said. “So under that scenario we could lose around 2,000 megawatts of electricity, which is roughly equal to what would power a quarter of  a million homes.”

The issue in a scenario like this is not the availability of power, but maintaining grid stability while solar is curtailing sharply and ramping up before and after the eclipse. In an effort to keep the grid balanced, Duke has arranged for the dispatch of alternative energy sources to maintain grid reliability and fill the demand gap during the eclipse.

“The grid is kind of a balancing act, and, in a sense, you’re using electricity as soon as it’s generated,” Wheeless said. “We just have to make sure we’re balancing the power plants we have online, knowing that solar will come back online after the eclipse is over.”

And natural gas and hydroelectric power are flexible enough to step into this balancing act.

“Natural gas plants can run at 100 percent, but then they can also throttle down to run at 60 percent,” Wheeless said. “So those plants will slowly decline their output as the solar begins to increase its output.”

And while the Great American Eclipse of 2017 has required careful planning and preparation, its financial impact is expected to be minimal for the energy industry.

“You do lose solar energy that day,” Wheeless said. “And, because the operating cost of solar is so low, you do incur some expense because you have to run natural gas instead. Still, I think it’s pretty minimal overall.”