Energy Blog: Noticeable Eclipse Impact on Electricity Sector

Energy Blog: Noticeable Eclipse Impact on Electricity Sector

The moon's shadow produced a dip in solar power production, but not enough to cause a detrimental effect on the electric grid.
I don’t know what the sky looked like where you were during the afternoon of April 8, but in central Indiana, it got progressively darker until the moon totally eclipsed the sun. It was quite the show. Aside from some traffic tie-ups and a crowd at the airport as eclipse tourists (like my family and me) headed home, there was little in the way of noticeable impact.

That wasn’t the case in Texas, at least in the electricity sector. According to the U.S. Energy Information Administration, the state “lost approximately 8.9 gigawatts (GW) of solar capacity during the eclipse,” as the shadow of the moon reduced the intensity of the sunlight falling on photovoltaic panels.

To get the full effect of a total eclipse, you have to be in the right place at the right time. The moon covers the entire disk of the sun only briefly, and the shadow travels from west to east so rapidly that any given spot will fall in the shadow for just a few minutes at a time. (This time around, the longest duration was around four and a half minutes, whereas the 2017 eclipse maxed out at less than three minutes.) It’s enough time for some spooky effects, such as stars and planets appearing in a now-dark sky and farm animals returning to their barns, but a four-minute blip shouldn’t make much difference to the electric grid.

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While the human impact is very much centered on the moments of total eclipse, the effect on solar power production lasts much longer. The partial obscuring of the sun’s disk is difficult for human observers to make out at first, but it begins to have an impact on solar power production almost immediately. Instead of getting the full 1,000 watts per square meter of radiation falling on a photovoltaic panel, the moon’s shadow starts reducing that over the course of nearly 90 minutes, before gradually allowing the sun to return to full strength.

Data from the Electric Reliability Council of Texas (ERCOT) shows that from a peak of around noon on the April 8 of 12.6 GW, the amount of solar power dropped to only 3.4 GW during the hour between 2:00 pm and 3:00 as the entire state was 80 percent shaded or more. By contrast, the afternoon of April 12 saw solar power production peak at 18.6 GW.

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It wasn’t just Texas. The band of at least 70 percent shading covered nearly the entire eastern part of the United States, and grids in those areas also saw temporary declines in solar power. The Midcontinent Independent System Operator, which manages the grid through much of the Midwest, saw solar power production drop from 3.8 GW to 1.3 GW over the course of the eclipse.

Fortunately, the dip in power didn’t cause any blackouts. Part of that was because the eclipse happened in April, a time of year when electric loads are light. But it also helped that ERCOT wasn’t counting on solar power. The weather forecast was for partly cloudy weather, which cuts the available light and thus solar power. Natural gas power plants were able to come online and cover the power deficit for the duration of the eclipse.

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An eclipse that followed the same path on an afternoon in August might have had more serious effects.

It’s fortunate for the power system that the next total eclipse to cross the continental U.S. doesn’t occur until 2045. Those who love watching the celestial spectacle, however, will need a passport to see another eclipse anytime soon.

Jeffrey Winters is editor in chief of Mechanical Engineering magazine.

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