PJM Describes April 8 Eclipse Impacts

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PJM maintained reliable grid operations while managing significant impacts to solar energy production during the total solar eclipse that directly crossed over the PJM region April 8.

The PJM update (PDF) provided at the May 1 Market Implementation Committee showed how the eclipse affected electricity generation and demand as the moon obscured the sun that afternoon. Darkening of skies obscured grid-connected solar generation and increased electricity demand due to the lapse in rooftop solar and minor effects of consumer behavior.

Under partly cloudy skies, the 122-mile-wide “path of totality” briefly obscured the entire sun in much of Ohio and parts of western Pennsylvania. The eclipse dimmed skies to lesser degrees throughout the PJM region.

Solar Generation Decrease Exceeded Forecast

On the generation side, PJM observed losses 6,200 MW of metered or grid-connected solar output, compared to the day-ahead forecast loss of 4,800 MW. This was because the day was less cloudy than forecast, and the amount of solar generation was therefore higher than predicted before the eclipse.

“Because of this higher solar generation output, we saw a larger decrease as we headed into those eclipse hours,” said Michael Stewart, PJM Sr. Engineer – Load Forecasting

Overall, the sunnier-than-forecast conditions also resulted in a larger eclipse-driven decrease from behind-the-meter (rooftop) solar to 4,900 MW compared to the 4,200 MW forecast. Because rooftop solar production is not connected to the PJM grid or metered by PJM, the decline during the eclipse event had the effect of increasing demand from the customers normally served by their rooftop solar.

Demand Decreased Due to Temperature, Human Behavior

PJM’s forecast accurately captured expected impacts to load, Stewart said, particularly during the eclipse period between 1:50 p.m. and 4:20 p.m. That included a drop in demand based on cooler temperatures and changes in consumer behavior, with many school closings and increased travel by eclipse tourists, especially in the path of totality.

PJM doubled regulation reserves to prepare for the eclipse to control frequency during sudden changes in electricity generation and demand. Gas combustion turbines were dispatched to provide fast-start power to compensate for dropping solar and wind power. As events unfolded, PJM extended procurement of additional regulation reserves for an additional hour to ease the generation transition in the waning hours of the eclipse event.

System frequency “was maintained very well throughout the event,” said Kevin Hatch, PJM Sr. Manager – Dispatch. “This was a historic event. We were set up very well with the forecast going into this. We followed our plan very closely. We adjusted throughout the event as needed, and we appreciate all of the coordination from our members.”