By Jason Fordney, Tom Kleckner, Amanda Durish Cook, Rory D. Sweeney and Michael Kuser
FOLSOM, Calif. — CAISO and other electric grid operators across the country managed large and rapid swings in solar generation output Monday during the first continent-wide total solar eclipse in nearly a century.
ISOs and RTOs were well prepared for the event, especially in solar-heavy California where the obscuration of the sun took thousands of megawatts of utility and rooftop solar off the grid. CAISO had to ramp up hydro and natural gas generation as solar dropped off, then do the reverse more quickly than usual as the sun came back.
“We wanted to make sure we could make it if it was an extremely hot day, or if it was a mild day,” CAISO Executive Director of Operations Nancy Traweek said. She added that the ISO had reached out to solar and hydro operators and asked them to be prepared for the event.
The last total solar eclipse to occur in the continental U.S. was before the growth in solar power in 1979 and was viewable only from the Pacific Northwest, according to NASA. Monday’s was the first total eclipse since 1918 to span the width of the U.S.
As eyes equipped with protective glasses turned upward around the country, CAISO employees excitedly gathered outside the building, some with family members, to view the event.
CAISO said it would not be able to provide precise figures for how much solar generation dropped off its system until later this week.
“We forecasted 4,200 MW of utility-scale solar coming off. We believe that the actual will be more in the 3,000 to 3,500 MW range,” CAISO spokesman Steven Greenlee said.
CAISO data showed that the eclipse took a little more than 3,000 MW offline; in a briefing Monday morning, ISO officials said more than 3,000 MW of utility solar and 1,400 MW of rooftop solar could be lost.
Grid operators had to deal with two solar ramp-ups rather than just one.
About 10:50 a.m. PT, after totality, load was about 30,500 MW and solar generation was about 4,100 MW, with the grid stable. When the sun was nearly clear of the moon about 11:30, CAISO said load was about 29,300 MW and solar generation was about 6,800 MW. By about 1:30 p.m., solar generation in the ISO was back up to about 9,000 MW. There is about 10,000 MW of solar capacity on the ISO system.
CAISO had to manage not only the rapid loss of solar but also a steeper-than-usual climb of that resource compared with a normal day as the sun returned. CAISO predicted it would lose about 51 MW/minute, and as the blockage waned, solar generation came back at a rate of 93 to 100 MW/minute. On a normal morning, solar ramps about 29 MW/minute.
Wholesale prices briefly went negative as solar returned, as they normally do when there is excess generation on the grid. CAISO said that the 1,000-mile East-West span of the Western Energy Imbalance Market (EIM) allowed it to call on available resources as other areas ramped down.
About 860 MW of solar went off the grid in the EIM.
SPP, ERCOT See Little Impact
SPP had anticipated a peak load of approximately 45,000 MW across its system Monday but saw demand about 2,500 MW below that as air conditioning usage dropped and manufacturing facilities closed while employees observed the eclipse.
“In preparation for the relatively sudden and not entirely predictable drop in load, SPP utilized its day-ahead market processes beginning Aug. 20 to commit adequate reserves to accommodate load swings and the resulting impacts to frequency and interchange,” SPP said. The RTO increased its regulation service in preparation. An eclipse also slows wind speed by cooling air, causing a 1,200-MW swing in the RTO’s wind generation that also had to be managed.
“By increasing our regulation requirements, we essentially ‘widened the lanes’ of our system and operated more conservatively than we might have on a normal day to accommodate any unpredictable occurrences during this rare event,” Director of System Operations CJ Brown said.
This was a great learning opportunity for SPP,” said Vice President of Operations Bruce Rew. “And I’m proud that our staff and systems were able to ensure that, despite so many variables and the rarity of the solar eclipse, it was essentially a non-event electrically speaking.”
Utility-scale solar in the ERCOT system dropped from a peak of 760 MW to a low of 299 MW during the eclipse, while total system load dropped from 60,824 MW to 60,163 MW. The ISO said a number of factors could have contributed to the load decrease, including reduced air-conditioning demand.
Duke Loses 1,700 MW in NC
In North Carolina, Duke Energy reported that it lost about 1,700 MW of capacity during the height of the eclipse. “Given the weather conditions, we should have expected 1,808 MW of solar output during the afternoon. But at the height of the eclipse, we were getting only about 109 MW,” said spokesman Randy Wheeless.
North Carolina is the nation’s No. 2 state for solar capacity, with 2,500 MW connected to the Duke system.
Peak demand for Duke Energy Carolinas and Duke Energy Progress in North Carolina is around 22,500 MW on a typical summer day.
MISO has no Issues
MISO said it navigated the eclipse without reliability problems as it crossed its 15-state footprint, but operators did see a significant drop in load.
“Around 1:15 p.m. ET, demand for electricity in the region flattened out and then dropped during a two-hour period as the moon passed in front of the sun. Load began steadily increasing after 3 p.m.,” said spokesman Mark Adrian Brown. “Cooler-than-expected temperatures likely contributed to the drop in load as storms rolled through the Upper Midwest Monday afternoon. Decreased solar generation during the eclipse did not have a major impact on the numbers.”
Currently, MISO has about 180 MW of grid-scale solar and an estimated 350 MW of distributed solar in its footprint.
The RTO said before the event that it would be monitoring its distributed generation and learning lessons for the eclipse on April 8, 2024, when solar will make up more generation in the region.
Clouds in PJM
In the eastern half of the country, cloud cover and rain dampened the eclipse’s effects. At PJM headquarters in Valley Forge, Pa., more than 50 people filtered through an onsite auditorium to try and view the eclipse as it passed across the continent and approached its footprint, the RTO said.
Peak load was expected to be 137,800 MW on Monday, with temperatures near 90 degrees Fahrenheit across much of the Mid-Atlantic.
PJM saw grid solar generation drop by about 520 MW from before the eclipse until its peak. Behind-the-meter solar dropped by 1,700 MW. Solar represents less than 1% of PJM’s 185,000 MW of generation capacity.
The RTO had expected the drop in solar production to result in an increase in net load. But “because of a variety of potential factors, including reduced air conditioning, increased cloud cover and changes in human behavior related to the event,” it saw a net decrease in demand of about 5,000 MW during the eclipse.
Temperatures dropped by an average of 2 degrees Fahrenheit, with the Chicago area hit by storms after the eclipse began.
“Substantial cloud cover largely obscured the event at PJM’s offices, but stakeholders and staff gathered outside with special glasses and homemade viewing apparatuses to catch whatever views were available,” PJM said. The grid operator carried about 1,000 MW of regulation service instead of the usual 800 MW.
PJM will use lessons from Monday’s event for April 8, 2024, when the RTO’s footprint will be in the path of a total eclipse between Texas and Maine.
Minimal Effects in New England
ISO-NE had sufficient resources available to meet the rise in electricity demand resulting from a drop in output from the region’s 2,000 MW of solar PV systems during the partial eclipse. New England saw peak obscuration at around 2:45 p.m., when the moon blocked about 65% of the sun. Skies were generally clear across the region during the eclipse.
ISO-NE reported in June that PV generation would face a less extreme reduction in output because the angle of the sun is lower in late August than earlier in the summer, and the eclipse would occur almost two hours after the solar noon peak.
“To precisely balance electricity supply and demand minute-to-minute during the partial eclipse, ISO system operators must consider three major factors that will affect PV output,” said the report: obscuration percentage, angle of the sun and cloud cover.
The grid operator cited human behavior as another factor that could dampen the dip in solar output: “When there’s an eclipse, people typically stop what they’re doing and watch,” which lowers demand for electricity, it said.
New York not Fazed
New York experienced the partial eclipse under clear skies. NYISO said it had minimal impacts on electric load and that it did not need to take any special transmission operating actions.
NYISO Vice President of Operations Wes Yeomans on Friday posted a YouTube video in which he explained that peak totality of roughly 80% would be strongest from 2:30 to 2:45 pm.
New York has approximately 850 MW of rooftop solar, but solar generation peaks at 625 MW because the panels are not aligned in the same direction, Yeomans said. Solar output peaks between noon and 1 p.m. on very sunny days.
The last significant solar eclipse in New York occurred on May 10, 1994, when there were very few solar devices in the state.