When the moon blots out the sun during the Aug. 21 solar eclipse, Duke Energy will face its trial by darkness.
The trouble: solar farms.
North Carolina has the nation’s second-highest power capacity in solar farms, exceeding 3,000 megawatts. With that amount of solar energy going dark, a total eclipse will be comparable to several nuclear reactors rapidly shutting down. This abrupt drop-off in electricity could cause a major disruption to the power grid, creating a surge in demand for backup power, with the potential for local power outages. But a Duke spokeswoman says the utility’s customers should see no interruption of power.
The eclipse presents an engineering conundrum that few electric companies have faced before, and will present special problems in North Carolina and California, the top two states for solar power output. The last total eclipse to pass over North Carolina took place in 1970, long before the state’s solar power boom, which now supplies Duke with enough solar energy to power 600,000 homes on a sunny day.
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Charlotte-based Duke is preparing for the planetary alignment by notifying the owners of solar farms to prepare to be temporarily disconnected in advance of the eclipse. Duke is planning to replace the electricity solar farms inject into the grid with backup power from natural gas plants.
The switch from sunshine to natural gas would go into effect only if the day of the eclipse is a brilliantly cloudless afternoon. If it’s cloudy or raining, nature would take care of the phase-out problem for Duke’s engineers.
“We’re going to simulate twilight,” said Duke spokeswoman Tammie McGee of the engineering strategy. “We’re going to simulate the gradual ramp-down of the solar generation just as it would happen at sunset.”
In California, a state that depends on solar power for 9 percent of its electricity, the state Public Utilities Commission is urging utility customers to conserve power for two hours during the Great Solar Eclipse by turning off lights and appliances in a collective effort to save 3,500 megawatts. “If millions of Californians turn off appliances and power strips to unplug from the grid during the eclipse, we can let our hard working sun take a break,” the agency said in a statement.
The eclipse will obscure the sun within 90 minutes, while an ordinary sunset in North Carolina takes five hours to incrementally reduce solar farm production from peak output to zero. The speed of the eclipse will require Duke to fire up natural gas-powered plants ahead of time to compensate for the loss of solar power.
“They’ll be sitting there in idle, ready to catch this steep ramp,” said Sammy Roberts, Duke’s director of system operations. “We’re not going to roll the dice on system reliability.”
This month’s eclipse will occur over 2 hours and 53 minutes during the afternoon, when North Carolina’s solar farms are operating at maximum capacity. The center of the eclipse will pass over Columbia, S.C., creating a 100 percent sun blockage for about two and a half minutes; Charlotte will be under 95 percent darkness and Raleigh falls into the 90 percent region.
Duke’s preparations are based on the experience in Germany, where 70 percent obscuration during a March 2015 eclipse shut down 1,800 megawatts of solar power output, Roberts told the N.C. Utilities Commission in a July 24 presentation about Duke’s preparations for the eclipse.
Duke accounts for about 2,450 megawatts of solar power connected to North Carolina’s power grid, some through direct ownership of solar farms but most through long-term power purchase contracts from independent operators, such as Chapel Hill’s Strata Solar. During the eclipse, solar output on Duke’s system is expected to drop to about 200 megawatts or less.
Strata Solar is prepared to unplug its 130 solar farms in North Carolina whenever Duke needs the assistance.
“It’s good to be cautious when there’s any chance of a sudden disruption to the plant or to the grid,” said Mike Loeser, director of operations for Strata Solar Services.
All in all, a critical mass of solar energy is not such a bad problem to have, and solar will likely not present a technological problem when the next eclipse darkens North Carolina’s skies, said Stephen Kalland, director of the N.C. Clean Energy Technology Center at N.C. State University.
“It’s not an issue that’s going to take months to resolve like pipeline repairs or the trunk line out in the Outer Banks that has to be repaired,” Kalland said. “And the next time it’ll be totally a non-issue because there will be so much energy storage out there that it’ll just be managed that way.”