(Bloomberg) -- US grid operators are set to face their largest controlled experiment for dealing with big swings in renewable power during this week’s “ring of fire” eclipse. Skies will darken on Oct. 14, quickly knocking out as much as 28,300 gigawatts of solar generation. 

That’s the equivalent of one in nine Americans’ homes temporarily going dark as the sun becomes obscured, compared to an otherwise clear day. No grid operator is expecting power shortages and many are bolstering backup supply. But this month’s eclipse will give operators insights into what resources they’ll need ahead of next year’s even bigger solar eclipse and — more pressingly — as more renewables come online.  

The annular eclipse taking place on Saturday will make most but not all solar generation in its path unavailable because the sun will not be entirely obscured by the moon. The eclipse will move diagonally across the country from Oregon to Texas, and solar power generation along its path will be impacted. The impact will be staggered by time zones. 

The California grid may see as much as 14,500 megawatts of solar power idled at about 9:30 a.m. local time relative to clear sky conditions. About 20 minutes later in Texas, about 11,900 megawatts of solar generation may go idle. Grid-scale solar farms will be responsible for most of the dip, though about a fifth of the total power going offline includes panels on households and businesses. 

“It’s a 6,000 mph shadow that’s moving across the country,” said Electric Reliability Council of Texas Chief Executive Officer Pablo Vegas. “As that shadow moves across Texas solar farms, it’s a rapid ramp that we are going to have to manage.”

California and other parts of the country are expected to see more clouds than during the 2017 eclipse, which means not as much solar would be online anyway. The event is also taking place on a Saturday in fall, when demand is typically lower than in winter or summer or any given weekday. The impact of the solar eclipse is based on preliminary estimates. 

“The good news is we know exactly what is going to happen,” said Barbara Clemenhagen, vice president of market intelligence at Customized Energy Solutions Ltd., noting that eclipses can have major impacts on load. “Solar eclipses are something that every region will be looking at."

Solar generation predictably wakes from sleep mode as soon as the first rays of sun hit panels and then dramatically ramps up over two to three hours. The California Independent System Operator, which oversees the state’s grid, estimates that solar output will drop at a rate of 85 megawatts a minute heading into the peak of the eclipse, according to grid data. It will then ramp back up at a rate of 120 megawatts a minute as the eclipse passes because the sun will be stronger. For comparison, the maximum ramp up California sees on a regular day is 24 megawatts per minute. 

In Texas, the peak of the eclipse is near the tail-end of the ramp-up of solar power, around 11:50 a.m. Central Time, according to the Electric Reliability Council of Texas, or Ercot as the state grid operator is known. Grid-scale solar may drop to a low of about 2,200 megawatts in the hour ending at noon, or about 9,900 megawatts lower than the solar output expected during the same hour on a Sunday. 

California and Texas have the most installed solar generation in the country. Grid operators in both states are used to performing the intricate dance of managing renewables’ intermittency, including having fleets of coal and natural gas units that can come online very quickly, a growing amount of battery storage and even the ability to ask households and businesses to throttle their energy usage. All those will be in play during the eclipse. California ISO said it plans to use battery, hydro and gas-powered resources to help meet demand. 

The eclipse will have a dual impact that will be especially dramatic in those two states because not only do they have significant large-scale solar but large amounts of rooftop solar installed behind the meter. The drop in rooftop solar generation could increase demand by more than 4,800 megawatts in California near the height of the eclipse and 2,000 megawatts on Texas’ main grid, according to estimates based on public data. These numbers do not account for any consumer-owned batteries that may be able to kick in during the event. The California ISO also estimated that wind speeds could dip by 2 to 6 mph during the eclipse, which would cut wind generation by about 10%. 

The peak of the eclipse — or annularity — will last about 4 to 5 minutes. Parts of Nevada, Utah, Arizona, Colorado and New Mexico will also be in the direct path of the eclipse and face similar challenges. There are at least another 16,700 megawatts of solar installed in the broader West, according to the California ISO, which trades spot power with these regions. The eclipse will reduce solar generation by 65% to 90% there. That would mean as much as 10,900 megawatts would be idled on an otherwise clear, sunny day.  No estimates were readily available showing the eclipse's impact on this region based on geography, though. 

Even states that won’t see near-full coverage of the sun will feel the eclipse’s impacts. 

The central US grid operated by the Midcontinent Independent System Operator may see up to 1,000 megawatts go offline — or about a third of the grid’s solar capacity — all concentrated in the southern part of the grid from about 9:30 a.m. to 12:30 p.m. Central Time. While New York isn’t in the direct path of the October eclipse, the state grid operator there expects 730 megawatts of utility-scale and behind-the-meter solar will be unavailable during the eclipse. 

Ultimately, the fall annular eclipse is a test run for the full eclipse taking place next April, when the sun will be completely obscured. Texas is directly in the path of both events. The trajectory of the spring eclipse will go from the Lone Star State diagonally northeast through New England. Texas, which is in the middle of a solar-generation boom, will have even more supply impacted, Ercot data shows. 

Even though these events are predictable, the California and Texas power markets could see some price volatility because of the sheer amount of solar on the grids now versus an August 2017 eclipse. Energy prices fell across the country during the full eclipse, but then California spot power plunged to negative levels — when some generators have to pay others to take their power — as solar generation surged back online. Prices could also spike in areas if there is a brief crunch and more expensive supply needs to be brought online quickly. Luis Lugo, head Ercot power trader at commodity merchant Mercuria Energy America, expects power prices in the day-ahead market to be elevated during the eclipse, though it may ultimately be a “non-event” pricing-wise. 

“It’s probably going to be more interesting from an operational standpoint,” he added.

Eclipses are an opportunity to test demand responses and how consumers curtail their energy usage for short periods to reduce stress on the grid in a way that is predictable and controllable, according to Michael Lee, chief executive officer of power retailer Octopus Energy, which enrolls households in programs to cut usage in exchange for compensation or credits. 

“Texans have underinvested in flexible demand because we always talk about new power plant supply,” Lee said.  “This event is only for an hour, and managing millions of thermostats while people are typically going to be outside looking at the eclipse is low-hanging fruit.”

The transition to clean energy means that renewables will make up an ever-greater share of electricity generation in the coming decades. That will require finding different ways to manage the grid, including dealing with lulls in generation. While grids already quickly react to sudden swings in supply or demand by ramping back-up plants or batteries up or down, the swings are going to get far bigger with more wind and solar on the grid, said Amber Motley, director of short-term forecasting at the California ISO. 

The eclipse “is a way to see how you have increased ramp rates, which will happen as the renewable penetration grows on the system, how sources can be dispatched and show their flexibility,” she said.  “It’s a glimpse into the future.”

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