Sunny California will see its skies darken for about three hours in mid-October by a rare annular eclipse, and officials now are working overtime with regional authorities, as well as natural gas interstate and intrastate suppliers, to ensure there are adequate resources available to make up for the expected solar shortfall.
An all-hands effort is underway, led by the California Independent System Operator (CAISO), for the partial eclipse, which is set to darken large swaths of the state beginning at 8 a.m. PT on Oct. 14. As the eclipse moves east, maximum obscuration in California is estimated to arrive at 9:30 a.m.
“On the eclipse day, partial obscuration of the sun will reduce output of rooftop solar and increase load by 4,843 MW or 29.6% at 9:15 a.m. relative to normal clear sky conditions,” according to CAISO.
In the state’s footprint, more than 14,350 MW of behind-the-meter (BTM) rooftop solar capacity is installed.
“California has an ambitious renewable energy goal, with a mandate for getting half of its electricity from renewable sources by 2030,” grid officials noted. “With approximately 10,000 MW of utility-scale solar production, solar covers as much as 30-40% of the grid’s energy needs on some days.” About 6,000 MW in BTM rooftop capacity also is installed.
“Since the eclipse occurs on a Saturday, load levels are lower than if the eclipse occurred on a weekday. Typically in October, the marine layer can serve to reduce rooftop solar output during the hours of the eclipse, so results from the eclipse clear-sky scenario should be viewed as a high impact scenario.”
No Sun? No Problem For Natural Gas
Natural gas, while a smaller share of California’s generation load, will play a role in providing reliable resources, state officials said.
CAISO is setting in place plans to work with “interstate and intrastate gas suppliers to ensure the availability of sufficient supplies during the solar eclipse.” It also is coordinating with thermal generators “to ensure they have procured enough gas to handle generation deviations during the day of the solar eclipse.”
In addition, the grid operator plans to analyze the impacts of generator and transmission outages “prior to approving outages for Oct. 14 to ensure needed resources are available to replace the loss of solar capacity.”
How much of the sun is covered will vary relative to the eclipse path, said CAISO senior energy meteorologist Jessica Stewart and lead forecast modeler John Rudolph of the Short-term Forecasting Unit.
“Most areas within the Western Energy Imbalance Market (WEIM) will experience obscuration between 65-90%.” The WEIM is composed of 22 participants and 21 balancing authority areas, including CAISO. The real-time wholesale energy trading market enables participants to buy and sell energy when needed.
Here Goes The Sun…
Unlike a total solar eclipse, where the sun is 100% covered, the moon will obscure only a portion of the sun later this month. However, as California has sharply boosted its solar capacity, the annular eclipse will take out a considerable amount of generation load.
“Rapid growth in grid-scale and distributed energy resources have transformed the western electricity market in recent years,” Stewart and Rudolph noted. “With more weather-driven supply resources, accurate short-term forecasts and uncertainty products have become increasingly important for managing this additional complexity as the West transitions toward a more carbon-free grid.”
A total solar eclipse occurred on Aug. 21, 2017, which reduced California’s utility-based solar production of 6,000 MW, or about two-thirds of typical levels. It also led to the loss of BTM production of about 1,460 MW.
Since then, California and western grid operations “have changed considerably,” said Stewart and Rudolph. In CAISO alone, “grid-scale and rooftop BTM solar capacity have grown by about 65% and 150%, respectively…And other western states have significantly increased their solar capacity over that same time period.
“This means that the upcoming eclipse will have a larger impact” on CAISO and most of the 21 WEIM participants versus 2017. Planning efforts are considered essential “to manage the potentially steep ramping conditions in renewables generation and load during the eclipse.” The term “ramping” refers to the rate of change in power flow over a given time period.
“Assuming clear-sky conditions at the time of the eclipse maximum, the grid-scale solar output will be reduced by 9,687 MW, or about 75% of our usual available capacity, compared to a sunny October day at the same time,” the CAISO forecasters said.
“From the eclipse maximum to the eclipse end, solar production will increase by 10,800 MW over the course of 90 minutes. This is an increase of about 120 MW of grid-scale solar per minute – 10 times the normal rate of solar increase, or ramp.”
Following the typical morning peaking period, the eclipse is expected to cause a second ramping period from 8:15-10:45 a.m. During the ramp-up period from 8:15-9:15 a.m., load is forecast to increase by 2,374 MW. The subsequent ramp-down suggests the load will drop by 6,643 MW from 9:15-10:45 a.m.
Texas Prepping Too
The Electric Reliability Council of Texas (ERCOT), the grid operator for 90% of the state, also is preparing for any challenges the annular solar eclipse may bring.
The most significant impacts in Texas are expected between 10:15 a.m. and 1:40 p.m. CT, according to ERCOT. Total system photovoltaic generation resources (PVGR) are likely to see the clear sky capability reduced to a minimum of 13% at the time of maximum impact. Typically, PVGRs experience a maximum coverage of sun from 76-90%, according to the grid operator.
“ERCOT is working with solar forecast vendors to ensure the forecasting models account for the impact of the eclipse,” said ERCOT’s Luke Butler of the Resource Forecasting and Analysis unit. “ERCOT will pre-posture the system as necessary to meet both the down and up solar ramps and use ancillary services for additional balancing needs.”
In the days leading up to the eclipse, the electric grid operator plans to assess sufficient capacity margin to determine whether changes are needed.
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