California ISO Prepares for a Solar Eclipse

Robert Rapier

A solar eclipse is expected to temporarily slash US solar power. Here's how the California ISO is preparing.

The California Independent System Operator (California ISO) manages most of the high-voltage transmission lines in the state and in portions of Nevada. One of the ISO's challenges is in dealing with the intermittency of a rapidly growing portfolio of renewable energy sources like wind and solar power, which is usually considered in terms of regularly occurring events. For example, solar photovoltaic power output will drop to zero every night, and it will be reduced during times of cloud cover.

The ISO and power plant managers must also plan around intermittency caused by rare events, however, such as a solar eclipse.

A Rare Solar Event

On August 21, 2017, parts of the United States will experience a total eclipse. The so-called "Great American Total Solar Eclipse" will darken skies all the way from Oregon to South Carolina, along a stretch of land about 70 miles (113 kilometers) wide. A total eclipse is certainly a rare event. The last total eclipse that was visible from the continental United States took place in 1979, and the last time one was visible across the entire contiguous United States was in 1918.

Although California isn't in the path of the total eclipse, the entire state will experience a partial eclipse, ranging from about 60 percent in the southern part of the state to more than 90 percent in the far northern part of the state.

The Impact on California

Because California is home to 40 percent of the country's utility-scale solar photovoltaic (PV) power, the Energy Information Administration (EIA) expects that it will be the state that is most impacted by the eclipse by far. California has 8.8 gigawatts (GW) of utility-scale solar power that will be affected by the eclipse; in comparison, the second-most-affected state, North Carolina, expects an impact on only 2.8 GW of solar power.

To make matters worse, the eclipse will occur late in the morning, which is near peak generation for solar PV systems. It will also take place during what has historically been one of the hottest periods of the year, which means demand for air conditioning will be high.

The California ISO estimates that the eclipse will cause a loss of 4.2 GW of utility-scale solar power and projects another 1.4 GW will be lost from rooftop solar generation. A net load of 6.0 GW will have to be filled using other sources of generation.

California Learns From Germany

The California ISO has been planning for the eclipse for more than a year. Part of the planning included studying the grid impacts of a total eclipse in Europe in 2015. In particular, they studied the impacts on Germany's electric grid, because Germany (like California) was home to nearly half of Europe's installed solar power during the 2015 eclipse. This event was the first of its kind in Germany with the potential to have a major impact on solar PV there, and asset managers and engineers prepared well in advance of the event, according to Deutsche Welle. Through careful coordination with power plant operators across the country, Germany successfully ramped dispatchable power supplies—primarily hydropower and natural gas plants—up and down as the eclipse began and ended. Germany successfully navigated the event, so the ISO learned from its experience.

Responding to the speed at which power production will be lost is one of the challenges in managing this event. Germany managed this by having ample supplies of dispatchable power available during the event. Power is expected to be lost during the eclipse at a rate of 70 megawatts (MW) per minute. The California ISO notes that a typical ramp-up rate in the morning is around 29 MW per minute, so plant managers must be prepared for much faster ramp rates than normal. Fast-responding, flexible backup power from hydropower and natural gas plants will be called upon during the eclipse. As Germany learned, close coordination between power plants was also a key to success.

Unknown Factors

The ISO's preparations have included market simulations, modeling of the expected grid impacts, operator training, and coordination with natural gas companies in preparation for an expected surge in natural gas demand during the eclipse.

Although there has been plenty of time to plan and prepare for the eclipse, one of the biggest unknowns will be the impact of the loss of residential solar power. The exact amount of residential solar power is unknown, so the demand for backup power could be more or less than anticipated. On the plus side, the eclipse will provide a data point on rooftop solar, which will identify a more accurate system mapping.

In turn, having a better idea of the amount of rooftop solar will also help utilities better manage requirements under California's Renewables Portfolio Standard (RPS). Understanding how much solar power is actually in the system will help utilities and grid operators better model and plan for the impact of intermittency at varying solar PV penetration levels.

Although the eclipse on August 21st is predictable, its exact impact isn't. Further, while utility-scale energy storage solutions may eventually render intermittency a concern of the past, this eclipse is a good reminder of the need for backup power, even in the case of an infrequent event.


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