On August 21, 2017 a rare total solar eclipse will sweep across the United States, starting in western Oregon and passing southeast across the country to South Carolina. During this time, the sun will appear either partially or completely blocked by the moon, depending on your location. The “Great American Total Solar Eclipse” will be the first total solar eclipse to span across the United States since 1918. This event also marks the first time where the U.S. electric grid will be significantly impacted by a solar eclipse.
Figure 1. The path of August’s total solar eclipse. Source.
The eclipse is expected to cause a major dip in solar production for a period of hours on this day, especially on the west coast. California, for example, is expecting to lose about 6,000 MW from the grid due to the lack of sunlight, which California ISO (CAISO) is planning to make up for via natural gas and hydro generation. The Washington Post article goes on to discuss how another challenge for CAISO is ensuring the substitute generators are able to ramp up and down quick enough to handle the changes in solar generation. For instance, as the moon begins to block the sun, solar energy collection is expected to decrease at a rate of 70 MW/minute. Similarly, ramp up rates of around 90 MW/minute are expected once the sunlight begins to come back.
This total solar eclipse will mark the first one to be visible on any part of the contiguous United States since 1979, long before solar power held any share of market generation. It will also be the first solar eclipse of any kind in the United States since May 2012, and solar has grown at record rates since then. Luckily, Europe witnessed a similar total solar eclipse in March 2015 to give us a better context of what to expect. Germany, who alone accounts for ~40% of European solar capacity, saw a drop of solar output from 21.7 GW to 6.2 GW during the eclipse. Reuters also reported that to make up the loss of generation, they looked to gas, coal, nuclear and hydroelectric pumped storage energy, and that overall, Europe experienced a reduction of 17 GW of solar power during the eclipse and did an excellent job of successfully weathering the event through proper planning ahead of time.
Back in the U.S., solar power accounted for 9% of California’s generation in 2016 and the state is home to nearly half of the nation’s total solar capacity. On August 21, California is expected to lose 50 to 75% of its solar production during the five or so hours. We will then see for the first time how the United States electric grid as a whole will adapt to its first significant dip in solar energy caused by a natural phenomenon.