California Increased Electricity Imports and Natural Gas Generation During Solar Eclipse
During the solar eclipse that passed across the continental United States on August 21, 2017, solar output on the California Independent System Operator (CAISO) electric system dropped while the sun was partially obscured. Much of the decrease in solar output was made up by increased electricity imports and increased generation from thermal units, most of which is fueled by natural gas.
Although California was not in the path of totality during the eclipse, meaning the sun was not completely obscured, California contains 43% of the national total for utility-scale solar and 40% of small-scale solar, (as of May 2017). Much of the state’s solar capacity is located in areas where sunlight was obscured by as much as 60%–70% during the eclipse.
Based on an average of the previous five weekdays, CAISO’s solar power output typically increases to about 9.1 gigawatts (GW) between 10:00 a.m. and 11:00 a.m. Pacific Time, or to roughly 31% of total load. On August 21, as the moon partially obscured sunlight, CAISO’s solar power output fell to a low of 3.6 GW for that hour, about 60% lower than normal.
CAISO oversees the operation of the bulk power system in much of California, operating 89% of California’s 10.0 GW of installed utility-scale solar capacity (based on data as of May 2017), including larger solar photovoltaic and solar thermal systems of at least one megawatt (MW) in capacity. In addition, California contains 5.7 GW of small-scale PV capacity (based on data as of May 2017). Electricity generation from small-scale capacity is not directly captured by CAISO but is reflected in a smaller overall load.
CAISO worked with other participants in the Western Energy Imbalance Market (EIM) to include the impact of the eclipse in their forecasts so that Tuesday’s planning accurately reflected available electricity supply. The EIM allowed for balancing across much of the Western Interconnect as the eclipse moved across the region and affected solar output in different places at different times. CAISO also worked with natural gas generators in California to ensure sufficient fuel supplies ahead of the eclipse.
CAISO planned for the rapid reduction and resumption of net load caused by the loss of solar during the eclipse by procuring an additional 100 MW to 150 MW of regulation capacity for the eclipse period. Regulation capacity is a market-based ancillary service that is used to cover short-term gaps between demand and supply.