Congestion is causing a rise in wind and solar power curtailments in the California Independent System Operator region, according to the Energy Information Administration.
The output of wind and solar generators are reduced either through price signals or through an order to reduce output, either during periods of congestion when power lines do not have enough capacity to deliver available energy or during periods of oversupply when generation exceeds customer electricity demand.
In CAISO, which operates most of the state’s power grid, curtailment is largely a result of congestion, the EIA said, noting that congestion-related curtailments have increased significantly since 2019 because solar generation has been outpacing upgrades in transmission capacity.
In 2022, CAISO curtailed 2.4 million megawatt hours of utility-scale wind and solar output, a 63 percent increase from the amount of electricity curtailed in 2021, the EIA said, citing CAISO data. As of September, CAISO has curtailed more than 2.3 million MWh of wind and solar output so far this year. Solar accounts for almost all of the energy curtailed in CAISO, 95 percent in 2022 and 94 percent in the first seven months of 2023, the EIA noted.
In most cases, CAISO tends to curtail the most solar in the spring when solar output is relatively high and electricity demand is relatively low because spring temperatures create less demand for heating or air conditioning.
CAISO has increasingly curtailed renewable generation as renewable capacity has grown in California, the EIA said, noting that in 2014, there were 9 gigawatts of wind and solar capacity in California. As of July 2023, there was 17.6 GW of wind and solar capacity, and developers plan to add another 3 GW by year-end 2024, the EIA said.
The EIA said CAISO is exploring and implementing various solutions to its increasing curtailment of renewables, including:
- Greater use of the Western Energy Imbalance Market, a real-time market that allows participants outside of CAISO to buy and sell energy to balance demand and supply. In 2022, more than 10 percent of total possible curtailments were avoided by trading within the WEIM, the EIA said, noting that a day-ahead market is expected to be in operation in spring 2025.
- Expanding CAISO transmission capacity to reduce congestion. CAISO’s 2022–23 Transmission Planning Process includes 45 transmission projects to accommodate load growth and a larger share of generation from renewable energy sources.
- Promoting the development of flexible resources, such as battery storage devices, that can quickly respond to sudden increases and decreases in demand such as battery storage facilities that can be charged with electricity that would otherwise have been curtailed.
California has 4.9 GW of battery storage, and developers plan to add another 7.6 GW by year-end 2024, according the EIA’s survey of recent and planned capacity changes.
