Where is Electricity Curtailment Happening in California? Opportunities for Underground Energy Storage

Abstract Curtailed electricity is a reduction of electricity output and leads to the loss of potentially useful renewable electricity, and therefore lost opportunities for reducing carbon dioxide emissions. The power imbalance is determined by a combination of economic factors, but most importantly demand. Other factors include the bidding strategy for electricity, and technical factors, such as limitations in transmission capacity and energy storage. The methodology to understand curtailed electricity temporally and geospatially includes data collection from publicly available sources such as the California Independent System Operator (CAISO) and directly from the Open Access Same-Time Information System (OASIS) platform. CAISO tracks the shorter-term and year-by-year patterns of curtailment and has shown that most curtailment occurs in the middle of the day in the months of February-June when solar and wind generation is high, and electricity demand is low due to moderate temperatures across California. In this work, we have identified the geo-spatial distribution of these curtailed units and the amount of curtailed electricity unit by unit. The amount of curtailed electricity is increasing dramatically year over year in California from 1.5 TWh in 2021 to 3.4 TWh in 2024, for example. The results show curtailment most strongly affects solar (photo-voltaic PV) units compared to wind farms. The units with the most curtailed electricity were identified based on rated capacity and total electricity generation. Most curtailed PV-electricity is within or adjacent to geological formations with favorable characteristics for subsurface energy storage in the form of hydrogen. Calculations show that the storage capacity of these reservoirs is more than sufficient to accommodate volumes of produced hydrogen equivalent to the amount of curtailed electricity.