CAISO Curtailments

Wind and solar curtailments by month.

The California Independent System Operator said 2019 was a "banner year" for integrating renewables, but the influx of clean energy is not without problems—data also showed that the peak level of curtailments of wind and solar surged by about 74 percent from the year before.

CAISO said a new renewables record was set last year when 80.3 percent of electricity supply came from renewables on May 15 at 2:45 p.m. At the time, nonrenewables served just 20.1 percent of demand, while utility solar served 51.1 percent, wind 18.8 percent, geothermal 4.8 percent, small hydro 2.7 percent, biomass 1.6 percent and biogas 0.9 percent. The new renewables mark surpassed the 73.9-percent peak of supply set on May 26 of the previous year. CAISO hit a historic solar generation peak of 11.473 MW on July 2, 2019, and a historic wind generation peak of 5,309 MW on May 8.

Many renewables are being added to the grid to meet California policy goals such as SB 100 and other efforts to move toward zero-carbon resources. This has also created a need for faster-ramping resources to fill the gap as demand rises in evening hours and utility and behind-the-meter solar begins to wane.

The peak for curtailments of wind and solar surged to 223,195 MWh—in May 2019—from a peak of 94,778 MWh in March 2018, a 74-percent increase. CAISO refers to curtailments, in which the market automatically reduces the production of renewables during periods of surplus energy, as "an acceptable operational tool." But it said it is seeking solutions to curtailments, including more energy storage and demand response; time-of-use rates that charge consumers higher rates during peak demand times; expansion of the Western Energy Imbalance Market; better regional coordination; electric vehicles that can respond to changing grid conditions; reduced minimum operating levels for existing generation; and investment in more flexible resources.

Curtailments also surged dramatically in January of this year compared with the same month last year, to more than 138,000 MWh, compared with about 12,800 MWh in January 2019. Of the January 2019 total, more than 126,000 MWh were "local economic" curtailments, and about 11,329 MWh were "system economic," with about 280 MWh classified as "local self-scheduled."

Local economic curtailments refer to market dispatch of generators with economic bids at times of oversupply and congestion, while "system economic" curtailments are bids to mitigate systemwide congestion. Local self-schedules are used to mitigate local congestion. Congestion is when the available, lowest-priced energy cannot be delivered to a load point because there is not sufficient transmission capacity.

There were "many factors" that contributed to the year-over-year increase in January curtailments, CAISO spokeswoman Anne Gonzales told California Energy Markets in a Feb. 6 email.

"Curtailments depend on an interplay of demand level, must-run hydro, weather conditions that impacts load, wind and solar production, and the increase of renewables on the grid," Gonzales said. Curtailments are rising in general as wind and solar are added. "This January in particular there was transmission congestion in the system, which doesn't necessarily follow the same seasonal pattern of renewable resources, but is dependent on specific transmission conditions."

Peak demand in 2019 in CAISO was 44,301 MW, set Aug. 15 at 5.50 p.m., a time at which renewables served 26.5 percent of load, natural gas 42.4 percent, imports 14.9 percent, nuclear 5.1 percent and large hydro 11.1 percent, data shows. The peak was slightly lower than the 46,427-MW demand peak set on July 25, 2018, at 5:33 p.m.

The EIM has provided significant value in avoiding curtailments, according to a recent study commissioned by the WIRES group, but the rate of renewables additions over the past year is "far outpacing" the ability of the EIM to absorb them.

"If renewables continue to be overbuilt at increasing rates, they will drive real-time prices lower, distorting price signals in the market," says the report by ScottMadden Management Consultants. Also, diminishing economics of renewables could lead to projects not getting built. Transmission, demand response and energy storage are zero-emission solutions to those risks, according to the study.

There are more than 11 GW of utility-scale and 6.5 GW of distributed solar in the CAISO footprint, with distributed solar due to grow to more than 12 GW by 2022. Renewables outside the state represent 25 percent of total renewables capacity reported as qualifying to meet California's renewables portfolio standard requirements. There are six interregional transmission projects being considered to move out-of-state renewables into California, the report says, and congestion continues to be an issue across California, driving a need for new transmission in some areas.