Flexibility is needed for California’s solar integration

May 26 was a special moment for renewable energy in California. Utility-scale solar output on the California Independent System Operator (CAISO) grid peaked at under 10 GW, and this was buoyed by nearly 4 GW of wind. This results to large-scale solar peaking at 64.6% of demand shortly after 2 PM.  In the middle of the day, the state’s gas plants ramped down to around 1 GW of total output. A total of 7 gigawatt-hours (GWh) of wind and solar power wasted over the day. The prices also dipped into the negative, like the month of May, negative prices were experienced during 9.5% of 5-minute intervals. While this was happening, the CAISO grid was still showing a net import of electricity, even during the hours of maximum solar output.

Recurring negative power prices of solar are realities on the California grid. Although negative power prices is a sign of a healthy market, curtailment is wasted power that increases rates. This displays the challenges that California is having on integrating penetrations of renewable energy. This concern grows  as the state strives to fulfill its 50% by 2030 renewable energy mandate.
 

Flexibility is key to integrate renewables energy 

Integrating wind and solar with minimal curtailment, changes must be made to sustain this system. Researchers have been working on this concern for decades. Deploying energy storage, quicker trading of electricity, and upgraded forecasting are methods assessed for resolution. A 2017 report by the Climate Policy Initiative: Flexibility, the key to low cost, low carbon grids sheds its insight. Grids with “near-total” renewable energy operate at a lower cost, as long as activities and resources were optimized towards the goal of maximum flexibility. This flexibility is required for renewables and the electricity from neighboring grids. At this time, there’s a gap  between  academic discussions of flexibility and the   practice of how grids are operated in California.
 

Moving forward with options 

CAISO focuses on the process of in-state resources being more flexible, with a mixed success. In some instances, there are restrictions with specific resources. The least flexible is the nuclear power plant, Diablo Canyon whose full output of around 2.3 GW 24/7 until it shuts down for maintenance, refueling or emergencies, and doesn’t respond to price signals. Such ramping places wear and tear on the systems plants. Diablo Canyon is scheduled to shut down in 2025 which solves this problem. In addition, the states hydroelectric fleet displays restrictions of flexibility in practice. Even on days of the highest solar penetrations, it ramps by about 1/3, rising from 2.4 GW during mid-day to 3.9 GW during the evening peak on May 26. Most of the states fleets are of smaller dams and run-of-river hydro plants. California’s fleet of fossil fuel plants are mostly of natural gas generation fill the gaps. These gas plants are vital for the evening ramp when net power demand is at its highest.

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