California’s solar and wind integration challenge
As a leader in the global energy transition, California is putting some of the highest levels of solar and wind on its grid in the world to date. And while the state’s grid operator has made some progress, the integration of these resources is currently limited not by physics, but by market rules and operational practices.
May 26 was a big day for renewable energy in California. Utility-scale solar output on the California Independent System Operator (CAISO) grid peaked at just under 10 GW, and this was buoyed by nearly 4 GW of wind. This led to large-scale wind and solar peaking at 64.6% of demand shortly after 2 PM, a new record.
During the middle of the day, the state’s gas plants ramped down significantly, falling to around 1 GW of total output. Even with this, as much as 1 GW of solar and wind was curtailed, for a total of 7 gigawatt-hours (GWh) of wind and solar power wasted over the course of the day. It is likely that prices also dipped into the negative, as during the month of May negative prices were experienced during 9.5% of 5-minute intervals, the highest portion in 2018 so far.
As all this was happening, the CAISO grid was still showing a net import of electricity, even during the hours of maximum solar and wind output.
A similar dynamic was seen a month earlier, when large-scale wind and solar set its previous record of 63.8% of all power. These were exceptional days, as weekend days in the Spring bring a combination of low overall power demand and high solar generation. But they were not the days of highest solar output, as large-scale solar now peaks at over 10 GW regularly. And this does not count the multiple GW of rooftop and other “behind-the-meter” solar, into which CAISO has no visibility.
Recurring negative power prices and curtailment of wind and solar are both realities on the California grid. And while negative power prices can be a healthy market signal, curtailment is wasted power, and increasing rates of both show the difficulty that California is having integrating large penetrations of renewable energy. This challenge will only get more significant as the state moves to meet its 50% by 2030 renewable energy mandate.
Flexibility
It is not enough to simply add wind and solar to a grid with physical and market structures designed for the characteristics of conventional power. In order to integrate wind and solar with minimal curtailment, changes must be made.
Researchers have been working on this issue for decades, and there is a significant body of technical literature describing how this can be done. There are many ways to skin this cat, including deploying energy storage, more rapid trading of electricity, and enhanced forecasting – all of which have been the subject of reports by the U.S. Department of Energy’s National Renewable Energy Laboratories (NREL) that look at California and the Western United States.
And while deploying massive volumes of batteries and building hundreds of miles of transmission is an expensive undertaking, many of the other measures to enable this transition are not.
This point was underlined in a 2017 report by the Climate Policy Initiative: Flexibility, the key to low cost, low carbon grids. This report found that grids with “near-total” renewable energy could operate at a lower overall system cost, as long as activities and resources were optimized towards the goal of maximum flexibility in the rest of the system. This flexibility is needed not only in the region where renewables are added, but also in the import and export of electricity from neighboring grids.
Unfortunately, there can be a lot of distance between these academic discussions of flexibility and the actual practice of how grids are operated, particularly in California.
Local flexibility, or lack thereof
In looking at the situation of curtailment and negative prices, writers including Vox’s David Roberts have concluded that California has “too much solar power”. This is an intellectually lazy take on a complex problem, as it fails to consider the role of other resources on California’s grid.
CAISO has been working to make the operation of in-state resources more flexible, with a mixed degree of success. In some cases, there are limitations with specific resources. The least flexible is California’s last remaining nuclear power plant, Diablo Canyon. Diablo Canyon runs at its full output of around 2.3 GW 24/7 until it shuts down for maintenance, refueling or emergencies, and does not respond to price signals including negative power prices.
Diablo Canyon is hardly unique. pv magazine has written in detail about why nuclear power plants are not flexible in practice, even when they have the technical ability to ramp. Not only does the high capex / low opex nature of nuclear power provide a powerful disincentive to produce at less than 100% output, but more importantly such ramping puts wear on the systems of these plants. At best this leads to increased maintenance costs and downtime. Diablo Canyon is scheduled to shut down in 2025, which will remove this problem.
California’s hydroelectric fleet is also showing limited flexibility in practice. Even on days of the highest solar and wind penetrations, the fleet is ramping only by about 1/3, rising from 2.4 GW during mid-day to 3.9 GW during the evening peak on May 26.
This is counter-intuitive given the experience of other nations and regions such as Scandinavia, Uruguay and Costa Rica, where the flexibility inherent in hydroelectric dams has enabled some of the world’s highest penetrations of wind and of renewable energy overall.
However, the majority of California’s fleet is not comprised of large hydro dams, but of smaller dams and run-of-river hydro plants. CAISO notes that the latter has a “fixed generation profile”, and is not dispatchable.
Given the relative inflexibility of these other two resources, California’s fleet of fossil fuel plants, which is almost exclusively natural gas generation, helps to fill the gaps. These gas plants are particularly critical for the evening ramp when net power demand is at its highest.
