Distributed Energy Resources

Calif. sets record for solar, renewable curtailments

California recently set two related but seemingly contradictory energy records.

On June 1, the California Independent System Operator (CAISO) set an all-time peak for instantaneous solar generation, 11,363 MW. The previous peak, set on May 4, was 11,358 MW.

In May, the CAISO also curtailed a record amount of solar and wind power, shutting off 223,195 MWh. In the first five months of the year, CAISO curtailed 630,864 MWh of wind and solar generation compared with 287,057 MWh in the same period in 2018.

When there is too much renewable energy coming into the CAISO system, particularly solar power in the middle of the day, the grid operator signals operators to turn off or turn down their plants to avoid overloading the electrical system.

Curtailments generally rise in the “shoulder” months of the spring and fall, CAISO spokeswoman Anne Gonzales said. April and May often see high levels of curtailments, for instance, because temperatures are not low enough to prompt a lot of people to turn on their heaters or high enough for them to switch on their air conditioners. Curtailments generally decline in the summer and winter when heating and cooling demand is higher.

That historical pattern is likely to persist, she said.

Another factor in the historically high levels of curtailments is a comparatively high level of hydropower generation. Lat year, California was going through a drought, leaving a generation gap that was able to be filled, at least partially, by wind and solar power. A wet and snowy winter has helped relieve that drought and left less of a role for other renewable resources.

California has been adding renewable resources to its electric power system at a rapid pace in order to meet the state’s environment goals. In the near term, California has a target of achieving 60% renewables by 2030. Longer term, the state has a target of achieving 100% clean energy – meaning that hydropower and nuclear power could count – by 2045.

Generating a record amount of solar power and dumping renewable power might seem contradictory. Why build more solar power plants, for instance, if so much power is being dumped or wasted?

From a policy perspective, however, it is not so clear how much of a problem curtailment poses. A May 2018 study the MIT Technology Review argues that curtailment could hamper California’s ability to meet its environmental goals. Over supply of power suppresses wholesale power prices, reducing the incentive for developers to step up and build more renewable power projects. Expressed another way, the more renewable energy resources come on to a system the less valuable that energy becomes.

In California, at least, that problem becomes even more complicated by the fact that the state’s build-out of renewable resources is driven in large part by programs and incentives that are funded by retail power prices. The result is that “the retail price is paying for excess capacity that’s driving the wholesale price down,” James Bushnell, an energy economist at the University of California, Davis, said in the MIT study.

But another scholarly article presents a different perspective. In a recent Solar Energy study, the authors argue that rather than a last resort measure to be avoided, curtailment is “critical to achieving intermittency mitigation and delivering firm PV generation at the lowest cost.”

The authors argue that there is a premium to be paid to transform low cost, intermittent solar power into a firm, dispatchable power source. The fundamental ingredient to minimizing that premium is overbuilding and, when necessary, curtailing solar power installations, they say. That strategy could enable solar power resources to be built for costs comparable costs of current conventional generation sources. They also note, however, that “marginal PV remuneration systems in place today may not be ideal to foster this lowest cost solar potential.”

On its website, the CAISO calls curtailment “an acceptable operational tool” and says, “oversupply conditions are expected to occur more often” as increasing amounts of renewable energy come on to its system. Nevertheless, the grid operator says it is seeking solutions “to avoid or reduce the amount of curtailment of renewable power to maximize the use of clean energy sources.”

One solution that has been presented would be to increase energy storage resources, but some experts say that energy storage is still too expensive to provide a solution on the scale needed.

Another solution that has been put forward to address curtailment is the creation of a regional grid that would tie California’s wholesale electric market with others in the Western United States. Proponents argue that by creating a larger, unified balancing market, excess renewable generation in one region could be shipped to another region with demand sufficient to use that renewable energy.

California has taken a lead in efforts to create an interconnected Western regional grid, but the process has been slow, complicated by the number of separate entities – many of whom are wary of giving up local control – that have to come together to make the concept a reality.