Behind the meter, we’re ducked.
In colder parts of the country, March comes in like a lion. Here in California, March comes in like a duck. Days are often sunny but not so hot. Solar electricity generation levels are high, but cooling demand is low.
Ten years ago, some prescient energy analysts at CAISO were thinking about how an increase in wind and solar generation might impact grid operations on sunny-but-cool California days. They projected electricity demand net of wind and solar generation (aka “net load”) in a future with increasing solar PV penetration. These projections gave rise to a now-familiar duck:
The CAISO duck curve caught people’s attention because it brought some looming renewable energy integration challenges into clearer focus. One of these challenges lies in the duck’s growing belly. As solar PV penetration increases, net load can bump up against the minimum generation levels of other grid-connected generators. If grid operators can’t find enough demand to soak up the sun, they have to curtail the excess solar generation.
Ten years later, these once-hypothetical over-generation challenges are here for real. Spring after spring, we set new records for renewable energy curtailment. Last April, CAISO reports that a record 596,175 MWh was curtailed. That’s like producing an entire extra day’s worth of electricity in April and then throwing it away.
It’s important to note that solar curtailment need not be a sign of inefficiency. On a highly decarbonized grid, there will be times when our solar supply exceeds our ability to cost-effectively soak it up. But it seems likely that the rates of solar curtailment we’re seeing today are higher than they need to be. So California has been scaling up its efforts to deal with these duck belly blues.
As we head into the 2023 duck season, I’ve been super-curious to see how these efforts have been impacting solar curtailment. This week’s blog post takes a preliminary look at how the duck charts are shaping up this year. And asks how three key curtailment-mitigation strategies are playing out.
A daily duck comparison
Since April 2018, CAISO has been posting – in real-time– hourly demand for grid electricity and the corresponding net load over the course of the day. These data can be used to track our renewable energy integration progress. This year, our March weather has been more lion than duck. But we’ve had a few sunny days. The graph below compares this “daily duck” data from last week (Tuesday, March 7) against a similar (temperature-wise) March day four years ago (Thursday, March 7, 2019).
This comparison is a fast-and-loose way to assess how our renewable energy integration challenges are unfolding. That said, you can clearly see that the mid-day dip in net demand (in purple) has been growing as utility-scale solar PV generation – shown in yellow—has been increasing. Last Tuesday, PV generation was more than 1,500 MW higher in the middle of the day as compared to March 7, 2019.
What these graphs do not show is solar curtailment. CAISO has been releasing daily reports on wind and solar curtailment since 2019. Over 22,000 MWh of solar was curtailed last Tuesday. This is an order of magnitude higher than the solar curtailment we saw in early March of 2019.
It’s discouraging to see solar curtailments rising fast. But perhaps we’d be curtailing even more if not for our efforts on three key fronts: batteries that can store excess supply for later use; regional market integration to help export excess renewable generation to places that can use it; and demand-side efforts to shift/increase load in the duck belly hours.
Battery storage to the rescue?
Solar PV capacity has increased significantly over the past five years. So, too, has battery storage! CAISO posts sub-hourly data on battery storage, so we can see how these batteries are being charged and discharged on ducky spring days. The graph below shows how batteries are assisting with duck belly management.
The blue line tracks battery storage activity in March 2019. Nothing to see here. The orange line summarizes data from last Tuesday. Very cool! You can see batteries storing electricity in the belly of the day and discharging them when the sun goes down.
To put this 2023 battery storage activity into perspective, the dashed yellow line plots the difference in daily utility-scale solar PV across the 2019-2023 March days. In this two-duck comparison, 2023 battery storage goes a long way toward offsetting the increase in utility-scale solar generation. Looking ahead, we should see batteries playing a growing role in mitigating curtailment.
Enhanced grid integration holding steady
As regional grid operations become more integrated, it should be getting easier to export our excess solar if and when there’s excess demand outside of California. The graph below tracks estimates of curtailment avoided in the increasingly integrated Western interconnect, thanks to the Western Energy Imbalance Market (EIM).
Avoided curtailment has been pretty steady over time. I found this a little surprising given that the WEIM membership/market size has been increasing over this time period. However, a limitation of the WEIM market is that participating utilities only make a fraction of their transmission capacity available for the generation re-optimization that occurs through the market. This can put a binding constraint on California’s renewable energy exports. With the newly approved Extended Day-Ahead Market (EDAM) participants will make all their transmission capacity available, and I would expect renewable curtailments to decrease further.
Losing ground on the demand side?
Another curtailment mitigation strategy involves “load shifting” into the belly of the day – think smart EV charging and pre-cooling your house. The graph below plots March 7, 2019, and March 7, 2023 grid-electricity demand profiles (i.e. the two green lines from our duck comparison above) in the same graph. What emerges from this comparison is a behind-the-meter (BTM) duck! Electricity consumption net of rooftop solar production is dipping ever-deeper mid-day.
What I take from this picture is that any successes we’ve had with load shifting into middle-of-the-day have been more than met by the increase in rooftop solar. As more BTM solar is installed, reductions in demand for grid electricity in the middle of the day will increase utility-side solar curtailment.
More battery capacity is coming online in California which will help reduce renewable energy curtailment. Enhanced regional grid coordination should also help us export when we’re over-supplied. But progress behind the meter will remain underwhelming so long as consumer-facing electricity prices offer limited incentives to move flexible demand into the belly of the day.
California is thinking about smarter and more dynamic retail rates that could help coordinate renewables integration more efficiently. More dynamic retail rate structures will be somewhat more complicated for consumers…but critical! We need all hands on duck (last duck pun, I promise) to tackle our mounting renewable energy integration challenges.
Suggested citation: Fowlie, Meredith, “California’s Duck-Belly Blues”, Energy Institute Blog, UC Berkeley, March 13, 2023, https://energyathaas.wordpress.com/2023/03/13/californias-duck-belly-blues/
Keep up with Energy Institute blog posts, research, and events on Twitter @energyathaas.