California has its iconic Hollywood sign, the Golden Gate Bridge, Yosemite Park’s Half Dome and the Death Valley, to name a few (photos). But when it comes to its electric grid, it is the Duck Curve that stands out as a symbolic icon – and a troublesome one at that. Since it was identified a decade ago by the California Independent System Operator (CAISO), the belly of the duck has become more pronounced, resulting in extended periods when CAISO experiences negative net load – that is (mostly) solar generation exceeding demand during sunny hours, especially in cool spring days when air conditioning is non-existent.
Historically, flexible natural gas fired plants did most of the heavy lifting during the critical ramping periods in the mid to late afternoon as the sun begins to set in the Pacific Ocean making up for the vanishing solar generation. But as the state continues to invest in utility-scale batteries, they have been assuming an ever growing role, storing the excess solar in the morning to mid-afternoon and discharging it later in the day and during the critical peak demand periods, which have progressively shifted toward the 6-9 pm. Short duration batteries, running 2-4 hours, offer the perfect cure for CAISO’s duck curve ailment.
Historically, flexible natural gas fired plants did most of the heavy lifting during the critical ramping periods in the late afternoon as the sun begins to set in the Pacific Ocean making up for the vanishing solar generation. But as the state continues to invest in utility-scale batteries, they have been assuming an ever-growing role, storing the excess solar in the morning to mid-afternoon and discharging it later in the day and during the critical peak demand periods, which have progressively shifted toward the 6-10 pm. Short duration batteries, running 2-4 hours, offer the perfect cure for CAISO’s duck curve ailment.
The inevitable recently happened when batteries became the single biggest source of generation during the evening’s ramping period. On a cool sunny day on 16 April 2024, during the evening peak period battery storage become the largest source of supply in the California grid – the first time that has happened – to the delight of geeks and non-geeks alike. The contribution of battery storage in the evening peak exceeded 6 GW, a new record, more than the contribution of natural gas, hydro, nuclear and renewables for approximately 2 hours as the batteries discharged their saved energy during the evening peak period.
Even more iconic: California’s negative net load
Source: CAISO
According to unofficial data, batteries contributed 6,177 MW around 8:10 pm, beating the prior record of 5,625 MW reached on 15 February 202
The rise of battery storage capacity has been impressive in the recent years. As recently as 5 years ago CAISO had as little as 120 MW of battery storage capacity at its disposal.
The state now has over 6.6 GW of battery storage – mostly utility-scale while adding nearly another 2 GW. By 2045, the California Energy Commission (CEC) reckons it’ll need as much as 52 GW of battery storage if it is to meet the state’s carbon-neutrality target.
Aside from utility-scale batteries and other large-scale options such as pumped hydro, California regulators and policymakers are keen on small-scale storage such as consumer-owned stationary batteries as well as the untapped storage capacity of the growing fleets of electric vehicles (EVs), vans, buses, trucks and so on. Numerous studies suggest that the combined storage capacity of these distributed resources will be significant and highly cost-effective.
Aside from the new battery storage record, CAISO has been setting and breaking numerous other records in recent weeks including a more frequent and longer number of hours when renewables supply 100% of the state’s electricity demand. These episodes, lasting from 15 minutes to 6 hours are becoming commonplace during the cool sunny spring days. In mid- April 2024, CAISO experienced a period when this happened in 30 out of 38 consecutive days. On one particularly sunny day utility scale solar output reached 17.786 GW on a cool day when CAISO load was low.
The grid operators in California and elsewhere are learning how to cope with the inherent variability of renewables as described in the following article.
This article originally appeared in the June 2024 issue of EEnergy Informer, a monthly newsletter edited by Fereidoon Sioshansi who may be reached at [email protected]"