The
duck curve is a term used to describe electricity demand during a 24-hour period after subtracting out solar and wind generated power during the middle of the day. Figure 1 shows an example duck curve for the California Independent Service Operator (CAISO) that manages the electrical grid in most of the state of California. The curve is for a spring day and shows electricity demand less renewable solar and wind generation for 2012 and 2013 (actual) through 2020 (projected).
The CAISO public affairs group first came up with this graphical device in 2012 to illustrate the issues the grid operator was facing with the rapid influx of solar photovoltaic (PV) generation into California's electricity mix. With a little imagination one can see the shape of a floating duck (see Video 1). The most obvious feature is the "belly" created by loss of demand during peak daylight hours as PV generation hits the grid. The "tail" is caused by the ramp up in demand in the early morning hours when people start waking up and using electricity. And the "head" is caused by the increase in the evening when the sun goes down, PV generation drops off, and people arrive home after work and begin cooking, watching television, and using appliances. The rapid increase in wind generated electricity starting about 2008 and solar about 2013, as shown in Figure 2, has created the ever-deepening belly in the duck curve.
From a grid operator's standpoint the ideal curve would be a flat line. Electricity demand would be constant all day long, and power plants could run continuously. This would be pure "base load" power. Almost as good would be a changing demand but at a very slow rate. The operator could scale generation up or down at individual plants to follow the demand curve, or, if necessary, start or stop additional plants that are available as standby providers, or "peaker" plants.
The grid operator dealing with a duck curve demand profile has a number of challenges to overcome to
maintain grid reliability. These include:
- oversupply risk - when more electricity is supplied than is needed to satisfy real-time electricity demand (see Video 2);
- short, steep ramps - when the grid operator must bring on or shut down generation resources to meet an increasing or decreasing electricity demand quickly; and
- decreased frequency response - when less resources are operating and available to automatically adjust electricity production to maintain grid reliability.
To
manage the grid and ensure reliability during such variable conditions, the grid operator needs resources with ramping flexibility and the ability to start and stop multiple times per day. The deeper the belly, the more challenging the job of the system operator.
The penetration of solar photovoltaic generation into the California system by 2020, as projected by CAISO in 2013, was realized 4 years early. In recognition of this progress, in 2018 the state adopted more aggressive targets with the passage of
Senate Bill 100, which mandated 60% renewables by 2030 and 100 percent zero-carbon electricity (i.e., including nuclear power) by 2045, which has put more pressure on the grid operator. In fact, Figure 3 shows a duck curve for the lowest actual net load day in the spring through 2023, with an absolute minimum load that hits zero. Theoretically, all electrical demand could be met by solar and wind power. In reality, CAISO cannot shut down all traditional power generation facilities and is forced to curtail power from solar and wind farms.
One major tool for the grid operator to manage the duck curve is storage capacity that allows electricity generated during the peak hours of sunshine to be used later in the day. By mid-2023,
California storage capacity surpassed 5,000 MW, which was a 10-fold increase from 2020. Nearly all of this capacity is based on lithium-ion 4-hour duration batteries. This battery storage capacity is in addition to approximately
4,000 MW of pumped hydroelectric storage capacity (see OWOE:
What is pumped hydroelectric storage?).
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