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. 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.
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.
However, the rapid increase in wind generated electricity starting about 2008 and solar about 2013, as shown in Figure 2, has created the deep belly in the duck curve. In fact, the penetration of solar photovoltaic generation into the system by 2020, as projected by CAISO in 2013, was realized 4 years early. And with California now working toward a target of 50% renewables by 2030 (the
Renewables Portfolio Standard), the challenges around managing the potential disconnects between supply and demand are only going to get greater.
The grid operator dealing with a duck curve demand profile must deal with key issues of:
- oversupply risk - when more electricity is supplied than is needed to satisfy real-time electricity demand (video);
- short, steep ramps - when the grid operator must bring on or shut down generation resources to meet an increasing or decreasing electricity demand quickly, over a short period of time; and
- decreased frequency response - when less resources are operating and available to automatically adjust electricity production to maintain grid reliability.
To ensure reliability under such variable conditions, the grid operator needs resources with ramping flexibility and the ability to start and stop multiple times per day. The CAISO has been able to manage the situation thus far through a combination of market driven policies and curtailments. However, very low electricity prices and curtailments are not desireable in the long-term as they devalue the investments that owners have made in their facilities.
It should also be noted that at least a half-dozen versatile gas-fired plants that sit along California's coast, but use seawater for once-through cooling, are scheduled for early retirement due to stricter environmental regulations. This will add further pressure on the remaining more traditional base load plants.
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