The Duck Curve Becomes Extreme in California
by Bob Shively, Enerdynamics President and Lead Facilitator
The “duck curve” is a name given to the shape of the net load curve in a market with a significant penetration of solar energy. The net load curve is the demand curve less all renewable generation. Hence, the net curve demonstrates the amount of load remaining to be served by non-renewable generation after loads have been served with all available renewable generation. The term was first widely used in California as system planners studied the potential effects of increasing solar power.
Source: California ISO
The curve is so named because it was observed that the net load curve looks like the shape of a duck with a tail during the morning, a belly in the middle of the day as solar output is at a maximum, and with a head in the early evening as solar power wanes and overall system demand is at its peak. As shown in the above graphic, the “belly” becomes increasingly pronounced as the amount of solar grows year by year.
The fact that the duck curve is not just theoretical is demonstrated by actual data from the California ISO (CAISO) for April 24, 2021, shown on the following graph. On this day, net loads in the middle of the day dropped to just over 1,000 MW. Note that during the four-hour period between the hours of 1600 and 2000, the CAISO need to ramp almost 16,000 MW of non-renewable resources to match supply and demand.
The duck curve is observed in other regions besides California. In recent years it has been seen in states such as Texas and even Massachusetts.
The duck curve demonstrates the operational changes required to manage an electric system with growing penetrations of solar energy. System operators require increased amounts of flexible resources that ramp up or down quickly to match changes in solar output. Such resources include:
- Controllable hydro power
- Hydro pumped storage
- Natural gas combustion and combined-cycle turbines
- Battery electric storage
- Flexible loads where customers are willing to adjust the timing of their consumption
Also important is the ability to import or export power from neighboring systems. In the above example of April 24, 2021, California was exporting over 2,600 MW at 1 p.m., but it was importing over 6,000 MW during the evening hours.
As solar power grows around the world, system operators will find the need for increasing amounts of flexible resources and will increasingly find the need to trade power across large geographical regions in real time. This will continue to lead to the evolution of technology and industry practices.
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