On the duck’s 10th birthday, here’s how to keep it from eating the power system
In 2008, the Duck Curve revealed the high solar penetration threat; has it been met?
A fat, slow-moving, 10-year-old duck threatens the U.S. power grid, despite the many engineering minds that have been and are still hunting it.
Ten years ago, researchers began thinking about the impact of rising renewables penetrations on the power system. They noticed solar creates a unique challenge because it can take over for less variable generation during the day but fades just when demand peaks in the evening.
Where solar penetrations rose fastest, power system operators and researchers saw increasing reason for concern. It led them to discover new levels of grid flexibility that are still taking shape.
What eventually became the duck was first noticed in graphs of power system load profiles where penetrations of solar photovoltaic (PV) generation were rising. The graphs were derived by National Renewable Energy Laboratory (NREL) researchers in February 2008. Their paper’s simulations revealed how supply and demand curves would be affected by a then purely theoretical high solar growth.
By 2013, California was working toward its 33% renewables by 2020 mandate and California Independent System Operator (CAISO) engineers had done more detailed studies. They named the resulting graph the “Duck Curve” because the deep dip in midday demand for system power looked like a duck’s belly when followed by a sharp evening demand spike that looked like a duck’s neck and bill.
On Feb. 18, California’s duck had its fattest belly ever when minimum demand dropped to a record-setting 7,149 MW midday low. On March 4, the duck held its head higher than ever with a record-setting three-hour evening ramp of 14,777 MW.
This is crucial because CAISO had projected no more than a 13,000 MW ramp and no less than a 12,000 MW minimum demand by 2020. The new numbers show the over-generation has gotten greater and the ramp has gotten steeper at much faster rates than the CAISO anticipated.
This doesn’t mean the duck will eat the power system. Forward thinking engineers are successfully devising ways to control its appetite by discovering new power system possibilities.
The duck starts to quack
NREL Principal Analyst Paul Denholm led the team that did the original simulations as part of planning for “integration of significant quantities” of solar. The new load profile emerged when PV generation took up to 10% of annual electricity from conventional power plants on the western U.S. grid.
This new load profile reported by NREL revealed a new challenge for system operators, Denholm told Utility Dive. To use the midday output from high levels of installed solar, they would have to back conventional plants down so far that the fast ramp-up to meet peak evening demand would strain their operational capability.
The alternative was curtailment of solar generation, which solves that problem for system operators, but adds cost to solar and reduces its impact on emissions.
“Up to the 10% penetration case, the net load shapes created by PV appear to fall well within the operational capabilities,” NREL concluded. At 2008’s installed solar cost, 10% solar penetrations seemed far off. But Denholm and other NREL analysts saw cost reductions coming and began studying grid impacts of higher penetrations.
NREL’s studies were likely the first modeling of high customer solar adoption, he said. “This weird shaped curve meant we needed to start thinking about the grid implications of high solar penetrations.”
“We shouldn’t confuse a challenge with an insurmountable problem. Engineers love challenges. The Duck Curve requires a new way of operating the power system. It doesn’t mean we can’t operate it that way.”
Paul Denholm
Principal Analyst, National Renewable Energy Research Laboratory
Solar grew fastest in California. By 2010, CAISO was doing detailed minute-by-minute studies, according to Senior Renewable Energy Integration Advisor Clyde Loutan. In 2013, CAISO’s work produced a load profile graph that looked even more like a duck. That’s when the Duck Curve got its name.
“In 2008, when NREL wrote their paper, nobody knew solar would grow so big so fast,” Loutan told Utility Dive. CAISO’s 2013 studies revealed “the duck would require a fleet of solutions,” Loutan said. “We realized that if we did not do something, the problem would get worse.”
Sacramento Municipal Utility District (SMUD) is a publicly-owned utility independent of the CAISO grid. But as a California electricity provider, it faced many of the same forces driving solar growth, according to Distributed Energy Resources Manager Obadiah Batholomy.
“We started doing simulations of significant amounts of solar on our system in 2011 or 2012,” he told Utility Dive. “We were starting to see solar costs fall on a trajectory that would mean a really significant uptake of solar and our research department was forecasting higher levels of solar than anything we had seen.”
