Duck curve

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The Duck Curve
Blue curve: Demand for electrical power
Orange curve: (the duck curve) supply of electrical power from dispatchable sources,
Gray curve: supply of solar electrical power
Data is for the State of California on October 22, 2016 (a Saturday),[1] a day when the wind power output was low and steady throughout the day.
The orange curve rises steeply from 17:00 to 18:00 as the sun sets, requiring about 5 gigawatt of generating capacity from dispatchable sources to come on line within one hour.

The duck curve is a graph of power production over the course of a day that shows the timing imbalance between peak demand and renewable energy production. Used in utility-scale electricity generation, the term was coined in 2012 by Karen Edson of the California Independent System Operator.[2][3]

Solar power[edit]

In some energy markets, daily peak demand occurs after sunset, when solar power is no longer available. In locations where a substantial amount of solar electric capacity has been installed, the amount of power that must be generated from sources other than solar or wind displays a rapid increase around sunset and peaks in the mid-evening hours, producing a graph that resembles the silhouette of a duck.[4][5] In Hawaii, significant adoption of solar generation has led to the more pronounced curve known as the Nessie curve.[6][7]

Without any form of energy storage, after times of high solar generation generating companies must rapidly increase other forms of power generation around the time of sunset to compensate for the loss of solar generation, a major concern for grid operators where there is rapid growth of photovoltaics.[8] Storage can fix these issues if it can be implemented. Flywheels[relevance questioned] have shown to provide excellent frequency regulation,[9] but have low energy storage and thus short duration. Short term use batteries, at a large enough scale of use, can help to flatten the duck curve and prevent generator use fluctuation and can help to maintain voltage profile.[10] However, cost is a major limiting factor for energy storage as each technique is expensive to produce at scale.[citation needed]

Mitigation strategies[edit]

Methods for coping with the rapid increase in demand at sunset reflected in the duck curve, which becomes more serious as the penetration of solar generation grows, include:[10]

A major challenge is deploying mitigating capacity at a rate that keeps up with the growth of solar energy production. The effects of the duck curve have happened faster than anticipated.[14]

Duck curve in California[edit]

Sources of electricity generation in California in 2020. Because these graphs do not display energy demand, they are not Duck Curves themselves, but demonstrate daily and seasonal variation in power production.
  Natural gas
  Solar power

The California Independent System Operator (CAISO) has been monitoring and analyzing the Duck Curve and its future expectations for about a half a century now and their biggest finding is the growing gap between morning and evening hours prices relative to midday hours prices.[1] According to their 2016 study, the U.S. Energy Information Administration, found that the wholesale energy market prices over the past six months during the 5 pm to 8 pm period (the "neck" of the duck) have increased to $60 per megawatt-hour, compared to about $35 per megawatt-hour in the same time frame in 2016.[4] However, on the other side they have measured a drastic decrease in the midday prices, nearing $15 per megawatt-hour.[4][needs update] These high peaks and deep valleys are only showing continued trends of going further apart making this Duck Curve even more prevalent as renewable energy production continues to grow.[5][better source needed][15]

A crucial part of this curve comes from the Net Load ("the difference between expected load and anticipated electricity production from the range of renewable energy sources").[4] In certain times of the year (namely Spring and Summer), the curves create a "belly" appearance in the midday that then drastically increases portraying an "arch" similar to the neck of a duck, consequently the name "The Duck Chart.[16]" This "neck" represents a ramp speed of between 10 and 17 GW in 3 hours (afternoon) in 2020 which has to be supplied by flexible generation.[17] During the midday, large amounts of solar energy are created, which partially contributes to lower demand for additional electricity.[18] Curtailment impacts the curve.[17] Increasing battery storage can mitigate the issues of solar abundance during the day. When excess solar energy is stored during the day and used in the evening, the price disparity between inexpensive midday and expensive evening energy can be reduced. Enough total solar technology exists to power the world, but there is a current lack of infrastructure to store solar energy for later use.[8] An oversupply of energy during low demand coupled with a lack of supply during high demand explains the large disparity between midday and evening energy prices. As of 2022, up to 6 GWh is shifted per day from low price to high price periods.[19]

Common misconceptions[edit]

One misconception related to the duck curve is that solar photovoltaic power does not help supply peak demand and therefore cannot replace other power plants. For example, in California, solar output is low at 7 pm when daily demand usually peaks.[20] This fact leads some to believe that solar power cannot reduce the need for other power plants, as they will still be needed at 7 pm when solar power output is low. However, California's annual demand peaks usually occur around 3 pm to 5 pm,[21] when solar power output is still substantial.[20] The reason that California's annual peak tends to be earlier than the daily peak is that California's annual peak usually occurs on hot days with large air conditioning loads, which tend to run more during midday.[22] As a result, solar power does in fact help supply peak demand and therefore can substitute for other sources of power.

See also[edit]


  1. ^ a b "California ISO - Renewables Reporting".
  2. ^ Roberts, David (20 March 2018). "Solar power's greatest challenge was discovered 10 years ago. It looks like a duck". Vox. Retrieved 20 March 2018.
  3. ^ Staple, Gregory. "California's Grid Geeks: Flattening the 'duck curve'". Green Biz. Retrieved 9 May 2021.
  4. ^ a b c d Paul Denholm, Matthew O'Connell, Gregory Brinkman, and Jennie Jorgenson. "Overgeneration from Solar Energy in California: A Field Guide to the Duck Chart" NREL/TP-6A20-65023. National Renewable Energy Laboratory, November 2015
  5. ^ a b Wirfs-Brock, Jordan (2 October 2014). "IE Questions: Why Is California Trying To Behead The Duck?". Inside Energy. Retrieved 29 October 2016.
  6. ^ "Charting Hawaii's Spectacular Solar Growth". The Energy Collective. Retrieved 4 February 2015.
  7. ^ "Hawaii's Solar-Grid Landscape and the 'Nessie Curve'". 10 February 2014. Retrieved 10 January 2017.
  8. ^ a b "What the Duck Curve Tells Us About Managing A Green Grid" (PDF). California ISO. Retrieved 29 April 2015.
  9. ^ Lazarewicz, Matthew; Rojas, Alex (10 June 2004). "Grid Frequency Regulation by Recycling Electrical Energy in Flywheels". Power Engineering Society General Meeting. 2: 2038–2042. doi:10.1109/PES.2004.1373235. ISBN 0-7803-8465-2. S2CID 20032334.
  10. ^ a b Lazar, Jim. "Teaching the "Duck" to Fly" (PDF). RAP. Retrieved 29 April 2015.
  11. ^ a b Vorrath, Sophie (30 August 2020). "Solar tariffs reshaped to favour batteries, EVs, and west-facing panels". RenewEconomy. even out the "solar duck curve". . install batteries and west-facing panels, which helps stretch solar generation into the afternoon-evening peak.
  12. ^ "It's time to start wasting solar energy". Retrieved 31 December 2020.
  13. ^ Pyper, Julia (9 May 2019). "Electric Ridesharing Benefits the Grid, and EVgo Has the Data to Prove It". Archived from the original on 18 October 2020. By charging up in the middle of the day, LDV fleets on EVgo's network also help to address the duck curve — where midday net load drops, driven by lots of solar flooding onto the grid
  14. ^ "The California Duck Curve Is Real, and Bigger Than Expected". 3 November 2016. Retrieved 10 January 2017.
  15. ^ "2021 Summer Loads and Resources Assessment" (PDF). California ISO. 23 May 2021. p. 36. Archived (PDF) from the original on 12 May 2021. The growing amount of photovoltaic solar generation that is interconnected to the ISO grid continues to change the ISO’s net load profile and creates more challenges and uncertainty for ISO operations. The result is a constantly increasing ramping requirement, significantly more than what has been required from the generat ion fleet in the past, both upward and downward. Furthermore, solar generation does not provide significant power at the hours ending 19:00 to 21:00, which leads to reliance on gas and other non-solar generation after sunset. The continuing decline in dispatchable generation in the ISO as dispatchable units retire is beginning to challenge the ISO system’s ability to meet net peak demand after sunset and flexible capacity requirements.
  16. ^ "EIA Data Reveals California's Real and Growing Duck Curve". Retrieved 1 December 2017.
  17. ^ a b "Final Flexible Capacity Needs Assessment for 2022" (PDF). California ISO. 14 May 2021. p. 9-10. Archived (PDF) from the original on 7 January 2022.
  18. ^ "A world turned upside down". The Economist. Retrieved 1 December 2017.
  19. ^ Murray, Cameron (13 April 2022). "Battery storage load shifting up to 6GWh a day on CAISO grid; operator eyes SoC-linked prices". Energy Storage News. Archived from the original on 29 April 2022.
  20. ^ a b "California ISO > Today's Outlook". 6 September 2021. Archived from the original on 18 December 2017.
  21. ^ "California ISO Peak Load History 1998 through 2020" (PDF). California ISO. 2020. Archived (PDF) from the original on 2 May 2012.
  22. ^ Hodge, Tyler (21 February 2021). "Hourly electricity consumption varies throughout the day and across seasons". U.S. Energy Information Administration. Archived from the original on 23 February 2020.

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