An Introduction to the Duck Curve

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An Introduction to the Duck Curve

As solar electricity increases, the daily balance of supply and demand is changing dramatically

Posted on Nov 11 2013 by Allison A. Bailes III, PhD

You may think there's no more boring topic than electric utilities. Power plants. Transmission lines. Engineers with flat top haircuts and pocket protectors full of pens in their white short-sleeved shirts.

Well, let me tell you two words that might help make them more interesting: duck curve. If you haven't heard this term yet, you're not alone.

A primer on utility load curves

The graph below shows a typical daily load profile for an electric utility. This one's for a day in New England in 2010, but the basic pattern is similar to what most electric utility load curves look like.

Demand for electricity is at its lowest during the night. When people wake up and start their day, demand rises (morning ramp). Then it stays up at a higher level through the day but rises to a peak after work ends and everyone returns home.

[Image credit: U.S. Energy Information Administration]

That's what a typical day looks like in a lot of places. In addition, there are annual changes and utilities are often classified as either summer peaking or winter peaking. That's just what you think: which season the utility hits its highest peak load for the year.

Utilities meet that demand in two ways. They have baseload plants and peaking plants. Baseload plants are mostly coal-fired here in the Southeast and mostly hydro in the Northwest. Peaking plants are often fueled by natural gas.

Now, what electric utility companies would like their load curve to look like is shown below.

[Image credit: Energy Vanguard]

If the demand were perfectly flat, a utility's job would become trivial. (Of course, there's all that messy stuff about storms and growth and plant outages, but let's ignore that here.) Push the button and let the generating plants run to deliver that unchanging amount of electricity. They could meet all the load with cheaper electricity from baseload plants and never have to turn on the peaking plants.

But things are changing, and quickly. California is one of the first states to be experiencing it, but it's coming to other states, too. What's happening is that cheap photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. modules (a.k.a, solar panels, although that’s an ambiguous term) are reducing the demand during the sunny part of the day. The morning ramp is flattening out, and now the evening ramp is getting steeper. The chart below shows the trend, actual and projected. (See Greentech Media for .)

[Image credit: California Independent System Operator]

As you can see here, the load during the middle of the day gets smaller because of all the PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.-generated electricity entering the grid.

As that electricity starts dropping off in late afternoon and evening, the evening peak begins to hit. That means that utility power plants that had been resting during the PV-saturated afternoon hours suddenly need to ramp up quickly. As you know, any large-scale operation has trouble turning on a dime, so the duck’s neck in the evolving load profile presents problems.

The world of electricity generation is changing rapidly. From smart meters to microgrids to the , there's a lot happening. The duck curve is an indicator.

Matt Golden, a policy guy in California who came from the home performance world, raised the issue of dramatic change in a comment on In it, he talked about the "new world where EE [energy efficiency] is dead." (There's more to it than meets the eye, though, so don't rush to judgment here yet.)

Could that be true? Will the duck curve kill energy efficiency? Or instead, is it time "to declare as a retired California Public Utility Commissioner suggested?

I'll be taking a deeper look into this issue next week, so be sure to check back then.

of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the . Check out his in-depth course, at Heatspring Learning Institute, and follow him on Twitter at .

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Image Credits:

  1. Image #1: Chris Hunkeler /

Nov 18, 2013 10:12 AM ET

Not so boring..
by Dan Kolbert

Thanks, AB3 - finally getting a chance to read this. I seem to be one of the few people who found the keynote at last year's NESEA conference, a panel on utilities, fascinating, so I may not be the best judge.

Nov 18, 2013 10:28 AM ET

Response to Dan Kolbert
by Allison A. Bailes III, PhD

You're welcome, Dan. Great seeing you in Portland last week!

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