The Duck Curve Part 1: A Challenge of Overbuilding Renewables

Part 1 of a 2-Part Series
Read Part 2: “Smoothing Out the Curve”

What is a Duck Curve, and what does it have to do with renewable energy?

One of the more interesting terms unique to the energy industry is the ‘Duck Curve’ – when taken at first glance one wonders what a duck, an electric grid, demand flexibility, and renewable energy all have in common.

The ‘Duck Curve’ is a term used to describe the shape of the demand curve (which displays how much electricity is needed from the power grid to meet fluctuating customer demand throughout a 24-hour period) when a large amount of renewables, particularly solar, are part of the power system.

To understand where the Duck Curve graph comes from, it’s important to know what factors go into shaping it. Let’s start with the concept of Net Load. Net Load is the difference between the amount of electricity we predict to use and how much electricity we end up producing from renewables. Thus, the Net Load will tell us how much power needs to come from traditional power plants; like those running on coal, gas, nuclear, etc.

The below graph comes from California Independent System Operator (ISO)

The Duck Curve graph shows the need for energy demand flexibility.

(Chart from Energy Alabama, May 2017)

How does the Duck Curve happen?

Look first at the line in 2012, above – this line shows a more traditional demand curve. To be clear, this is what energy system operators in the past would use as a baseline forecast when scheduling the amount of electricity their power plants would need to generate every day.

Before the introduction of variable resources (like renewable energy), the forecasted Net Load was fairly accurate and easy to predict. Over the years, however, the amount of renewable generation has increased significantly and the ability to forecast and predict demand has become increasingly difficult. The need for demand flexibility is higher than ever. This is clear when observing the Net Load for other years on the above graph.

The decrease in Net Load for 2014 and onward is a result of introducing of more and more renewables (particularly solar) into the system. You’ll immediately notice that there is minimal, if any, of the load that needs to be met by power plants during the middle hours of the day. However, as the sun goes down and evening demand begins to increase (people going home, cooking, turning on their TVs, charging their phones, cars, etc.) there is a tremendous amount of strain on the system as plant operators must ramp up all available power plants to keep the lights on.

Still don’t see a duck? How about now?

Duck superimposed on a duck curve

(Chart from Berkeley News, January 2018)

The shape of the power demand curve has changed to the sinking curve in the middle of the day because more power is being met by solar or wind generation. As a result, less power is needed from utility fossil fuel or nuclear power plants as the sun shines and the wind blows.

On the flip side, as we get into the evening hours, more power from coal, oil, gas and nuclear plants is required—and required quickly—to ramp up to meet peaking customer demand as the sun goes down.

Why is this steep ramping every day a problem?

If we stick with the California ISO example from earlier, we see that California over the last decade has been, hands down, the leader of solar installations in the U.S.: as a whole, the state surpassed 11.2 GW of installed solar capacity by the end of 2017. Introducing this large amount of solar energy is what causes the “Duck Curve” along with the evening ramp-up challenges utilities face when the sun sets each day in states like California.

The key here is to keep in mind that traditional power plants (those running on fossil fuels) are not very flexible and cannot just be “switched on” like a light switch every evening to meet this increased demand.

The end result? Plant operators are forced to keep inflexible plants that run on coal, oil, and gas operating all day, so they’re still burning fuels and producing emissions in order to be ready to ramp-up their generation when the sun goes down. This inflexibility is why California’s traditional fossil fuel plants are forced to run as much as they are in spite of all the solar.

This is the solar power dilemma that California is facing. This same challenge is seen in other places, like Hawaii, where a large amount of solar generation has been installed into a power system that is made up of inflexible fossil fuel plants.

Drilling down by the hour: how power is generated to meet customer demand.

People use the most electricity from 6-9 a.m. and 2-7 p.m.

When you think about this, it makes sense because most households need their homes warm in the winter and cool in the summer when they are preparing for work or school from 6-9 a.m. The second peak is when they return from work and school between 2-7 p.m. Most businesses and plants require the bulk of their power during the day, so residential demands are dropping off as solar input is going up.

Traditional oil, gas, and coal power plants are cycled up and down throughout the day to meet demand. It puts wear and tear on the plant equipment and adds pollution to the environment. Some may say “just add storage” – and while yes, some of this commercial power storage technology does exist, it has not progressed to a point that it solves all the problems. Given the current technology of storage as it is, it doesn’t make economic sense to install at a scale that would be necessary to cover the gap.

The Duck Curve highlights how, when we add solar and wind energy to the mix, we must rethink how we meet fluctuating demand in the smartest way possible. The goals we must shoot for are

  1. keeping energy costs affordable,
  2. maintaining system reliability and
  3. minimizing the need for fossil fuel power generation and resulting environmental effects.

Continue reading in Part 2, where we explore some steps we can take to smooth out the Duck Curve.