What’s All This About “Duck Curves”?



By: Peter Kelly-Detwiler & Richard E. Harrison

Ten years ago, energy researchers at the National Renewable Energy Laboratory were examining the challenges of integrating large quantities of solar energy into the power grid.

They quickly determined that since solar panels are limited to producing power over a roughly six-hour period in the middle of the day, solar deployments would have a pronounced impact on the net load (demand). The following graph shows how the output of solar at various levels were expected to affect that load.

The researchers then looked at the issue from the supply side. They determined that large quantities of solar energy would change the way the generation fleet would have to be dispatched because net peak demand would be pushed to later in the day.

The California Independent System Operator (CAISO) knew this would have profound impacts. Perhaps the most critical was that solar would reduce the need for other power plants to generate for certain hours of the day. It was also going to require plants that traditionally had operated all day to cycle off during the middle of the day when solar was abundant.

In 2012, CAISO created a graph to show the anticipated impact of solar on an extreme spring day. (Spring was chosen because demand is low and solar output – with panels typically oriented at 90 degrees – is high during hours of output.) The illustration soon became known as the “Duck Curve” for obvious reasons.

What CAISO underestimated was the rapidity with which the duck would grow to maturity. The expectation was that, by 2020, generation (net of solar) required to meet demand would be 12,000 megawatts (MW). The reality has been shockingly different. Two years ahead of that date, net generation required in mid-February 2018 was just under 7,000 MW.

The influx of solar also resulted in wholesale electricity market prices moving into negative territory. This occurs especially in years when spring hydropower – a function of melting winter snowpack – is also abundant.

California isn’t the only state dealing with the duck curve. As more solar has been installed on the Hawaiian island of Kauai, daytime demand for electricity (shown by the purple curve in the graph below) had dropped significantly. Other Hawaiian islands are either facing or will soon face similar situations.

Even New England’s system operator is seeing a similar dynamic. Solar output there recently created a new first: net nighttime demand exceeding that of the afternoon.

All of these graphs point to similar dynamics and challenges for planners and grid operators. At scale, solar energy poses an integration challenge.

It also opens the door – and creates a market – for other resources to flatten out curves, e.g., batteries. Batteries can absorb energy during midday, release it at night, and bring stability to the grid.

Without complementary resources, too much solar can soon become too much of a good thing. At some point, to continue effectively incorporating more cheap and clean solar energy, we will have to combine with other cost-effective resources. Put another way, we will need to “tummy tuck the duck.”

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