Solar Power’s Greatest Challenge Was Discovered 10 Years Ago. It Looks Like a Duck. - By David Roberts

A researcher discusses the “duck curve” he helped discover.

Back in 2008 a group of researchers at the National Renewable Energy Laboratory (NREL) noticed a funny-looking shape in their modeling.

They were starting to take solar photovoltaic (PV) panels seriously, running projections of what might happen if PV were deployed at scale.  They noticed that large-scale deployment had a peculiar effect on the electricity “load curve,” the shape that electricity demand takes throughout the day.

A typical load curve looks something like this:

As you can see, demand spikes in the morning (when everyone wakes up) and again in the evening (when everyone gets home from work), before declining at night.

This curve happens to be from New England, in the fall.  The curve looks somewhat different in different regions and during different seasons — there are different-size peaks and ramps — but it usually falls within a fairly narrow and predictable range.

Until solar PV comes along!

Total load minus renewable energy is known as “net load.”  That’s the target utilities have to hit with their dispatchable resources.

As more and more solar PV is integrated into the grid, it starts dramatically suppressing net load during midday, when the sun is out.  The net load curve sags in the middle of the day (like a belly) and then swoops back up when the sun goes down (like a neck).  
It’s just like that first load curve I showed you — a peak in the morning, a peak in the evening — only in between, there’s an enormous sag that gets bigger as more solar is added.

At the time, the California Independent Service Operator (CAISO, the operator of the California energy grid) was noticing the same thing.  The realized that high levels of solar penetration start generating a net load curve that looks ... well, like a duck.  Thus, the “duck curve.”

Can’t quite see it?  Here, let me help:

It has now been 10 years since NREL’s fateful discovery, and in the interim, the duck curve has become a serious threat to solar and a shared obsession among the clean energy community.

If it doesn’t get solved, things could get ugly.  In the near future, utilities could regularly be forced to ramp up their dispatchable plants for a morning peak, then scale back or shut down almost all of those plants while the sun is out, and then bring them all back online (quickly) when the sun goes down.

All that ramping and stop-starting is expensive and unfamiliar to the operators of many fossil fuel power plants.  If the tension gets too high, solar expansion could be choked off.

Luckily, solutions to the duck curve abound — all kinds of options for making the grid more flexible and softening the peaks and ramps.  In part thanks to early warnings from energy modelers, utilities and grid operators are beginning to awaken to the issue and take steps to address it.

Read more at Solar Power’s Greatest Challenge Was Discovered 10 Years Ago. It Looks Like a Duck.