The solar eclipse in Europe will be an unusual test for solar power

On the morning of Friday, March 20, a major solar eclipse will hit Europe. Most people will get to enjoy a fun astronomical spectacle, as the moon wanders directly between us and the sun, casting a shadow on Earth.

But the eclipse will create an odd headache for electric utilities in Germany.

That's because Germany now gets roughly 7 percent of its electricity from solar power. And, for about 75 minutes, the moon will block much of the sun, decreasing sunlight by as much as three-quarters. All those solar panels will suddenly see a sharp decline in generation. Then, just as quickly, the eclipse will end, and those panels will produce a massive surge of electricity.

(Opower)

"Solar output could fall up to 2.7 times faster in Germany than it ever normally does," explains Barry Fischer, an analyst at Opower, who has taken an in-depth look at a partial eclipse that similarly affected the western United States in October 2014. (He supplied the chart above.) "This is going to require a very precise response."

This eclipse may seem like a rare and unusual situation. But it's actually a great glimpse into what a future heavy on renewables might look like. If solar power becomes more widespread, utilities and grid operators will face more and more situations where solar output fluctuates greatly — from eclipses, but also from simple sunrises and sunsets, or even from unexpected cloud cover.

This sort of intermittency is one of the biggest challenges with trying to scale up power sources like wind and solar. And how power companies grapple with this will go a long ways toward determining the future of renewables.

How Germany can deal with the March solar eclipse

Photovoltaic panels at the site of former airport in Neuhardenberg, eastern Germany. (Patrick Pleul/AFP/Getty Images)

As it turns out, German researchers have been giving this a fair amount of thought — eclipses, after all, are very predictable. One report from the University of Applied Sciences in Berlin suggests that the country's grid operators can take several actions to make sure the country doesn't see a wave of temporary blackouts when the eclipse hits. Like:

1. Make use of pump storage. Whenever German grid operators have a bit of excess electricity (say, from wind turbines spinning at night, when no one needs them), they sometimes use the extra juice to pump water up a hill. Then, when they need a bit of extra power, they can let that water fall back downhill, spinning turbines to generate electricity. This is known as "pump storage," and it will come in handy for an occasion like this.
2. Modulate energy demand. The eclipse is a problem because it will reduce electric output right as households and companies are using lots of power. So one possibility is to ask (or pay) certain factories or companies to use less power when the eclipse comes.
3. Turn on natural gas plants or import electricity. Germany does have some spare natural gas plants that can be fired up if necessary — or it can always import some excess electricity from neighboring countries like France.

"The consensus among people who have looked at this is that Germany should be able to overcome this," Fischer told me. "But they definitely can't sit on their hands — and they'll have to do things they don't normally have to do." The problem will be particularly tricky if cloud cover unexpectedly changes the rate at which solar power declines.

What's more, Fischer notes, the country's grid operators will also have to deal with the fallout when the eclipse ends and solar power ramps back up — they may, for instance, have to ask factories to start using electricity at a faster rate, or to export some of that juice.

The western US just faced a (smaller) eclipse dilemma

This isn't the first time an eclipse has messed with solar power — and it won't be the last.

The western United States experienced something similar on October 23, 2014, Fischer explains. This was only a partial eclipse — blocking out just 30 to 50 percent of the sun. And California, Arizona, Nevada, and New Mexico only have about one-third the solar capacity that Germany does (about 12 gigawatts, compared to Germany's 38).

But it was still a hassle. An analysis that Opower shared with me of more than 5,000 homes in the region found that rooftop solar panels suddenly saw a 41 percent drop in electricity that they were sending to the grid:

(Opower)

So how did US grid operators replace that lost power? Power companies appeared to turn on a bunch of fast-start natural gas plants during this time. "We saw this huge bump in thermal power," Fischer says. "It was way out of the ordinary."

So far, so good. But it helped that solar power is still just a small portion of US electricity. What happens when it becomes more and more important, and these sorts of dips and bumps become harder to deal with? (For those keeping count, the United States will get hit by more eclipses in 2017, 2023, and 2024.)

Solar fluctuations could become a much bigger issue

Solar panels in a row at sunrise. (Photo by Frank Bienewald/LightRocket via Getty Images)

This is where things get interesting. Both Germany and California are trying to expand solar power massively. The German government has set a goal of increasing solar capacity from 38 gigawatts to 66 gigawatts between now and 2030.

If solar expands that much, one analysis has found, even an ordinary sunrise and sunset on a clear day will have the same effect on Germany's grid as the March 20 eclipse will. Indeed, even a bout of particularly cloudy weather could create a big dip in production. All of these fluctuations in sunlight matter much more when you're dealing with twice as many solar panels.

The same goes for the United States — especially with California aiming to get 33 percent of its electricity from renewables in 2020. Sunsets will increasingly pose a big challenge, as solar panels wind down for the day but electricity demand soars as people get home, turning on their lights and appliances.

This is known in energy circles as the "duck curve." The duck-shaped chart below shows how non-solar power will, in theory, have to ramp up and down as solar power expands and contracts throughout the day. The more solar you have, the duckier the chart gets:

(California ISO)

Is this fatal to solar's future? Not necessarily. Experts have noted that there are all sorts of possible ways power companies could try to alleviate this hurdle — by, essentially, flattening the duck curve.

So, for instance, power companies could orient more solar panels toward the west, so that the panels produce more power during peak demand hours. (Right now a lot of panels face south, maximizing output but exacerbating the duck curve.) They could deploy more storage capacity — like flywheels, pump storage, or batteries — in order to store excess solar power during the day and release it in the evening, when it's needed. Along similar lines, there are air conditioners that use ice to "store" energy, helping juggle demand.

On top of that, utilities could expand programs that encourage companies to reduce or delay electricity consumption right at the moments when the grid would otherwise be stressed. (The US already does a lot of this, but one analysis by Jim Lazar of the Regulatory Assistance Project suggested that the US could reduce demand by another 3 percent with further programs.)

Whatever the solution, this is going to be something that electric utilities and grid operators will increasingly have to think harder about. And the eclipse in Germany on March 20 is a good early test case to see how well they can manage it.

Further reading

-- The cost of wind and solar power keeps dropping all around the world.

-- Solar power is now growing so fast that older energy companies are trying to stop it.

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