When the Sun doesn't shine and the wind is calm, humankind still needs power.

When the Sun is blazing and the wind is blowing, Germany’s solar and wind power plants swing into high gear. For nine days in July 2023, renewables produced more than 70 percent of the electricity generated in the country; there are times when wind turbines even need to be turned off to avoid overloading the grid.

But on other days, clouds mute solar energy down to a flicker and wind turbines languish. For nearly a week in January 2023, renewable energy generation fell to less than 30 percent of the nation’s total, and gas-, oil- and coal-powered plants revved up to pick up the slack.

Germans call these periods Dunkelflauten, meaning “dark doldrums,” and they can last for a week or longer. They’re a major concern for doldrum-afflicted places like Germany and parts of the United States as nations increasingly push renewable-energy development. Solar and wind combined contribute 40 percent of overall energy generation in Germany and 15 percent in the US and, as of December 2024, both countries have goals of becoming 100 percent clean-energy-powered by 2035.

The challenge: how to avoid blackouts without turning to dependable but planet-warming fossil fuels.

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