Minimize to nav 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. Solving the variability problem of solar and wind energy requires reimagining how to power our world, moving from a grid where fossil fuel plants are turned on and off in step with energy needs to one that converts fluctuating energy sources into a continuous power supply. The solution lies, of course, in storing energy when it’s abundant so it’s available for use during lean times. But the increasingly popular electricity-storage devices today—lithium-ion batteries—are only cost-effective in bridging daily fluctuations in sun and wind, not multiday doldrums. And a decades-old method that stores electricity by pumping water uphill and recouping the energy when it flows back down through a turbine generator typically works only in mountainous terrain. The more solar and wind plants the world installs to wean grids off fossil fuels, the more urgently it needs mature, cost-effective technologies that can cover many locations and store energy for at least eight hours and up to weeks at a time. Engineers around the world are busy developing those technologies—from newer kinds of batteries to systems that harness air pressure, spinning wheels, heat or chemicals like hydrogen. It’s unclear what will end up sticking. “The creative part … is happening now,” says Eric Hittinger, an expert on energy policy and markets at Rochester Institute of Technology who coauthored a 2020 deep dive in the Annual Review of Environment and Resources on the benefits and costs of energy storage systems. “A lot of it is going to get winnowed down as front-runners start to show themselves.” Finding viable storage solutions will help to shape the overall course of the energy transition in the many countries striving to cut carbon emissions in the coming decades, as well as determine the costs of going renewable—a much-debated issue among experts. Some predictions imply that weaning the grid off fossil fuels will invariably save money, thanks to declining costs of solar panels and wind turbines, but those projections don’t include energy storage costs. Other experts stress the need to do more than build out new storage, like tweaking humanity’s electricity demand. In general, “we have to be very thoughtful about how we design the grid of the future,” says materials scientist and engineer Shirley Meng of the University of Chicago. Reinventing the battery The fastest-growing electricity storage devices today—for grids as well as electric vehicles, phones and laptops—are lithium-ion batteries. Recent years have seen massive installations of these around the globe to help balance electricity supply and demand and, more recently, to offset daily fluctuations in solar and wind. One of the world’s largest battery grid storage facilities, in California’s Monterey County, reached its full capacity in 2023 at a site with a natural-gas-powered plant. It can now store 3,000 megawatt-hours and is capable of providing 750 megawatts—enough to power more than 600,000 homes every hour for up to four hours. Lithium-ion batteries convert electrical energy into chemical energy by using electricity to fuel chemical reactions at two lithium-containing electrode surfaces, storing and releasing energy. Lithium became the material of choice because it stores a lot of energy relative to its weight. But the batteries have shortcomings, including their fire risk, their need for air-conditioning in hot climates, and a finite global supply of lithium. Importantly, lithium-ion batteries aren’t suitable for long-duration storage, explains Meng. Despite monumental price declines in recent years, they remain costly due to their design and the price of mining and extracting lithium and other metals. The battery cost is above $100 per kilowatt-hour—meaning that a battery container supplying one megawatt (enough for about 800 homes) every hour for five hours would cost at least $500,000. Providing electricity for longer would quickly become economically unfeasible, Meng says. “I think four to eight hours is really a sweet spot for balancing cost and performance,” she says. For longer durations, “we want energy storage that costs one tenth of what it does today—or maybe, if we could, one hundredth,” Hittinger says. “If you can’t make it extremely cheap, then you don’t have a product.” One way of cutting costs is to switch to cheaper ingredients. Several companies in the US, Europe and Asia are working to commercialize sodium-ion batteries that replace lithium with sodium, which is more abundant and cheaper to extract and purify. Different battery architectures are also being developed—such as “redox flow” batteries, in which chemical reactions take place not at electrode surfaces but in two fluid-filled tanks that act as electrodes. With this kind of design, capacity can be enlarged by increasing tank size and electrolyte amount, which is much cheaper than increasing the expensive electrode material of lithium-ion batteries. Redox-flow batteries could supply electricity over days or weeks, Meng says. US-based company Form Energy, meanwhile, just opened a factory in West Virginia to make “iron-air” batteries. These harness the energy released when iron reacts with air and water to form iron hydroxide—rust, in other words. “Recharging the battery is taking rust and unrusting it,” says William Woodford, Form’s chief technical officer. Because iron and air are cheap, the batteries are inexpensive. The downside with both iron-air and redox-flow batteries is that they give back up to 60 percent less energy than is put into them, partly because they gradually discharge with no current applied. Meng thinks both battery types have yet to resolve these issues and prove their reliability and cost-effectiveness. But the efficiency loss of iron-air batteries could be dealt with by making them larger. And since long-duration batteries supply energy at times when solar and wind power is scarce and more costly, “there’s more tolerance for a little bit of loss,” Woodford says.