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A mountain, a tower, a thermos of molten salt. These are the batteries that could power our renewable future.

Climate change is pushing the power grid to the limit. Energy storage could help.

Transmission lines on a blue background are seen connecting to a jar with solar panels and wind turbines inside of it. Amanda Northrop/Vox

Blackouts are a devastating reality of our climate-changed world. An unprecedented winter storm in 2021 knocked out power for millions of Texans for days, killing hundreds, and this summer Californians managed to barely save their state’s power grid from the brink of collapse during a record-breaking heat wave.

Some blackouts are caused by storms destroying infrastructure like transmission lines and substations — just look at what’s happened in Puerto Rico after Hurricanes Maria and, more recently, Fiona.

But many blackouts can also be blamed on how the electric system works. Namely: The goal of the power grid is to deliver electricity to your home as soon as it’s been generated at a power plant. There isn’t a great pool of electricity waiting in reserve for when demand spikes. Experts say that needs to change.

“Electricity systems are real-time systems,” said Eric Fournier, research director at UCLA’s Institute of the Environment and Sustainability. There’s little room for error.

In the past, there was an easy fix. If grid operators ever needed more power, they’d just burn more fossil fuels, in real time, to meet demand. But that makes climate change worse (electricity generation is responsible for 25 percent of total greenhouse gas emissions in the United States). It’s a vicious cycle: Climate change is what’s pushing our grids to the limit.

Switching to clean energy is the obvious solution. But while wind and solar power are efficient, they’re not always available: Solar power turns off at night, and wind turbines can’t generate power on a still day. With renewables, demand can still outpace supply.

We need a way to store renewable electricity. That sounds like ... a battery. But batteries — at least the kind found in our cellphones and cars — aren’t necessarily the best solution. Lithium-ion batteries, which have become the de facto standard for rechargeable batteries and are used in everything from phones and laptops to electric cars, are expensive to produce and might be better suited for those portable applications than sitting static in storage racks.

“We need to think about solutions that go beyond conventional lithium-ion batteries,” said Dharik Mallapragada, a principal research scientist at the MIT Energy Initiative and co-author of a recent study on the future of energy storage. Money is on everybody’s mind at COP27, the UN climate negotiations currently underway in Egypt, and the world needs affordable solutions that can work for wealthy and poor countries alike.

“No single technology is going to make this happen,” Mallapragada said. “We have to think about it as a jigsaw puzzle, where every piece plays its role in the system.”

The power grid is a massive machine. To make a battery for it, we have to think big — and weird. On this week’s episode of Unexplainable, Vox’s podcast about unanswered questions, we explore what the future batteries of the grid might look like, from the time-tested to the fantastical. There are many ways to bottle lightning.

A battery built into a mountain

A blue-green lake fills the bottom-right half of the photograph, running to the wall of a dam. Further below is another lake. Mountains rise on either side of both lakes.
The Verbund Moosersperre Dam in Kaprun, Austria, which has a capacity of 830 megawatts — enough to power almost 100,000 homes for more than a week.
Akos Stiller/Bloomberg via Getty Images

On a very basic level, all batteries work by taking electricity, storing it as a different form of energy, and turning that energy back into electricity (or, to be extremely technical, electric energy) when it’s needed again.

Lithium-ion batteries are chemical batteries, which means they store electricity as chemical energy. They’re very efficient; they can generally release upward of 90 percent of the energy put into them.

But a battery doesn’t have to be based on chemical energy — there are all kinds of other energy types we can convert that electricity into. Take, for example, pumped hydro.

Pumped-storage hydropower, or pumped hydro, is the biggest kind of grid-storage battery currently in operation in the United States. It’s also the oldest; the first pumped hydro facility in the country opened in New Milford, Connecticut, in 1930.

The concept behind pumped hydro is pretty straightforward. Sometimes power plants — especially renewable power plants like wind — generate more electricity than we can use, and grid operators end up having to simply dump that energy in a process called “curtailment.”

But if those renewable power facilities were hooked up to pumped hydro, that excess energy could be used to pump water up a hill or mountain and fill a reservoir. That movement uphill raises the water’s potential energy; when the energy is needed, the water is released and sent through a hydroelectric turbine, turning the potential energy back into electricity.

Pumped hydro took off in the United States during the 1970s and ’80s when the country saw a boom in nuclear power. Nuclear plants were very good at constantly generating a steady amount of electricity around the clock, Mallapragada said, but there wasn’t an easy way to increase or decrease their output.

To be able to respond to fluctuating demand, grid operators built pumped-hydro stations to store the excess energy generated by nuclear power plants during times of low energy use; without pumped hydro, that energy could have gone to waste. (It’s a similar dilemma to the one faced by solar and wind power plants, and recently some pumped-hydro stations have seen their energy sources shift from nuclear energy to renewables.)

It’s a time-tested, efficient solution. So building more pumped-hydro plants could work well for the future of clean energy.

But pumped hydro isn’t perfect: It requires specific geographies (like mountains, but any terrain with an elevation difference would work), and building a pumped-hydro station often requires hollowing out rugged landscapes in order to install the pumps and other infrastructure that move water up to the reservoir.

That’s an energy-intensive, resource-hungry process. So while pumped hydro could work in some circumstances, especially when existing facilities are being transitioned to renewable energy, it’s only one part of Mallapragada’s jigsaw puzzle of energy storage solutions. One of those pieces, Mallapragada said, could even be taking the principles behind pumped hydro and applying them outside of mountains.

Don’t have a mountain? Build one.

Pumped hydro, at its core, uses the force of gravity to pull water downhill and transform potential energy into electricity.

But if there isn’t a mountain in sight to build the plant, engineers can essentially build a mountain of their own. This is called gravity storage: Instead of lifting and dropping water, these “batteries” would lift and lower solid blocks of some heavy material like concrete.

The most attention-getting version of this technology comes from a company called Energy Vault. A prototype built in Switzerland involved a multi-armed crane that uses renewable energy to pick up 35-ton concrete blocks, slowly building a concrete tower around itself and storing solar and wind power as potential energy. When energy is needed, the process is reversed: The cranes let the blocks drop, unspooling their cables and powering a motor that generates electricity.

More recent versions of Energy Vault’s storage solution look a bit more staid — the cranes have been replaced with warehouse-esque buildings full of 30-ton bricks riding elevators, which you can see in the video below — but the underlying concept remains the same, using excess renewable energy to power the mechanisms that lift the blocks and dropping them when renewables aren’t available.

The concept isn’t limited to building towers or bricks on elevators. Other companies are exploring using abandoned mine shafts as potential gravity storage sites that can help stabilize the grid during energy spikes.

Gravity storage sidesteps the mountain-sized hurdle getting in the way of new pumped-hydro stations, and experts say it should be just as efficient as pumped hydro. Yet it, too, isn’t perfect: Building gravity storage systems is also energy-intensive, and the costs might outweigh the benefits — especially if lithium-ion batteries continue to get cheaper.

Energy Vault says it’s working to get costs down, particularly in terms of raw materials: the company told WIRED’s Matt Reynolds that their new bricks can be made out of waste materials rather than concrete, reducing the energy load of setting up the system.

Giant thermoses can repurpose old fossil fuel technology

A key problem in the energy transition equation — and currently occupying the minds of the international negotiators at COP27 — is how to pay for it, especially in countries that installed fossil fuel power plants relatively recently.

Unlike the United States, which has an aging fossil fuel power plant fleet that is nearing (or, in some cases, well past) retirement age, countries like India and China are home to coal-power plants that have plenty of shelf life left.

That’s where thermal energy storage comes in. In a thermal storage system, renewable electricity coming from sources such as wind turbines or solar panels is used to heat up a material that’s particularly good at capturing heat, like a molten salt, and surrounding it with insulation to essentially make a giant thermos. That heat can then be released to create steam or hot air and drive a turbine, just like a coal or nuclear plant today.

A man bends over a field of leafy green vegetables in the foreground, while steam rises from a coal plant in the background.
A coal plant in Tongling, Anhui province, China, in January 2019. Thermal energy storage could be used to repurpose coal plants to store renewable energy.
Qilai Shen/Bloomberg via Getty Images

Mallapragada’s particularly excited about the potential for thermal storage because it can potentially be used in existing fossil fuel plants by swapping out, say, a coal burner for a thermal storage unit. This solves multiple problems: A large portion of the existing infrastructure can remain in place. Coal plants are already attached to the power grid, which saves on costs, and many of the existing jobs at those power plants will transfer over to a plant powered by stored thermal energy — which means a more equitable energy transition.

For developing countries, that could be a game-changer. But — and you might be noticing a theme here — thermal storage has some downsides too. Gravity storage and pumped hydro are very efficient; you can usually recover somewhere in the vicinity of 70 to 85 percent of the energy stored. Thermal storage is much less efficient, so it couldn’t be relied on alone. It’s just another piece slotting into the jigsaw, rather than the complete picture.

Rust can be our friend

Every solution discussed so far has been something other than a chemical battery. But even if lithium-ion might not be the best solution for the grid, there are still some chemical batteries worth considering. One of them even uses something that, unlike lithium, is ubiquitous: rust.

Rust is usually a nuisance. But a type of battery called an iron-air battery turns that idea on its head.

Unlike the rest of the solutions in this story, iron-air batteries are the most similar to what we traditionally think of as batteries: they rely on chemical reactions to store and release energy, just like lithium-ion batteries. But traditional batteries are usually a combination of two or more chemicals inside one battery casing. In iron-air batteries, one of the chemicals is iron — one of the most abundant metals on our planet.

The other chemical? The oxygen in the air around it.

Iron rusts. We all know this. But rusting is a chemical reaction, called oxidation, and just like the reaction in a lithium-ion battery it can be reversed. That is to say, we can charge up rusted iron.

It’s as if the electricity is being used to polish the iron: Charging the iron keeps the oxidation at bay, but the iron’s natural state is to want to rust. Letting the iron rust essentially pushes the electrons out of the metal, discharging the battery. When it’s time to charge the battery again, the process is reversed.

It all sounds a bit sci-fi, but the idea is quickly becoming a reality; a company called Form Energy recently signed a deal with a Georgia utility to build an iron-air battery that can store 100 hours’ worth of energy. Eventually, the company hopes to build farms of iron-air batteries, each the size of a side-by-side washer and dryer, that can scale in size according to the needs of a community.

Mallapragada cautions though, building rust batteries is “easier said than done.” It’s hard to find the sweet spot of a chemical combination that can charge and discharge without losing too much energy along the way.

Rusting iron isn’t the most efficient battery in the world — its 40 to 60 percent efficiency range pales in comparison to the ultra-efficient lithium’s 90-plus percent — but it’s much, much cheaper and easier to make. Lithium-ion batteries require all sorts of limited metals; iron is everywhere.

Putting the pieces together

Energy storage isn’t going to be simple, and there isn’t going to be any single solution that’s going to get us to a place where we can be free of fossil fuels. Some of the batteries mentioned here might not even work; many are still in early testing. But all of these solutions, put together with many more we didn’t touch on here, are still important steps in the right direction.

If they’re implemented across the country — mountain batteries in some places, perhaps thermal tanks or warehouses full of rusting iron in others — they would be the key to both stopping and living with climate change and its threats to the power grid. Even as extreme weather gets worse, stored energy could help us quite literally weather the storm.

“I’m quite optimistic that we can solve this problem, because we have all these solutions,” said Mallapragada. “The right answer will look very different for California than it would for the Northeast or other parts of the country or even other parts of the world. We have all the pieces. We just have to figure out how to make them work together.”