How To Turn Caves Into Giant BatteriesS

How do we build a world that's less dependent on fossil fuels? One solution is right under your feet. Compressed Air Energy Storage (CAES) is a very boring name for a very awesome system that turns the Earth itself into a giant battery. Experts say it's one of the most cost-effective ways to store energy on a massive scale—something that we'll have to do if we want to use more wind and solar power, and less coal and natural gas. Here's how it works.

Every day, in more ways than we even realize, you and I depend on the reliability of our electric grid-the network of wires that connect power plants, houses, and businesses in an endless circuit. But that grid is flawed in a lot of ways, some of which hinder our ability to make energy more sustainable. For instance, the grid has no storage, at least not enough to matter. And that means that it's not as reliable as we think it is.

At any given moment, there must be almost exactly the same amount of electricity being produced as there is being consumed. If the balance tilts either way-even by a fraction of a percent-it could lead to a blackout. To simply keep the lights on, the grid has to be constantly monitored, with controllers predicting demand and making small adjustments, minute-by-minute, to supply. This happens 24 hours a day, 7 days a week. The job is hard enough with coal, natural gas, nuclear, and hydro power plants, which can increase or decrease production more-or-less on demand. As we rely more on wind and solar power, though, it'll get harder. That's because the output from those energy sources is more dependent on the weather than on what the grid needs. If you've got a lot of electric demand, but not much wind, you can't ask a wind farm to produce more electricity.

This isn't a big problem now, but it will become a problem in the future. When I was researching my book, Before the Lights Go Out, engineers and other grid experts told me that the United States could get somewhere between 20% and 30% of it's electric capacity from wind and solar power before we'll have to make some big changes to the way the grid operates.

Energy storage is one of the key tools we have that can solve this problem. But adding storage to the grid is not as simple as it sounds. That's because scale matters. Chemical batteries, like the ones in our cars or our iPhones, are the first things most of us think of when we think about storing energy. But by the time you're talking about socking away enough energy to light and power a few hundred homes for just half a day, chemical batteries have to be ridiculously large. At this scale, a chemical battery is the size of a semi truck, costs millions of dollars, and must be shipped over from Japan. Nobody in the Western Hemisphere makes them.

That's where CAES comes in. CAES systems store energy underground in the form of compressed air, but to make it work you have to start with the right kind of geology. In particular, you need a space that's airtight. This means that you can't just pump air into the sort of cave you've toured while on vacation. Instead, you have to find a hollowed-out space underground that used to hold something naturally-such as a natural gas reservoir that's had all of the gas pumped out of it. You can also use porous rock-material that looks solid, but is actually full of little holes.

Once you've identified your cave, then you build a wind farm nearby. These turbines have to be connected into the grid, but they're also connected to big air compressors. During the day, the system works like a normal wind farm, but at night, when the wind is plentiful and electric demand is decidedly not, the turbines stop sending electricity to the grid. Instead, that electricity is used to power the air compressors. During the course of the night, the compressors pump the cave full of air. Then the next day, as electric demand rises, that compressed air is released and used to help run an on-site generator, which is also partially powered by natural gas.

How To Turn Caves Into Giant Batteries

The air stored down there is under pressure, but it can't explode because air doesn't explode. A high-pressure air leak is a risk, and would be dangerous, says Jared Garrison, a Ph.D. student at the University of Texas at Austin who studies the integration of CAES, wind power, and the grid. But the geology helps you avoid that. "You look for sites that have a dome-shaped impermeable caprock above the space where you'll store the air," he said. "Pressurized gas only moves up, not down. So the caprock naturally holds the gas in place, where you put it. The only way for air to escape is back up the pipe."

Nothing like this exists anywhere in the world today. There are two CAES systems in operation, one in Germany and one in Alabama. But they aren't connected to wind power. Instead, they're just used to provide backup to the grid in general. It's been 20 years since the CAES facility in Alabama was built. It's only recently-as more people have started to think seriously about what it will take to get us off fossil fuels-that CAES has become more interesting. Right now, there are several places in the United States where people have proposed building a CAES system linked to wind power. The furthest along: Two sites in Texas that Garrison thinks will be breaking ground in the near future.

The company developing those sites, Apex Energy, is in a good position to make a CAES-and-wind combo successful. Texas has more installed wind capacity than any other state, and most of those wind turbines are built in places where the wind is strongest at night. Demand for electricity is highest during the day, so a CAES system would allow Texas to get more useable energy from wind turbines that already exist. Instead of letting a windy night go to waste, that energy could be stored and used the next day.

But success is still a moving target. A few years ago, Iowa looked like it was on track to become the site of the first CAES system in two decades. But that project didn't happen. The reason: After taking a closer look at the geology, researchers realized that the Iowa site wouldn't be able to store as much compressed air as they needed it to.

Maggie Koerth-Baker is the author of Before the Lights Go Out: Conquering the Energy Crisis Before It Conquers Us (find out more here), and is the science editor of BoingBoing.