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Go to Brilliant.org/SciShow to learn more. [♪ INTRO]. Renewable energy sources like wind and solar are awesome, but they have a downside—they're intermittent.

That means, if we want to make our grids more sustainable, we need to store some of the energy they produce for later. Not everybody loves the idea of using electrochemical batteries for this, though, because they're expensive, and they're not all that great for the environment—which kind of defeats the purpose of switching to renewables in the first place. Luckily, there are other options, like mechanical batteries: ones that store energy using the physical position or motion of things rather than chemicals.

Many of these were initially designed ages ago, but they're making a bit of a comeback. And that's because, while we might be able to come up with all sorts of completely new ways to store energy, the ones that have worked for centuries can continue to be put to good use—especially with some modern sprucing. Let's start with one you're probably familiar with: compressed air.

Ever since the invention of the first air pump in 1650, compressed air has been used to power things like railway brakes and noisy pneumatic tools such as nail guns and jack hammers. Turns out this idea can be scaled up a lot to be used for energy storage. First, a high-pressure compressor pumps air into a huge chamber, like a salt cave or an abandoned mine.

Then, when we need the power back, the air is released to run an electricity-generating turbine. Thanks to their ginormous size, compressed-air batteries offer the large-scale capacity needed by power plants — up to 321 megawatts at a time. They're also pretty cheap, and can start pumping juice in about twelve minutes, which is less than half of the response time of conventional fossil-fuel turbines.

Plus, they can store the energy for over a year. But there are some drawbacks. Thanks to our friend thermodynamics, we know some of the energy used to compress the air becomes heat—which dissipates over time into the chamber's walls.

And since the air also becomes cold when it's decompressed, it needs to be reheated so that it doesn't damage the turbines —so energy is also lost on the way out, as well. That means the ratio between the energy we put in and the energy we can get out—the battery's round-trip efficiency—is lower for compressed-air batteries than top-of-the-line chemical batteries. Those have round-trip efficiencies around ninety percent.

But even modern compressed-air facilities, which reuse the heat from the compression process to warm the outgoing air, only reach sixty to seventy percent efficiency. Engineers might be able to improve upon that, but for now, we might want to turn to other retro battery technologies, like rail energy storage. That's basically a battery powered by gravity in the form of a weighted train.

The process is simple: a locomotive pulls train cars weighted with rock or cement up a slope. More cars can be brought up to store additional energy as needed. Then, when the cars roll downhill, the gravitational energy is converted into electricity through regenerative braking, a technology already used in some hybrid and electric cars.

We've used gravity to store energy since at least the fourteenth-century, when mechanical turret clocks powered by suspended weights were invented. And people have come up with several types of modern gravity batteries, like ones which use cranes to raise and suspend giant blocks. But the only large-scale gravity battery in development so far consists of this sort of train on a hill setup.

With a capacity of about fifty megawatts, rail batteries offer less per-battery storage than compressed-air systems, but they also have a bunch of advantages. Like, you don't need a Batcave to house all that energy, because rail storage can work anywhere with an incline of at least five percent. And they have decent round trip efficiency, at about seventy-eight percent.

Plus, their startup time of five to ten seconds is an order of magnitude better than what compressed-air storage can offer. But that's slow compared to flywheel energy storage. Flywheels are basically just wheels that take considerable energy to put in motion.

Each rotates around a shaft, and because we know objects in motion tend to stay that way, these spinning wheels can store most of the effort put into moving them as kinetic energy. Then, that energy can be siphoned off with deceleration —kind of like regenerative braking. Flywheels are super old-school tech.

They have kept potter's wheels turning evenly since at least the third millennium BCE. And you have a flywheel to thank if you've ever ridden in a non-electric car, because they smooth out the explosive bursts of energy in internal-combustion engines. But flywheels can also be used as batteries.

The first flywheel created specifically for energy storage was developed in 1833 to power the first self-propelled torpedo. They have some drawbacks, of course. Like, they're less useful for large-scale power-plant storage, since they can lose as much as twenty percent of their charge per day to friction.

But modern flywheels use space-age materials like carbon-fiber composites, and spin in a vacuum while levitating on superconductive magnetic bearings. This makes them lighter and smaller, limits the loss of the stored energy from friction, and makes it possible to increase their rotation speed and expand their storage capacity. It also makes them super expensive to make, especially when compared with compressed air or rail storage batteries.

Still, they're probably worth the investment, since their round-trip efficiency can be as high as ninety percent. And even though they're still slower than chemical batteries, they have response times counted in hundreds of milliseconds. Also, with all of that space-age technology, their relatively small size makes it possible to use them in things like uninterruptible power-supply systems in server farms, trains, and hybrid race-cars.

These three ancient battery technologies show that there really is truth to that age-old maxim: if it ain't broke, don't fix it. And one thing's for sure—clean, cheap, and reliable energy storage technologies like these are the way of the future— as retro as they may be. Speaking of renewable energy: if you've ever found yourself wishing you could learn more about them — well I have some good news!

With a premium subscription to Brilliant.org, you can take their course on Solar Energy and learn all about the physics of capturing sun power. By the end of it, you'll be able to explain multi-junction cells and doped semiconductors like a pro. Plus, it's just one of the dozens of interactive, expert-designed courses that Brilliant has to offer.

So if you love learning new things, you can head on over to Brilliant.org/SciShow to learn more. And, as a thank you for being a SciShow Viewer, the first 200 people to sign up at that link will get 20% off an annual Premium subscription. [♪ OUTRO].