Regenerative Braking & Energy Storage Without Batteries? Hobbyists Are Figuring It Out

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One of the best thing about electric and hybrid vehicles is that the energy doesn’t get completely wasted when you need to brake. By using an electric motor as a generator, you can slow a vehicle down and put some of that kinetic energy into a battery pack so you can use it again later. Sure, there are conversion losses both going into the battery and coming back out to the wheels, so you don’t get a lot more than half of the energy back, but an ICE vehicle turns all of that energy into heat, which gets dissipated into the air.

Electric bikes, scooters, and other micromobility options can do regenerative braking, too. This is great for getting better range and doing fewer brake jobs, just like in a car.

But can this be done without batteries and electric motors? Can you store energy away for later use? It turns out that you can, and this guy built a bicycle that does it.

What he built is often called a Kinetic Energy Recovery System, or KERS, and if you’re familiar with Formula One racing, you may have heard of this.

Instead of taking mechanical energy, converting it to electrical energy, and then using that to charge a battery (this then becomes chemical energy), these sorts of KERS systems use a flywheel to store the kinetic energy. For those familiar with ICE, most engines already use a flywheel to smooth out an engine’s power delivery by storing energy in a spinning disc on front of the engine between the times a piston pushes, so it’s an established technology.

Using a clutch or CVT, gears can take power from the wheels and use it to spin up a flywheel that’s not directly attached to a vehicle’s engine. This adds drag to the vehicle’s wheel, and brakes the vehicle without simply putting all of that energy into the air as heat the way friction brakes do.

By using low friction bearings and making the flywheel turn at over 60,000 RPM, an automotive KERS system can hold energy for a while and allow it to be used later for a quick burst of power. If used on the street, an ICE vehicle could potentially use this technology to save fuel the same way a hybrid does.

Why Don’t We See This Used In Street Vehicles?

Mechanical systems like this haven’t been implemented widely, though. Beyond research projects by automotive companies, only some British buses have actually used the system on the streets. Why? Because it’s expensive, adds mechanical parts, and if something goes wrong mechanically with something turning at such high RPM (in excess of 60,000), it could be like a grenade going off. For these reasons, and the fact that automotive electrification is the main path forward, it makes sense to just use batteries to store energy instead of using them.

For something small like a bicycle, it made sense to experiment with the system, though. The energies involved are very low (no risk of something blowing up like a grenade next to one’s crotch), and the use of a mechanical bicycle doesn’t pollute the way an ICE vehicle does. Getting an extra bit of power during acceleration or to climb a hill would be nice for bikes without an electric motor, right?

It Didn’t Really Work Out That Well For The Bike

Even in this case, it didn’t work out that well for the guy in the video.

The idea was solid. He had some pulley gears that could pull power from the rear wheel and use it to spin up a steel disc, and used a bicycle brake to act as a clutch to transfer the energy without needing a CV (continuously variable) gear of some kind. When he wanted to stop, he could pull the brake lever just like a a regular brake lever, and it would start transferring energy into the disc. When he wanted power again, he could reconnect the disc to the gears with his brake/clutch and it would start pushing the bike forward.

The problem is that the speeds involved weren’t quite enough to get good energy storage. The flywheel quickly would rob the bike of enough energy to spin up, and then couldn’t spin any faster than the bike, so it stopped slowing the bike down. If he used the pedals to charge the flywheel up, it could store more energy, but that’s kind of a waste in this case.

It’s a good way to see how this kind of energy storage system works, even it if wasn’t practical.

Still Good Potential For Stationary Energy Storage Without Batteries

A mechanical flywheel doesn’t care what kind of energy gets it to spinning. You could use rockets the way Adam Savage did with his Panjandrum project. You could use a moving vehicle’s kinetic energy to turn a flywheel. You could also use an electric motor to get a flywheel spinning.

This is something that Adam Savage also experimented with before later using rockets:

To test the flywheels, Adam’s team used a 72-volt electric motor to get them spun up. Because they weigh 450 pounds each, these huge flywheels ended up being able to store a LOT of energy even at 300 RPM, about a third of their target speed.

The same guy who built the flywheel bike decided to see if he could use a flywheel in place of batteries for energy storage, and he had a great idea to reduce friction.

By using magnetic levitation, friction losses were very low, and he set up an alternator to be turned by the levitating flywheel. At one point, he used an electric drill to spin the whole flywheel up, producing significant voltage.

For stationary storage, a number of companies as well as NASA are working on commercializing the technology because it has important advantages over batteries. Not only would it have better longevity than lithium batteries, but it would also have less problems with pollution and disposal at end of lifetime. For those reasons, it makes a lot of sense to experiment with flywheels.

On the other hand, a mechanical failure of such a system, assuming it’s large enough to power something like your home, could be catastrophic. A small vehicle-based system might store enough energy to be like a hand grenade should it fail. A home system designed to turn a generator and power everything in your home all night could be like having a bomb go off that sends shrapnel into your neighbor’s homes, too. For a flywheel big enough for utility storage, you’re talking about some serious damage should the thing malfunction.

For this reason, there needs to be some fairly hefty shielding around an energy storage flywheel, or it needs to be stored underground where it won’t hurt anyone if it “grenades.”

Clearly the technology has some great potential, but it needs some serious work to be ready for prime time.

Featured image: Screenshot from this video, which shows a flywheel energy storage system.

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Jennifer Sensiba

Jennifer Sensiba is a long time efficient vehicle enthusiast, writer, and photographer. She grew up around a transmission shop, and has been experimenting with vehicle efficiency since she was 16 and drove a Pontiac Fiero. She likes to get off the beaten path in her "Bolt EAV" and any other EVs she can get behind the wheel or handlebars of with her wife and kids. You can find her on Twitter here, Facebook here, and YouTube here.

Jennifer Sensiba has 1988 posts and counting. See all posts by Jennifer Sensiba