What kind of a flywheel would you need to be able to store 40-80 kWH in a car?

What kind of a flywheel would you need to be able to store 40-80 kWH in a car?

Vision: you drive into a supercharger station in your Tesla RedDwarf (Gen V :o) and park over a charging rig. Two or more chargers automatically click into place. Some begin to top up your batteries while others very quickly spin up the cylindrical flywheels integrated into the car's battery platform. Within a minute or two you're off again. For the next 20 minutes, the flywheels are used to top up the car-batteries. For the remainder of the trip they're used in a way similar to eg. the KERS system in place of regular electric regenerative braking.

Is this vision realistic when you get down to the numbers and the science? One number I've seen floating about on the internet is an energy-density (on the lowish end) of around 100 Wh/kg for a flywheel.

hiawatha | September 30, 2013

That didn't work as expected... here's the beginning of the post!

Quora, for those who prefer it:

I have no knowledge of physics, engineering or materials science whatsoever, so any and all information would be useful :o

With present or very-near-future technology, would you be able to quickly store the equivalent of 40-80 kWh in one or more flywheels in an electric car?

What kind of materials would be realistic? What kinds of speeds? What volume and mass?

Timo | October 1, 2013

100Wh/kg is low. Batteries can handle quite a bit higher charging speed than what SC gives, so flywheels don't really add anything to this.

PorfirioR | October 2, 2013

@hiawatha, I could not follow your link as it required an account. So my apologies if I am repeating anything here.

It appears that the flywheel idea is not as far-fetched as it sounded to me originally. It looks like Volvo already has a system in development for their S60 model:

On the surface, it makes more sense to me in gas-electric hybrids where the flywheel coupled with regenerative braking takes the place of a battery.

However, the idea is quite intriguing to me, especially if there could be a way to somehow inductively "charge"/spin the flywheel (i.e. magnetically) while the vehicle is in motion. For examplie, driving along a magnetic "rail". Otherwise, the idea of pulling in somewhere to get a spin sounds a lot, at least logistically, like battery swapping.

PorfirioR | October 2, 2013

I just realized that I said something wrong. A gas-electric hybrid would cease to be gas-electric if the flywheel replaces the electric components and instead would be simply a more efficient gas vehicle. So the Volvo S60 concept would not be technically a hybrid.

Dramsey | October 3, 2013

With present or very-near-future technology, would you be able to quickly store the equivalent of 40-80 kWh in one or more flywheels in an electric car?

No. Simple, huh?

It is simple physics. Under ideal conditions, about 80kWh of power will move a 4,700-pound Model S about 260 miles. Consider the amount of kinetic energy that much mass moving that distance represents. That's the amount of energy you have to put into the flywheel.

Without working through the math (because I'm lazy), it would be a LOT of mass spinning VERY fast. Personally I'd be very nervous having that much momentum in a spinning mass inside the card. I'd worry about gyroscopic effects, too!

Josh_D | October 5, 2013

^ That. I could run the numbers if anyone really wants me to, but I don't think it would be practical.

Dwdnjck@ca | October 6, 2013

It would work if the flywheel were made of Kryptonite.

Timo | October 7, 2013

Using wiki I found this:

Electric power backup[6] 600kg 50cm 30,000RPM 92.0MJ 26.0kWh

In order to store 85kWh similar flywheel would require mass of nearly two tons, IE. mass of the entire car. Gyroscopic effect would be rather massive too.

nwdiver93 | October 7, 2013

Yeah... any flywheel large enough to store enough energy would make it nearly impossible to turn.

polyphase | October 7, 2013

The physics on this one seem kind of tough, but I am picturing a 1934 "Popular Mechanics" cover that might feature it! A 3-story gantry perched on the trunk supporting a massive flywheel spinning perilously close to the occupant family's heads (convertible, natch).
"Car that travels 1000 miles with no gas tank coming soon, scientists say."

Timo | October 7, 2013

nwdiver93, turning would not be a problem if the flywheel spins in horizontal level, but tilting car would be near-impossible. Climbing hills would be rather...interesting. OTOH it would be fun to see car seemingly hanging in mid-air just one set of wheels touching the ground once other set is driven over some hole.

Earl and Nagin ... | October 9, 2013

You'll find that Tesla's Chief Technology Officer, JB Straubel spent a lot of time working on flywheel energy storage at Rosen Motors ( in his early days ( From that, you can be pretty sure Tesla is very familiar with what a flywheel can and can't do.
Flywheels have good power density (W/kg) but not good energy mass density (wh/kg), making them good for KERS (Kinetic Energy Recovery Systems) in racing but not so good for providing driving range in an EV. In other words, they can provide short bursts of a lot of power but not for a long time as is needed for an EV to get good range.