# Flywheel generators in vehicles.

1. May 28, 2012

### Robin07

Is it that, flywheel electric generators are not used in vehicles because? The momentum of the wheel will tend to have the vehicle continue in a straight line rather than turning a corner with that vehicle? If so, what would we expect if the flywheels' orientation changes with the direction of the vehicles path?

Thanks

2. May 28, 2012

### phyzguy

I think they are mainly not used because they simply don't store enough energy. Let's take a simple example. A 200 kg flywheel 1 m in diameter spinning at 12,000 RPM stores an energy of 1/2 I omega^2 = (M*R^2)/4*omega^2~20MJ of energy. This is about the energy in 1/2 liter of gasoline. So if your car is very efficient, it might go 10 miles before you have to stop and spin up again.

3. May 28, 2012

### Robin07

Thanks, I was thinking more along the lines of a continuously spinning wheel, generating electricity from a photo array. And I guess what I was more curious about is the momentum associated with the flywheel. I suspected that the momentum of the flywheel would work against the motion of the vehicle turning. I tested a home build electromagnetically pulsed gyroscope and tested to see if this was true, when I turned the corner, in my truck, I didn't notice any conflict in momentum. Perhaps it was note spinning fast enough or I didn't turn hard enough or a combination of both.

Anyways thanks for the response.

4. May 28, 2012

### Staff: Mentor

I'm not sure if you recognize, but flywheels are batteries, not generators.

5. May 28, 2012

### Robin07

Yes, thanks. Flywheels are batteries, storing kinetic energy. I've housed a flywheel around a permanent magnet rotor which aids in generating electricity. It's all R+D/hobby at this time. I was surprised to see what you can do with stuff around the house. It works pretty well, as far as the mechanics are concerned anyways. I haven't measured the out put voltage yet or increased the weight of the rim to test out my suspicions that its momentum will interfere with stearing or not.

6. May 29, 2012

### AlephZero

The gyroscopic effects of the flywheel are not so important if the axis is vertical not horizontal. But as others have said, considering the amount of extra mass you have to carry around in case the flywheel bursts or its bearings fail, the efficiency sums don't add up to much.

7. May 30, 2012

### Robin07

My axis rotates in a horizontal plane but can be adopted to vertical if need be. I find that spinning the rotor in a vertical plane suits my design. The axle spins with the rotor, in a vertical fasion as well as turning in a horizontal guide-way. Essentially flipping end for end very much like a "power-ball" or what we used to call a Dynabee wrist exerciser or the newest one that I've run into called a Dynamax. All of these utilize the gyroscopic effect for resistance training.

It's not tapping into the stored energy that's in the flywheel, as per say. Although it does aid in the momentumum of the wheel, it's more the increase in rim speed that I was trying to accomplish without increasing the voltage.

Thanks Phyguy, I wasn't aware that a flywheel is that inefficient.
Thanks Alphzero, the concerns you have are being considered as well as other factors involved. That is not to say I know all that needs to be taken into account. I'm all ears.

I'm not as clear in understanding the differences that come into play when a gyroscope is spun in a vertical fashion compared to a horizontal plane and its related momentium if carried in a straight line and then turned from that straight path. Would you clarify this for me Alephzero?

8. May 31, 2012

### Danger

In a round-about way, the regenerative braking in an electric or hybrid car could be considered a flywheel effect. The angular momentum of the wheels is partially converted to electricity.

9. May 31, 2012

### xxChrisxx

Flywheels work perfectly well when used correctly.

You can't run a car with one, but you can use it as a hybrid, either a full hybrid or a patrial hybrid. It's also not worth tapping energy from a flywheel with the method you described, it's best to either stick it into a generator or use a CVT pully.

see: http://www.flybridsystems.com/

Vehicles produce a good proportion of CO2 when pulling away, the idea of partial hybrids using flywheels is that they provide the power to pull away so a few HP for a few seconds. Which can give a decent saving for CO2/km on the duty cycle.

For storing high amounts of energy we need 2 things:
http://en.wikipedia.org/wiki/Rotational_energy.

1. A high J value. (Angular mass).

High angular masses mean more energy is stored per RPM. It also means that the outer edge of the material stores the most. This means we want out flywheel to be like a doughnut.

However from the power equation we can see that w^2 vs J. This means rotational speed is actually more important than J for achieving high energy storage. The angular speed increases stress on the flywheel, so we want to make it out of something with ver high strength but low weight. It's why we make them from carbon fibre.

2. High rotational speed.

High speeds, means that the flywheel needs to be run in a vacuum for efficiency. This is obviously difficult if you need to attach it to a shaft to tap the energy. So one of the areas of development is a magnetic coupling (essentially a fixed magnetic gear ratio).

http://www.ricardo.com/en-GB/News--...nd-generation-high-speed-flywheel-technology/

http://phys.org/news/2011-09-flybus-prototype-hybrid-bus-future.html

10. May 31, 2012

### phyzguy

I wouldn't say that they are inefficient. An internal combustion engine is only 20-30% efficient at converting stored chemical energy into energy of motion, while I think a flywheel can achieve efficiencies of 80-90% or more. It's just very difficult to get them to store as much energy as a tank of gasoline.

11. Jun 11, 2012

### benny61

I was just going to butt in and say it can act as a perfect hybrid.

My main concern with hybrids is their value. If you look at how much money they save, versus how much you pay for them, you'll see you need almost 30 years for a return.

At the moment only eco-enthusiasts buy them.

12. Jun 13, 2012