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How Heavy Does a Flywheel Need to Be to Turn a Generator For an Hour?

03/02/2012 12:19 AM

How heavy does a flywheel need to be to turn at a slow speed (100 rpm) in order to turn a generator for an hour at 2500 rpm's?

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#1

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 12:40 AM

Welcome to the forum Chuck.

In a soft way, I'll try to say that your question has no valid answer. Even if there were no friction to slow down the system, once you start taking energy from the generator, the flywheel (and generator) will begin to slow down.

There is another area you need to research as well. Look up flywheels and you will find that it's not just how heavy they are, but whether the mass is at the outer edge (like a bicycle wheel) or evenly distributed (like a disk cut from a flat sheet of metal).

I am sure that others here will give you links to specific sites where you can add to your body of knowledge on flywheels and such. I wish you well.

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#2

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 12:56 AM

And to add further explantaion, the weight of the flywheel (and as mentioned the mass distribution) with the rpm, gives the momentum of the flywheel. And the momentum gives us how much energy it has stored.

The transfer of this stored energy to electricity through the generator, depends on the size of the generator and the efficiency of the generator (at various rpm - as mentioned the rpm will slow as the energy stored in the flywheel reduces).

So in the perfect world, the mechanical energy in, will equal the electrical energy out, allowing for efficieny losses in the generator and in the mechanical transfer of the energy to the generator.

My advice; start with how much energy you need (ie how many kW?) Then size the generator, and obtain the efficiency / rpm curve. Then you can calculate the energy requirements of the generator to achieve the outputs you need.

Hope this helps,

Anthony

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#3

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 1:13 AM

Thanks for the start. To be a little more clear, I have a 7kw portable generator that I would like to run off a flywheel instead of the motor. I am aware of the dangers of high speed failures so I would like to turn the flywheel at lower speeds thus the heavy weight The generator came with a 5 hp motor that no longer works and the little boy in me wants to see if I can get it to produce 7 kw for one hour using a flywheel.

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#4

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 1:42 AM

The speed would be in a curve, the flywheel under no load would begin to slow on its own once the force driving it was removed.....a generator generally speaking, must be run at a constant speed to be useful, unless you are storing the energy say for instance in a battery...A 7kw generator would require about 12hp, this would require a force of 12 x 550 ft·lbf/s = 6600 ft·lbf/s.... this would take a flywheel that weighed several million tons....

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#8

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 9:11 AM

His 7 kw generator came with a 5 HP engine. Something doesn't compute.

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#9

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 9:12 AM

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#10

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 9:20 AM

Point taken.

However the OP wants 7kw continuous, I believe. Or, at least he's under the impression that his generator is capable of this.

A tall order for a 100RPM flywheel in either case.

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#11

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 9:23 AM

Much better than 33 1/3 rpm. :)

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#12

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 9:45 AM

There's a whole generation out there going, "What's that?"

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#21

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 4:27 PM

I was shouting at the radio last night as I listened to Eddie Izzard saying that whale song was "on 78...if you put it on 45" and he demonstrated that the frequency of the sound would rise and the words would run together to be intelligible as English.

A good joke ruined....

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#22

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 4:36 PM

I think I have that song on an 8-track tape, somewhere.

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#23

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 6:34 PM

Half-heard that - didn't believe my ears. <sad face>.

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#14

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 10:47 AM

You could of course put the flywheel output through a continuously variable transmission system in order to drive the generator at constant speed.

The questioner might like to Google the Oerlikon Gyrobus, which extracted electricity directly from a spinning flywheel and could propel the bus for about 1 hour. But .... the flywheel spun at about 3,000 rpm. On the other hand it didn't weigh millions of tons, or even 48 tons. You may note that the flywheel recharge time was between 30 secs. and 3 mins. - a hell of a lot faster than any battery recharge. I know various companies have re-investigated this idea, but I have never seen any feedback regarding its success.

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#15

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 11:15 AM

Just a word on a couple of your points (not that I disagree with you): the energy stored goes up as the square of the angular veocity, so at 3000 rpm, it would store 900 times as much energy as at 100rpm. Also, the flywheel mass can be reduced (as previously mentioned by Just an Engineer) by concentrating the mass as much as possible near the rim. An "ideal" flywheel with all the mass at the rim would store twice as much energy as a disk-shaped one of the same mass and at the same angular velocity.

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#17

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 12:07 PM

And F1 stores energy at about 22,000 rpm. The F1 kinetic energy recovery system generates up to 50+ kW of energy.

Second, any transmission introduces loss in the way of friction.

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#19

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 1:05 PM

Re: transmission losses - I quite agree, and have never suggested otherwise.

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#18

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 12:49 PM

Good points!

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#16

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 11:17 AM

Thanks for being helpful. I am a firefighter by trade and have spent my life helping random people that needed a helping hand for no other reason that I could. So with that said. Thanks I understand that increasing the speed is more efficient than increasing the weight is this correct?

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#5

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 3:18 AM

There is a whole bunch of necessary information missing from the original post, which displays no understanding at all of relevant physics relationships.

On a very wild guess, such a flywheel might need to weigh a few tens of tons.

Where will the energy come from to "spin up" said flywheel?

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#6

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 8:01 AM

He's cranking it by hand?

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#7

Re: How heavy does a flywheel need to be to turn a generator for an hour?

03/02/2012 8:23 AM

Or... just run on top of it like a Flintstone tread mill.

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#13

Re: How Heavy Does a Flywheel Need to Be to Turn a Generator For an Hour?

03/02/2012 9:50 AM

Back-of-an-envelope calculation, just to get an idea (figures are approximate):

Ignoring all losses, your target figure is 7kW for 1 hour, i.e. 7kWh.

7kWh ≈ 25 x 106 joules.

The energy stored in the flywheel is ½Iω2, where I is the moment of inertia and ω is the angular velocity.

ω = 100rpm ≈ 10.5 rad/second, so setting ½Iω2 = 25 x 106, we need I = 460,000 kgm2.

Consider a solid steel disk-shaped flywheel, 100mm thick (got to start somewhere!). The moment of inertia = mr2/2, where m is the mass and r the radius. The mass of the disk = volume x density = pi x r2 x h x 7850 kg, where h is the thickness (0.1m) and 7850 is the density of steel (in kg/m3).

Substituting this mass into the formula for the moment of inertia, and setting it equal to the value for I previously calculated, we get

pi x r4 x 785 /2 = 460,000

Solving for r gives a flywheel radius of about 4.4m, which works out at a mass of about 48 tonnes.

Anyone fancy checking my figures?

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#20

Re: How Heavy Does a Flywheel Need to Be to Turn a Generator For an Hour?

03/02/2012 3:00 PM

Thanks JohnDG for providing an answer that presents the OP with some elucidating information related to his question.

To simplify things a little more for someone who has little exposure to physics / engineering, JohnDG's equation of Energy Stored in the Flywheel = ½Iω2 boils down to:

Energy ≈ rpm2 * mass * diameter2

So Chuck, you can see that the mass of the flywheel, which is what you originally asked about, has less of an impact on the energy stored in the flywheel than the rpm or the diameter.

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