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Microwatt

Thu Aug 18 2011, 06:04AM

Registered Member #3282 Joined: Wed Oct 06 2010, 05:01PM
Location:
Posts: 224

Patrick wrote ...

Also one last thing, dont exclude other options that arnt like your manhole idea, remember kinetic energy of a rotating body can be calculated with radius and mass with velocity and integration or some such thing. Ask yourself this: does it have to be a thin rotating wheel? or can it be a long slim rolling shaft? does it have to be light and spinning fast? or can it be massive and spinning slowly? one way may be easier/cheaper/faster to build than another way.

The reason why I won't consider anything else becuase a thin rotating heavy wheel is the most optimum geometry to store rotational kinetic energy. And a manhole cover is the cheapest piece of steel you can find that is big and disk shaped.

Don't you want to have megawatt levels of power?

Forty

Thu Aug 18 2011, 03:33PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

I'd find a brand new manhole cover if you do use one. a weathered cover could be misshaped or have tiny cracks in it. the latter could cause it to fly apart at high speed.

Ash Small

Thu Aug 18 2011, 03:59PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Look on Ebay etc. for big, single cylinder diesel engines.

They are often dirt cheap (a few dollars) and have HUGE flywheels (around manhole cover size, or even bigger), you might also get some useful bearings with it.

Finn Hammer

Thu Aug 18 2011, 05:14PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Microwatt wrote ...

The reason why I won't consider anything else becuase a thin rotating heavy wheel is the most optimum geometry to store rotational kinetic energy. And a manhole cover is the cheapest piece of steel you can find that is big and disk shaped.

Don't you want to have megawatt levels of power?

You may very well find that a manhole cover is not steel at all, but instead cast iron, lacking much in tensile strength.

Cheers, Finn Hammer

2Spoons

Thu Aug 18 2011, 11:51PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

Getting a manhole cover balanced will be fun. You might be better to simply spend some $ and get a disc cut out of steel gauge plate. It will be stronger and easier to balance and attach a shaft to.

Last time I was involved in a flywheel system the flywheel was a 10kg composite 'barrel' shape with a carbon fiber outer wrap. This was intended to run at 40,000rpm, in a vacuum. Stored energy would have been 3MJ, if it had ever got up to speed before the bearings died.

Patrick

Thu Aug 25 2011, 07:06AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

2Spoons wrote ...

Getting a manhole cover balanced will be fun. You might be better to simply spend some $ and get a disc cut out of steel gauge plate. It will be stronger and easier to balance and attach a shaft to.

Last time I was involved in a flywheel system the flywheel was a 10kg composite 'barrel' shape with a carbon fiber outer wrap. This was intended to run at 40,000rpm, in a vacuum. Stored energy would have been 3MJ, if it had ever got up to speed before the bearings died.

I was told that some college and nuclear weapons labs had flywheels that could spin at 1,000,000+ RPM, and weighed thousands of pounds, for extremely high energy impulse they could go from high RPM to low in just several revolutions! One of my professors said the one he worked on as a college kid was really scary, at a million RPM if something went wrong they new they would all be dead pretty quick. it was built in an underground cement/steel blockhouse and there were all kinds of safety rules and such.

2Spoons

Thu Aug 25 2011, 11:57PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

The system I worked on had containment walls made from aluminium 1.5 inches thick, in order to deal with catastrophic failure of the flywheel.
1,000,000 rpm seems improbable for that kind of weight.

Forty

Fri Aug 26 2011, 12:48AM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

5000lbs and .5m radius cylinder at 1,000,000rpm would hold about 3 terajoules. that'd be scary.
The hardest part about building a compulsator is setting up the coils and connections, so you should probably do that first and worry about what chunk of metal to use as your flywheel later.

are you going to go with a rewound alternator? is that the best starting point for a compulsator or would some kind of large motor work? It sure is hard to find information about compulsator design.

Myke

Fri Aug 26 2011, 01:32AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

They use a large flywheel for nuclear fusion research at MIT since they can't suddenly pull that much power from the system without large problems iirc. I highly doubt they've made many things that can go 1000000 RPM. I think I'd belief a couple tons at a few 10000s RPM though.

EDIT: 1000000 RPM was finally reached in 2008 but is for only very tiny things. I'm not sure if it has a been used in any applications since it was made.

Bored Chemist

Fri Aug 26 2011, 09:08AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

I simply don't believe that anything i strong enough to make a macroscopic million RPM flywheel.
Here's a bit from WIKI that explains why
"The amount of energy that can safely be stored in the rotor depends on the point at which the rotor will warp or shatter. The hoop stress on the rotor is a major consideration in the design of a flywheel energy storage system.
Ïƒt =Ï r^2 Ï‰^2
Where:
Ïƒt is the tensile stress on the rim of the cylinder
Ï is the density of the cylinder
r is the radius of the cylinder, and
Ï‰ is the angular velocity of the cylinder."

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