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Picture a massive flywheel tethered within a much less massive flywheel.
Now picture the inner massive flywheel as being divided into equal segments, each only allowed to move inwards, (the center of the circle), or outwards, (the less massive flywheel), on tracks, and that its circular shape is
held in place by massive springs within the outer less massive flywheel.
When the massive inner flywheel is sped up to maximum velocity, then drawing from this spin would normally be the extent of the power possible to be taken from it.
This completely disregards the amount of potential energy which could be stored from the exerted cetrifugal force of a heavy ring spinning at high speed.
At a high enough rate of speed each segment of the inner flywheel would expand constricting each of these springs, and when power is taken from the spin of the flywheel the decreasing size caused from the expanding springs energy would be tapped before the previously possible energy storage of the flywheel itself would even begin to be tapped.
bicycle with speed governor
bicycle_20with_20speed_20governor
shameless self promotion [xaviergisz, Oct 01 2010]
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Thanks eh. You're pretty quick on the draw I hadn't finished tweaking yet. |
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Boy, really needs a visual aid for my brane to get it.
Sounds very interesting though. (brain spelled that
way on porpoise) |
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a flywheel with a constant speed rim ? |
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So, like the governor on an old steam engine then? |
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how does this store more energy than a solid fly-
wheel of the larger size? |
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I was unexpectedly taken drunk last night, which makes it
currrently impossible for me to appreciate all the details
here. But, is the gist of it that you store some energy by the
compression of springs using centripugal force? |
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The advantage with this is not in energy storage, but in the grenade-like design, which will kill everyone in the vicinity when it fails. |
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The maximum energy you could store in the compression of the springs is a function of their modulus and yield strength. |
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The energy you could store in the same mass of steel by spinning it is a function of it's mass, the radius of arc and the square of the angular velocity. |
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Your extra steel would be more useful as flywheel mass than springs. |
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An effecient flywheel has most of it's mass concentrated at the rim. Since speed is of the essence, it needs to be as strong as possible.
Moving parts within the flywheel are a bad thing.
(-) |
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I think I get how it works! But it's too complicated to explain... |
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"The advantage with this is not in energy storage, but in the grenade-like design, which will kill everyone in the vicinity when it fails" |
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//is the gist of it that you store some energy by the compression of springs using centripugal force?// |
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Yes exactly. It is an (I think) untapped energy source. |
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The maximum energy you could store in the compression of the springs is a function of their modulus and yield strength. // |
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Um... No can do. Not for a bit here yet anyway. I'll look up modulus and yeild strength when I get back from work. |
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//The energy you could store in the same mass of steel by spinning it is a function of it's mass, the radius of arc and the square of the angular velocity.// |
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Well, let's think about that for a second without math. (sorry 'tsall I've got)
If I build two flywheels of equal mass and one can expand to store centrifugal energy in springs then when they are both spun up to equal speeds they should produce the same amount of energy when slowed to a complete halt yes? |
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If that is the case then the still depressed springs in the expanded flywheel have excess energy over and above that of the flywheel itself.
Perhaps they could be used to get the flywheel back up to a decent speed from rest. |
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//Your extra steel would be more useful as flywheel mass than springs.// |
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The springs would be flywheel mass. |
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I know this thing would have the potential to wreak havoc but... don't engineers eat that stuff for breakfast? |
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According to Mr. Wikipedia, a regenerative braking
flywheel for trains has a mass of 600kg, spins at 8000 RPM
and stores 30MJ. It also has a diameter of 50cm (which
seems small, but hey), or a radius of 25cm. |
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Now, the centripugal force experienced by the flywheel
(and I'm taking it at a radius of 20cm, assuming that the
disc is solid, as an average) will be about 14,000G. Hence,
in a very crude sense, the total force acting outwards on
the 600kg rotor is something like 600 x 14,000 x 9.8 =
80MN. |
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Now suppose that we allow the segments of the rotor to
move outwards by 10cm (which is a lot, compared to the
size of the rotor), against some sort of spring. In this
case, the total energy stored in the spring will be: |
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80x10^6 x 0.1 x 0.5 (ie, force times half the distance, all in
newtons and metres) = 4MJ. |
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This is a significant fraction of the energy stored as
rotation, but is not a *huge* extra amount (it's an extra 10%
or so). |
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So, my guess is that this is not worth it. |
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Yes, all the calculations are very crude (and yes, if the
diameter of the flywheel changes...yada yada). But I
suspect it's not worth it. |
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Nevertheless, it's only not-worth-it by an order of
magnitude, which is pretty good, so I'm giving a [+] for
good intuitive thinking. |
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I tried the algebra but got bogged down... in summary, the weights are latched inboard, the wheel is spun up, and it now has kinetic energy E. |
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The weights are unlatched , and move outwards against the spring. As the weights move outwards, angular momentum is conserved, so the flywheel slows down, and loses kinetic energy. It now has kinetic energy e. |
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Meanwhile the weights have compressed the springs, which now have potential energy P |
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Yes, P+e<E, but the aim of the idea, I think, was to be able
to store more energy (ie, you put extra energy in to spin the
flywheel back up, after the weight-movement has caused it
to slow down), so you store energy as both rotation and
spring-compression. |
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But, as per previous annotation, the amount of extra energy
that can be stored is only a small percentage. As others have
pointed out, the inclusion of a mechanism would weaken the
flywheel anyway, meaning that it wouldn't be worthwhile. |
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I am having a hard time understanding the figures for energy storage in the springs. Wouldn't the amount be entirely dependent on the strength of the springs and the rotational speed of the flywheel? |
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The other negative aspect of a flywheel like this is having it made from keystone segments on sliders making this inherently unstable. That part I get, but I can't help but wonder if this could be a positive thing. Flywheels are dangerous. If anything goes wrong you suddenly have a massive wheel which will stand up and tear through anything until its energy is expended or it explodes spectacularly in a very uncontrolled manner.
With this design if something goes wrong the amount of energy needed to be contained is divided by the number of segments making up the flywheel. You know exactly where each piece will end up in an accident and can build the housing to withstand them. It might also allow for some small amount of control to dampen vibrations as they begin before resonance causes failure. |
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That's the way I see it spinning in my head anyway. Probably have to build the thing from fullerene though. |
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// Wouldn't the amount be entirely dependent on the
strength of the springs and the rotational speed of the
flywheel?// |
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Yes, sort of. However, it really all boils down to rotational
speed (and mass). Basically, the rotational speed and mass
determine the force on the segments of the rotor. You then
choose a spring which is strong enough to *just* be fully
compressed (or stretched) by this force. |
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I think the closest thing to this in the real world
would be a speed governor on a steam engine.
They have balls that swing out and up pushing
against gravity instead of springs, but it's a similar
situation. (It's where we get the term: "Balls out"
for going fast) I think you're talking about pulling
back some rotation from the balls falling down as
well as their spinning to use the analogy. Might
help you visualize how much work you'd actually
be able to achieve and if it was worth it. |
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There's no extra energy you're getting out of this
though, you put extra work in to push those
weights out and the energy in the compressed
springs is what you'd get out at the other
end of the cycle. So if you had a ten pound
compressible spring, you'd need to put ten pounds
into it via rotation and you'd have ten pounds to
play with and translate back into rotation
at the end. I think technically you'd be talking
about pound-foot units (not foot-pounds) but I'm
not sure. Somebody feel free to slap me with a
dueling glove if I'm wrong. |
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Not sure if it would be worth it but croissant for
the interesting idea at least. |
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[doctorremulac3] //where we get the term: "Balls out" for
going fast// Magnificent phrase, provided your etymology is
correct, and that's uncertain. Other sources suggest the root
metaphor is "moving so fast your scrotum escapes your
athletic supporter." If you're right, then "balls-in" should also
be common. If, for example, I could find a plot of torque vs.
angular velocity with the X axis running from "balls-in" to
"balls-out" I would gratefully admit "balls-in" to my idiolect.
But Google suggests that "balls-in" is rare to nonexistent as
an antonym of "balls-out" |
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//Other sources suggest the root metaphor is
"moving so fast your scrotum escapes your
athletic
supporter."// |
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And can we be honest for a second? Who among us
didn't think that's what it meant? I pictured it a
step further, with the unit in question hanging
out of the pants completely and flying around as
the athlete, intent on glory, ran on oblivious.
"Boy, look at him run! He's balls-out!" |
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Boy that turned from science to gutter humor
quick. |
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I'd picture "balls in" being an admonition to "cool
it" or "take it down a notch" as in "Hey Doc, might
want to go balls-in a little on the potty mouth.
This is a respectable web site." |
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//Who among us didn't think that's what it meant?// |
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<embarrassed> Me. <\embarrassed> Apparently, I had a pure,
innocent mind until [doctorremulac3] corrupted it. |
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//Yes, P+e<E, but the aim of the idea, I think, was to be able to store more energy// So, how about mounting the springs static, on a frame beside the axle mountings of the flywheel? |
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//There's no extra energy you're getting out of this though, you put extra work in to push those weights out and the energy in the compressed springs is what you'd get out at the other end of the cycle.// |
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Ah. I thought that it was strictly a function of mass and that the centrifugal force was being wasted in a trapped system, (honestly I'm still not entirely convinced that it isn't). |
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...and what [mouseposture] said last. I always wondered what was meant by balls out |
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The steam engine governer is cool. What a waste of power though. |
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//the centrifugal force was being wasted in a trapped
system, (honestly I'm still not entirely convinced that it
isn't).// |
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Force isn't energy. So, you can't really "waste" the
centripugal force, and it can't be "trapped" in a regular
flywheel. |
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Force acting over a distance is energy. In a normal
flywheel, the centripugal force doesn't act over a distance.
In yours, it does act over a distance (to move the weights
against the springs), and you have to input that energy to
drive up the flywheel. |
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In a steel flywheel, as it spins up the heavy rim exerts an outward force on the steel spokes. This causes the spokes to stretch very slightly, storing energy, but the rim has now moved outwards very slightly, robbing kinetic energy from the wheel. |
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AS the wheel slows (as its energy is tapped), the force reduces, so the spokes contract (releasing their stored elastic energy), and so the rim moves very slightly inwards, which increases the kinetic energy of the wheel. |
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So one could argue this is, inadvertenty and in a very small way, baked by every real flywheel ever made, except those made from 100% inelasticum. |
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I guess the thing that I can't get my head around is that, the springs are part of the mass and become the spinning rim of a flywheel which is the same diameter and mass of a wheel which didn't expand, and that compression would be an excess energy storage without any extra input. Not that it has created any more energy, just stored a different force of inertia that wasn't being utilised. |
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I believe you, I just need to figure out why I'm snagging on this to... give the old mental projector a smack. |
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//and that compression would be an excess energy storage
without any extra input. Not that it has created any more
energy, just stored a different force of inertia that wasn't
being utilised.// |
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You're still confusing force and energy. |
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But, what happens to the energy is much as others have
already explained. If the weights move out, then it's like
an iceskater extending their arms whilst spinning: they
slow down (it has to do with the conversation of angry
moments). |
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Imagine now that your weights are held in place by
latches, which are released once the flywheel is spinning
(it's easier to envisage this way). Suppose also that the
weights can move out by a certain distance, and are
stopped from travelling further by restraints. |
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If you have no springs, and the weights move freely, then
they will fly outward and slam into their restraints,
releasing energy as heat and noise and whatever - just the
same as if you drop a weight onto the floor. This energy is
lost from the spinning energy of the flywheel which, as
noted already, slows down. |
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If you have springs, (and if the springs are just stiff
enough) then the weights will fly out as before but,
instead of slamming against their restraints, are instead
slowed by the springs so that they just come to rest
against the restraints. Energy is still lost from the
spinninness of the flywheel (which still slows down) but,
instead of being dissipated as noise and heat by the
weights hitting the restraints, it is now stored as spring
compression, and can be recovered. |
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Got it (finally), thanks.
hmmmm |
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This idea would work like clockwork, which is to say very inefficiently. According to Wikipedia's energy-density page, spring power is the heaviest way to store energy. A flywheel is over a thousand times better than a spring, per unit of weight. |
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This idea does give a novel way to store power in a thick, short spring, instead of a long spiral, and does allow the inefficiency of a spring to be balanced out by adding weight to a flywheel. But it buggers up the flywheel, adding complexity and reducing the safe top speed. |
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It could act to provide a more constant speed to the flywheel, as it would act like the governors described above. [ ] |
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//that compression would be an excess energy
storage without any extra input.// |
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But not so, here's another way to look at it. You
spin this thing at say 20 rpm with no springs and
that's enough to move the weights from the
center to the outer part of the flywheel. Now you
put the springs in so you're going to have to spin it
faster to move those weights. How much faster?
Depends on how stiff the
springs are. And once the springs are compressed,
you're not storing anything else in them no matter
how
fast you go beyond that point. |
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Like MB said, when those weights expand out from
the center of gravity, you're already going to have
a harder time turning it, you're just adding the
additional hurdle (and potential kinetic energy
storage) of the springs. |
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Bummer.
Back to the drawing board. |
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Been there Bro. Maaany times. |
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I would also like to note, that the radius of a flywheel is usually directly related to the tensile strength and yield strength of the metal (or increasingly so carbon fiber) that is to say, modern flywheels spin on the order of thousands of rpms if they spun much faster the metal would deform and then violently explode. |
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Springs basically being hardened metal would yield under these stresses and simply lose all their energy storing capacity. Also the reason you don't see many springs used as energy storage is that it's inefficient only a small portion is returned. Might as well make the flywheel somewhat thicker and call it a day |
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Just make the flywheel out of rubber. |
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