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Flywheel Energy Storage

  1. May 29, 2012 #1
    HI All!,
    I'm a thesis student in Industrial Design and need some physics help!
    My thesis project is to make a system that stores energy in the form of kinetic energy in a flywheel to power common products. This system would be plugged in all the time and would begin working immediately after the electricity flow stops(power outage) because of the nature of flywheels.

    My biggest issue is deciding what is the best generator to use and how much weight would be needed to power three different categories of household products, they are: (cellphones, alarm clock, small lamps ect,) next, (hair dryers, and straighteners, garage doors, ext) lastly the major appliances like refrigerators, furnaces microwaves ect......I know that I would need somewhere in the 600lbs range at about 100RPM to power a refrigerator for an hour(or so I was told) and 20 tons to power a house. My target weight is somewhere around 30 - 50lbs so that it's still marketable and to target the smaller electronics because I'm gearing this towards inner city apartments where you cannot have a gas powered generator. Also does anyone know how I could figure out how much electricity is need to keep this system spinning at full speed?

    Also currently I'm using an old Technics turntable that the record sits on a disc that has a fixed magnet in it and is powered by brushless electromagnets to prove that I can generate power. I by-passed a series of resisters sending about 24VAC to the motor and added about 25 lbs to the center that is almost perfectly centered. At this current set up I'm getting about 7.5VAC that reduces to 0 in about 5 minutes. Does the diameter of the disc(flywheel) change the kinetic energy? or if the majority of the weight was around the outer most point and not in the center?

    I know that in my current set up I'm loosing a lot of power in friction, and bad circuit design but for this stage I just need to prove that it's possible and to be able to light some LED's or HOPEFULLY wire a rectifier and capacitor with a voltage regulator to stabilize the about 7.5VAC that I'm getting now into 5DC.

    Sorry this was long winded but I'm looking for formulas that can help me determine the best set up for the scenarios that I mentioned before. Any help what so ever would be greatly appreciated!!
  2. jcsd
  3. May 30, 2012 #2


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    Let's say you have 20k and a 40cm dia. For max energy storage, yes, you want as much mass on the perimeter as possible. But commercial flywheels spin so fast there's a risk of flying apart, so they tend to be lens-shaped. Let's take it as a uniform disc.
    The M.I. is MR2/2 = 0.4 kg.m2.
    If you want to supply 50W for 3 minutes, that's 9000 joules. The rotational speed required is given by w2 = 9000 / 0.4 s-2, w = 150 rev/s, or 9000rpm.
    All that's with no losses. I've no idea how low you could reasonably make the idling loss.
  4. May 30, 2012 #3
    Back in the '70's Popular Mechanics had an issue about something like this - flywheel drive cars.

    The flywheels were hundreds of pounds and encased in housings that sustained a vacuum. The RPMs were up in the thousands. The losses during idle were minimal and allowed for a car to be driven around for hours with respectable acceleration.

    Of course there were issues about collisions or fancy driving being effected by the super gyroscopic motors.
  5. May 30, 2012 #4
    You could look up some old steam engine books. They should have all the mechanical info you need. The electrical side is a different ball game. I've found all different kinds of alternator setups hooked up to flywheels searching on the net.
  6. May 30, 2012 #5
    Thanks everyone! One last question, say I found a generator that operates at 200 RPM, is there a way to determine the best possible dimensions for the flywheel to maximize the power output? or would it not be that crucial because of the lower RPM speed?
  7. May 30, 2012 #6
    Well, I'm not a scientist but just a home tinkerer but I would say the biggest one that you can get that will fit within the size constraints you need to use. After that it's how fast you can turn it safely without centrifugal force making it come apart as Haruspex stated.

    If you find an alternator that can put out useful power at 200 rpm I would really like to know what type it is as that would help me on a project I'm working on. So far I've found about 600 rpm for an alt is the slowest speed I could use on my project effectively. Bringing it down to 200 would save me quite a bit of money on.
  8. Jul 7, 2012 #7
    i have an assembly of 7.5 hp dc motor attached to gearbox
    which makes a flywheel of 250 kg to rotate at 1500 rpm
    the dimension of flywheel is 22" diameter and 4" thickness
    so what will be the final torque output if a connect something to the flywheel
  9. Jul 7, 2012 #8


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    That depends on the output power.

    The kinetic energy of a solid disk is [itex]E=\frac{m}{4} \omega^2 r^2[/itex] with [itex]\omega = 25 \cdot 2 \pi /s[/itex] and r=0.28m.

    Using these values, E = 121kJ = 0.034kWh. This is enough to power a 34W-device for 1 hour or a 1kW-device for 2 minutes.
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