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Flywheel energy storage alternative

  1. Aug 3, 2005 #1
    Flywheels are interesting but dangerous if they malfunction because the
    energy is concentrated in the vicinity of the rotor.

    My question to you ME's is this- could you engineer a practical and/or
    efficient energy storage system that relied on gravity?

    I envision an enourmous mass on a rope. The rope is connected to
    the shaft of an electric motor-generator. You "charge" the device
    by running power to the motor/generator and lifting the mass. You
    get energy back out in the obvious way.

    It's safe because in the worst case the mass "falls" but probably doesn't
    kill anyone.

    Is this a feasible alternative to lead-acid batteries and flywheels?
  2. jcsd
  3. Aug 3, 2005 #2


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    A similar principle is used in pumped storage hydroelectric power stations, the difference being that the mass is in liquid form. These have two reservoirs at different heights. When demand for electricity is peaking, the turbines run normally as the water goes from the top reservoir to the bottom one. When demand is low, the turbines are reversed (essentially becoming pumps), and the water is returned to the top reservoir.

    Potential problems with your idea are that you'd need either an absolutely enormous mass, or an absolutely enormous height over which it can fall. Both of these scenarios have their own engineering difficulties to address, as I'm sure you can imagine. For a large energy storage capacity, you'd likely be talking extremely high numbers for things like tension in the rope, which would itself have some notable safety issues.
  4. Aug 4, 2005 #3


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    You might be best to go with some sort of heat storage or the aforementioned fluid tower approach. The weights in a grandfather clock are pretty much along the line of your original idea, but on a far smaller scale. You would indeed require some honkin' huge mass for the scale of operation that you're proposing.
    The fluid approach is far more efficient than heat storage, if only by virtue of it not needing to be 'preserved'. Heat will disappear if not used in time, whereas water or mercury or whatever will wait until needed.
    On the other hand, about the only hazard from a huge flywheel would be bearing failure. If you can minimize that risk, then it still might be the most practical way to go.
  5. Aug 4, 2005 #4
    Good points.

    I did some further investigating and discovered that the energy density
    of compressed-gas storage is on the same order as the hydrocarbons-
    or about 100 times greater than that of a flywheel.

    I hadn't thought of it, but there is enourmous pressure possible for a gas at room
    temperature before it liquifies and the mass is very low leading to a high energy density.
  6. Aug 4, 2005 #5


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    A gaseous form of a hydraulic accumulator...
  7. Aug 5, 2005 #6


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    Been there... done that... will never order the breakfast burritos again...
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