Q: How much energy can a household store conveniently using a
flywheel, eg to store summer solar energy for the winter?
[14 May 1997]
A: Consider a vertical-axis concrete flywheel in the basement, say
5 m in radius, 1 m thick. Its mass is about
density*pi*radius2*thickness = 5000*3.14*52*1 = 400,000 kg.
If we want the rim to be subsonic then the speed of rotation, omega,
can't exceed 10.7 revs per second. At this speed the energy stored
0.25*mass*omega2*radius2 = 0.25*400000*672*52 = 1e10 J = 3000 kWh.
A person consumes about 2 kW (in industrialised countries, and counting only electrical usage - car usage may consume up to as much again), so this would last one person about 60 days.
Note however that at this speed the rim acceleration is
radius*omega2 = 22000 m/s2 = 2300g,
which seems impractically high.
There are various other problems too: making bearings that can support 400 tonnes, friction in the bearings, windage, gyroscopic effects, and also how to get the energy into and out of the the wheel [Colin McEwen]. Tilting the wheel so that the axle's parallel to the earth's axis or using contra-rotating wheels might be possible, but neither magnetic levitation nor evacuating the basement seems very practical.
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