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.
Back to puzzles
This page is maintained by Thomas Bending,
and was last modified on 7 March 2017.
Comments, criticisms and suggestions are welcome. Copyright © Thomas Bending 2017.