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Home energy storage

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 is
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.