Why Metal Flywheels Are Dumb
Updated: Jul 11, 2022
Qnetic is unequivocally targeting one thing: the transition to renewable energy and the end of fossil fuels. When the sun isn’t shining or the wind isn’t blowing (thankfully the tides always ebb and flow), we need enough stored energy to see us through.
To do this, civilization needs an unfathomably huge amount of stationary storage. Therefore, the first design criteria an energy storage system requires is very large capacity. The storage medium in a flywheel is the rotor. The amount of kinetic energy stored in the rotor is controlled by this equation:
I is the moment of inertia of the rotor: this basically refers to its shape and weight. Heavy weight and wide diameter increase I and increase the kinetic energy. ω is omega, which is the rotational speed of the rotor. Both I and ω should be increased to store more energy but actually ω matters much more because it is set to the power of 2. Double the weight and you’ll double the KE, but double the speed and you’ll quadruple the KE. Speed matters most in this equation.
The limit to energy storage capacity of any given rotor is the strength of the material to withstand centrifugal forces. For a given speed, the higher the mass, the higher the centrifugal force. Therefore, to go faster, it is better to keep the mass nice and low and this is where metal will disappoint you.
Trust us, we have done the maths: if you wanted to build a two megawatt capacity flywheel from metal, the rotor would weigh about 300 tonnes compared to Qnetic’s 14 tonnes. Apart from this horrible amount of material, the bearings would need to be huge to support that weight and the losses would be unacceptable.
A kWh-sized flywheel with a metal rotor makes sense but unfortunately it doesn’t scale to the megawatt-hour regime and won’t contribute enough to the mission: full transition to renewables.