# Flywheel design

With all the press coverage of Formula One KERS systems, you might be forgiven for thinking that all racing hybrid systems are of the 'conventional' electric-hybrid type. This is far from the case, and if you look a little deeper, there are a number of systems that are based on mechanical technology rather than electric machines, power electronics and batteries

If you want to see real diversity of technology in terms of energy recovery then you need look no further than sportscar racing, where two mechanical systems will be racing in 2012. The unique DeltaWing 'technology car' will race at Le Mans, and we hope very much that Porsche will continue to enter its hybrid 911 in selected races in 2012. Ironically, both systems have their roots in design studies for Formula One projects.

The two systems, and an unknown number that are not yet racing, are all based on flywheel energy storage. Rather than converting kinetic energy into electrical energy, these systems maintain energy in the form of kinetic energy, and this is stored/used by changing the speed of the flywheel. If you studied physics at school, you might recall the formula for the energy contained in a rotating body:

In this formula, E is the energy, I is the moment of inertia of the body and ? is the rotational speed. We can see that if we want to store a lot of energy, we need to increase inertia and speed (especially speed). In order to increase inertia, we need to either increase mass or increase the radius at which the mass is distributed. Increasing speed is something we all understand. So, it seems very easy - take something heavy, and spin it very fast.

When we do so, however, the problems soon become apparent. The centrifugal forces acting on a body (as students of physics will again recall) increase in proportion to the mass, and to the square of the rotational speed. As we get towards the centre of the flywheel, the material has to support a force due to a large wedge of material. If you work through the mathematics, you will soon see that you run into trouble at fairly low levels of energy storage. A metallic disc just won't do the job.

If you look at any of the current racing flywheel energy storage devices, you will see a metallic hub that supports a thick rim of carbon-fibre reinforced polymer (CFRP) composite. What might surprise you is that neither of these components is capable, on its own, of working at the design speed. Both would fail in a spectacular manner at a speed much lower than their design speed, due to huge centrifugal forces leading to stresses that exceed the strength of the materials used for their construction.

If ever there was a match made in heaven, it is this. The key here is to use a heavy interference, which puts the flywheel rim and the hub into radial compression. This means the flywheel can reach a high speed without the inner fibres of the rim being put into radial tension.

Fig. 1 - A typical racing energy storage flywheel, with a metallic hub and CFRP rim

Written by Wayne Ward