One of the reasons we built Trev was to get people thinking about the energy they use for mobility. Most people appreciate that using tonnes of machinery to transport a 75 kg person uses a lot more energy than necessary. But some people get a little too inspired: Why don’t you put generators on the wheels?
Generators on the wheels is not a bad idea. The electric motors used in most electric and hybrid cars can act as motors converting electrical power to mechanical power, or as generators converting mechanical power to electrical power. Working as a generator, the motor can convert the kinetic (movement) energy of the car into electrical energy, and this energy can be used to partly recharge the battery. But converting the kinetic energy of the car to electrical energy slows the car. It is called regenerative braking, and the braking force that can be achieved is about the same as the driving force that can be achieved by converting electricity to motion.
Regenerative braking cannot convert all of the kinetic energy of the car into electrical energy—some of the energy is dissipated by resistance forces in the tyres and bearings and by aerodynamic drag, and some is dissipated as heat in the motor/generator and in the electronic controllers. But some of the kinetic energy can be converted to electricity and stored for later use—which is better than occurs with normal friction braking, where all of the kinetic energy is dissipated and none of it can be recovered.
But the proponents of generators on the wheels often want to go beyond regenerative braking to generate electricity without slowing the car. Not necessarily perpetual motion, where the generators generate more than enough electricity to power the motors, but enough to reduce the power required from the battery.
It doesn’t work—at least, not in our universe.
To see why it doesn’t work, try writing power values in each of the empty boxes in the diagram below. Suppose the car is travelling on a flat road at constant speed, so the power values are not changing. The total power into the motor must be the same as the total power out of the motor, and the total power into the generator must be the same as the total power out of the generator. It is like Power Sodoku—everything has to add up.
- Write in the power required for propulsion. This is the power required to overcome rolling resistance and aerodynamic drag. Trev uses about 4000 W at 70-80 km/h.
- Write in the motor losses. Losses in an electric drive system are about 10% of the propulsion power.
- Write in how much power will be transferred from the motor to the generator, and generator losses (about 10% of the power into the generator).
- Calculate the ‘battery out’ power and the ‘battery in’ power.
The total energy from the battery is (battery out – battery in). What do you have to do to minimise (battery out – battery in)?