I am surprised how often this question comes up. I was presenting a seminar on electric vehicles at the Institute of the Motor Industry last Wednesday and somebody asked this exact question.
The issue is that there is no such thing as ‘free’ energy - it requires energy to make energy: so for a wind turbine to generate electricity it requires wind, for a coal-fired power station to generate electricity it requires coal, and so on.
If you take a regular car alternator off the shelf, you can easily spin it around by hand. If you connect an volt-meter to the alternator when you spin it around by hand you can see that you register a voltage. Therefore, I can understand people assuming that you can therefore generate electricity from a moving vehicle.
However, when you start putting an electrical load on the alternator and try and spin it around, you find an awful lot more resistance - you really have to work in order to turn the alternator over. In terms of effort, around 5-10% of a petrol engine’s power is used to turn the alternator - that is an awful lot of horsepower being used to generate a few hundred watts of electricity.
And that is the problem, you end up putting so much energy into turning the alternator, you end up loosing out.
An electric car does have an alternator of a sorts… its an electric motor. When you stop accelerating, the electric motor starts getting turned around by the wheels and starts generating electricity to put back into the batteries. Most electric cars use this to provide ‘regenerative braking’ - using the momentum of the car to slow the car down whilst putting power back into the batteries.
I was driving a Tesla the other day and you can stop the car quite quickly using regenerative braking alone. The same is true with the G-Wiz - the first 30% of braking is all done using regenerative brakes (which in effect means you hardly ever use the mechanical braking system in normal driving).
Once you’ve experienced the braking effect of an electric car using the electric motor as an alternator, you appreciate quite how much friction is generated by an alternator when it is generating electricity - quite simply, enough to stop the car from high speed quite rapidly.
The same is true if you want to use a wind turbine on the car to generate electricity when you are going along - the additional wind resistance and friction means more energy is wasted by having to have more powerful motors to drive the car than is ever regained by the wind turbines.
All is not lost, however. There is an answer for a self-propelling electric vehicle: it’s called Solar Power.
Three electric car manufacturers have already announced solar powered cars. One is already in production whilst the other two are on the way. Meanwhile, you can buy plans for a solar powered trike on eBay with a range of 20-30 miles per day and an enterprising guy in India has recently built his own solar powered cart with a range of around 15 miles a day at speeds of up to 30mph.
Here’s an article I wrote on solar powered cars a few months ago - A solar car revolution in 2010?