So many questions I get about the EV are regarding “charging while driving”. This basically all comes back to the idea of perpetual motion and is proved impossible via the first and second laws of thermodynamics (wikipedia). However, it comes up quite often the the post below is a very good response to the question “couldn’t you just put a wind generator on the car to charge while you’re driving?”.

This is a post from www.diyelectriccar.com:

I’ll give you a few examples. Let’s say it takes 10,000 Watts to keep your car at 50 mph. And we’re striving to maintain speed.

First example…
Say you put the wind generator attached to the outside of the car. Wind generators work by creating drag on free flowing air to convert the wind energy into rotational then into electrical. Thus the wind generator can’t produce any more energy than the drag it creates on the air. So if you installed a 100% efficiency 1,000 Watt wind generator on your car, your car will then need 11,000 Watts of power to maintain speed. Your wind generator is supplying 1,000 Watts of that, so you’re back to 10,000 Watts to maintain speed. Hence without even looking at efficiency losses, there is no gain.

Second example, slightly more complex…
Your car needs the 10,000 Watts to maintain speed because of the drag on the body and wheel friction. We’ll just look at the drag on the body as at high speeds it is much larger than the wheel friction. Now imagine you removed the radiator under the hood as you don’t need it for your electric car. Reason stands you could duct the air from your car’s grill into a wind generator where your radiator once was.

Let’s assume again everything is 100% efficient. The car is already suffering from drag because of the air being stopped under the hood so we’ll say drag is the same with the wind generator, 10,000 Watts. Let’s imagine you can fit a 1,000 Watt generator underneath the hood driven purely by the frontal area of the grill. With everything perfectly efficient, you will deliver 1,000 Watts additional to your motor, effectively decreasing your energy needs down to 9,000 Watts.

Sounds good right? Well in actuality there are inefficiencies. 95% loss from motor, 95% loss from controllers, 95% loss from transmission. 1,000 * .95 * .95 * .95 = 857 Watts. So now 10,000 – 857 = 9,143 Watts. Still better than nothing.

Now let’s approach this differently. Instead of trying to harness the drag we’re creating, let’s try to get rid of it. Buy a $20 sheet of aluminum and place it over your grill. Your drag has reduced by 1,000 Watts, meaning your car only need 9,000 Watts to maintain speed. Much easier, simpler, and cheaper than a complex ducting/generator system.

Third example…
We’ve been assuming no external wind. Say like your car was parked with your wind generator running. It would generate power from the external wind and store it into the battery. You then bring in the generator to remove the drag and begin driving. That is completely doable and why many people install wind generators at their house (it’d be a pain in the butt to haul it around in the car).