Every once in a while,  I’ve heard people request the following question, something along the lines of:
“Why don’t’ they stick a windmill on top of our cars?  Then when we drive at 60 mph, it would cause the blades to spin, which we could hook up to a generator.   This could manufacture electricity, which we could store in a battery, to power the car.”
Ummm….that would be referred to as a Perpetual Motion Machine, as well as those only exist in Lah-Lah Land.
Remember as kid, how difficult it was to pedal your bike when it was hooked up to one of those cheezy night-light generators?    Suddenly, it’s a lot more work.
Same thing would apply to your car.  Spinning a windmill takes work.  It would slow you down, acting as a big brake.   You’d end up burning far more energy in gasoline than whatever you’d gain back from any electricity you’d made.
But what if you had a special high-efficiency windmill blades?   What if you had an estimated perfectly frictionless windmill bearings?
Nope.   Still wouldn’t work.
Even under the an estimated all optimum conditions, you wouldn’t even break even.    You’d ALWAYS burn more gasoline with a windmill-generator, than without.
Nature says there’s no such thing as a Free Lunch.
Thank the 2nd Law of Thermodynamics on behalf of that.
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Let’s continue the discussion on cars as well as braking.
What if, instead of a windmill on your roof,  your wheels were connected to an electric generator, that only turned on when you applied the brakes?
This is a whole different story.
When you braked,  the kinetic energy from your car’s mass as well as speed would now be converted to making the generators spin as well as make electricity.    This would slow down your car (just like the windmill on your roof would), as well as THEN you could store this energy in a battery on behalf of later use.
Don’t worry, though.   This doesn’t violate any Laws of Thermodynamics.    Remember, there’s no such thing as a Free Lunch.
When you’re cruising on the highway, you’re burning the same amount of gas, regardless.   But it’s HOW the braking is applied, that makes the difference.
With conventional braking,  ALL your kinetic energy is converted into friction on the brake pads as well as is lost as heat.    All that speed you had, all that gas you burned to get there…Pffft!    Gone! …Never to be re-used again.   And now the atmosphere is slightly warmer.  (Al Gore is crying, as we speak.)
But with electrical-generator braking, the one big difference is that you’d at least recover SOME of your kinetic energy back as electrical energy.   You wouldn’t be creating any more energy, you’d just be wasting LESS.
Which is a huge improvement from before.
This is what’s known as  regenerative braking.
Hybrid cars utilize this technology.
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Posted in Fried Science