Posted tagged ‘heat’

Perfessor Friar Applies the Brakes

April 13, 2009

Every once in a while,  I’ve heard people ask 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 make electricity, which we could store in a battery, to power the car.”

Ummm….that would be called a Perpetual Motion Machine, and 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 almost perfectly frictionless windmill bearings?

Nope.   Still wouldn’t work.

Even under the most ideal 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 for that.


Let’s continue the discussion on cars and 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 and speed would now be converted to making the generators spin and make electricity.    This would slow down your car (just like the windmill on your roof would), and THEN you could store this energy in a battery for 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 and 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 use this technology.