Perfessor Friar Applies the Brakes

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.


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26 Comments on “Perfessor Friar Applies the Brakes”

  1. Mike Says:

    “there’s no such thing as a Free Lunch” or tanstaafl

    ‘”There Ain’t No Such Thing As A Free Lunch” originating in the 1940s and later popularized by science fiction writer Robert A. Heinlein in his 1966 novel The Moon Is a Harsh Mistress,’ which is where I first became acquainted with it.

    and thinking about entropy (2nd law) always makes my head hurt, but not as bad as when I had to figure it out in class for problems I never used in the real world.

    Cool post!

  2. Friar Says:

    TANSTAAFL. I love it! 🙂

    I read most of Heinlein’s early stuff, including that book. But that was a while ago, I forgot that he popularized that expression.

    Hmph. Regarding all those Thermo problems we did in class. I hear ya. We did tons of them in 3rd Year Engineering. That was in 1985…I don’t think I’ve had to do one since, in the real world.

    I vaguely remember “Steam Tables” too. 😉

  3. t. sterling Says:

    I’ve never heard such a question/idea of windmills on top of a car. I know I wouldn’t do it since I’d be concerned about passing under bridges… or it just falling off for whatever random reason. My car is missing items already that I may or maynot be aware of where they have gone. Except for my bumper…

  4. Mike Says:

    “steam tables” – I actually had one or more copies in my desk drawer for the entire time I was an instructor — 1983 to 2007. I can’t think of a time I ever used it, though.

    The first time I had to work through thermo was at Naval Nuclear Power School in Mare Island, California. Without given away the date, we did our calculations using slide rules — for real! 😉

  5. Friar Says:

    Well, I admit, that question hasn’t been asked to me in a while, though I have heard it asked (or certain variations thereof) on more than one occasion.

    A windmill on a car is something that’s just ASKING to be stolen, when you come to think of it.

    I know OF slide rules, though I’ve never used one.

    I was in High School in 1979….even back then, our teacher said he refused to teach us to use one. Calculators were starting to become popular. The writing was on the wall.

    I had a Hewlett-Packard 15C Programmable calculator in 1985. Worked on Reverse-Polish Notation. It was pretty advanced, even by today’s standards. In fact, I still use it!

  6. Patricia Says:

    Hey, I am smarter than I think? Nope, we own a Hybrid car at the office and I can see it storing energy particularly when I go up hill and down.

    I did just get a free Kindle II….from my rewards card.
    But I don’t suppose that is as neat? a deal or trick

    My husband says well we really paid for it because we paid out over $37k in sales tax this year – I will not go into medical bills…
    So I guess it is not really a free Kindle II But I like it very much.
    Sounds like skiing in Vermont built up energy for you! Good job

  7. Friar Says:

    Well, skiing relaxed me mentally. But physically, I’m exhausted.

    But I prefer it this way, rather than the other way around.

  8. Captain Push Says:

    What will the crazy left-wingers come out with next?
    The Flintstone Mobile? I can see our big fat feet sticking out of the floorboards to “supplement” our tiny engines while going up hills and provide more braking during downhills.

    How about sails with computerized “tacking?”

  9. Tom Says:

    Far more efficient than a Flintsone stone vehicle and much closer to a perpetual motion machine is a bicycle! I have a new one that weigh only 15 lbs and wheels with exceedingly efficient bearings. And I think it makes me feel far better when I arrive at my destination – not to mention how much more of the route I get to see! I also think it’s far easier than dealing with sails and days of no wind.

  10. Friar Says:

    Though, as an engineer, I must admit, I’m impressed with regenerative braking. It’s not a pie-in-the-sky save-the-planet idea. It’s an elegant design that recovers a lot of wasted energy. Dunno why they didn’t come up with this sooner.

    By the way, I always wondered how the wheels of Fred’s car stayed in place . The axles didn’t seem right. (Howcum the front wheel didn’t roll away when he stopped?)

    Don’t they say the bicycle is the most efficient means of conveying human muscle power into motion? (Something like 80-90%, I believe?).

    Though I don’t understand how some people go overboard. Paying $5000-plus for their bikes. Unless you’re on the Tour de France, I think that’s a bit crazy for a vehicle that doesn’t even have an internal combustion engine.

  11. Kyddryn Says:

    Panoz Racing developed a prototype race car that used the power generated by braking for the turbo charger. We affectionately called it Sparky (the non-hybrid one was the Batmobile – nothing but original, we corner-workers!). This was before the Prius and the Civic Hybrid and all those newfangled cars came out. It still races, now and then.

    I never thought about a windmill atop the car, but I have been known to hold a whirligig out the window…wonder what I could power with that?

    Shade and Sweetwater,

  12. Friar Says:


    Just be aware that whatever you power with a whirligig out the window, would be indirectly be coming from the gasoline burning in your car’s engine. 😉

  13. Eyeteaguy Says:

    I may be in the UK, but I can still be an a$$ from here.

    KERS is in F1 this years and BMW is planning on putting it on their cars in 2012. Now that is not a free lunch but it comes with a coupon and is subsidized.

    We are getting there.

    Have you seen the oil tankers with the sails! Very cool.

  14. Tom Says:

    I beg to differ with you about the cost of a bicycle. If you can feel the difference between a $15,000 Ford Focus and a $50,000 Mercedes, there is at least as much difference between a $1000 bike and a $5000 one. It makes far more sense to me than the Mercedes which continues to heat & destroy our planet. And it’s only 1/10th the cost! I have ridden a $5,000 bike and it definitely improves my distance and speed and joy for being out on the road. It may be my “mid-life crisis sports vehicle of choice” but it’s doing far more for my body and for the planet than a Mercedes ever could – to say nothing of the friends I meet and events I discover when bicycling rather than driving the infernal internal combustion machine.

  15. Tom Says:

    To Eyeteaguy in the UK:
    An oil tanker with sails! That sounds like an oxymoron. Using wind to deliver the oil to heat the planet and make our climate more fierce! I can’t think of anything crazier. But perhaps it makes the tankers an easier catch for the pirates of Somalia.

  16. Friar Says:

    Hey…it takes talent to be an a$$ from across the Atlantic.

    My blog must be really important to you, if you’re taking the time out of your busy schedule to check me out while you’re traveling.

    Either that, or you’re very BORED.

    I admit, I have several family members who are really seriously into cycling and having the expensive bikes. They’d agree with you 100% (especially Friar’s Mom).

    I agree, cheap bikes are awful. But I wonder at what point do you get to diminishing returns?

    But I’m pretty sure I could have a pretty good time with a $2000 bike, and meet the same cyclists I could with a $5000 one.

    If people want to spend that kind of money…fine. But I just don’t have that kind of cash.

  17. Friar Says:

    Tom and Eyeteaguy
    Sounds like an Oxymoron, but I can see the point.

    They’re going to use the oil-tankers to deliver oil anyway, whether we like it or not.

    So might as well put sails on them and save SOME of the fuel costs.

  18. Tom Says:

    Sorry, I don’t know what KERS or F1 are or why they come with coupons or would be subsidized.

  19. Friar Says:

    Me either. (Good question!) 😉

  20. Eyeteaguy Says:

    KERS kinetic energy recovery system. F1 is supposed to be the pinnacle of automotive technology but it has been citicized for not being green. So they are forcing the introduction of a system that stores eneregy and then can be used to boost power. This year they get 80 extra horse power for 6 seconds a lap and then 12 next year and so on. There are 2 basic designs, one using batteries and one using a flywheel. It looks like the flywheel may be the better one. Lighter, safer, simpler.

    And as for coupons etc. “Now” should have read “Not”. I was being an A$$ again. 7 hour flight and 4 hours of sleep.

    And your blog is entertainment to me so why wouldn’t I read it on vacation! The cockeral woke me early anyway, and the sheep, and the horse. Bloody english countryside. But everything is GREEN here! What a novel colour! I’d forgotten what it looked like.


  21. Friar Says:

    I heard about those “kinetic batteries”. Like a flywheel on those toys cars we used to play with.

    (Imagine if the fly-wheel ever got loose, though?) Tell your dog: “Go get it, Rover!”

    Actually, though, I must say I’m quite disappointed in you. All this talk about saving energy and helping the planet.

    Yet you flew to the UK for your vacation. What a huge carbon footprint…tsk tsk.

    You could have saved a lot of CO2 and probably have had just as good a vacation, by staying at a B&B in Guelph for a week.

  22. Eyeteaguy Says:

    10,000 rpm is what it spins at. Its not very big, its a big electro magnet that runs an electric motor when the button is pushed.

    I flew to the UK. So shoot me. As Brett commented on in my post, flying is better than driving. If I flew my own plane here and I was the only passenger….. then you’d have a point.

    Besides. My family is all here, not in Guelph.

  23. Friar Says:


    You can shoot ME…because I drove over 1000 km this weekend.

    Just so I can go to a mountain, and have complicated machinery lift me to the top, just so I can put slippery boards on my feet and slide down again. Again and again.

    (Of course, Tony already gave me flack for that on Monday…). So maybe I’m off the hook.

  24. Ed Says:

    Very interesting post. I really enjoy your science stories. A comment about the windmill on the roof of the car. What if it was used like the sail of a sailboat? Of course it would only work when the wind was blowing. Imagine a car with no gas motor at all and have the windmill coupled somehow to the car drive wheels. A control system would be required to face the windmill into the wind and the vehicle speed would be limited by the amount of wind and the wind resistance drag of the entire system. Perhaps batteries or some energy storage system could fill-in when the wind is not blowing. Of course this may not be practical on our roads but I just wanted to mention that theoretically, it is possible to power a car with a windmill.

  25. Friar Says:

    Yes, theoretically, if used to harness wind power, kind of the equivalent of a sail, a windmill could be used to power a car.

    I guess you’d have to know how many watts of energy per square meter the wind generated, and convert that to enough power to run the car. I wonder how big the propeller blades would have to be? I suspect it would have to be some pretty huge windmill.

    Interesting thought experiment. I wonder of anyone’s tried to figure this out? (Brett would probably know)

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