Kind of an odd power generation question.

[Traab]

With a lot of focus on electric cars, I couldnt help but wonder, if you attached a wind turbine to the front end of your car, as an example, implant it in the grill on your front end, or near the bumper, so its being blown by the air as you drive, could it produce a useable amount of power to be worth the effort to develop? Would it provide enough supplemental power to make a noticeable difference on mileage? Or would it have to be too big and bulky to be practical?

[Dodom]

It would not help mileage because all the power generated would be at the cost of extra drag.

But it doesn't mean it can't be done for other reasons, if you can think of an use for power generation that doesn't rely on the alternator, it is feasible.

[Peelee]

OK, even though it's not at all what you suggested, now I just want propellar cars.

[factotum]

I'm with Dodom--the power generated from this wind turbine has to come from somewhere, and in this case, it's coming from extra drag being applied to the car as it moves, meaning the car needs more power to move it and thus negating the benefits of having the turbine in the first place. You would be far better off improving the car's streamlining than trying to generate additional power in this way.

[Douglas]

Yes, the additional drag would make this not merely not worth the cost, but actively detrimental. I think I remember an xkcd what if related to this, but I haven't managed to find it yet.

[ace rooster]

You sometimes see light aircraft with little props generating electricity for radios and the like, with the advantage being that it will still work even if the engine stops. Car systems tend to be less critical, and even if they were, the generators would be much better off attached to the wheels directly.

As for whether the system could work, it might if it is windy. At that point you are basically sailing a car though, with all the same constraints.

[BannedInSchool]

You sometimes see light aircraft with little props generating electricity for radios and the like,
I knew some vehicle used wind generators, but it just wasn't coming to me. When I remembered someone got there before me. :smallsmile: Also, I think commercial airliners have an emergency generator they can deploy. Hmm, and there's a tickle of memory of some wind-powered weapons pod on a military aircraft. Ah, so there could be an automotive application in providing electricity to a towed camper without having to run power lines, or your roof-mounted minigun.

[factotum]

A plane has the advantage it will keep moving pretty fast through the air even if the engine fails, though, making an emergency wind turbine a reasonable idea. This is not generally true of a car. :smallwink:

[Alent]

If you're willing to sacrifice all electric and go hybrid, this idea is not that far off from a Hybrid Turbocharger (http://en.wikipedia.org/wiki/Hybrid_turbocharger).

I have no familiarity with the above system, but the configuration sounded close enough to a supercharger I found myself wondering if anyone had put a supercharger or turbocharger on a hybrid car and stumbled across the above.

[Tyndmyr]

With a lot of focus on electric cars, I couldnt help but wonder, if you attached a wind turbine to the front end of your car, as an example, implant it in the grill on your front end, or near the bumper, so its being blown by the air as you drive, could it produce a useable amount of power to be worth the effort to develop? Would it provide enough supplemental power to make a noticeable difference on mileage? Or would it have to be too big and bulky to be practical?


No.

Anything that collects energy from the forward momentum of a vehicle will always slow you down. Because no machine is perfectly efficient, this will always represent a net power loss.

See also, thermodynamics, and why all "free energy" schemes cannot work.

[BannedInSchool]

Anything that collects energy from the forward momentum of a vehicle will always slow you down. Because no machine is perfectly efficient, this will always represent a net power loss.
Can't...resist...nitpick...

Unless you're using your brakes (wind turbine brakes? Umm...probably not a good idea) to generate electrical power. Then collecting energy from your own motion is a gain over just using brakes that convert it to heat.

Hey, I said it was a nitpick. :smallsmile:

[Douglas]

Can't...resist...nitpick...

Unless you're using your brakes (wind turbine brakes? Umm...probably not a good idea) to generate electrical power. Then collecting energy from your own motion is a gain over just using brakes that convert it to heat.

Hey, I said it was a nitpick. :smallsmile:
It's still a net energy loss relative to coasting. The thing that makes it a good idea is that slowing down is, at that moment, an actual positive objective.

[Tyndmyr]

It's still a net energy loss relative to coasting. The thing that makes it a good idea is that slowing down is, at that moment, an actual positive objective.

Precisely!

Though telling the policeman that running the stopsign was efficient may not be of help, truthful or not. =)

[BannedInSchool]

Now with the 24 Hours of Le Mans just this weekend and those cars now using a variety of energy recovery systems I feel compelled to at least mention that they're running to an energy efficiency formula with hybrid technologies. They've got limitations per lap on how much petroleum-based fuel they can use and how many stored megaJules.

Hmm, now if you could choose between either starting with a bunch of speed and kinetic energy and no power or starting at zero velocity with the same amount of stored energy you could expend, intuitively I'd like to say you'd eventually go further at a slower speed using stored energy considering spherical cows of a perfectly flat path and no rolling resistance but with air resistance. I say that because air resistance is such a bitch, but I don't know that the longer time spent wouldn't make it a wash even with lower air resistance at the lower speed. Well, I can't work it out right now, I'm not quite compelled enough to investigate, and it's not important, but there's the query anyway. :smallbiggrin:

[Domino Quartz]

...considering spherical cows...
What do spherical cows have to do with anything?

[factotum]

Hmm, now if you could choose between either starting with a bunch of speed and kinetic energy and no power or starting at zero velocity with the same amount of stored energy you could expend, intuitively I'd like to say you'd eventually go further at a slower speed using stored energy

Well, yes, that goes without saying--it takes exponentially more power to go faster, mainly down to air resistance. For instance, my little Nissan Micra has an official top speed of 106mph from its 86bhp diesel engine. In order to get a car that goes twice as fast as that you need a lot more than twice as much power--as an example, a Ferrari 458 will just crack 200mph (official top speed is 202), but needs nearly 600bhp to achieve that. However, we're talking the Le Mans 24 hour here--they don't go slowly!

[ace rooster]

No.

Anything that collects energy from the forward momentum of a vehicle will always slow you down. Because no machine is perfectly efficient, this will always represent a net power loss.

See also, thermodynamics, and why all "free energy" schemes cannot work.

Though momentum is always relative, so your momentum relative to the air can be different from your momentum relative to the road.

Consider moving 10m/s into a 5m/s headwind. Relative to the wind you are moving at 15m/s so if you use a turbine to reduce your velocity to 14m/s you can harvest 29j/kg energy, and that can go into charging a battery. If you then use your wheels to accelerate you from 9m/s back up to 10m/s you need to expend 19j/kg from the battery. This is a net gain of energy.

Note that this is not free energy; it is using the velocity differential between the air and the road. Wind power basically.

Sorry if this feels nitpicky, I just feel the need to call out anyone who uses "always" lightly.



Well, yes, that goes without saying--it takes exponentially more power to go faster, mainly down to air resistance. For instance, my little Nissan Micra has an official top speed of 106mph from its 86bhp diesel engine. In order to get a car that goes twice as fast as that you need a lot more than twice as much power--as an example, a Ferrari 458 will just crack 200mph (official top speed is 202), but needs nearly 600bhp to achieve that. However, we're talking the Le Mans 24 hour here--they don't go slowly!


It is not exponential, it is cubic*. Worth pointing out though is that the energy per meter is only quadratic. ie, you go twice as fast and your mpg will drop by a factor of 4. You will need 8 times the power, but your journey only takes half the time.

[Yora]

Well, ^2 is an exponent, isn't it?

[BannedInSchool]

What do spherical cows have to do with anything?
Long joke cut short, a farmer with a cow problem of milking or butchering them, goes to the local university to consult with a physicist, because he heard they're the smartest of the smartest, and gets back an answer that starts with, "Consider a spherical cow...". :smallbiggrin:

[shawnhcorey]

Perfectly feasible. It's called, Downwind Faster Than The Wind (http://www.wired.com/2010/06/downwind-faster-than-the-wind/). Video (http://www.youtube.com/watch?v=5CcgmpBGSCI)

[Icewraith]

It depends on the numbers. However, for every case where the wind is helping, consider the cases where there's little relative wind (or the wind is blowing perpendicular to the propeller) so the extra weight and equipment for the propeller and charging system is adding to the mass of the car with no benefit.

This worked great for Link until he got pulled over for using a baton while driving.

[aspi]

Well, ^2 is an exponent, isn't it?
Exponential growth means that your variable is in the exponent, i.e. 2^x. If your variable is the base, i.e. x^2, then it's "just" polynomial growth.

[mantia]

kind of a different idea but why dont electric cars have solar panels instead of a normal roof? seems like wasted space.

[Tev]

kind of a different idea but why dont electric cars have solar panels instead of a normal roof? seems like wasted space.

I don't know how much electricity those would make (I bet very little, and you have to pay extra for them), but I know that car with solar-panels-as-a-roof will be quite ugly.

[Alent]

kind of a different idea but why dont electric cars have solar panels instead of a normal roof? seems like wasted space.

A few reasons, but primarily practicality. The assumption at least in the US is that you'll be using your roof (infrequently) to mount a luggage rack. Even if you're not, the solar roof isn't going to add much range to an electric car. It'll let it recharge while stationary, but my understanding is that it would take longer than the standard 4 hour charge.

Occasionally you'll see proof of concept all solar cars that can actually Drive from their solar panels, but those tend to be extremely light stripped down single productions that don't have the safety features and crash testing required to be road legal in most countries.

[factotum]

Yeah, it's mainly down to power supplied. Even at noon on the sunniest day a solar panel small enough to fit on a car roof won't supply enough on its own to power a normal-sized car, and they're obviously largely useless in overcast weather.

[Lord Torath]

kind of a different idea but why dont electric cars have solar panels instead of a normal roof? seems like wasted space.Direct sunlight provides (IIRC) about 3 HP per square meter. You can get maybe 2 square meters of roof space if you're being generous (3 or 4 for a van), which gives you 6 HP for your car, assuming perfect solar-to-electrical conversion. An idea one of my classmates came up with was a roof solar panel to power a small window fan to cool your car on hot days. I seem to recall that using a computer fan and a 9v battery (because he didn't have a solar panel), he managed to keep the interior temperature within 5-10 degrees of the outside temperature, and a bit better than that with tinted windows. Add a windshield block, and your car's interior will be pretty close to ambient.

[factotum]

But...the car already has a perfectly good electrical system and fans? Struggling to see where the advantage of your friend's approach would be, especially since the power consumed by a fan is absolutely tiny compared to the power required to keep the car moving forward. (As an example, a full-size desk fan, which is considerably larger than anything you'll find in a car, pulls about 20W of power--that's about one-fortieth of a horsepower!).

[Lord Torath]

But...the car already has a perfectly good electrical system and fans? Struggling to see where the advantage of your friend's approach would be, especially since the power consumed by a fan is absolutely tiny compared to the power required to keep the car moving forward. (As an example, a full-size desk fan, which is considerably larger than anything you'll find in a car, pulls about 20W of power--that's about one-fortieth of a horsepower!).Is that directed at me? The solar window fan was designed to keep your car from superheating when you leave it parked in the hot sun, not for use while you're driving. There may be a better way of implementing this, maybe a way of tying the roof panel directly into the existing electrical system; it was a primarily a project for a Heat Transfer class. Someone else in the class used an empty milk jug, some aquarium tubing, and a windshield wiper-fluid pump to create a swamp cooler for your car (again, not something you'd use if your car has air conditioning, but great for the kind of old beater a college student might drive).

[tomandtish]

Well, Ford is taking the next step (http://fordrack.com/2015-ford-c-max-solar-energi-concept-release-date/), or trying to anyway, with solar panels that actually charge the drive batteries. Whether or not it will actually be successful....

[Chen]

Is that directed at me? The solar window fan was designed to keep your car from superheating when you leave it parked in the hot sun, not for use while you're driving. There may be a better way of implementing this, maybe a way of tying the roof panel directly into the existing electrical system; it was a primarily a project for a Heat Transfer class. Someone else in the class used an empty milk jug, some aquarium tubing, and a windshield wiper-fluid pump to create a swamp cooler for your car (again, not something you'd use if your car has air conditioning, but great for the kind of old beater a college student might drive).

Where is the air being pulled in from by the fan? If you add it to the window wouldn't there be a fairly big hole someone would use to much easily break into the car with?

[Lord Torath]

Where is the air being pulled in from by the fan? If you add it to the window wouldn't there be a fairly big hole someone would use to much easily break into the car with?The idea was to crack your windows about an inch, inch-and-a-half, and his contraption would fit into one of them, so there wasn't a gap large enough to reach your arm through. I think a better idea would be to tie your solar panels into the car's electrical system, and use the solar power to run the interior fans. Or even the AC if you can generate enough power. Maybe include a switch to disconnect the battery so you don't risk draining it if you're not pulling enough juice from the sun.

[Chen]

Ah gotcha. And yeah connecting to the main interior fans does make a lot more sense to me too.