Can you answer this challenging energy/physics puzzle?

Question

I've recently heard of a device that generates electricity to charge a cellphone. It is a small plastic wind turbine that is attached to your car's side-view mirror, that generates electricity while you are driving. The gadget is sold as a "green" way to charge your cellphone.

My puzzle is this:

Is this really a "green" device? Since the car's engine is ultimately "pushing" the turbine through the air, is this not just a way of using gas to charge your cellphone?

Another question:

A small amount of "extra" (marginal) gas is required to push the turbine through the air (for both the weight of the device and the wind resistance of pushing it through the air). Is it possible that the energy generated by the device is greater than this "extra" amount of gas consumed to use it?

I've argued that it is not, as it would "generate" energy from nothing (i.e., a perpetual motion machine). Others have argued that it is just recouping energy that would otherwise be lost. who is right?

Answer 1

My cell-phone is charged in my  auto through the cigarette lighter. Since I don't smoke it is really my cell-phone charger. no energy       lost!!!

Answer 2

very good question. dude, the power required to run this turbine will not effect the gas consumption to a very large extent and it can be assumed to draw negligble gas for the operation of this turbine. consider the weight of the car, the people sitting in it and then the weight and resistance to air offered by this small wind turbine.It is just negligible. hope I cleard my point. Thanks.

Answer 3

It is not truly green.  As you yourself have already noted, the added weight and drag it produces on the car mean that it is just using gas power instead of a wall socket's power to charge the cell phone.  Granted the weight and air drag are negligible and you would never feel notice it driving the car, but you cannot get something for nothing.

Answer 4

The only way that it is recouping energy that would otherwise be lost is if you ONLY use this when braking (engine or actual brakes) is occuring.  So if you would like to take this device and put it out only when you are trying to stop, it could be considered somewhat green.

The fact is that it takes energy to spin the turbine and that has to come from somewhere and it is going to come from your gas engine.  It's basically a similar concept to the old lights that they used to have on bicycles where a generator ran off of your back tire.  You got electricity but it wasn't free.  You had to peddle a lot harder to get that light.  Now in this case, it's your engine that has to work harder.  Realistically, there would be a non noticeable difference in your MPG but you really aren't being green.

With regard to your second question, you could design a permanent magnet wind generator to power your phone and theoretically make it more efficient than your alternator, the requirements to make it operate at all velocity ranges (0-100 MPH including headwinds?) would prove difficult.

I'm not an electrical engineer, but I think your car is typically generating more electricty than you are using so when you plug in your phone, you are really tapping into energy that otherwise wouldn't be used.

Answer 5

I have heard of these gadgets, only I have seen ones that you strap on your arm as you run.  Most likely you are not changing the aerodynamics of your car enough to cause a noticeable decrease in gas millage.  It would be equivalent to sticking your hand out of the car or opening the window.  It is a money saver because you are using the motion you are already creating and capturing more energy.  You could also do this by plugging it into your car charge port.  The only difference is that the wind turbine could be used other places to charge the phone, like your arm or just outside if the wind is brisk.

Answer 6

Of course it's not "green." The energy to spin the turbine comes from burning gasoline in your car. It would be much more efficient to just plug in a charger at the car's power port.