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  Car Wind Generator

 car wind generatorSome machines claim to provide 'free' energy but in fact they only work because they take power from another machine or external source that is not immediately apparent. It actually confirms the first and second laws of thermodynamics. A common suggestion for example is a propeller or turbine mounted on a vehicle. As the vehicle moves it turns the propeller and generates electricity just like a stationary wind turbine. 
There is no doubt that electricity will be generated so why isn’t this a good idea? One reason is that small wind turbines of the size that could easily be mounted on a vehicle don’t generate much power. See the article on wind generators. But that is not the main reason.
The main reasons are the thermodynamic laws. The first law says that energy can only be converted from one form to another. So to get some electrical energy from the passing air we must be extracting energy from it. A propeller or wind turbine does this by slowing the air down by getting it to turn the propeller. It doesn’t really matter what type of turbine it is. There are many different types that all have their strengths and weaknesses. For more about the definitions of energy and power click here.
The next part needs a bit of thinking about. The air is stationary so it has no energy. It is the vehicle that is moving. So we can’t extract energy from the air because it is already stationary. The only source of energy is the vehicle.
The vehicle is forcing the propeller through the air and as it does so the air resists it. This resistance acting on the shaped surface of the propeller causes it to turn and generate electricity. Some of the moving energy of the vehicle is being converted into electricity and the drag caused by pushing the propeller through the air is causing the vehicle to slow down.
So using any form of wind device that uses the wind stream passing a vehicle to extract energy is going to slow the vehicle down. Some may be more efficient than others but in all cases the effect is going to be negative.

The second law of thermodynamics makes it clear that you cannot convert energy from one form to another in a way that is 100% efficient. So the very best that you could hope for is that the amount of electricity produced would be a tiny amount less than the energy that was being taken from the vehicle by slowing it down. In reality it will be a lot less.
The only time that this may be a useful effect is when you are actually trying to slow the vehicle down. Regenerative braking is already used on electric vehicles to recover energy that would otherwise be lost when the vehicle slows. It is possible to imagine a wind turbine that is only exposed when the vehicle is deliberately reducing speed. However quite a large area of turbine is needed to generate useful amounts of power.
A wind turbine 1 metre diameter will generate about 1 kW of power if it is mounted on a vehicle travelling at about 60 miles per hour (~ 25 metres per second). This is about equal to the amount of power generated by the alternator on a large vehicle.
Because of the aerodynamic and electrical generation inefficiencies it would take about 3 kW of vehicle power to generate it.  So it’s quite a good brake but not a very good source of power and driving around with a 1 metre diameter turbine on the vehicle is going to create its own problems.