Aerodynamic drag on a car and recovering some of that energy using a fan

[Andrea Vironda]

I think that driving at 130-140 km/h a car could dissipate 5-10 kW of energy only because of aerodynamic drag.
Why cars don't have a fan somewhere to exploit the aerodynamic drag?

For example a Pelton turbine

 

[Lnewqban]

Because that fan will increase the drag resistance to the forward movement of the car, generating less energy in return than the amount absorbed.
We have to burn fuel to maintain a forward force that overcomes the resistive force of drag, which increases with the square of the relative speed between car and air steam.

During the times the car is desired to slowdown from high speeds, yes, anything that recovers energy, like some regenerative brakes of electric cars, would save fuel.
Nevertheless, the gain is always in competition with the cost and extra weight of any recovering system.

 

[jack action]

The 5-10 kW of energy comes from the power source of the vehicle ... plus the inefficiencies of the powertrain. So assuming you have battery-powered car, it would be more efficient to plug your load directly on the battery, rather than:

1. putting a turbine in front of the car to power that load (that would increase the drag power proportionally to its power output + inefficiencies);
2. that extra power would be delivered by the powertrain (+ inefficiencies);
3. ... to finally have the power coming from the battery anyway!

One exception is the use of ram-air intake systems. The reason is that you can achieve a compression without the use of a compressor. You still have an increase in drag, but you save on weight, complexity and cost. There is probably no efficiency gain by using the most efficient compressor you can find either.

 

[Baluncore]

I think that driving at 130-140 km/h a car could dissipate 5-10 kW of energy only because of aerodynamic drag.

Even if it could work, what percentage of the time does a vehicle travel at or over 130 km/h ? Is it worth carrying weight and complexity when it is used for so little time? You have plenty of time to think about that next time you are driving up a hill, or sitting in a traffic jam.

 

[jrmichler]

What @Lnewqban and @jack action said, but in my words: If car designers could eliminate aerodynamic drag, they would. Adding a turbine to extract energy from the airstream only adds drag. And it adds more drag than the energy produced by the turbine. There is no way around this.

It is possible, however, to reduce the drag of a motor vehicle. Since the OP is interested, and has a master's degree in mechanical engineering, I suggest the following reading:

1) Aerodynamics of Road Vehicles, by Wolf-Heinrich Hucho.
2) Fluid-Dynamic Drag, by Sighard F. Hoerner.
3) Modifying the Aerodynamics of Your Road Car, by Julian Edgar.

The first two are well known classics, the third is recommended as being very good. A course in fluid dynamics is highly recommended to get the most out of the first two references, while a high school education should be enough for the third. Disclaimer: the Julian Edgar book has a couple pages about modifications to my truck.