If you can cut a hole through the car to reduce air resistance, then that should be done. That's free efficiency. Although it's interesting that no car manufacturers do this and there's probably a reason for that. But don't put a turbine there. Newton's third law states that for every reaction there's an opposite and equal reaction. Put simply getting the fan to move means you have to put energy into moving the car forwards, and the energy needed to turn the fan will be needed from energy you're putting in. You're now having to put energy into moving the car forward and turn a turbine. And because nothing is 100% efficient, you'd lose energy. I suppose you could stick a wind turbine on the car when it's on a car park, but it'd probably be better to stick the turbines on the roof of the car park instead (and even then I think they'd be too small to power much). Solar energy is free. But as a rough calculation say 5 sq m at 200 W/m^2 = 1kW. I chose 5 sq m to make the numbers easier. In truth getting 5 sq m on car all facing the sun seems very optimistic! 200W/m^2 is pretty optimistic for the uk too, but let's allow technology to improve a bit. We need 450kJ (thanks to 2shortplanks for reminding me about the 1/2) to get 1000kg to 70mph (we could reduce this by making the car lighter). So that would take 450 seconds (7 and a half minutes) for solar power to get us to 70mph (even with my very generous solar panels). This is ignoring friction and air resistance in getting the car up to 70mph though, so it'd really take much longer than this. There again, if your car could collect and store that energy whilst sitting on the driveway, then that's a good thing. Free energy is nice (free if we ignore the cost of the panels), but again you'd probably just put the solar panels on the garage roof instead. But back to DA's point. Braking wastes a lot of energy, so it's a good place to collect it. Convoy systems and better traffic management can also reduce the wasted energy that cars brake away, by reducing the need for so many braking events.