All three systems require large amounts of electricity, either you generate hydrogen, or compress air, or just charge the car.
The cleannest sorce of electricity is solar. Solar takes a large area of land and good weather. Also it is one of the least used forms of power generation.
Next is wind, which is clean, but slightly enviromentally damaging. It seems, at least with the wind farms on the way to Stockton from San Jose, they have a problem of birds (some endangered) flying into the spinning blades and being killed. Also, there is very little power generated in this fashion.
Then we have hydro-electric dams. Depending on the age of the damn it can pollute very little, to be horrendously poluting, as they least lubricating oil. It also floods valleys, endangers fish runs, and inhibits yearly floods, causing silt build up.
Third, we have coal power, the worst of the lot, and still one of the most common forms of generating electricity.
A new form of power generation that will be tested soon, is using buoys attached via chain to generators on the bottom of the ocean, and the surf motion generates the power. It uses little space, generates huge amounts of power, and is fairly clean. I think the only worries will be lubricant and rust. But mass production in this fashion is still many years away.
So, are these alternative fuels really cleaner, or are we just moving the mess?