The efficiency of compressed air back to mechanical energy is usually only about 1/3 or less. When you compress air you heat it. Unless the compressor, lines and storage tank are heavily insulated, you lose 1/2 of the energy you put into compressing the air right up front as waste heat. You can never get that back.
Then when you decompress the air you lose more potential energy because air cools as it is expanded. Unless you have a source of waste heat to inject to keep the temperature stable, you lose again as the tank and lines cool and the pressure drops faster than it would have otherwise.
The Amish are known to use mills for compressed air because they are allowed to use compressed air but not electricity. It isn't done for the sake of efficiency.
If you setup a mill to begin compressing air mechanically when the batteries are full or it reaches peak speed that might work, but it would make for a much more complex mill.
Look at the pics of the homemade mills here and you will see how difficult it would be to add another device to the mill without blocking the wind and also still raise and lower the whole contraption for required maintenance.
The power available in the air goes up with the cube of the wind speed. But the wind in a given area blows in a certain range of wind speeds only a certain amount of time. The best use of a mill needs to consider both. Plus the efficiency of a mill is not linear with wind speed. If you live where the wind blows steady, then sizing and designing a mill can be straight forward. Otherwise it may not be possible at all. You just built the biggest mill you can, get it as high up in the wind as you can and take what comes your way.
http://zebu.uoregon.edu/disted/ph162/l11.html Has a good simple explanation of this. The 59% theoretical possible he mentions is called Betz' Law most places.
http://en.wikipedia.org/wiki/Betz%27_lawAll that said, if you could pump water instead of compress air and store the water in a raised tank for off peak power production, you might get usable efficiencies because water is not compressible and it has much higher mass density than air.
