A turbocharger would have to spin many thousands of RPM's to get any appreciable compression. I doubt that this would prove to be a useful route (and I'm a fan of turbo's).

I recall an adapter that allowed a motorcycle enthusiast to use the electric starter to spin the engine and get air pressure through the spark plug hole of one of the cylinders to fill an air mattress when camping, so this line of logic is "do-able".

I also recall that the firing order of a Ford 302 allowed one bank to operate as a smooth firing 4-cylinder, while the other bank of the engine functioned as a 4-cyl air compressor.

http://www.southern-tool.com/store/grimm-schmidt_gas_compressors.html http://www.ytmag.com/toolt/messages/143175.html

There is also a kit that involves a custom cam, and several other adapters that converted a VW air-cooled 4-cylinder bug engine so that two cylinders functioned as an engine, and two of the cylinders acted as an air compressor.

http://www.angelfire.com/biz/dunnright/DR3.html

I am a fan of efficiency, but efficiency is a relative term, in that, if the wind/water is free, then, if an IN-efficient device made from free junk can perform a job that you need...start out in-efficient and work your way up as time and money allow.