i may have misread the initial post, but it sounded like you were thinking the car A/C compressor runs off gas. If you didn't mean that, then disregard this post

the compressor runs off a belt driven by the engine which when the clutch is engaged it puts more draw on the engine which takes more gas to run...
You could probably make a 12v winch motor or something turn it, but then you'd be taking probably hundreds of amps to run, which would kill the alternator, and still put strain on the engine, not to mention it would get extremely hot

again, sorry if that wasn't what you were thinking