I thought of that.....in fact, one of my first 'How come...?' was why aren't car AC units run from the heat of the manifold. (it's gotta be stationary, BUT they work in sail boats. Go figure.
Having just become the proud owner of a 1944 vintage Servel-Electrolux, propane refrigerator that last ran in a sheep camp in 1968....I got real interested in them. It takes 335 degrees to make it work. That's doable on the exhaust, but not for the cooling water.
It took a while to find these links, but they show the process AND sell parts and pieces to make your own.
http://www.rvmobile.com/http://nucold.com/<side note> My grandad was an 'observer' of the strange, odd and unusual..... "Why are Blackberries red? Because they're still green."
He noticed in 1958 that Ford was advertizing heavily about their 'rattle-free bodies' and solid cars, when everybody knew they started rattling at the first RR track from the showroom. He said always look critically at what the advertizers tell you...what they tout is usually a problem.
Check out the Servel-Electrolux refrig. ads of the '30s and '40s. All about how quiet they are. "Quiet as a dawn at sea...", "The best neighbor a family ever had...". "You can whisper in the kitchen now!"..
DON'T believe it! This thing sounds like a weed burner for the first ten hours then tapers off to a sound like a BBQ on high, but it makes things COLD..... in fact it's still freezing Diet Cokes on 'low', so I may have the wrong jets in it.