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It's possible he couldn't get one anyway depending on how exactly it works. Ammonia-based refrigeration is very old technology: https://en.wikipedia.org/wiki/Icyball (In some of the other coverage he even mentions being inspired by this.)

It's also relatively easy to build one, if you're not put off by ammonia or the high pressures involved:

http://crosleyautoclub.com/IcyBall/HomeBuilt/HomeBuilt.html

If you publish, you effectively void the novelty condition of an invention and prevent patentability.

True, but manufacturers may be less than willing to tool up to produce them if anyone can jump in on it and under cut them. He may have a higher purpose in mind but almost certainly the manufacturer doesn't, and just wants to get paid.

Last time I checked you couldn't get a patent on something that already exists, and he already owns the copyright.

In fact he needs to make sure he explicitly grants other people permission to copy his design, otherwise it will be protected under copyright law, whether he wants it to or not.

Wouldn't prior art prevent that? (I'm generally ignorant on this stuff)

You cant patent something that has prior art.