I can't believe that one can do better than this heat exchanger here:
$1000 solar water heating system -- storage tank
Like others, I would be concerned about cross contamination of anti-freeze with the drinking water at the solder points whereas, if one uses intact coil of pex tubing, it has plenty(!) of surface area for heat transfer, it holds a good bit of water in the tubing as a reserve, and I wouldn't have reservations about its integrity of the tubing and cross contamination (although they don't use anti-freeze, the water in the storage tank is not exactly potable having sat in the tank for a year or more).