I'm working on a design for a solar dehydrator that I'm hoping will get closer to 24 hour operation by storing heat in a water tank, and as part of that design I'm trying to get at the best way to handle heating the water. I've seen some stuff about using air as a heat transfer fluid (basically the solar panel heats the air, which then flows up into a heat exchanger in the water tank and heats the water). That is very attractive to me because of the freeze resistance and simplicity. I was wondering what kind of other problems might crop up as a result of taking that approach though -- would efficiency be lower, how does it perform in less-than-perfect conditions, etc? Are there any major gotchas for using air as the heat transfer fluid in a water heater?
The problem that will happen is that when it gets below freezing outside the cold air will freeze your heat exchanger. I do not suggest using air collectors to heat water. Liquid type flat plate collectors are reliable, not that expensive and are easy to use in a freeze-proof system.