Was looking into biogas generation, and I think I got a good handle on that, but during my research into that I came across some articles about Solar Refrigeration. The upshot is that using an adsorbant material, like carbon or zeolite, and the application of heat you can create a cycle for cooling or refrigerating a liquid in a vacuum. (Not a very good explaination, but I'll try to elaborate.)
Essentially, you heat the adsorbant material to boil the liquid out of it, desorbtion. The vapor goes through a condensor where it cools and liquifies and collects into a reservoir. When the heat is removed the adsorbant material begins adsorbing the liquid from the reservoir. This generates heat at the adsorbant material and quickly draws heat out of the liquid thus causing it to chill quickly.
Look into CoolKeg, you open a valve on the keg and the zeolite adsorbs the water and chills the beer for 8 hours.
Ideas for uses; solar refrigerators, novelty camping mugs that will chill your drink and can be recharged by placing it in the fire, off grid heating, cooling and refrigeration (this could get crazy complex, but scavenging the heat of the adsorbant material as it's adsorbing and the recharge cycle for use as space heating)...
I've read that one square meter of solar panels can generate 4-5Kgs of ice per day. Thats an adequate amount for food storage and some simple space cooling.
Is anyone interested in playing with this stuff? I'm going to be making a vacuum pump in the near future and I'll be using it for all kinds of things, this being one of the more interesting. I'm going to try it on a small scale first, just some pvc and some simple valves.