This is a well known technology. The classic example uses 2 clay pot's, sand, and water.<p><a href="http://www.seed.slb.com/en/scictr/lab/pot_refrigerator/index.htm" rel="nofollow">http://www.seed.slb.com/en/scictr/lab/pot_refrigerator/index...</a><p>PS: Clay pot fr get's google to popup "Clay pot Frige" with 88k responses.