A while back I remember reading about the Icyball concept in survival settings. Using a mix of amonia and water, heat applied to one side causes the ammonia and water to seperate(ammonia faster due to lower boiling temp) and the other side to freeze when placed on a bucket of water. I've heard of people who actually took Igloo containers and modified them to use the Icyball setups. Reports of it working incredibly well were the norm. But certain safety issues were recommended. I'll post back when I gather the info again.
Check this page out. Lots of info.