I like wagonman's idea of an absorption cooler:
Solar air conditioning - Wikipedia, the free encyclopedia
Inexpensive Home Building: Passive Cooling Solar Cooling: Use Heat To Cool
Home Built IcyBall
Building Your Own Larry Hall Icyball
HowStuffWorks "Gas and Propane Refrigerators"
Size, weight and safety are issues to consider but I like the idea of using all that "free" exhaust heat. Also, it probably wouldn't cool very fast (or until more heat is generated) at first. But the principle is sound. It is practical and is on the commercial market (see gas/propane refrigerators)