I have received an assignment which requires me to build a device that can cool two liters of water (at room temperature) as much as possible. The only source of energy is me; I can drive the device in whatever way I can. I am allowed to run the device for 30 minutes, and the lowest temperature reached counts. The device can't be bigger than 2x2x2 metres and all of it has to be at room temperature at the beginning. Also, it can't cost more than €100 in materials. I have some ideas for it, and was wondering if anyone has criticisms or other ideas. 1. use the Joule-Thompson cycle to cool the water. I would drive the compressor with my legs (so like a bicycle). 2. use a fridge-like system, with a phase change. Again, I would drive the compressor. 3. use a piston. If liquid water is forced to be in a low perssure environment, it will want to evaporate. This costs energy which is taken out of the remaining water, which would freeze. This can be done with several liquids, each being cooled in advance by the previous one. Then a very cold liquid can be dropped in the water which will cool the water. These are my ideas. Any suggestions or criticisms would be much appreciated.