I think everyone knows about “Drinking bird” toy (https://en.wikipedia.org/wiki/Drinking_bird). Here I would like present ”modern”, solid state analog 🙂
We take a Peltier element and put wet rag on one side and big heat sink on the other side.
Water will be evaporating from the rag and it will be cooling.
This creates small (about one degree) temperature difference for several hours.
Using modern DC-DC converter for energy harvesting (like LTC3108) some small electronic device can be powered e.g. clock.
I also believe that it is possible build similar system with ”closed cycle” where water (or perhaps some other coolant circulate inside device). This, however, presents much more serious engineering challenge.