I'm a big fan of alternative energy. While solar and wind are the big players in modern alternative energy, I think geothermal energy should be looked into more.
I plan is to use geothermal energy in an unconventional way. Usually geothermal is used to cool and heat buildings or used to make electricity via steam. I am hoping to produce electricity directly via a thermoelectric device.
Don't forget that at different times of the day and year, the temperature difference may be inverted (hotter on top, or colder on top) -- and the scavenging circuit needs to handle that. Also, it would be best to charge all the time and store to a capacitor or battery for nighttime lighting.