This is a little sun powered board with ultra low power consumption and zero maintenance. It measures temperature, humidity, pressure and sends it every 15 minutes with a RF module. The small solar panel (8 x 6 cms) harvest energy to charge two small and cheap super capacitors of 5F each. It can run alone almost forever, at least until the caps die. But supercapacitors have a much longer lifespan than batteries. It can survive without sun about 80 hours, more than 3 days! I tested it with the solar panel disconnected. This is also environmental friendly as it produces it's own energy and don't use batteries with it's 'dangerous good regulations' (DGR) for handling, disposing, etc.

It seems like this project would be a good way to learn about using supercaps to run things.