A Peltier module allows you to convert heat into electricity. Because you can place it in areas that are normally warm anyway, electricity created is "free" in some sense, although it works best when one side of the module is cold and the other is hot. In other words, all you will need for this project is the Peltier module and a cooler surface such as earth or water and a warm area like a well-lit window or a warm tray.