https://www.youtube.com/watch?v=xzV3p2hEat0&feature=player_embedded
http://www.activistpost.com/2013/02/student-invents-device-that-charges.html
Student Invents Device that Charges Batteries with Radio and WIFI Waves
Depending on the strength of the electromagnetic field it is possible to charge a small battery within one day. The system is meant to be an option for granting access to already existing but unheeded energy sources. By exploring these sources it can create a new awareness of the invisible electromagnetic spaces while giving them a spatial dimension. There are two types of harvester for different electromagnetic fields: a smaller harvester that is suitable for lower frequencies below 100Hz which you can find in the general mains (50/60Hz, 16,7Hz) and a bigger one that is suitable for lower and higher frequencies like radio broadcast (~100MHz), GSM (900/1800MHz) up to Bluetooth and WLAN (2,4GHz).