This guys sell components that enable battery free operation(according to their claims) using rf energy: http://www.powercastco.com/ http://www.powercastco.com

This guys sell components that enable battery free operation(according to their claims) using rf energy: http://www.powercastco.com/ http://www.powercastco.com/ BTW, rf energy is generally among the lowest energy harvesting sources, and in many cases environmental light, vibration(for example on a machine) ,and heat are much stronger energy harvesting sources, and most likely available at low cost.