Generating power from electricity in the air

Energy harvesting technologies use energy from the ambient environment as a primary energy source and convert it into electricity. Power can be taken from the air via various radiation conversions: e.g., ultrasonic sounds, infrared beams, magnetic radiation for induction, radio frequency signals, Wi-Fi, ... 

The number of start-up companies in the field of electro-magnetic wave energy harvesting and energy transport through the air increased significantly few years ago. However, ultrasonic and radiofrequency still present low performance and have not experienced any significant development. Conversely inductive and infrared technologies are doing well with applications currently dedicated to the military sector (charging of submarine batteries, PV panel in space to charge devices in desert locations...). 

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