For the past one hundred or so years, the most common way to transmit electricity to homes and businesses has been through the use of electrical cables, and the extensive power grid that runs throughout the United States and the rest of the world.  As a result of recent research done by the Massachusetts Institute of Technology (MIT), that may change fairly soon.

             In the late 1800’s Nikola Tesla performed experiments to send large amounts of electricity through the air. Some of his experiments proved successful, although the efficiency rating for sending the electricity was very low.

 

Above: A Tesla Coil

             In 2006, MIT researchers developed a way to send electrical power with a much higher efficiency than Tesla was able to achieve. The theory behind this idea is resonance. Most FM and AM radios use resonance to tune in on a specific station; FM uses frequency modulation, tuning in on radio waves with different frequencies, and AM uses amplitude modulation, tuning in on radio waves with different amplitudes.  All radio waves carry some amount of power, which can be observed using a crystal AM radio, a radio that has no power source and uses only the power contained in the waves to power its circuit. When an inductor is used to transmit power (for example, in a transformer), the magnetic field that it produces is very weak, mainly because it sends a magnetic field in all directions, so it does not concentrate the field. This means that any device that is receiving the power must be within a very short distance of the inductor. An electric toothbrush uses this basic idea. Most rechargeable electric toothbrushes have an inductor in the charging base, and an inductor in the bottom of the toothbrush. The inductor in the charging base induces current in the inductor in the bottom of the toothbrush, and in turn charges the battery. This is called inductive coupling. For this method, the toothbrush must be very close to the charging base because inductive coupling is fairly inneficient, and only carries electricity over short distances.

 

Above: Inductive Coupling with a Rechargeable Toothbrush

             The idea behind this new form of wireless power transfer is based upon the theory of resonance and the circuitry of the common AM or FM radio. Every radio uses a basic tuning circuit, composed of a capacitor and an inductor. This circuitry allows the radio to tune in on certain frequencies by increasing or decreasing the inductance or capacitance in the circuit. Using this basic principle of the common radio, wireless power transfer can be made much more efficient. If you send power in frequencies, along radio waves, you can greatly improve the efficiency over inductive coupling. If you send power in the form of a radio wave, you are only sending it to certain receivers that are tuned to the specific frequency that the power transmitter is tuned to. This means that instead of sending these radio waves everywhere, (and wasting a lot of electricity) you can send power to a select few devices within a certain range, usually about two meters. This is beneficial more for in-home use, rather than widespread power transfer.

Above: Wireless Power transfer through Resonance      

              For long distance power transfer, microwaves (the form of electromagnetic radiation, not the kitchen appliance) are more sensible. Microwaves have a much longer range than normal radio waves and are therefore much more efficient in terms of wireless power transfer. The only major drawback with microwaves is that they aren’t entirely safe; microwaves contain high amounts of radiation, which could be a problem with radiation poisoning and cancer. Overall, you can expect the power grids to stay , but the ability to eliminate wires for common household devices is already here.

http://electronics.howstuffworks.com/wireless-power.htm

 Devin Mullen

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