Wireless PowerBy Jenna Rock and Loren Schwappach For Jing Guo – Electromagentics March 2010
Overview History Recent Developments Technical Details Demonstration Advantages and Disadvantages Variants and Evolution of Technology
History Nikola Tesla’s experiments (1900s) Experiments conducted by Nikola Tesla over 100 years ago. The Chicago World’s Fair Demonstration in 1893. Required a clear line-of-site. Second proposed system used the earth’s ionosphere. Investors could not see a way to manage and profit from Tesla’s ideas. Wireless power via highly elliptical antennas (1970s) June 5, 1975 NASA JPL Goldstone demonstration. 34kw of power at a distance of 1.5km; 82% efficiency. Seen as impractical due to atmospheric absorption and free space loss. Wireless power in the Twentieth Century (2000s) Today the most popular approach to wireless power uses inductive charging. Considered a type of short distance wireless energy transfer. Impractical for separation of more than a few inches.
Recent DevelopmentsMassachusetts Institute of Technology (MIT) Experiment Project Lead: Marin Soljačić Solution: wireless power transfer via strongly coupled magnetic resonances. Concept: “WiTricity” (Wireless Electricity) Overcomes several major drawbacks Free space loss and atmospheric absorption. Requirement for unobstructed line-of-sight (Lasers & HDAs). Close-range and very low-power energy transfer limitations. “WiTricity” concept 106 times better than magnetic induction.
Technical Details Magnetic Induction Loop or coil of conductive material like copper, carrying an AC current, to generate an oscillating magnetic field. When second conducting loop (receiver) is brought close enough to the first, it captures a portion of the oscillating magnetic field, inducing an electric current in the second coil. The current it drives around the circuit opposes the change in magnetic flux (Lenz’s Law). When the current reverses direction, the magnetic field also reverses its direction. Faradays law of Coupling electromagnetic induction: Conductors are referred to as inductively or E= -dφB/dt magnetically coupled when they are configured such E = electromotive force. that change in current flow through one induces a φB = magnetic flux. voltage across the ends of the other through electromagnetic induction. A simple example is a locomotive pulling a train car.
Technical Details (Continued) Resonance: The tendency of a system to oscillate at larger amplitude at some frequencies than at others. These are known as the systems resonant frequencies. At these frequencies, even small periodic driving forces can produce large amplitude oscillations. Opera singer example, swing example. Resonant Magnetic Coupling: Magnetic coupling occurs when two objects exchange energy through their oscillating magnetic fields. All resonators have a Q (Quality) factor characterizes a resonators bandwidth relative to its center frequency, a tuning fork has a resonance of approx 1000.
Technical Details (Continued)The MIT Experiment Used two self-resonant coils, single copper loops (r =25 cm). One coil (the source coil) is coupled inductively to an oscillating circuit; the other (the device coil) is coupled inductively to a resistive load. The transmitting coil output a 9.9MHz resonating magnetic field. The resulting resonant frequency is: 1 f0 2 LC Both coils were separated by a distance of 2m with a 60W light connected to the receiving coil.
Advantages and Disadvantages Pros No negative effect on humans. “WiTricity” is using higher frequencies than pacemakers use, for example. Efficient power transfer is only received by like resonant devices. “WiTricity” magnetic field is << than earths. No negative effects on the environment. No more batteries ending up in landfills. “WiTricity” is several thousand times more efficient than batteries and a million times more efficient than induction. Cons Smallpower transmission waste due to coils (thermal energy).
Variants and Evolution of theTechnology Products and Applications Consumer: phones, laptops, flat screen TV’s, digital pictures, home theater systems, speakers, and desktop PC’s and peripherals for use in home and “WiTricity” enabled hot spots. Industrial: power interconnections across rotating and moving “joints” (think robotics, packing machinery, assembly machinery, machine tools), power interconnections in harsh environments (drilling, mining, underwater), robotics, and automatic guided vehicles. Transportation: Automatic wireless powering for personal and commercial hybrid and future all electric vehicles and high tech military systems (mobile robotics, aircrafts, etc). Evolution of “WiTricity” technology Smaller scale WiTricity fixed receivers, capable of enabling the WiTricity transmitters within an area. WiTricity on a larger scale (street “WiTricity” transmitters), recreational use.
Conclusion Wireless power transfer is quickly becoming a viable reality. “WiTricity” products expected in 2011 “WiTricity” offers an extremely efficient alternative to previous attempts at providing wireless power. Future improvements in wireless power technology offer world changing implications.
ReferencesF. Hadley,. (2007). “Goodbye wires”. (PDF) “MIT_WiTricity_Press_Release.pdf”J. Dix. (2010). “Wireless power” Retrieved March 17, 2010, from http://www.networkworld.com/news/2010/011210-witricity.html?page=1A. Kurs., A. Karalis., R. Moffatt., P. Fisher., & M. Soljacic. (2007). “Wireless power transfer via strongly coupled magnetic resonances”. Science Express. (n.d.).A. Kurs. (2007). “Wireless power transfer via strongly coupled magnetic resonances. Science. 317(7), 83,A. Karalis., J. Joannopoulos., & Marin Soljacic. (2007). Efficient wireless non- radiative mid-range energy transfer. Annals of Physics. 323(2008), 34-48.