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  1. 1. Wireless Power 1 Wireless Power (March 2010) Jenna Rock and Loren Schwappach highly directional antennas have achieved efficiencies greater Abstract: The idea of using strongly coupled magnetic than 95 percent. However, practical problems with thisresonances to wirelessly transfer power over large distances will approach include the large antenna sizes required and thebe introduced. This new idea was discovered by a group of MIT losses due to atmospheric absorption and free space.professors and has far reaching implications on how power will Another approach to providing wireless power which hasbe transferred in future years. Along with presenting this idea, atop level understanding of how this method of wireless power become quite popular this century is the use of inductivetransfer is provided. charging to transfer energy between two systems. Because there must be a short gap between the two inducting coils, this is considered a type of short distance wireless energy transfer I. INTRODUCTION and impractical for separation of more than a few inches. I MAGINE a future in which wireless power transfer redefined the way people live and work. Imagine a worldwhere cell phones, household robots, music and multimedia III. RECENT DEVELOPMENTSplayers, computers, automobiles, medical devices, and other In 2006 a bright group of Massachusetts Institute offixed and portable electronics were powered without ever Technology (MIT) theoretical physics professors led by Marinbeing plugged into the wired power grid. This imagined world Soljačić developed a possible solution, using wireless poweris much closer to reality than one might think. A “tutorial” of transfer via strongly coupled magnetic resonances. Theira method for providing wireless power via strongly coupled approach overcomes several major drawbacks that previouslymagnetic resonances will be presented along with a discussion plagued the transmission of power wirelessly. Theseof the implications. drawbacks can be summarized as: large energy wasted into free space when using an omni-directional transmitter; unobstructed line-of-sight required by lasers and highly II. HISTORICAL INFORMATION directional antennas; and very close-range or very low-power The concept of transferring power to the world wirelessly energy transfer as limited by magnetic induction. Theirwas first experimented with by Nikola Tesla who lived from concept, which they refer to as “WiTricity” (as in Wireless10 July 1856 – 7 January 1943. His experiments were with Electricity), uses strongly coupled magnetic resonances andhigh power electromagnetic waves and he even managed to has been shown to improve the distance of magnetic inductiondemonstrate lighting a bulb using wireless power at The power transfer by a factor of 106. In other words, non-Chicago World’s Fair in 1893. Unfortunately, Tesla’s system resonant magnetic induction is almost a million times lessfor providing wireless power required a clear line-of-site efficient than WiTricity. The significantly improved efficiencybetween the transmitter and the receiver. Tesla even proposed of this approach greatly expands the potential applicationsa system of using the earth’s ionosphere to store and which can harness this form of wireless energy. In order topropagates wireless energy via electrical conduction, understand how wireless power transfer via strongly coupledeffectively lightning bolts. However, a means of controlling magnetic resonances works, an understanding of magneticand profiting from Tesla’s unconventional ideas could not be induction, inductive coupling, resonance, and resonantenvisioned in his time and thus were thought impractical and magnetic coupling is first needed.financially irresponsible by his investors. Eventually Tesla’sworld changing ideas were abandoned and Tesla was labeledas a mad scientist, bankrupted by his investors, and driven IV. TECHNICAL DETAILSslowly into poverty and depression. A. Magnetic Induction After Tesla’s death the driving force behind wireless Magnetic Induction is a fundamental idea used throughoutpower transmission was the idea of using radio and microwave electronics. It is the process of developing a voltage across apower transmission systems. NASA JPL Goldstone conductor (like copper) positioned in a shifting magnetic fielddemonstrated long distance wireless power transmission by (the field moves around a fixed conductor), or conversely, asuccessfully transmitting 34kw of electrical power at a conductor moving through a stationary magnetic field. It usesdistance of 1.5km with an efficiency of 82 percent on June 5, a loop or coil of conductive material like silver, gold or copper1975. Since the demonstration, individual experiments using to carry an AC current and generate an oscillating (corresponding to the alternating current) magnetic field as shown in Figure 1. As electric current, I, flows though the
  2. 2. Wireless Power 2wire, it produces to a changing magnetic field, B, which wraps induction (toothbrushes, etc.), yet the applications are limitedaround the wire. As the current reverses in direction, the to a very small range. Michael Faraday, who is credited withmagnetic field also reverses its direction, thus achieving the the discovery of magnetic induction, stated that thechanging field required. electromotive force (EMF) produced around a closed loop is directly related to the changing amount of the magnetic flux through any surface bounded by its path. Faradays Law of electromagnetic induction is: E= -dφB/dt, where E is known in physics as the electromotive force and φB is the magnetic flux. Lenzs Law also states that: “the current in the loop is always in such a direction as to oppose the change of magnetic Figure 1: Magnetic field created on a single AC flux Ф(t) that produced it” [7]. The change in direction derived conducting wire. [6] by Lenzs Law can be thought of as the result of the minus sign in Faraday’s Law. If a conductive loop (the transmitter) is driven by an ACpower source, it will quickly generate an oscillating magnetic B. Couplingfield around the vicinity of the loop as shown by Figure 2. Theblue lines in the illustration represent the magnetic field that is The phenomenon of coupling is observed when energy isbeing created when current flows through the loop. When the dynamically exchanged between two objects, whethercurrent reverses in direction, the magnetic field will also mechanical, thermal, electrical, or otherwise. Generically,reverse in direction, thus providing an alternating field. coupling is the connection between a source of energy and a consumer of energy – a mechanical example of coupling is a car pulling a trailer via a ball coupling. Magnetic coupling (or inductive coupling) is the use of magnetism to accomplish this transfer of power. When a changing current flow in one conductor induces a voltage across a second conductor, coupling has been observed. Mutual inductance is a measure Figure 2: Magnetic field on a coil. [6] of the amount of coupling that has occurred between two such conductors. Wireless electricity, WiTricity, relies on magnetic When a second conductive loop (the receiver) is brought coupling to eliminate all physical connections linking thewithin close enough proximity to the first, it will quickly begin source and receiver and thus allows a wireless powerto capture a portion of the original loop’s oscillating magnetic connection between the two devices.field. Thus the original (transmitting) loop will then induce orgenerate an electrical current (flow of electrical energy) in the C. Resonancesecond (receiving) coil as shown in Figure 3. The second Resonance is the inclination of an object or system tocoil’s generated current may then be used to power connected oscillate (vibrate) at larger amplitudes at some frequenciesdevices. An electrical transformer (a device made of two coils rather than at other frequencies. The natural frequencies arein close proximity) is a device that uses this idea of magnetic called the object or system’s resonant frequencies and can beinduction to transfer energy from a primary coil to its used to accomplish amazing things. At resonant frequencies,secondary coil without the coils being electrically wired even microscopic, periodic, forces can eventually producetogether. It is used to “transform” the AC at one voltage to AC large amplitude oscillations in like-resonant receiving a different voltage, in other words it is used to step a voltage The property of resonance exists in numerous differentup or down. physical systems. Nikola Tesla described resonance as the frequency at which energy can be most effectively added to an oscillating system (like the earth’s Ionosphere). A child’s playground swing is an outstanding example of an oscillating system involving both potential and kinetic energy. This is when a child swings back and forth at a constant rate determined by the length of the swing. The child can increase the energy in the swing system and thus make the swing go higher by aligning his/her body with the movement of the swing. In this example, the swing is moving at its mechanical Figure 3: Basic Transformer. [6] resonant frequency and the child’s synchronized movements are effectively transferring energy into the swing system. This type of electrical power transfer from one loop to the Another popular example of resonance is an opera singer in another is known as magnetic induction and is the basis for opera house shattering several wine glasses as she produces aelectrical transformers and electric generators. Many of magnificent single note (frequency) for a sufficiently longtoday’s electronic devices already use the concept of magnetic
  3. 3. Wireless Power 3duration. All objects have natural resonant frequencies and in and the receiving coil was wired to a resistive load of a 60Wthe opera singer example the wine glasses are acting as natural bulb. They used two single copper loops each with aresonate receivers. The sound waves produced by the opera matching radius of 25 cm. The transmitting wire coil output asinger are captured by the wine glasses and as the wine glasses 9.9MHz resonating magnetic field. The coils were separatedabsorb the energy from the singer’s notes, they begin vibrating by a distance of 2 meters. Since the coils were resonant, theand eventually shatter in an amazing display. current and charge density profiles were 90 degrees out of phase from each other. Using electromagnetic theory, the MIT group defined an effective inductance and effective D. Resonant Magnetic Coupling capacitance as follows: Resonant magnetic coupling uses the efficient energy µ0 drdr ( J (r ) J (r ))transfer property of resonance in conjunction with oscillatingmagnetic fields. Resonance requires the transmitter and L= 4π I 0 2 ∫∫ r − r ,receiver to have the same natural frequency. Coupling 1 1 drdr ( ρ ( r ) ρ ( r ))describes the transfer of energy. And magnetic waves enablethis coupling to be done wirelessly. The ability to wirelessly = C 4πε 0 q 0 2 ∫∫ r − r .transmit power using this effect is the focus of the researchand demonstration by the MIT team in 2006. Furthermore, this coupling can occur even around and Where J(r) is the spatial current and ρ(r) is the chargethrough objects as illustrated in Figure 4. Non-conductive density. The group stated that the energy contained in the coilmaterials simply do not affect the magnet waves. And is thus expressed as:magnetic waves wrap around conductive materials but do not 1 2 1 2 U= L I0 = q0 .impact them as the natural frequencies are not similar. 2 2C The resulting resonant frequency is then defined as: 1 f0 = . 2π LC Using their design constraints the MIT group was able to build a demonstration, shown in Figure 5, and light up the receiving 60W light bulb, effectively demonstrating wireless power with approximately 50% efficiency. This 50% efficiency is already several thousand times more efficient than today’s batteries. Figure 4: Wireless power transfer around an object. [6] E. WiTricity WiTricity, as discovered by a group of MIT professors leadby Professor Marin Soljačić, uses a scheme where non-radiating magnetic coupling is implemented using the non-lossy stationary near-field (the region within a radius r << λ,where λ = wavelength). WiTricity transmitters and receiversuse specially designed magnetic resonators as shown in Figure5 that efficiently transfer power over relatively large distancesvia the magnetic near-field. The MIT group revealed thatwhen the permittivity ε is large and field variations are slow,the material absorption is related to the material loss tangent,which allows the resonating field to couple at incredibleefficiencies. Figure 5: Professor Soljacic and the WiTricity team. [1] F. The MIT Experiment The original MIT experiment validating the concept of G. EfficiencyWiTricity was demonstrated in 2006 and used two self- One of the most spectacular aspects of this system ofresonant coils. A self-resonating coil has its own natural transferring power wirelessly is that it is relatively highlyresonant frequencies. The transmitting coil was wired to an efficient. The MIT team used and confirmed the equation:oscillating circuit; the two coils were coupled magnetically;
  4. 4. Wireless Power 4 PW devices that can be utilized in and out of the vicinity of a η= , WiTricity power source. These two definitions can be applied PS + PD + PW to the different products that use this calculate the efficiency of the system. The graph in Figure The possible applications of WiTricity in automatically6 shows the theoretical and experimental efficiencies the MIT powering and charging today’s electronics are found in their system. Applications for consumer electronics include automatic charging and recharging of portable electronics (I-pods, cell phones, controllers, laptops, etc.) in automobiles, homes, offices, and WiTricity-enabled hot spots. Other consumer applications include direct powering of immobile devices such as digital pictures, home theater systems, flat screen televisions, speakers, and desktop PC’s and peripherals. Industrial applications include direct wireless power interconnections across rotating and moving “joints” (e.g., robotics, packing equipment, assembly machine lines, and tools) thus, doing away with costly and problematic wiring. Other industrial applications include providing power interconnections in harsh environments (mining, underwater, etc.), and a wireless source for moving robotics and automatic guided vehicles eliminating the need for expensive power Figure 6: Theoretical and Experimental System wiring, complex mechanisms, and labor intensive charging. Efficiency. [5] Applications in transportation include automatic wireless powering for personal and commercial hybrid and future all- The graph in Figure 6 compares three different things electric vehicles and eliminating the need for costly harnessesrelated to the efficiency of the system. The teal shaded area and slip rings. There are also unconventional applications likeshows the theoretical prediction of maximum efficiency. The the powering of implantable medical devices and high techblack squares show the calculated maximum. And the red military systems (e.g., military robotics, aircrafts, etc).bars show the actual experimental values. WiTricity Corporation is hoping to bring such WiTricity If the distance between the source and the receiving coil is transmitters and receivers to the consumer market as early as“d”, then about 95% of the power can be transferred (with 2011.appropriate coils and frequencies). However, if there is alonger distance say “5d” only about 10% of the power can betransferred. However, these are still impressive results, V. ADVANTAGES AND DISADVANTAGESeverything considered. As one of the MIT professors One of the things that will probably be on the tip of mostexplained, “If Im replacing an alkaline battery, if its only 1% people’s tongue when they hear about this new way ofefficient it will still beat the pants off of a battery because transferring power is caution on the potential negative impactsdisposable battery power costs 350x grid power.” [2]. This this new method may have on the environment, especiallyincredible amount of efficiency for transferring energy humans. The answer to this obvious question is another greatwirelessly has exciting implications. selling point for the technology. The WiTricity method of providing wireless power transfer has insignificant effects on H. WiTricity Products and Applications the environment and has almost no effect on humans since they look like air to the resonating magnetic field. Figure 5 After the impressive research results were recorded by the shows the MIT group standing between their huge inductiveWiTricity research team, a corporation was founded in 2007 to coils, clearly not being affected in any way by the magneticdevelop ways for commercializing this new technology. The field.WiTricity Corporation has two different definitions in how For the environmental side of things, the way these coilstheir wireless project can be used. Their first definition is for work in transferring energy the electricity stays on the sendingDirect Wireless Power. Direct Wireless Power is when all of or receiving coils instead of radiating harmfully into thethe power that a system or device requires is provided without environment, like many of today’s devices (take cell phoneswires, and without a chemical energy source such as batteries. for example). Also, since the power is transferred by theDirect Wireless Power is used when a device is always within magnetic field there are no physical electrons beingthe range of a WiTricity power source. The next definition is transmitted into the air. The only thing that is escaping intothat of Automatic Wireless Charging. Automatic Wireless the environment is the heat caused by the small resistance ofCharging is used when a system/device with a rechargeable the wire as the current flows through the coils. Therefore, allbattery is able to charge itself, either while powered up or not, of the power built on the coils that has not been absorbed bywithout the need for a physical cord or battery change. the receiving coils, is slowly dissipated as heat loss. InAutomatic Wireless Charging is for mobile systems and addition, as WiTricity does not use radio waves, it does not
  5. 5. Wireless Power 5interfere with Wi-Fi, cell phones, or other methods of [2] J. Dix. (2010). “Wireless power” Retrieved March 17,communication. The reason why transferring power 2010, fromwirelessly like this does not hurt humans is again because no are being radiated into the air. Since the power is witricity.html?page=1being transferred by a magnetic field much smaller than the [3] A. Kurs., A. Karalis., R. Moffatt., P. Fisher., &’s magnetic field it is perfectly safe for all earthly life, Soljacic. (2007). Wireless power transfer via stronglyincluding human life. Furthermore, since it is extremely coupled magnetic resonances”. Science Express. (n.d.).difficult to get two unlike objects to perfectly resonate [4] A. Kurs. (2007). Wireless power transfer via stronglymagnetically with each other it is extremely unlikely for coupled magnetic resonances. Science. 317(7), 83, [5] A. Karalis., J. Joannopoulos., & Marin Soljacic. (2007).unwanted objects to interfere with the WiTricity method of Efficient wireless non-radiative mid-range energywireless power transfer, making WiTricity safe for pace transfer. Annals of Physics. 323(2008), 34-48.makers and other small medical devices. This protection is [6] WiTricity Corporation. (2010). WiTricity technology: thealso ensured because WiTricity Corporation technology uses basics. Retrieved March 20, 2010, from,frequencies that are untouched by most of today’s devices. [7] Ulaby, F. Fundamentals of applied electromagnetics. Upper Saddle River, NJ; Pearson Education Inc., 2007, VI. VARIANTS AND EVOLUTION OF THE TECHNOLOGY pp. 528 While the development of “WiTricity” applications alreadyseems limitless, the concept is still in its birthing stage andthere are sure to be many improvements developed in thetechnology in the coming years. During the development ofthis report, the authors could already envision potentialadditional applications/extensions. For example, an automaticwireless charging device could transmit an “I need power”signal periodically and a WiTricity transmission system couldlisten for those requests and provide power as needed. Thiscould eliminate the need to have the WiTricity transmittercontinuously powered on. Another area that didn’t appear tobe explored is relay stations. The idea is to havereceive/transmit stations to extend the range of power withoutadditional wires – could such an idea replace wires betweenpower poles? What about power provided to a vehicle while itwas moving? Just as WiMax first was only a fixed wirelesstechnology before being enhanced to support mobileapplicaions, WiTricity might be enhanced to provide power tovehicles while being driven. These are just a few rough ideas;the WiTricity concept is bound to have numerousimprovements in the years ahead. VII. CONCLUSION WiTricity uses non-radiative magnetic resonant coupling toachieve wireless power transfer over a million times moreefficient than inductive coupling. The idea of usingcomponents’ natural resonant frequencies to couple inductionfields ensures efficient energy transfer and has globalapplications. The MIT group that researched andexperimentally validated this concept in 2006 has alreadybegun developing applications for this technology. TheWiTricity Corporation hopes to bring this technology to theconsumer by 2011. REFERENCES[1] F. Hadley,. (2007). Goodbye wires. (PDF) “MIT_WiTricity_Press_Release.pdf”