Wireless power

ing capacitive coupling between electrodes.[5][8] Applica- tions of this type are electric toothbrush chargers, RFID tags, smartcards, and chargers for implantable medical devices like artificial cardiac pacemakers, and inductive powering or charging of electric vehicles like trains or buses.[9][11] A current focus is to develop wireless sys- tems to charge mobile and handheld computing devices such as cellphones, digital music player and portable com- puters without being tethered to a wall plug. In radiative or far-field techniques, also called power beaming, power is transmitted by beams of electromagnetic radiation, like microwaves or laser beams. These techniques can trans- port energy longer distances but must be aimed at the receiver. Proposed applications for this type are solar power satellites, and wireless powered drone aircraft.[9] An important issue associated with all wireless power sys- tems is limiting the exposure of people and other living things to potentially injurious electromagnetic fields (see Electromagnetic radiation and health).[9]

1 Overview

Antennas or Inductive charging pad for LG smartphone, using the Qi (pro- Coupling Devices nounced 'Chi') system, an example of near-field wireless trans- fer. When the phone is set on the pad, a coil in the pad creates a magnetic field which induces a current in another coil, in the Vs phone, charging its battery. Power Source Transmitter Receiver Load Wireless power transfer (WPT)[1] or wireless energy transmission is the transmission of electrical power from Generic block diagram of a wireless power system a power source to a consuming device without using solid wires or conductors.[2][3][4][5] It is a generic term “Wireless power transmission” is a collective term that that refers to a number of different power transmis- refers to a number of different technologies for trans- sion technologies that use time-varying electromagnetic mitting power by means of time-varying electromagnetic fields.[1][5][6][7] Wireless transmission is useful to power fields.[1][5][8] The technologies, listed in the table below, electrical devices in cases where interconnecting wires differ in the distance over which they can transmit power are inconvenient, hazardous, or are not possible. In wire- efficiently, whether the transmitter must be aimed (di- less power transfer, a transmitter device connected to rected) at the receiver, and in the type of electromagnetic a power source, such as the mains power line, trans- energy they use: time varying electric fields, magnetic mits power by electromagnetic fields across an interven- fields, radio waves, microwaves, or infrared or visible ing space to one or more receiver devices, where it is con- light waves.[8] [1] verted back to electric power and utilized. In general a wireless power system consists of a “transmit- Wireless power techniques fall into two categories, non- ter” device connected to a source of power such as mains radiative and radiative.[1][6][8][9][10] In near-field or non- power lines, which converts the power to a time-varying radiative techniques, power is transferred over short dis- electromagnetic field, and one or more “receiver” devices tances by magnetic fields using inductive coupling be- which receive the power and convert it back to DC or tween coils of wire or in a few devices by electric fields us- AC electric power which is consumed by an electrical

1 2 2 FIELD REGIONS

load.[1][8] In the transmitter the input power is converted technologies are used for transmitting power: to an oscillating electromagnetic field by some type of "antenna" device. The word “antenna” is used loosely • Near-field or nonradiative region - This means here; it may be a coil of wire which generates a magnetic the area within about 1 wavelength (λ) of the field, a metal plate which generates an electric field, an antenna.[1][4][10] In this region the oscillating electric antenna which radiates radio waves, or a laser which gen- and magnetic fields are separate[6] and power can erates light. A similar antenna or coupling device in the be transferred via electric fields by capacitive cou- receiver converts the oscillating fields to an electric cur- pling (electrostatic induction) between metal elec- rent. An important parameter which determines the type trodes, or via magnetic fields by inductive cou- of waves is the frequency f in hertz of the oscillations. pling (electromagnetic induction) between coils of The frequency determines the wavelength λ = c/f of the wire.[5][6][8][9] These fields are not radiative,[10] waves which carry the energy across the gap, where c is meaning the energy stays within a short distance the velocity of light. of the transmitter.[18] If there is no receiving de- Wireless power uses much of the same fields and waves vice or absorbing material within their limited range [18] as wireless communication devices like radio,[6][12] an- to “couple” to, no power leaves the transmitter. other familiar technology which involves power trans- The range of these fields is short, and depends on mitted without wires by electromagnetic fields, used in the size and shape of the “antenna” devices, which cellphones, radio and television broadcasting, and WiFi. are usually coils of wire. The fields, and thus In radio communication the goal is the transmission of the power transmitted, decrease exponentially with [4][17][19] information, so the amount of power reaching the re- distance, so if the distance between the two ceiver is unimportant as long as it is enough that the signal “antennas” Dᵣₐₑ is much larger than the diameter to noise ratio is high enough that the information can of the “antennas” Dₐ very little power will be re- be received intelligibly.[5][6][12] In wireless communica- ceived. Therefore these techniques cannot be used tion technologies generally only tiny amounts of power for long distance power transmission. reach the receiver. By contrast, in wireless power, the amount of power received is the important thing, so the Resonance, such as resonant inductive efficiency (fraction of transmitted power that is received) coupling, can increase the coupling be- is the more significant parameter.[5] For this reason wire- tween the antennas greatly, allowing ef- less power technologies are more limited by distance than ficient transmission at somewhat greater wireless communication technologies. distances,[1][4][6][9][20][21] although the fields still decrease exponentially. Therefore the These are the different wireless power range of near-field devices is conventionally technologies:[1][8][9][13][14] devided into two categories: • Short range - up to about one antenna di- ameter: Dᵣₐₑ ≤ Dₐ.[18][20][22] This is 2 Field regions the range over which ordinary nonreso- nant capacitive or inductive coupling can Electric and magnetic fields are created by charged parti- transfer practical amounts of power. cles in matter such as electrons. A stationary charge cre- • Mid-range - up to 10 times the antenna ates an electrostatic field in the space around it. A steady diameter: Dᵣₐₑ ≤ 10 Dₐ.[20][21][22][23] current of charges (direct current, DC) creates a static This is the range over which resonant ca- magnetic field around it. These fields contain energy. pacitive or inductive coupling can trans- The above fields cannot carry power because they are fer practical amounts of power. static, but time-varying fields can.[16] Accelerating elec- tric charges, such as are found in an alternating current • Far-field or radiative region - Beyond about 1 (AC) of electrons in a wire, create time-varying electric wavelength (λ) of the antenna, the electric and mag- and magnetic fields in the space around them. These fields netic fields are perpendicular to each other and prop- can exert oscillating forces on the electrons in a receiving agate as an electromagnetic wave; examples are “antenna”, causing them to move back and forth. These radio waves, microwaves, or light waves.[1][4][9] This represent alternating current which can be used to power part of the energy is radiative,[10] meaning it leaves a load. the antenna whether or not there is a receiver to ab- The oscillating electric and magnetic fields surrounding sorb it. The portion of energy which does not strike moving electric charges in an antenna device can be di- the receiving antenna is dissipated and lost to the vided into two regions, depending on distance Dᵣₐₑ from system. The amount of power emitted as electro- the antenna.[1][4][6][8][9][10][17] The boundary between the magnetic waves by an antenna depends on the ratio regions is somewhat vaguely defined.[8] The fields have of the antenna’s size Dₐ to the wavelength of the different characteristics in these regions, and different waves λ,[24] which is determined by the frequency: 3.2 Capacitive coupling 3

λ = c/f. At low frequencies f where the antenna is much smaller than the size of the waves, Dₐ << λ, very little power is radiated. Therefore the B near-field devices above, which use lower frequen- cies, radiate almost none of their energy as electro- Vs magnetic radiation. Antennas about the same size Power Oscillator L1 L2 Rectifier Load as the wavelength Dₐ ≈ λ such as monopole or Source dipole antennas, radiate power efficiently, but the electromagnetic waves are radiated in all directions Generic block diagram of an inductive wireless power system. (omnidirectionally), so if the receiving antenna is far away, only a small amount of the radiation will hit it.[10][20] Therefore these can be used for short range, by an electric current to induce a current in a second con- inefficient power transmission but not for long range ductor. This effect occurs in the electromagnetic near transmission.[25] field, with the secondary in close proximity to the pri- mary. As the distance from the primary is increased, However, unlike fields, electromagnetic radia- more and more of the primary’s magnetic field misses the tion can be focused by reflection or refraction secondary. Even over a relatively short range the induc- into beams. By using a high-gain antenna tive coupling is grossly inefficient, wasting much of the or optical system which concentrates the ra- transmitted energy.[30] diation into a narrow beam aimed at the re- This action of an electrical transformer is the simplest ceiver, it can be used for long range power form of wireless power transmission. The primary coil [20][25] transmission. From the Rayleigh crite- and secondary coil of a transformer are not directly con- rion, to produce the narrow beams necessary nected; each coil is part of a separate circuit. Energy to focus a significant amount of the energy on transfer takes place through a process known as mutual a distant receiver, an antenna must be much induction. Principal functions are stepping the primary larger than the wavelength of the waves used: voltage either up or down and electrical isolation. Mobile [26][27] Dₐ >> λ = c/f. Practical beam power phone and electric toothbrush battery chargers, are exam- devices require wavelengths in the centime- ples of how this principle is used. Induction cookers use ter region or below, corresponding to frequen- this method. The main drawback to this basic form of cies above 1 GHz, in the microwave range or wireless transmission is short range. The receiver must [1] above. be directly adjacent to the transmitter or induction unit in order to efficiently couple with it. Common uses of resonance-enhanced electrodynamic 3 Near-field or non-radiative tech- induction[31] are charging the batteries of portable devices niques such as laptop computers and cell phones, medical im- plants and electric vehicles.[32][33][34] A localized charg- [35] Main article: Coupling (electronics) ing technique selects the appropriate transmitting coil in a multilayer winding array structure.[36] Resonance is used in both the wireless charging pad (the transmitter The near-field components of electric and magnetic circuit) and the receiver module (embedded in the load) fields die out quickly beyond a distance of about one to maximize energy transfer efficiency. Battery-powered diameter of the antenna (Dₐ). Outside very close devices fitted with a special receiver module can then be ranges the field strength and coupling is roughly pro- charged simply by placing them on a wireless charging −3 [17] portional to (Dᵣₐₑ/Dₐ) . Since power is propor- pad. It has been adopted as part of the Qi wireless charg- tional to the square of the field strength, the power trans- ing standard. ferred decreases with the sixth power of the distance (Dᵣₐₑ/Dₐ)−6.[6][19][28][29] or 60 dB per decade. In other This technology is also used for powering devices with words, doubling the distance between transmitter and re- very low energy requirements, such as RFID patches and ceiver causes the power received to decrease by a factor contactless smartcards. Instead of relying on each of the of 26 = 64. many thousands or millions of RFID patches or smart- cards to contain a working battery, electrodynamic induc- tion can provide power only when the devices are needed. 3.1 Inductive coupling

Main articles: Inductive coupling and Resonant inductive 3.2 Capacitive coupling coupling The electrodynamic induction wireless transmission Main article: Capacitive coupling technique relies on the use of a magnetic field generated 4 4 FAR-FIELD OR RADIATIVE TECHNIQUES

In capacitive coupling (electrostatic induction), the dual by the rotating magnets produce less electromagnetic in- of inductive coupling, power is transmitted by electric terference to nearby electronic devices than the high fre- fields[5] between electrodes such as metal plates. The quency magnetic fields produced by inductive coupling transmitter and receiver electrodes form a capacitor, with systems. A prototype system charging electric vehicles the intervening space as the dielectric.[5][6][9][37][38] An has been in operation at University of British Columbia alternating voltage generated by the transmitter is ap- since 2012. Other researchers, however, claim that the plied to the transmitting plate, and the oscillating electric two energy conversions (electrical to mechanical to elec- field induces an alternating potential on the receiver plate trical again) make the system less efficient than electrical by electrostatic induction,[5] which causes an alternat- systems like inductive coupling.[13] ing current to flow in the load circuit. The amount of power transferred increases with the frequency[37] and the capacitance between the plates, which is proportional to the area of the smaller plate and (for short distances) in- 4 Far-field or radiative techniques versely proportional to the separation.[5] Capacitive coupling has only been used practically in a Far field methods achieve longer ranges, often multi- few low power applications, because the very high volt- ple kilometer ranges, where the distance is much greater ages on the electrodes required to transmit significant than the diameter of the device(s). The main reason for power can be hazardous,[6][9] and can cause unpleasant longer ranges with radio wave and optical devices is the side effects such as noxious ozone production. In addi- fact that electromagnetic radiation in the far-field can be tion, in contrast to magnetic fields,[20] electric fields in- made to match the shape of the receiving area (using high teract strongly with most materials, including the human directivity antennas or well-collimated laser beams). The body, due to dielectric polarization.[38] Intervening ma- maximum directivity for antennas is physically limited by terials between or near the electrodes can absorb the en- diffraction. ergy, in the case of humans possibly causing excessive In general, visible light (from lasers) and microwaves electromagnetic field exposure.[6] However capacitive (from purpose-designed antennas) are the forms of elec- coupling has a few advantages over inductive. The field tromagnetic radiation best suited to energy transfer. is largely confined between the capacitor plates, reducing The dimensions of the components may be dictated by the interference, which in inductive coupling requires heavy distance from transmitter to receiver, the wavelength and ferrite “flux confinement” cores.[5][38] Also, alignment re- the Rayleigh criterion or diffraction limit, used in stan- quirements between the transmitter and receiver are less dard radio frequency antenna design, which also applies critical.[5][6][37] Capacitive coupling has recently been ap- to lasers. Airy’s diffraction limit is also frequently used plied to charging battery powered portable devices[39] and to determine an approximate spot size at an arbitrary dis- is being considered as a means of transferring power be- tance from the aperture. Electromagnetic radiation ex- tween substrate layers in integrated circuits.[40] periences less diffraction at shorter wavelengths (higher frequencies); so, for example, a blue laser is diffracted 3.3 Magnetodynamic coupling less than a red one. The Rayleigh criterion dictates that any radio wave, mi- In this method, power is transmitted between two rotat- crowave or laser beam will spread and become weaker ing armatures, one in the transmitter and one in the re- and diffuse over distance; the larger the transmitter an- ceiver, which rotate synchronously, coupled together by tenna or laser aperture compared to the wavelength of ra- a magnetic field generated by permanent magnets on the diation, the tighter the beam and the less it will spread as armatures.[13] The transmitter armature is turned either a function of distance (and vice versa). Smaller antennae by or as the rotor of an electric motor, and its magnetic also suffer from excessive losses due to side lobes. How- field exerts torque on the receiver armature, turning it. ever, the concept of laser aperture considerably differs The magnetic field acts like a mechanical coupling be- from an antenna. Typically, a laser aperture much larger tween the armatures.[13] The receiver armature produces than the wavelength induces multi-moded radiation and power to drive the load, either by turning an electric gen- mostly collimators are used before emitted radiation cou- erator or by using the receiver armature as the rotor in an ples into a fiber or into space. induction generator . Ultimately, beamwidth is physically determined by This device has been proposed as an alternative to induc- diffraction due to the dish size in relation to the wave- tive power transfer for noncontact charging of electric ve- length of the electromagnetic radiation used to make the hicles.[13] A rotating armature embedded in a garage floor beam. or curb would turn a receiver armature in the underside of the vehicle to charge its batteries.[13] It is claimed that Microwave power beaming can be more efficient than this technique can transfer power over distances of 10 to lasers, and is less prone to atmospheric attenuation caused 15 cm (4 to 6 inches) with high efficiency, over 90%.[13] by dust or water vapor. Also, the low frequency stray magnetic fields produced Then the power levels are calculated by combining the 4.2 Lasers 5

above parameters together, and adding in the gains Following World War II, which saw the development of and losses due to the antenna characteristics and the high-power microwave emitters known as cavity mag- transparency and dispersion of the medium through netrons, the idea of using microwaves to transmit power which the radiation passes. That process is known as cal- was researched. By 1964, a miniature helicopter pro- culating a link budget. pelled by microwave power had been demonstrated.[45] Japanese researcher Hidetsugu Yagi also investigated 4.1 Microwaves wireless energy transmission using a directional array an- tenna that he designed. In February 1926, Yagi and his Main article: Microwave power transmission colleague Shintaro Uda published their first paper on the Power transmission via radio waves can be made more tuned high-gain directional array now known as the Yagi antenna. While it did not prove to be particularly use- ful for power transmission, this beam antenna has been widely adopted throughout the broadcasting and wireless telecommunications industries due to its excellent perfor- mance characteristics.[46] Wireless high power transmission using microwaves is well proven. Experiments in the tens of kilowatts have been performed at Goldstone in California in 1975[47][48][49] and more recently (1997) at Grand Bassin on Reunion Island.[50] These methods achieve distances on the order of a kilometer. Under experimental conditions microwave conversion ef- ficiency was measured to be around 54%.[51] An artist’s depiction of a solar satellite that could send electric energy by microwaves to a space vessel or planetary surface. More recently a change to 24 GHz has been suggested as microwave emitters similar to LEDs have been made with directional, allowing longer distance power beaming, very high quantum efficiencies using negative resistance with shorter wavelengths of electromagnetic radiation, i.e. Gunn or IMPATT diodes and this would be viable typically in the microwave range.[41] A rectenna may for short range links. be used to convert the microwave energy back into electricity. Rectenna conversion efficiencies exceeding 95% have been realized. Power beaming using mi- 4.2 Lasers crowaves has been proposed for the transmission of en- ergy from orbiting solar power satellites to Earth and the beaming of power to spacecraft leaving orbit has been considered.[42][43] Power beaming by microwaves has the difficulty that for most space applications the required aperture sizes are very large due to diffraction limiting antenna direc- tionality. For example, the 1978 NASA Study of so- lar power satellites required a 1-km diameter transmit- ting antenna, and a 10 km diameter receiving rectenna, for a microwave beam at 2.45 GHz.[44] These sizes can be somewhat decreased by using shorter wavelengths, al- though short wavelengths may have difficulties with at- mospheric absorption and beam blockage by rain or wa- ter droplets. Because of the "thinned array curse,” it is not possible to make a narrower beam by combining the beams of several smaller satellites.

For earthbound applications a large area 10 km diameter With a laser beam centered on its panel of photovoltaic cells, receiving array allows large total power levels to be used a lightweight model plane makes the first flight of an aircraft while operating at the low power density suggested for powered by a laser beam inside a building at NASA Marshall human electromagnetic exposure safety. A human safe Space Flight Center. power density of 1 mW/cm2 distributed across a 10 km diameter area corresponds to 750 megawatts total power In the case of electromagnetic radiation closer to the vis- level. This is the power level found in many modern elec- ible region of the spectrum (tens of micrometers to tens tric power plants. of nanometres), power can be transmitted by converting 6 6 HISTORY electricity into a laser beam that is then pointed at a pho- In the context of wireless power, energy harvesting, also tovoltaic cell.[52] This mechanism is generally known as called power harvesting or energy scavenging, is the con- “power beaming” because the power is beamed at a re- version of ambient energy from the environment to elec- ceiver that can convert it to electrical energy. tric power, mainly to power small autonomous wireless [69] Compared to other wireless methods:[53] electronic devices. The ambient energy may come from stray electric or magnetic fields or radio waves from nearby electrical equipment, light, thermal energy • Collimated monochromatic wavefront propagation (heat), or kinetic energy such as vibration or motion allows narrow beam cross-section area for transmis- of the device.[69] Although the efficiency of conversion sion over large distances. is usually low and the power gathered often minuscule • Compact size: solid state lasers fit into small prod- (milliwatts or microwatts),[69] it can be adequate to run ucts. or recharge small micropower wireless devices such as remote sensors, which are proliferating in many fields.[69] • No radio-frequency interference to existing radio This new technology is being developed to eliminate the communication such as Wi-Fi and cell phones. need for battery replacement or charging of such wire- • Access control: only receivers hit by the laser receive less devices, allowing them to operate completely au- power. tonomously.

Drawbacks include: 6 History • Laser radiation is hazardous. Low power levels can blind humans and other animals. High power levels In 1826 André-Marie Ampère developed Ampère’s cir- can kill through localized spot heating. cuital law showing that electric current produces a mag- • Conversion between electricity and light is ineffi- netic field.[70] Michael Faraday developed Faraday’s law cient. Photovoltaic cells achieve only 40%–50% of induction in 1831, describing the electromagnetic efficiency.[54] (Efficiency is higher with monochro- force induced in a conductor by a time-varying magnetic matic light than with solar panels). flux. In 1862 James Clerk Maxwell synthesized these and other observations, experiments and equations of elec- • Atmospheric absorption, and absorption and scat- tricity, magnetism and optics into a consistent theory, tering by clouds, fog, rain, etc., causes up to 100% deriving Maxwell’s equations. This set of partial differ- losses. ential equations forms the basis for modern electromag- • Requires a direct line of sight with the target. netics, including the wireless transmission of electrical energy.[14][71] Maxwell predicted the existence of elec- Laser “powerbeaming” technology has been mostly ex- tromagnetic waves in his 1873 A Treatise on Electricity [72] plored in military weapons[55][56][57] and aerospace[58][59] and Magnetism. In 1884 John Henry Poynting devel- applications and is now being developed for commercial oped equations for the flow of power in an electromag- and consumer electronics. Wireless energy transfer sys- netic field, Poynting’s theorem and the Poynting vector, tems using lasers for consumer space have to satisfy laser which are used in the analysis of wireless energy trans- [14][71] safety requirements standardized under IEC 60825. fer systems. In 1888 Heinrich Rudolf Hertz dis- [60] covered radio waves, confirming the prediction of elec- Other details include propagation, and the coherence tromagnetic waves by Maxwell.[72] and the range limitation problem.[61] Geoffrey Landis[62][63][64] is one of the pioneers of solar power satellites[65] and laser-based transfer of energy es- 6.1 Tesla’s experiments pecially for space and lunar missions. The demand for safe and frequent space missions has resulted in propos- Serbian-born American inventor Nikola Tesla performed als for a laser-powered space elevator.[66][67] the first experiments in wireless power transmission at the turn of the 20th century,[71][73] and may have done more NASA’s Dryden Flight Research Center demonstrated a to popularize the idea than any other individual. In the lightweight unmanned model plane powered by a laser period 1891 to 1904 he experimented with spark-excited beam.[68] This proof-of-concept demonstrates the feasi- radio frequency resonant transformers, now called Tesla bility of periodic recharging using the laser beam system. coils, which generated high AC voltages on elevated ca- pacitive terminals.[71][73][74] With these he was able to transmit power for short distances without wires. In 5 Energy harvesting demonstrations before the American Institute of Electri- cal Engineers[74] and at the 1893 Columbian Exposition Main article: Energy harvesting in Chicago he lit light bulbs from across a stage.[73] He found he could increase the distance by using a receiv- 6.2 Microwaves 7

firmation of this putative demonstration;[76][86][90] Tesla did not mention it,[86] and it does not appear in his metic- ulous laboratory notes.[90][91] It originated in 1944 from Tesla’s first biographer, John J. O'Neill,[76] who said he pieced it together from “fragmentary material... in a num- ber of publications”.[92] In the 110 years since Tesla’s experiments, efforts using similar equipment have failed to achieve long distance power transmission,[73][76][86][88] and the scientific consensus is his World Wireless sys- tem would not have worked.[14][71][75][81][86][93][94][95][96] Tesla’s world power transmission scheme remains today what it was in Tesla’s time, a fascinating dream.[14][81]

Tesla demonstrating wireless power transmission in a lecture at Columbia College, New York, in 1891. The two metal sheets 6.2 Microwaves are connected to his Tesla coil oscillator, which applies a high radio frequency oscillating voltage. The oscillating electric field Before World War 2, little progress was made in wireless between the sheets ionizes the low pressure gas in the two long power transmission.[87] Radio was developed for com- Geissler tubes he is holding, causing them to glow by fluorescence, similar to neon lights. munication uses, but couldn't be used for power trans- mission due to the fact that the relatively low-frequency radio waves spread out in all directions and little energy reached the receiver.[14][71][87] In radio communication, ing LC circuit tuned to resonance with the transmitter’s at the receiver, an amplifier intensifies a weak signal using LC circuit.[75] At his Colorado Springs laboratory during energy from another source. For power transmission, ef- 1899-1900, by using voltages of the order of 20 mega- ficient transmission required transmitters that could gen- volts generated by an enormous coil, he was able to light erate higher-frequency microwaves, which can be focused three incandescent lamps by resonant inductive coupling in narrow beams towards a receiver.[14][71][87][94] [76][77] at a distance of about 100 feet (30 m). Coupling The development of microwave technology during World between resonant circuits by electric or magnetic fields War 2, such as the klystron and magnetron tubes and is now a familiar technology used throughout electronics, parabolic antennas[87] made radiative (far-field) methods and is currently of interest again as a means of short-range practical for the first time, and the first long-distance wireless power transmission.[73][78] As mentioned above [73] wireless power transmission was achieved in the 1960s it is a "near-field" effect, so it is not able to transmit by William C. Brown.[14][71] In 1964 Brown invented the power over long distances. rectenna which could efficiently convert microwaves to However, Tesla claimed to be able to transmit power on DC power, and in 1964 demonstrated it with the first a worldwide scale, using a method that involved conduc- wireless-powered aircraft, a model helicopter powered tion through the Earth and atmosphere.[79][80][81][82] The by microwaves beamed from the ground.[14][87] A major proposal suggested that receiving stations would consist motivation for microwave research in the 1970s and 80s of terminals suspended in the air at above 30,000 feet was to develop a solar power satellite.[71][87] Conceived (9,100 m) in altitude, where the pressure is lower than in 1968 by Peter Glaser, this would harvest energy from at sea level.[79] At this altitude, Tesla claimed, electricity sunlight using solar cells and beam it down to Earth as could be sent at high voltages (millions of volts) over long microwaves to huge rectennas, which would convert it distances. to electrical energy on the electric power grid.[14][97] In In 1901, Tesla began construction of a large high- landmark 1975 high power experiments, Brown demon- voltage coil facility, the Wardenclyffe Tower at Shore- strated short range transmission of 475 W of microwaves ham, New York, intended as a prototype transmitter at 54% DC to DC efficiency, and he and Robert Dick- for a "World Wireless System" that was to transmit inson at NASA’s Jet Propulsion Laboratory transmitted power worldwide, but he lost funding by 1904 and 30 kW DC output power across 1.5 km with 2.38 GHz the facility was never completed.[81][83] Although Tesla microwaves from a 26 m dish to a 7.3 x 3.5 m rectenna array.[14][98] The incident-RF to DC conversion efficiency claimed his ideas were proven, he had a history of fail- [14][98] ing to confirm his ideas by experiment,[84][85] and there of the rectenna was 80%. In 1983 Japan launched seems to be no evidence that he ever transmitted sig- MINIX (Microwave Ionosphere Nonlinear Interation Ex- periment), a rocket experiment to test transmission of nificant power beyond the short-range demonstrations [14] above,[14][71][75][76][85][86][87][88][89] perhaps 300 feet (91 high power microwaves through the ionosphere. m). The only report of long-distance transmission by In recent years a focus of research has been the develop- Tesla is a claim, not found in reliable sources, that in ment of wireless-powered drone aircraft, which began in 1899 he wirelessly lit 200 light bulbs at a distance of 1959 with the Dept. of Defense’s RAMP (Raytheon Air- 26 miles (42 km).[76][86] There is no independent con- borne Microwave Platform) project[87] which sponsored 8 8 FURTHER READING

Brown’s research. In 1987 Canada’s Communications 7 See also Research Center developed a small prototype airplane called Stationary High Altitude Relay Platform (SHARP) • Beam-powered propulsion to relay telecommunication data between points on earth similar to a communication satellite. Powered by a • Beam Power Challenge – one of the NASA rectenna, it could fly at 13 miles (21 km) altitude and Centennial Challenges stay aloft for months. In 1992 a team at Kyoto University built a more advanced craft called MILAX (MIcrowave • Differential capacitance Lifted Airplane eXperiment). In 2003 NASA flew the • Dispersion relation first laser powered aircraft. The small model plane’s mo- tor was powered by electricity generated by photocells • Distributed generation from a beam of infrared light from a ground based laser, while a control system kept the laser pointed at the plane. • Electricity distribution

• Electric power transmission

• Electromagnetic compatibility

• Electromagnetic radiation and health 6.3 Near-field technologies • Energy harvesting • Inductive power transfer between nearby coils of wire is Fermi gas an old technology, existing since the transformer was de- • Free electron model veloped in the 1800s. Induction heating has been used for 100 years. With the advent of cordless appliances, • Friis transmission equation inductive charging stands were developed for appliances used in wet environments like electric toothbrushes and • Microwave power transmission electric razors to reduce the hazard of electric shock. • Multidimensional systems One field to which inductive transfer has been applied is to power electric vehicles. In 1892 Maurice Hutin and • Resonant inductive coupling Maurice Leblanc patented a wireless method of powering railroad trains using resonant coils inductively coupled to • Surface plasmon a track wire at 3 kHz.[99] The first passive RFID (Radio • Surface plasmon polariton Frequency Identification) technologies were invented by [100] [101] Mario Cardullo (1973) and Koelle et al. (1975) • Surface wave and by the 1990s were being used in proximity cards and contactless smartcards. • Thinned array curse The proliferation of portable wireless communication de- • Transmission medium vices such as cellphones, tablet, and laptop computers in recent decades is currently driving the development of • Wardenclyffe Tower wireless powering and charging technology to eliminate the need for these devices to be tethered to wall plugs dur- • Wave vector ing charging.[102] The Wireless Power Consortium was • established in 2008 to develop interoperable standards Zenneck wave across manufacturers.[102] Its Qi inductive power standard published in August 2009 enables charging and powering of portable devices of up to 5 watts over distances of 4 8 Further reading cm (1.6 inches).[103] The wireless device is placed on a flat charger plate (which could be embedded in table tops Books and Articles at cafes, for example) and power is transferred from a flat coil in the charger to a similar one in the device. • Agbinya, Johnson I., Ed. (2012). Wireless Power In 2007, a team led by Marin Soljačić at MIT used cou- Transfer. River Publishers. ISBN 8792329233. pled tuned circuits made of a 25 cm resonant coil at 10 Comprehensive, theoretical engineering text MHz to transfer 60 W of power over a distance of 2 me- ters (6.6 ft) (8 times the coil diameter) at around 40% • Shinohara, Naoki (2014). Wireless Power Trans- efficiency.[73][104] This technology is being commercial- fer via Radiowaves. John Wiley & Sons. ISBN ized as WiTricity. 1118862961. Engineering text 9

• Tomar, Anuradha; Gupta, Sunil (July 2012). [7] Wilson, Tracy V. (2014). “How Wireless Power Works”. “Wireless power Transmission: Applications and How Stuff Works website. InfoSpace LLC. Retrieved De- Components”. International Journal of Engineering cember 15, 2014. Research & Technology (ESRSA Publications Pvt. [8] Sun, Tianjia; Xie, Xiang; Zhihua, Wang (2013). Wireless Ltd.) 1 (5): 1–8. ISSN 2278-0181. Brief survey of Power Transfer for Medical Microsystems. Springer Sci- state of wireless power and applications ence & Business Media. pp. 5–6. ISBN 1461477026.

• Kurs, André; Karalis, Aristeidis; Moffatt, Robert [9] Valtchev, Stanimir S.; Baikova, Elena N.; Jorge, Luis R. (July 2007). “Wireless Power Transfer via Strongly (December 2012). “Electromagnetic Field as the Wireless Coupled Magnetic Resonances”. Science (American Transporter of Energy”. Facta Universitatis Ser. Electri- cal Engineering (Serbia: University of Niš) 25 (3): 171– Association for the Advancement of Science) 317: 181. doi:10.2298/FUEE1203171V. Retrieved December 83–85. doi:10.1126/science.1143254. ISSN 1095- 15, 2014. 9203. Landmark paper on MIT team’s 2007 devel- opment of mid-range resonant wireless transmission [10] Agbinya, Johnson I. (2012). Wireless Power Transfer. River Publishers. pp. 1–2. ISBN 8792329233. • Thibault, G. (2014). Wireless Pasts and Wired Fu- [11] New Scientist:Wireless charging for electric vehicles hits tures. In J. Hadlaw, A. Herman, & T. Swiss (Eds.), the road Theories of the Mobile Internet. Materialities and Imaginaries. (pp. 126–154). London: Routledge. [12] Shinohara 2014 Wireless Power Transfer via Radiowaves, A short cultural history of wireless power p. 27 [13] Ashley, Steven (November 20, 2012). “Wireless recharg- Patents ing: Pulling the plug on electric cars”. BBC website. British Broadcasting Corp. Retrieved December 10, 2014. • U.S. Patent 4,955,562, Microwave powered aircraft, John E. Martin, et al. (1990). [14] Tomar, Anuradha; Gupta, Sunil (July 2012). “Wireless power Transmission: Applications and Components”. In- • U.S. Patent 3,933,323, Solid state solar to mi- ternational Journal of Engineering Research & Technology crowave energy converter system and apparatus, 1 (5). ISSN 2278-0181. Retrieved November 9, 2014. Kenneth W. Dudley, et al. (1976). [15] “short”, “midrange”, and “long range” are defined below • U.S. Patent 3,535,543, Microwave power receiving [16] Coleman, Christopher (2004). An Introduction to Radio antenna, Carroll C. Dailey (1970). Frequency Engineerin. Cambridge University Press. pp. 1–3. ISBN 1139452304. [17] Agbinya (2012) Wireless Power Transfer, p. 126-129 9 References [18] Umenei, A. E. (June 2011). “Understanding Low Fre- quency Non-radiative Power Transfer”. Fulton Innova- [1] Shinohara, Naoki (2014). Wireless Power Transfer via tion, Inc. Retrieved January 3, 2015. Radiowaves. John Wiley & Sons. pp. ix–xiii. ISBN [19] Schantz, Hans G. (June 2007). “A Real-Time Loca- 1118862961. tion System Using Near-Field Electromagnetic Ranging”. [2] Bush, Stephen F. (2014). Smart Grid: Communication- 2007 IEEE Antennas and Propagation Society Interna- Enabled Intelligence for the Electric Power Grid. John Wi- tional Symposium, Honolulu, Hawaii, USA. Inst. of Elec- ley & Sons. p. 118. ISBN 1118820231. trical and Electronic Engineers. pp. 3792–3795. Re- trieved January 2, 2015. [3] “Wireless energy transfer”. Encyclopedia of terms. PC [20] Karalis, Aristeidis; Joannopoulos, J.D.; Soljačić, Marin Magazine Ziff-Davis. 2014. Retrieved December 15, (January 2008). “Efficient wireless non-radiative mid- 2014. range energy transfer”. Annals of Physics 323 (1): 34–48. Retrieved January 3, 2015. [4] Rajakaruna, Sumedha; Shahnia, Farhad; Ghosh, Arindam (2014). Plug In Electric Vehicles in Smart Grids: In- [21] Wong, Elvin (2013). “Seminar: A Review on Technolo- tegration Techniques. Springer. pp. 34–36. ISBN gies for Wireless Electricity”. HKPC. The Hong Kong 981287299X. Electronic Industries Association Ltd. Retrieved January 3, 2015. [5] Gopinath, Ashwin (August 2013). “All About Transfer- ring Power Wirelessly”. Electronics For You E-zine (EFY [22] "Typically, an inductive coupled system can transmit Enterprises Pvt. Ltd.): 52–56. Retrieved January 16, roughly the diameter of the transmitter."(p. 4) "...mid- 2015. range is defined as somewhere between one and ten times the diameter of the transmitting coil."(p. 2) Baar- [6] Sazonov, Edward; Neuman, Michael R (2014). Wearable man, David W.; Schwannecke, Joshua (December 2009). Sensors: Fundamentals, Implementation and Applications. “White paper: Understanding Wireless Power”. Fulton Elsevier. pp. 253–255. ISBN 0124186661. Innovation. Retrieved January 3, 2015. 10 9 REFERENCES

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11 Text and image sources, contributors, and licenses

11.1 Text

• Wireless power Source: http://en.wikipedia.org/wiki/Wireless%20power?oldid=645517593 Contributors: Jimbo Wales, Bryan Derk- sen, Heron, Edward, Skysmith, Ronz, Julesd, Feedmecereal, Rob.derosa, Reddi, Omegatron, AnonMoos, Twang, Fuelbottle, Xanzzibar, Wjbeaty, Gil Dawson, Wolfkeeper, Fleminra, Ssd, Remy B, Bobblewik, Beland, Icairns, GaryPeterson, JamesTeterenko, Punga, Shadanan, Discospinster, Rich Farmbrough, Pjacobi, YUL89YYZ, Pjrich, Adambro, Jordin, John Vandenberg, Jag123, Sparkgap, Bert Hick- man, Jeodesic, Kundor, Pearle, Justinc, Passw0rd, Merenta, Hcm1955, Gary, Hackwrench, Wouterstomp, SlimVirgin, Metron4, BRW, Cromwellt, Wtshymanski, Bsadowski1, Cfrjlr, Richwales, Gatewaycat, Smark33021, Noz92, JALockhart, Mindmatrix, Radderz, Ar- mando, Benbest, CharlesC, Phoenix-forgotten, Edison, Josh Parris, Rjwilmsi, Ansend, Vegaswikian, Tdowling, NeonMerlin, JanSuchy, Xero, Docbug, Nihiltres, Ewlyahoocom, Kolbasz, Fresheneesz, Diza, Srleffler, Idaltu, Bgwhite, Manscher, Roboto de Ajvol, The Ram- bling Man, Peregrine Fisher, RussBot, Shawn81, Gaius Cornelius, Salsb, Yrithinnd, Ospalh, Dv82matt, Eurosong, WAS 4.250, 2over0, Peter Judge, Back ache, Geoffrey.landis, Mossig, AGToth, Tierce, Katieh5584, Tom Morris, SmackBot, Prebys, Thierry Caro, Luketheob- scure, Jagged 85, Alan McBeth, BiT, Gilliam, Chris the speller, TimBentley, Thumperward, Oli Filth, Neo-Jay, Quackslikeaduck, A. B., Theneokid, Achilles03, Trekphiler, Rrburke, Worrydream, Addshore, GVnayR, NOS, Ohconfucius, LN2, Rjmorris, Teneriff, Sbmehta, Jaganath, Soumyasch, Shawnborgia, Mr. Blake, BillFlis, Akitstika, Peter Horn, Kvng, Hu12, Iridescent, Cbrown1023, MGlosenger, Ben- plowman, Dikasths, SamerZiadeh, Tawkerbot2, G-W, Chetvorno, Chris55, MightyWarrior, Mikiemike, CmdrObot, Sulfis, GALVATRON, N2e, Joechao, Casper2k3, User6985, Cydebot, Dougweller, Calorus, Marx1980, Gralo, Chrisdab, Tellyaddict, Nick Number, AntiVan- dalBot, Jj137, Fritz Jörn, Alphachimpbot, Lfstevens, Hayesgm, Magioladitis, VoABot II, SHCarter, Roches, Tobogganoggin, Joecoolaug, RicarDog, Indon, Tjameson, Dontdoit, MartinBot, Xtraplanetary, Jim.henderson, Jwagnerhki, RockMFR, J.delanoy, Hans Dunkelberg, Ayecee, Hnc14, LordAnubisBOT, GandalfDaGraay, Gripen40k, Rocket71048576, Fountains of Bryn Mawr, Atropos235, WinterSpw, Xnuala, VolkovBot, Gmoose1, AeoniosHaplo, Philip Trueman, TXiKiBoT, GLPeterson, WI Keith, IPSOS, Anna Lincoln, Gillyweed, J Casanova, YordanGeorgiev, KarusaUK, Legoktm, Demize, Hazel77, HybridBoy, Sylvia chamorro, SieBot, Moonriddengirl, Qfli78, Caltas, Eagleal, Bhimaji, Nopetro, BobShair, Ericjul, Afernand74, Wikiaristos, Wuhwuzdat, Denisarona, Escape Orbit, Kanonkas, Efege, Sfan00 IMG, Oneforlogic, ClueBot, PipepBot, Krish raja, PhilDWraight, Auntof6, Sun Creator, SchreiberBike, El bot de la dieta, Scotttroyer, DumZiBoT, InternetMeme, XLinkBot, Dthomsen8, Pathexplorer, MystBot, RyanCross, Rogimoto, Leeman0, Addbot, DOI bot, Power- Beam, Diegobarone, Sergei, Leszek Jańczuk, 1archie99, Mdr5546, 5 albert square, Wireless friend, Rojypala, Suzumebachisecret, Yobot, TaBOT-zerem, Mandm, Gdewilde, AnomieBOT, Jim1138, Powerbeam08, Visionary77, Frkandris, Henriqueqc, Обедающий философ, Aaagmnr, Materialscientist, Citation bot, Knowledge Incarnate, LilHelpa, Obersachsebot, Konor org, Capricorn42, Drilnoth, Gensanders, Gilo1969, Inferno, Lord of Penguins, StealthCopyEditor, Dranthonyjthomson, Kirsted, Brett Lally, GliderMaven, FrescoBot, Magnagr, Freonjake, Afshin23, Doylespace, BenzolBot, Citation bot 1, DrilBot, Drjokerman, Pinethicket, I dream of horses, Jonesey95, Gabriel- Hug, JamesOKAY, Confuxous, Yunshui, Vr3690, DoinkerBoink, Torjusf, RjwilmsiBot, Agent Smith (The Matrix), Enauspeaker, John of Reading, Carsten.erickson, GoingBatty, Wikipelli, Hhhippo, Fæ, N90p, Bushben, Greatpouya2, Kunihura, JosJuice, EricWesBrown, Ran- goon11, VictorianMutant, Teapeat, Rememberway, ClueBot NG, Incompetence, A520, Ronhui, Rknelissen, Widr, MerlIwBot, Helpful Pixie Bot, Bibcode Bot, Earleon, BG19bot, Artticlesnet1, Rijinatwiki, Neøn, Great Tasting Snacks, Xlicolts613, SAuhsoj, Klilidiplomus, BattyBot, Obarac, Khazar2, AutomaticStrikeout, Akram84, Nikolas369, Mogism, DJBitterbarn, Andyhowlett, Beastly789, Malfactor22, Veredai, Camyoung54, Gunasekarkvct, Jeffrey Bosboom, MMcGehee, Stamptrader, JaconaFrere, Wyn.junior, Hulk1992amr, Monkbot, Filedelinkerbot, Tigercompanion25, BethNaught, Antenna Designer, Tanuj22, A-dizz-a, Aluchsinger, Cartheur, Bullsmaniac and Anony- mous: 414

11.2 Images

• File:Crystal_energy.svg Source: http://upload.wikimedia.org/wikipedia/commons/1/14/Crystal_energy.svg License: LGPL Contributors: Own work conversion of Image:Crystal_128_energy.png Original artist: Dhatfield • File:ED03-0249-18.jpg Source: http://upload.wikimedia.org/wikipedia/commons/9/9a/ED03-0249-18.jpg License: Public domain Con- tributors: http://www.dfrc.nasa.gov/Gallery/Photo/Power-Beaming/HTML/ED03-0249-18.html Original artist: Tom Tschida • File:Inductive_charging_of_LG_smartphone_(2).jpg Source: http://upload.wikimedia.org/wikipedia/commons/5/5c/Inductive_ charging_of_LG_smartphone_%282%29.jpg License: CC BY 2.0 Contributors: LG, Original artist: LG • File:Suntower.jpg Source: http://upload.wikimedia.org/wikipedia/commons/8/84/Suntower.jpg License: Public domain Contributors: ? Original artist: ? • File:TeslaWirelessPower1891.png Source: http://upload.wikimedia.org/wikipedia/commons/3/39/TeslaWirelessPower1891.png Li- cense: Public domain Contributors: Electrical World, May 20, 1891. Transferred from en.wikipedia Original artist: Original uploader was Reddi at en.wikipedia • File:Wireless_power_system.svg Source: http://upload.wikimedia.org/wikipedia/commons/8/8f/Wireless_power_system.svg License: CC0 Contributors: Own work Original artist: Chetvorno • File:Wireless_power_system_-_inductive_coupling.svg Source: http://upload.wikimedia.org/wikipedia/commons/b/b5/Wireless_ power_system_-_inductive_coupling.svg License: CC0 Contributors: Own work Original artist: Chetvorno

11.3 Content license

• Creative Commons Attribution-Share Alike 3.0