Ministry of Education and Youth of Republic of MoldovaTechnical University of MoldovaThe specialty of Engineering and Management Quality

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Theme: Wireless power

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Checked up by Sup.Lector M.AbabiiChisinau 2015Wireless power transfer(WPT)orwireless energy transmissionis the transmission ofelectrical powerfrom apower sourceto a consuming device without using solid wires orconductors.[2][3][4][5]It is a generic term that refers to a number of different power transmission technologies that use time-varyingelectromagnetic fields.[1][5][6][7]Wireless transmission is useful to power electrical devices in cases where interconnectingwiresare inconvenient, hazardous, or are not possible. In wireless power transfer, a transmitter device connected to a power source, such as themains powerline, transmits power byelectromagnetic fieldsacross an intervening space to one or more receiver devices, where it is converted back to electric power and utilized.Wireless power techniques fall into two categories, non-radiative and radiative.Innear-fieldornon-radiativetechniques, power is transferred over short distances bymagnetic fieldsusinginductive couplingbetweencoils of wireor in a few devices byelectric fieldsusingcapacitive couplingbetweenelectrodes.Applications of this type areelectric toothbrushchargers,RFIDtags,smartcards, and chargers for implantable medical devices likeartificial cardiac pacemakers, and inductive powering or charging ofelectric vehicleslike trains or buses.[9][11]A current focus is to develop wireless systems to charge mobile and handheld computing devices such ascellphones,digital music playersand portable computers without being tethered to a wall plug.Inradiativeorfar-fieldtechniques, also calledpower beaming, power is transmitted by beams ofelectromagnetic radiation, likemicrowavesorlaserbeams. These techniques can transport energy longer distances but must be aimed at the receiver. Proposed applications for this type aresolar power satellites, and wireless powereddrone aircraft.[9]An important issue associated with all wireless power systems is limiting the exposure of people and other living things to potentially injuriouselectromagnetic fields(seeElectromagnetic radiation and health).[9]

Contents 1Overview 2Field regions 3Near-field or non-radiative techniques 3.1Inductive coupling 3.1.1Resonant inductive coupling 3.2Capacitive coupling 3.3Magnetodynamic coupling 4Far-field or radiative techniques 4.1Microwaves 4.2Lasers 5Energy harvesting 6History 6.1Tesla's experiments 6.2Microwaves 6.3Near-field technologies

Overview[edit]

Generic block diagram of a wireless power system"Wireless power transmission" is a collective term that refers to a number of different technologies for transmitting power by means of time-varyingelectromagnetic fields.[1][5][8]The technologies, listed in the table below, differ in the distance over which they can transmit power efficiently, whether the transmitter must be aimed (directed) at the receiver, and in the type of electromagnetic energy they use: time varyingelectric fields,magnetic fields,radio waves,microwaves, orinfraredor visiblelight waves.[8]In general a wireless power system consists of a "transmitter" device connected to a source of power such asmains powerlines, which converts the power to a time-varying electromagnetic field, and one or more "receiver" devices which receive the power and convert it back to DC or AC electric power which is consumed by anelectrical load.[1][8]In the transmitter the input power is converted to an oscillatingelectromagnetic fieldby some type of "antenna" device. The word "antenna" is used loosely here; it may be a coil of wire which generates amagnetic field, a metal plate which generates anelectric field, anantennawhich radiates radio waves, or alaserwhich generates light. A similar antenna or coupling device in the receiver converts the oscillating fields to an electric current. An important parameter which determines the type of waves is thefrequencyfinhertzof the oscillations. The frequency determines thewavelength=c/fof the waves which carry the energy across the gap, wherecis thevelocity of light.Wireless power uses the same fields and waves aswireless communicationdevices likeradio,[6][12]another familiar technology which involves power transmitted without wires by electromagnetic fields, used incellphones,radioandtelevision broadcasting, andWiFi. Inradio communicationthe goal is the transmission of information, so the amount of power reaching the receiver is unimportant as long as it is enough that thesignal to noise ratiois high enough that the information can be received intelligibly.[5][6][12]In wireless communication technologies generally only tiny amounts of power reach the receiver. By contrast, in wireless power, the amount of power received is the important thing, so theefficiency(fraction of transmitted power that is received) is the more significant parameter.[5]For this reason wireless power technologies are more limited by distance than wireless communication technologies.These are the different wireless power technologies:[1][8][9][13][14]TechnologyRange[15]Directivity[8]FrequencyAntenna devicesCurrent and or possible future applications

Inductive couplingShortLowHz - MHzWire coilsElectric tooth brush and razor battery charging, induction stovetops and industrial heaters.

Resonant inductive couplingMid-LowMHz - GHzTuned wire coils, lumped element resonatorsCharging portable devices (Qi,WiTricity), biomedical implants, electric vehicles, powering busses, trains, MAGLEV,RFID,smartcards.

Capacitive couplingShortLowkHz - MHzElectrodesCharging portable devices, power routing in large scale integrated circuits, Smartcards.

Magnetodynamic[13]ShortN.A.HzRotating magnetsCharging electric vehicles.

MicrowavesLongHighGHzParabolic dishes,phased arrays,rectennasSolar power satellite, powering drone aircraft.

Light wavesLongHighTHzLasers, photocells, lensesPowering drone aircraft, powering space elevator climbers.

Field regions[edit]Electricandmagnetic fieldsare created bycharged particlesin matter such aselectrons. A stationary charge creates anelectrostatic fieldin the space around it. A steadycurrentof charges (direct current, DC) creates a staticmagnetic fieldaround it. The above fields containenergy, but cannot carrypowerbecause they are static. However time-varying fields can carry power.[16]Accelerating electric charges, such as are found in analternating current(AC) of electrons in a wire, create time-varying electric and magnetic fields in the space around them. These fields can exert oscillating forces on the electrons in a receiving "antenna", causing them to move back and forth. These represent alternating current which can be used to power a load.The oscillating electric and magnetic fields surrounding moving electric charges in an antenna device can be divided into two regions, depending on distanceDrangefrom the antenna.[1][4][6][8][9][10][17]The boundary between the regions is somewhat vaguely defined.[8]The fields have different characteristics in these regions, and different technologies are used for transmitting power: Near-fieldornonradiativeregion- This means the area within about 1wavelength() of the antenna.[1][4][10]In this region the oscillatingelectricandmagnetic fieldsare separate[6]and power can be transferred via electric fields bycapacitive coupling(electrostatic induction) between metal electrodes, or via magnetic fields byinductive coupling(electromagnetic induction) between coils of wire.[5][6][8][9]These fields are notradiative,[10]meaning the energy stays within a short distance of the transmitter.[18]If there is no receiving device or absorbing material within their limited range to "couple" to, no power leaves the transmitter.[18]The range of these fields is short, and depends on the size and shape of the "antenna" devices, which are usually coils of wire. The fields, and thus the power transmitted, decreaseexponentiallywith distance,[4][17][19]so if the distance between the two "antennas"Drangeis much larger than the diameter of the "antennas"Dantvery little power will be received. Therefore these techniques cannot be used for long distance power transmission.Resonance, such asresonant inductive coupling, can increase the coupling between the antennas greatly, allowing efficient transmission at somewhat greater distances,[1][4][6][9][20][21]although the fields still decrease exponentially. Therefore the range of near-field devices is conventionally devided into two categories: Short range- up to about one antenna diameter:DrangeDant.[18][20][22]This is the range over which ordinary nonresonant capacitive or inductive coupling can transfer practical amounts of power. Mid-range- up to 10 times the antenna diameter:Drange10Dant.[20][21][22][23]This is the range over which resonant capacitive or inductive coupling can transfer practical amounts of power. Far-fieldorradiativeregion- Beyond about 1 wavelength () of the antenna, the electric and magnetic fields are perpendicular to each other and propagate as anelectromagnetic wave; examples areradio waves,microwaves, orlight waves.[1][4][9]This part of the energy isradiative,[10]meaning it leaves the antenna whether or not there is a receiver to absorb it. The portion of energy which does not strike the receiving antenna is dissipated and lost to the system. The amount of power emitted as electromagnetic waves by an antenna depends on the ratio of the antenna's sizeDantto the wavelength of the waves,[24]which is determined by the frequency:=c/f. At low frequenciesfwhere the antenna is much smaller than the size of the waves,Dant=c/f.[26][27]Practicalbeam powerdevices require wavelengths in the centimeter region or below, corresponding to frequencies above 1 GHz, in themicrowaverange or above.[1]Near-field or non-radiative techniques[edit]Main article:Coupling (electronics)The near-field components of electric and magnetic fields die out quickly beyond a distance of about one diameter of the antenna (Dant). Outside very close ranges the field strength and coupling is roughly proportional to (Drange/Dant)3[17][28]Since power is proportional to the square of the field strength, the power transferred decreases with the sixth power of the distance (Drange/Dant)6.[6][19][29][30]or 60dB per decade. In other words, doubling the distance between transmitter and receiver causes the power received to decrease by a factor of 26= 64.Inductive coupling[edit]

Generic block diagram of an inductive wireless power system.

(right)A light bulb powered wirelessly by induction, in 1910.(left)Modern inductive power transfer, an electric toothbrush charger. A coil in the stand produces a magnetic field, inducing an AC current in a coil in the toothbrush, which is rectified to charge the batteries.Ininductive coupling(electromagnetic induction[9][31]orinductive power transfer, IPT), power is transferred betweencoils of wireby amagnetic field.[6]The transmitter and receiver coils together form atransformer[6][9](see diagram). Analternating current(AC) through the transmitter coil(L1)creates an oscillatingmagnetic field(B)byAmpere's law. The magnetic field passes through the receiving coil(L2), where it induces an alternatingEMF(voltage) byFaraday's law of induction, which creates an AC current in the receiver.[5][31]The induced alternating current may either drive the load directly, or berectifiedtodirect current(DC) by arectifierin the receiver, which drives the load. A few systems, such as electric toothbrush charging stands, work at 50/60Hz so ACmains currentis applied directly to the transmitter coil, but in most systems anelectronic oscillatorgenerates a higher frequency AC current which drives the coil, because transmission efficiency improves withfrequency.[31]Inductive coupling is the oldest and most widely used wireless power technology, and virtually the only one so far which is used in commercial products. It is used ininductive chargingstands forcordlessappliances used in wet environments such aselectric toothbrushes[9]and shavers, to reduce the risk of electric shock.[7]Another application area is "transcutaneous" recharging of biomedicalprosthetic devicesimplantedin the human body, such ascardiac pacemakersandinsulin pumps, to avoid having wires passing through the skin.[32][33]It is also used to chargeelectric vehiclessuch as cars and to either charge or power transit vehicles like buses and trains.[9][14]However the fastest growing use is wireless charging pads to recharge mobile and handheld wireless devices such aslaptopandtablet computers,cellphones,digital media players, andvideo game controllers.[14]The power transferred increases with frequency[31]and themutual inductanceMbetween the coils,[5]which depends on their geometry and the distanceDrangebetween them. A widely-used figure of merit is thecoupling coefficient.[31][34]This dimensionless parameter is equal to the fraction ofmagnetic fluxthroughL1that passes throughL2. If the two coils are on the same axis and close together so all the magnetic flux fromL1passes throughL2,k=1 and the link efficiency approaches 100%. The greater the separation between the coils, the more of the magnetic field from the first coil misses the second, and the lowerkand the link efficiency are, approaching zero at large separations.[31]The link efficiency and power transferred is roughly proportional tok2.[31]In order to achieve high efficiency, the coils must be very close together, a fraction of the coil diameterDant,[31]usually within centimeters,[25]with the coils' axes aligned. Wide, flat coil shapes are usually used, to increase coupling.[31]Ferrite"flux confinement" cores can confine the magnetic fields, improving coupling and reducinginterferenceto nearby electronics,[31][32]but they are heavy and bulky so small wireless devices often use air-core coils.Ordinary inductive coupling can only achieve high efficiency when the coils are very close together, usually adjacent. In most modern inductive systemsresonant inductive coupling(described below)is used, in which the efficiency is increased by usingresonant circuits.[10][21][31][35]This can achieve high efficiencies at greater distances than nonresonant inductive coupling.

Prototype inductive electric car charging system at 2011 Tokyo Auto Show

Powermatinductive charging spots in a coffee shop. Customers can set their phones and computers on them to recharge.Wireless powered access card.Resonant inductive coupling[edit]Main article:Resonant inductive coupling

Diagram of the resonant inductive wireless power system demonstrated byMarin Soljai's MIT team in 2007. Theresonant circuitswere coils of copper wire which resonated with their internal capacitance (dotted capacitors) at 10 MHz. Power was coupled into the transmitter resonator, and out of the receiver resonator into the rectifier, by small coils which also served forimpedance matching.Resonant inductive coupling(electrodynamic coupling,[9]evanescent wave couplingorstrongly coupled magnetic resonance[20]) is a form of inductive coupling in which power is transferred by magnetic fields(B, green)between tworesonant circuits(tuned circuits), one in the transmitter and one in the receiver(see diagram, right).[6][7][9][10][35]Each resonant circuit consists of a coil of wire connected to acapacitor, or aself-resonantcoil or otherresonatorwith internal capacitance. The two are tuned to resonate at the sameresonant frequency. The resonance between the coils can greatly increase coupling and power transfer, analogously to the way a vibratingtuning forkcan inducesympathetic vibrationin a distant fork tuned to the same pitch.Nikola Teslafirst discovered resonant coupling during his pioneering experiments in wireless power transfer around the turn of the 20th century,[36][37][38]but the possibilities of using resonant coupling to increase transmission range has only recently been explored.[39]In 2007 a team led byMarin Soljaiat MIT used two coupled tuned circuits each made of a 25cm self-resonant coil of wire at 10MHz to achieve the transmission of 60 W of power over a distance of 2 meters (6.6ft) (8 times the coil diameter) at around 40% efficiency.[7][9][20][37][40]The concept behind resonant inductive coupling is that highQ factorresonatorsexchange energy at a much higher rate than they lose energy due to internaldamping.[20]Therefore by using resonance, the same amount of power can be transferred at greater distances, using the much weaker magnetic fields out in the peripheral regions ("tails") of the near fields (these are sometimes calledevanescentfields[20]). Resonant inductive coupling can achieve high efficiency at ranges of 4 to 10 times the coil diameter (Dant).[21][22][23]This is called "mid-range" transfer,[22]in contrast to the "short range" of nonresonant inductive transfer, which can achieve similar efficiencies only when the coils are adjacent. Another advantage is that resonant circuits interact with each other so much more strongly than they do with nonresonant objects that power losses due to absorption in stray nearby objects are negligible.[10][20]A drawback of resonant coupling is that at close ranges when the two resonant circuits are tightly coupled, the resonant frequency of the system is no longer constant but "splits" into two resonant peaks, so the maximum power transfer no longer occurs at the original resonant frequency and the oscillator frequency must be tuned to the new resonance peak.[21]Resonant technology is currently being widely incorporated in modern inductive wireless power systems.[31]The MIT team is commercializing their version asWiTricity. One of the possibilities envisioned for this technology is area wireless power coverage. A coil in the wall or ceiling of a room might be able to wirelessly power lights and mobile devices anywhere in the room, with reasonable efficiency.[7]An environmental and economic benefit of wirelessly powering small devices such as clocks, radios, music players andremote controlsis that it could drastically reduce the 6 billionbatteriesdisposed of each year, a large source oftoxic wasteand groundwater contamination.[25]Capacitive coupling[edit]Main article:Capacitive couplingIncapacitive coupling(electrostatic induction), the dual of inductive coupling, power is transmitted by electric fields[5]betweenelectrodessuch as metal plates. The transmitter and receiver electrodes form acapacitor, with the intervening space as thedielectric.[5][6][9][32][41]An alternating voltage generated by the transmitter is applied to the transmitting plate, and the oscillatingelectric fieldinduces an alternatingpotentialon the receiver plate byelectrostatic induction,[5][41]which causes an alternating current to flow in the load circuit. The amount of power transferred increases with thefrequency[41]and thecapacitancebetween the plates, which is proportional to the area of the smaller plate and (for short distances) inversely proportional to the separation.[5]Capacitive coupling has only been used practically in a few low power applications, because the very high voltages on the electrodes required to transmit significant power can be hazardous,[6][9]and can cause unpleasant side effects such as noxiousozoneproduction. In addition, in contrast to magnetic fields,[20]electric fields interact strongly with most materials, including the human body, due todielectric polarization.[32]Intervening materials between or near the electrodes can absorb the energy, in the case of humans possibly causing excessive electromagnetic field exposure.[6]However capacitive coupling has a few advantages over inductive. The field is largely confined between the capacitor plates, reducing interference, which in inductive coupling requires heavy ferrite "flux confinement" cores.[5][32]Also, alignment requirements between the transmitter and receiver are less critical.[5][6][41]Capacitive coupling has recently been applied to charging battery powered portable devices[42]and is being considered as a means of transferring power between substrate layers in integrated circuits.[43]Capacitive wireless power systems

Bipolar

UnipolarTwo types of circuit have been used: Bipolar design:[44]In this type of circuit, there are two transmitter plates and two receiver plates. Each transmitter plate is coupled to a receiver plate. The transmitteroscillatordrives the transmitter plates in opposite phase (180 phase difference) by a high alternating voltage, and the load is connected between the two receiver plates. The alternating electric fields induce opposite phase alternating potentials in the receiver plates, and this "push-pull" action causes current to flow back and forth between the plates through the load. A disadvantage of this configuration for wireless charging is that the two plates in the receiving device must be aligned face to face with the charger plates for the device to work. Unipolar design:[5][41]In this type of circuit, the transmitter and receiver have only one active electrode, and either thegroundor a large inactive capacitive electrode serves as the return path for the current. The transmitter oscillator and the load is connected between the electrodes and agroundconnection. inducing an alternating potential on the nearby receiving electrode with respect to ground, causing alternating current to flow through the load connected between it and ground.Resonance can also be used with capacitive coupling to extend the range. At the turn of the century,Nikola Tesladid the first experiments with both resonant electrostatic and magnetic coupling.Magnetodynamic coupling[edit]In this method, power is transmitted between two rotatingarmatures, one in the transmitter and one in the receiver, which rotate synchronously, coupled together by amagnetic fieldgenerated bypermanent magnetson the armatures.[13]The transmitter armature is turned either by or as the rotor of anelectric motor, and its magnetic field exertstorqueon the receiver armature, turning it. The magnetic field acts like a mechanical coupling between the armatures.[13]The receiver armature produces power to drive the load, either by turning a separateelectric generatoror by using the receiver armature itself as the rotor in a generator.This device has been proposed as an alternative to inductive power transfer for noncontact charging ofelectric vehicles.[13]A rotating armature embedded in a garage floor or curb would turn a receiver armature in the underside of the vehicle to charge its batteries.[13]It is claimed that this technique can transfer power over distances of 10 to 15cm (4 to 6 inches) with high efficiency, over 90%.[13]Also, the low frequency stray magnetic fields produced by the rotating magnets produce lesselectromagnetic interferenceto nearby electronic devices than the high frequency magnetic fields produced by inductive coupling systems. A prototype system charging electric vehicles has been in operation atUniversity of British Columbiasince 2012. Other researchers, however, claim that the two energy conversions (electrical to mechanical to electrical again) make the system less efficient than electrical systems like inductive coupling.[13]Far-field or radiative techniques[edit]Far fieldmethods achieve longer ranges, often multiple kilometer ranges, where the distance is much greater than the diameter of the device(s). The main reason for longer ranges with radio wave and optical devices is the fact that electromagnetic radiation in thefar-fieldcan be made to match the shape of the receiving area (using highdirectivityantennas or well-collimated laserbeams). The maximum directivity for antennas is physically limited bydiffraction.In general,visible light(from lasers) andmicrowaves(from purpose-designed antennas) are the forms of electromagnetic radiation best suited to energy transfer.The dimensions of the components may be dictated by the distance fromtransmittertoreceiver, thewavelengthand theRayleigh criterionordiffractionlimit, used in standardradio frequencyantennadesign, which also applies to lasers.Airy's diffraction limitis also frequently used to determine an approximate spot size at an arbitrary distance from theaperture. Electromagnetic radiation experiences less diffraction at shorter wavelengths (higher frequencies); so, for example, a blue laser is diffracted less than a red one.TheRayleigh criteriondictates that any radio wave, microwave or laser beam will spread and become weaker anddiffuseover distance; the larger the transmitter antenna or laser aperture compared to thewavelengthof radiation, the tighter the beam and the less it will spread as a function of distance (and vice versa). Smaller antennae also suffer from excessive losses due toside lobes. However, the concept oflaser apertureconsiderably differs from an antenna. Typically, a laser aperture much larger than the wavelength inducesmulti-modedradiation and mostlycollimatorsare used before emitted radiation couples into a fiber or into space.Ultimately,beamwidthis physically determined by diffraction due to the dish size in relation to the wavelength of the electromagnetic radiation used to make the beam.Microwave power beaming can be more efficient than lasers, and is less prone to atmosphericattenuationcaused by dust orwater vapor.Then the power levels are calculated by combining the above parameters together, and adding in thegainsandlossesdue to the antenna characteristics and thetransparencyanddispersionof the medium through which the radiation passes. That process is known as calculating alink budget.Microwaves[edit]Main article:Microwave power transmission

An artist's depiction of asolar satellitethat could send electric energy by microwaves to a space vessel or planetary surface.Power transmission via radio waves can be made more directional, allowing longer distance power beaming, with shorter wavelengths of electromagnetic radiation, typically in themicrowaverange.[45]Arectennamay be used to convert the microwave energy back into electricity. Rectenna conversion efficiencies exceeding 95% have been realized. Power beaming using microwaves has been proposed for the transmission of energy from orbitingsolar power satellitesto Earth and thebeaming of power to spacecraftleaving orbit has been considered.[46][47]Power beaming by microwaves has the difficulty that, for most space applications, the required aperture sizes are very large due todiffractionlimiting antenna directionality. For example, the 1978NASAStudy of solar power satellites required a 1-km diameter transmitting antenna and a 10km diameter receiving rectenna for a microwave beam at2.45GHz.[48]These sizes can be somewhat decreased by using shorter wavelengths, although short wavelengths may have difficulties with atmospheric absorption and beam blockage by rain or water 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 10km diameter receiving array allows large total power levels to be used while operating at the low power density suggested for human electromagnetic exposure safety. A human safe power density of 1mW/cm2distributed across a 10km diameter area corresponds to 750 megawatts total power level. This is the power level found in many modern electric power plants.Following World War II, which saw the development of high-power microwave emitters known ascavity magnetrons, the idea of using microwaves to transmit power was researched. By 1964, a miniature helicopter propelled by microwave power had been demonstrated.[49]Japanese researcherHidetsugu Yagialso investigated wireless energy transmission using a directional array antenna that he designed. In February 1926, Yagi and his colleagueShintaro Udapublished their first paper on the tuned high-gain directional array now known as theYagi antenna. While it did not prove to be particularly useful for power transmission, this beam antenna has been widely adopted throughout the broadcasting and wireless telecommunications industries due to its excellent performance characteristics.[50]Wireless high power transmission using microwaves is well proven. Experiments in the tens of kilowatts have been performed atGoldstonein California in 1975[51][52][53]and more recently (1997) at Grand Bassin onReunion Island.[54]These methods achieve distances on the order of a kilometer.Under experimental conditions, microwave conversion efficiency was measured to be around 54%.[55]More recently, a change to 24GHz has been suggested as microwave emitters similar to LEDs have been made with very high quantum efficiencies using negative resistance, i.e. Gunn or IMPATT diodes, and this would be viable for short range links.Lasers[edit]

With a laser beam centered on its panel of photovoltaic cells, a lightweight model plane makes the first flight of an aircraft powered by a laser beam inside a building at NASA Marshall Space Flight Center.In the case of electromagnetic radiation closer to the visible region of the spectrum (tens ofmicrometersto tens ofnanometres), power can be transmitted by converting electricity into alaserbeam that is then pointed at a photovoltaic cell.[56]This mechanism is generally known as "power beaming" because the power is beamed at a receiver that can convert it to electrical energy.Compared to other wireless methods:[57] Collimatedmonochromaticwavefrontpropagation allows narrow beam cross-section area for transmission over large distances. Compact size:solid state lasersfit into small products. Noradio-frequencyinterference to existing radio communication such asWi-Fiand cell phones. Access control: only receivers hit by the laser receive power.Drawbacks include: Laser radiation is hazardous. Low power levels can blind humans and other animals. High power levels can kill through localized spot heating. Conversion between electricity and light is inefficient. Photovoltaic cells achieve only 40%50% efficiency.[58](Efficiency is higher with monochromatic light than with solar panels). Atmospheric absorption, and absorption and scattering by clouds, fog, rain, etc., causes up to 100% losses. Requires a direct line of sight with the target.Laser "powerbeaming" technology has been mostly explored inmilitary weapons[59][60][61]andaerospace[62][63]applications and is now being developed for commercial andconsumer electronics. Wireless energy transfer systems using lasers for consumer space have to satisfylaser safetyrequirements standardized under IEC 60825.[citation needed]Other details includepropagation,[64]and thecoherence and the range limitation problem.[65]Geoffrey Landis[66][67][68]is one of the pioneers ofsolar power satellites[69]and laser-based transfer of energy especially for space and lunar missions. The demand for safe and frequent space missions has resulted in proposals for a laser-poweredspace elevator.[70][71]NASA'sDryden Flight Research Centerdemonstrated a lightweight unmanned model plane powered by a laser beam.[72]This proof-of-concept demonstrates the feasibility of periodic recharging using the laser beam system.Energy harvesting[edit]Main article:Energy harvestingIn the context of wireless power,energy harvesting, also calledpower harvestingorenergy scavenging, is the conversion of ambient energy from the environment to electric power, mainly to power small autonomous wireless electronic devices.[73]The ambient energy may come from stray electric or magnetic fields or radio waves from nearby electrical equipment, light,thermal energy(heat), orkinetic energysuch as vibration or motion of the device.[73]Although the efficiency of conversion is usually low and the power gathered often minuscule (milliwatts or microwatts),[73]it can be adequate to run or recharge small micropower wireless devices such asremote sensors, which are proliferating in many fields.[73]This new technology is being developed to eliminate the need for battery replacement or charging of such wireless devices, allowing them to operate completely autonomously.History[edit]In 1826Andr-Marie AmpredevelopedAmpre's circuital lawshowing that electric current produces a magnetic field.[74]Michael FaradaydevelopedFaraday's law of inductionin 1831, describing the electromagnetic force induced in a conductor by a time-varying magnetic flux. In 1862James Clerk Maxwellsynthesized these and other observations, experiments and equations of electricity, magnetism and optics into a consistent theory, derivingMaxwell's equations. This set ofpartial differential equationsforms the basis for modern electromagnetics, including the wireless transmission of electrical energy.[14][35]Maxwell predicted the existence of electromagnetic waves in his 1873A Treatise on Electricity and Magnetism.[75]In 1884John Henry Poyntingdeveloped equations for the flow of power in an electromagnetic field,Poynting's theoremand thePoynting vector, which are used in the analysis of wireless energy transfer systems.[14][35]In 1888Heinrich Rudolf Hertzdiscoveredradio waves, confirming the prediction of electromagnetic waves by Maxwell.[75]Tesla's experiments[edit]

Tesla demonstrating wireless power transmission in a lecture atColumbia College, New York, in 1891. The two metal sheets are connected to hisTesla coiloscillator, which applies a highradio frequencyoscillating voltage. The oscillating electric field between the sheetsionizesthe low pressure gas in the two longGeissler tubeshe is holding, causing them to glow byfluorescence, similar toneon lights.

(left)Experiment in resonant inductive transfer by Tesla at Colorado Springs 1899. The coil is in resonance with Tesla's magnifying transmitter nearby, powering the light bulb at bottom.(right)Tesla's unsuccessful Wardenclyffe power station.InventorNikola Teslaperformed the first experiments in wireless power transmission at the turn of the 20th century,[35][37]and may have done more to popularize the idea than any other individual. In the period 1891 to 1904 he experimented with transmitting power by inductive and capacitive coupling using spark-excitedradio frequencyresonant transformers, now calledTesla coils, which generated high AC voltages.[35][37][76]With these he was able to transmit power for short distances without wires. In demonstrations before the American Institute of Electrical Engineers[76]and at the 1893 Columbian Exposition in Chicago he lit light bulbs from across a stage.[37]He found he could increase the distance by using a receivingLC circuittuned toresonancewith the transmitter'sLC circuit.[36]usingresonant inductive coupling.[37][38]At his Colorado Springs laboratory during 1899-1900, by using voltages of the order of 20 megavolts generated by an enormous coil, he was able to light three incandescent lamps at a distance of about one hundred feet.[77][78]Theresonant inductive couplingwhich Tesla pioneered is now a familiar technology used throughout electronics; its use in wireless power has been recently rediscovered and it is currently being widely applied to short-range wireless power systems.[37][79]The inductive and capacitive coupling used in Tesla's experiments is a "near-field" effect,[37]so it is not able to transmit power long distances. However, Tesla was obsessed with developing a wireless power distribution system that could transmit power directly into homes and factories, as proposed in a visionary 1900 article inCenturymagazine.[80][81][82][83]and believed that resonance was the key. He claimed to be able to transmit power on aworldwidescale, using a method that involved conduction through the Earth and atmosphere.[81][82][83][84]Tesla was vague about his methods. One of his ideas was to use balloons to suspend transmitting and receiving terminals in the air above 30,000 feet (9,100m) in altitude, where the pressure is lower.[84]At this altitude, Tesla claimed, an ionized layer would allow electricity to be sent at high voltages (millions of volts) over long distances.

Resonant wireless power demonstration at theFranklin Institute, Philadelphia, 1937. Visitors could adjust the receiver's tuned circuit(right)with the two knobs. When theresonant frequencyof the receiver was out of tune with the transmitter, the light would go out.In 1901, Tesla began construction of a large high-voltage coil facility, theWardenclyffe Towerat Shoreham, New York, intended as a prototype transmitter for a "World Wireless System" that was to transmit power worldwide, but by 1904 his investors had pulled out, and the facility was never completed.[82][85]Although Tesla claimed his ideas were proven, he had a history of failing to confirm his ideas by experiment,[86][87]and there seems to be no evidence that he ever transmitted significant power beyond the short-range demonstrations above.[14][35][36][77][87][88][89][90][91]The only report of long-distance transmission by Tesla is a claim, not found in reliable sources, that in 1899 he wirelessly lit 200 light bulbs at a distance of 26 miles (42km).[77][88]There is no independent confirmation of this putative demonstration;[77][88][92]Tesla did not mention it,[88]and it does not appear in his meticulous laboratory notes.[92][93]It originated in 1944 from Tesla's first biographer, John J. O'Neill,[77]who said he pieced it together from "fragmentary material... in a number of publications".[94]In the 110 years since Tesla's experiments, efforts using similar equipment have failed to achieve long distance power transmission,[37][77][88][90]and the scientific consensus is his World Wireless system would not have worked.[14][35][36][82][88][95][96][97][98]Tesla's world power transmission scheme remains today what it was in Tesla's time, a fascinating dream.[14][82]Microwaves[edit]Before World War 2, little progress was made in wireless power transmission.[89]Radiowas developed for communication uses, but couldn't be used for power transmission due to the fact that the relatively low-frequencyradio wavesspread out in all directions and little energy reached the receiver.[14][35][89]In radio communication, at the receiver, anamplifierintensifies a weak signal using energy from another source. For power transmission, efficient transmission requiredtransmittersthat could generate higher-frequencymicrowaves, which can be focused in narrow beams towards a receiver.[14][35][89][96]The development ofmicrowavetechnology during World War 2, such as theklystronandmagnetrontubes andparabolic antennas[89]made radiative (far-field) methods practical for the first time, and the first long-distance wireless power transmission was achieved in the 1960s byWilliam C. Brown.[14][35]In 1964 Brown invented therectennawhich could efficiently convert microwaves to DC power, and in 1964 demonstrated it with the first wireless-powered aircraft, a model helicopter powered by microwaves beamed from the ground.[14][89]A major motivation for microwave research in the 1970s and 80s was to develop asolar power satellite.[35][89]Conceived in 1968 byPeter Glaser, this would harvest energy from sunlight usingsolar cellsand beam it down to Earth asmicrowavesto huge rectennas, which would convert it to electrical energy on theelectric power grid.[14][99]In landmark 1975 high power experiments, Brown demonstrated short range transmission of 475 W of microwaves at 54% DC to DC efficiency, and he and Robert Dickinson at NASA's Jet Propulsion Laboratory transmitted 30kW DC output power across 1.5km with 2.38GHz microwaves from a 26 m dish to a 7.3 x 3.5 m rectenna array.[14][100]The incident-RF to DC conversion efficiency of the rectenna was 80%.[14][100]In 1983 Japan launched MINIX (Microwave Ionosphere Nonlinear Interation Experiment), a rocket experiment to test transmission of high power microwaves through the ionosphere.[14]In recent years a focus of research has been the development of wireless-powered drone aircraft, which began in 1959 with the Dept. of Defense's RAMP (Raytheon Airborne Microwave Platform) project[89]which sponsored Brown's research. In 1987 Canada's Communications Research Center developed a small prototype airplane calledStationary High Altitude Relay Platform(SHARP) to relay telecommunication data between points on earth similar to acommunication satellite. Powered by a rectenna, it could fly at 13 miles (21km) altitude and stay aloft for months. In 1992 a team at Kyoto University built a more advanced craft called MILAX (MIcrowave Lifted Airplane eXperiment). In 2003 NASA flew the first laser powered aircraft. The small model plane's motor was powered by electricity generated byphotocellsfrom a beam of infrared light from a ground based laser, while a control system kept the laser pointed at the plane.Near-field technologies[edit]Inductive power transfer between nearby coils of wire is an old technology, existing since thetransformerwas developed in the 1800s.Induction heatinghas been used for 100 years. With the advent ofcordlessappliances, inductive charging stands were developed for appliances used in wet environments likeelectric toothbrushesandelectric razorsto reduce the hazard of electric shock.One field to which inductive transfer has been applied is to power electric vehicles. In 1892 Maurice Hutin and Maurice Leblanc patented a wireless method of powering railroad trains using resonant coils inductively coupled to a track wire at 3kHz.[101]The first passiveRFID(Radio Frequency Identification) technologies were invented by Mario Cardullo[102](1973) and Koelle et al.[103](1975) and by the 1990s were being used inproximity cardsand contactlesssmartcards.The proliferation of portable wireless communication devices such ascellphones,tablet, andlaptop computersin recent decades is currently driving the development of wireless powering and charging technology to eliminate the need for these devices to be tethered to wall plugs during charging.[104]TheWireless Power Consortiumwas established in 2008 to develop interoperable standards across manufacturers.[104]ItsQiinductive power standard published in August 2009 enables charging and powering of portable devices of up to 5 watts over distances of 4cm (1.6 inches).[105]The wireless device is placed on a flat charger plate (which could be embedded in table tops at cafes, for example) and power is transferred from a flat coil in the charger to a similar one in the device.In 2007, a team led by Marin Soljai at MIT used coupled tuned circuits made of a 25cm resonant coil at 10MHz to transfer 60 W of power over a distance of 2 meters (6.6ft) (8 times the coil diameter) at around 40% efficiency.[37][40]This technology is being commercialized asWiTricity.