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Wireless power transfer

December 18, 2023

Wireless power transfer (WPT), wireless power transmission, wireless energy transmission (WET), or electromagnetic power transfer is the transmission of electrical energy without wires as a physical link. In a wireless power transmission system, an electrically powered transmitter device generates a time-varying electromagnetic field that transmits power across space to a receiver device; the receiver device extracts power from the field and supplies it to an electrical load. The technology of wireless power transmission can eliminate the use of the wires and batteries, thereby increasing the mobility, convenience, and safety of an electronic device for all users.[2] Wireless power transfer is useful to power electrical devices where interconnecting wires are inconvenient, hazardous, or are not possible.

Inductive charging pad for a smartphone as an example of near-field wireless transfer. When the phone is set on the pad, a coil in the pad creates a magnetic field[1] which induces a current in another coil, in the phone, charging its battery.

Wireless power techniques mainly fall into two categories: near field and far-field.[3] In near field or non-radiative techniques, power is transferred over short distances by magnetic fields using inductive coupling between coils of wire, or by electric fields using capacitive coupling between metal electrodes.[4][5][6][7] Inductive coupling is the most widely used wireless technology; its applications include charging handheld devices like phones and electric toothbrushes, RFID tags, induction cooking, and wirelessly charging or continuous wireless power transfer in implantable medical devices like artificial cardiac pacemakers, or electric vehicles.

In far-field or radiative techniques, also called power beaming, power is transferred by beams of electromagnetic radiation, like microwaves[8] or laser beams. These techniques can transport energy longer distances but must be aimed at the receiver. Proposed applications for this type include solar power satellites and wireless powered drone aircraft.[9][10][11]

An important issue associated with all wireless power systems is limiting the exposure of people and other living beings to potentially injurious electromagnetic fields.[12][13]

Overview edit

Further information: Coupling (electronics)
 
Generic block diagram of a wireless power system

Wireless power transfer is a generic term for a number of different technologies for transmitting energy by means of electromagnetic fields.[14][15][16] The technologies, listed in the table below, differ in the distance over which they can transfer power efficiently, whether the transmitter must be aimed (directed) at the receiver, and in the type of electromagnetic energy they use: time varying electric fields, magnetic fields, radio waves, microwaves, infrared or visible light waves.[17]

In general a wireless power system consists of a "transmitter" device connected to a source of power such as a mains power line, 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 current which is used by an electrical load.[14][17] At the transmitter the input power is converted to an oscillating electromagnetic field by some type of "antenna" device. The word "antenna" is used loosely here; it may be a coil of wire which generates a magnetic field, a metal plate which generates an electric field, an antenna which radiates radio waves, or a laser which generates light. A similar antenna or coupling device at the receiver converts the oscillating fields to an electric current. An important parameter that determines the type of waves is the frequency, which determines the wavelength.

Wireless power uses the same fields and waves as wireless communication devices like radio,[18][19] another familiar technology that involves electrical energy transmitted without wires by electromagnetic fields, used in cellphones, radio and television broadcasting, and WiFi. In radio communication the goal is the transmission of information, so the amount of power reaching the receiver is not so important, as long as it is sufficient that the information can be received intelligibly.[15][18][19] In wireless communication technologies only tiny amounts of power reach the receiver. In contrast, with wireless power transfer the amount of energy received is the important thing, so the efficiency (fraction of transmitted energy that is received) is the more significant parameter.[15] For this reason, wireless power technologies are likely to be more limited by distance than wireless communication technologies.

Wireless power transfer may be used to power up wireless information transmitters or receivers. This type of communication is known as wireless powered communication (WPC). When the harvested power is used to supply the power of wireless information transmitters, the network is known as Simultaneous Wireless Information and Power Transfer (SWIPT);[20] whereas when it is used to supply the power of wireless information receivers, it is known as a Wireless Powered Communication Network (WPCN).[21][22][23]

These are the different wireless power technologies:[14][17][24][25]

Technology Range[26] Directivity[17] Frequency Antenna devices Current and/or possible future applications
Inductive coupling Short Low Hz – MHz Wire coils Electric tooth brush and razor battery charging, induction stovetops and industrial heaters.
Resonant inductive coupling Mid- Low kHz – GHz Tuned wire coils, lumped element resonators Charging portable devices (Qi), biomedical implants, electric vehicles, powering buses, trains, MAGLEV, RFID, smartcards.
Capacitive coupling Short Low kHz – MHz Metal plate electrodes Charging portable devices, power routing in large-scale integrated circuits, Smartcards, biomedical implants.[5][6][7]
Magnetodynamic coupling Short N.A. Hz Rotating magnets Charging electric vehicles,[25] biomedical implants.[27]
Microwaves Long High GHz Parabolic dishes, phased arrays, rectennas Solar power satellite, powering drone aircraft, charging wireless devices
Light waves Long High ≥THz Lasers, photocells, lenses Charging portable devices,[28] powering drone aircraft, powering space elevator climbers.

Field regions edit

Electric and magnetic fields are created by charged particles in matter such as electrons. A stationary charge creates an electrostatic field in the space around it. A steady current of charges (direct current, DC) creates a static magnetic field around it. The above fields contain energy, but cannot carry power because they are static. However time-varying fields can carry power.[29] Accelerating electric charges, such as are found in an alternating 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 distance Drange from the antenna.[14][17][18][24][30][31][32] The boundary between the regions is somewhat vaguely defined.[17] The fields have different characteristics in these regions, and different technologies are used for transferring power:

  • Near-field or nonradiative region – This means the area within about 1 wavelength (λ) of the antenna.[14][30][31] In this region the oscillating electric and magnetic fields are separate[18] and power can be transferred via electric fields by capacitive coupling (electrostatic induction) between metal electrodes,[4][5][6][7] or via magnetic fields by inductive coupling (electromagnetic induction) between coils of wire.[15][17][18][24] These fields are not radiative,[31] meaning the energy stays within a short distance of the transmitter.[33] If there is no receiving device or absorbing material within their limited range to "couple" to, no power leaves the transmitter.[33] 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, decrease exponentially with distance,[30][32][34] so if the distance between the two "antennas" Drange is much larger than the diameter of the "antennas" Dant very little power will be received. Therefore, these techniques cannot be used for long range power transmission.
Resonance, such as resonant inductive coupling, can increase the coupling between the antennas greatly, allowing efficient transmission at somewhat greater distances,[14][18][24][30][35][36] although the fields still decrease exponentially. Therefore the range of near-field devices is conventionally divided into two categories:
  • Short range – up to about one antenna diameter: Drange ≤ Dant.[33][35][37] 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: Drange ≤ 10 Dant.[35][36][37][38] This is the range over which resonant capacitive or inductive coupling can transfer practical amounts of power.
  • Far-field or radiative region – Beyond about 1 wavelength (λ) of the antenna, the electric and magnetic fields are perpendicular to each other and propagate as an electromagnetic wave; examples are radio waves, microwaves, or light waves.[14][24][30] This part of the energy is radiative,[31] 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 size Dant to the wavelength of the waves λ,[39] which is determined by the frequency: λ = c/f. At low frequencies f where the antenna is much smaller than the size of the waves, Dant << λ, very little power is radiated. Therefore the near-field devices above, which use lower frequencies, radiate almost none of their energy as electromagnetic radiation. Antennas about the same size as the wavelength Dant ≈ λ such as monopole or dipole antennas, radiate power efficiently, but the electromagnetic waves are radiated in all directions (omnidirectionally), so if the receiving antenna is far away, only a small amount of the radiation will hit it.[31][35] Therefore, these can be used for short range, inefficient power transmission but not for long range transmission.[40]
However, unlike fields, electromagnetic radiation can be focused by reflection or refraction into beams. By using a high-gain antenna or optical system which concentrates the radiation into a narrow beam aimed at the receiver, it can be used for long range power transmission.[35][40] From the Rayleigh criterion, to produce the narrow beams necessary to focus a significant amount of the energy on a distant receiver, an antenna must be much larger than the wavelength of the waves used: Dant >> λ = c/f.[41] Practical beam power devices require wavelengths in the centimeter region or below, corresponding to frequencies above 1 GHz, in the microwave range or above.[14]

Near-field (nonradiative) techniques edit

At large relative distance, the near-field components of electric and magnetic fields are approximately quasi-static oscillating dipole fields. These fields decrease with the cube of distance: (Drange/Dant)−3[32][42] Since power is proportional to the square of the field strength, the power transferred decreases as (Drange/Dant)−6.[18][34][43][44] or 60 dB per decade. In other words, if far apart, increasing the distance between the two antennas tenfold causes the power received to decrease by a factor of 106 = 1000000. As a result, inductive and capacitive coupling can only be used for short-range power transfer, within a few times the diameter of the antenna device Dant. Unlike in a radiative system where the maximum radiation occurs when the dipole antennas are oriented transverse to the direction of propagation, with dipole fields the maximum coupling occurs when the dipoles are oriented longitudinally.

Inductive coupling edit

Main article: Inductive charging
 
Generic block diagram of an inductive wireless power system
 
 
(left) Modern inductive power transfer, an electric toothbrush charger. A coil in the stand produces a magnetic field, inducing an alternating current in a coil in the toothbrush, which is rectified to charge the batteries.
(right) A light bulb powered wirelessly by induction, in 1910.

In inductive coupling (electromagnetic induction[24][45] or inductive power transfer, IPT), power is transferred between coils of wire by a magnetic field.[18] The transmitter and receiver coils together form a transformer[18][24] (see diagram). An alternating current (AC) through the transmitter coil (L1) creates an oscillating magnetic field (B) by Ampere's law. The magnetic field passes through the receiving coil (L2), where it induces an alternating EMF (voltage) by Faraday's law of induction, which creates an alternating current in the receiver.[15][45] The induced alternating current may either drive the load directly, or be rectified to direct current (DC) by a rectifier in the receiver, which drives the load. A few systems, such as electric toothbrush charging stands, work at 50/60 Hz so AC mains current is applied directly to the transmitter coil, but in most systems an electronic oscillator generates a higher frequency AC current which drives the coil, because transmission efficiency improves with frequency.[45]

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 in inductive charging stands for cordless appliances used in wet environments such as electric toothbrushes[24] and shavers, to reduce the risk of electric shock.[46] Another application area is "transcutaneous" recharging of biomedical prosthetic devices implanted in the human body, such as cardiac pacemakers and insulin pumps, to avoid having wires passing through the skin.[47][48] It is also used to charge electric vehicles such as cars and to either charge or power transit vehicles like buses and trains.[24]

However the fastest growing use is wireless charging pads to recharge mobile and handheld wireless devices such as laptop and tablet computers, computer mouse, cellphones, digital media players, and video game controllers.[citation needed] In the United States, the Federal Communications Commission (FCC) provided its first certification for a wireless transmission charging system in December 2017.[49]

The power transferred increases with frequency[45] and the mutual inductance   between the coils,[15] which depends on their geometry and the distance   between them. A widely used figure of merit is the coupling coefficient  .[45][50] This dimensionless parameter is equal to the fraction of magnetic flux through the transmitter coil   that passes through the receiver coil   when L2 is open circuited. If the two coils are on the same axis and close together so all the magnetic flux from   passes through  ,   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 lower   and the link efficiency are, approaching zero at large separations.[45] The link efficiency and power transferred is roughly proportional to  .[45] In order to achieve high efficiency, the coils must be very close together, a fraction of the coil diameter  ,[45] usually within centimeters,[40] with the coils' axes aligned. Wide, flat coil shapes are usually used, to increase coupling.[45] Ferrite "flux confinement" cores can confine the magnetic fields, improving coupling and reducing interference to nearby electronics,[45][47] 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 systems resonant inductive coupling (described below) is used, in which the efficiency is increased by using resonant circuits.[31][36][45][51] This can achieve high efficiencies at greater distances than nonresonant inductive coupling.

 
Prototype inductive electric car charging system at 2011 Tokyo Auto Show
 
Powermat inductive charging spots in a coffee shop. Customers can set their phones and computers on them to recharge.
 
Wireless powered access card.
 
GM EV1 and Toyota RAV4 EV inductively charging at a now-obsolete Magne Charge station

Resonant inductive coupling edit

Main article: Resonant inductive coupling
Further information: Tesla coil § Resonant transformer

Resonant inductive coupling (electrodynamic coupling,[24] strongly coupled magnetic resonance[35]) is a form of inductive coupling in which power is transferred by magnetic fields (B, green) between two resonant circuits (tuned circuits), one in the transmitter and one in the receiver (see diagram, right).[18][24][31][46][51] Each resonant circuit consists of a coil of wire connected to a capacitor, or a self-resonant coil or other resonator with internal capacitance. The two are tuned to resonate at the same resonant frequency. The resonance between the coils can greatly increase coupling and power transfer, analogously to the way a vibrating tuning fork can induce sympathetic vibration in a distant fork tuned to the same pitch.

Nikola Tesla first discovered resonant coupling during his pioneering experiments in wireless power transfer around the turn of the 20th century,[52][53][54] but the possibilities of using resonant coupling to increase transmission range has only recently been explored.[55] In 2007 a team led by Marin Soljačić at MIT used two coupled tuned circuits each made of a 25 cm self-resonant coil of wire at 10 MHz to achieve the transmission of 60 W of power over a distance of 2 meters (6.6 ft) (8 times the coil diameter) at around 40% efficiency.[24][35][46][53][56]

The concept behind resonant inductive coupling systems is that high Q factor resonators exchange energy at a much higher rate than they lose energy due to internal damping.[35] 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.[35] Resonant inductive coupling can achieve high efficiency at ranges of 4 to 10 times the coil diameter (Dant).[36][37][38] This is called "mid-range" transfer,[37] 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.[31][35]

A drawback of resonant coupling theory 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,[57][58][59] 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.[36][60]

Resonant technology is currently being widely incorporated in modern inductive wireless power systems.[45] 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.[46] An environmental and economic benefit of wirelessly powering small devices such as clocks, radios, music players and remote controls is that it could drastically reduce the 6 billion batteries disposed of each year, a large source of toxic waste and groundwater contamination.[40]

A study for the Swedish military found that 85kHz systems for dynamic wireless power transfer for vehicles can cause electromagnetic interference at a radius of up to 300 kilometers.[61]

Capacitive coupling edit

Main article: Capacitive coupling

Capacitive coupling also referred to as electric coupling, makes use of electric fields for the transmission of power between two electrodes (an anode and cathode) forming a capacitance for the transfer of power.[62] In capacitive coupling (electrostatic induction), the conjugate of inductive coupling, energy is transmitted by electric fields[4][15][5][7] between electrodes[6] such as metal plates. The transmitter and receiver electrodes form a capacitor, with the intervening space as the dielectric.[6][15][18][24][47][63] An alternating voltage generated by the transmitter is applied to the transmitting plate, and the oscillating electric field induces an alternating potential on the receiver plate by electrostatic induction,[15][63] which causes an alternating current to flow in the load circuit. The amount of power transferred increases with the frequency[63] the square of the voltage, and the capacitance between the plates, which is proportional to the area of the smaller plate and (for short distances) inversely proportional to the separation.[15]

Capacitive wireless power systems
 
Bipolar coupling
 
Monopolar coupling

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,[18][24] and can cause unpleasant side effects such as noxious ozone production. In addition, in contrast to magnetic fields,[35] electric fields interact strongly with most materials, including the human body, due to dielectric polarization.[47] Intervening materials between or near the electrodes can absorb the energy, in the case of humans possibly causing excessive electromagnetic field exposure.[18] However capacitive coupling has a few advantages over inductive coupling. The field is largely confined between the capacitor plates, reducing interference, which in inductive coupling requires heavy ferrite "flux confinement" cores.[15][47] Also, alignment requirements between the transmitter and receiver are less critical.[15][18][63] Capacitive coupling has recently been applied to charging battery powered portable devices[4] as well as charging or continuous wireless power transfer in biomedical implants,[5][6][7] and is being considered as a means of transferring power between substrate layers in integrated circuits.[64]

Two types of circuit have been used:

  • Transverse (bipolar) design:[5][7][65][66] In this type of circuit, there are two transmitter plates and two receiver plates. Each transmitter plate is coupled to a receiver plate. The transmitter oscillator drives 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.[16]
  • Longitudinal (unipolar) design:[15][63][66] In this type of circuit, the transmitter and receiver have only one active electrode, and either the ground or a large passive electrode serves as the return path for the current. The transmitter oscillator is connected between an active and a passive electrode. The load is also connected between an active and a passive electrode. The electric field produced by the transmitter induces alternating charge displacement in the load dipole through electrostatic induction.[67]

Resonant capacitive coupling edit

Resonance can also be used with capacitive coupling to extend the range. At the turn of the 20th century, Nikola Tesla did the first experiments with both resonant inductive and capacitive coupling.

Electrodynamic Wireless Power Transfer edit

An electrodynamic wireless power transfer (EWPT) system utilizes a receiver with a mechanically resonating or rotating permanent magnet.[68][69] When subjected to a time-varying magnetic field, the mechanical motion of the resonating magnet is converted into electricity by one or more electromechanical transduction schemes (e.g. electromagnetic/induction, piezoelectric, or capacitive).[70][71] In contrast to inductive coupling systems which usually use high frequency magnetic fields, EWPT uses low-frequency magnetic fields (<1 kHz),[72][73][74] which safely pass through conductive media and have higher human field exposure limits (~2 mTrms at 1 kHz),[75][76] showing promise for potential use in wirelessly recharging biomedical implants. For EWPT devices having identical resonant frequencies, the magnitude of power transfer is entirely dependent on critical coupling coefficient, denoted by  , between the transmitter and receiver devices. For coupled resonators with same resonant frequencies, wireless power transfer between the transmitter and the receiver is spread over three regimes – under-coupled, critically coupled and over-coupled. As the critical coupling coefficient increases from an under-coupled regime ( ) to the critical coupled regime, the optimum voltage gain curve grows in magnitude (measured at the receiver) and peaks when   and then enters into the over-coupled regime where   and the peak splits into two.[77] This critical coupling coefficient is demonstrated to be a function of distance between the source and the receiver devices.[78][79]

Magnetodynamic coupling edit

In this method, power is transmitted between two rotating armatures, one in the transmitter and one in the receiver, which rotate synchronously, coupled together by a magnetic field generated by permanent magnets on the armatures.[25] The transmitter armature is turned either by or as the rotor of an electric motor, and its magnetic field exerts torque on the receiver armature, turning it. The magnetic field acts like a mechanical coupling between the armatures.[25] The receiver armature produces power to drive the load, either by turning a separate electric generator or 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 of electric vehicles.[25] 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.[25] It is claimed that this technique can transfer power over distances of 10 to 15 cm (4 to 6 inches) with high efficiency, over 90%.[25][80] Also, the low frequency stray magnetic fields produced by the rotating magnets produce less electromagnetic interference to nearby electronic devices than the high frequency magnetic fields produced by inductive coupling systems. A prototype system charging electric vehicles has been in operation at University of British Columbia since 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.[25]

Zenneck Wave Transmission edit

A new kind of system using the Zenneck type waves was shown by Oruganti et al., where they demonstrated that it was possible to excite Zenneck wave type waves on flat metal-air interfaces and transmit power across metal obstacles.[81][82][83] Here the idea is to excite a localized charge oscillation at the metal-air interface, the resulting modes propagate along the metal-air interface.[81]

Far-field (radiative) techniques edit

Far field methods achieve longer ranges, often multiple kilometer ranges, where the distance is much greater than the diameter of the device(s). High-directivity antennas or well-collimated laser light produce a beam of energy that can be made to match the shape of the receiving area. The maximum directivity for antennas is physically limited by diffraction.

In general, visible light (from lasers) and microwaves (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 from transmitter to receiver, the wavelength and the Rayleigh criterion or diffraction limit, used in standard radio frequency antenna design, which also applies to lasers. Airy's diffraction limit is also frequently used to determine an approximate spot size at an arbitrary distance from the aperture. Electromagnetic radiation experiences less diffraction at shorter wavelengths (higher frequencies); so, for example, a blue laser is diffracted less than a red one.

The Rayleigh limit (also known as the Abbe diffraction limit), although originally applied to image resolution, can be viewed in reverse, and dictates that the irradiance (or intensity) of any electromagnetic wave (such as a microwave or laser beam) will be reduced as the beam diverges over distance at a minimum rate inversely proportional to the aperture size. The larger the ratio of a transmitting antenna's aperture or laser's exit aperture to the wavelength of radiation, the more can the radiation be concentrated in a compact beam

Microwave power beaming can be more efficient[clarification needed] than lasers, and is less prone to atmospheric attenuation caused by dust or aerosols such as fog.

Here, the power levels are calculated by combining the above parameters together, and adding in the gains and losses due to the antenna characteristics and the transparency and dispersion of the medium through which the radiation passes. That process is known as calculating a link budget.

Microwaves edit

 
An artist's depiction of a solar satellite that could send 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 the microwave range.[84] A rectenna may be used to convert the microwave energy back into electricity. Rectenna conversion efficiencies exceeding 95% have been realized.[citation needed] Power beaming using microwaves has been proposed for the transmission of energy from orbiting solar power satellites to Earth and the beaming of power to spacecraft leaving orbit has been considered.[85][86]

Power beaming by microwaves has the difficulty that, for most space applications, the required aperture sizes are very large due to diffraction limiting antenna directionality. For example, the 1978 NASA study of solar power satellites required a 1-kilometre-diameter (0.62 mi) transmitting antenna and a 10-kilometre-diameter (6.2 mi) receiving rectenna for a microwave beam at 2.45 GHz.[87] 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 10 km 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 1 mW/cm2 distributed across a 10 km diameter area corresponds to 750 megawatts total power level. This is the power level found in many modern electric power plants. For comparison, a solar PV farm of similar size might easily exceed 10,000 megawatts (rounded) at best conditions during daytime.

Following World War II, which saw the development of high-power microwave emitters known as cavity magnetrons, the idea of using microwaves to transfer power was researched. By 1964, a miniature helicopter propelled by microwave power had been demonstrated.[88]

Japanese researcher Hidetsugu Yagi also investigated wireless energy transmission using a directional array antenna that he designed. In February 1926, Yagi and his colleague Shintaro Uda published their first paper on the tuned high-gain directional array now known as the Yagi 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.[89]

Wireless high power transmission using microwaves is well proven. Experiments in the tens of kilowatts have been performed at the Goldstone Deep Space Communications Complex in California in 1975[90][91][92] and more recently (1997) at Grand Bassin on Reunion Island.[93] These methods achieve distances on the order of a kilometer.

Under experimental conditions, microwave conversion efficiency was measured to be around 54% across one meter.[94]

A change to 24 GHz 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.

In 2013, inventor Hatem Zeine demonstrated how wireless power transmission using phased array antennas can deliver electrical power up to 30 feet. It uses the same radio frequencies as WiFi.[95][96]

In 2015, researchers at the University of Washington introduced power over Wi-Fi, which trickle-charges batteries and powered battery-free cameras and temperature sensors using transmissions from Wi-Fi routers.[97][98] Wi-Fi signals were shown to power battery-free temperature and camera sensors at ranges of up to 20 feet. It was also shown that Wi-Fi can be used to wirelessly trickle-charge nickel–metal hydride and lithium-ion coin-cell batteries at distances of up to 28 feet.

In 2017, the Federal Communications Commission (FCC) certified the first mid-field radio frequency (RF) transmitter of wireless power.[99] In 2021 the FCC granted a license to a over-the-air (OTA) wireless charging system that combines near-field and far-field methods by using a frequency of about 900 MHz. Due to the radiated power of about 1 W this system is intended for small IoT devices as various sensors, trackers, detectors and monitors.[100]

Lasers edit

 
A laser beam centered on a panel of photovoltaic cells provides enough power to a lightweight model airplane for it to fly.

In the case of electromagnetic radiation closer to the visible region of the spectrum (.2 to 2 micrometers), power can be transmitted by converting electricity into a laser beam that is received and concentrated onto photovoltaic cells (solar cells).[101][102] This mechanism is generally known as 'power beaming' because the power is beamed at a receiver that can convert it to electrical energy. At the receiver, special photovoltaic laser power converters which are optimized for monochromatic light conversion are applied.[103]

Advantages compared to other wireless methods are:[104]

  • Collimated monochromatic wavefront propagation allows narrow beam cross-section area for transmission over large distances. As a result, there is little or no reduction in power when increasing the distance from the transmitter to the receiver.
  • Compact size: solid state lasers fit into small products.
  • No radio-frequency interference to existing radio communication such as Wi-Fi and cell phones.
  • Access control: only receivers hit by the laser receive power.

Drawbacks include:

  • Laser radiation is hazardous. Without a proper safety mechanism, low power levels can blind humans and other animals. High power levels can kill through localized spot heating.
  • Conversion between electricity and light is limited. Photovoltaic cells achieve a maximum of 40%–50% efficiency.[105]
  • 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. (Instead of being beamed directly onto the receiver, the laser light can also be guided by an optical fiber. Then one speaks of power-over-fiber technology.)

Laser 'powerbeaming' technology was explored in military weapons[106][107][108] and aerospace[109][110] applications. Also, it is applied for the powering of various kinds of sensors in industrial environments. Lately, it is developed for powering commercial and consumer electronics. Wireless energy transfer systems using lasers for consumer space have to satisfy laser safety requirements standardized under IEC 60825.[citation needed]

The first wireless power system using lasers for consumer applications was demonstrated in 2018, capable of delivering power to stationary and moving devices across a room. This wireless power system complies with safety regulations according to IEC 60825 standard. It is also approved by the US Food and Drugs Administration (FDA).[111]

Other details include propagation,[112] and the coherence and the range limitation problem.[113]

Geoffrey Landis[114][115][116] is one of the pioneers of solar power satellites[117] 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-powered space elevator.[118][119]

NASA's Dryden Flight Research Center has demonstrated a lightweight unmanned model plane powered by a laser beam.[120] This proof-of-concept demonstrates the feasibility of periodic recharging using a laser beam system.

Scientists from the Chinese Academy of Sciences have developed a proof-of-concept of utilizing a dual-wavelength laser to wirelessly charge portable devices or UAVs.[121]

Atmospheric plasma channel coupling edit

See also: Electrolaser

In atmospheric plasma channel coupling, energy is transferred between two electrodes by electrical conduction through ionized air.[122] When an electric field gradient exists between the two electrodes, exceeding 34 kilovolts per centimeter at sea level atmospheric pressure, an electric arc occurs.[123] This atmospheric dielectric breakdown results in the flow of electric current along a random trajectory through an ionized plasma channel between the two electrodes. An example of this is natural lightning, where one electrode is a virtual point in a cloud and the other is a point on Earth. Laser Induced Plasma Channel (LIPC) research is presently underway using ultrafast lasers to artificially promote development of the plasma channel through the air, directing the electric arc, and guiding the current across a specific path in a controllable manner.[124] The laser energy reduces the atmospheric dielectric breakdown voltage and the air is made less insulating by superheating, which lowers the density ( ) of the filament of air.[125]

This new process is being explored for use as a laser lightning rod and as a means to trigger lightning bolts from clouds for natural lightning channel studies,[126] for artificial atmospheric propagation studies, as a substitute for conventional radio antennas,[127] for applications associated with electric welding and machining,[128][129] for diverting power from high-voltage capacitor discharges, for directed-energy weapon applications employing electrical conduction through a ground return path,[130][131][132][133] and electronic jamming.[134]

Energy harvesting edit

Main article: Energy harvesting

In the context of wireless power, energy harvesting, also called power harvesting or energy scavenging, is the conversion of ambient energy from the environment to electric power, mainly to power small autonomous wireless electronic devices.[135] The ambient energy may come from stray electric or magnetic fields or radio waves from nearby electrical equipment, light, thermal energy (heat), or kinetic energy such as vibration or motion of the device.[135] Although the efficiency of conversion is usually low and the power gathered often minuscule (milliwatts or microwatts),[135] it can be adequate to run or recharge small micropower wireless devices such as remote sensors, which are proliferating in many fields.[135] 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.[136][137]

History edit

19th century developments and dead ends edit

The 19th century saw many developments of theories, and counter-theories on how electrical energy might be transmitted. In 1826, André-Marie Ampère discovered a connection between current and magnets. Michael Faraday described in 1831 with his law of induction the electromotive force driving a current in a conductor loop by a time-varying magnetic flux. Transmission of electrical energy without wires was observed by many inventors and experimenters,[138][139][140] but lack of a coherent theory attributed these phenomena vaguely to electromagnetic induction.[141] A concise explanation of these phenomena would come from the 1860s Maxwell's equations[51] by James Clerk Maxwell, establishing a theory that unified electricity and magnetism to electromagnetism, predicting the existence of electromagnetic waves as the "wireless" carrier of electromagnetic energy. Around 1884 John Henry Poynting defined the Poynting vector and gave Poynting's theorem, which describe the flow of power across an area within electromagnetic radiation and allow for a correct analysis of wireless power transfer systems.[51][142] This was followed on by Heinrich Rudolf Hertz' 1888 validation of the theory, which included the evidence for radio waves.[142]

During the same period two schemes of wireless signaling were put forward by William Henry Ward (1871) and Mahlon Loomis (1872) that were based on the erroneous belief that there was an electrified atmospheric stratum accessible at low altitude.[143][144] Both inventors' patents noted this layer connected with a return path using "Earth currents"' would allow for wireless telegraphy as well as supply power for the telegraph, doing away with artificial batteries, and could also be used for lighting, heat, and motive power.[145][146] A more practical demonstration of wireless transmission via conduction came in Amos Dolbear's 1879 magneto electric telephone that used ground conduction to transmit over a distance of a quarter of a mile.[147]

Tesla edit

 
Tesla demonstrating wireless transmission by "electrostatic induction" during an 1891 lecture at Columbia College.  The two metal sheets are connected to a Tesla coil oscillator, which applies high-voltage radio frequency alternating current.  An oscillating electric field between the sheets ionizes the low-pressure gas in the two long Geissler tubes in his hands, causing them to glow in a manner similar to neon tubes.

After 1890, inventor Nikola Tesla experimented with transmitting power by inductive and capacitive coupling using spark-excited radio frequency resonant transformers, now called Tesla coils, which generated high AC voltages.[51][53][148] Early on he attempted to develop a wireless lighting system based on near-field inductive and capacitive coupling[53] and conducted a series of public demonstrations where he lit Geissler tubes and even incandescent light bulbs from across a stage.[53][148][149] He found he could increase the distance at which he could light a lamp by using a receiving LC circuit tuned to resonance with the transmitter's LC circuit.[52] using resonant inductive coupling.[53][54] Tesla failed to make a commercial product out of his findings[150] but his resonant inductive coupling method is now widely used in electronics and is currently being applied to short-range wireless power systems.[53][151]

 
 
(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.

Tesla went on to develop a wireless power distribution system that he hoped would be capable of transmitting power long distance directly into homes and factories. Early on he seemed to borrow from the ideas of Mahlon Loomis,[152][153] proposing a system composed of balloons to suspend transmitting and receiving electrodes in the air above 30,000 feet (9,100 m) in altitude, where he thought the pressure would allow him to send high voltages (millions of volts) long distances. To further study the conductive nature of low pressure air he set up a test facility at high altitude in Colorado Springs during 1899.[154][155][156] Experiments he conducted there with a large coil operating in the megavolts range, as well as observations he made of the electronic noise of lightning strikes, led him to conclude incorrectly[157][147] that he could use the entire globe of the Earth to conduct electrical energy. The theory included driving alternating current pulses into the Earth at its resonant frequency from a grounded Tesla coil working against an elevated capacitance to make the potential of the Earth oscillate. Tesla thought this would allow alternating current to be received with a similar capacitive antenna tuned to resonance with it at any point on Earth with very little power loss.[158][159][160] His observations also led him to believe a high voltage used in a coil at an elevation of a few hundred feet would "break the air stratum down", eliminating the need for miles of cable hanging on balloons to create his atmospheric return circuit.[161][162] Tesla would go on the next year to propose a "World Wireless System" that was to broadcast both information and power worldwide.[163][164] In 1901, at Shoreham, New York he attempted to construct a large high-voltage wireless power station, now called Wardenclyffe Tower, but by 1904 investment dried up and the facility was never completed.

Near-field and non-radiative technologies edit

Inductive power transfer between nearby wire coils was the earliest wireless power technology to be developed, existing since the transformer was developed in the 1800s. Induction heating has been used since the early 1900s and is used for induction cooking.[165]

With the advent of cordless devices, induction charging stands have been developed for appliances used in wet environments, like electric toothbrushes and electric razors, to eliminate the hazard of electric shock. One of the earliest proposed applications of inductive transfer was to power electric locomotives. 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 3 kHz.[166]

In the early 1960s resonant inductive wireless energy transfer was used successfully in implantable medical devices[167] including such devices as pacemakers and artificial hearts. While the early systems used a resonant receiver coil, later systems[168] implemented resonant transmitter coils as well. These medical devices are designed for high efficiency using low power electronics while efficiently accommodating some misalignment and dynamic twisting of the coils. The separation between the coils in implantable applications is commonly less than 20 cm. Today resonant inductive energy transfer is regularly used for providing electric power in many commercially available medical implantable devices.[169]

The first passive RFID (Radio Frequency Identification) technologies were invented by Mario Cardullo[170] (1973) and Koelle et al.[171] (1975) and by the 1990s were being used in proximity cards and contactless smartcards.

The proliferation of portable wireless communication devices such as mobile phones, tablet, and laptop computers in recent decades is currently driving the development of mid-range wireless powering and charging technology to eliminate the need for these devices to be tethered to wall plugs during charging.[172] The Wireless Power Consortium was established in 2008 to develop interoperable standards across manufacturers.[172] Its Qi inductive power standard published in August 2009 enables high efficiency charging and powering of portable devices of up to 5 watts over distances of 4 cm (1.6 inches).[173] The wireless device is placed on a flat charger plate (which can 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 Soljačić at MIT used a dual resonance transmitter with a 25 cm diameter secondary tuned to 10 MHz to transfer 60 W of power to a similar dual resonance receiver over a distance of 2 meters (6.6 ft) (eight times the transmitter coil diameter) at around 40% efficiency.[53][56]

In 2008 the team of Greg Leyh and Mike Kennan of Nevada Lightning Lab used a grounded dual resonance transmitter with a 57 cm diameter secondary tuned to 60 kHz and a similar grounded dual resonance receiver to transfer power through coupled electric fields with an earth current return circuit over a distance of 12 meters (39 ft).[174] In 2011, Dr. Christopher A. Tucker and Professor Kevin Warwick of the University of Reading, recreated Tesla's 1900 patent 0,645,576 in miniature and demonstrated power transmission over 4 meters (13 ft) with a coil diameter of 10 centimetres (3.9 in) at a resonant frequency of 27.50 MHz, with an effective efficiency of 60%.[175]

Microwaves and lasers edit

Before World War II, little progress was made in wireless power transmission.[92] Radio was developed for communication uses, but could not be used for power transmission since the relatively low-frequency radio waves spread out in all directions and little energy reached the receiver.[51][92] In radio communication, at the receiver, an amplifier intensifies a weak signal using energy from another source. For power transmission, efficient transmission required transmitters that could generate higher-frequency microwaves, which can be focused in narrow beams towards a receiver.[51][92][176]

The development of microwave technology during World War II, such as the klystron and magnetron tubes and parabolic antennas,[92] made radiative (far-field) methods practical for the first time, and the first long-distance wireless power transmission was achieved in the 1960s by William C. Brown.[51] In 1964, Brown invented the rectenna which 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.[92] A major motivation for microwave research in the 1970s and 1980s was to develop a solar power satellite.[51][92] Conceived in 1968 by Peter Glaser, this would harvest energy from sunlight using solar cells and beam it down to Earth as microwaves to huge rectennas, which would convert it to electrical energy on the electric power grid.[177] In landmark 1975 experiments as technical director of a JPL/Raytheon program, Brown demonstrated long-range transmission by beaming 475 W of microwave power to a rectenna a mile away, with a microwave to DC conversion efficiency of 54%.[178] At NASA's Jet Propulsion Laboratory, he and Robert Dickinson transmitted 30 kW DC output power across 1.5 km with 2.38 GHz microwaves from a 26 m dish to a 7.3 x 3.5 m rectenna array. The incident-RF to DC conversion efficiency of the rectenna was 80%.[179] In 1983 Japan launched Microwave Ionosphere Nonlinear Interaction Experiment (MINIX), a rocket experiment to test transmission of high power microwaves through the ionosphere.[citation needed]

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[92] which sponsored Brown's research. In 1987 Canada's Communications Research Center developed a small prototype airplane called Stationary High Altitude Relay Platform (SHARP) to relay telecommunication data between points on earth similar to a communications satellite. Powered by a rectenna, it could fly at 13 miles (21 km) 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 by photocells from a beam of infrared light from a ground-based laser, while a control system kept the laser pointed at the plane.

See also edit

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Further reading edit

Books and articles edit

  • de Rooij, Michael A. (2015). Wireless Power Handbook. Power Conversion Publications. ISBN 978-0996649216. Latest work on AirFuel Alliance class 2 and class 3 transmitters, adaptive tuning, radiated EMI, multi-mode wireless power systems, and control strategies.
  • Agbinya, Johnson I., Ed. (2012). Wireless Power Transfer. River Publishers. ISBN 978-8792329233.{{cite book}}: CS1 maint: multiple names: authors list (link) Comprehensive, theoretical engineering text
  • Shinohara, Naoki (2014). Wireless Power Transfer via Radiowaves. John Wiley & Sons. ISBN 978-1118862964. Engineering text
  • Kurs, A.; Karalis, A.; Moffatt, R.; Joannopoulos, J. D.; Fisher, P.; Soljacic, M. (6 July 2007). "Wireless Power Transfer via Strongly Coupled Magnetic Resonances". Science. 317 (5834): 83–86. Bibcode:2007Sci...317...83K. CiteSeerX 10.1.1.418.9645. doi:10.1126/science.1143254. PMID 17556549. S2CID 17105396.
  • Thibault, G. (2014). Wireless Pasts and Wired Futures. In J. Hadlaw, A. Herman, & T. Swiss (Eds.), Theories of the Mobile Internet. Materialities and Imaginaries. (pp. 126–154). London: Routledge. A short cultural history of wireless power

Patents edit

  • U.S. Patent 4,955,562, Microwave powered aircraft, John E. Martin, et al. (1990).
  • U.S. Patent 3,933,323, Solid state solar to microwave energy converter system and apparatus, Kenneth W. Dudley, et al. (1976).
  • U.S. Patent 3,535,543, Microwave power receiving antenna, Carroll C. Dailey (1970).

External links edit

  • Howstuffworks "How Wireless Power Works" – describes near-range and mid-range wireless power transmission using induction and radiation techniques.
  • , – its history before 1980.
  • The Stationary High Altitude Relay Platform (SHARP), – microwave beam powered.
  • Marin Soljačić's MIT WiTricity – wireless power transmission pages.
  • Rezence – official site of a wireless power standard promoted by the Alliance for Wireless Power
  • Qi – official site of a wireless power standard promoted by the Wireless Power Consortium
  • PMA – official site of a wireless power standard promoted by the Power Matters Alliance
  • WiPow – official site of the WiPow Coalition, promoting standardized wireless power for medical, mobility and wheeled devices

December 18, 2023
wireless, power, transfer, wireless, power, transmission, wireless, energy, transmission, electromagnetic, power, transfer, transmission, electrical, energy, without, wires, physical, link, wireless, power, transmission, system, electrically, powered, transmit. Wireless power transfer WPT wireless power transmission wireless energy transmission WET or electromagnetic power transfer is the transmission of electrical energy without wires as a physical link In a wireless power transmission system an electrically powered transmitter device generates a time varying electromagnetic field that transmits power across space to a receiver device the receiver device extracts power from the field and supplies it to an electrical load The technology of wireless power transmission can eliminate the use of the wires and batteries thereby increasing the mobility convenience and safety of an electronic device for all users 2 Wireless power transfer is useful to power electrical devices where interconnecting wires are inconvenient hazardous or are not possible Inductive charging pad for a smartphone as an example of near field wireless transfer When the phone is set on the pad a coil in the pad creates a magnetic field 1 which induces a current in another coil in the phone charging its battery Wireless power techniques mainly fall into two categories near field and far field 3 In near field or non radiative techniques power is transferred over short distances by magnetic fields using inductive coupling between coils of wire or by electric fields using capacitive coupling between metal electrodes 4 5 6 7 Inductive coupling is the most widely used wireless technology its applications include charging handheld devices like phones and electric toothbrushes RFID tags induction cooking and wirelessly charging or continuous wireless power transfer in implantable medical devices like artificial cardiac pacemakers or electric vehicles In far field or radiative techniques also called power beaming power is transferred by beams of electromagnetic radiation like microwaves 8 or laser beams These techniques can transport energy longer distances but must be aimed at the receiver Proposed applications for this type include solar power satellites and wireless powered drone aircraft 9 10 11 An important issue associated with all wireless power systems is limiting the exposure of people and other living beings to potentially injurious electromagnetic fields 12 13 Contents 1 Overview 2 Field regions 3 Near field nonradiative techniques 3 1 Inductive coupling 3 1 1 Resonant inductive coupling 3 2 Capacitive coupling 3 2 1 Resonant capacitive coupling 3 3 Electrodynamic Wireless Power Transfer 3 4 Magnetodynamic coupling 3 5 Zenneck Wave Transmission 4 Far field radiative techniques 4 1 Microwaves 4 2 Lasers 5 Atmospheric plasma channel coupling 6 Energy harvesting 7 History 7 1 19th century developments and dead ends 7 2 Tesla 7 3 Near field and non radiative technologies 7 4 Microwaves and lasers 8 See also 9 References 10 Further reading 10 1 Books and articles 10 2 Patents 11 External linksOverview editFurther information Coupling electronics nbsp Generic block diagram of a wireless power systemWireless power transfer is a generic term for a number of different technologies for transmitting energy by means of electromagnetic fields 14 15 16 The technologies listed in the table below differ in the distance over which they can transfer power efficiently whether the transmitter must be aimed directed at the receiver and in the type of electromagnetic energy they use time varying electric fields magnetic fields radio waves microwaves infrared or visible light waves 17 In general a wireless power system consists of a transmitter device connected to a source of power such as a mains power line 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 current which is used by an electrical load 14 17 At the transmitter the input power is converted to an oscillating electromagnetic field by some type of antenna device The word antenna is used loosely here it may be a coil of wire which generates a magnetic field a metal plate which generates an electric field an antenna which radiates radio waves or a laser which generates light A similar antenna or coupling device at the receiver converts the oscillating fields to an electric current An important parameter that determines the type of waves is the frequency which determines the wavelength Wireless power uses the same fields and waves as wireless communication devices like radio 18 19 another familiar technology that involves electrical energy transmitted without wires by electromagnetic fields used in cellphones radio and television broadcasting and WiFi In radio communication the goal is the transmission of information so the amount of power reaching the receiver is not so important as long as it is sufficient that the information can be received intelligibly 15 18 19 In wireless communication technologies only tiny amounts of power reach the receiver In contrast with wireless power transfer the amount of energy received is the important thing so the efficiency fraction of transmitted energy that is received is the more significant parameter 15 For this reason wireless power technologies are likely to be more limited by distance than wireless communication technologies Wireless power transfer may be used to power up wireless information transmitters or receivers This type of communication is known as wireless powered communication WPC When the harvested power is used to supply the power of wireless information transmitters the network is known as Simultaneous Wireless Information and Power Transfer SWIPT 20 whereas when it is used to supply the power of wireless information receivers it is known as a Wireless Powered Communication Network WPCN 21 22 23 These are the different wireless power technologies 14 17 24 25 Technology Range 26 Directivity 17 Frequency Antenna devices Current and or possible future applicationsInductive coupling Short Low Hz MHz Wire coils Electric tooth brush and razor battery charging induction stovetops and industrial heaters Resonant inductive coupling Mid Low kHz GHz Tuned wire coils lumped element resonators Charging portable devices Qi biomedical implants electric vehicles powering buses trains MAGLEV RFID smartcards Capacitive coupling Short Low kHz MHz Metal plate electrodes Charging portable devices power routing in large scale integrated circuits Smartcards biomedical implants 5 6 7 Magnetodynamic coupling Short N A Hz Rotating magnets Charging electric vehicles 25 biomedical implants 27 Microwaves Long High GHz Parabolic dishes phased arrays rectennas Solar power satellite powering drone aircraft charging wireless devicesLight waves Long High THz Lasers photocells lenses Charging portable devices 28 powering drone aircraft powering space elevator climbers Field regions editElectric and magnetic fields are created by charged particles in matter such as electrons A stationary charge creates an electrostatic field in the space around it A steady current of charges direct current DC creates a static magnetic field around it The above fields contain energy but cannot carry power because they are static However time varying fields can carry power 29 Accelerating electric charges such as are found in an alternating 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 distance Drange from the antenna 14 17 18 24 30 31 32 The boundary between the regions is somewhat vaguely defined 17 The fields have different characteristics in these regions and different technologies are used for transferring power Near field or nonradiative region This means the area within about 1 wavelength l of the antenna 14 30 31 In this region the oscillating electric and magnetic fields are separate 18 and power can be transferred via electric fields by capacitive coupling electrostatic induction between metal electrodes 4 5 6 7 or via magnetic fields by inductive coupling electromagnetic induction between coils of wire 15 17 18 24 These fields are not radiative 31 meaning the energy stays within a short distance of the transmitter 33 If there is no receiving device or absorbing material within their limited range to couple to no power leaves the transmitter 33 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 decrease exponentially with distance 30 32 34 so if the distance between the two antennas Drange is much larger than the diameter of the antennas Dant very little power will be received Therefore these techniques cannot be used for long range power transmission Resonance such as resonant inductive coupling can increase the coupling between the antennas greatly allowing efficient transmission at somewhat greater distances 14 18 24 30 35 36 although the fields still decrease exponentially Therefore the range of near field devices is conventionally divided into two categories Short range up to about one antenna diameter Drange Dant 33 35 37 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 Drange 10 Dant 35 36 37 38 This is the range over which resonant capacitive or inductive coupling can transfer practical amounts of power Far field or radiative region Beyond about 1 wavelength l of the antenna the electric and magnetic fields are perpendicular to each other and propagate as an electromagnetic wave examples are radio waves microwaves or light waves 14 24 30 This part of the energy is radiative 31 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 size Dant to the wavelength of the waves l 39 which is determined by the frequency l c f At low frequencies f where the antenna is much smaller than the size of the waves Dant lt lt l very little power is radiated Therefore the near field devices above which use lower frequencies radiate almost none of their energy as electromagnetic radiation Antennas about the same size as the wavelength Dant l such as monopole or dipole antennas radiate power efficiently but the electromagnetic waves are radiated in all directions omnidirectionally so if the receiving antenna is far away only a small amount of the radiation will hit it 31 35 Therefore these can be used for short range inefficient power transmission but not for long range transmission 40 However unlike fields electromagnetic radiation can be focused by reflection or refraction into beams By using a high gain antenna or optical system which concentrates the radiation into a narrow beam aimed at the receiver it can be used for long range power transmission 35 40 From the Rayleigh criterion to produce the narrow beams necessary to focus a significant amount of the energy on a distant receiver an antenna must be much larger than the wavelength of the waves used Dant gt gt l c f 41 Practical beam power devices require wavelengths in the centimeter region or below corresponding to frequencies above 1 GHz in the microwave range or above 14 Near field nonradiative techniques editAt large relative distance the near field components of electric and magnetic fields are approximately quasi static oscillating dipole fields These fields decrease with the cube of distance Drange Dant 3 32 42 Since power is proportional to the square of the field strength the power transferred decreases as Drange Dant 6 18 34 43 44 or 60 dB per decade In other words if far apart increasing the distance between the two antennas tenfold causes the power received to decrease by a factor of 106 1000000 As a result inductive and capacitive coupling can only be used for short range power transfer within a few times the diameter of the antenna device Dant Unlike in a radiative system where the maximum radiation occurs when the dipole antennas are oriented transverse to the direction of propagation with dipole fields the maximum coupling occurs when the dipoles are oriented longitudinally Inductive coupling edit Main article Inductive charging nbsp Generic block diagram of an inductive wireless power system nbsp nbsp left Modern inductive power transfer an electric toothbrush charger A coil in the stand produces a magnetic field inducing an alternating current in a coil in the toothbrush which is rectified to charge the batteries right A light bulb powered wirelessly by induction in 1910 In inductive coupling electromagnetic induction 24 45 or inductive power transfer IPT power is transferred between coils of wire by a magnetic field 18 The transmitter and receiver coils together form a transformer 18 24 see diagram An alternating current AC through the transmitter coil L1 creates an oscillating magnetic field B by Ampere s law The magnetic field passes through the receiving coil L2 where it induces an alternating EMF voltage by Faraday s law of induction which creates an alternating current in the receiver 15 45 The induced alternating current may either drive the load directly or be rectified to direct current DC by a rectifier in the receiver which drives the load A few systems such as electric toothbrush charging stands work at 50 60 Hz so AC mains current is applied directly to the transmitter coil but in most systems an electronic oscillator generates a higher frequency AC current which drives the coil because transmission efficiency improves with frequency 45 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 in inductive charging stands for cordless appliances used in wet environments such as electric toothbrushes 24 and shavers to reduce the risk of electric shock 46 Another application area is transcutaneous recharging of biomedical prosthetic devices implanted in the human body such as cardiac pacemakers and insulin pumps to avoid having wires passing through the skin 47 48 It is also used to charge electric vehicles such as cars and to either charge or power transit vehicles like buses and trains 24 However the fastest growing use is wireless charging pads to recharge mobile and handheld wireless devices such as laptop and tablet computers computer mouse cellphones digital media players and video game controllers citation needed In the United States the Federal Communications Commission FCC provided its first certification for a wireless transmission charging system in December 2017 49 The power transferred increases with frequency 45 and the mutual inductance M displaystyle M nbsp between the coils 15 which depends on their geometry and the distance D range displaystyle D text range nbsp between them A widely used figure of merit is the coupling coefficient k M L 1 L 2 displaystyle k M sqrt L 1 L 2 nbsp 45 50 This dimensionless parameter is equal to the fraction of magnetic flux through the transmitter coil L 1 displaystyle L1 nbsp that passes through the receiver coil L 2 displaystyle L2 nbsp when L2 is open circuited If the two coils are on the same axis and close together so all the magnetic flux from L 1 displaystyle L1 nbsp passes through L 2 displaystyle L2 nbsp k 1 displaystyle k 1 nbsp 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 lower k displaystyle k nbsp and the link efficiency are approaching zero at large separations 45 The link efficiency and power transferred is roughly proportional to k 2 displaystyle k 2 nbsp 45 In order to achieve high efficiency the coils must be very close together a fraction of the coil diameter D ant displaystyle D text ant nbsp 45 usually within centimeters 40 with the coils axes aligned Wide flat coil shapes are usually used to increase coupling 45 Ferrite flux confinement cores can confine the magnetic fields improving coupling and reducing interference to nearby electronics 45 47 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 systems resonant inductive coupling described below is used in which the efficiency is increased by using resonant circuits 31 36 45 51 This can achieve high efficiencies at greater distances than nonresonant inductive coupling nbsp Prototype inductive electric car charging system at 2011 Tokyo Auto Show nbsp Powermat inductive charging spots in a coffee shop Customers can set their phones and computers on them to recharge nbsp Wireless powered access card nbsp GM EV1 and Toyota RAV4 EV inductively charging at a now obsolete Magne Charge station Resonant inductive coupling edit Main article Resonant inductive coupling Further information Tesla coil Resonant transformer Resonant inductive coupling electrodynamic coupling 24 strongly coupled magnetic resonance 35 is a form of inductive coupling in which power is transferred by magnetic fields B green between two resonant circuits tuned circuits one in the transmitter and one in the receiver see diagram right 18 24 31 46 51 Each resonant circuit consists of a coil of wire connected to a capacitor or a self resonant coil or other resonator with internal capacitance The two are tuned to resonate at the same resonant frequency The resonance between the coils can greatly increase coupling and power transfer analogously to the way a vibrating tuning fork can induce sympathetic vibration in a distant fork tuned to the same pitch Nikola Tesla first discovered resonant coupling during his pioneering experiments in wireless power transfer around the turn of the 20th century 52 53 54 but the possibilities of using resonant coupling to increase transmission range has only recently been explored 55 In 2007 a team led by Marin Soljacic at MIT used two coupled tuned circuits each made of a 25 cm self resonant coil of wire at 10 MHz to achieve the transmission of 60 W of power over a distance of 2 meters 6 6 ft 8 times the coil diameter at around 40 efficiency 24 35 46 53 56 The concept behind resonant inductive coupling systems is that high Q factor resonators exchange energy at a much higher rate than they lose energy due to internal damping 35 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 35 Resonant inductive coupling can achieve high efficiency at ranges of 4 to 10 times the coil diameter Dant 36 37 38 This is called mid range transfer 37 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 31 35 A drawback of resonant coupling theory 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 57 58 59 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 36 60 Resonant technology is currently being widely incorporated in modern inductive wireless power systems 45 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 46 An environmental and economic benefit of wirelessly powering small devices such as clocks radios music players and remote controls is that it could drastically reduce the 6 billion batteries disposed of each year a large source of toxic waste and groundwater contamination 40 A study for the Swedish military found that 85kHz systems for dynamic wireless power transfer for vehicles can cause electromagnetic interference at a radius of up to 300 kilometers 61 Capacitive coupling edit Main article Capacitive coupling Capacitive coupling also referred to as electric coupling makes use of electric fields for the transmission of power between two electrodes an anode and cathode forming a capacitance for the transfer of power 62 In capacitive coupling electrostatic induction the conjugate of inductive coupling energy is transmitted by electric fields 4 15 5 7 between electrodes 6 such as metal plates The transmitter and receiver electrodes form a capacitor with the intervening space as the dielectric 6 15 18 24 47 63 An alternating voltage generated by the transmitter is applied to the transmitting plate and the oscillating electric field induces an alternating potential on the receiver plate by electrostatic induction 15 63 which causes an alternating current to flow in the load circuit The amount of power transferred increases with the frequency 63 the square of the voltage and the capacitance between the plates which is proportional to the area of the smaller plate and for short distances inversely proportional to the separation 15 Capacitive wireless power systems nbsp Bipolar coupling nbsp Monopolar coupling 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 18 24 and can cause unpleasant side effects such as noxious ozone production In addition in contrast to magnetic fields 35 electric fields interact strongly with most materials including the human body due to dielectric polarization 47 Intervening materials between or near the electrodes can absorb the energy in the case of humans possibly causing excessive electromagnetic field exposure 18 However capacitive coupling has a few advantages over inductive coupling The field is largely confined between the capacitor plates reducing interference which in inductive coupling requires heavy ferrite flux confinement cores 15 47 Also alignment requirements between the transmitter and receiver are less critical 15 18 63 Capacitive coupling has recently been applied to charging battery powered portable devices 4 as well as charging or continuous wireless power transfer in biomedical implants 5 6 7 and is being considered as a means of transferring power between substrate layers in integrated circuits 64 Two types of circuit have been used Transverse bipolar design 5 7 65 66 In this type of circuit there are two transmitter plates and two receiver plates Each transmitter plate is coupled to a receiver plate The transmitter oscillator drives 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 16 Longitudinal unipolar design 15 63 66 In this type of circuit the transmitter and receiver have only one active electrode and either the ground or a large passive electrode serves as the return path for the current The transmitter oscillator is connected between an active and a passive electrode The load is also connected between an active and a passive electrode The electric field produced by the transmitter induces alternating charge displacement in the load dipole through electrostatic induction 67 Resonant capacitive coupling edit Resonance can also be used with capacitive coupling to extend the range At the turn of the 20th century Nikola Tesla did the first experiments with both resonant inductive and capacitive coupling Electrodynamic Wireless Power Transfer edit An electrodynamic wireless power transfer EWPT system utilizes a receiver with a mechanically resonating or rotating permanent magnet 68 69 When subjected to a time varying magnetic field the mechanical motion of the resonating magnet is converted into electricity by one or more electromechanical transduction schemes e g electromagnetic induction piezoelectric or capacitive 70 71 In contrast to inductive coupling systems which usually use high frequency magnetic fields EWPT uses low frequency magnetic fields lt 1 kHz 72 73 74 which safely pass through conductive media and have higher human field exposure limits 2 mTrms at 1 kHz 75 76 showing promise for potential use in wirelessly recharging biomedical implants For EWPT devices having identical resonant frequencies the magnitude of power transfer is entirely dependent on critical coupling coefficient denoted by k displaystyle k nbsp between the transmitter and receiver devices For coupled resonators with same resonant frequencies wireless power transfer between the transmitter and the receiver is spread over three regimes under coupled critically coupled and over coupled As the critical coupling coefficient increases from an under coupled regime k lt k c r i t displaystyle k lt k crit nbsp to the critical coupled regime the optimum voltage gain curve grows in magnitude measured at the receiver and peaks when k k c r i t displaystyle k k crit nbsp and then enters into the over coupled regime where k gt k c r i t displaystyle k gt k crit nbsp and the peak splits into two 77 This critical coupling coefficient is demonstrated to be a function of distance between the source and the receiver devices 78 79 Magnetodynamic coupling edit In this method power is transmitted between two rotating armatures one in the transmitter and one in the receiver which rotate synchronously coupled together by a magnetic field generated by permanent magnets on the armatures 25 The transmitter armature is turned either by or as the rotor of an electric motor and its magnetic field exerts torque on the receiver armature turning it The magnetic field acts like a mechanical coupling between the armatures 25 The receiver armature produces power to drive the load either by turning a separate electric generator or 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 of electric vehicles 25 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 25 It is claimed that this technique can transfer power over distances of 10 to 15 cm 4 to 6 inches with high efficiency over 90 25 80 Also the low frequency stray magnetic fields produced by the rotating magnets produce less electromagnetic interference to nearby electronic devices than the high frequency magnetic fields produced by inductive coupling systems A prototype system charging electric vehicles has been in operation at University of British Columbia since 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 25 Zenneck Wave Transmission edit A new kind of system using the Zenneck type waves was shown by Oruganti et al where they demonstrated that it was possible to excite Zenneck wave type waves on flat metal air interfaces and transmit power across metal obstacles 81 82 83 Here the idea is to excite a localized charge oscillation at the metal air interface the resulting modes propagate along the metal air interface 81 Far field radiative techniques editFar field methods achieve longer ranges often multiple kilometer ranges where the distance is much greater than the diameter of the device s High directivity antennas or well collimated laser light produce a beam of energy that can be made to match the shape of the receiving area The maximum directivity for antennas is physically limited by diffraction In general visible light from lasers and microwaves 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 from transmitter to receiver the wavelength and the Rayleigh criterion or diffraction limit used in standard radio frequency antenna design which also applies to lasers Airy s diffraction limit is also frequently used to determine an approximate spot size at an arbitrary distance from the aperture Electromagnetic radiation experiences less diffraction at shorter wavelengths higher frequencies so for example a blue laser is diffracted less than a red one The Rayleigh limit also known as the Abbe diffraction limit although originally applied to image resolution can be viewed in reverse and dictates that the irradiance or intensity of any electromagnetic wave such as a microwave or laser beam will be reduced as the beam diverges over distance at a minimum rate inversely proportional to the aperture size The larger the ratio of a transmitting antenna s aperture or laser s exit aperture to the wavelength of radiation the more can the radiation be concentrated in a compact beamMicrowave power beaming can be more efficient clarification needed than lasers and is less prone to atmospheric attenuation caused by dust or aerosols such as fog Here the power levels are calculated by combining the above parameters together and adding in the gains and losses due to the antenna characteristics and the transparency and dispersion of the medium through which the radiation passes That process is known as calculating a link budget Microwaves edit nbsp An artist s depiction of a solar satellite that could send 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 the microwave range 84 A rectenna may be used to convert the microwave energy back into electricity Rectenna conversion efficiencies exceeding 95 have been realized citation needed Power beaming using microwaves has been proposed for the transmission of energy from orbiting solar power satellites to Earth and the beaming of power to spacecraft leaving orbit has been considered 85 86 Power beaming by microwaves has the difficulty that for most space applications the required aperture sizes are very large due to diffraction limiting antenna directionality For example the 1978 NASA study of solar power satellites required a 1 kilometre diameter 0 62 mi transmitting antenna and a 10 kilometre diameter 6 2 mi receiving rectenna for a microwave beam at 2 45 GHz 87 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 10 km 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 1 mW cm2 distributed across a 10 km diameter area corresponds to 750 megawatts total power level This is the power level found in many modern electric power plants For comparison a solar PV farm of similar size might easily exceed 10 000 megawatts rounded at best conditions during daytime Following World War II which saw the development of high power microwave emitters known as cavity magnetrons the idea of using microwaves to transfer power was researched By 1964 a miniature helicopter propelled by microwave power had been demonstrated 88 Japanese researcher Hidetsugu Yagi also investigated wireless energy transmission using a directional array antenna that he designed In February 1926 Yagi and his colleague Shintaro Uda published their first paper on the tuned high gain directional array now known as the Yagi 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 89 Wireless high power transmission using microwaves is well proven Experiments in the tens of kilowatts have been performed at the Goldstone Deep Space Communications Complex in California in 1975 90 91 92 and more recently 1997 at Grand Bassin on Reunion Island 93 These methods achieve distances on the order of a kilometer Under experimental conditions microwave conversion efficiency was measured to be around 54 across one meter 94 A change to 24 GHz 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 In 2013 inventor Hatem Zeine demonstrated how wireless power transmission using phased array antennas can deliver electrical power up to 30 feet It uses the same radio frequencies as WiFi 95 96 In 2015 researchers at the University of Washington introduced power over Wi Fi which trickle charges batteries and powered battery free cameras and temperature sensors using transmissions from Wi Fi routers 97 98 Wi Fi signals were shown to power battery free temperature and camera sensors at ranges of up to 20 feet It was also shown that Wi Fi can be used to wirelessly trickle charge nickel metal hydride and lithium ion coin cell batteries at distances of up to 28 feet In 2017 the Federal Communications Commission FCC certified the first mid field radio frequency RF transmitter of wireless power 99 In 2021 the FCC granted a license to a over the air OTA wireless charging system that combines near field and far field methods by using a frequency of about 900 MHz Due to the radiated power of about 1 W this system is intended for small IoT devices as various sensors trackers detectors and monitors 100 Lasers edit nbsp A laser beam centered on a panel of photovoltaic cells provides enough power to a lightweight model airplane for it to fly In the case of electromagnetic radiation closer to the visible region of the spectrum 2 to 2 micrometers power can be transmitted by converting electricity into a laser beam that is received and concentrated onto photovoltaic cells solar cells 101 102 This mechanism is generally known as power beaming because the power is beamed at a receiver that can convert it to electrical energy At the receiver special photovoltaic laser power converters which are optimized for monochromatic light conversion are applied 103 Advantages compared to other wireless methods are 104 Collimated monochromatic wavefront propagation allows narrow beam cross section area for transmission over large distances As a result there is little or no reduction in power when increasing the distance from the transmitter to the receiver Compact size solid state lasers fit into small products No radio frequency interference to existing radio communication such as Wi Fi and cell phones Access control only receivers hit by the laser receive power Drawbacks include Laser radiation is hazardous Without a proper safety mechanism low power levels can blind humans and other animals High power levels can kill through localized spot heating Conversion between electricity and light is limited Photovoltaic cells achieve a maximum of 40 50 efficiency 105 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 Instead of being beamed directly onto the receiver the laser light can also be guided by an optical fiber Then one speaks of power over fiber technology Laser powerbeaming technology was explored in military weapons 106 107 108 and aerospace 109 110 applications Also it is applied for the powering of various kinds of sensors in industrial environments Lately it is developed for powering commercial and consumer electronics Wireless energy transfer systems using lasers for consumer space have to satisfy laser safety requirements standardized under IEC 60825 citation needed The first wireless power system using lasers for consumer applications was demonstrated in 2018 capable of delivering power to stationary and moving devices across a room This wireless power system complies with safety regulations according to IEC 60825 standard It is also approved by the US Food and Drugs Administration FDA 111 Other details include propagation 112 and the coherence and the range limitation problem 113 Geoffrey Landis 114 115 116 is one of the pioneers of solar power satellites 117 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 powered space elevator 118 119 NASA s Dryden Flight Research Center has demonstrated a lightweight unmanned model plane powered by a laser beam 120 This proof of concept demonstrates the feasibility of periodic recharging using a laser beam system Scientists from the Chinese Academy of Sciences have developed a proof of concept of utilizing a dual wavelength laser to wirelessly charge portable devices or UAVs 121 Atmospheric plasma channel coupling editSee also Electrolaser In atmospheric plasma channel coupling energy is transferred between two electrodes by electrical conduction through ionized air 122 When an electric field gradient exists between the two electrodes exceeding 34 kilovolts per centimeter at sea level atmospheric pressure an electric arc occurs 123 This atmospheric dielectric breakdown results in the flow of electric current along a random trajectory through an ionized plasma channel between the two electrodes An example of this is natural lightning where one electrode is a virtual point in a cloud and the other is a point on Earth Laser Induced Plasma Channel LIPC research is presently underway using ultrafast lasers to artificially promote development of the plasma channel through the air directing the electric arc and guiding the current across a specific path in a controllable manner 124 The laser energy reduces the atmospheric dielectric breakdown voltage and the air is made less insulating by superheating which lowers the density p displaystyle p nbsp of the filament of air 125 This new process is being explored for use as a laser lightning rod and as a means to trigger lightning bolts from clouds for natural lightning channel studies 126 for artificial atmospheric propagation studies as a substitute for conventional radio antennas 127 for applications associated with electric welding and machining 128 129 for diverting power from high voltage capacitor discharges for directed energy weapon applications employing electrical conduction through a ground return path 130 131 132 133 and electronic jamming 134 Energy harvesting editMain article Energy harvesting In the context of wireless power energy harvesting also called power harvesting or energy scavenging is the conversion of ambient energy from the environment to electric power mainly to power small autonomous wireless electronic devices 135 The ambient energy may come from stray electric or magnetic fields or radio waves from nearby electrical equipment light thermal energy heat or kinetic energy such as vibration or motion of the device 135 Although the efficiency of conversion is usually low and the power gathered often minuscule milliwatts or microwatts 135 it can be adequate to run or recharge small micropower wireless devices such as remote sensors which are proliferating in many fields 135 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 136 137 History edit19th century developments and dead ends edit The 19th century saw many developments of theories and counter theories on how electrical energy might be transmitted In 1826 Andre Marie Ampere discovered a connection between current and magnets Michael Faraday described in 1831 with his law of induction the electromotive force driving a current in a conductor loop by a time varying magnetic flux Transmission of electrical energy without wires was observed by many inventors and experimenters 138 139 140 but lack of a coherent theory attributed these phenomena vaguely to electromagnetic induction 141 A concise explanation of these phenomena would come from the 1860s Maxwell s equations 51 by James Clerk Maxwell establishing a theory that unified electricity and magnetism to electromagnetism predicting the existence of electromagnetic waves as the wireless carrier of electromagnetic energy Around 1884 John Henry Poynting defined the Poynting vector and gave Poynting s theorem which describe the flow of power across an area within electromagnetic radiation and allow for a correct analysis of wireless power transfer systems 51 142 This was followed on by Heinrich Rudolf Hertz 1888 validation of the theory which included the evidence for radio waves 142 During the same period two schemes of wireless signaling were put forward by William Henry Ward 1871 and Mahlon Loomis 1872 that were based on the erroneous belief that there was an electrified atmospheric stratum accessible at low altitude 143 144 Both inventors patents noted this layer connected with a return path using Earth currents would allow for wireless telegraphy as well as supply power for the telegraph doing away with artificial batteries and could also be used for lighting heat and motive power 145 146 A more practical demonstration of wireless transmission via conduction came in Amos Dolbear s 1879 magneto electric telephone that used ground conduction to transmit over a distance of a quarter of a mile 147 Tesla edit nbsp Tesla demonstrating wireless transmission by electrostatic induction during an 1891 lecture at Columbia College The two metal sheets are connected to a Tesla coil oscillator which applies high voltage radio frequency alternating current An oscillating electric field between the sheets ionizes the low pressure gas in the two long Geissler tubes in his hands causing them to glow in a manner similar to neon tubes After 1890 inventor Nikola Tesla experimented with transmitting power by inductive and capacitive coupling using spark excited radio frequency resonant transformers now called Tesla coils which generated high AC voltages 51 53 148 Early on he attempted to develop a wireless lighting system based on near field inductive and capacitive coupling 53 and conducted a series of public demonstrations where he lit Geissler tubes and even incandescent light bulbs from across a stage 53 148 149 He found he could increase the distance at which he could light a lamp by using a receiving LC circuit tuned to resonance with the transmitter s LC circuit 52 using resonant inductive coupling 53 54 Tesla failed to make a commercial product out of his findings 150 but his resonant inductive coupling method is now widely used in electronics and is currently being applied to short range wireless power systems 53 151 nbsp nbsp 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 Tesla went on to develop a wireless power distribution system that he hoped would be capable of transmitting power long distance directly into homes and factories Early on he seemed to borrow from the ideas of Mahlon Loomis 152 153 proposing a system composed of balloons to suspend transmitting and receiving electrodes in the air above 30 000 feet 9 100 m in altitude where he thought the pressure would allow him to send high voltages millions of volts long distances To further study the conductive nature of low pressure air he set up a test facility at high altitude in Colorado Springs during 1899 154 155 156 Experiments he conducted there with a large coil operating in the megavolts range as well as observations he made of the electronic noise of lightning strikes led him to conclude incorrectly 157 147 that he could use the entire globe of the Earth to conduct electrical energy The theory included driving alternating current pulses into the Earth at its resonant frequency from a grounded Tesla coil working against an elevated capacitance to make the potential of the Earth oscillate Tesla thought this would allow alternating current to be received with a similar capacitive antenna tuned to resonance with it at any point on Earth with very little power loss 158 159 160 His observations also led him to believe a high voltage used in a coil at an elevation of a few hundred feet would break the air stratum down eliminating the need for miles of cable hanging on balloons to create his atmospheric return circuit 161 162 Tesla would go on the next year to propose a World Wireless System that was to broadcast both information and power worldwide 163 164 In 1901 at Shoreham New York he attempted to construct a large high voltage wireless power station now called Wardenclyffe Tower but by 1904 investment dried up and the facility was never completed Near field and non radiative technologies edit Inductive power transfer between nearby wire coils was the earliest wireless power technology to be developed existing since the transformer was developed in the 1800s Induction heating has been used since the early 1900s and is used for induction cooking 165 With the advent of cordless devices induction charging stands have been developed for appliances used in wet environments like electric toothbrushes and electric razors to eliminate the hazard of electric shock One of the earliest proposed applications of inductive transfer was to power electric locomotives 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 3 kHz 166 In the early 1960s resonant inductive wireless energy transfer was used successfully in implantable medical devices 167 including such devices as pacemakers and artificial hearts While the early systems used a resonant receiver coil later systems 168 implemented resonant transmitter coils as well These medical devices are designed for high efficiency using low power electronics while efficiently accommodating some misalignment and dynamic twisting of the coils The separation between the coils in implantable applications is commonly less than 20 cm Today resonant inductive energy transfer is regularly used for providing electric power in many commercially available medical implantable devices 169 The first passive RFID Radio Frequency Identification technologies were invented by Mario Cardullo 170 1973 and Koelle et al 171 1975 and by the 1990s were being used in proximity cards and contactless smartcards The proliferation of portable wireless communication devices such as mobile phones tablet and laptop computers in recent decades is currently driving the development of mid range wireless powering and charging technology to eliminate the need for these devices to be tethered to wall plugs during charging 172 The Wireless Power Consortium was established in 2008 to develop interoperable standards across manufacturers 172 Its Qi inductive power standard published in August 2009 enables high efficiency charging and powering of portable devices of up to 5 watts over distances of 4 cm 1 6 inches 173 The wireless device is placed on a flat charger plate which can 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 Soljacic at MIT used a dual resonance transmitter with a 25 cm diameter secondary tuned to 10 MHz to transfer 60 W of power to a similar dual resonance receiver over a distance of 2 meters 6 6 ft eight times the transmitter coil diameter at around 40 efficiency 53 56 In 2008 the team of Greg Leyh and Mike Kennan of Nevada Lightning Lab used a grounded dual resonance transmitter with a 57 cm diameter secondary tuned to 60 kHz and a similar grounded dual resonance receiver to transfer power through coupled electric fields with an earth current return circuit over a distance of 12 meters 39 ft 174 In 2011 Dr Christopher A Tucker and Professor Kevin Warwick of the University of Reading recreated Tesla s 1900 patent 0 645 576 in miniature and demonstrated power transmission over 4 meters 13 ft with a coil diameter of 10 centimetres 3 9 in at a resonant frequency of 27 50 MHz with an effective efficiency of 60 175 Microwaves and lasers edit Before World War II little progress was made in wireless power transmission 92 Radio was developed for communication uses but could not be used for power transmission since the relatively low frequency radio waves spread out in all directions and little energy reached the receiver 51 92 In radio communication at the receiver an amplifier intensifies a weak signal using energy from another source For power transmission efficient transmission required transmitters that could generate higher frequency microwaves which can be focused in narrow beams towards a receiver 51 92 176 The development of microwave technology during World War II such as the klystron and magnetron tubes and parabolic antennas 92 made radiative far field methods practical for the first time and the first long distance wireless power transmission was achieved in the 1960s by William C Brown 51 In 1964 Brown invented the rectenna which 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 92 A major motivation for microwave research in the 1970s and 1980s was to develop a solar power satellite 51 92 Conceived in 1968 by Peter Glaser this would harvest energy from sunlight using solar cells and beam it down to Earth as microwaves to huge rectennas which would convert it to electrical energy on the electric power grid 177 In landmark 1975 experiments as technical director of a JPL Raytheon program Brown demonstrated long range transmission by beaming 475 W of microwave power to a rectenna a mile away with a microwave to DC conversion efficiency of 54 178 At NASA s Jet Propulsion Laboratory he and Robert Dickinson transmitted 30 kW DC output power across 1 5 km with 2 38 GHz microwaves from a 26 m dish to a 7 3 x 3 5 m rectenna array The incident RF to DC conversion efficiency of the rectenna was 80 179 In 1983 Japan launched Microwave Ionosphere Nonlinear Interaction Experiment MINIX a rocket experiment to test transmission of high power microwaves through the ionosphere citation needed 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 92 which sponsored Brown s research In 1987 Canada s Communications Research Center developed a small prototype airplane called Stationary High Altitude Relay Platform SHARP to relay telecommunication data between points on earth similar to a communications satellite Powered by a rectenna it could fly at 13 miles 21 km 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 by photocells from a beam of infrared light from a ground based laser while a control system kept the laser pointed at the plane See also edit nbsp energy portal nbsp Physics portalBeam powered propulsion Beam Power Challenge one of the NASA Centennial Challenges Electricity distribution Electric power transmission Electromagnetic compatibility Electromagnetic radiation and health Energy harvesting Friis transmission equation Microwave power transmission Qi standard Space based solar power Resonant inductive coupling Thinned array curse uBeam acoustic energy transfer system Wardenclyffe Tower Wi Charge far field infrared wireless power World Wireless SystemReferences edit The pad senses when a phone is on it and turns on the field The pad uses a small amount of energy when not in use however in modern wireless systems this off power is very small compared to the power used when charging Hoffman Chris 15 September 2017 How Does Wireless Charging Work How To Geek How To Geek LLC Retrieved 11 January 2018 Ibrahim F N Jamail N A M Othman N A 2016 Development of wireless electricity transmission through resonant coupling 4th IET Clean Energy and Technology Conference CEAT 2016 pp 33 5 doi 10 1049 cp 2016 1290 ISBN 978 1 78561 238 1 Kracek Jan Mazanek Milos June 2011 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Page amp Cooper New York City 1916 Cited in Anderson Leland 1992 Nikola Tesla on His Work with Alternating Currents and Their Application to Wireless Telegraphy Telephony and Transmission of Power An Extended Interview Sun Publishing Company p 110 ISBN 978 1893817012 At that time I was absolutely sure that I could put up a commercial plant if I could do nothing else but what I had done in my laboratory on Houston Street but I had already calculated and found that I did not need great heights to apply this method My patent says that I break down the atmosphere at or near the terminal If my conducting atmosphere is 2 or 3 miles above the plant I consider this very near the terminal as compared to the distance of my receiving terminal which may be across the Pacific That is simply an expression I saw that I would be able to transmit power provided I could construct a certain apparatus and I have as I will show you later I have constructed and patented a form of apparatus which with a moderate elevation of a few hundred feet can break the air stratum down Carlson W Bernard 2013 Tesla Inventor of the Electrical Age Princeton University Press pp 302 367 ISBN 978 1400846559 Tesla Nikola June 1900 The Problem of Increasing Human Energy Century Magazine Retrieved 20 November 2014 Rudnev Valery Loveless Don Cook Raymond L 14 July 2017 Handbook of Induction Heating Second ed CRC Press ISBN 978 1351643764 United States 527857A Maurice Hutin amp Maurice Leblanc Transformer system for electric railways issued 23 October 1894 Schuder J C 2002 Powering an artificial heart Birth of the inductively coupled radio frequency system in 1960 Artificial Organs 26 11 909 915 doi 10 1046 j 1525 1594 2002 07130 x PMID 12406141 SCHWAN M A Troyk P R November 1989 High efficiency driver for transcutaneously coupled coils Images of the Twenty First Century Proceedings of the Annual International Engineering in Medicine and Biology Society pp 1403 1404 doi 10 1109 IEMBS 1989 96262 S2CID 61695765 a href Template Cite book html title Template Cite book cite book a journal ignored help What is a cochlear implant Cochlearamericas com 30 January 2009 Archived from the original on 24 December 2008 Retrieved 4 June 2009 United States 3713148A Mario W Cardullo amp William L Parks Transponder apparatus and system issued 23 January 1973 Koelle A R Depp S W Freyman R W 1975 Short range radio telemetry for electronic identification using modulated RF backscatter Proceedings of the IEEE 63 8 1260 1261 doi 10 1109 proc 1975 9928 a b Sayer Peter 19 December 2008 Wireless Power Consortium to Unleash Electronic Gadgets PCWorld Retrieved 8 December 2014 Global Qi Standard Powers Up Wireless Charging PRNewswire UBM plc 2 September 2009 Retrieved 8 December 2014 Leyh G E Kennan M D 28 September 2008 Efficient wireless transmission of power using resonators with coupled electric fields PDF 2008 40th North American Power Symposium NAPS 2008 40th North American Power Symposium Calgary 28 30 September 2008 IEEE pp 1 4 doi 10 1109 NAPS 2008 5307364 ISBN 978 1 4244 4283 6 Retrieved 20 November 2014 Tucker Christopher A Warwick Kevin Holderbaum William 2013 A contribution to the wireless transmission of power International Journal of Electrical Power amp Energy Systems 47 235 242 doi 10 1016 j ijepes 2012 10 066 Curty Jari Pascal Declercq Michel Dehollain Catherine Joehl Norbert 2006 Design and Optimization of Passive UHF RFID Systems Springer p 4 ISBN 978 0387447100 Glaser Peter E 22 November 1968 Power from the Sun Its future PDF Science 162 3856 857 861 Bibcode 1968Sci 162 857G doi 10 1126 science 162 3856 857 PMID 17769070 Retrieved 4 November 2014 Friend Michael Parise Ronald J Cutting the Cord ISTF 07 1726 Mainland High School Daytona Beach Florida Retrieved 7 October 2016 Dickinson R M 1976 Performance of a High Power 2 388 GHZ Receiving Array in Wireless Power Transmission over 1 54 km MTT S International Microwave Symposium Digest 76 139 141 doi 10 1109 mwsym 1976 1123672 Further reading editBooks and articles edit de Rooij Michael A 2015 Wireless Power Handbook Power Conversion Publications ISBN 978 0996649216 Latest work on AirFuel Alliance class 2 and class 3 transmitters adaptive tuning radiated EMI multi mode wireless power systems and control strategies Agbinya Johnson I Ed 2012 Wireless Power Transfer River Publishers ISBN 978 8792329233 a href Template Cite book html title Template Cite book cite book a CS1 maint multiple names authors list link Comprehensive theoretical engineering text Shinohara Naoki 2014 Wireless Power Transfer via Radiowaves John Wiley amp Sons ISBN 978 1118862964 Engineering text Kurs A Karalis A Moffatt R Joannopoulos J D Fisher P Soljacic M 6 July 2007 Wireless Power Transfer via Strongly Coupled Magnetic Resonances Science 317 5834 83 86 Bibcode 2007Sci 317 83K CiteSeerX 10 1 1 418 9645 doi 10 1126 science 1143254 PMID 17556549 S2CID 17105396 Thibault G 2014 Wireless Pasts and Wired Futures In J Hadlaw A Herman amp T Swiss Eds Theories of the Mobile Internet Materialities and Imaginaries pp 126 154 London Routledge A short cultural history of wireless power Patents edit U S Patent 4 955 562 Microwave powered aircraft John E Martin et al 1990 U S Patent 3 933 323 Solid state solar to microwave energy converter system and apparatus Kenneth W Dudley et al 1976 U S Patent 3 535 543 Microwave power receiving antenna Carroll C Dailey 1970 External links editHowstuffworks How Wireless Power Works describes near range and mid range wireless power transmission using induction and radiation techniques Microwave Power Transmission its history before 1980 The Stationary High Altitude Relay Platform SHARP microwave beam powered Marin Soljacic s MIT WiTricity wireless power transmission pages Rezence official site of a wireless power standard promoted by the Alliance for Wireless Power Qi official site of a wireless power standard promoted by the Wireless Power Consortium PMA official site of a wireless power standard promoted by the Power Matters Alliance WiPow official site of the WiPow Coalition promoting standardized wireless power for medical mobility and wheeled devices Retrieved from https en wikipedia org w index php title Wireless power transfer amp oldid 1189520771, wikipedia, wiki, book, books, library,

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