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Wireless power transfer (WPT) or wireless energy transmission is the transmission of electrical power from a power source to a consuming device without using solid wires or conductors. It is a generic term that refers to a number of different power transmission technologies that use time-varying electromagnetic fields. Wireless transmission is useful to power electrical devices in cases where interconnecting wires are inconvenient, hazardous, or are not possible. In wireless power transfer, a transmitter device connected to a power source, such as the mains power line, transmits power by electromagnetic fields across an intervening space to one or more receiver devices, where it is converted back to electric power and utilized.

Wireless power techniques fall into two categories, non-radiative and radiative. In near-field or non-radiative techniques, power is transferred over short distances by magnetic fields using inductive coupling between coils of wire or in a few devices by electric fields using capacitive coupling between electrodes. Applications of this type are electric toothbrush chargers, RFID tags, smartcards, and chargers for implantable medical devices like cardiac pacemakers, and inductive powering or charging of electric vehicles like trains or busses. A current focus is to develop wireless systems to charge mobile and handheld computing devices such as cellphones, digital music player and portable computers without being tethered to a wall plug. With the radiative or far-field techniques, also called power beaming, electrical energy is transmitted by beams of electromagnetic radiation, like microwaves or laser beams. These techniques can transport energy longer distances but must be aimed at the receiver. Proposed applications for this type are solar power satellites, and wireless powered drone aircraft. An important issue associated with all wireless power systems is limiting the exposure of people and other living things to potentially injurious electromagnetic fields (see Electromagnetic radiation and health).

Overview

"Wireless power transfer" is a collective term that refers to a number of different technologies for the transmission of electrical energy. The technologies are listed in the table below. They differ widely in the distance over which they can transmit power efficiently and in the type of field energy they use: a time-varying magnetic field, a time-varying electric field, a rotating magnetic field, a bound-mode EM surface wave, or electromagnetic radiation in the form of radio waves, microwaves, infrared radiation or visible light.

A typical wireless power system consists of a source of electrical energy, such as an AC power system, connected to a "transmitter" that converts the power to electrical field energy and one or more "receivers" that interact with the transmitted field energy and convert it back to electrical power that is consumed by an electrical load. On the transmitter side the input power is processed and then converted to field energy by an interface component, which may be a coil of wire that produces a magnetic field, terminal electrodes that produce an electric field, a permanent magnet that produces a magnetic field, an antenna that radiates radio waves, or a laser that emits light. A similar or complimentary interface component on the receiver side converts the field energy back to electrical power.

An important parameter that determines the type of wave is the frequency f in hertz of the oscillations. The frequency determines the wavelength λ = c/f of the waves which carry the energy across the gap, where c is the velocity of light. Two additional parameters instrumental in determining the type of wave are the time-variation of the wave (given by its angular frequency ω) and the spatial variation of the wave (given by its wave vector kx). Purely transverse electromagnetic space waves, with synchronized electric and magnetic fields perpendicular to the direction of propagation, can only exist for ω > ωp (the plasma frequency). ωp is the resonant frequency of free electrons in the conductor or conductors in response to an electrical excitation. For ω < ωp, the wave-vector becomes imaginary, giving an exponentially decaying surface wave instead of a propagating space wave. The field intensity of the surface wave is at a maximum at the earth-atmosphere interface and exponentially decays away from the surface. Both of these electromagnetic waves can be mathematically described by solving Maxwell's equations at a metal-dielectric interface.

Radiative wireless power systems use the same propagation mode as wireless communication systems, like radio and television broadcasting, cellular telephone systems, and WiFi; everyday technologies that involve the transmission of electrical energy without wires by means of electromagnetic radiation. In the case of wireless telecommunications the goal is the transmission of information, and the amount of power reaching the receiver is not so important, as long as the signal to noise ratio is high enough that the data can be received intelligibly. With most present day wireless telecommunications technologies, only a small amount of the transmitted energy reaches the receiver. By contrast, in wireless power the amount of energy received is of greater significance, so the efficiency (percentage of transmitted energy that is received) is the more important parameter. A large portion of the energy sent out by the transmitter must arrive at the receiver or receivers to make the system economical. For this reason a wireless power technology may be limited by distance more than wireless telecommunication technologies.

These are the different wireless power technologies:>

Field regions

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. These fields contain energy. The above fields cannot carry power because they are static , but time-varying fields can. 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 D_range from the antenna. The fields have different characteristics in these regions, and different technologies are used for transmitting power:

• Near-field or nonradiative region - This means the area within about 1 wavelength (λ) of the antenna. In this region the oscillating electric and magnetic fields are separate and power can be transferred via electric fields by capacitive coupling (electrostatic induction) between metal electrodes, or via magnetic fields by inductive coupling (electromagnetic induction) between coils of wire. These fields are not radiative, meaning the energy stays within a short distance of the transmitter. If there is no receiving device or absorbing material within their limited range to "couple" to, no power leaves the transmitter. 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, so if the distance between the two "antennas" D_range is much larger than the diameter of the "antennas" D_ant very little power will be received. Therefore these techniques cannot be used for long distance power transmission.

Resonance, such as resonant inductive coupling, can increase the coupling between the antennas greatly, allowing efficient transmission at somewhat greater distances, although the fields still decrease exponentially. Therefore the range of near-field devices falls into one of two categories:

• • Short range - up to about one antenna diameter: D_range ≤ D_ant. 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: D_range ≤ 10 D_ant. This is the range over which resonant coupling can transfer practical amounts of power.
  • Medium range or reactive near-field zone – This is the distance up to the outer boundary of the reactive near-field region, commonly considered to be a distance of 1∕2π times the wavelength λ (λ∕2π or 0.159 × λ) from the antenna surface.

• 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. This part of the energy is radiative, 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 D_ant to the wavelength of the waves λ, 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, D_ant << 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 D_ant ≈ 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. Therefore these can be used for short range, inefficient power transmission but not for long range transmission.

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. 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: D_ant >> λ = c/f. Practical beam power devices require wavelengths in the centimeter region or below, corresponding to frequencies above 1 GHz, in the microwave range or above.

Non-radiative techniques

Electromagnetic induction

There are two forms of energy transfer by electromagnetic induction. These are magnetic inductive coupling and capacitive inductive coupling. Magnetic coupling is further classified as inductive coupling and resonant inductive coupling.

Magnetic Inductive coupling

Inductive coupling

The direct inductive coupling technique relies on the use of a magnetic field produced by an electric current in a wire coil, called the primary, to induce a current in a second coil in close proximity, called the secondary. This action of an electrical transformer is the simplest form of wireless power transmission. The primary coil and secondary coil of a transformer are not directly connected; each coil is part of a separate circuit. Energy transfer takes place through a process known as mutual induction. The principal functions are stepping the primary voltage either up or down and electrical isolation. As the spacing between the primary and secondary is increased, more and more of the primary's magnetic field misses the secondary. Even over a relatively short distance, direct inductive coupling is grossly inefficient, wasting much of the transmitted energy. The main drawback to this basic form of wireless transmission is its extremely short range. The receiver coil must be concentric with or directly adjacent to the transmitter coil or induction unit in order to efficiently couple with it. Applications of the induction technique includes electric toothbrush and electric razor chargers, induction stove tops and industrial induction heaters.

Resonant inductive coupling

The resonant inductive coupling or electrodynamic induction technique also relies on the use of a magnetic field produced by an electric current in a primary coil to induce a current in a secondary coil. When resonant coupling is used, both the transmitter and receiver coils are tuned to a common resonant frequency by the addition of parallel capacitors, forming a pair of LC circuits. The application of resonance increases the transmission range. Performance can be further improved by modifying the drive current from a sinusoidal to a non-sinusoidal transient waveform. In this way significant power can be transmitted between two mutually-attuned LC circuits having a relatively low coefficient of coupling.

A common use of this technique is the charging of battery powered mobile or handheld devices, such as digital music players, smart phones, tablets, and laptop computers without being tethered to an plug-in AC/DC adapter battery charger. A localized charging technique selects the appropriate transmitting coil in a multilayer winding array structure. Resonance is used in both the wireless charging pad (the transmitter circuit) and the receiver module (embedded in the load) to maximize energy transfer efficiency. Battery-powered devices fitted with a special receiver module can then be charged simply by placing them on a wireless charging pad. Resonant inductive coupling has been adopted as part of the Qi wireless charging standard. Some additional applications are RFID tag and reader systems, smartcard and scanner systems, charging systems for implantable battery-powered medical devices like cardiac pacemakers, the stationary charging of battery-powered electric vehicles such as electric cars, and the powering of trains and rail cars. This technology is also used for powering passive devices with very low energy requirements, such as RFID tags and contactless smartcards. Instead of relying on each of many thousands or millions of RFID tags or smartcards to contain a working battery, the method can provide power as needed, as the device is being scanned.

Capacitive coupling

Electrostatic induction or capacitive coupling is the passage of electric field energy through a dielectric. The action of a capacitor involves the transfer of energy between two conductive plates through a dielectric by means of an electric field. If a time-varying voltage is applied across the leads of a capacitor, a displacement current can flow. When a high-voltage, high-frequency alternating current is applied to two metal plates separated by a distance, a cold cathode gas discharge or fluorescent tube positioned in proximity of the two charged surfaces can be illuminated because the electrostatic field energy ionizes the gas in the tube creating plasma. One low power application of this technology is energy transfer between substrate layers on large-scale integrated circuit devices.

Magnetodynamic coupling

Any permanent magnet that is exposed to an external magnetic field will be subject to a force which, as well as moving the permanent magnet, acts to align the magnetic field in the permanent magnet with the field of the external force. This is described by the equation for force on a dipole as magnetic torque. If the allowed motion of the permanent magnet is restricted, such as a magnet restricted to motion along an axis and magnetized along that axis, then a degree of motion and rotation will be allowed within limits. If the external magnetic field is time-varying then the permanent magnet will move within its allowed range of motion. In the example of a magnet restricted to a single axis, producing an alternating magnetic field along this axis will cause the permanent magnet to travel backward and forward on the axis. If a coil is placed near this permanent magnet, the change in magnetic flux will induce an electromotive force in the coil according to Faraday's law of induction, to which a load may be connected in order to cause current flow, using the same principle as an alternator. The external field in a magnetically-coupled system may also be the field produced by a permanent magnet. Here the field produced by this magnet is approximated as a magnetic dipole with some magnetization, m, aligned in a given direction. For the second magnet, which is allowed to move freely, there will be a force of attraction and a force acting to rotate the magnet.

In the case of two magnets which are restricted to rotate around parallel axes, when the first magnet is rotated a torque will be produced on the second magnet causing it to align with the first magnet. This can be described similarly to a system of gears, where the magnets are essentially two meshed gears with a 1:1 ratio. As the first magnet continues to rotate, the second magnet will also rotate synchronously. In this kind of a system, the power used to rotate the first magnet can be extracted as electrical energy through the coils surrounding the second magnet. The amount of power transferred across the gap between magnets is a function of the torque, which is a function of magnetic moment, and the rotating frequency of the magnets. In this way, electrical power may be transferred across an air gap at high efficiency, equivalent to or greater than that of a resonant inductively coupled system, and has been demonstrated previously at the kW scale over short distances

Bound-mode EM surface wave

The wireless transmission of electrical energy is by a bound-mode EM surface wave between ground terminal electrodes with an equivalent time-varying electrical displacement associated with paired air terminal electrodes. This technique depends upon the electrical conductivity of Earth, that is to say, the spherical conducting terrestrial transmission line. Energy transmission is achieved by charging and discharging the air terminal electrode of a grounded resonance transformer electrical oscillator transmitter, generating an alternating electric field. This electric field energy can couple with the air terminal electrode of a similarly designed grounded resonance transformer electrical energy receiver tuned to the same frequency. Electrical energy is transferred between the transmitter and receiver by electrical conduction between the ground terminal electrodes when this coupling is established. This form of wireless transmission, in which alternating current electricity passes through the earth with an equivalent electrical displacement through the air above it, was demonstrated in 2008 over distances up to 12 meters, achieving power transmission efficiencies superior to the resonant electrical induction method.

Far-field or radiative techniques

Far field methods achieve longer ranges, often multiple kilometer ranges, where the distance is much greater than the diameter of the device(s). The main reason for longer ranges with radio wave and optical devices is the fact that electromagnetic radiation in the far-field can be made to match the shape of the receiving area (using high directivity antennas or well-collimated laser beams). 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 criterion dictates that any radio wave, microwave or laser beam will spread and become weaker and diffuse over distance; the larger the transmitter antenna or laser aperture compared to the wavelength of radiation, the tighter the beam and the less it will spread as a function of distance (and vice versa). Smaller antennae also suffer from excessive losses due to side lobes. However, the concept of laser aperture considerably differs from an antenna. Typically, a laser aperture much larger than the wavelength induces multi-moded radiation and mostly collimators are used before emitted radiation couples into a fiber or into space.

Ultimately, beamwidth is physically determined by diffraction due to the dish size in relation to the wavelength of the electromagnetic radiation used to make the beam.

Microwave power beaming can be more efficient than lasers, and is less prone to atmospheric attenuation caused by dust or water vapor losing atmosphere to vaporize the water in contact.

Then 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

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. A rectenna may be used to convert the microwave energy back into electricity. Rectenna conversion efficiencies exceeding 95% have been realized. Power beaming using microwaves has been proposed for the transmission of energy from orbiting solar power satellites to Earth and the beaming of power to spacecraft leaving orbit has been considered.

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-km diameter transmitting antenna, and a 10 km diameter receiving rectenna, for a microwave beam at 2.45 GHz. 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/cm 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.

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

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.

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

Under experimental conditions microwave conversion efficiency was measured to be around 54%.

More recently 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.

Lasers

In the case of electromagnetic radiation closer to the visible region of the spectrum (tens of micrometers to tens of nanometres), power can be transmitted by converting electricity into a laser beam that is then pointed at a photovoltaic cell. This mechanism is generally known as "power beaming" because the power is beamed at a receiver that can convert it to electrical energy.

Compared to other wireless methods:

• Collimated monochromatic wavefront propagation allows narrow beam cross-section area for transmission over large distances.
• 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. Low power levels can blind humans and other animals. High power levels can kill through localized spot heating.
• Conversion between electricity and light is inefficient. Photovoltaic cells achieve only 40%–50% efficiency. (Efficiency is higher with monochromatic light than with solar panels).
• Atmospheric absorption, and absorption and scattering by clouds, fog, rain, etc., causes up to 100% losses.
• Requires a direct line of sight with the target.

Laser "powerbeaming" technology has been mostly explored in military weapons and aerospace applications and is now being developed for commercial and consumer electronics. Wireless energy transfer systems using lasers for consumer space have to satisfy laser safety requirements standardized under IEC 60825.

Other details include propagation, and the coherence and the range limitation problem.

Geoffrey Landis is one of the pioneers of solar power satellites 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.

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

History

In 1862 James Clerk Maxwell synthesized previous observations, experiments and equations of electricity, magnetism and optics into a consistent theory, deriving Maxwell's equations. This set of partial differential equations forms the basis for modern electromagnetics including the wireless transmission of electrical energy. In 1884 John Henry Poynting developed equations for the flow of power in an electromagnetic field, Poynting's theorem and the Poynting vector, which are used in the analysis of wireless power systems. In 1888 Heinrich Rudolf Hertz experimentally confirmed the existence of electromagnetic radiation. Hertz’s apparatus for generating electromagnetic waves was a VHF or UHF radio wave spark gap transmitter.

Tesla’s experiments

Nikola Tesla may have done more to popularize the idea of wireless transmission than any other person of the 20th century. He began with the development of a radio frequency resonant transformer, known as the Tesla coil in 1891. Between 1891 and 1899 he demonstrated wireless energy transmission both publicly during lectures in New York, Chicago, St. Louis, Philadelphia, London, and Paris, and privately at his Manhattan laboratories by means of electrodynamic induction, electrostatic induction, electromagnetic radiation or radio waves, and the bound-mode EM surface wave. In demonstrations before the American Institute of Electrical Engineers and at the 1893 Columbian Exposition in Chicago he lit light bulbs from across a stage. He found the transmission-reception distance could be increased by tuning the receiver to resonate with the transmitter.

In 1899 Tesla shifted his wireless transmission research to Colorado Springs, Colorado to work out data for the construction of Wardenclyffe, a large commercial plant to be built on Long Island, New York. The facility was designed for trans-Atlantic wireless telecommunications based upon disturbed charge of ground and air method engineering and the related patents.

The plant in Colorado was merely designed in the same sense as a naval constructor designs first a small model to ascertain all the quantities before he embarks on the construction of a big vessel.

In one demonstration at the Colorado Springs Experimental Station, three incandescent lamps were lit by resonant inductive coupling at a distance of about 100 feet (30 m). Coupling between resonant circuits by electric or magnetic fields is now a familiar technology used throughout electronics. Resonant inductive coupling is once again of interest for short-range wireless power transmission. As mentioned above it is a "near-field" effect,, so, as Tesla discovered in 1899, it is not suitable for the transmission of electrical energy over long distances. While in Colorado he wrote, "the inferiority of the induction method would appear immense as compared with the disturbed charge of ground and air method."

In 1900 Tesla received the patents SYSTEM OF TRANSMISSION OF ELECTRICAL ENERGY and APPARATUS FOR TRANSMISSION OF ELECTRICAL ENERGY. These two patents describe hypothetical wireless stations with air terminal electrodes raised to more than 30,000 feet (9,100 m) elevation, along with the claim that electric field energy can be made to pass over long distances by conduction between elevated terminals maintained at this altitude. Another claim was that such high elevation of the air terminals is not needed. Modern demonstrations of this wireless power transfer method show that incandescent lamps can be lit at medium-range distances. The transmitted energy can be detected at great distances.

Tesla’s theory of operation states, the periodic charging and discharging of a resonance transformer transmitter's air terminal electrode periodically alters Earth's electrostatic charge.

Starting from two facts that the earth is a conductor insulated in space, and that a body cannot be charged without causing an equivalent displacement of electricity in the earth, I undertook to construct a machine suited for creating as large a displacement as possible of the earth's electricity.

This redistribution of charge results in the passage of electric current through the ground along with an accompanying guided surface wave. Tesla believed that with sufficient transmitter power output, Earth’s electrostatic potential can disturbed over its entire surface.

In 1901 Tesla began construction of the Wardenclyffe power plant and tower, a wireless telecommunications facility in Shoreham, New York, intended as the prototype station for the World Wireless System, based upon the principle of terrestrial electrical conductivity and his theory of earth resonance.

The only known report of the long-distance transmission and reception of electrical energy by Tesla himself is a statement made to attorney Drury W. Cooper in 1916 that in 1899 he collected quantitative transmission-reception data at a distance of about 10 miles (16 km). Two reports by others of Tesla having achieved long-distance power transmission have been found. The first is the purported wireless operation of lamps and electric motors at a distance of 15 miles (24 km). The second is an assertion by Tesla biographer John J. O'Neill, said to be pieced together from "fragmentary material . . . in a number of publications," that in 1899 Tesla lit 200 incandescent lamps at a distance of 26 miles (42 km). There is no independent confirmation of these two supposed demonstrations. Tesla did not mention them, and they do not appear in his meticulously kept laboratory notes.

Over one-hundred years have passed since his original work and there is no documentation of the Tesla wireless system apparatus ever having been replicated, other than by Leyh and Kennan, and no published reports exist of any attempt to achieve long distance wireless energy transfer by this means. A number of individuals have expressed their opinion that Tesla wireless system technology cannot possibly work. While Tesla's wireless energy transfer scheme remains only a fascinating dream for some, modern demonstrations have validated the basic concept over medium range distancesand mathematical analysis suggest that long distance wireless telecommunications by its means is feasible.

Microwaves

Before World War 2, little progress was made in wireless power transmission. Radio was developed for communication uses, but couldn't be used for power transmission due to the fact that the relatively low-frequency radio waves spread out in all directions and little energy reached the receiver. 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.

The development of microwave technology during World War 2, such as the klystron and magnetron tubes and parabolic antennas 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. 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. A major motivation for microwave research in the 1970s and 80s was to develop a solar power satellite. 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. In landmark 1975 high power experiments, Brown demonstrated short range transmission of 475 W of microwaves at 54% DC to DC efficiency, and he and Robert Dickinson at NASA's Jet Propulsion Laboratory transmitted 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%. In 1983 Japan launched MINIX (Microwave Ionosphere Nonlinear Interation Experiment), a rocket experiment to test transmission of high power microwaves through the ionosphere.

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 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 communication 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.

Near-field technologies

Inductive power transfer between nearby coils of wire is an old technology, existing since the transformer was developed in the 1800s. Induction heating has been used for 100 years. With the advent of cordless appliances, inductive charging stands were developed for appliances used in wet environments like electric toothbrushes and electric razors to reduce the hazard of electric shock.

One field to which inductive transfer has been applied is to power electric vehicles. In 1892 Maurice Hutin and Maurice Leblanc patented a wireless method of powering railroad trains using resonant coils inductively coupled to a track wire at 3 kHz. The first passive RFID (Radio Frequency Identification) technologies were invented by Mario Cardullo (1973) and Koelle et al. (1975) and by the 1990s were being used in proximity cards and contactless smartcards.

The proliferation of portable wireless communication devices such as cellphones, tablet, and laptop computers in recent decades is currently driving the development of wireless powering and charging technology to eliminate the need for these devices to be tethered to wall plugs during charging. The Wireless Power Consortium was established in 2008 to develop interoperable standards across manufacturers. Its Qi inductive power standard published in August 2009 enables charging and powering of portable devices of up to 5 watts over distances of 4 cm (1.6 inches). The wireless device is placed on a flat charger plate (which could be embedded in table tops at cafes, for example) and power is transferred from a flat coil in the charger to a similar one in the device.

In 2007, a team led by Marin Soljačić at MIT used coupled tuned circuits made of a 25 cm resonant coil at 10 MHz to transfer 60 W of power over a distance of 2 meters (6.6 ft) (8 times the coil diameter) at around 40% efficiency. This technology is being commercialized as WiTricity.

See also

• Beam-powered propulsion
• Beam Power Challenge – one of the NASA Centennial Challenges
• Differential capacitance
• Dispersion relation
• Distributed generation
• Electricity distribution
• Electric power transmission
• Electromagnetic compatibility
• Electromagnetic radiation and health
• Energy harvesting
• Fermi gas
• Free electron model
• Friis transmission equation
• Microwave power transmission
• Multidimensional systems
• Resonant inductive coupling
• Surface plasmon
• Surface plasmon polariton
• Surface wave
• Thinned array curse
• Transmission medium
• Wardenclyffe Tower
• Wave vector
• Zenneck wave

Further reading

Books and Articles

• Steinmetz, C. P. (1914). Elementary lectures on electric discharges, waves and impulses, and other transients. New York: McGraw-Hill Book Co., Inc. An historic electrical engineering treatise.
• Agbinya, Johnson I., Ed. (2012). Wireless Power Transfer. River Publishers. ISBN 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 1118862961. Engineering text
• Tomar, Anuradha; Gupta, Sunil (July 2012). "Wireless power Transmission: Applications and Components". International Journal of Engineering Research & Technology. 1 (5). ESRSA Publications Pvt. Ltd.: 1–8. ISSN 2278-0181. Brief survey of state of wireless power and applications
• Kurs, André; Karalis, Aristeidis; Moffatt, Robert (July 2007). "Wireless Power Transfer via Strongly Coupled Magnetic Resonances" (PDF). Science. 317. American Association for the Advancement of Science: 83–85. doi:10.1126/science.1143254. ISSN 1095-9203. Landmark paper on MIT team's 2007 development of mid-range resonant wireless transmission
• 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

• U.S. patent 787,412, Art of transmitting electrical energy through the natural mediums, Nikola Tesla (1905).
• U.S. patent 1,119,732, Apparatus for transmitting electrical energy, Nikola Tesla (1914).
• U.S. patent 3,535,543, Microwave power receiving antenna, Carroll C. Dailey (1970).
• 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 4,955,562, Microwave powered aircraft, John E. Martin, et al. (1990).
• U.S. patent 7,164,255, Inductive battery charger system with primary transformer windings formed in a multi-layer structure, Shu-Yuen Ron Hui (2007).

References

1. ^ Shinohara, Naoki (2014). Wireless Power Transfer via Radiowaves. John Wiley & Sons. pp. ix–xiii. ISBN 1118862961.
2. Bush, Stephen F. (2014). Smart Grid: Communication-Enabled Intelligence for the Electric Power Grid. John Wiley & Sons. p. 118. ISBN 1118820231.
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    A high-frequency and high-voltage driver source excites the resonant transmitter to generate an alternating electric field which can couple with the resonant receiver. Energy will be delivered as soon as this coupling relation is set up.

44. 2008 North American Power Symposium.
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     Figure 7, "EXPERIMENT TO ILLUSTRATE AN INDUCTIVE EFFECT OF AN ELECTRICAL OSCILLATOR OF GREAT POWER - The photograph shows three ordinary incandescent lamps lighted to full candle-power by currents induced in a local loop consisting of a single wire forming a square of fifty feet each side, which includes the lamps, and which is at a distance of one hundred feet from the primary circuit energized by the oscillator. The loop likewise includes an electrical condenser, and is exactly attuned to the vibrations of the oscillator, which is worked at less than five percent of its total capacity.

101. 5 June 1899, ‘‘Nikola Tesla  Colorado Springs Notes  1899–1900’’, Nolit, 1978
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     My experiments showed that at a height of 5 miles the air was in a condition to transmit the energy in this way, but my experiments in Colorado showed that at a height of 1 mile it is plenty enough rarefied to break down under the stress and conduct the current to the distant points.
     

         I have to say here that when I filed the applications of September 2, 1897, for the transmission of energy in which this method was disclosed, it was already clear to me that I did not need to have terminals at such high elevation, but I never have, above my signature, announced anything that I did not prove first. That is the reason why no statement of mine was ever contradicted, and I do not think it will be, because whenever I publish something I go through it first by experiment, then from experiment I calculate, and when I have the theory and practice meet I announce the results.
     

         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. . . . 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. . . .

103. Cooper, Drury W., internal document of the law firm Kerr, Page & Cooper, New York City, 1916.

     Counsel: What was the distance of the receiver from the sending station in the Colorado test?
     

     Tesla: Well, these distances were small, for the reason that they were merely intended to give me quantitative data.
     Counsel: Could you give the number of miles, approximately?
     

     Tesla: Oh, 10 miles or so.

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External links

• Howstuffworks "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 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

• Emerging technologies
• Energy development
• Electric power distribution
• Electromagnetic compatibility
• Microwave transmission
• Wireless energy transfer