Wireless power

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Wireless power or wireless energy transmission is the transmission of electrical energy from a power source to an electrical load without man-made conductors. Wireless transmission is useful in cases where interconnecting wires are inconvenient, hazardous, or impossible. The problem of wireless power transmission differs from that of wireless telecommunications, such as radio. In the latter, the proportion of energy received becomes critical only if it is too low for the signal to be distinguished from the background noise. With wireless power, efficiency is the more significant parameter.[citation needed] A large part of the energy sent out by the generating plant must arrive at the receiver or receivers to make the system economical.

The most common form of wireless power transmission is carried out using direct induction followed by resonant magnetic induction. Other methods under consideration are electromagnetic radiation in the form of microwaves or lasers[1] and electrical conduction through natural media.[2]

Electric energy transfer[edit]

An electric current flowing through a conductor, such as a wire, carries electrical energy. When an electric current passes through a circuit there is an electric field in the dielectric surrounding the conductor; magnetic field lines around the conductor and lines of electric force radially about the conductor.[3]

In a direct current circuit, if the current is continuous, the fields are constant; there is a condition of stress in the space surrounding the conductor, which represents stored electric and magnetic energy, just as a compressed spring or a moving mass represents stored energy. In an alternating current circuit, however, the fields also alternate; that is, with every half wave of current and of voltage, the magnetic and the electric field start at the conductor and run outwards into space with the speed of light.[4] Where these alternating fields impinge on another conductor a voltage and a current are induced.[3] respectively in any dielectric substance, a field of charges is enforced, with a current in relaxation.

Any change in the electrical conditions of the circuit, whether internal[5] or external[6] involves a readjustment of the stored magnetic and electric field energy of the circuit, that is, a so-called transient. A transient is of the general character of a condenser discharge through an inductive circuit. The phenomenon of the condenser discharge through an inductive circuit therefore is of the greatest importance to the engineer, as the foremost cause of high-voltage and high-frequency troubles in electric circuits.[7]

Electromagnetic induction is proportional to the intensity of the current and voltage in the conductor which produces the fields and to the frequency. The higher the frequency the more intense the inductive effect. Energy is transferred from a conductor that produces the fields (the primary) to any conductor on which the fields impinge (the secondary). Part of the energy of the primary conductor passes inductively across space into secondary conductor and the energy decreases rapidly along the primary conductor. A high frequency current does not pass for long distances along a conductor but rapidly transfers its energy by induction to adjacent conductors. Higher induction resulting from the higher frequency is the explanation of the apparent difference in the propagation of high frequency disturbances from the propagation of the low frequency power of alternating current systems. The higher the frequency the more preponderant become the inductive effects that transfer energy from circuit to circuit across space. The more rapidly the energy decreases and the current dies out along the circuit, the more local is the phenomenon.[3]

The flow of electric energy thus comprises phenomena inside the conductor[8] and phenomena in the space outside the conductor—the electric field—which, in a continuous current circuit, is a condition of steady magnetic and dielectric stress, and in an alternating current circuit is alternating, that is, an electric wave launched by the conductor[3] to become far-field electromagnetic radiation traveling through space with the speed of light.

In electric power transmission and distribution, the phenomena inside the conductor are of main importance, and the electric field of the conductor is usually observed only incidentally.[9] Inversely, in the use of electric power for radio telecommunications it is only the electric and magnetic fields outside of the conductor, that is far-field electromagnetic radiation, which is of importance in transmitting the message. The phenomenon in the conductor, the current in the launching structure, is not used.[3]

The electric charge displacement in the conductor produces a magnetic field and resultant lines of electric force. The magnetic field is a maximum in the direction concentric, or approximately so, to the conductor. That is, a ferromagnetic body[10] tends to set itself in a direction at right angles to the conductor. The electric field has a maximum in a direction radial, or approximately so, to the conductor. The electric field component tends in a direction radial to the conductor and dielectric bodies may be attracted or repelled radially to the conductor.[11]

The electric field of a circuit over which energy flows has three main axes at right angles with each other:

  1. The magnetic field, concentric with the conductor.
  2. The lines of electric force, radial to the conductor.
  3. The power gradient, parallel to the conductor.

Where the electric circuit consists of several conductors, the electric fields of the conductors superimpose upon each other, and the resultant magnetic field lines and lines of electric force are not concentric and radial respectively, except approximately in the immediate neighborhood of the conductor. Between parallel conductors they are conjugate of circles. Neither the power consumption in the conductor, nor the magnetic field, nor the electric field, are proportional to the flow of energy through the circuit. However, the product of the intensity of the magnetic field and the intensity of the electric field is proportional to the flow of energy or the power, and the power is therefore resolved into a product of the two components i and e, which are chosen proportional respectively to the intensity of the magnetic field and of the electric field. The component called the current is defined as that factor of the electric power which is proportional to the magnetic field, and the other component, called the voltage, is defined as that factor of the electric power which is proportional to the electric field.[11]

In radio telecommunications the electric field of the transmit antenna propagates through space as a radio wave and impinges upon the receive antenna where it is observed by its magnetic and electric effect.[11] Radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X rays and gamma rays are shown to be the same electromagnetic radiation phenomenon, differing one from the other only in frequency of vibration.[3][12]

Electromagnetic induction[edit]

Energy transfer by electromagnetic induction is typically magnetic but capacitive coupling can also be achieved.

Electrodynamic induction method[edit]

The electrodynamic induction wireless transmission technique is near field over distances up to about one-sixth of the wavelength used. Near field energy itself is non-radiative but some radiative losses do occur. In addition there are usually resistive losses. With electrodynamic induction, electric current flowing through a primary coil creates a magnetic field that acts on a secondary coil producing a current within it. Coupling must be tight in order to achieve high efficiency. As the distance from the primary is increased, more and more of the magnetic field misses the secondary. Even over a relatively short range the inductive coupling is grossly inefficient, wasting much of the transmitted energy.[13]

This action of an electrical transformer is the simplest form of wireless power transmission. The primary and secondary circuits of a transformer are not directly connected. Energy transfer takes place through a process known as mutual induction. Principal functions are stepping the primary voltage either up or down and electrical isolation. Mobile phone and electric toothbrush battery chargers, and electrical power distribution transformers are examples of how this principle is used. Induction cookers use this method. The main drawback to this basic form of wireless transmission is short range. The receiver must be directly adjacent to the transmitter or induction unit in order to efficiently couple with it.

The application of resonance increases the transmission range somewhat. When resonant coupling is used, the transmitter and receiver inductors are tuned to the same natural frequency. Performance can be further improved by modifying the drive current from a sinusoidal to a nonsinusoidal transient waveform.[14] In this way significant power may be transmitted between two mutually-attuned LC circuits having a relatively low coefficient of coupling. Transmitting and receiving coils are usually single layer solenoids or flat spirals with parallel capacitors, which, in combination, allow the receiving element to be tuned to the transmitter frequency.

Common uses of resonance-enhanced electrodynamic induction[15] are charging the batteries of portable devices such as laptop computers and cell phones, medical implants and electric vehicles.[16][17][18] A localized charging technique[19] selects the appropriate transmitting coil in a multilayer winding array structure.[20] 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. This approach is suitable for universal wireless charging pads for portable electronics such as mobile phones. It has been adopted as part of the Qi wireless charging standard.

It is also used for powering devices having no batteries, such as RFID patches and contactless smartcards, and to couple electrical energy from the primary inductor to the helical resonator of Tesla coil wireless power transmitters.

Electrostatic induction method[edit]

Main article: Capacitive coupling
The illumination of two exhausted tubes by means of a powerful, rapidly alternating electrostatic field created between two vertical metal sheets suspended from the ceiling on insulating cords. This involves the physics of electrostatic induction.[21][22][23]

Electrostatic induction or capacitive coupling is the passage of electrical energy through a dielectric. In practice it is an electric field gradient or differential capacitance between two or more insulated terminals, plates, electrodes, or nodes that are elevated over a conducting ground plane. The electric field is created by charging the plates with a high potential, high frequency alternating current power supply. The capacitance between two elevated terminals and a powered device form a voltage divider.

The electric energy transmitted by means of electrostatic induction can be utilized by a receiving device, such as a wireless lamp.[24][25][26] Nikola Tesla demonstrated the illumination of wireless lamps by energy that was coupled to them through an alternating electric field.[21][27][28]

Electromagnetic radiation[edit]

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 beam) thereby delivering almost all emitted power at long ranges. The maximum directivity for antennas is physically limited by diffraction.

Beamed power, size, distance, and efficiency[edit]

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. In addition to the Rayleigh criterion Airy's diffraction limit is also frequently used to determine an approximate spot size at an arbitrary distance from the aperture.

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.

Microwave method[edit]

An artist's depiction of a solar satellite that could send electric energy by microwaves to a space vessel or planetary surface.

Power transmission via radio waves can be made more directional, allowing longer distance power beaming, with shorter wavelengths of electromagnetic radiation, typically in 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.[1][29]

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.[30] 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.

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.[31]

Japanese researcher Hidetsugu Yagi also investigated wireless energy transmission using a directional array antenna that he designed. In February 1926, Yagi and 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.[32]

Wireless high power transmission using microwaves is well proven. Experiments in the tens of kilowatts have been performed at Goldstone in California in 1975[33][34][35] and more recently (1997) at Grand Bassin on Reunion Island.[36] These methods achieve distances on the order of a kilometer.

Under experimental conditions microwave conversion efficiency was measured to be around 54%.[37]

Laser method[edit]

With a laser beam centered on its panel of photovoltaic cells, a lightweight model plane makes the first flight of an aircraft powered by a laser beam inside a building at NASA Marshall Space Flight Center.

In the case of electromagnetic radiation closer to the visible region of the spectrum (tens 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.[38] 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:[39]

Drawbacks include:

Laser "powerbeaming" technology has been mostly explored in military weapons[41][42][43] and aerospace[44][45] 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.[citation needed]

Other details include propagation,[46] and the coherence and the range limitation problem.[47]

Geoffrey Landis[48][49][50] is one of the pioneers of solar power satellites[51] 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.[52][53]

NASA's Dryden Flight Research Center demonstrated a lightweight unmanned model plane powered by a laser beam.[54] This proof-of-concept demonstrates the feasibility of periodic recharging using the laser beam system.

Magnetodynamic coupling[edit]

Any permanent magnet which 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.


Atmospheric conduction[edit]

U.S. Patent 649,621, "APPARATUS FOR TRANSMISSION OF ELECTRICAL ENERGY"

The World Wireless System, dependent upon the high electrical conductivity of plasma and the high electrical conductivity of the earth, was proposed as early as 1904.[55][56] The project never achieved commercial operation.

In Nikola Tesla's theory, the atmospheric conduction method passes electrical current through the earth, and through the upper troposphere and the stratosphere.[57] Current flow is induced by electrostatic induction up to an elevation of approximately 3 miles (4.8 km) above Earth's surface.[58][59] Electrical conduction and the flow of current through the upper atmospheric strata starting at a barometric pressure of approximately 130 millimeters of mercury is made possible by the creation of capacitively coupled discharge plasma through the process of atmospheric ionization.[60][61][62] Tesla's plan was that in this way electric lamps can be lit and electric motors turned at moderate distances. The transmitted energy can be detected at much greater distances.[63]

Timeline of wireless power[edit]

See also[edit]

Further reading[edit]

Books
  • Fleming, J. A. (1916) The principles of electric wave telegraphy and telephony. London: Longmans, Green and Co.
  • Fleming, J. A. (1911). Propagation of electric currents in telephone & telegraph conductors. New York: Van Nostrand.
  • Franklin, W. S. (1909). Electric waves: An advanced treatise on alternating-current theory. New York: Macmillan Co.
  • General Electric Co. (1915). General Electric review, Volume 18. "Wireless Transmission of Energy" By Elihu Thomson. General Electric Company, Lynn. (ed. Lecture by Professor Thomson, National Electric Light Association, New York.)
  • Hu, A. P. (2009). Wireless/Contactless power supply: Inductively coupled resonant converter solutions. Saarbrücken, Germany: VDM Verlag Dr. Müller.
  • Kennelly, A. E. (1912). The application of hyperbolic functions to electrical engineering problems: Being the subject of a course of lectures delivered before the University of London in May and June 1911. London: University of London Press.
  • Louis Cohen (1913). Formulae and tables for the calculation of alternating current problems. McGraw-Hill.
  • Orlich, E. M. (1912). Die Theorie der Wechselströme.
  • Steinmetz, C. P. (1914). Elementary lectures on electric discharges, waves and impulses, and other transients. New York: McGraw-Hill book co., inc.
  • Walker, J., Halliday, D., & Resnick, R. (2011). Fundamentals of physics. Hoboken, NJ: Wiley.
Patents

References[edit]

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