Transmisión inalámbrica de energía

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Plataforma de carga inductiva para un teléfono inteligente LG, utilizando el sistema de Qi (pronunciado "chi"), un ejemplo de transferencia inalámbrica de campo cercano. Cuando el teléfono está configurado en la plataforma, una bobina en la almohadilla crea un campo magnético que induce una corriente en otra bobina, en el teléfono, cargando la batería

Transmisión de energía inalámbrica[1] o transmisión inalámbrica de energía es la transmisión de energía eléctrica de una Fuente de alimentación a un dispositivo sin la utilización de cables o conductor eléctrico.[2] [3] [4] [5] Es un término genérico utilizado para referirse a un distinto numero de tecnologías de transmisión de energía que usan una variable de tiempo de campo electromagnético.[1] [5] [6] [7] La transmisión inalámbrica es útil para los dispositivos de potencial eléctrico en casos en donde la utilización de cables es inconveniente, peligrosa, o no es posible. En la transmisión inalámbrica de energía, un dispositivo emisor conectado a una fuente de poder tal como una fuente de electricidad doméstica, transmite energía por un campo electromagnético a través de un espacio intermedio a uno o más dispositivos receptores, donde es convertida de vuelta a energía eléctrica y utilizada.[1]

Las técnicas de transferencia de energía pueden ser de dos clases, la no-radiativa y la radiativa.[1] [6] [8] [9] [10] En las técnicas de campo cercano o no-radiativas, la energía es transferida a través de cortas distancias por campos magnéticos usando un acoplamiento magnético entre electrones.[5] [8] Este tipo se aplica a cepillos dental eléctricos, cargadores, etiquetas RFID, tarjetas inteligentes, cargadores para dispositivos médicos implantables como marcapasos, y potencia inductiva o cargadores de vehículos eléctricos como trenes o autobuses.[9] [11] Su enfoque actual es el de desarrollar sistemas inalámbricos para cargar dispositivos informáticos portátiles y móviles como teléfonos celulares o reproductores digitales de música y computadoras portátiles sin estar atado a un enchufe de pared. En las técnicas radiativas o de campo cercano y lejano, también llamadas, radiantes de energía, la energía es transmitida por haces de radiación electromagnética, como microondas o haces de láser. Estas técnicas pueden transportar la energía por una distancia mayor pero deben ser dirigidas en el receptor. Las aplicaciones propuestas para este tipo son la de satélites de energía solar y vehículos aéreos no tripulados de energía inalámbrica. [9] Un importante problema asociado a todos los sistemas de energía inalámbrica es limitar la exposición de las personas y otros seres vivos a posibles dañinos campos electromagnéticos. [9]

Resumen[editar]

Diagrama de bloques genérico de un sistema de energía inalámbrica

"La transmisión de energía inalámbrica es un término colectivo que se refiere a un diferente número de tecnologías de transmisión de energía por medio de campos electromagnéticos de tiempo-variable. [1] [5] [8] Las tecnologías, listadas en la tabla inferior, difieren de la distancia en que pueden transmitir la energía de manera eficiente, si el emisor debe ser dirigido al receptor, y el tipo de energía electromagnética que utilizan: tiempo variable campos eléctricos, campos magnéticos, ondas de radio, microondas o infrarrojo o luz visible. [8]

En general, un sistema de energía inalámbrica consiste en un dispositivo "emisor" conectado a una fuente de energía tal como una línea de electricidad doméstica, la cual convierte la energía a un campo electromagnético de tiempo-variable, y uno o más dispositivos "receptores", los cual reciben la energía y la convierten en corriente directa o alterna la cual es consumida por una carga eléctrica. [1] [8] En el transmisor, la energía de entrada es convertida a un campo electromagnético oscilante por alguna clase de dispositivo de antena. La palabra "antena" es utilizada libremente aquí, podría ser una bobina de alambre que genere un campo magnético, una placa metálica la cual genere un campo eléctrico, una antena que irradie ondas de radio, o un láser que genere luz. Una antena similar o dispositivos unidos en el receptor convierten los campos oscilantes en corriente eléctrica. Un parámetro importante el cual determina el tipo de ondas es la frecuencia f en hertz de las oscilaciones. La frecuencia determina la longitud de onda λ = c/f de las ondas que llevan la energía a través de la brecha, donde c es la velocidad de la luz

La energía inalámbrica usa muchos de los mismos campos y ondas como los dispositivos de comunicación inalámbricos como la radio, [6] [12] otra tecnología familiar la cual implica la energía transmitida sin cables por campos electromagnéticos, utilizados en teléfonos, difusión de radio y televisión, y WiFi. En las radiocomunicaciones el objetivo es la transmisión de información, por lo tanto, la cantidad de energía que llega al receptor no es tan importante mientras sea suficiente para que la relación señal/ruido sean tan alta como para que la información pueda ser recibida inteligiblemente. [5] [6] [12] En las tecnologías de comunicación inalámbrica, generalmente sólo delgadas cantidades de energía llegan al receptor. Por contraste, en la energía inalámbrica, las cantidades de energía recibida son algo importante, por lo que la eficiencia (fracción de la energía transmitida que es recibida) es el parámetro más significativo. [5] Por esta razón, las tecnologías de energía inalámbrica están más limitadas por la distancia que las tecnologías de comunicación inalámbricas

Estos son las diferentes tecnologías de energía inalámbrica:[1] [8] [9] [13] [14]

Tecnología Rango[15] Directividad[8] Frequencia Antena Aplicaciones actuales o posibles en el futuro
Acoplamiento inductivo Corta Baja Hz - MHz Bobinas de alambre Carga de batería de máquinas de afeitar y cepillos dentales eléctricos, placa de cocina de inducción y calentadores industriales.
Acoplamiento inductivo resonante Media Baja MHz - GHz Bobinas de alambre Tuned, resonadores de elementos concentrados Carga de dispositivos portátiles (Qi, Witricidad), implantes biomédicos, vehículos eléctricos, alimentación de autobuses, trenes, MAGLEV, RFID, tarjetas inteligentes.
El acoplamiento capacitivo Corta Baja kHz - MHz Electrodos Carga de dispositivos portátiles, enrutamiento de potencia en circuitos integrados a gran escala, tarjetas inteligentes.
Magnetodinámica[13] Corta N.A. Hz Rotación de magnetos Carga de vehículos eléctricos.
Microondas Larga Alta GHz Platos parabólicos, antenas en fase, rectenna Satélites de energía solar, alimentación de aeronaves no tripuladas.
Ondas de Luz Larga Alta ≥THz Láser, fotocélulas, lentes Alimentación de aeronaves no tripuladas, alimentación de escaladores de elevadores espaciales.

Regiones de Campo[editar]

Los campos eléctricos y campos magnéticos son creados por una partícula cargada en la materia tales como los electrones. Una carga estacionaria crea un campo electrostático en el espacio alrededor de ella. Una carga estable de corriente eléctrica, corriente directa crea un campo magnético alrededor de ella. Los campos anteriores contienen energía pero no pueden llevar energía eléctrica porque son estáticas. De todas maneras, los campos de tiempo-variable pueden llevar energía [16] Acelerando las cargas eléctricas, tal como se encuentran en una corriente alterna de electrones en un cable, crea campos eléctricos y magnéticos de tiempo-variable en el espacio alrededor de ellas. Estos campos pueden ejercer fuerzas oscilantes en los electrones y en la "antena" receptora, causando que se muevan de atrás hacia adelante. Estas representan una corriente alternativa que puede ser usada para generar una carga. Los campos eléctricos y magnéticos oscilantes, circundan moviendo cargas eléctricas en el "dispositivo antena" que pueden ser divididos en dos regiones, dependiendo de la distancia Drange de la antena.[1] [4] [6] [8] [9] [10] [17] El límite entre las regiones está vagamente definido.[8] Los campos tienen diferentes características en estas regiones, y diferentes tecnologías son usada para transmitir energía:

  • Los Campos cercanos o regiones no radiativas- Esto significa que el área dentro de aproximadamente 1 longitud de onda (λ) de la antena.[1] [4] [10] En esta región, los campos eléctricos oscilantes y campos magnéticos están separados[6] y la energía puede ser transferida a través de los campos eléctricos por la capacidad de acoplamiento de la inducción electrostática entre electrodos metálicos o a través de campos magnéticos por acoplamiento inductivo de inducción electromagnética entre rollos de almabre. [5] [6] [8] [9] Estos campos no son radiativos, [10] lo que significa que la energía se mantiene dentro de una distancia cercana a la del transmisor.[18] Si no hay dispositivos receptores o material absorbente dentro del rango limitado para "emparejarlo", la energía no deja al emisor. [18] El rango de estos campos es corto, y depende del tamaño y forma de los dispositivos "antena", los cuales son comúnmente rollos de alambre. Los campos y la energía transmitida, disminuye exponecialmente con la distancia [4] [17] [19] así, si la distancia entre dos "antenas" Drange es mucho mayor que el diametro de "antenas" Dant muy poca energía será recibida. Por lo tanto, éstas técnicas no pueden ser usadas para transmisiones de energía de larga distancia
La Resonancia, tal como la resonancia de acoplamiento inductiva, puede incrementar grandiosamente el acoplamiento entre antenas, permitiendo una transmisión eficiente en distancias mayores, [1] [4] [6] [9] [20] [21] aunque los campos aún disminuyen exponecialmente. Por lo tanto, el rango de los dispositivos de campo-cercano es convencionalmente dividido en dos categorías:
  • Rango Corto - hasta aproximadamente el diámetro de una antena: Drange ≤ Dant.[18] [20] [22] Este es el rango alrededor del cual la capacidad no resonante ordinaria o acoplamiento inductivo puede transferir cantidades prácticas de energía
  • Rango medio - por encima de 10 veces el diámetro de la antena: Drange ≤ 10 Dant.[20] [21] [22] [23] Este es el rango alrededor del cual la capacidad resonante o acoplamiento inductivo puede transferir cantidades prácticas de energía
  • Campos lejanos o regiones radiativas - más allá de aproximadamente 1 longitud de onda (λ) de la antena, los campos eléctricos y magnéticos son perpendiculares a cada uno y se propagan como ondas electromagnéticas; algunos ejemplos son las ondas de radio, microondas, o las ondas de luz.[1] [4] [9] Esta parte de la energía es radiativa,[10] lo que significa que deja la antena haya o no un receptor que la absorba. La porción de energía la cual no golpea la antena receptora es disipada y perdida. La cantidad de energía emitida como ondas electromagnéticas por una antena depende del tamaño del radio de la antena Dant para la longitud de onda de las ondas λ,[24] la cual es determinada por la frecuencia: λ = c/f. En bajas frecuencias f donde la antena es mucho más pequeña que el tamaño de las ondas, Dant << λ, muy poca energía es irradiada. Por lo tanto, los dispositivos de campo cercano de arriba, los cuales usan frecuencias más bajan, no irradian casi nada de su energía como radiación electromagnética. Las antenas de aproximadamente el mismo tamaño que en la longitud de ondas Dant ≈ λ tales como las antenas de un polo o dos polos, irradian energía eficientemente, pero las ondas electromagnéticas son irradiadas en todas direcciones, así, si la antena receptora está lejos, sólo una pequeña cantidad de la radiación la golpeará.[10] [20] Por lo tanto, estas pueden ser usasadas para transmisiones de energía ineficientes de corto rango pero no para transmisiones de gran rango.[25]
Sin embargo, a diferencia de los campos, la radiación electromagnética puede ser concentrada por Reflexión o Refracción en haces. Mediante el uso de una antena de alta-ganancia o un sistema óptico el cual concentre la radiacón en un haz estrecho dirigido en el receptor, puede ser usado para transmisiones de energía de gran rango.[20] [25] Desde el criterio de Rayleigh, para producir los haces estrechos necesarios para concentrar una cantidad significativa de energía en un receptor distante, la antena debe ser mucho más larga que la longitud de onda de las ondas usadas: Dant >> λ = c/f.[26] [27] Unos dispositivos de "haces de energía" prácticos, requieren de una longitud de ondas en la región de un centímetro o menor, correspondiente a frencuencias por encima de 1 Ghz, en el rango de las microondas o mayor.[1]

Técnicas de Campo cercano o no radiativas[editar]

Los componentes de campos cercanos de campos eléctricos y magnéticos desaparecen rápidamente más allá de una distancia de alrededor de un diámetro de la antena (Dant). Fuera de rangos muy cercanos, la fuerza del campo y acoplamiento es aproximadamente proporcional a (Drange/Dant)−3[28] [17] Puesto que la energía es proporcional al cuadrado de la intensidad del campo, la energía transferida disminuye con la sexta parte de la distancia de la energía (Drange/Dant)−6.[6] [19] [29] [30] o 60 dB por década. En otras palabras, duplicar la distancia entre el emisor y receptor, causa que la energía recibida disminuya por el factor de 26 = 64.

Acoplamiento Inductivo[editar]

Diagrama de bloques genérico de un sistema inductivo de energía inalámbrica.

Esta técnica de transmisión inalámbrica se basa en el uso de un campo magnético generado por una corriente eléctrica para inducir una corriente en un segundo conductor. Este efecto se produce en el campo electromagnético cercano, con el secundario en estrecha proximidad al primario. A medida que aumenta la distancia desde el primario, más y más del campo magnético del primario esquiva al secundario Incluso en un rango relativamente corto el acoplamiento inductivo es muy ineficiente, perdiendo mucha de la energía transmitida.

La técnica de inducción electrodinámica de transmisión inalámbrica se basa en el uso de un campo magnético generado por una corriente eléctrica para inducir una corriente en un segundo conductor. Este efecto ocurre en el campo cercano de energía electromagnética, con el secundario, cercano al primario. Como la distancia desde el primario fue incrementada, más y más del campo electromagnético del primario esquiva al secundario. Incluso alrededor de un relativo corto rango, el acoplamiento inductivo es gravemente ineficiente, desperdiciando mucha de la energía transmitida.[31]

Esta acción de un transformador eléctrico es la más sencilla forma de transmisión de energía inalámbrica. La bobina primaria y la bobina secundaria de un transformador no están directamente conectadas; cada bobina es parte de un circuito separado. La transferencia de energía toma lugar a través de un proceso conocido como inducción mutua. Las principales funciones son pasar el voltaje primario ya sea hacia arriba o hacia abajo y el aislamiento eléctrico. Los cargadores de cepillos dentales eléctricos y teléfonos móviles, son ejemplos de como es usado este principio. La Cocina de inducción utiliza este método. El principal inconveniente de este método básico de transmisión de energía es el corto rango. El receptor debe estar directamente adyacente al transmisor o unidad de inducción a fin de que se acople eficientemente con él

Los usos comunes de la inducción electrodinámica de resonancia mejorada[32] son cargar la batería de dispositivos portátiles tales teléfonos celulares y computadora, implantes biomédicos y vehículos eléctricos.[33] [34] [35] Una técnica de carga localizada[36] seleccionando la bobina transmisora apropiada en una estructura con matriz multicapa.[37] La resonancia es usada en ambas almohadillas de carga inalámbrica (el circuito trnasmisor) y el módulo receptor (incrustado en la carga) para maximizar la transferencia de energía. Los dispositivos de pilas equipados con un módulo receptor especial pueden ser cargados simplemente con colocarlos en una almohadilla de carga inalámbrica. Esto ha sido adoptado como parte del Qi (estándar de electricidad por inducción).

Esta tecnología también es utilizada para los dispositivos eléctricos con bajos requerimientos de energía, tales como los parches RFID y las tarjetas inteligentes sin contacto. En lugar de confiar en que cada uno de los muchos miles o millones de parches RFDI o tarjetas inteligentes contengan una batería en trabajo constante, la inducción electrodinámica puede proveer energía sólo cuando los dispositivos se necesiten

Acoplamiento capacitivo[editar]

En la inducción electrostática de acoplamiento capacitivo, el dual del acoplamiento inductivo, la energía es transmitida por campos eléctricos entre placas metálicas. Los electrodos emisor y receptor de un condensasor, con el espacio intermedio como el dieléctrico.[5] [6] [9] [38] [39] 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,[5] which causes an alternating current to flow in the load circuit. The amount of power transferred increases with the frequency[38] 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.[5]

Capacitive coupling has only been used practically in a few low power applications, because the very high voltages on the electrodes required to transmit significant power can be hazardous,[6] [9] and can cause unpleasant side effects such as noxious ozone production. In addition, in contrast to magnetic fields,[20] electric fields interact strongly with most materials, including the human body, due to dielectric polarization.[39] Intervening materials between or near the electrodes can absorb the energy, in the case of humans possibly causing excessive electromagnetic field exposure.[6] However capacitive coupling has a few advantages over inductive. The field is largely confined between the capacitor plates, reducing interference, which in inductive coupling requires heavy ferrite "flux confinement" cores.[5] [39] Also, alignment requirements between the transmitter and receiver are less critical.[5] [6] [38] Capacitive coupling has recently been applied to charging battery powered portable devices[40] and is being considered as a means of transferring power between substrate layers in integrated circuits.[41]

Magnetodynamic coupling[editar]

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.[13] 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.[13] 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.[13] A rotating armature embedded in a garage floor or curb would turn a receiver armature in the underside of the vehicle to charge its batteries.[13] It is claimed that this technique can transfer power over distances of 10 to 15 cm (4 to 6 inches) with high efficiency, over 90%.[13] 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.[13]

Far-field or radiative techniques[editar]

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.

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[editar]

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.[42] 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.[43] [44]

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.[45] 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.[46]

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.[47]

Wireless high power transmission using microwaves is well proven. Experiments in the tens of kilowatts have been performed at Goldstone in California in 1975[48] [49] [50] and more recently (1997) at Grand Bassin on Reunion Island.[51] These methods achieve distances on the order of a kilometer.

Under experimental conditions, microwave conversion efficiency was measured to be around 54%.[52]

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[editar]

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.[53] 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:[54]

  • 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.[55] (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[56] [57] [58] and aerospace[59] [60] 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.[cita requerida]

Other details include propagation,[61] and the coherence and the range limitation problem.[62]

Geoffrey Landis[63] [64] [65] is one of the pioneers of solar power satellites[66] 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.[67] [68]

NASA's Dryden Flight Research Center demonstrated a lightweight unmanned model plane powered by a laser beam.[69] This proof-of-concept demonstrates the feasibility of periodic recharging using the laser beam system.

Energy harvesting[editar]

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.[70] 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.[70] Although the efficiency of conversion is usually low and the power gathered often minuscule (milliwatts or microwatts),[70] it can be adequate to run or recharge small micropower wireless devices such as remote sensors, which are proliferating in many fields.[70] This new technology is being developed to eliminate the need for battery replacement or charging of such wireless devices, allowing them to operate completely autonomously.

History[editar]

In 1826 André-Marie Ampère developed Ampère's circuital law showing that electric current produces a magnetic field.[71] Michael Faraday developed Faraday's law of induction in 1831, describing the electromagnetic force induced in a conductor by a time-varying magnetic flux. In 1862 James Clerk Maxwell synthesized these and other 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.[14] [72] Maxwell predicted the existence of electromagnetic waves in his 1873 A Treatise on Electricity and Magnetism.[73] 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 energy transfer systems.[14] [72] In 1888 Heinrich Rudolf Hertz discovered radio waves, confirming the prediction of electromagnetic waves by Maxwell.[73]

Tesla's experiments[editar]

Tesla demonstrating wireless power transmission in a lecture at Columbia College, New York, in 1891. The two metal sheets are connected to his Tesla coil oscillator, which applies a high radio frequency oscillating voltage. The oscillating electric field between the sheets ionizes the low pressure gas in the two long Geissler tubes he is holding, causing them to glow by fluorescence, similar to neon lights.

Inventor Nikola Tesla performed the first experiments in wireless power transmission at the turn of the 20th century,[72] [74] and may have done more to popularize the idea than any other individual. In the period 1891 to 1904 he experimented with transmitting power by inductive and capacitive coupling using spark-excited radio frequency resonant transformers, now called Tesla coils, which generated high AC voltages.[72] [74] [75] With these he was able to transmit power for short distances without wires. In demonstrations before the American Institute of Electrical Engineers[75] and at the 1893 Columbian Exposition in Chicago he lit light bulbs from across a stage.[74] He found he could increase the distance by using a receiving LC circuit tuned to resonance with the transmitter's LC circuit.[76] using resonant inductive coupling. At his Colorado Springs laboratory during 1899-1900, by using voltages of the order of 20 megavolts generated by an enormous coil, he was able to light three incandescent lamps at a distance of about one hundred feet.[77] [78] The resonant inductive coupling which Tesla pioneered is now a familiar technology used throughout electronics; its use in wireless power has been recently rediscovered and it is currently being widely applied to short-range wireless power systems.[74] [79]

The inductive and capacitive coupling used in Tesla's experiments is a "near-field" effect,[74] so it is not able to transmit power long distances. However, Tesla was obsessed with developing a wireless power distribution system that could transmit power directly into homes and factories, as proposed in a visionary 1900 article in Century magazine.[80] [81] [82] [83] and believed that resonance was the key. He claimed to be able to transmit power on a worldwide scale, using a method that involved conduction through the Earth and atmosphere.[84] [81] [82] [83] Tesla was vague about his methods. One of his ideas was to use balloons to suspend transmitting and receiving terminals in the air above 30,000  (Expresión errónea: falta operando para * m) in altitude, where the pressure is lower.[84] At this altitude, Tesla claimed, an ionized layer would allow electricity to be sent at high voltages (millions of volts) over long distances.

In 1901, Tesla began construction of a large high-voltage coil facility, the Wardenclyffe Tower at Shoreham, New York, intended as a prototype transmitter for a "World Wireless System" that was to transmit power worldwide, but by 1904 his investors had pulled out, and the facility was never completed.[82] [85] Although Tesla claimed his ideas were proven, he had a history of failing to confirm his ideas by experiment,[86] [87] and there seems to be no evidence that he ever transmitted significant power beyond the short-range demonstrations above.[14] [72] [76] [77] [87] [88] [89] [90] [91] The only report of long-distance transmission by Tesla is a claim, not found in reliable sources, that in 1899 he wirelessly lit 200 light bulbs at a distance of 26  (Expresión errónea: falta operando para * km).[77] [88] There is no independent confirmation of this putative demonstration;[77] [88] [92] Tesla did not mention it,[88] and it does not appear in his meticulous laboratory notes.[92] [93] It originated in 1944 from Tesla's first biographer, John J. O'Neill,[77] who said he pieced it together from "fragmentary material... in a number of publications".[94] In the 110 years since Tesla's experiments, efforts using similar equipment have failed to achieve long distance power transmission,[74] [77] [88] [90] and the scientific consensus is his World Wireless system would not have worked.[14] [72] [76] [82] [88] [95] [96] [97] [98] Tesla's world power transmission scheme remains today what it was in Tesla's time, a fascinating dream.[14] [82]

Microwaves[editar]

Before World War 2, little progress was made in wireless power transmission.[89] 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.[14] [72] [89] 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.[14] [72] [89] [96]

The development of microwave technology during World War 2, such as the klystron and magnetron tubes and parabolic antennas[89] made radiative (far-field) methods practical for the first time, and the first long-distance wireless power transmission was achieved in the 1960s by William C. Brown.[14] [72] 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.[14] [89] A major motivation for microwave research in the 1970s and 80s was to develop a solar power satellite.[72] [89] 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.[14] [99] In landmark 1975 high power experiments, Brown demonstrated short range transmission of 475 W of microwaves at 54% DC to DC efficiency, and he and Robert Dickinson at NASA's Jet Propulsion Laboratory transmitted 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.[14] [100] The incident-RF to DC conversion efficiency of the rectenna was 80%.[14] [100] In 1983 Japan launched MINIX (Microwave Ionosphere Nonlinear Interation Experiment), a rocket experiment to test transmission of high power microwaves through the ionosphere.[14]

In recent years a focus of research has been the development of wireless-powered drone aircraft, which began in 1959 with the Dept. of Defense's RAMP (Raytheon Airborne Microwave Platform) project[89] which sponsored Brown's research. In 1987 Canada's Communications Research Center developed a small prototype airplane 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[editar]

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.[101] The first passive RFID (Radio Frequency Identification) technologies were invented by Mario Cardullo[102] (1973) and Koelle et al.[103] (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.[104] The Wireless Power Consortium was established in 2008 to develop interoperable standards across manufacturers.[104] 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).[105] The wireless device is placed on a flat charger plate (which could be embedded in table tops at cafes, for example) and power is transferred from a flat coil in the charger to a similar one in the device.

In 2007, a team led by Marin 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  (Expresión errónea: falta operando para * ) (8 times the coil diameter) at around 40% efficiency.[74] [106] This technology is being commercialized as WiTricity.

See also[editar]

3

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Further reading[editar]

Books and Articles

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Patents

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References[editar]

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  3. «Wireless energy transfer». Encyclopedia of terms. PC Magazine Ziff-Davis. 2014. Consultado el December 15, 2014. 
  4. a b c d e f Rajakaruna, Sumedha; Shahnia, Farhad; Ghosh, Arindam (2014). Plug In Electric Vehicles in Smart Grids: Integration Techniques. Springer. pp. 34–36. ISBN 981287299X. 
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  6. a b c d e f g h i j k l m Sazonov, Edward; Neuman, Michael R (2014). Wearable Sensors: Fundamentals, Implementation and Applications. Elsevier. pp. 253–255. ISBN 0124186661. 
  7. Wilson, Tracy V. (2014). «How Wireless Power Works». How Stuff Works website. InfoSpace LLC. Consultado el December 15, 2014. 
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  15. "short", "midrange", and "long range" are defined below
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External links[editar]