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Wireless energy transfer

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Wireless energy transfer is the transfer of electromagnetic energy for power to do work via conduction, induction, or transmission without a physical connection.

Contents

Description

Wireless energy transfer, by definition, does not require a physical medium through which to flow. Anyone can demonstrate that energy can be transmitted without a direct physical connection by simply touching the ends of a wire, briefly, to the ends of a small battery. Hold the wire near a compass needle while you do this and you'll see the compass needle twitch. It takes energy to make something move, so you've transmitted energy wirelessly. The needle moves because the electric current which briefly flows through the wire generates a magnetic field which acts on the needle (see electromagnetic induction and magnetic field for more information).

Another, more dramatic demonstration of wireless energy uses a radio transmitter generating more than a few watts, such as an amateur radio transmitter. A Fluorescent lamp with no wires attached to it, held near the antenna, will glow when the transmitter is activated. Send "SOS" and the light blinks "SOS". The transmitted energy causes the gas inside the lamp to glow, like the northern lights.

Once the basic principle is established, then the problem is to concentrate the energy of the transmission so that most of it is received where it can be converted into useful power. Think of a flashlight that lets you focus the light beam or spread it out so that it's very wide. If you wanted to shine the flashlight on a solar cell, you'd want the flashlight's beam to be tightly focussed to reduce the amount of energy which does not fall on the receiver and is lost.

Early systems (such as Tesla proposed) were incapable of such "focusing" of transmitted energy, because the necessary antenna size is impractically large at low frequencies (see extremely low frequency radio waves). Without focusing, much of the energy would be lost to the atmosphere.

The advent of technology for much higher transmission frequencies (such as those used by microwave transmitters) created the possibility of "beaming" the energy through the use of directional antennas, such as the one invented by Hidetsugu Yagi. Lasers, which create a coherent and tightly "beamed" form of light energy, are even more appropriate.

In most cases the cost of such solutions is much higher than simply using copper wire. Wireless energy transfer is therefore most applicable to situations where the energy receiver cannot be copper-tethered to the energy source — such as sending energy to an airplane or spacecraft, or another planet, or in the case of solar power satellites, from orbital satellites to a rectenna on Earth.

History

As the wireless art developed during the turn of the 20th century, industry was looking toward a method of wireless energy transfer. At the St. Louis World's Fair (1904) a prize was offered for a successful attempt to drive an 0.1 Horsepower air-ship motor by energy transmitted through space at a distance of least 100 ft. (The Electrician (London), September 1902, pages 814-815)

Hertz

A precursor of this technology can be found in the works of Heinrich Rudolf Hertz in the late nineteenth century. In 1888, Hertz experimented with pulsed power transmission at 500 megahertz.

Tesla

The development of wireless energy transfer began in earnest with the lectures and patents of the electrical engineer Nikola Tesla (and is described in his 1916 deposition on the history of wireless and radio technology). In experiments around 1899, Tesla was able to light lamps filled with gas (similar to neon) from over 25 miles away without using wires. Tesla used a high frequency current (Prodigal Genius, O'Neill; pg 193). During his experiments in Colorado, he lit ordinary incandescent lamps at 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 was at a distance of one-hundred feet from the primary circuit energized by the oscillator (Century Magazine, June 1900).

The construction of a global wireless energy distribution system centred on his Wardenclyffe Tower was started almost a century ago by Tesla but was abandoned due to a lack of investment funds. The Wardenclyffe facility was meant to be the start of a national (and later global) system of towers broadcasting power to users as electromagnetic waves. There is some evidence that Wardenclyffe might have used extremely low frequency signals combined with higher frequency signals. In practice, the transmitter electrically influences both the earth and the space above it. He made a point of describing the process as being essentially the same as passing electricity through a wire by conduction. Tesla believed that energy could be efficiently transmitted from the facility via longitudinal "non-Hertzian" (or non-Maxwellian) waves (ed. see waves in plasmas for examples). Powered by an industrial alternator, the tower was apparently intended to inject large amounts of energy into a natural Earth circuit, using the Earth-Ionosphere circuit as the transmission conduit. Tesla called his wireless technique the "disturbed charge of ground and air method."

In various writings, Tesla explained that the Earth itself would behave as a resonant LC circuit that could be electrically excited at predescribed frequencies. However, Earth resonance would be of a very low frequency (about 7 Hz) which would utilize Schumann resonance. Alternatively, a surface or ground wave, similar to the Zenneck wave could have been utilized. Others believe that earth currents were to be utilized. According to Tesla, the planet's large cross-sectional area provides a low resistance path for the flow of earth currents. The greatest losses are apt to occur at the points where the transmitting and receiving stations are connected with the ground. This is why Tesla stated,

"You see the underground work is one of the most expensive parts of the tower. In this system that I have invented it is necessary for the machine to get a grip of the earth, otherwise it cannot shake the earth. It has to have a grip on the earth so that the whole of this globe can quiver, and to do that it is necessary to carry out a very expensive construction." ["Nikola Tesla On His Work With Alternating Currents and Their Application to Wireless Telegraphy, Telephony and Transmission of Power", p. 203]
To close the circuit, in theory, a second path would be established between the plants' elevated high-voltage terminals through rarefied upper-level atmospheric strata. The connection would be made by electrostatic induction or conduction through plasma. Tesla firmly believed that Wardenclyffe would permit wireless transmission and reception across large distances with negligible losses.

Yagi

In Japan, Hidetsugu Yagi attempted wireless power transmission. In February 1926, Yagi and Uda published their first report on the wave projector directional antenna, later known as the yagi antenna. Yagi managed to demonstrate a proof of concept, but the engineering problems proved to be more onerous than conventional systems. [link]

Uses

The wireless transfer of energy is used in various devices, such as electric toothbrushes (to recharge their batteries), the transcutaneous energy transfer (TET) systems in artificial hearts like AbioCor and most notably in mobile phones.

See also

Related patents

External articles, references, and further reading

Nikola Tesla
Books, essays, and papers


  • Thomas W. Benson, "[Wireless Transmission of Power now Possible]". Electrical experimenter, March 1920.
  • Ahamid Aidinejad and James F. Corum, "The Transient Propagation of ELF Pulses in the Earth-Ionosphere Cavity".
  • Toby Grotz, "Artificially Stimulated Resonance of the Earth's Schumann Cavity Waveguide".   Proceedings of the Third International New Energy Technology Symposium/Exhibition, June 25th-28th, 1988.
  • James O. McSpadden
  • * "Collection of Power from Space, [References]".
  • * "[Wireless Power Transmission Demonstration]".
  • "[Inverse Rectennas for Two-Way Wireless Power Transmission]; Suitable rectennas under reverse bias can be made to act as transmitters". NASA's Jet Propulsion Laboratory, Pasadena, California.
  • PlanetAnalog, "[Cutting the Last Wire], True wireless devices require untethered power distribution". 13 December 2005.
  • "Radiant Energy -- [Wireless Transformer of High Power Lines?]". PES Network, Inc., 2005.
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