Wireless energy transfer

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Wireless energy transfer is the transfer of significant amounts of electromagnetic energy without a physical connection for the purpose of doing work. This is distinct from electromagnetic transmission for the purpose of transferring information (radio), where the amount of power is not as important as the integrity of the signal, though similar physics are usually involved.

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[edit] Description

The principle that energy can be transmitted without a direct physical connection is revealed by simple demonstration: Briefly touching the ends of a wire to the poles of a small battery while holding the body of the wire near a compass needle will induce motion of the needle twitching; by the Newtonian axiom that energy is required to induce movement, this simple experiment observes the transmission of energy wirelessly. The needle moves because the electrical current which briefly flows through the wire generates a coaxial magnetic field which acts on the needle by magnetic induction.

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: for instance, relaying the abbreviation "SOS" by Morse code will cause the light to correspondingly blink "SOS" as electromagnetic induction by the transmitted signal causes the gas inside the lamp (acting as a natural receiver) to glow, not unlike the causal mechanism in the aurora borealis.

With the basic principle thus established, the challenge then is to channel the energy of transmission to ensure efficient reception, whereupon it can be converted into useful power — by analogy, a flashlight beam focused narrowly (by a lens) onto a solar cell will mimimize the amount of energy which does not fall on the receiver and is ambiently lost. Early systems were incapable of such focused manipulation of electromagnetic energy because the necessary antenna size is impractically large at extremely low radio wave frequencies; lacking focus, much of the transmitted energy would be lost to the atmosphere.

The advent of technology for much higher transmission frequencies, like those used by microwave transmitters, created the possibility of relaying electromagnetic energy through the application of directional antennas, such as the one invented by Hidetsugu Yagi. Lasers, which create a coherent and tightly confined beam of light energy, are even more appropriate.

In most cases, such solutions are not economical as compared to the simpler standard of power transmission by 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 transmitted between planetary bodies, or from orbital solar power satellites to a rectenna on Earth.

[edit] 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).

[edit] 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.

[edit] Tesla

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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 gas discharge lamps (similar to neon signs) 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 system for wireless telegraphy, telephony, and transmission of power centered on his Wardenclyffe Tower, started over a century ago by Tesla but abandoned due to a lack of investment funds. The Wardenclyffe facility was the prototype for a system of interconnected towers that would broadcast electrical energy to users in the form of electromagnetic waves such as those which are associated with the flow of electrical energy through a transmission line, rather than electromagnetic radiation. There is considerable evidence that Wardenclyffe would 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. Tesla made a point of describing the process as being essentially the same as passing electricity through a wire by conduction. He believed that energy could be efficiently transmitted from the facility by true electrical conduction through the earth, and either displacement currents, i.e., electrostatic induction, or, with a high-power system, true electrical conduction through plasma. Tesla called his wireless technique the "disturbed charge of ground and air method."

The ground-air system depends upon passage of electrical current through both the earth and the atmosphere. To accommodate this, the Wardenclyffe-type World System transmitter/receiver facility includes both an air and a ground connection, each being called a "terminal". Tesla clearly specified the earth as being one of the conducting media involved in ground and air system technology. The other specified medium is the atmosphere above 5 miles elevation. While not an ohmic conductor, in this region of the troposphere and upwards, the density or pressure is sufficiently reduced to so that, according to Tesla's theory, the atmosphere's insulating properties can be easily impaired, allowing an electric current to flow. His theory further states that the conducting region is developed through the process of atmospheric ionization, in which the affected portions thereof are changed to plasma. The presence of the magnetic fields developed by each plant's helical resonator suggests that an embedded magnetic field and flux linkage is also involved. Flux linkage with the Earth's natural magnetic field is also a possibility, especially in the case of an earth resonance transmission system.

The atmosphere below 5 miles is viewed as a propagating medium for a portion of the above-ground circuit, and, being an insulating medium, electrostatic induction would be involved rather than true electrical conduction. Tesla felt that with a sufficiently high electrical potential on the elevated terminal the practical limitation imposed upon its height could be overcome. He anticipated that a highly energetic transmitter, as was intended at Wardenclyffe, would charge the elevated terminal to the point where the atmosphere around and above the facility would break down and become ionized, leading to a flow of true conduction currents between the two terminals by a path up to and through the troposphere, and back down to the other facility. The ionization of the atmosphere directly above the elevated terminals could be facilitated by the use of an ionizing beam of ultraviolet radiation to form what might be called a high-voltage plasma transmission line.

In 1935 Tesla spoke about the transmission of propulsive power to ships at sea "through the stratosphere" using this technique.

The principles of this high tension power, generated by shore plants and transmitted through the upper reaches of the air, illuminating the sky, turning night into day and at the same time supplying power, have occupied Dr. Tesla's attention on and off now for the past thirty-five years...

There is a method of conveying great power to ships at sea which would be able to propel them across oceans at high speed...

The principle is this. A ray of great ionizing power is used to give to the atmosphere great powers of conduction. A high tension current of 10,000,000 to 12,000,000 volts is then passed along the ray to the upper strata of the air, which strata can be broken down very readily and will conduct electricity very well.

A ship would have to have equipment for producing a similar ionizing ray. The current which has passed through the stratosphere will strike this ray, travel down it and pass into the engines which propel the ship.

—"Faster Liners is Tesla's Dream", New York Sun, June 5, 1935

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, perhaps, 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.

[edit] 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. [1]

[edit] 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.

A fully developed World System would, conceivably, allow for the removal of many existing high-tension power transmission lines, and facilitate the interconnection of electrical generation plants on a global scale.

[edit] See also

[edit] Related patents

[edit] External articles, references, and further reading

Nikola Tesla
Books, essays, and papers
Other history
  • Frank E. Little, James O. McSpadden, Kai Chang, and Nobuyuki Kaya, "Toward space solar power: Wireless energy transmission experiments past, present and future". AIP Conference Proceedings, January 15, 1998, Volume 420, Issue 1, pp. 1225-1233.
  • Lewis Coe, "Wireless Radio: A History". McFarland & Company, Jul 1, 1996. ISBN 0-7864-0259-8
  • W. C. Brown,
    • "The history of wireless power transmission". Solar Energy, Vol. 56, No. 1, pp. 3-21, 1996.
    • "The History of Power Transmission by Radio Waves". IEEE Transactions on Microwave Theory and Techniques, 1984.

[edit] External links

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