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PowerPedia:Wireless transmission of electricity

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Wireless energy transfer also known as wireless energy transmission is the process that takes place in any system where electromagnetic energy is transmitted from a power source (such as a Tesla coil) to an electrical load, without interconnecting wires. Wireless transmission is employed in cases where interconnecting wires are inconvenient, hazardous, or impossible. Though the physics can be similar (pending on the type of wave used), there is a distinction from electromagnetic transmission for the purpose of transferring information (radio), where the amount of power transmitted is only important when it affects the integrity of the signal.



Wireless energy transfer, the transmission of electrical energy without wires, has been around since about 1856 in the form of mutual induction. Using induction it is possible to transmit and receive signals over a considerable distance. However, to draw significant power in that way, the two inductors must be placed fairly close together. If resonant coupling is used, where inductors are tuned to a mutual frequency, power may be transmitted over a range of many meters. Another form of wireless energy transfer is electromagnetic radiation, such as in radio waves. For example, a fluorescent tube held near an active radio transmitter radiating more than a few watts (such as an amateur radio transmitter) will glow. The reason behind this phenomenon is similar to the physics involved in the aurora borealis.

As wireless telecommunications technologies were being developed during the early 1900s, researchers were investigating different wireless energy transfer methods to power more significant loads than the high-resistance sensitive devices that were being used to detect the received energy. 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 feet.(The Electrician (London), September 1902, pages 814-815).)

Except for RFID tags, use of wireless energy transfer for powering devices over room-sized or community-sized ranges has not been widely implemented to date. It has been assumed by some that broadcasting electrical energy sufficient for powering electrical devices would have negative health implications. With focused beams of microwave radiation there are definite health and safety risks. The physical alignment and targeting of devices to receive the energy beam is problematic.

Wireless energy transfer methods

At least four methods exist by which electrical energy can be transferred from a source to a load without the use of manmade conductors. These are: electromagnetic induction, electromagnetic radiation, evanescent wave coupling, and electrical conduction. The second method is radiative; the others are non-radiative.

Electromagnetic induction

The electrical transformer is probably the simplest example of wireless energy transfer. The primary and secondary circuits of a transformer are electrically isolated from each other. The transfer of energy takes place by electromagnetic coupling through a process known as induction. (An added benefit is the capability to step the primary voltage either up or down.) The induction cooker is an example of how this principle can be used. In the induction cooker, electrical energy is wirelessly transferred into the cookware, where it is converted ohmically into heat for cooking. Electric toothbrush chargers work in a similar way. The main drawback to induction, however, is the short range. The receiver must be in relatively close proximity to the transmitter (or “induction unit") in order to inductively couple with it.

Electromagnetic radiation

Electromagnetic radiation in the form of either radio waves or light can also be used to transfer energy wirelessly. While systems based upon this method are used mostly for information transfer, a high degree of efficiency in power transmission is also achievable under certain circumstances.

The earliest work in the area of wireless transmission via radio waves was performed by Heinrich Rudolf Hertz in 1888. A few years later Guglielmo Marconi worked with a modified form of Hertz's transmitter.

Nikola Tesla also investigated radio transmission and reception but unlike Marconi, Tesla designed his own transmitter -- one with power-processing capability some five orders-of-magnitude greater than those of its predecessors. He would use this same coupled-tuned-circuit oscillator to implement his non-radiative conduction-based wireless energy transfer method as well. Both of these wireless methods employ a minimum of four tuned circuits, two at the transmitter and two at the receiver.

Japanese researcher Hidetsugu Yagi also investigated wireless transmission. In February 1926, Yagi and Uda published their first paper on the tuned high gain directional array known as the Yagi antenna. This beam antenna has been widely adopted throughout the broadcasting and wireless telecommunications industries due to its exceptional performance characteristics and robustness such as a Yagi antenna. ("Scanning the Past: A History of Electrical Engineering from the Past, Hidetsugu Yagi")

Efficient power transmission via radio waves can be achieved by using shorter wavelengths of electromagnetic radiation, typically in the microwave range. A rectenna may be used to convert the microwave energy back into electricity. Conversion efficiencies exceeding 95% have been achieved in this manner. 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. In the case of light, power can be transmitted by converting electricity into a laser beam that is then fired at a solar cell receiver. This is generally known as "power beaming." Its drawbacks are as follows:

  1. Conversion to light, such as a laser, is usually very inefficient (although quantum cascade lasers improve this)
  2. Conversion back into electricity is also typically very inefficient, with the absolute best modern solar cells achieving 40% efficiency.
  3. Atmospheric absorption causes losses.
  4. As with microwave beaming, this method requires a direct line of sight with the target.

Photovoltaic cells can also be used to receive energy from Earth's strongest natural source of electromagnetic radiation, the Sun. A new company, Powercast introduced wireless power transfer technology using RF energy at the 2007 Consumer Electronics Show, winning best Emerging Technology. ("CES Best of 2007")

Evanescent wave coupling

Researchers at MIT believe they have rediscovered a way to wirelessly transfer power using non-radiative electromagnetic energy resonant tunneling. By sending electromagnetic waves around in a highly angular waveguide, evanescent waves are produced which carry no energy. Evanescent wave coupling is a process by which electromagnetic waves are transmitted from one medium to another by means of the evanescent (or decaying) electromagnetic field(s). This is usually accomplished by placing two or more waveguides close together so that the evanescent field does not decay much in the vicinity of the other waveguide. Assuming the receiving waveguide can support mode(s) of the appropriate frequency, the evanescent field gives rise to propagating wave mode(s), thereby connecting (or coupling) the wave from one waveguide to the next.

If a proper resonant waveguide is brought near the transmitter, the evanescent waves can allow the energy to tunnel (specifically evanescent wave coupling, the electromagnetic equivalent of tunneling) to the power drawing waveguide, where they can be rectified into DC power. Since the electromagnetic waves would tunnel, they would not propagate through the air to be absorbed or dissipated, and would not disrupt electronic devices or cause physical injury like microwave or radio wave transmission might. Researchers anticipate up to 5 meters of range for the initial device, and are currently working on a functional prototype. ("'Evanescent coupling' could power gadgets wirelessly",

Evanescent coupling is always associated with matter, i.e. with the induced currents and charges within a partially reflecting surface. This coupling is directly analogous to the nearfield, non-radiative coupling between the primary and secondary coils of a transformer, or between the two plates of a capacitor. Mathematically, the process is the same as that of quantum tunneling, except with electromagnetic waves instead of quantum-mechanical wavefunctions. Evanescent wave coupling is used to excite dielectric microsphere resonators among other things. A new application could be wireless energy transfer, useful, for instance, for charging electronic gadgets without wires. [1]

Break down of Evanescent wave coupling

This method of resonant inductive coupling has key implications in the solution of the two main problems associated with non-resonant inductive coupling and electromagnetic radiation, one of which is caused by the other; distance and efficiency. Electromagnetic induction works on the principle of a primary coil generating a predominantly magnetic field and a secondary coil being within that field so a current is induced within its coils. This causes the relatively short range due to the amount of power required to produce an electromagnetic field. Over greater distances the non-resonant induction method is inefficient and wastes much of the transmitted energy, just to increase range. This is where the resonance comes in and helps efficiency dramatically by "tunneling" the magnetic field to a receiver coil that resonates at the same frequency. Unlike the multiple-layer secondary of a non-resonant transformer, such receiving coils are single layer solonoids with closely spaced capacitor plates on each end, which, combined, allow the coil to be tuned into a certain frequency thereby eliminating the wide energy wasting wave problem and allowing the energy used to focus on a certain frequency increasing the range.

Earlier experiments were performed with the wireless transmission of energy by non-radiative electromagnetic resonant induction in the early 1890s. This work started at 35 South 5th Ave., New York City and was subsequently adopted for lighting purposes at another laboratory at 46 Houston St. (Nikola Tesla: Guided Weapons & Computer Technology, Leland Anderson, Ed., Twenty First Century Books, Breckenridge, 1998, p. 62.) The induction energy transmission method was also used at Colorado Springs, to compare its efficacy with another energy transfer method that was under development (see method #4 below). In this case the resonant induction transmitter contained three tuned circuits, and the receiver had a single tuned circuit comprised of a one-turn inductance, a capacitor and a resistive load.

Here is a tuned circuit, you see, out in the field with three incandescent lamps and a condenser. The energy is transmitted inductively, from the oscillator. In this case, I have the primary supply circuit, the energizing condenser circuit, the primary inducing circuit, and the secondary in the field as in the fourth circuit, all tuned—four circuits in resonance.(Nikola Tesla On His Work With Alternating Currents and Their Application to Wireless Telegraphy, Telephony, and Transmission of Power, Twenty First Century Books, 1992, pp. 93-94.)
It is found that with the above circuits and under such conditions, about 1 mile communications should be possible. With circuits 1000 meters square, about 30 miles. From this, the inferiority of the induction method would appear to be immense as compared with disturbed charge of ground and air method.(Marincic, Aleksandar, ed., Nikola Tesla—Colorado Springs Notes, 1899-1900 Nolit, 1978, p. 29.)

Electrical conduction

Early type Tesla Magnifier
Early type Tesla Magnifier
Later type Tesla Magnifier
Later type Tesla Magnifier

From experiments performed between 1888 and 1907 Nikola Tesla concluded that the earth is an excellent electrical conductor, and an electric current can be made to propagate undiminished for distances of thousands of miles. It was also found that the earth’s natural electrical charge can be made to oscillate, "by impressing upon it [very low frequency] current waves of certain lengths, definitely related to its diameter." (The Future of the Wireless Art Wireless Telegraphy & Telephony, Walter W. Massie & Charles R. Underhill, 1908, pp. 67-71)

It was also discovered that the resistance of the earth is negligible due to its immense cross sectional area and relative shortness as compared to its diameter.

A [conducting] sphere of the size of a little marble offers a greater impediment to the passage of a current than the whole earth. . . . The resistance is only at the point where you get into the earth with your current. The rest is nothing.(Nikola Tesla On His Work With Alternating Currents and Their Application to Wireless Telegraphy, Telephony, and Transmission of Power, pp. 134-135)

126 x-Q. In this system, then, as you have described it, the current actually flows from the transmitter through the ground to the receiver; is that so?"

"Yes, sir; it does, in accordance with my understanding. In my Patent No. 649,621, “Apparatus for Transmission of Electrical Energy", [May 15, 1900] it is stated distinctly: “It is to be noted that the phenomenon here involved in the transmission of electrical energy is one of true conduction and is not to be confounded with the phenomena of electrical radiation, etc." (Nikola Tesla: Guided Weapons & Computer Technology, Leland Anderson, Twenty First Century Books, p. 82)

Tesla envisioned the development of a "world system" based upon these principles that would combine wireless telecommunications and electrical power transmission.

The currents are proportionate to the potentials which are developed under otherwise equal conditions. If you have an antenna of a certain capacity charged to 100,000 volts, you will get a certain current; charged to 200,000 volts, twice the current. When I spoke of these enormous potentials [on the order of 12 million volts], I was describing an industrial plant on a large scale because that [industrial power transmission] was the most important application of these principles, but I have also pointed out in my patents that the same principles can be applied to telegraphy and other purposes. That is simply a question of how much power you want to transmit. (Nikola Tesla On His Work With Alternating Currents and Their Application to Wireless Telegraphy, Telephony, and Transmission of Power, p. 145)

The communications component was his initial goal. While electrical power transmission was viewed as being of greater importance, the attempt at its large-scale implementation would have taken place only after feasibility of the basic concept had been established. In 1901 work began on a prototype world wireless station known as Wardenclyffe, that would have been the first in a system of interconnected towers designed for this purpose. The second facility was planned for the southern coast of England. Wardenclyffe was not completed due to financial difficulties.

Wireless energy transfer applications

The transmission of information

Several 20th century technology that use wireless power (and are in widespread use) included AM, FM, and TV broadcasting. Telecommunications and wireless internet was an application that began in the last decade of the 20th century. Wireless transmission of electricity aids navigation by the Global Positioning System.

The transmission of power

Devices using this principle to charge portable consumer electronics such as cell phones are commercially available.(SplashPower; Battery powered devices can be charged by placing them on an induction mat.) The Powercast system, unveiled in 2007, is applicable for a number of devices with low power requirements. This could include LEDs, computer peripherals, wireless sensors, and medical implants. A company called eCoupled unveiled their own take on inductive coupling, which will soon be used on "Herman Miller" desks to recharge devices wirelesly. Example include the transcutaneous energy transfer (TET) systems in artificial hearts like [[AbioCor] and induction stove tops (and microwave ovens).

A method for, "the transmission of electrical energy without wires" that depends upon electrical conduction through the earth was announced in 1904. In the distant future this system could allow for the elimination of many existing high-tension power transmission lines and facilitate the interconnection of electrical generation plants on a global scale.("The Transmission of Electrical Energy Without Wires," Electrical World, March 5, 1904)


Tesla transmission of electrical energy.
Tesla transmission of electrical energy.

Related patents

External articles, references, and further reading

Nikola Tesla
Books, essays, and papers
Other history
  • Little, Frank E., 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.
  • Coe, Lewis, "Wireless Radio: A History". McFarland & Company, Jul 1, 1996. ISBN 0-7864-0259-8
  • Brown, W. C.
    • "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.

External sites

See also

- PowerPedia main index
- PESWiki home page

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