Wireless energy transmission is the process that takes place in any system where electromagnetic energy is transferred from an energy source to an energy absorber or energy sink without any interconnecting wires. Such systems are used for communications, remote sensing, and electrical power transmission. Wireless transmission is employed in those cases where interconnecting wires are inconvenient, impossible, or deemed to be hazardous.
Introduction
The transmission of electrical energy without wires was first demonstrated around 1856 in the form of mutual induction. Using induction it is possible to transmit and receive energy over a considerable distance. However, to transfer power the two inductors must be placed fairly close together. If pulse-excited resonant coupling is used, where both inductors are tuned to a mutual frequency, a considerable amount of energy may be transmitted over a range of many meters. Another form of wireless energy transmission is by means of electromagnetic radiation, such as used in radio. Energy can also be transmitted by means of electrical currents made to flow through a naturally existing conductor, or a natural medium that can be made conducting.
As wireless technologies were being developed during the early 1900s, researchers further investigated these different transmission methods. The original goal was simply to generate an effect locally and detect it at a distance. Around the same time, efforts were made to power more significant loads than the high-resistance sensitive devices that were being used to simply detect the received energy.
Except for RFID tags and certain devices for covert surveillance, use of wireless energy transmission for powering devices over room-sized or community-sized ranges has not yet been widely implemented.
Wireless energy transmission methods
These include, but are not limited to the following:
Electromagnetic induction
The action of an electrical transformer is probably the simplest example of wireless energy transmission. The primary and secondary circuits of a transformer are electrically isolated from each other. The transmission 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.)
Thomas Edison developed an induction-based wireless communications system.
Electric toothbrush chargers use this principle, as do induction cookers. In the latter, the electrical energy is wirelessly transmitted into the metal cookware, where the potential is converted into heat.
Electromagnetic radiation
Electromagnetic radiation in the form of either radio waves or light is also used to wirelessly transfer energy. While systems based upon this method are used mostly for communications, a relatively 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 conduction-based wireless energy transmission 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. [1] [2]
Power transmission via radio waves is by achieved by using shorter wavelengths of electromagnetic radiation, typically in the microwave range. A rectenna is used to convert microwave energy back into electricity. Conversion efficiencies exceeding 95% have been achieved in this manner.
Light has also been used as a medium for wireless transmission. The heliograph is an early example of a light-based system. Data transmission via fiber-optic cable is a current example.
Power can be transmitted by converting electricity into a laser beam that is then directed towards a photovoltaic receiver. This is generally known as "power beaming." Photovoltaic cells can also be used to receive energy from Earth's strongest natural source of electromagnetic radiation, the Sun.
Resonant electromagnetic induction or evanescent wave coupling
Tesla performed experiments with wireless transmission by pulse-excited resonant inductive coupling 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. [3] The induction energy transmission method was also used at Colorado Springs, to compare its efficacy with another energy transmission 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.[4]
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.[5]
Researchers at MIT rediscovered this way to wirelessly transmit energy. According to their model the transfer of energy takes place by electromagnetic resonant tunneling. Electromagnetic waves sent around a highly angular waveguide produce evanescent waves that carry no energy. If an identical resonant waveguide receiver 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.
Pulse-excited resonant inductive coupling has key implications in the solution of the two main problems associated with non-resonant inductive coupling and electromagnetic radiation, 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 it. The relatively short range is due to the amount of power required to produce field. Over greater distances the non-resonant induction method wastes much of the transmitted energy just to increase range. Resonance improves efficiency by "tunneling" the magnetic field to a receiver coil that resonates at the exact same frequency. Unlike the multiple-layer secondary of a non-resonant transformer, such receiving coils have just one turn with closely spaced capacitor plates positioned on each end.
Electrical conduction
From experiments performed between 1888 and 1907 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." [6]
It was concluded 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. . . . [6]
The resistance is only at the point where you get into the earth with your current. The rest is nothing.[7]
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.” [8]
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. [9]
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.
Tesla publicly announced his method for "the transmission of electrical energy without wires" in 1904. In the distant future this system could be used to augment or perhaps even replace our existing wireless broadcasting and P2P telecomunications systems, and allow for the elimination of many existing high-tension power transmission lines and facilitate the interconnection of electrical generation plants on a global scale.[10]
References
- ^ Yagi antenna
- ^ "Scanning the Past: A History of Electrical Engineering from the Past, Hidetsugu Yagi"
- ^ Nikola Tesla: Guided Weapons & Computer Technology, Leland Anderson, Ed., Twenty First Century Books, Breckenridge, 1998, p. 62.
- ^ 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.
- ^ Marincic, Aleksandar, ed., Nikola Tesla—Colorado Springs Notes, 1899-1900 Nolit, 1978, p. 29.
- ^ a b The Future of the Wireless Art Wireless Telegraphy & Telephony, Walter W. Massie & Charles R. Underhill, 1908, pp. 67-71
- ^ Nikola Tesla On His Work With Alternating Currents and Their Application to Wireless Telegraphy, Telephony, and Transmission of Power, pp. 134-135
- ^ Nikola Tesla: Guided Weapons & Computer Technology, Leland Anderson, Twenty First Century Books, p. 82
- ^ Nikola Tesla On His Work With Alternating Currents and Their Application to Wireless Telegraphy, Telephony, and Transmission of Power, p. 145
- ^ "The Transmission of Electrical Energy Without Wires," Electrical World, March 5, 1904