Wireless energy transfer

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An artist's depiction of a solar satellite, which could send energy wirelessly to a space vessel or planetary surface.

Wireless energy transfer or wireless power transmission is the process that takes place in any system where electrical energy is transmitted from a power source to an electrical load, without interconnecting wires. Wireless transmission is employed in cases where instantaneous or continuous energy transfer is needed, but interconnecting wires are inconvenient, hazardous, or impossible.

Though the physics are related, this is distinct from electromagnetic transmission for the purpose of transferring information, where the amount of power transmitted is only important when it affects the integrity of the signal.

[edit] Introduction

In 1825 William Sturgeon invented the electromagnet, a conducting wire wrapped around an iron core. The principle of EM induction — that a changing magnetic field can induce an electrical current in an adjacent wire — was discovered by Michael Faraday in 1831. Combining these two discoveries, Nicholas Joseph Callan was the first to demonstrate the transmission and reception of electrical energy without wires. Callan’s 1836 induction coil apparatus consisted of two insulated coils — called the primary and secondary windings — both placed around a common iron core. A battery intermittently connected to the primary would ‘induce’ a voltage in the longer secondary causing a spark to jump across its free terminals.^[1][2]

In an induction coil or electrical transformer, which can have either an iron core or an air core, the transmission of energy takes place by simple electromagnetic coupling through a process known as mutual induction. With this method it is possible to transmit and receive energy 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 and the input current is modified from a sinusoidal into a rectangular waveform, significant power may be transmitted over a range of many meters.

Another means of wireless energy transfer is by electromagnetic radiation. In 1864 James Clark Maxwell mathematically modeled the behavior of electromagnetic radiation. Some early work in the area of wireless transmission via radio waves was done in 1888 by Heinrich Hertz who performed experiments that validated Maxwell’s mathematical model. Hertz’s apparatus for generating electromagnetic waves is generally acknowledged as the first radio transmitter. A few years later Guglielmo Marconi worked with a modified form of the Hertz-wave transmitter, the main improvement being the addition of an elevated conductor and a ground connection. Both of these elements can be traced back to the 1749 work of Benjamin Franklin and that of Mahlon Loomas in 1864.

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.^[3] 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.

As wireless technologies were being developed during the early 1900s, researchers further investigated these different wireless transmission methods. The goal was simply to generate an effect locally and detect it at a distance. Around the same time, efforts began to power more significant loads than the high-resistance sensitive devices that were being used to simply detect the received energy. At the St. Louis World's Fair (1904), a prize was offered for a successful attempt to drive a 0.1 horsepower (75 W) air-ship motor by energy transmitted through space at a distance of least 100 feet (30 m).^[4]

Except for RFID tags, wireless power transmission over room-sized or community-sized distances has not been widely implemented. Rightly or not, it has been assumed by some that any system for broadcasting energy to power electrical devices will have negative health implications. With focused beams of microwave radiation there are definite health and safety risks. Considering the hazards associated with powerful ionizing radiation the physical alignment and targeting of devices to receive the energy beam is particularly worrying.

[edit] Wireless energy transmission methods

These include but are not necessarily limited to the following:

[edit] Electromagnetic induction

Main article: Electromagnetic induction

The action of an electrical transformer is the simplest instance 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 mutual induction. (An added benefit is the capability to step the primary voltage either up or down.) The electric toothbrush charger is an example of how this principle can be used. The main drawback to induction, however, is the short range. The receiver must be in very close proximity to the transmitter or induction unit in order to inductively couple with it.

[edit] Applications

• The electric toothbrush battery charger
• The induction cooker stovetop

It can be argued the cookware part of an induction cooker is not a secondary in the strictest sense of the term. It is more accurately described as the non-laminated core of an alternating-current electromagnet, in which eddy currents are induced resulting in the heating effect.

• Transcutaneous energy transfer (TET) systems in artificial hearts like AbioCor and other surgically implanted devices.
• Devices using induction to charge portable consumer electronics such as cell phones.^[5][6]

[edit] Electromagnetic radiation

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.

Main article: Electromagnetic radiation

Electromagnetic radiation in the form of either radio waves or light can also be used to for power transmission. While systems using this method are mostly for communications, a modicum of efficiency in power transmission is achievable under certain circumstances.

[edit] Radio and microwave

Main article: Microwave power transmission

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 using a greatly improved transmitter.

Japanese researcher Hidetsugu Yagi also investigated wireless energy transmission using a directional array antenna that he designed. In February 1926, Yagi and 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 exceptional performance characteristics and robustness.^[7][8]

Power transmission via radio waves is 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 realized 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.

[edit] Low power

A new company, Powercast introduced wireless power transfer technology using RF energy at the 2007 Consumer Electronics Show, winning best Emerging Technology.^[9] The Powercast system is applicable for a number of devices with low power requirements. This could include LEDs, computer peripherals, wireless sensors, and medical implants. Currently, it achieves a maximum output of 6 volts for a little over one meter. It is expected for arrival late 2007.^[10]

[edit] High power

Wireless Power Transmission (using microwaves) is well proven. Experiments in the tens of killowatts have been performed at Goldstone in California in 1975^[11][12] and more recently (1997) at Gand Bassin on Reunion Island^[13]

[edit] Light

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:

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 under ideal conditions.
3. Atmospheric absorption causes losses.
4. As with microwave beaming, this method requires a direct line of sight with the target.

NASA has demonstrated flight of lightweight model plane powered by a laser beam.

[edit] Size and power level

The size of the components is dictated by:

• distance from transmitter to receiver
• the wavelength of the radiation
• the laws of physics, specifically the Rayleigh Criterion or Diffraction limit, used in standard RF (Radio Frequency) antenna design, which also applies to lasers. These laws dictate that any beam will spread (microwave or laser) and become weaker and more diffuse over greater distance. The larger the transmitter antenna or laser aperture, the tighter the beam and the less it will spread as a function of distance (and vice versa). Smaller antennas also suffer from excessive losses due to sidelobes.

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 of the media through which the radiation passes. That process is known as calculating a Link Budget.

[edit] Efficiency issues

This article or section may be confusing or unclear for many readers. Please improve the article or discuss this issue on the talk page. This article has been tagged since March 2007.

Microwave power beaming achieves much higher conversion efficiency than lasers, and is less prone to atmospheric attenuation.

The most efficient laser power beaming system today has photovoltaic panels on the climber optimized to the wavelength of the laser. With the best (and most expensive) current usable technology, efficiency is around 0.5%. If climbers are to be disposable, the most expensive photoelectric panels may not be an option. Losses due to atmospheric spreading can be reduced by the use of adaptive optics, or power beaming from outside the atmosphere, and losses due to absorption can be reduced by a properly chosen laser wavelength, although laser power beaming from the ground does not work through clouds.

Although laser and photovoltaic technologies have been rapidly advancing, it is unknown what transmission efficiency improvement is possible. Further optimization of photovoltaics, for example, typically rely on enhancing the absorption of particular wavelengths, which may not match up with the wavelengths of more efficient lasers. The most efficient lasers — laser diode arrays, which can surpass 50% efficiency — currently have poor coherence, and could not be used, leaving as available options standard chemical lasers with efficiencies of a few percent or less. Only the advent of high-coherence diode laser arrays or a similar technology would allow for notably improved power usage efficiency, as laser efficiency comprises most of the energy loss, and there is only a limited amount of improvement possible in photovoltaic panels.

Taking the theoretical example of transferring 50 MJ of energy from one place to another (see space elevator and space elevator economics): The base cost of payload transfer, given the current power grid rate of about US$0.11/kW·h = about US$0.03/MJ,^[14] is around US$1.74/kg. Factoring for transmission losses, assuming current laser efficiencies of 2%, solar cell efficiencies of 30%, and atmospheric losses of about 20%, this works out to about 0.5% overall efficiency, or $350/kg.

[edit] Evanescent wave coupling

The factual accuracy of this section is disputed. Please see the relevant discussion on the talk page

Main article: Evanescent wave coupling

In 2006, Marin Soljačić and other researchers at the Massachusetts Institute of Technology discovered a new way to wirelessly transfer power using non-radiative electromagnetic energy resonant tunneling.^[15][16][17] Their theoretical analysis showed that by sending electromagnetic waves around in a highly angular waveguide, evanescent waves are produced which carry no energy. 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.^[15]

While the work of Soljačić, et al was only theoretical, Nikola Tesla actually experimented with this very same method of wireless power transmission in the early 1890s.^{[dubious — see talk page]} His work started at 35 South 5th Ave., New York City and the method was subsequently adopted for lighting purposes at another laboratory at 46 Houston St.^[18] The induction power transmission method was also experimented with at Colorado Springs. The resonant induction transmitter contained three tuned circuits. The induction receiver had a single tuned circuit comprised of a one-turn inductance, a capacitor and a resistive load.

"Resonant inductive coupling" has key implications in solving 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 solenoids with closely spaced capacitor plates on each end, which in combination allow the coil to be tuned to the transmitter frequency thereby eliminating the wide energy wasting "wave problem" and allowing the energy used to focus in on a specific frequency increasing the range.

[edit] Electrical conduction

Main article: Electrical conduction

Electrical energy can also be transmitted by means of electrical currents made to flow through naturally existing conductors, specifically the earth, lakes and oceans, and through the atmosphere — a natural medium that can be made conducting if the breakdown voltage is exceeded and the gas becomes ionized. For example, when a high voltage is applied across a neon tube the gas becomes ionized and a current passes between the two internal electrodes.

In a practical wireless energy transmission system using this principle, a high-power ultraviolet beam might be used to form a vertical ionized channel in the air directly above the transmitter-receiver stations. The same concept is used in virtual lightning rods, the electrolaser electroshock weapon^[19] and has been proposed for disabling vehicles.^[20][21][22]

A "world system" for "the transmission of electrical energy without wires" that depends upon the electrical conductivity of the earth was proposed by Tesla in 1904.^[23] The Tesla effect (named in honor of Nikola Tesla) is an archaic term for an application of a type of electrical conduction (that is, the movement of energy through matter; not just the production of voltage across a conductor).^{[24][25] [26]} Through longitudinal waves, an operator uses the Tesla effect in the wireless transfer of energy to a receiving device. The Tesla effect is a type of high field gradient between electrode plates for wireless energy transfer. The Tesla effect uses high frequency alternating current potential differences transmitted between two plates or nodes. The electrostatic forces through natural media across a conductor situated in the changing magnetic flux can transfer power to the conducting receiving device (such as Tesla's wireless bulbs).

Currently, the effect has been appropriated by some in the fringe scientific community as an effect which purportedly causes man-made earthquakes from electromagnetic standing waves, for example Tesla's teleforce via mechanical earth-resonance concepts.^{[27] [28]} A number of modern writers have "reinterpreted" and expanded upon Tesla's original writings. In the process, they have invoked behavior and phenomena that are often inconsistent with experimental observation and mainstream science. The wireless system would combine electrical power transmission along with broadcasting and wireless telecommunications, allowing for the elimination of many existing high-tension power transmission lines and facilitate the interconnection of electrical generation plants on a global scale, but has never been constructed or proven feasible.

[edit] See also

energy Portal

• Transmission medium
• Electric power transmission
  • Electric power transmission#Alternate transmission methods
• Spacecraft propulsion
• Distributed generation
• Electricity distribution
• Electricity market
• Electric power transmission
• HV-AC, High voltage alternating current
• Radiant energy
• Electrodynamic tether
• Rectenna
• Solar power satellite

[edit] Related patents

• U.S. Patent 649621 , "Apparatus for Transmission of Electrical Energy".
• U.S. Patent 685953 , "Apparatus for Utilizing Effects Transmitted from a Distance to a Receiving Device through Natural Media".
• U.S. Patent 685954 , "Method of Utilizing Effects Transmitted through Natural Media".
• U.S. Patent 514168 , "Means for Generating Electric Currents".
• U.S. Patent 593138 , "Electrical Transformer".
• U.S. Patent 685955 , "Apparatus for Utilizing Effects Transmitted From A Distance To A Receiving Device Through Natural Media".
• U.S. Patent 685956 , "Apparatus for Utilizing Effects Transmitted through Natural Media".
• U.S. Patent 685957 , "Apparatus for the Utilization of Radiant Energy".
• U.S. Patent 685958 , "Method of Utilizing of Radiant Energy".
• U.S. Patent 787412 , "Art of Transmitting Electrical Energy through the Natural Mediums".
• U.S. Patent 1119732  , "Apparatus for Transmitting Electrical Energy".
• U.S. Patent 1990977 , "Energy transmission system".
• U.S. Patent 2415688 , "Induction device".
• U.S. Patent 3781647 , "Method And Apparatus For Converting Solar Radiation To Electrical Power".
• U.S. Patent 6798716 , "System and method for wireless electrical power transmission ".
• U.S. Patent 6906495 , "Contact-less power transfer".

[edit] References

1. ^ Reville, William, “Nicholas Callan – Priest Scientist at Maynooth,” University College, Cork
2. ^ The original induction coil was invented in 1836 by Nicholas Callan (1799-1864), a priest and the professor of natural philosophy at St. Patrick's College at Maynooth, County Kildare, Ireland.
3. ^ Nikola Tesla's Priority In the Invention of Radio
4. ^ The Electrician (London), September 1902, pages 814-815).
5. ^ SplashPower; Battery powered devices can be charged by placing them on an induction mat.
6. ^ eCoupled unveiled their own take on inductive coupling, which will soon be used on [http://www.hermanmiller.com "Herman Miller" desks to recharge devices wirelessly]
7. ^ Yagi antenna
8. ^ "Scanning the Past: A History of Electrical Engineering from the Past, Hidetsugu Yagi"
9. ^ "CES Best of 2007"
10. ^ EE Times: Practical apps in works for wireless energy transfer - R. Colin Johnson 01/22/2007
11. ^ Wireless Power Transmission for Solar Power Satellite (SPS) (Second Draft by N. Shinohara), Space Solar Power Workshop, Georgia Institute of Technology
12. ^ Brown., W. C. (September 1984). "The History of Power Transmission by Radio Waves". Microwave Theory and Techniques, IEEE Transactions on (Volume: 32, Issue: 9 On page(s): 1230- 1242 + ISSN: 0018-9480). 
13. ^ POINT-TO-POINT WIRELESS POWER TRANSPORTATION IN REUNION ISLAND 48th International Astronautical Congress, Turin, Italy, 6-10 October 1997 - IAF-97-R.4.08 J. D. Lan Sun Luk, A. Celeste, P. Romanacce, L. Chane Kuang Sang, J. C. Gatina - University of La Réunion - Faculty of Science and Technology.
14. ^ Cost of lavish Christmas lights display offset by simple measures - Oak Ridge National Laboratory Dec. 20, 2002
15. ^ ^{a b} "'Evanescent coupling' could power gadgets wirelessly", NewScientist.com news service, November 15, 2006 Accessed: January 8, 2007
16. ^ Karalis, Aristeidis; J.D. Joannopoulos, Marin Soljačić (November 2006). "Efficient wireless non-radiative mid-range energy transfer". arXiv:physics/0611063. Retrieved on 2007-02-24. 
17. ^ Wireless energy could power consumer, industrial electronics — MIT press release
18. ^ Nikola Tesla: Guided Weapons & Computer Technology, Leland Anderson, Ed., Twenty First Century Books, Breckenridge, 1998, p. 62.
19. ^ A Survey of Laser Lightning Rod Techniques - Barnes, Arnold A., Jr. ; Berthel, Robert O.
20. ^ What is LIPC? - Ionatron directed-energy weapons
21. ^ Frequently Asked Questions - HSV Technologies
22. ^ Vehicle Disabling Weapon by Peter A. Schlesinger, President, HSV Technologies, Inc. - NDIA Non-Lethal Defense IV 20-22 Mar 2000
23. ^ "The Transmission of Electrical Energy Without Wires," Electrical World, March 5, 1904
24. ^ Norrie, H. S., "Induction Coils: How to make, use, and repair them". Norman H. Schneider, 1907, New York. 4th edition.
25. ^ Electrical experimenter, January 1919. pg. 615
26. ^ Tesla: Man Out of Time By Margaret Cheney. Page 174.
27. ^ Bearden, T. E., Tesla's Secret and the Soviet Tesla Weapons.
28. ^ Vassilatos, Gerry, Secrets of Cold War Technology

[edit] External links

[edit] Nikola Tesla

• Cheney, Margaret "Tesla: Man Out of Time". Simon and Schuster, Oct 2, 2001. ISBN 0-7432-1536-2
• Grotz, Toby, "Project Tesla: Wireless Transmission of Power; Resonating Planet Earth". Theoretical Electromagnetic Studies and Learning Association, Inc.
• "Tesla: Life and legacy; Colorado Springs". PBS.
• 1931 Electric Pierce Arrow Anecdote

[edit] Books, essays, and papers

• Benson, Thomas W., "Wireless Transmission of Power now Possible". Electrical experimenter, March 1920.
• Aidinejad, Ahamid and James F. Corum, "The Transient Propagation of ELF Pulses in the Earth-Ionosphere Cavity".
• Grotz, Toby, "Artificially Stimulated Resonance of the Earth's Schumann Cavity Waveguide". Proceedings of the Third International New Energy Technology Symposium/Exhibition, June 25th-28th, 1988.
• McSpadden, James O.
  • "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.
• Karalis, Aristeidis, J. D. Joannopoulos and Marin Soljačić- "Wireless Non-Radiative Energy Transfer", Massachusetts Institute of Technology, November 2006.

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.

[edit] Websites

• Project Tesla
• History of Microwave Power Transmission before 1980
• The SHARP's Stationary High Altitude Relay Platform, microwave beam powered
• MIT project
• MIT at Wireless Power
• Wireless energy could power consumer, industrial electronics
• Powercast
• Howstuffworks "How Wireless Power Works" Describes wireless power transmission over short, medium, and very long distances.