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Wireless power 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 in an electrical grid. Wireless transmission is ideal in cases where instantaneous or continuous energy transfer is needed, but interconnecting wires are inconvenient, hazardous, or impossible.

Though the physics of both are related, this is distinct from wireless transmission for the purpose of transferring information (such as radio) through waves, where the percentage of the power that is received is only important if it becomes too low to successfully recover the signal. With wireless energy transfer, the efficiency is a more critical parameter and this creates important differences in these technologies.

The most common and the most viable form of wireless power transfer is carried out using Inductive Power Transfer.

History of Inductive Power Transfer

  • 1820: André-Marie Ampère describes Ampere’s law showing that electric current produces a magnetic field
  • 1831: Michael Faraday describes Faraday’s law of induction an important basic law of electromagnetism
  • 1864: James Clerk Maxwell mathematically modeled the behavior of electromagnetic radiation.
  • 1888: Heinrich Rudolf Hertz confirmed the existence of electromagnetic radiation. Hertz’s "apparatus for generating electromagnetic waves" is generally acknowledged as the first radio transmitter.
  • 1893: Nikola Tesla demonstrated the illumination of vacuum bulbs wirelessly (without any wires connected to the bulbs) at the World Columbian Exposition in Chicago.
  • 1894: Hutin & LeBlanc, espouse long held view that inductive energy transfer should be possible, they file a US Patent describing a system for power transfer at 3 kHz
  • 1894: Jagdish Chandra Bose ignited gunpowder and rang a bell at a distance using electromagnetic waves, showing that communication signals can be sent without using wires.[1][2]
  • 1895: Jagdish Chandra Bose transmitted signals over a distance of nearly a mile.[1][2]
  • 1897: Guglielmo Marconi had transmitted Morse code signals over a distance of about 6 km.
  • 1897: Nikola Tesla (inventor of radio[3], microwaves and Alternating current motor) filed his first patents dealing with Wardenclyffe tower.
  • 1900: Marconi failed to get a patent for Radio in the United States. The patent office mentioned "Marconi's pretended ignorance of the nature of a "Tesla oscillator" being little short of absurd..."[citation needed].
  • 1901: Guglielmo Marconi first transmitted and received signals across the Atlantic Ocean. Engineer Otis Pond working for Tesla, said, "Looks as if Marconi got the jump on you." Tesla replied, "Marconi is a good fellow. Let him continue. He is using seventeen of my patents."
  • 1904: at the St. Louis World's Fair, 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]
  • 1926: Shintaro Uda and Hidetsugu Yagi published their first paper on Uda's "tuned high-gain directional array"[5] better known as the Yagi antenna.
  • 1964: William C. Brown demonstrated on CBS News with Walter Cronkite a microwave-powered model helicopter that received all the power needed for flight from a microwave beam. Between 1969 and 1975 Brown was technical director of a JPL Raytheon program that beamed 30 kW over a distance of 1 mile at 84% efficiency.
  • 1971: Prof. Don Otto develops a small trolley powered by Inductive Power Transfer at The University of Auckland, in New Zealand.
  • 1975: Goldstone Deep Space Communications Complex did experiments in the tens of kilowatts. [6][7][8]
  • 1988: A power electronics group led by Prof. John Boys at The University of Auckland in New Zealand, develop an inverter using novel engineering materials and power electronics and conclude that inductive power transmission should be achievable. A first prototype for a contact-less power supply is built. Auckland Uniservices, the commercial company of The University of Auckland, Patents the Technology.
  • 1989: Daifuku, a Japanese company, engages Auckland Uniservices Ltd to develop the technology for car assembly plants and materials handling providing challenging technical requirements including multiplicity of vehicles
  • 1990: Prof. John Boys team develops novel technology enabling multiple vehicles to run on the same inductive power loop and provide independent control of each vehicle. Auckland UniServices Patents the technology.
  • 1996: Auckland Uniservices develops an Electric Bus power system using Inductive Power Transfer to charge(30-60kW) opportunistically commencing implementation in New Zealand. Prof John Boys Team commission 1st commercial IPT Bus in the world at Whakarewarewa, in New Zealand.
  • 2004: Inductive Power Transfer, developed at The University of Auckland and Patented by Auckland Uniservices Ltd, is now used by 90 per cent of the US$1 billion clean room industry for materials handling equipment in semiconductor, LCD and plasma screen manufacture. Daifuku, a Licensee of Auckland Uniservices, own more than 50% of market share
  • 2005: Prof Boys' team at The University of Auckland, refines 3-phase IPT Highway and pick-up systems allowing transfer of power to moving vehicles in the lab
  • 2007: A physics research group, led by Prof. Marin Soljacic, at MIT confirm the earlier(1980's) work of Prof. John Boys by wireless powering of a 60W light bulb with 40% efficiency at a 2m (7ft) distance using two 60cm-diameter coils.
  • 2008: Intel reproduces Prof. John Boys group's 1980's experiments by wirelessly powering a light bulb with 75% efficiency.

Size, distance, and efficiency

The size of the components is dictated by the distance from transmitter to receiver, the wavelength and the Rayleigh Criterion or Diffraction limit, used in standard RF (Radio Frequency) antenna design, which also applies to lasers.

The Rayleigh Criterion dictates that any beam will spread (microwave or laser) and become weaker and diffuse over 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 antennae also suffer from excessive losses due to side lobes.

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 medium through which the radiation passes. That process is known as calculating a link budget.

Ultimately, beamwidth is physically determined by diffraction due to the dish size in relation to the wavelength of the electromagnetic radiation used to make the beam. Microwave power beaming can be more efficient than lasers, and is less prone to atmospheric attenuation caused by dust or water vapor losing atmosphere to vaporize the water in contact.

Near field

These are wireless transmission techniques over distances comparable to, or a few times the diameter of the device(s).

Induction

Main article: Inductive coupling

The action of an electrical transformer is the simplest instance of wireless energy transfer. The primary and secondary circuits of a transformer are not directly connected. 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 battery charger of an electric toothbrush is an example of how this principle can be used. The main drawback to induction, however, is the short range. The receiver must be very close to the transmitter or induction unit in order to inductively couple with it.

Resonant induction

In November 2006, Marin Soljačić and other researchers at the Massachusetts Institute of Technology applied the near field behaviour well known in electromagnetic theory to a wireless power transmission concept based on strongly-coupled resonators.[9][10][11] In a theoretical analysis (see Ref: Annals of Physics), they demonstrate that, by designing electromagnetic resonators that suffer minimal loss due to radiation and absorption and have a near field with mid-range extent (namely a few times the resonator size), mid-range efficient wireless energy-transfer is possible. The reason is that, if two such resonant objects are brought in mid-range proximity, their near fields (consisting of so-called 'evanescent waves') couple (evanescent wave coupling) and can allow the energy to tunnel/transfer from one object to the other within times much shorter than all loss times, which were designed to be long, and thus with the maximum possible energy-transfer efficiency. Since the resonant wavelength is much larger than the resonators, the field can circumvent extraneous objects in the vicinity and thus this mid-range energy-transfer scheme does not require line-of-sight. By utilizing in particular the magnetic field to achieve the coupling, this method can be safe, since magnetic fields interact weakly with living organisms.

On June 7, 2007, it was reported that a prototype system had been implemented.[12][13] In an experimental demonstration (see Ref: Science), the MIT researchers successfully demonstrated the ability to power a 60-watt light bulb wirelessly using two copper coils of 60cm diameter that were 2m (7ft) away at roughly 45% efficiency. The coils were designed to resonate together at 10MHz and were oriented along the same axis. One was connected inductively to a power source, and the other one to a bulb. The setup powered the bulb on, even when the direct line of sight was blocked using a wooden panel.

"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.

Beginning in the early 1960s resonant inductive wireless energy transfer was used successfully in implantable medical devices [14] including such devices as pacemakers and artificial hearts. While the early systems used a resonant receiver coil later systems [15] implemented resonant transmitter coils as well. These medical devices are designed for high efficiency using low power electronics while efficiently accommodating some misalignment and dynamic twisting of the coils. The separation between the coils in implantable applications is commonly less than 20 cm. Today resonant inductive energy transfer is regularly used for providing electric power in many commercially available medical implantable devices.[16]

Wireless electric energy transfer for experimentally powering electric automobiles and buses is a higher power application (>10kW) of resonant inductive energy transfer. High power levels are required for rapid recharging and high energy transfer efficiency is required both for operational economy and to avoid negative environmental impact of the system. An experimental electrified roadway test track built circa 1990 achieved 80% energy efficiency while recharging the battery of a prototype bus at a specially equipped bus stop [17] [18]. The bus could be outfitted with a retractable receiving coil for greater coil clearance when moving. The gap between the transmit and receive coils was designed to be less than 10 cm when powered. In addition to buses the use of wireless transfer has been investigated for recharging electric automobiles in parking spots and garages as well.

Some of these wireless resonant inductive devices operate at low milliwatt power levels and are battery powered. Others operate at higher kilowatt power levels. Current implantable medical and road electrification device designs achieve more than 75% transfer efficiency at an operating distance between the transmit and receive coils of less than 10 cm.

Far field

Means for long conductors of electricity forming part of an electric circuit and electrically connecting said ionized beam to an electric circuit. (U.S. patent 1,309,031)

These methods achieve longer ranges, often multiple kilometre ranges, where the distance is much greater than the diameter of the device(s).

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 later Guglielmo Marconi worked with a modified form of Hertz's transmitter. Nikola Tesla also investigated radio transmission and reception.

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 excellent performance characteristics.[5]

Power transmission via radio waves can be made more directional, allowing longer distance power beaming, with shorter wavelengths of electromagnetic radiation, typically in the microwave range. A rectenna may be used to convert the microwave energy back into electricity. Rectenna conversion efficiencies exceeding 95% have been realized. 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.[19][20]

Power beaming by microwaves has the difficulty that for most space applications the required aperture sizes are very large. For example, the 1978 NASA Study of solar power satellites required a 1-km diameter transmitting antenna, and a 10 km diameter receiving rectenna, for a microwave beam at 2.45 GHz. These sizes can be somewhat decreased by using shorter wavelengths, although short wavelengths may have difficulties with atmospheric absorption and beam blockage by rain or water droplets. Because of the Thinned array curse, it is not possible to make a narrower beam by combining the beams of several smaller satellites.

For earthbound applications a large area 10 km diameter receiving array allows large total power levels to be used while operating at the low power density suggested for human electromagnetic exposure safety. A human safe power density of 1 mW/cm2 distributed across a 10 km diameter area corresponds to 750 megawatts total power level. This is the power level found in many modern electric power plants.

High power

Wireless Power Transmission (using microwaves) is well proven. Experiments in the tens of kilowatts have been performed at Goldstone in California in 1975[6][7][21] and more recently (1997) at Grand Bassin on Reunion Island.[22]

These methods achieve distances on the order of a kilometer.

Low power

A new company, Powercast introduced wireless power transfer technology using RF energy at the 2007 Consumer Electronics Show, winning best Emerging Technology.[23] 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.[24]

A different low-power wireless power technology has been proposed by Landis.[25]

Laser

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.

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 "powerbeaming". Its drawbacks are:

  1. Conversion to light, such as with a laser, is moderately inefficient (although quantum cascade lasers improve this)
  2. Conversion back into electricity is moderately inefficient, with photovoltaic cells achieving 40%-50% efficiency.[26] (Note that conversion efficiency is rather higher with monochromatic light than with insolation of solar panels).
  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 a lightweight model plane powered by a laser beam (while it's not using the system described above but the solar sail technic)

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[27] and has been proposed for disabling vehicles.[28][29][30]

The Tesla effect.[31][32][33]. A "world system" for "the transmission of electrical energy without wires" that depends upon electrical conductivity was proposed by Tesla.[34] Through longitudinal waves, an operator uses the Tesla effect in the wireless transfer of energy to a receiving device.

A "world system" for "the transmission of electrical energy without wires" that depends upon the electrical conductivity was proposed by Nikola Tesla as early as 1904.[34] The Tesla effect is the application of a type of electrical conduction (that is, the movement of energy through space and matter; not just the production of voltage across a conductor).[35][36][37] Tesla stated,

Instead of depending on induction at a distance to light the tube [... the] ideal way of lighting a hall or room would [...] be to produce such a condition in it that an illuminating device could be moved and put anywhere, and that it is lighted, no matter where it is put and without being electrically connected to anything. I have been able to produce such a condition by creating in the room a powerful, rapidly alternating electrostatic field. For this purpose I suspend a sheet of metal a distance from the ceiling on insulating cords and connect it to one terminal of the induction coil, the other terminal being preferably connected to the ground. Or else I suspend two sheets as [...] each sheet being connected with one of the terminals of the coil, and their size being carefully determined. An exhausted tube may then be carried in the hand anywhere between the sheets or placed anywhere, even a certain distance beyond them; it remains always luminous.[38][39]

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.

Wireless transmission of power and energy demonstration during his high frequency and potential lecture of 1891.

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, related to Tesla's telegeodynamics mechanical earth-resonance concepts.[40][41] A number of modern writers have "reinterpreted" and expanded upon Tesla's original writings. In the process, they have sometimes invoked behavior and phenomena that are inconsistent with experimental observation. On the other hand, a number of researchers have experimented with Tesla's basic wireless energy transmission system design and made physical observations that are inconsistent with some basic tenets of mainstream science [citation needed].

The Tesla world 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. However, a close reading of Tesla's patents suggests that he may have misinterpreted the 25-70 km nodal structures associated with lightning that he observed during his 1899 Colorado Springs experiments in terms of circumglobally propagating standing waves instead of as the well known local interference between direct and reflected waves between the ground and the ionosphere (not known to exist at the time). Many of the properties of the real earth-ionosphere cavity that have subsequently been mapped in great detail were unknown to Tesla, and a consideration of the earth-ionosphere waveguide propagation parameters as they are known today shows that Tesla's concept of a global wireless power grid is not practically realizable.[citation needed]

Tesla patents

Tesla coil transformer wound in the form of a flat spiral. This is the transmitter form as described in U.S. patent 645,576.
Further information: Tesla patents

Nikola Tesla had multiple patents disclosing long distance power transmission. Tesla, in U.S. patent 0,645,576 System of Transmission of Electrical Energy and U.S. patent 0,649,621 Apparatus for Transmission of Electrical Energy, described new and useful combinations of transformer coils. The transmitting coil or conductor arranged and excited to cause currents or oscillation to propagate in the medium of conduction through the natural medium from one point to another remote point therefrom and a receiver coil or conductor of the transmitted signals.[42] The production of currents at very high potential could be attained in these coils. U.S. patent 0,787,412 Art of Transmitting Electrical Energy through the Natural Mediums describes a combined system for wireless telecommunications and electrical power distribution achieved through the use of earth-resonance principles.

See also


Notes

  1. ^ a b "The Work of Jagdish Chandra Bose: 100 years of mm-wave research". tuc.nrao.edu.
  2. ^ a b "Jagadish Chandra Bose", ieeeghn.org.
  3. ^ Nikola Tesla's Priority In the Invention of Radio
  4. ^ The Electrician (London), 1904).
  5. ^ a b "Scanning the Past: A History of Electrical Engineering from the Past, Hidetsugu Yagi"
  6. ^ a b NASA Video, date/author unknown
  7. ^ a b Wireless Power Transmission for Solar Power Satellite (SPS) (Second Draft by N. Shinohara), Space Solar Power Workshop, Georgia Institute of Technology
  8. ^ Brown., W. C. (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). {{cite journal}}: |issue= has extra text (help); Unknown parameter |month= ignored (help)
  9. ^ "Wireless electricity could power consumer, industrial electronics". MIT News. 2006-11-14. {{cite web}}: Check date values in: |date= (help)
  10. ^ "Gadget recharging goes wireless". Physics World. 2006-11-14. {{cite web}}: Check date values in: |date= (help)
  11. ^ "'Evanescent coupling' could power gadgets wirelessly". NewScientist.com news service. 2006-11-15. {{cite web}}: Check date values in: |date= (help)
  12. ^ "Goodbye wires…". MIT News. 2007-06-07. {{cite web}}: Check date values in: |date= (help)
  13. ^ "Wireless power a reality". Physics World. 2007-06-07. {{cite web}}: Check date values in: |date= (help)
  14. ^ J. C. Schuder, “Powering an artificial heart: Birth of the inductively coupled-radio frequency system in 1960,” Artificial Organs, vol. 26, no. 11, pp. 909–915, 2002.
  15. ^ SCHWAN M. A. and P.R. Troyk, "High efficiency driver for transcutaneously coupled coils" IEEE Engineering in Medicine & Biology Society 11th Annual International Conference, November 1989, pp. 1403-1404.
  16. ^ What is a cochlear implant?
  17. ^ Systems Control Technology, Inc, "Roadway Powered Electric Vehicle Project, Track Construction and Testing Program". UC Berkeley Path Program Technical Report: UCB-ITS-PRR-94-07, http://www.path.berkeley.edu/PATH/Publications/PDF/PRR/94/PRR-94-07.pdf
  18. ^ Shladover, S.E., “PATH at 20: History and Major Milestones”, Intelligent Transportation Systems Conference, 2006. ITSC '06. IEEE 2006, pages 1_22-1_29.
  19. ^ G. A. Landis, "Applications for Space Power by Laser Transmission," SPIE Optics, Electro-optics & Laser Conference, Los Angeles CA, January 24-28 1994; Laser Power Beaming, SPIE Proceedings Vol. 2121, 252-255.
  20. ^ G. Landis, M. Stavnes, S. Oleson and J. Bozek, "Space Transfer With Ground-Based Laser/Electric Propulsion" (AIAA-92-3213) NASA Technical Memorandum TM-106060 (1992).
  21. ^ Brown., W. C. (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). {{cite journal}}: |issue= has extra text (help); Unknown parameter |month= ignored (help); line feed character in |issue= at position 45 (help)
  22. ^ 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.
  23. ^ "CES Best of 2007"
  24. ^ EE Times: Practical apps in works for wireless energy transfer - R. Colin Johnson 01/22/2007
  25. ^ G. A. Landis, "Charging of Devices by Microwave Power Beaming," U.S. Patent 6,967,46, November 22 2005) link
  26. ^ power transmission via lasers
  27. ^ A Survey of Laser Lightning Rod Techniques - Barnes, Arnold A., Jr. ; Berthel, Robert O.
  28. ^ What is LIPC? - Ionatron directed-energy weapons
  29. ^ Frequently Asked Questions - HSV Technologies
  30. ^ Vehicle Disabling Weapon by Peter A. Schlesinger, President, HSV Technologies, Inc. - NDIA Non-Lethal Defense IV 20-22 March 2000
  31. ^ Norrie, H. S., "Induction Coils: How to make, use, and repair them". Norman H. Schneider, 1907, New York. 4th edition.
  32. ^ Electrical experimenter, January 1919. pg. 615
  33. ^ Tesla: Man Out of Time By Margaret Cheney. Page 174
  34. ^ a b "The Transmission of Electrical Energy Without Wires," Electrical World, March 5, 1904
  35. ^ Norrie, H. S., "Induction Coils: How to make, use, and repair them". Norman H. Schneider, 1907, New York. 4th edition.
  36. ^ January 1919. pg. 615, Electrical Experimenter
  37. ^ Tesla: Man Out of Time By Margaret Cheney. Page 174.
  38. ^ Martin, T. C., & Tesla, N. (1894). The inventions, researches and writings of Nikola Tesla, with special reference to his work in polyphase currents and high potential lighting. New York: The Electrical Engineer. Page 188.
  39. ^ Experiments With Alternating Currents of Very High Frequency, and Their Application to Methods of Artificial Illumination (excerpt). Retrieved April 2007.
  40. ^ Vassilatos, Gerry, Secrets of Cold War Technology
  41. ^ Bearden, T. E., Tesla's Secret and the Soviet Tesla Weapons.
  42. ^ Peterson, Gary, "Comparing the Hertz-wave and Tesla wireless systems". Feed Line No. 9 Article

References

General information

  • Cheney, Margaret "Tesla: Man Out of Time". Simon and Schuster, October 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
  • 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.
  • "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.
  • 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, July 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.
  • Brown, W. C., "The History of Power Transmission by Radio Waves". IEEE Transactions on Microwave Theory and Techniques, 1984.

Nikola Tesla

  • US patent 0645576, Nikola Tesla, "System of Transmission of Electrical Energy", issued 1900-03-20 
  • US patent 0649621, Nikola Tesla, "Apparatus for Transmission of Electrical Energy", issued 1900-05-15 
  • US patent 0787412, Nikola Tesla, "Art of Transmitting Electrical Energy through the Natural Mediums", issued 1905-04-18 

M.I.T. team

  • Aristeidis Karalis (2008). "Efficient wireless non-radiative mid-range energy transfer". Annals of Physics. 323: 34–48. doi:10.1016/j.aop.2007.04.017. Published online: April 2007 {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  • Andre Kurs (2007). "Wireless power transfer via strongly coupled magnetic resonances". Science. 317: 83–86. doi:10.1126/science.1143254. PMID 17556549. Published online: June 2007 {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
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