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.
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), 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.

Contents

[edit] History

[edit] Modern day usage

Except for RFID tags, wireless power transmission over room-sized or community-sized distances has not been widely implemented. Correctly 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[citation needed]. Considering the hazards associated with powerful radiation, the physical alignment and targeting of devices to receive the energy beam is of particular concern. However, with the use of resonant coupling, wavelengths produced are longer, making it no more dangerous than being exposed to radio waves.

[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 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 antennas 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.

[edit] Efficiency

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The efficiency of an energy transferring system is the percentage of energy sent which reaches the destination. Sending energy through wires is often more efficient because wires represent a low loss way to confine and guide the energy to where it is needed. Still, generally, wireless energy transfer works very well at short range; and efficient long-distance transfer is possible if the transmitters and receivers are physically large, or if the energy can be formed into a tight beam, such as with lasers or large microwave dishes. 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. However, microwaves have much longer wavelengths than visible light, and require proportionately larger transmitters and receivers to deal with diffraction over long distances.

The most efficient laser power beaming system today has photovoltaic panels optimized to the wavelength of the laser. Losses due to atmospheric spreading can be reduced by the use of adaptive optics, and losses due to absorption can be reduced by a properly chosen laser wavelength. Laser power beaming does not work well through clouds.

Although laser and photovoltaic technologies have been rapidly advancing, it is unknown what transmission efficiency improvement is possible. The most efficient lasers are laser diode arrays, which can surpass 50% efficiency, but such lasers do not have mutual coherence. Other options include standard chemical lasers with efficiencies of a few percent or less. The development of high-coherence diode laser arrays would allow for notably improved power usage efficiency, as laser inefficiency comprises most of the energy loss.

[edit] An example

To calculate the efficiency of transferring 50 MJ of electrical energy by laser beam, simply multiply the efficiencies of all the components together. Assume current laser efficiencies of 25%, monochromatic solar cell conversion efficiencies of 50%, power conditioning efficiency of 80%, receiver efficiency of 75%, and atmospheric transmission efficiency of 90%. This works out to 6.75% end-to-end efficiency.

Given the current power grid rate of about US$0.11/kW·h (about US$0.03/MJ),[1] if the required energy cost of payload transfer is around US$1.75/kg, this must be multiplied by a factor of 15 (1/.0675) to account for the efficiency, and thus the actual power cost would be about $25/kg.

[edit] Near field

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

[edit] 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 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.

Applications
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.
  • The electromagnetic card reader The act of swiping the card induces eddy currents in the reader which are measured.
  • Transcutaneous energy transfer (TET) systems in artificial hearts and other surgically implanted devices.
  • Devices using induction to charge portable consumer electronics such as cell phones.[2][3]

[edit] Resonant induction

Main article: Evanescent wave coupling

In 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 transfer concept based on coupled resonators.[4][5][6] In a short theoretical analysis they demonstrate 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[citation needed]) 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.[4]

On June 7, 2007, it was reported that a prototype system had been implemented. The MIT researchers successfully demonstrated the ability to power a 60-watt light bulb from a power source that was seven feet (2 meters) away at roughly 40% efficiency.

"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 [7] including such devices as pacemakers and artificial hearts. While the early systems used a resonant receiver coil later systems [8] 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.[9]

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 [10] [11]. 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 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.

[edit] 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 )
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).

[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.

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.[12]

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.[13][14]

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[15][16][17] and more recently (1997) at Grand Bassin on Reunion Island.[18]

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.[19] 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.[20]

A different low-power wireless power technology has been proposed by Landis.[21]

[edit] Light

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.
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.[22] (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.

[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[23] and has been proposed for disabling vehicles.[24][25][26]

The Tesla effect.
The Tesla effect.

A "world system" for "the transmission of electrical energy without wires" that depends upon the electrical conductivity was proposed by Nikola Tesla as late as 1904.[27] 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).[28][29][30] 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.[31][32]

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.
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.[33][34] 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.

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.

[edit] 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 .
Tesla coil transformer wound in the form of a flat spiral. This is the transmitter form as described in U.S. Patent 645,576 .

Nikola Tesla had multiple patents that he claimed would enable 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 through conduction through the natural medium from one point to another remote point therefrom and a receiver coil or conductor of the transmitted signals.[35] 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.

[edit] See also

energy Portal


[edit] Notes

  1. ^ Cost of lavish Christmas lights display offset by simple measures - Oak Ridge National Laboratory December 20, 2002
  2. ^ SplashPower; Battery powered devices can be charged by placing them on an induction mat.
  3. ^ 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]
  4. ^ a b "'Evanescent coupling' could power gadgets wirelessly", NewScientist.com news service, November 15, 2006 Accessed: January 8, 2007
  5. ^ Karalis, Aristeidis; J.D. Joannopoulos, Marin Soljačić (November 2006). "Efficient wireless non-radiative mid-range energy transfer". arXiv:physics/0611063. 
  6. ^ Wireless energy could power consumer, industrial electronics — MIT press release
  7. ^ 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.
  8. ^ 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.
  9. ^ http://www.cochlearamericas.com/Products/11.asp
  10. ^ 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
  11. ^ Shladover, S.E., “PATH at 20: History and Major Milestones”, Intelligent Transportation Systems Conference, 2006. ITSC '06. IEEE 2006, pages 1_22-1_29.
  12. ^ "Scanning the Past: A History of Electrical Engineering from the Past, Hidetsugu Yagi"
  13. ^ 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.
  14. ^ 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).
  15. ^ NASA Video, date/author unknown
  16. ^ Wireless Power Transmission for Solar Power Satellite (SPS) (Second Draft by N. Shinohara), Space Solar Power Workshop, Georgia Institute of Technology
  17. ^ 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). 
  18. ^ 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.
  19. ^ "CES Best of 2007"
  20. ^ EE Times: Practical apps in works for wireless energy transfer - R. Colin Johnson 01/22/2007
  21. ^ G. A. Landis, "Charging of Devices by Microwave Power Beaming," U.S. Patent 6,967,46, November 22 2005) link
  22. ^ power transmission via lasers
  23. ^ A Survey of Laser Lightning Rod Techniques - Barnes, Arnold A., Jr. ; Berthel, Robert O.
  24. ^ What is LIPC? - Ionatron directed-energy weapons
  25. ^ Frequently Asked Questions - HSV Technologies
  26. ^ Vehicle Disabling Weapon by Peter A. Schlesinger, President, HSV Technologies, Inc. - NDIA Non-Lethal Defense IV 20-22 March 2000
  27. ^ "The Transmission of Electrical Energy Without Wires," Electrical World, March 5, 1904
  28. ^ Norrie, H. S., "Induction Coils: How to make, use, and repair them". Norman H. Schneider, 1907, New York. 4th edition.
  29. ^ January 1919. pg. 615, Electrical Experimenter
  30. ^ Tesla: Man Out of Time By Margaret Cheney. Page 174.
  31. ^ 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.
  32. ^ Experiments With Alternating Currents of Very High Frequency, and Their Application to Methods of Artificial Illumination (excerpt). Retrieved April 2007.
  33. ^ Vassilatos, Gerry, Secrets of Cold War Technology
  34. ^ Bearden, T. E., Tesla's Secret and the Soviet Tesla Weapons.
  35. ^ Peterson, Gary, "Comparing the Hertz-wave and Tesla wireless systems". Feed Line No. 9 Article

[edit] References

[edit] General information

[edit] Transmission and efficiency readings

[edit] Related patents

[edit] External links