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 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 identical, 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 differences in these technologies.

The most common form of wireless power transfer is carried out using Inductive Power Transfer. Other viable technologies for wireless power are based on microwaves and lasers ^{[1] [2]}.

[edit] History of wireless energy 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 synthesizes the previous observations, experiments and equations of electricity, magnetism and optics into a consistent theory, and mathematically models the behavior of electromagnetic radiation.
• 1888: Heinrich Rudolf Hertz confirms the existence of electromagnetic radiation. Hertz’s "apparatus for generating electromagnetic waves" is generally acknowledged as the first radio transmitter.
• 1891: Nikola Tesla improves on Hertz's primitive radio-frequency power supply in U.S. Patent No. 454,622, "System of Electric Lighting."
• 1893: Nikola Tesla demonstrates 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 U.S. Patent describing a system for power transfer at 3 kHz
• 1894: Nikola Tesla wirelessly lights up vacuum tubes at the 35 South Fifth Avenue and 46 E. Houston Street laboratory in New York by means of the electrodynamic resonance effects ^[3].
• 1894: Jagdish Chandra Bose ignites gunpowder and rings a bell at a distance using electromagnetic waves, showing that communication signals can be sent without using wires.^[4][5]
• 1895: Jagdish Chandra Bose transmits signals over a distance of nearly a mile.^[4][5]
• 1896: Nikola Tesla transmits electrical energy over a distance of about 48 km (30 miles) from the Houston St. laboratory to West Point.^[6]
• 1897: Guglielmo Marconi uses Hertz's radio transmitter to transmit Morse code signals over a distance of about 6 km.
• 1897: Nikola Tesla files the first of his patent applications dealing with Wardenclyffe tower.
• 1900: Guglielmo Marconi fails to get a patent for radio in the United States.
• 1901: Guglielmo Marconi first transmits and receives signals across the Atlantic Ocean using Nikola Tesla's wireless energy transmitter.
• 1904: At the St. Louis World's Fair, a prize is 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).^[7]
• 1926: Shintaro Uda and Hidetsugu Yagi publishes their first paper on Uda's "tuned high-gain directional array"^[8] better known as the Yagi antenna.
• 1961: William C. Brown publishes article that explores possibilities of microwave power transmission. ^[9][10]
• 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.
• 1968: Peter Glaser proposes wirelessly transferring Solar energy captured in Space using "Powerbeaming" technology ^{[11] [12]}.
• 1971: Prof. Don Otto develops a small trolley powered by Inductive Power Transfer at The University of Auckland, in New Zealand.
• 1973: World first passive RFID demonstrated at Los-Alamos National Lab. ^[13]
• 1975: Goldstone Deep Space Communications Complex does experiments in the tens of kilowatts. ^[14][15][16]
• 1988: A power electronics group led by Prof. John Boys at The University of Auckland in New Zealand, develops 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 used by 90 per cent of the US$1 billion clean room industry for materials handling equipment in semiconductor, LCD and plasma screen manufacture.
• 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 60 cm-diameter coils.
• 2008: Bombardier offers new wireless transmission product PRIMOVE, a power system for use on trams and light-rail vehicles.^[17]
• 2008: Industrial designer Thanh Tran, at Brunel University made a wireless light bulb powered by a high efficiency 3W LED.
• 2008: Intel reproduces Nikola Tesla's 1894 implementation and Prof. John Boys group's 1988's experiments by wirelessly powering a light bulb with 75% efficiency.

[edit] Size, distance, and efficiency

The size of the components may be 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. In addition to the Rayleigh criterion Airy's diffraction limit is also frequently used to determine an approximate spot size at an arbitrary distance from the aperture. However, the above mathematics does not account for atmospheric absorption which is quite a severe damping effect on propagating energy in addition to causing severe Fading and loss of QoS.

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 compared to the wavelength of radiation, 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. However, the concept of laser aperture considerably differs from an antenna. Typically, a laser aperture much larger than the wavelength induces multi-moded radiation and mostly collimators are used before emitted radiation couples into a fiber or into space.

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.

[edit] Near field

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

[edit] Induction

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 a mobile phone or the transformers in the street are examples 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.

[edit] Resonant induction

"Resonant inductive coupling" or the "electrodynamic inductive effect" has key implications in solving the main problem associated with non-resonant inductive coupling and electromagnetic radiation; specifically, the dependence of efficiency on transmission distance. 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 in the secondary. This results in a 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. 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. 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 or transient waveform, significant power may be transmitted over a range of many meters.^[18] 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.

In the early 1960s resonant inductive wireless energy transfer was used successfully in implantable medical devices ^[19] including such devices as pacemakers and artificial hearts. While the early systems used a resonant receiver coil, later systems ^[20] 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.^[21]

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 ^{[22] [23]}. 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.

In November 2006, Marin Soljačić and other researchers at the Massachusetts Institute of Technology applied the near field behavior well known in electromagnetic theory to a wireless power transmission concept based on strongly-coupled resonators.^[24][25][26] In a theoretical analysis,^[27] 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 circuits tuned to the same frequency are within a fraction of a wavelength, their near fields (consisting of 'evanescent waves') couple by means of evanescent wave coupling (which is related to quantum tunneling). Standing waves develop between the inductors, which can allow the energy to 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.

[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 )

Far field methods achieve longer ranges, often multiple kilometer ranges, where the distance is much greater than the diameter of the device(s). The main reason for longer ranges is the fact that radiation in the far-field can be made to match the shape of the receiving area (using high Directivity antennas or well collimated Laser Beam) thereby delivering almost all emitted power at long ranges.

[edit] Radio and microwave

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.^[8]

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.^[28][29]

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^[14][15][30] and more recently (1997) at Grand Bassin on Reunion Island.^[31]

These methods achieve distances on the order of a kilometer.

[edit] 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 electromagnetic radiation closer to visible region of spectrum (10s of microns (um) to 10s of nm), power can be transmitted by converting electricity into a laser beam that is then pointed at a solar cell receiver. This mechanism is generally known as "PowerBeaming" because the Power is Beamed at a receiver that can convert it to usable electrical energy.

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.^[32] (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.

There are also a few unique advantages of Laser based energy transfer that outweigh the disadvantages.

1. collimated monochromatic wavefront propagation allows narrow beam cross-section area for energy confinement over large ranges.
2. compact size of solid state lasers-photovoltaics semiconductor diodes allows ease of integration into products with small form factors.
3. ability to operate with zero radio-frequency interference to existing communication devices i.e. wi-fi and cell phones.
4. control of Wireless Energy Access, instead of omnidirectional transfer where there can be no authentication before transferring energy.

These allow laser-based Wireless Energy Transfer concept to compete with conventional energy transfer methods.

The Laser "PowerBeaming" technology has been mostly explored in military weapons^{[33] [34] [35]} and aerospace ^{[36] [37]} applications and is now being developed for commercial and consumer electronics Low-Power applications. Wireless energy transfer system using laser for consumer space has to satisfy Laser safety requirements standardized under IEC 60825.

To develop an understanding of the trade-offs of Laser "a special type of light wave" based system:^{[38][39][40][41]}

1. Propagation of a laser beam ^{[42] [43] [44]} (on how Laser beam propagation is much less affected by diffraction limits)
2. Coherence and the range limitation problem (on how spatial and spectral coherence characteristics of Lasers allows better distance-to-power capabilities ^[45])
3. Airy disk (on how most fundamentally wavelength dictates the size of a disk with distance)
4. Applications of laser diodes (on how the laser sources are utilized in various industries and their sizes are reducing for better integration)

Geoffrey Landis ^{[46] [47] [48]} is one of the pioneers of Solar Power Satellite ^[49] and Laser-based transfer of energy especially for Space and Lunar missions. The continuously increasing demand for safe and frequent space missions has resulted in serious thoughts on a futuristic Space elevator^{[50] [51]} that would be powered by Lasers. NASA's Space elevator need wireless power to be beamed to it, for it to climb a tether ^[52].

NASA's Dryden Flight Research Center has demonstrated flight of a lightweight unmanned model plane powered by a laser beam ^[53]. This concept allows a periodic recharging using Laser beam and an unlimited time in air.

[edit] Electrical conduction

The Tesla effect.^{[54] [55][56]}. A "world system" for "the transmission of electrical energy without wires" that depends upon electrical conductivity was proposed by Tesla.^[57] Through longitudinal waves, an operator uses the Tesla effect in the wireless transfer of energy to a receiving device.

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

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

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^[58] and has been proposed for disabling vehicles.^[59][60][61]

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.^[57] 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).^[62][63][64] 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.^[65][66]

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, related to Tesla's telegeodynamics mechanical earth-resonance concepts.^[67][68] 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 wireless energy transmission system design and made physical observations that may be inconsistent with a basic tenet of mainstream physics related to the scalar derivatives of the electromagnetic potentials, which are presently considered as being nonphysical. ^{[69] [70] [71][citation needed]}.

A 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 or concentric spherical shell waveguide propagation parameters as they are known today shows that wireless energy transfer by the direct excitation of a Schumann cavity resonance mode is not realizable.^{[72][citation needed]}

On the other hand, Tesla's concept of a global wireless power grid based upon energy transmission by means of a spherical conductor transmission line with an upper half-space return circuit is feasible, defying none of the known laws of physics. Energy transmission by means of a spherical conductor “single-wire” surface wave transmission line may be possible, a feasibility study using a sufficiently powerful and properly tuned Tesla coil earth-resonance transmitter being called for.

[edit] 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.^[73] 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

• WiTricity
• Wireless Resonant Energy Link
• Thinned array curse
• Transmission medium
• Distributed generation
• Electricity distribution
• Electric power transmission
• Microwave power transmission

[edit] References

[edit] External links

• Project Tesla
• History of Microwave Power Transmission before 1980
• The SHARP's Stationary High Altitude Relay Platform, microwave beam powered
• MIT Prof. Marin Soljačić Wireless Power Transmission page
• Howstuffworks "How Wireless Power Works" — describes wireless power transmission over short, medium, and long distances
• 'Drilling Up' Into Space for Energy — news article about possible use of space power in Palau
• Is Wireless Energy Closer Than We Think?
• Anticipating MIT Witricity — Nikola Tesla demonstrated the electrodynamic inductive effect over 100 years ago.
• Wireless Energy Transmission Tech Group