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Wireless energy transfer
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 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.
Additional recommended knowledge
In 1825 William Sturgeon invented the electromagnet, a conducting wire wrapped around an iron core. The principle of electromagnetic 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.
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, significant power may be transmitted over a range of many meters.
In 1864 James Clerk 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. 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).
William C. Brown demonstrated in 1964 on the CBS Walter Cronkite news a microwave-powered model helicopter that received all the power needed for flight from a microwave beam. Between 1969 and 1975 Bill Brown was technical director of a JPL Raytheon program that beamed 30 kW over a distance of 1 mile at 84% efficiency.
Modern day usage
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 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.
Size and power level
The size of the components is dictated by:
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.
The efficiency of wireless power is the ratio between power that reaches the receiver and the power supplied to the transmitter. Generally wirelessly transmitted energy is dispersed as the energy radiates into the environment or is lost as heat at the transmitter or receiver. Wired transmission techniques on the other hand lose less power, as wires confine and guide the energy to where it is needed. Generally, wireless energy transfer works best at short range; although long distances are possible if the transmitters and receivers are physically large, or the energy can be formed into a tight beam, such as with lasers or large microwave dishes. The ultimate beamwidth is limited by diffraction.
When phased arrays are used for wireless transmission, the phased array normally needs to be contiguous due to a phenomenon called the thinned array curse; gaps in the array act as a diffraction grating and cause side lobes that lose energy.
Microwave power beaming often achieves higher conversion efficiency than lasers, and is less prone to atmospheric attenuation. However microwaves have far 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 — laser diode arrays, 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. High-coherence diode laser arrays or a similar technology would allow for notably improved power usage efficiency, as laser inefficiency comprises most of the energy loss.
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, 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.
Short distance induction
These methods can reach at most a few centimetres.
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.)
WiPower  technology is a very recent example of inductive charging technology. The charging pad allow users to charge multiple electronic devices that are placed on its surface. It is insensitive to the position or orientation of the devices under charge. Unlike most inductive charging systems, the WiPower system uses air-core technology which allows the system to be integrated into very small electronic devices. The efficiency of the system actually exceeds many corded chargers which have a median efficiency of 57%.
The electric toothbrush charger is another 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.
These methods achieve distances of a few meters
A new company, Powercast introduced wireless power transfer technology using RF energy at the 2007 Consumer Electronics Show, winning best Emerging Technology. 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..
A different low-power wireless power technology has been proposed by Landis.
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.  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) 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.
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.
Nikola Tesla had two patents that he claimed would enable long distance power transmission.
Tesla, in U.S. Patent 0,645,576 The production of currents of very high potential could be attained in these coils.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.
These methods achieve multiple kilometre ranges.
Radio and microwave
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.
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 ,.
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.
Wireless Power Transmission (using microwaves) is well proven. Experiments in the tens of kilowatts have been performed at Goldstone in California in 1975  and more recently (1997) at Grand Bassin on Reunion Island.
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:
NASA has demonstrated flight of a lightweight model plane powered by a laser beam.
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 and has been proposed for disabling vehicles.
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. The Tesla effect (named in honor of Tesla) is an archaic term for an 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). Tesla stated,
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. 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.
Transmission and efficiency readings
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Wireless_energy_transfer". A list of authors is available in Wikipedia.|