Wireless energy transfer

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In the wireless power transmission , also called non-contact power transmission , wireless power transfer or non-contact power transmission referred to, is electrical energy transferred contactlessly from one object to another. An essential feature is that the electrical energy required for operation is not supplied along electrical lines and by means of electrical contacts, but rather through non-wired electromagnetic fields . The latter also includes light, for example in the form of a laser beam .

The most widespread method is inductive energy transfer. In the close range of a few centimeters, this has a comparatively high efficiency of 90%. Application examples are the charging of batteries in mobile devices such as electric toothbrushes or cell phones , as well as the transfer of energy between stationary and moving machine parts or between the route and vehicles moving on it.

A carbon filament lamp is made to glow without contact. A wireless (inductive) energy transfer over a few centimeters around 1910.

principle

Principle of wireless energy transfer between a transmitter (left) and consumer (right)

A distinction is made between two principles for wireless energy transmission, which differ in terms of their physical properties:

  1. Wireless energy transmission in the near field , also known as non-radiative coupling. This includes, for example, inductive coupling based on magnetic flux . The term wireless energy transmission is often used synonymously for inductive energy transmission, as this plays a dominant role in practical applications. With the non-radiating coupling in the near field, wave phenomena play no role.
  2. The energy transfer in the far field , also known as radiant energy transfer, based on electromagnetic waves. In addition to light, this also includes radio technology which, in addition to its main application in the area of ​​signal or message transmission, can in principle also be used for energy transmission.

The differences between the near-field coupling and the radiating far-field are mainly in the range: In technical applications, near-field coupling is limited to short distances in the range of a few centimeters to a few meters, while the transmission of energy via the far field can bridge significantly larger distances, but in technical applications the transmission of comparatively very small powers is limited. The reason lies in the so-called free space attenuation , which generally allows a very low degree of efficiency well below 1%.

The main principles of wireless energy transmission are shown below.

Near field transmissions

Inductive coupling

Principle of inductive energy transfer

For inductive energy transmission, an alternating magnetic field is generated in the transmitter by means of an oscillator . The transmission takes place by means of mutual induction between two coils , a coil L1 in the transmitter and a coil L2 in the receiver. In the receiving coil, the alternating current in the transmitting coil induces an alternating voltage, which is rectified in applications such as the charging of accumulators and fed as direct voltage to the consumer such as a charge regulator. The operating principle corresponds to that of a transformer with loose coupling of the two coils. The magnetic field lines of the magnetic flux density B are drawn in green in the schematic diagram .

The distance between the two coils represents the wireless transmission path and should be as small as possible - a distance of a few centimeters to a few 10 cm is typical. If the distance between the two coils is greater, the leakage flux increases significantly, which means that the inductive coupling decreases and the efficiency deteriorates. Typical distances that can be bridged with this method are approximately the coil diameter up to twice the coil diameter, the frequency range used extends from a few 10 kHz to the MHz range. Typical applications in this area are RFID transponders, contactless chargers or the energy supply between moving machine parts or between special rail systems and electrically operated vehicles such as the Transrapid .

Resonant inductive coupling

Principle of resonant inductive energy transfer

The resonant inductive coupling represents an extension of the inductive coupling with the aim of increasing the short range. For this purpose, as shown in the schematic diagram opposite, one or more free oscillating circuits are attached in the free space between the transmitter and receiver coils . Each of these resonant circuits consists of a capacitor C and a coil L , the resonance frequency of which is matched to the transmission frequency. The resonance between the oscillating circuits leads to an improved magnetic coupling between the transmitting and receiving coils at the transmission frequency. The resonant circuits should have the highest possible quality factor . The result is a greater range and better efficiency. A wireless energy transmission is thus possible over a distance in the order of 4 to 10 times the coil diameter.

For example, in 2007 at the Massachusetts Institute of Technology, under ideal laboratory conditions with a coil diameter of 25 cm and a distance of 2 m, an electrical power of 60  W was transmitted with an efficiency of 40%. The resonant inductive coupling is marketed commercially under brand names such as WiTricity .

In 2013 a work was published which, among other things, shed light on the possibilities of increasing efficiency through the use of resonant inductive couplers. This shows that the efficiency of the energy transmission in a coupled transmission system in the near field can only be increased or even maximized by choosing the complex load impedance. If the transmitted power is also to be maximized, an adaptation to the source is necessary in addition to adapting the load. From this point of view, the mode of operation of the schematic diagram opposite can be understood approximately in such a way that the two outer coils effect the adaptation to the source and load and the middle, loosely coupled pair of coils is used for energy transmission.

The division into the two inner, loosely coupled energy transmission coils and the outer adaptation coils make it clear that the adaptation does not necessarily have to be carried out by additional inductive couplings. Rather, by choosing appropriate matching networks, it is also possible to achieve the same or higher efficiency of the energy transmission with only two coils.

Capacitive coupling

Principle of the capacitive coupling

The capacitive coupling is based on a similar basic structure as the inductive transmission, only the electric field E is used for wireless energy transmission between two metal plates. The design of these metal plates represents an electrical capacitor C ; the area between the two plates is the route for wireless energy transmission. The two capacitors are fed with alternating voltage obtained from an oscillator on the transmitter side. Rectification takes place on the consumer side and the direct voltage is fed to the actual consumer.

The capacitive coupling is of little practical importance, since high electrical voltages occur in the space between the metal plates when higher powers are transmitted. The distances between the plates should also be kept as small as possible in order not to reduce the efficiency too much.

Far field transmissions

Electromagnetic waves are used for electromagnetic energy transmission , the principle corresponds to the procedure used for the transmission of radio signals . The energy transmission in the far field can also be a directed laser beam, for example. The laser beam is directed at a photocell as a receiver, which converts the optical power into electrical power.

While far field transmission is well suited for information and signal transmission in technical systems, wireless energy transmission is associated with high losses due to the free field attenuation and the losses during conversion, as in the case of a laser or photocell, with very low overall efficiencies. A practical example of energy transmission would be the detector-receiver , a simple radio receiver for medium waves that draws its power supply only from the radio signal in the vicinity of powerful transmitters and does not require an additional power supply such as a battery for operation. Because the overall efficiency is far below 1%, technically feasible wireless energy transmissions in the far field have almost no practical significance, apart from a few special applications.

Standards and norms

Mainly for use in the field of mobile devices such as cell phones and for wireless charging of built-in batteries, there are various industry standards that are widely used and are shown below.

Qi

On December 17th, 2008 the international Wireless Power Consortium (WPC) was founded, which provides for the introduction of a global standard called Qi (Chinese word for " life energy ", pronunciation : [ ˈt͡ʃiː ]) for charging electronic products with inductive coupling. The more than 200 members of the Wireless Power Consortium include manufacturers from the fields of cell phones, entertainment electronics and batteries, but also network operators, furniture manufacturers and automotive suppliers. The Qi industry standard for inductive energy transmission to mobile devices up to 5 watts has existed since mid-2010. The Qi standard 1.2 now supports an output of up to 15 watts. However, "Fast Wireless Charging" is only supported by some current Samsung Galaxy models so far. Both current smartphones and some older models support the Qi standard for wireless charging.

AirFuel inductive (Powermat)

The Power Matters Alliance (PMA), founded in March 2012 by Procter & Gamble and Powermat Technologies, among others, supports a standard developed by Powermat Technologies for charging end devices using inductive coupling. There are currently only a few devices that have built the technology directly themselves, including the Samsung Galaxy S6 / S6 Edge (+) to Galaxy S8 (+). Rather, corresponding charging cradles and batteries are sold here, which are inserted into the respective devices. The Powermat technology is prominent on the supply side because charging options are offered in various Starbucks and McDonald's branches.

AirFuel Resonant (Rezence)

The process of the Alliance for Wireless Power, initially known as English Wireless Power and meanwhile as "Rezence", uses a resonant magnetic coupling. Various industrial companies, including Intel , Qualcomm , Samsung , Broadcom and Integrated Device Technology , have joined forces in the Alliance for Wireless Power . In February 2014, the previously competing consortia of the Powermat and Rezence processes announced that they would ensure greater interoperability in wireless charging by making their standards compatible.

Electric vehicles

In the German Commission for Electrical, Electronic and Information Technologies (DKE) there is the working group AK 353.0.1, which at the end of 2010 drafted an application rule for inductive charging of electric vehicles.

In September 2015, two inductive charging systems were shown at the International Motor Show : Qualcomm presented the Halo system, Bombardier Transportation presented a PRIMOVE system for 3.6 kW and Audi presented the Audi Wireless Charging (AWC).

Emission of electromagnetic fields

In the case of wireless energy transmission, the standards and norms for electromagnetic compatibility (EMC) and for the electromagnetic effect on users within the scope of electromagnetic environmental compatibility (EMC) and their limit values, such as the limit values ​​of the ICNIRP in particular, are the basis for many local ones Standards are.

See also

Individual evidence

  1. Nejila Parspour: Contactless energy transfer - state of the art. (No longer available online.) VDE IALB, archived from the original on July 5, 2010 ; accessed on December 17, 2018 .
  2. a b Johnson I. Agbinya (ed.): Wireless Power Transfer . River Publishers Series in Communications, 2012, ISBN 978-87-92329-23-3 .
  3. Stanimir S. Valtchev, Elena N. Baikova, Luis R. Jorge: Electromagnetic field as the wireless transporter of energy . Faculty of Science and Technology, UNL, Campus Caparica, Portugal, 2012, doi : 10.2298 / FUEE1203171V ( Online [PDF]).
  4. ^ André Kurs, Aristeidis Karalis, Robert Moffatt: Wireless Power Transfer via Strongly Coupled Magnetic Resonances . In: American Association for the Advancement of Science (Ed.): Science . 317, July 2007, ISSN  1095-9203 , pp. 83-85. bibcode : 2007Sci ... 317 ... 83K . doi : 10.1126 / science.1143254 . PMID 17556549 .
  5. Dominik Huwig: Energy transfer through near field coupling. etatronix.de, accessed on June 20, 2015 . P. 53.
  6. Members of the Wireless Power Consortium , accessed March 4, 2017.
  7. Wireless Power Consortium website , accessed August 4, 2011.
  8. QI Standard 1.2 of the Wireless Power Consortium
  9. ^ Alliance for Wireless Power and Power Matters Alliance Join Forces , Wall Street Journal Europe, accessed February 13, 2014
  10. Intel plans to charge wirelessly via A4WP in the future. In: heise online. June 20, 2013, accessed February 13, 2014 .
  11. More collaboration on wireless charging. In: heise online. February 12, 2014, accessed February 13, 2014 .
  12. ^ Bombardier Mannheim: Experts convinced by PRIMOVE solution for cars . Bombardier. September 17, 2015. Archived from the original on April 5, 2016. Retrieved on September 17, 2015.
  13. AUDI: Fast charging and Audi wireless charging . AUDI. September 17, 2015. Retrieved September 17, 2015.
  14. International Commission on Non-Ionizing Radiation Protection (ICNIRP): Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields. Health Physics April 1998, Volume 74, Number 4.