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wireless power transfer (WPT)

Electrical energy is usually transferred through wires and conductors, but there are some developments that deal with wireless and cable-free energy transfer. There are several concepts for the technology, also known as wireless energy transfer or wireless charging

, based on near-field and far-field transmission. The near-field communication methods are magnetic induction (MI), magnetic resonance (MR), and capacitive field coupling, and far-field communication

methods

are RF charging, which are based on radio frequencies in various ISM bands, infrared light

, or laser technology

The Different Concepts of

Wireless Power Transfer Transfer Principles

Wireless Power Transfer Transfer Principles

forWireless Power In the near-field communication based

transfer methods with electrodynamic and electrostatic coupling, the energy is transferred over a few centimeters distance. This results in a high coupling coefficient. In inductive coupling with magnetic induction, the energy is transmitted from the primary coil of the energy transmitter, the Power Transmitting Unit (PTU), to the secondary coil of the energy receiver, the Power Receiving Unit (PRU), via magnetic fields. In this principle, an RF alternating current with a frequency of 100 kHz to 205 kHz generates a magnetic field in the primary coil, which is induced into the secondary coil, where it generates an induction voltage. The method quickly reaches its limits because optimum energy transmission is only guaranteed if the coils are precisely aligned with each other. The transmission efficiency is proportional to the coupling factor (k) multiplied by the quality (Q) of the coils (k x Q). As the coupling factor decreases, the efficiency falls. Well-known examples of this technology include electric toothbrushes and RFID

.More recent developments use magnetic induction (MI) to charge batteries in mobile devices, iPods, iPhones, smartphones, and cell phones. These techniques use charging mats with inserted flat coils on which the devices are placed, known as powermats

or power pads. However, it is a prerequisite that the devices are equipped with flat coils. An example of this is Nokia's Lumia smartphones

Wireless power transfer of high power

Wireless Power Transfer (WPT) is a wireless charging technology used in charging batteries in electric vehicles

. It is based on magnetic induction (MI). Energy transfers of 100 kW have already been realized with this method. The energy transfer takes place between a primary coil, which is mounted as a floor plate on the garage floor or embedded in the floor of parking spaces, and the secondary coil, which is located under the vehicle floor

Wireless charging with floor plate, photo: motorauthority.com

Wireless charging with floor plate, photo: motorauthority.com

The electric vehicle is parked above the primary coil and inductive charging technology is used to charge the battery. To achieve efficient energy transfer, the distance between the primary and secondary coils should be as small as possible. To keep the ground plates as flat as possible, inductive energy transfer operates at about 100 kHz. The advantage is that the secondary coil has little weight and no charging station or charging cable

is needed

Block diagram of a wireless power transmission

Block diagram of a wireless power transmission

The technology of inductive power transmission is driven by the Wireless Power Consortium (WPC), which also developed the Qi standard for this technology, and the Power Matters Alliance (PMA) with its PMA standard. The Society of Automotive Engineers (SAE) has standardized the technology under the SAE J2954standard

. According to the standard, there are three charging classes WPT1 with a charging power of 3.7 kW, WPT2 with 7.7 kW and WPT3 with 11 kW. In single-phase operation with powers of 3.7 kW, an efficiency of over 90% is achieved.Other induction principles use electric fields or the resonance principle of magnetic resonance (MR) for energy transfer. As far as energy transfer via electric fields is concerned, a quasi-static electric field is used between two devices. In this method, both devices - the charger and the device to be charged - contain coupling plates that act as a capacitor. If the devices are close enough to each other, then the capacitor plate is used for energy transfer. There are already implemented concepts for the resonant principle with WiTricity and Wireless Resonant Energy Link (WREL) from Intel. The Alliance

for Wireless Power(A4WP) is working on magnetic resonance technology, and

inautumn 2015 the Power Matters Alliance (PMA) and the Alliance for Wireless Power (A4WP) merged to form the AirFuel Alliance

Experimental setup of a power transmission based on the resonance principle, photo: cnet.de

Experimental setup of a power transmission based on the resonance principle, photo: cnet.de

Wireless power transmission works with high-frequency electromagnetic waves, which are received and converted by the PRU units. In this RF charging, radio waves in the ISM band at 6.78 MHz

or higher ISM frequency bands are used for energy transmission. The received frequencies are rectified and supply power is obtained from the obtained DC voltage. The transmittable power is 33 W.Another radio technology concept uses

thebackscattering of

radio waves for power transmission.The obtained powers are in the microwatt range, but they are sufficient to supply low-power sensors in Wireless Sensor and Actor Networks (WSAN), as used in Narrow BandIoT

(NB-IoT).Airnergy is such a radio-based concept, but like all transmission techniques

, it is subject to physical laws. These limit the extension in that thefield strength decreases linearly with the distance between the energy transmitter and the energy receiver, and the power density

even quadratically.The laser-based concept

, known asPowerBeam and whose technology is called WiTrichy, beams energy using lasers to the energy receiver, which converts the heat of the laser beam into energy in a solar cell. The transmittable power is in the milliwatt range at distances of a few meters

. However, the efficiency is only a few percent, if that

Advantages of Wireless Power Transfer

Wireless power transfer is extremely convenient to use because there is no need to make a cable connection between the charger and

theterminal device. Also, the devices are insulated from each other and can be housed in fully enclosed cases, making them protected against dust, splashing water, and gases. This means that they can also be used in potentially explosive atmospheres. A disadvantage is the poorer energy balance compared to wired charging stations. This disadvantage can be compensated for by communication

between the charging station and the end device. However, wireless energy transmission on a radio basis is not entirely new. As early as the middle of the last century, there was a phenomenon in which light bulbs in houses near long-wave transmitters lit up without the power being switched on. This phenomenon was based on the fact that the electrical wiring formed a long-wave antenna and converted the high transmitting power, which amounted to several hundred kilowatts. In other cases, resourceful people used the garden fence as an antenna. Today, on the other hand, we are talking about transmission powers of a few watts.

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Englisch: wireless power transfer - WPT
Updated at: 28.01.2021
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Translations: DE