Wireless power transfer (WPT), wireless power transmission, wireless energy transmission (WET), or electromagnetic power transfer is the transmission of electrical energy without wires as a physical link. In a wireless power transmission system, a transmitter device, driven by electric power from a power source, generates a time-varying electromagnetic field, which transmits power across space to a receiver device, which extracts power from the field and supplies it to an electrical load. The technology of wireless power transmission can eliminate the use of the wires and batteries, thus increasing the mobility, convenience, and safety of an electronic device for all users.[2] Wireless power transfer is useful to power electrical devices where interconnecting wires are inconvenient, hazardous, or are not possible.
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Wireless power techniques mainly fall into two categories, near field and far-field.[3] In near field or non-radiative techniques, power is transferred over short distances by magnetic fields using inductive coupling between coils of wire, or by electric fields using capacitive coupling between metal electrodes.[4][5][6][7] Inductive coupling is the most widely used wireless technology; its applications include charging handheld devices like phones and electric toothbrushes, RFID tags, induction cooking, and wirelessly charging or continuous wireless power transfer in implantable medical devices like artificial cardiac pacemakers, or electric vehicles.
In far-field or radiative techniques, also called power beaming, power is transferred by beams of electromagnetic radiation, like microwaves[8] or laser beams. These techniques can transport energy longer distances but must be aimed at the receiver. Proposed applications for this type include solar power satellites and wireless powered drone aircraft.[9][10][11]
Wireless power transfer is a generic term for a number of different technologies for transmitting energy by means of electromagnetic fields.[14][15][16] The technologies, listed in the table below, differ in the distance over which they can transfer power efficiently, whether the transmitter must be aimed (directed) at the receiver, and in the type of electromagnetic energy they use: time varying electric fields, magnetic fields, radio waves, microwaves, infrared or visible light waves.[17]
In general a wireless power system consists of a "transmitter" device connected to a source of power such as a mains power line, which converts the power to a time-varying electromagnetic field, and one or more "receiver" devices which receive the power and convert it back to DC or AC electric current which is used by an electrical load.[14][17] At the transmitter the input power is converted to an oscillating electromagnetic field by some type of "antenna" device. The word "antenna" is used loosely here; it may be a coil of wire which generates a magnetic field, a metal plate which generates an electric field, an antenna which radiates radio waves, or a laser which generates light. A similar antenna or coupling device at the receiver converts the oscillating fields to an electric current. An important parameter that determines the type of waves is the frequency, which determines the wavelength.
Wireless power uses the same fields and waves as wireless communication devices like radio,[18][19] another familiar technology that involves electrical energy transmitted without wires by electromagnetic fields, used in cellphones, radio and television broadcasting, and WiFi. In radio communication the goal is the transmission of information, so the amount of power reaching the receiver is not so important, as long as it is sufficient that the information can be received intelligibly.[15][18][19] In wireless communication technologies only tiny amounts of power reach the receiver. In contrast, with wireless power transfer the amount of energy received is the important thing, so the efficiency (fraction of transmitted energy that is received) is the more significant parameter.[15] For this reason, wireless power technologies are likely to be more limited by distance than wireless communication technologies.
Wireless power transfer may be used to power up wireless information transmitters or receivers. This type of communication is known as wireless powered communication (WPC). When the harvested power is used to supply the power of wireless information transmitters, the network is known as Simultaneous Wireless Information and Power Transfer (SWIPT);[20] whereas when it is used to supply the power of wireless information receivers, it is known as a Wireless Powered Communication Network (WPCN).[21][22][23]
The oscillating electric and magnetic fields surrounding moving electric charges in an antenna device can be divided into two regions, depending on distance Drange from the antenna.[14][17][18][24][30][31][32] The boundary between the regions is somewhat vaguely defined.[17] The fields have different characteristics in these regions, and different technologies are used for transferring power:
In inductive coupling (electromagnetic induction[24][45] or inductive power transfer, IPT), power is transferred between coils of wire by a magnetic field.[18] The transmitter and receiver coils together form a transformer[18][24] (see diagram). An alternating current (AC) through the transmitter coil (L1) creates an oscillating magnetic field (B) by Ampere's law. The magnetic field passes through the receiving coil (L2), where it induces an alternating EMF (voltage) by Faraday's law of induction, which creates an alternating current in the receiver.[15][45] The induced alternating current may either drive the load directly, or be rectified to direct current (DC) by a rectifier in the receiver, which drives the load. A few systems, such as electric toothbrush charging stands, work at 50/60 Hz so AC mains current is applied directly to the transmitter coil, but in most systems an electronic oscillator generates a higher frequency AC current which drives the coil, because transmission efficiency improves with frequency.[45]
Inductive coupling is the oldest and most widely used wireless power technology, and virtually the only one so far which is used in commercial products. It is used in inductive charging stands for cordless appliances used in wet environments such as electric toothbrushes[24] and shavers, to reduce the risk of electric shock.[46] Another application area is "transcutaneous" recharging of biomedical prosthetic devices implanted in the human body, such as cardiac pacemakers and insulin pumps, to avoid having wires passing through the skin.[47][48] It is also used to charge electric vehicles such as cars and to either charge or power transit vehicles like buses and trains.[24]
The power transferred increases with frequency[45] and the mutual inductance M \displaystyle M between the coils,[15] which depends on their geometry and the distance D range \displaystyle D_\textrange between them. A widely used figure of merit is the coupling coefficient k = M / L 1 L 2 \displaystyle k\;=\;M/\sqrt L_1L_2 .[45][50] This dimensionless parameter is equal to the fraction of magnetic flux through the transmitter coil L 1 \displaystyle L1 that passes through the receiver coil L 2 \displaystyle L2 when L2 is open circuited. If the two coils are on the same axis and close together so all the magnetic flux from L 1 \displaystyle L1 passes through L 2 \displaystyle L2 , k = 1 \displaystyle k=1 and the link efficiency approaches 100%. The greater the separation between the coils, the more of the magnetic field from the first coil misses the second, and the lower k \displaystyle k and the link efficiency are, approaching zero at large separations.[45] The link efficiency and power transferred is roughly proportional to k 2 \displaystyle k^2 .[45] In order to achieve high efficiency, the coils must be very close together, a fraction of the coil diameter D ant \displaystyle D_\textant ,[45] usually within centimeters,[40] with the coils' axes aligned. Wide, flat coil shapes are usually used, to increase coupling.[45] Ferrite "flux confinement" cores can confine the magnetic fields, improving coupling and reducing interference to nearby electronics,[45][47] but they are heavy and bulky so small wireless devices often use air-core coils.
Resonant inductive coupling (electrodynamic coupling,[24] strongly coupled magnetic resonance[35]) is a form of inductive coupling in which power is transferred by magnetic fields (B, green) between two resonant circuits (tuned circuits), one in the transmitter and one in the receiver (see diagram, right).[18][24][31][46][51] Each resonant circuit consists of a coil of wire connected to a capacitor, or a self-resonant coil or other resonator with internal capacitance. The two are tuned to resonate at the same resonant frequency. The resonance between the coils can greatly increase coupling and power transfer, analogously to the way a vibrating tuning fork can induce sympathetic vibration in a distant fork tuned to the same pitch.
Nikola Tesla first discovered resonant coupling during his pioneering experiments in wireless power transfer around the turn of the 20th century,[52][53][54] but the possibilities of using resonant coupling to increase transmission range has only recently been explored.[55] In 2007 a team led by Marin Soljačić at MIT used two coupled tuned circuits each made of a 25 cm self-resonant coil of wire at 10 MHz to achieve the transmission of 60 W of power over a distance of 2 meters (6.6 ft) (8 times the coil diameter) at around 40% efficiency.[24][35][46][53][56]
The concept behind resonant inductive coupling systems is that high Q factor resonators exchange energy at a much higher rate than they lose energy due to internal damping.[35] Therefore, by using resonance, the same amount of power can be transferred at greater distances, using the much weaker magnetic fields out in the peripheral regions ("tails") of the near fields.[35] Resonant inductive coupling can achieve high efficiency at ranges of 4 to 10 times the coil diameter (Dant).[36][37][38] This is called "mid-range" transfer,[37] in contrast to the "short range" of nonresonant inductive transfer, which can achieve similar efficiencies only when the coils are adjacent. Another advantage is that resonant circuits interact with each other so much more strongly than they do with nonresonant objects that power losses due to absorption in stray nearby objects are negligible.[31][35] 2ff7e9595c
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