Editing Wireless power transfer (section)

{{short description|Electrical transmission without physical connection}}
[[File:Inductive charging of LG smartphone (2).jpg|thumb|[[Inductive charging|Inductive charging pad]] for a smartphone as an example of near-field wireless transfer.  When the phone is set on the pad, a coil in the pad creates a magnetic field<ref name="Hoffman">{{cite web |last=Hoffman |first=Chris |title=How Does Wireless Charging Work? |website=How-To Geek |publisher=How-To Geek LLC |date=15 September 2017 |url=https://www.howtogeek.com/162483/no-more-cables-how-wireless-charging-works-and-how-you-can-use-it-today/ |access-date=11 January 2018}}</ref> which induces a current in another coil, in the phone, charging its battery.]]
[[File:Wireless power system.svg|thumb|upright=1.7|Generic block diagram of a wireless power system]]

'''Wireless power transfer''' ('''WPT'''; also '''wireless energy transmission''' or '''WET''') is the transmission of [[electrical energy]] without [[wire]]s as a physical link. In a wireless power transmission system, an [[electric power source|electrically powered]] transmitter device generates a time-varying [[electromagnetic field]] that transmits power across space to a receiver device; the receiver device 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, thereby increasing the mobility, convenience, and safety of an electronic device for all users.<ref>{{cite book |doi=10.1049/cp.2016.1290 |chapter=Development of wireless electricity transmission through resonant coupling |title=4th IET Clean Energy and Technology Conference (CEAT 2016) |year=2016 |last1=Ibrahim |first1=F.N. |last2=Jamail |first2=N.A.M. |last3=Othman |first3=N.A. |pages=33 (5 .) |isbn=978-1-78561-238-1}}</ref> Wireless power transfer is useful to power electrical devices where interconnecting wires are inconvenient, hazardous, or are not possible.

Wireless power techniques mainly fall into two categories: [[Near and far field]].<ref>{{cite journal |last1=Kracek |first1=Jan |last2=Mazanek |first2=Milos |title=Wireless Power Transmission for Power Supply: State of Art |journal=Radioengineering |date=June 2011 |volume=20 |issue=2 |pages=457–463 |url=https://www.radioeng.cz/fulltexts/2011/11_02_457_463.pdf}}</ref> In ''near field'' or ''non-radiative'' techniques, power is transferred over short distances by [[magnetic field]]s using [[inductive coupling]] between [[electromagnetic coil|coils of wire]], or by [[electric field]]s using [[capacitive coupling]] between metal [[electrode]]s.<ref name="ECN2011"/><ref name="Trancutaneous Capacitive Wireless Power Transfer"/><ref name="Capacitive Elements for Wireless Power Transfer to biomedical implants"/><ref name="Capacitive Wireless Power Transfer to biomedical implants"/> Inductive coupling is the most widely used wireless technology; its applications include charging handheld devices like phones and [[electric toothbrush]]es, [[RFID]] tags, [[induction cooking]], and wirelessly charging or continuous wireless power transfer in implantable medical devices like [[artificial cardiac pacemaker]]s, or [[electric vehicle]]s. In ''far-field'' or ''radiative'' techniques, also called ''power beaming'', power is transferred by beams of [[electromagnetic radiation]], like [[microwave]]s<ref>{{cite journal |author1=Miguel Poveda-García |author2=Jorge Oliva-Sanchez |author3=Ramon Sanchez-Iborra |author4=David Cañete-Rebenaque |author5=Jose Luis Gomez-Tornero |title=Dynamic Wireless Power Transfer for Cost-Effective Wireless Sensor Networks using Frequency-Scanned Beaming |journal=IEEE Access |year=2019 |doi=10.1109/ACCESS.2018.2886448 |volume=7 |pages=8081–8094 |bibcode=2019IEEEA...7.8081P |doi-access=free}}</ref> 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 satellite]]s and wireless powered [[drone aircraft]].<ref name="Bush">{{cite book |last1=Bush |first1=Stephen F. |title=Smart Grid: Communication-Enabled Intelligence for the Electric Power Grid |publisher=John Wiley & Sons |date=2014 |page=118 |url=https://books.google.com/books?id=bUSMAgAAQBAJ&pg=PA118 |isbn=978-1118820230}}</ref><ref name="PCMag">{{cite web |title=Wireless energy transfer |website=Encyclopedia of terms |publisher=PC Magazine Ziff-Davis |year=2014 |url=https://www.pcmag.com/encyclopedia/term/57396/wireless-energy-transfer |access-date=15 December 2014}}</ref><ref>{{cite news |last1=Marks |first1=Paul |title=Wireless charging for electric vehicles hits the road |url=https://www.newscientist.com/article/mg22129534-900-wireless-charging-for-electric-vehicles-hits-the-road/ |work=New Scientist |date=22 January 2014}}</ref>

Wireless power transfer is a generic term for a number of different technologies for transmitting energy by means of electromagnetic fields.<ref name="Shinohara1">{{cite book |last1=Shinohara |first1=Naoki |title=Wireless Power Transfer via Radiowaves |publisher=John Wiley & Sons |date=2014 |pages=ix–xiii |url=https://books.google.com/books?id=TwegAgAAQBAJ&pg=PR9 |isbn=978-1118862964}}</ref><ref name="Gopinath">{{cite journal |last1=Gopinath |first1=Ashwin |title=All About Transferring Power Wirelessly |journal=Electronics for You E-zine |pages=52–56 |date=August 2013 |url=http://www.efymagonline.com/pdf/52_Wireless%20Power%20Transfer_EFY%20August%202013.pdf |access-date=16 January 2015 |url-status=dead |archive-url=https://web.archive.org/web/20150119044123/http://www.efymagonline.com/pdf/52_Wireless%20Power%20Transfer_EFY%20August%202013.pdf |archive-date=19 January 2015}}</ref><ref name="X. Lu">{{cite journal |last1=Lu |first1=X. |last2=Wang |first2=P. |last3=Niyato |first3=D. |last4=Kim |first4=D. I. |last5=Han |first5=Z. |year=2016 |title=Wireless Charging Technologies: Fundamentals, Standards, and Network Applications |journal=IEEE Communications Surveys and Tutorials |volume=18 |issue=2 |pages=1413–1452 |doi=10.1109/comst.2015.2499783 |arxiv=1509.00940 |s2cid=8639012}}</ref> The technologies 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 wave]]s, microwaves, [[infrared]] or [[visible light]] waves.<ref name="Sun">{{cite book |last1=Sun |first1=Tianjia |last2=Xie |first2=Xiang |last3=Zhihua |first3=Wang |title=Wireless Power Transfer for Medical Microsystems |publisher=Springer Science & Business Media |date=2013 |pages=5–6 |url=https://books.google.com/books?id=kTA_AAAAQBAJ&q=%22wireless+power%22&pg=PA6 |isbn=978-1461477020}}</ref>

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.<ref name="Shinohara1" /><ref name="Sun" /> At the transmitter the input power is converted to an oscillating electromagnetic field by some type of "[[antenna (radio)|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 (electronics)|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]],<ref name="Sazonov">{{cite book |last1=Sazonov |first1=Edward |last2=Neuman |first2=Michael R. |title=Wearable Sensors: Fundamentals, Implementation and Applications |publisher=Elsevier |date=2014 |pages=253–255 |url=https://books.google.com/books?id=wGJzAwAAQBAJ&pg=PA253 |isbn=978-0124186668}}</ref><ref name="Shinohara2" >{{cite book |url=https://books.google.com/books?id=TwegAgAAQBAJ&pg=PP27 |last=Shinohara |year=2014 |title=Wireless Power Transfer via Radiowaves |page=27 |publisher=John Wiley & Sons |isbn=9781118862964}}</ref> another familiar technology that involves electrical energy transmitted without wires by electromagnetic fields, used in [[cellphone]]s, [[radio broadcasting|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.<ref name="Gopinath" /><ref name="Sazonov" /><ref name="Shinohara2" /> 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.<ref name="Gopinath" /> 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). {{anchor|SWIPT}}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);<ref>{{Cite journal |arxiv=1409.0261 |doi=10.1109/MCOM.2014.6957150 |title=Simultaneous wireless information and power transfer in modern communication systems |journal=IEEE Communications Magazine |volume=52 |issue=11 |pages=104–110 |year=2014 |last1=Krikidis |first1=Ioannis |last2=Timotheou |first2=Stelios |last3=Nikolaou |first3=Symeon |last4=Zheng |first4=Gan |last5=Ng |first5=Derrick Wing Kwan |last6=Schober |first6=Robert |bibcode=2014arXiv1409.0261K |s2cid=3462059}}</ref> whereas when it is used to supply the power of wireless information receivers, it is known as a Wireless Powered Communication Network (WPCN).<ref>{{Cite journal |arxiv=1508.06366 |doi=10.1109/MWC.2016.7462480 |title=Wireless powered communication networks: An overview |journal=IEEE Wireless Communications |volume=23 |issue=2 |pages=10–18 |year=2016 |last1=Bi |first1=Suzhi |last2=Zeng |first2=Yong |last3=Zhang |first3=Rui |author4=Dong in Kim |last5=Han |first5=Zhu |s2cid=3504276}}</ref><ref>{{Cite arXiv |eprint=1807.05543 |last1=Lu |first1=Xiao |title=Maximizing Ergodic Throughput in Wireless Powered Communication Networks |last2=Wang |first2=Ping |last3=Niyato |first3=Dusit |author4=Dong in Kim |last5=Han |first5=Zhu |class=cs.IT |year=2018}}</ref><ref>{{Cite journal |doi=10.1109/MCOM.2015.7081084 |title=Wireless powered communication: Opportunities and challenges |journal=IEEE Communications Magazine |volume=53 |issue=4 |pages=117–125 |year=2015 |last1=Bi |first1=Suzhi |last2=Ho |first2=Chin Keong |last3=Zhang |first3=Rui |arxiv=1408.2335 |s2cid=7127575}}</ref>

An important issue associated with all wireless power systems is limiting the exposure of people and other living beings to potentially injurious electromagnetic fields.<ref name="Lu">{{cite book |last1=Lu |first1=Yan |last2=Ki |first2=Wing-Hung |title=CMOS Integrated Circuit Design for Wireless Power Transfer |publisher=Springer |date=2017 |pages=2–3 |url=https://books.google.com/books?id=7fwwDwAAQBAJ&q=%22wireless+power%22+safety+exposure&pg=PA3 |isbn=978-9811026157}}</ref><ref name="Sun3">{{cite book |last1=Sun |first1=Tianjia |last2=Xie |first2=Xiang |last3=Wang |first3=Zhihua |title=Wireless Power Transfer for Medical Microsystems |publisher=Springer Science and Business Media |date=2013 |url=https://books.google.com/books?id=kTA_AAAAQBAJ&q=%22wireless+power%22+safety+exposure&pg=PA32 |isbn=978-1461477020}}</ref>