Editing Wireless power transfer (section)

=== Microwaves ===
[[File:Suntower.jpg|thumb|right|An artist's depiction of a [[solar satellite]] that could send energy by microwaves to a space vessel or planetary surface]]

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 power transmission|microwave]] range.<ref name="Massa2013">{{cite journal |first1=A. Massa, G. Oliveri, F. Viani, and P. Rocca |title=Array designs for long-distance wireless power transmission – State-of-the-art and innovative solutions |journal=Proceedings of the IEEE |date=June 2013 |volume=101 |issue=6 |pages=1464–1481 |doi=10.1109/JPROC.2013.2245491 |last1=Massa |last2=Oliveri |first2=Giacomo |last3=Viani |first3=Federico |last4=Rocca |first4=Paolo |s2cid=2990114}}</ref> A [[rectenna]] may be used to convert the microwave energy back into electricity. Rectenna conversion efficiencies exceeding 95% have been realized.{{citation needed|date=September 2018}} Power beaming using microwaves has been proposed for the transmission of energy from orbiting [[solar power satellite]]s to Earth and the [[Beam-powered propulsion|beaming of power to spacecraft]] leaving orbit has been considered.<ref name=space>{{cite book |last1=Landis |first1=G. A. |title=Laser Power Beaming |chapter=Applications for space power by laser transmission |editor-first1=Jack V. |editor-first2=Edward E. |editor-last1=Walker |editor-last2=Montgomery Iv |year=1994 |volume=2121 |pages=252–255 |doi=10.1117/12.174188 |bibcode=1994SPIE.2121..252L |s2cid=108775324}}</ref><ref>{{cite journal |first1=G. |last1=Landis |title=Space Transfer With Ground-Based Laser/Electric Propulsion |journal=NASA Technical Memorandum |year=1992 |doi=10.2514/6.1992-3213 |url=https://arc.aiaa.org/doi/abs/10.2514/6.1992-3213 |hdl=2060/19930011426 |s2cid=109847404 |hdl-access=free}}</ref>

Power beaming by microwaves has the difficulty that, for most space applications, the required aperture sizes are very large due to [[diffraction]] limiting antenna directionality. For example, the 1978 [[NASA]] study of solar power satellites required a {{convert|1|km|mi|adj=mid|-diameter}} transmitting antenna and a {{convert|10|km|mi|adj=mid|-diameter}} receiving rectenna for a microwave beam at [[ISM band|2.45&nbsp;GHz]].<ref>{{cite book |doi=10.1109/WCPEC.2006.279877 |year=2006 |last1=Landis |first1=Geoffrey |title=2006 IEEE 4th World Conference on Photovoltaic Energy Conference |chapter=RE-Evaluating Satellite Solar Power Systems for Earth |pages=1939–1942 |isbn=1-4244-0016-3 |s2cid=22181565 |hdl=2060/20070005136 |hdl-access=free}}</ref> 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.

For earthbound applications, a large-area 10&nbsp;km diameter receiving array allows large total power levels to be used while operating at the low power density suggested for human electromagnetic exposure safety. A human safe power density of 1&nbsp;mW/cm<sup>2</sup> distributed across a 10&nbsp;km diameter area corresponds to 750 megawatts total power level. This is the power level found in many modern electric power plants. For comparison, a solar PV farm of similar size might easily exceed 10,000 megawatts (rounded) at best conditions during daytime.

Following World War II, which saw the development of high-power microwave emitters known as [[magnetron|cavity magnetrons]], the idea of using microwaves to transfer power was researched. By 1964, a miniature helicopter propelled by microwave power had been demonstrated.<ref>{{cite web |url=http://stinet.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=AD0474925 |title=Experimental Airborne Microwave Supported Platform |archive-url=https://web.archive.org/web/20100302204238/http://stinet.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=AD0474925 |archive-date=2 March 2010 |url-status=dead }}</ref>

Japanese researcher [[Hidetsugu Yagi]] also investigated wireless energy transmission using a directional array antenna that he designed. In February 1926, Yagi and his colleague [[Shintaro 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.<ref name="Yagi">{{cite web |url=http://ieee.cincinnati.fuse.net/reiman/05_2004.htm |title=Scanning the Past: A History of Electrical Engineering from the Past, Hidetsugu Yagi |publisher=Ieee.cincinnati.fuse.net |access-date=4 June 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090611154616/http://ieee.cincinnati.fuse.net/reiman/05_2004.htm |archive-date=11 June 2009}}</ref>

Wireless high power transmission using microwaves is well proven. Experiments in the tens of kilowatts have been performed at the [[Goldstone Deep Space Communications Complex]] in California in 1975<ref name=autogenerated3>{{cite web |url=http://www.spaceislandgroup.com/solarspace.html |archive-url=https://web.archive.org/web/20220122200330/http://www.spaceislandgroup.com/solarspace.html |archive-date=2022-01-22 |title=Space Solar Energy Initiative |publisher=Space Island Group |access-date=4 June 2009}}</ref><ref name=autogenerated1>{{cite journal |url=http://www.sspi.gatech.edu/wptshinohara.pdf |title=Wireless Power Transmission for Solar Power Satellite (SPS) |edition=Second Draft |first=N. |last=Shinohara |journal=Space Solar Power Workshop |publisher=Georgia Institute of Technology}}</ref><ref name="Brown1984">{{cite journal |last1=Brown |first1=W.C. |title=The History of Power Transmission by Radio Waves |journal=IEEE Transactions on Microwave Theory and Techniques |date=September 1984 |volume=32 |issue=9 |pages=1230–1242 |doi=10.1109/TMTT.1984.1132833 |bibcode=1984ITMTT..32.1230B |s2cid=73648082}}</ref> and more recently (1997) at Grand Bassin on [[Reunion Island]].<ref>{{cite web |url=http://www2.univ-reunion.fr/~lcks/Old_Version/PubIAF97.htm |archive-url=https://web.archive.org/web/20051023080942/http://www2.univ-reunion.fr/~lcks/Old_Version/PubIAF97.htm |archive-date=2005-10-23 |title=POINT-TO-POINT WIRELESS POWER TRANSPORTATION IN REUNION ISLAND |work=48th International Astronautical Congress |location=Turin, Italy |date=6–10 October 1997 |first1=J. D. |last1=Lan Sun Luk |first2=A. |last2=Celeste |first3=P. |last3=Romanacce |first4=L. |last4=Chane Kuang Sang |first5=J. C. |last5=Gatina |publisher=University of La Réunion – Faculty of Science and Technology}}</ref> These methods achieve distances on the order of a kilometer.

Under experimental conditions, microwave conversion efficiency was measured to be around 54% across one meter.<ref>{{cite journal |last1=Brown |first1=W.C. |last2=Eves |first2=E.E. |title=Beamed microwave power transmission and its application to space |journal=IEEE Transactions on Microwave Theory and Techniques |date=June 1992 |volume=40 |issue=6 |pages=1239–1250 |doi=10.1109/22.141357 |bibcode=1992ITMTT..40.1239B}}</ref>

A change to 24&nbsp;GHz has been suggested as microwave emitters similar to LEDs have been made with very high quantum efficiencies using [[negative resistance]], i.e., Gunn or IMPATT diodes, and this would be viable for short range links.

In 2013, inventor Hatem Zeine demonstrated how wireless power transmission using phased array antennas can deliver electrical power up to 30 feet. It uses the same radio frequencies as WiFi.<ref>{{Cite web |url=https://newatlas.com/cota-ossia-wireless-charging-microwave-phased-array/29217/ |title=Cota system transmits power wirelessly at up to 30 feet |website=newatlas.com |access-date=2018-01-05 |date=30 September 2013}}</ref><ref>{{Cite news |url=https://techcrunch.com/2013/09/09/cota-by-ossia-wireless-power/ |title=Cota By Ossia Aims To Drive A Wireless Power Revolution And Change How We Think About Charging |last=Etherington |first=Darrell |work=TechCrunch |access-date=2018-01-05}}</ref>

In 2015, researchers at the University of Washington introduced power over Wi-Fi, which trickle-charges batteries and powered battery-free cameras and temperature sensors using transmissions from Wi-Fi routers.<ref name=powifi>{{cite arXiv |eprint=1505.06815 |title=Powering the Next Billion Devices with Wi-Fi |last1=Talla |first1=Vamsi |last2=Kellogg |first2=Bryce |last3=Ransford |first3=Benjamin |last4=Naderiparizi |first4=Saman |last5=Gollakota |first5=Shyamnath |last6=Smith |first6=Joshua R. |class=cs.NI |year=2015}}</ref><ref>{{Cite web |url=https://www.technologyreview.com/s/538031/first-demonstration-of-a-surveillance-camera-powered-by-ordinary-wi-fi-broadcasts/ |title=First Demonstration of a Surveillance Camera Powered by Ordinary Wi-Fi Broadcasts |last=arXiv |first=Emerging Technology from the |access-date=2016-09-28}}</ref> Wi-Fi signals were shown to power battery-free temperature and camera sensors at ranges of up to 20 feet. It was also shown that Wi-Fi can be used to wirelessly trickle-charge nickel–metal hydride and lithium-ion coin-cell batteries at distances of up to 28 feet.

In 2017, the Federal Communications Commission (FCC) certified the first mid-field radio frequency (RF) transmitter of wireless power.<ref>{{Cite web |url=https://ir.energous.com/press-releases/detail/596/energous-receives-industry-first-fcc-certification-for |title=Energous Receives Industry-First FCC Certification for Over-the-Air, Power-at-a-Distance Wireless Charging :: Energous Corporation (WATT) |website=Energous Corporation |access-date=2018-01-05}}</ref> In 2021 the FCC granted a license to an over-the-air (OTA) wireless charging system that combines near-field and far-field methods by using a frequency of about 900&nbsp;MHz. Due to the radiated power of about 1 W this system is intended for small [[Internet of things|IoT]] devices as various sensors, trackers, detectors and monitors.<ref>{{Cite web |last=Emilio |first=Maurizio Di Paolo |date=2021-11-08 |title=Energous Enables Wireless Power Transfer Solutions at any Distance for U.S. and Europe |url=https://www.eetimes.eu/energous-enables-wireless-power-transfer-solutions-at-any-distance-for-u-s-and-europe/ |access-date=2021-11-11 |website=EE Times Europe}}</ref>