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Fuel cell
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===In practice=== Values are given from 40% for acidic, 50% for molten carbonate, to 60% for alkaline, solid oxide and PEM fuel cells.<ref>{{cite web|title=Fuel Cells |date=November 2015 |url=https://www.energy.gov/sites/prod/files/2015/11/f27/fcto_fuel_cells_fact_sheet.pdf |access-date=2022-12-27}}</ref> Fuel cells cannot store energy like a battery,<ref>{{cite web|url=https://www.loc.gov/rr/scitech/tracer-bullets/batteriestb.html#scope |title=Batteries, Supercapacitors, and Fuel Cells: Scope |access-date= 11 February 2009 |date=20 August 2007 |publisher=Science Reference Services }}</ref> except as hydrogen, but in some applications, such as stand-alone power plants based on discontinuous sources such as [[solar energy|solar]] or [[wind power]], they are combined with [[electrolysis|electrolyzers]] and storage systems to form an energy storage system. As of 2019, 90% of hydrogen was used for oil refining, chemicals and fertilizer production (where hydrogen is required for the [[HaberโBosch process]]), and 98% of hydrogen is produced by [[steam methane reforming]], which emits carbon dioxide.<ref>[https://www.power-technology.com/comment/standing-at-the-precipice-of-the-hydrogen-economy "Realising the hydrogen economy"] {{Webarchive|url=https://web.archive.org/web/20191105054643/https://www.power-technology.com/comment/standing-at-the-precipice-of-the-hydrogen-economy/ |date=5 November 2019 }},''Power Technology'', 11 October 2019</ref> The overall efficiency (electricity to hydrogen and back to electricity) of such plants (known as ''round-trip efficiency''), using pure hydrogen and pure oxygen can be "from 35 up to 50 percent", depending on gas density and other conditions.<ref>{{Cite news| last=Garcia| first= Christopher P.| title = Round Trip Energy Efficiency of NASA Glenn Regenerative Fuel Cell System |date=January 2006 | publisher = Preprint |page=5 | display-authors=1| author2=<Please add first missing authors to populate metadata.>| hdl= 2060/20060008706}}</ref> The electrolyzer/fuel cell system can store indefinite quantities of hydrogen, and is therefore suited for long-term storage. Solid-oxide fuel cells produce heat from the recombination of the oxygen and hydrogen. The ceramic can run as hot as {{cvt|800|C|F}}. This heat can be captured and used to heat water in a [[micro combined heat and power]] (m-CHP) application. When the heat is captured, total efficiency can reach 80โ90% at the unit, but does not consider production and distribution losses. CHP units are being developed today for the European home market. Professor Jeremy P. Meyers, in the [[Electrochemical Society]] journal ''Interface'' in 2008, wrote, "While fuel cells are efficient relative to combustion engines, they are not as efficient as batteries, primarily due to the inefficiency of the oxygen reduction reaction (and ... the oxygen evolution reaction, should the hydrogen be formed by electrolysis of water). ... [T]hey make the most sense for operation disconnected from the grid, or when fuel can be provided continuously. For applications that require frequent and relatively rapid start-ups ... where zero emissions are a requirement, as in enclosed spaces such as warehouses, and where hydrogen is considered an acceptable reactant, a [PEM fuel cell] is becoming an increasingly attractive choice [if exchanging batteries is inconvenient]".<ref name=Meyers1>Meyers, Jeremy P. [http://www.electrochem.org/dl/interface/wtr/wtr08/wtr08_p36-39.pdf "Getting Back Into Gear: Fuel Cell Development After the Hype"]. The Electrochemical Society ''Interface'', Winter 2008, pp. 36โ39, accessed 7 August 2011</ref> In 2013 military organizations were evaluating fuel cells to determine if they could significantly reduce the battery weight carried by soldiers.<ref name="The fuel cell industry review 2013">{{cite web| url = http://www.fuelcelltoday.com/media/1889744/fct_review_2013.pdf| title = The fuel cell industry review 2013}}</ref> ==== In vehicles ==== In a [[fuel cell vehicle]] the tank-to-wheel efficiency is greater than 45% at low loads<ref name=RSC>Eberle, Ulrich and Rittmar von Helmolt. [https://www.researchgate.net/publication/224880220_Sustainable_transportation_based_on_electric_vehicle_concepts_a_brief_overview "Sustainable transportation based on electric vehicle concepts: a brief overview"]. Energy & Environmental Science, [[Royal Society of Chemistry]], 14 May 2010, accessed 2 August 2011</ref> and shows average values of about 36% when a driving cycle like the NEDC ([[New European Driving Cycle]]) is used as test procedure.<ref name="status2007">{{Cite journal| title = Fuel Cell Vehicles:Status 2007 | journal=Journal of Power Sources | date = 20 March 2007 | doi = 10.1016/j.jpowsour.2006.12.073 | author = Von Helmolt, R. | volume = 165 | pages = 833โ843 | last2 = Eberle | first2 = U| issue = 2| bibcode = 2007JPS...165..833V }}</ref> The comparable NEDC value for a Diesel vehicle is 22%. In 2008 Honda released a demonstration fuel cell electric vehicle (the [[Honda FCX Clarity]]) with fuel stack claiming a 60% tank-to-wheel efficiency.<ref>{{cite web| url=http://automobiles.honda.com/fcx-clarity/fuel-cell-comparison.aspx| title=Honda FCX Clarity โ Fuel cell comparison| publisher=Honda| access-date=2009-01-02| archive-date=3 January 2009| archive-url=https://web.archive.org/web/20090103204930/http://automobiles.honda.com/fcx-clarity/fuel-cell-comparison.aspx| url-status=dead}}</ref> It is also important to take losses due to fuel production, transportation, and storage into account. Fuel cell vehicles running on compressed hydrogen may have a power-plant-to-wheel efficiency of 22% if the hydrogen is stored as high-pressure gas, and 17% if it is stored as [[liquid hydrogen]].<ref>{{cite web|title=Efficiency of Hydrogen PEFC, Diesel-SOFC-Hybrid and Battery Electric Vehicles |date=15 July 2003 |url=http://www.efcf.com/reports/E04.pdf |access-date=2007-05-23 |url-status=dead |archive-url=https://web.archive.org/web/20061021051748/http://www.efcf.com/reports/E04.pdf |archive-date=21 October 2006 }}</ref>
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