Power-to-X
Power-to-X (also P2X and P2Y) are electricity conversion, energy storage, and reconversion pathways from surplus renewable energy.<ref name="acatech-2016">acatech; Lepoldina; Akademienunion, eds. (2016). Flexibility concepts for the German power supply in 2050 : ensuring stability in the age of renewable energies (PDF). Berlin, Germany: acatech — National Academy of Science and Engineering. ISBN 978-3-8047-3549-1. Archived from the original (PDF) on 6 October 2016. Retrieved 10 June 2016.</ref><ref name="lund-etal-2015">Lund, Peter D; Lindgren, Juuso; Mikkola, Jani; Salpakari, Jyri (2015). "Review of energy system flexibility measures to enable high levels of variable renewable electricity" (PDF). Renewable and Sustainable Energy Reviews. 45: 785–807. doi:10.1016/j.rser.2015.01.057.</ref> Power-to-X conversion technologies allow for the decoupling of power from the electricity sector for use in other sectors (such as transport or chemicals), possibly using power that has been provided by additional investments in generation.<ref name="acatech-2016"/> The term is widely used in Germany and may have originated there.
The X in the terminology can refer to one of the following: power-to-ammonia, power-to-chemicals, power-to-fuel,<ref>Trakimavicius, Lukas (December 2023). "Mission Net-Zero: Charting the Path for E-fuels in the Military". NATO Energy Security Centre of Excellence.</ref> power-to-gas (power-to-hydrogen, power-to-methane) power-to-liquid (synthetic fuel), power to food,<ref>Sillman, J.; Uusitalo, V.; Ruuskanen, V.; Ojala, L.; Kahiluoto, H.; Soukka, R.; Ahola, J. (1 November 2020). "A life cycle environmental sustainability analysis of microbial protein production via power-to-food approaches". The International Journal of Life Cycle Assessment. 25 (11): 2190–2203. doi:10.1007/s11367-020-01771-3. ISSN 1614-7502.</ref> power-to-heat. Electric vehicle charging, space heating and cooling, and water heating can be shifted in time to match generation, forms of demand response that can be called power-to-mobility and power-to-heat.
Collectively power-to-X schemes which use surplus power fall under the heading of flexibility measures and are particularly useful in energy systems with high shares of renewable generation and/or with strong decarbonization targets.<ref name="acatech-2016"/><ref name="lund-etal-2015"/> A large number of pathways and technologies are encompassed by the term. In 2016 the German government funded a €30 million first-phase research project into power-to-X options.<ref name="rwth-2016">"Power-to-X: entering the energy transition with Kopernikus" (Press release). Aachen, Germany: RWTH Aachen. 5 April 2016. Retrieved 9 June 2016.</ref>
Power-to-fuel
Surplus electric power can be converted to gas fuel energy for storage and reconversion.<ref name="sternberg-etal-2015">Sternberg, André; Bardow, André (2015). "Power-to-What? — Environmental assessment of energy storage systems". Energy and Environmental Science. 8 (2): 389–400. doi:10.1039/c4ee03051f.</ref><ref name="agora-2014">Agora Energiewende (2014). Electricity storage in the German energy transition : analysis of the storage required in the power market, ancillary services market and the distribution grid (PDF). Berlin, Germany: Agora Energiewende. Retrieved 30 December 2018.</ref><ref name="sterner-etal-2014">Sterner, Michael; Eckert, Fabian; Thema, Martin; et al. (2014). Langzeitspeicher in der Energiewende — Präsentation [Long-term storage in the Energiewende — Presentation]. Regensburg, Germany: Forschungsstelle für Energienetze und Energiespeicher (FENES), OTH Regensburg. Retrieved 9 May 2016.</ref><ref name="ausfelder-etal-2016">Ausfelder, Florian; Beilmann, Christian; Bräuninger, Sigmar; Elsen, Reinhold; Hauptmeier, Erik; Heinzel, Angelika; Hoer, Renate; Koch, Wolfram; Mahlendorf, Falko; Metzelthin, Anja; Reuter, Martin; Schiebahn, Sebastian; Schwab, Ekkehard; Schüth, Ferdi; Stolten, Detlef; Teßmer, Gisa; Wagemann, Kurt; Ziegahn, Karl-Friedrich (May 2016). Energy storage systems: the contribution of chemistry — Position paper (PDF). Germany: Koordinierungskreis Chemische Energieforschung (Joint Working Group on Chemical Energy Research). ISBN 978-3-89746-183-3. Retrieved 9 June 2016.</ref> Direct current electrolysis of water (efficiency 80–85% at best) can be used to produce hydrogen which can, in turn, be converted to methane (CH4) via methanation.<ref name=sternberg-etal-2015/><ref name="pagliaro-and-konstandopoulos-2012"> Pagliaro, Mario; Konstandopoulos, Athanasios G (15 June 2012). Solar Hydrogen: Fuel of the Future. Cambridge, United Kingdom: RSC Publishing. doi:10.1039/9781849733175. ISBN 978-1-84973-195-9. S2CID 241910312. </ref> Another possibility is converting the hydrogen, along with CO2 to methanol.<ref>George Olah's renewable methanol plant</ref> Both these fuels can be stored and used to produce electricity again, hours to months later.
Storage and reconversion of power-to-fuel
Hydrogen and methane can be used as downstream fuels, fed into the natural gas grid, or used to make synthetic fuel.<ref name="koenig-etal-2014">König, Daniel Helmut; Baucks, Nadine; Kraaij, Gerard; Wörner, Antje (18–19 February 2014). "Entwicklung und Bewertung von Verfahrenskonzepten zur Speicherung von fluktuierenden erneuerbaren Energien in flüssigen Kohlenwasserstoffen" [Development and evaluation of process concepts for storing fluctuating renewable energy in liquid hydrocarbons]. Jahrestreffen der ProcessNet-Fachgruppe Energieverfahrenstechnik. Karlsruhe, Germany. Retrieved 9 May 2016.</ref><ref name="foit-etal-2016">Foit, Severin; Eichel, Rüdiger-A; Vinke, Izaak C; de Haart, Lambertus GJ (1 October 2016). "Power-to-Syngas – an enabling technology for the transition of the energy system? Production of tailored synfuels and chemicals using renewably generated electricity". Angewandte Chemie International Edition. 56 (20): 5402–5411. doi:10.1002/anie.201607552. ISSN 1521-3773. PMID 27714905.</ref> Alternatively they can be used as a chemical feedstock, as can ammonia (NH3).
Reconversion technologies include gas turbines, combined cycle plants, reciprocating engines and fuel cells. Power-to-power refers to the round-trip reconversion efficiency.<ref name=sternberg-etal-2015/> For hydrogen storage, the round-trip efficiency remains limited at 35–50%.<ref name="lund-etal-2015"/> Electrolysis is expensive and power-to-gas processes need substantial full-load hours to be economic.<ref name=acatech-2016/> However, while round-trip conversion efficiency of power-to-power is lower than with batteries and electrolysis can be expensive, storage of the fuels themselves is quite inexpensive.[citation needed] This means that large amounts of energy can be stored for long periods of time with power-to-power, which is ideal for seasonal storage. This could be particularly useful for systems with high variable renewable energy penetration, since many areas have significant seasonal variability of solar, wind, and run-of-the-river-hydroelectric generation.
Batteries
Despite it also being based fundamentally on electrolytic chemical reactions, battery storage is not normally considered a power-to-fuel concept.
Power-to-heat
The purpose of power-to-heat systems is to utilize excess electricity generated by renewable energy sources which would otherwise be wasted. Depending on the context, the power-to-heat can either be stored as heat, or delivered as heat to meet a need.<ref name="bloess-etal-2018">
Bloess, Andreas; Schill, Wolf-Peter; Zerrahn, Alexander (15 February 2018). "Power-to-heat for renewable energy integration: a review of technologies, modeling approaches, and flexibility potentials". Applied Energy. 212: 1611–1626. doi:10.1016/j.apenergy.2017.12.073. hdl:10419/200120. ISSN 0306-2619. 
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Heating systems
In contrast to simple electric heating systems such as night storage heating which covers the complete heating requirements, power-to-heat systems are hybrid systems, which additionally have traditional heating systems using chemical fuels like wood or natural gas.<ref>Sterner, Stadler, Michael, Ingo (2014). Energiespeicher – Bedarf, Technologien, Integration. Berlin and Heidelberg.{{cite book}}: CS1 maint: location missing publisher (link) CS1 maint: multiple names: authors list (link)</ref>: 124 When there are excess energy the heat production can result from electric energy otherwise the traditional heating system will be used. In order to increase flexibility power-to-heat systems are often coupled with heat accumulators. The power supply occurs for the most part in the local and district heating networks. Power-to-heat systems are also able to supply buildings or industrial systems with heat.<ref>Schweiger, Gerald (2017). "The potential of power-to-heat in Swedish district heating systems". Energy. 137: 661–669. doi:10.1016/j.energy.2017.02.075.</ref>
Power-to-heat involves contributing to the heat sector, either by resistance heating or via a heat pump. Resistance heaters have unity efficiency, and the corresponding coefficient of performance (COP) of heat pumps is 2–5.<ref name=sternberg-etal-2015/> Back-up immersion heating of both domestic hot water and district heating offers a cheap way of using surplus renewable energy and will often displace carbon-intensive fossil fuels for the task.<ref name="acatech-2016"/> Large-scale heat pumps in district heating systems with thermal energy storage are an especially attractive option for power-to-heat: they offer exceptionally high efficiency for balancing excess wind and solar power, and they can be profitable investments.<ref name="zakeri-etal-2016">Zakeri, Behnam; Rinne, Samuli; Syri, Sanna (31 March 2015). "Wind integration into energy systems with a high share of nuclear power – what are the compromises?". Energies. 8 (4): 2493–2527. doi:10.3390/en8042493. ISSN 1996-1073.</ref><ref name="salpakari-etal-2016">Salpakari, Jyri; Mikkola, Jani; Lund, Peter D (2016). "Improved flexibility with large-scale variable renewable power in cities through optimal demand side management and power-to-heat conversion". Energy Conversion and Management. 126: 649–661. doi:10.1016/j.enconman.2016.08.041. ISSN 0196-8904.</ref>
Heat storage systems
Other forms of power-to-X
Power-to-mobility refers to the charging of battery electric vehicles (BEV). Given the expected uptake of EVs, dedicated dispatch will be required. As vehicles are idle for most of the time, shifting the charging time can offer considerable flexibility: the charging window is a relatively long 8–12 hours, whereas the charging duration is around 90 minutes.<ref name="lund-etal-2015"/> The EV batteries can also be discharged to the grid to make them work as electricity storage devices, but this causes additional wear to the battery.<ref name="lund-etal-2015"/>
Impact
According to the German concept of sector coupling interconnecting all the energy-using sectors will require the digitalisation and automation of numerous processes to synchronise supply and demand.<ref>"Sector coupling – Shaping an integrated renewable energy system". Clean Energy Wire. 18 April 2018. Retrieved 6 March 2019.</ref>
A 2023 study examined to role that power‑to‑X could play in a highly‑renewable future energy system for Japan. The P2X technologies considered include water electrolysis, methanation, Fischer–Tropsch synthesis, and Haber–Bosch synthesis and the study used linear programming to determine least‑cost system structure and operation. Results indicate that these various P2X technologies can effectively shift electricity loads and reduce curtailment by 80% or more.<ref name="onodera-etal-2023">
Onodera, Hiroaki; Delage, Rémi; Nakata, Toshihiko (1 October 2023). "Systematic effects of flexible power-to-X operation in a renewable energy system: a case study from Japan". Energy Conversion and Management: X. 20: 100416. doi:10.1016/j.ecmx.2023.100416. ISSN 2590-1745. Retrieved 1 September 2023.
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