View source for Variable renewable energy

{{Short description|Class of renewable energy sources}}
[[File:Andasol Guadix 4.jpg|thumb|right|The 150 MW [[Andasol solar power station]] is a commercial [[parabolic trough]] [[solar thermal]] power plant, in [[Renewable energy in Spain|Spain]]. The Andasol plant uses tanks of molten salt to store solar energy so that it can continue generating electricity even after sunset.<ref>{{Cite journal|last=Cartlidge|first=Edwin|date=2011-11-18|title=Saving for a Rainy Day|url=https://www.science.org/doi/10.1126/science.334.6058.922|journal=Science|language=en|volume=334|issue=6058|pages=922–924|doi=10.1126/science.334.6058.922|issn=0036-8075|pmid=22096185|bibcode=2011Sci...334..922C}}</ref>]]
[[File:Renewables need flexible backup not baseload.png|thumb|Grids with high penetration of renewable energy sources generally need more flexible generation rather than baseload generation<ref>{{Cite web |date=2022-06-15 |title=Flexible Power Plant Operation to Enable High Renewable Energy Penetration |url=https://iesr.or.id/en/flexible-power-plant-operation-to-enable-high-renewable-energy-penetration |access-date=2022-11-21 |website=IESR |language=en-US}}</ref>]]
'''Variable renewable energy''' ('''VRE''') or '''intermittent renewable energy sources (IRES)''' are [[renewable energy]] sources that are not [[Dispatchable generation|dispatchable]] due to their fluctuating nature, such as [[wind power]] and [[solar power]], as opposed to controllable renewable energy sources, such as dammed [[hydroelectricity]] or [[biomass]], or relatively constant sources, such as [[Geothermal power|geothermal]] power.

The use of small amounts of intermittent power has little effect on [[Electrical grid|grid]] operations. Using larger amounts of intermittent power may require upgrades or even a redesign of the grid infrastructure.<ref name="All_Island_Grid_Overview">{{cite web|date=January 2008|title=All Island Grid Study|url=http://www.dcenr.gov.ie/NR/rdonlyres/E20F6E3D-8724-4E39-B315-0670B8C9841E/0/AllIslandGridStudyStudyOverviewJan08a2.pdf|url-status=dead|archive-url=https://web.archive.org/web/20090318231419/http://www.dcenr.gov.ie/NR/rdonlyres/E20F6E3D-8724-4E39-B315-0670B8C9841E/0/AllIslandGridStudyStudyOverviewJan08a2.pdf|archive-date=2009-03-18|access-date=2008-10-15|work=Department of Communications, Energy and Natural Resources|pages=3–5, 15}}</ref><ref name="CarbonDTI">{{cite web|date=January 2004|orig-year=commissioned June 2003|title=The Carbon Trust & DTI Renewables Network Impacts Study|url=http://www.ensg.gov.uk/assets/ct_impact_of_re_final_report.pdf|url-status=dead|archive-url=http://webarchive.nationalarchives.gov.uk/20100919181607/http://www.ensg.gov.uk/assets/ct_impact_of_re_final_report.pdf|archive-date=2010-09-19|access-date=2009-04-22|work=Carbon Trust and UK Department of Trade and Industry}}</ref> 

Options to absorb large shares of variable energy into the grid include using [[Energy storage|storage]], improved interconnection between different variable sources to smooth out supply, using dispatchable energy sources such as hydroelectricity and having overcapacity, so that sufficient energy is produced even when weather is less favourable. More connections between the energy sector and the building, transport and industrial sectors may also help.<ref>{{Cite report |publisher=Intergovernmental Panel on Climate Change |date=4 April 2022 |title=IPCC: Climate Change 2022, Mitigation of Climate Change, Summary for Policymakers |url=https://report.ipcc.ch/ar6wg3/pdf/IPCC_AR6_WGIII_SummaryForPolicymakers.pdf |access-date=2004-04-22 |website=ipecac.ch |archive-date=2022-08-07 |archive-url=https://web.archive.org/web/20220807023536/https://report.ipcc.ch/ar6wg3/pdf/IPCC_AR6_WGIII_SummaryForPolicymakers.pdf |url-status=dead }}</ref>{{Rp|page=55}}

==Background and terminology==
The penetration of intermittent renewables in most power grids is low: global electricity generation in 2021 was 7% wind and 4% solar.<ref name=":0">{{Cite web |date=2022-03-29 |title=Global Electricity Review 2022 |url=https://ember-climate.org/insights/research/global-electricity-review-2022/ |access-date=2022-03-31 |website=Ember |language=en-US}}</ref> However, in 2021 Denmark, Luxembourg and Uruguay generated over 40% of their electricity from wind and solar.<ref name=":0" /> Characteristics of variable renewables include their unpredictability, variability, and low operating costs.<ref>{{Cite journal |last1=Riesz |first1=Jenny |last2=Milligan |first2=Michael |date=May 2015 |title=Designing electricity markets for a high penetration of variable renewables |url=https://wires.onlinelibrary.wiley.com/doi/10.1002/wene.137 |journal=WIREs Energy and Environment |language=en |volume=4 |issue=3 |pages=279–289 |doi=10.1002/wene.137 |bibcode=2015WIREE...4..279R |s2cid=167079952 |issn=2041-8396}}</ref> These, along with renewables typically being [[Wide area synchronous grid|asynchronous]] generators, provide a challenge to [[Electric power transmission|grid operators]], who must make sure supply and demand are matched. Solutions include [[energy storage]], [[demand response]], availability of overcapacity and [[sector coupling]].<ref>{{Cite journal|date=2020-01-01|title=Challenges and solution technologies for the integration of variable renewable energy sources—a review|url=https://www.sciencedirect.com/science/article/pii/S0960148119309875|journal=Renewable Energy|language=en|volume=145|pages=2271–2285|doi=10.1016/j.renene.2019.06.147|issn=0960-1481|last1=Sinsel|first1=Simon R.|last2=Riemke|first2=Rhea L.|last3=Hoffmann|first3=Volker H.|hdl=20.500.11850/373407|s2cid=198480155|hdl-access=free}}</ref> Smaller isolated grids may be less tolerant to high levels of penetration.<ref name="All_Island_Grid_Overview" /><ref name="Czisch100Renew">{{cite web|last=Czisch|first=Gregor|author2=Gregor Giebel|title=Realisable Scenarios for a Future Electricity Supply based 100% on Renewable Energies|url=http://www.risoe.dk/rispubl/reports/ris-r-1608_186-195.pdf|url-status=dead|archive-url=https://web.archive.org/web/20140701230913/http://www.risoe.dk/rispubl/reports/ris-r-1608_186-195.pdf|archive-date=2014-07-01|access-date=2008-10-15|work=Institute for Electrical Engineering – Efficient Energy Conversion University of Kassel, Germany and Risø National Laboratory, Technical University of Denmark}}</ref>

Matching power demand to supply is not a problem specific to intermittent power sources. Existing power grids already contain elements of uncertainty including sudden and large changes in demand and unforeseen power plant failures. Though power grids are already designed to have some capacity in excess of projected peak demand to deal with these problems, significant upgrades may be required to accommodate large amounts of intermittent power.<ref name="IEA2005WindVar">{{cite web|year=2005|title=Variability of Wind Power and other Renewables: Management Options and Strategies|url=http://www.iea.org/Textbase/Papers/2005/variability.pdf|access-date=2008-10-15|work=IEA}}</ref>

Several key terms are useful for understanding the issue of intermittent power sources. These terms are not standardized, and variations may be used. Most of these terms also apply to traditional power plants.

* '''Intermittency''' or '''variability''' is the extent to which a power source fluctuates. This has two aspects: a predictable variability (such as the day-night cycle) and an unpredictable part (imperfect local weather forecasting).<ref>{{cite journal |last1=Widén |first1=Joakim |last2=Carpman |first2=Nicole |title=Variability assessment and forecasting of renewables: A review for solar, wind, wave and tidal resources |journal=Renewable and Sustainable Energy Reviews |date=1 April 2015 |volume=44 |pages=356–375 |doi=10.1016/j.rser.2014.12.019 |url=https://www.sciencedirect.com/science/article/pii/S1364032114010715 |language=en |issn=1364-0321}}</ref> The term ''intermittent'' can be used to refer to the unpredictable part, with ''variable'' then referring to the predictable part.<ref>{{cite journal |last1=Pommeret |first1=Aude |last2=Schubert |first2=Katheline |title=Energy Transition with Variable and Intermittent Renewable Electricity Generation |journal=Cesifo Working Paper Series |date=2019 |volume=7442 |page=2 |url=https://ideas.repec.org/p/ces/ceswps/_7442.html |series=CESifo Working Paper}}</ref>
* '''[[Dispatchable generation|Dispatchability]]''' is the ability of a given power source to increase and decrease output quickly on demand. The concept is distinct from intermittency; dispatchability is one of several ways system operators match supply (generator's output) to system demand (technical loads).<ref name=MEFlowBattery>{{cite web|url=http://www.memagazine.org/backissues/membersonly/oct05/features/rerere/rerere.html |title=renewable. rechargeable. remarkable. |access-date=2008-10-20 |last=Kuntz |first=Mark T. |author2=Justin Dawe |year=2005 |work=VRB Power Systems |publisher=Mechanical Engineering |url-status=dead |archive-url=https://web.archive.org/web/20090115212122/http://www.memagazine.org/backissues/membersonly/oct05/features/rerere/rerere.html |archive-date=2009-01-15 }}</ref>
* '''Penetration''' is the amount of electricity generated from a particular source as a percentage of annual consumption.<ref name="ieawind.org">[http://www.ieawind.org/AnnexXXV/Publications/Task25/Task%2025%20Design%20and%20Operation%20of%20Power%20Systems%20UWIG.pdf International Energy Agency Wind Task Force, "Design and Operation of Power Systems with Large Amounts of Wind Power"] {{Webarchive|url=https://web.archive.org/web/20071025113737/http://www.ieawind.org/AnnexXXV/Publications/Task25/Task%2025%20Design%20and%20Operation%20of%20Power%20Systems%20UWIG.pdf |date=2007-10-25 }} Oklahoma Conference Presentation, October 2006</ref>
* '''Nominal power''' or '''[[nameplate capacity]]''' is the theoretical output registered with authorities for classifying the unit. For [[Intermittent power source|intermittent power sources]], such as wind and solar, nameplate power is the source's output under ideal conditions, such as maximum usable wind or high sun on a clear summer day. 
* '''[[Capacity factor]]''', '''average capacity factor''', or '''load factor''' is the ratio of actual electrical generation over a given period of time (usually a year) to actual generation in that time period. Basically, it is the ratio between the how much electricity a plant produced and how much electricity a plant would have produced if were running at its nameplate capacity for the entire time period.
* '''[[Electrical grid#Capacity and firm capacity|Firm capacity]]''' or '''[[firm power]]''' is "guaranteed by the supplier to be available at all times during a period covered by a commitment".<ref>{{Cite web|title=firm power|url=https://www.ecowho.com/defn/f/firm%20power/2da42|access-date=2021-02-10|website=www.ecowho.com}}</ref>
* '''[[Capacity credit]]''': the amount of conventional (dispatchable) generation power that can be potentially removed from the system while keeping the reliability, usually expressed as a percentage of the nominal power.<ref name='WindCC'>{{cite web |url=https://core.ac.uk/download/pdf/13774867.pdf |title=Wind Powers Has a Capacity Credit |access-date=2008-10-16 |last=Giebel |first=Gregor |work=Risø National Laboratory |archive-url=https://web.archive.org/web/20090318231423/http://ejournal.windeng.net/3/01/GGiebel-CapCredLit_WindEngEJournal_2005_right_links.pdf |archive-date=2009-03-18 }}</ref>{{Example needed|date=April 2022}}{{Clarify|reason=please see talk page|date=April 2022}}
* '''Foreseeability''' or '''predictability''' is how accurately the operator can anticipate the generation:<ref>{{Cite journal|date=2020|title=Defining and Quantifying Intermittency in the Power Sector|url=https://www.mdpi.com/1996-1073/13/13/3366/pdf|journal=Energies|volume=13|doi=10.3390/en13133366|via=|doi-access=free|last1=Suchet|first1=Daniel|last2=Jeantet|first2=Adrien|last3=Elghozi|first3=Thomas|last4=Jehl|first4=Zacharie|issue=13|page=3366}}</ref> for example [[tidal power]] varies with the tides but is completely foreseeable because the orbit of the moon can be predicted exactly, and improved weather forecasts can make wind power more predictable.<ref>{{Cite web|date=2016-08-15|title=Volatile but predictable: Forecasting renewable power generation|url=https://www.cleanenergywire.org/factsheets/volatile-predictable-forecasting-renewable-power-generation|access-date=2021-02-10|website=Clean Energy Wire|language=en}}</ref>

==Sources==
{{See also|Renewable energy}}
Dammed hydroelectricity, biomass and geothermal are dispatchable as each has a store of potential energy; wind and solar without storage can be decreased (curtailed) but are not dispatchable.

===Wind power===
[[File:Windpowerprediction.png|thumb|Day ahead prediction and actual wind power]]
Grid operators use day ahead forecasting to determine which of the available power sources to use the next day, and weather forecasting is used to predict the likely wind power and solar power output available. Although [[Wind power forecasting|wind power forecasts]] have been used operationally for decades, {{as of|2019|lc=y}} the IEA is organizing international collaboration to further improve their accuracy.<ref>{{Cite web|url=http://www.ieawindforecasting.dk/|title=IEA wind task 36|website=iea wind forecasting|language=en|access-date=2019-07-25}}</ref>

[[File:Erie Shores Wind Farm output Aug-Jul 2008.gif|thumb|right|[[Erie Shores Wind Farm]] monthly output over a two-year period]]
[[File:Aralvaimozhy station.jpg|right|thumb|A [[Muppandal wind farm|wind farm]] in [[Muppandal]], [[Tamil Nadu]], [[India]]]]Wind-generated power is a variable resource, and the amount of electricity produced at any given point in time by a given plant will depend on wind speeds, air density, and turbine characteristics (among other factors). If wind speed is too low then the wind turbines will not be able to make electricity, and if it is too high the turbines will have to be shut down to avoid damage.  While the output from a single turbine can vary greatly and rapidly as local wind speeds vary, as more turbines are connected over larger and larger areas the average power output becomes less variable.<ref name="IEA2005WindVar"/>

* '''Intermittence:''' Regions smaller than [[Synoptic scale meteorology|synoptic scale]] (less than about 1000&nbsp;km long, the size of an average country) have mostly the same weather and thus around the same wind power, unless local conditions favor special winds. Some studies show that wind farms spread over a geographically diverse area will as a whole rarely stop producing power altogether.<ref name="connect">{{cite web|date=2007-11-21|title=The power of multiples: Connecting wind farms can make a more reliable and cheaper power source|url=http://www.eurekalert.org/pub_releases/2007-11/ams-tpo112107.php}}</ref><ref name="Archer2007">{{Cite journal|author1=Archer, C. L.|author2=Jacobson, M. Z.|year=2007|title=Supplying Baseload Power and Reducing Transmission Requirements by Interconnecting Wind Farms|url=http://www.stanford.edu/group/efmh/winds/aj07_jamc.pdf|journal=Journal of Applied Meteorology and Climatology|volume=46|issue=11|pages=1701–1717|bibcode=2007JApMC..46.1701A|citeseerx=10.1.1.475.4620|doi=10.1175/2007JAMC1538.1}}</ref> However this is rarely the case for smaller areas with uniform geography such as Ireland,<ref>{{cite web|url= http://www.inference.phy.cam.ac.uk/withouthotair/c26/page_187.shtml |title=Sustainable Energy - without the hot air. Fluctuations and storage|author=David JC MacKay}}</ref><ref>{{cite web |url=http://www.atom.edu.pl/index.php?option=com_content&task=view&id=92&Itemid=73 |title=Czy w Polsce wiatr wystarczy zamiast elektrowni atomowych? |trans-title=Can the wind suffice instead of nuclear power in Poland? |language=pl |publisher=atom.edu.pl |author=Andrzej Strupczewski |access-date=2009-11-26 |archive-url=https://web.archive.org/web/20110904155414/http://www.atom.edu.pl/index.php?option=com_content&task=view&id=92&Itemid=73 |archive-date=2011-09-04 |url-status=dead }}</ref><ref name='BaseFallacy'>{{cite web |url=http://www.sustainabilitycentre.com.au/BaseloadFallacy.pdf |title=The Base-Load Fallacy |access-date=2008-10-18 |last=Diesendorf |first=Mark |date=August 2007 |work=Institute of Environmental Studies |publisher=www.energyscience.org.au |archive-url=https://web.archive.org/web/20080708184613/http://www.sustainabilitycentre.com.au/BaseloadFallacy.pdf |archive-date=2008-07-08 |url-status=dead }}</ref> Scotland<ref>[http://www.windaction.org/posts/30544-report-questions-wind-power-s-ability-to-deliver-electricity-when-most-needed#.WHkNM7kSiyA "Analysis of UK Wind Generation"] 2011</ref> and Denmark which have several days per year with little wind power.<ref name="Denmark2002">{{Cite journal | title= Why wind power works for Denmark |journal = Proceedings of the Institution of Civil Engineers - Civil Engineering |volume = 158 |issue = 2 |pages = 66–72 |date = May 2005 | doi= 10.1680/cien.2005.158.2.66 |last1 = Sharman|first1 = Hugh}}</ref>
* '''Capacity factor:''' Wind power typically has an annual capacity factor of 25–50%, with offshore wind outperforming onshore wind.<ref>{{Cite web|title=Average annual capacity factors by technology, 2018 – Charts – Data & Statistics|url=https://www.iea.org/data-and-statistics/charts/average-annual-capacity-factors-by-technology-2018|access-date=2021-02-10|website=IEA|language=en-GB}}</ref>
* '''Dispatchability:''' Because wind power is not by itself dispatchable wind farms are sometimes built with storage.<ref>{{Cite web|title=How Dispatchable Wind Is Becoming a Reality in the US|url=https://www.greentechmedia.com/articles/read/how-dispatchable-wind-is-becoming-a-reality-in-the-us|access-date=2020-08-10|website=www.greentechmedia.com}}</ref><ref>{{Cite web|title=51MWh vanadium flow battery system ordered for wind farm in northern Japan|url=https://www.energy-storage.news/news/51mwh-vanadium-flow-battery-system-ordered-for-wind-farm-in-northern-japan|access-date=2020-08-10|website=Energy Storage News|date=20 July 2020|language=en}}</ref>
* '''Capacity credit:''' At low levels of penetration, the capacity credit of wind is about the same as the capacity factor. As the concentration of wind power on the grid rises, the capacity credit percentage drops.<ref name="BWEAMyth">{{cite web|date=February 2005|title=Blowing Away the Myths|url=http://www.bwea.com/pdf/ref_three.pdf|url-status=dead|archive-url=https://web.archive.org/web/20070710024744/http://www.bwea.com/pdf/ref_three.pdf|archive-date=2007-07-10|access-date=2008-10-16|work=The British Wind Energy Association}}</ref><ref name='TyndallCenter'>{{cite web|url= http://www.tyndall.ac.uk/research/theme2/final_reports/t2_24.pdf |title=Security assessment of future UK electricity scenarios |access-date=2008-10-20 |last=Nedic |first=Dusko |author2=Anser Shakoor |author3=Goran Strbac |author4=Mary Black |author5=Jim Watson |author6=Catherine Mitchell |date = July 2005|work=Tyndall Centre for Climate Change Research |archive-url = https://web.archive.org/web/20070111234301/http://www.tyndall.ac.uk/research/theme2/final_reports/t2_24.pdf |archive-date = January 11, 2007}}</ref>
* '''Variability:''' Site dependent.<ref name="Junling">{{cite journal |author1=Junling Huang |author2=Xi Lu |author3=Michael B. McElroy |url= http://dash.harvard.edu/bitstream/handle/1/10981611/Meteorologically%20defined%20limits%20to%20reduction%20in%20the%20variability%20of%20outputs%20from%20a%20coupled%20wind%20farm%20system%20in%20the%20Central%20US_1.pdf?sequence=6 |title=Meteorologically defined limits to reduction in the variability of outputs from a coupled wind farm system in the Central US|journal=Renewable Energy|volume=62 |pages=331–340 |year=2014 |doi=10.1016/j.renene.2013.07.022|s2cid=3527948 }}</ref>  Sea breezes are much more constant than land breezes.<ref name="IEA2005WindVar"/> Seasonal variability may reduce output by 50%.<ref>[https://web.archive.org/web/20170118042610/https://pdfs.semanticscholar.org/1709/4a682549e8e853be7b393e916f4cab91487a.pdf] Graham Sinden (1 December 2005). "Characteristics of the UK wind resource" pg4</ref>
* '''[[Reliability engineering|Reliability]]:''' A wind farm has high technical reliability when the wind blows. That is, the output at any given time will only vary gradually due to falling wind speeds or storms (the latter necessitating shut downs). A typical wind farm is unlikely to have to shut down in less than half an hour at the extreme, whereas an equivalent-sized power station can fail totally instantaneously and without warning. The total shutdown of wind turbines is predictable via weather forecasting. The average availability of a wind turbine is 98%, and when a turbine fails or is shut down for maintenance it only affects a small percentage of the output of a large wind farm.<ref>[http://www.iset.uni-kassel.de/abt/FB-I/publication/2006-02-09Reliability.pdf Reliability of Wind Turbines]{{dead link|date=November 2017 |bot=InternetArchiveBot |fix-attempted=yes }} {{Dead link|date=March 2022}}</ref>
* '''Predictability:''' Although wind is variable, it is also predictable in the short term. There is an 80% chance that wind output will change less than 10% in an hour and a 40% chance that it will change 10% or more in 5&nbsp;hours.<ref>{{cite web|url= http://www.nrel.gov/wind/systemsintegration/system_integration_basics.html |title=Wind Systems Integration Basics|archive-url= https://web.archive.org/web/20120607000124/http://www.nrel.gov/wind/systemsintegration/system_integration_basics.html |archive-date=7 June 2012}}</ref>

Because wind power is generated by large numbers of small generators, individual failures do not have large impacts on power grids. This feature of wind has been referred to as resiliency.<ref name='PEMBINAbrochure'>{{cite web |url=http://pubs.pembina.org/reports/renew-doable-brochure.pdf |title=renewable is doable A Smarter Energy Plan for Ontario (brochure version) |access-date=2008-10-17 |date=August 2007 |work=PEMBINA Institute |archive-date=2008-08-27 |archive-url=https://web.archive.org/web/20080827184239/http://pubs.pembina.org/reports/renew-doable-brochure.pdf |url-status=dead }}</ref>

===Solar power===
[[File:ATTParksolarpaneloutput.png|thumb|Daily solar output at [[AT&T Park]] in San Francisco]]
[[File:ATTParkannualoutput.png|250px|thumb|right|Seasonal variation of the output of the solar panels at AT&T park in San Francisco]]Intermittency inherently affects [[solar energy]], as the production of renewable electricity from solar sources depends on the amount of sunlight at a given place and time.  Solar output varies throughout the day and through the seasons, and is affected by dust, fog, cloud cover, frost or snow.  Many of the seasonal factors are fairly predictable, and some solar thermal systems make use of heat storage to produce grid power for a full day.<ref>[http://www.nationalgeographic.com.es/2011/10/25/gemasolar_energia_non_stop.html Gemasolar, energía non stop] {{Webarchive|url=https://web.archive.org/web/20130206152532/http://www.nationalgeographic.com.es/2011/10/25/gemasolar_energia_non_stop.html |date=2013-02-06 }} Spanish 26 October 2011</ref>

* '''Variability:''' In the absence of an [[energy storage system]], solar does not produce power at night, little in bad weather and varies between seasons. In many countries, solar produces most energy in seasons with low wind availability and vice versa.<ref>{{Cite journal|date=2020-01-01|title=A review on the complementarity of renewable energy sources: Concept, metrics, application and future research directions|journal=Solar Energy|language=en|volume=195|pages=703–724|doi=10.1016/j.solener.2019.11.087|issn=0038-092X|doi-access=free|last1=Jurasz|first1=J.|last2=Canales|first2=F.A.|last3=Kies|first3=A.|last4=Guezgouz|first4=M.|last5=Beluco|first5=A.|arxiv=1904.01667|bibcode=2020SoEn..195..703J}}</ref>
* '''Capacity factor''' Standard photovoltaic solar has an annual average capacity factor of 10-20%,<ref>{{Cite web|title=Average annual capacity factors by technology, 2018 – Charts – Data & Statistics|url=https://www.iea.org/data-and-statistics/charts/average-annual-capacity-factors-by-technology-2018|access-date=2021-02-10|website=IEA|language=en-GB}}</ref> but panels that move and track the sun have a capacity factor up to 30%.<ref>{{Cite report|url=https://www.worldenergy.org/assets/downloads/WEC_J1143_CostofTECHNOLOGIES_021013_WEB_Final.pdf|title=World Energy Perspective|date=2013|publisher=World Energy Council|page=21}}</ref> Thermal [[solar parabolic trough]] with storage 56%.<ref name='NRELSolar'>{{cite web|url=http://www.nrel.gov/docs/fy04osti/35060.pdf |title=Executive Summary: Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts |access-date=2016-11-07 |date = October 2003|work=National Renewable Energy Laboratory }}</ref> Thermal [[solar power tower]] with storage 73%.<ref name="NRELSolar"/>

[[File:SolarStirlingEngine.jpg|thumb|[[Solar thermal energy#Dish designs|Dish Stirling]]|left]]The impact of intermittency of solar-generated electricity will depend on the correlation of generation with demand. For example, solar thermal power plants such as [[Nevada Solar One]] are somewhat matched to summer peak loads in areas with significant cooling demands, such as the south-western United States. Thermal energy storage systems like the small Spanish [[Gemasolar Thermosolar Plant]] can improve the match between solar supply and local consumption. The improved capacity factor using thermal storage represents a decrease in maximum capacity, and extends the total time the system generates power.<ref>[http://www.iea.org/impagr/cip/pdf/issue36SolarP.pdf Spain Pioneers Grid-Connected Solar-Tower Thermal Power] p. 3. Retrieved December 19, 2008.</ref><ref>{{cite web|url=http://www.renewableenergyworld.com/rea/magazine/story?id=52693 |title=A solar-powered economy: How solar thermal can replace coal, gas and oil |access-date=2008-10-17 |last=Mills |first=David |author2=Robert G. Morgan |date = July 2008|work=RenewableEnergyWorld.com }}</ref><ref>{{cite web |url=http://www.farm-energy.ca/IReF/index.php?page=solar-air-cooling-ataglance |title=Solar Air Cooling |access-date=2008-10-17 |date=March 2008 |work=Integration of Renewable energy on Farms |archive-url=https://web.archive.org/web/20110706180243/http://www.farm-energy.ca/IReF/index.php?page=solar-air-cooling-ataglance |archive-date=2011-07-06 |url-status=dead }}</ref>

===Run-of-the-river hydroelectricity===
In many countries new large dams are no longer being built, because of the [[environmental impact of reservoirs]]. [[Run-of-the-river hydroelectricity|Run of the river projects]] have continued to be built.<ref>{{Cite web | url=http://keeyask.com/the-project/project-description/ | title=Project Description – Keeyask Hydropower Limited Partnership| date=10 February 2011}}</ref> The absence of a reservoir results in both seasonal and annual variations in electricity generated.

===Tidal power===
[[File:Tide type.svg|thumb|Types of tide]]
[[Tidal power]] is the most predictable of all the variable renewable energy sources. The tides reverse twice a day, but they are never intermittent, on the contrary they are completely reliable.<ref>{{Cite web |title=Energy Resources: Tidal power |url=http://www.darvill.clara.net/altenerg/tidal.htm |access-date=2022-03-31 |website=www.darvill.clara.net}}</ref>

===Wave power===
Waves are primarily created by wind, so the power available from waves tends to follow that available from wind, but due to the mass of the water is less variable than wind power. Wind power is proportional to the cube of the wind speed, while wave power is proportional to the square of the wave height.<ref>{{Cite web |url=http://www.mos.org/oceans/motion/wind.html |title=Wind and Waves |access-date=2012-06-04 |archive-date=2012-09-13 |archive-url=https://web.archive.org/web/20120913081744/http://www.mos.org/oceans/motion/wind.html |url-status=dead }}</ref><ref>{{Cite web |url=http://energetech.com.au:8080/attachments/ComparingWindSpeedWaveHeight.pdf |title=Comparing the Variability of Wind Speed and Wave Height Data |access-date=2012-06-04 |archive-url=https://web.archive.org/web/20120617081359/http://energetech.com.au:8080/attachments/ComparingWindSpeedWaveHeight.pdf |archive-date=2012-06-17 |url-status=dead }}</ref><ref>{{Cite web |url=http://www.savenkov.org/publications/Savenkov_on_the_truncated_weibull_distribution_2009.pdf |title=Savenkov, M 2009 'On the Truncated Weibull Distribution and its Usefulness in Evaluating the Theoretical Capacity Factor of Potential Wind (or Wave) Energy Sites', University Journal of Engineering and Technology, vol. 1, no. 1, pp. 21-25 |access-date=2014-11-30 |archive-url=https://web.archive.org/web/20150222120957/http://www.savenkov.org/publications/Savenkov_on_the_truncated_weibull_distribution_2009.pdf |archive-date=2015-02-22 |url-status=dead }}</ref>

==Solutions for their integration==
{{See also|Electric power transmission}}

The displaced dispatchable generation could be coal, natural gas, biomass, nuclear, geothermal or storage hydro.{{Cn|date=November 2022}}{{Clarify|reason=Why displace stored hydro?|date=November 2022}} Rather than starting and stopping nuclear or geothermal it is cheaper to use them as constant [[base load]] power. Any power generated in excess of demand can displace heating fuels, be converted to storage or sold to another grid. Biofuels and conventional hydro can be saved for later when intermittents are not generating power. Some forecast that “near-firm” renewables (batteries with solar and/or wind) power will be cheaper than existing nuclear by the late 2020s: therefore they say base load power will not be needed.<ref>{{Cite web |last=Harvey |first=George |date=2022-06-28 |title=We Don't Need Base Load Power |url=https://cleantechnica.com/2022/06/28/we-dont-need-base-load-power/ |access-date=2022-11-21 |website=CleanTechnica |language=en-US}}</ref> [[Fossil fuel phase-out|Alternatives to burning]] coal and natural gas which produce fewer [[greenhouse gas]]es may eventually make fossil fuels a [[stranded asset]] that is left in the ground. Highly integrated grids favor flexibility and performance over cost, resulting in more plants that operate for fewer hours and lower [[capacity factor]]s.<ref>Michael G. Richard: [https://www.treehugger.com/renewable-energy/death-capacity-factor-how-wind-solar-ultimately-win-game.html Death by 'capacity factor': Is this how wind and solar ultimately win the game?], 2015-10-06</ref>

All sources of electrical power have some degree of variability, as do demand patterns which routinely drive large swings in the amount of electricity that suppliers feed into the grid. Wherever possible, grid operations procedure are designed to match supply with demand at high levels of reliability, and the tools to influence supply and demand are well-developed. The introduction of large amounts of highly variable power generation may require changes to existing procedures and additional investments.

The capacity of a reliable [[renewable energy|renewable power]] supply, can be fulfilled by the use of [[Redundancy (engineering)|backup or extra infrastructure and technology]], using mixed renewables to produce electricity [[Fault-tolerant design|above the intermittent average]], which may be used to meet regular and unanticipated supply demands.<ref>{{cite web|url=http://www.renewableenergyworld.com/rea/news/print/article/2011/02/solar-solar-and-energy-storage-a-perfect-match|title=Solar and Energy Storage: A Perfect Match - Energy Storage to the Test|publisher=RenewableEnergyWorld.com|access-date=2011-03-08}}</ref> Additionally, the storage of energy to fill the shortfall intermittency or for emergencies can be part of a reliable power supply.

In practice, as the power output from wind varies, partially loaded conventional plants, which are already present to provide response and reserve, adjust their output to compensate. While low penetrations of intermittent power may use existing levels of response and spinning reserve, the larger overall variations at higher penetrations levels will require additional reserves or other means of compensation.

===Operational reserve===
{{See also|National Grid Reserve Service}}

All managed grids already have existing operational and "spinning" reserve to compensate for existing uncertainties in the power grid. The addition of intermittent resources such as wind does not require 100% "back-up" because operating reserves and balancing requirements are calculated on a system-wide basis, and not dedicated to a specific generating plant.

Some gas, or hydro power  plants are partially loaded and then controlled to change as demand changes or to replace rapidly lost generation. The ability to change as demand changes is termed "response".  The ability to quickly replace lost generation, typically within timescales of 30&nbsp;seconds to 30&nbsp;minutes, is termed "spinning reserve".

Generally thermal plants running as [[Peaking power plant|peaking plants]] will be less efficient than if they were running as [[Base load power plant|base load]]. Hydroelectric facilities with storage capacity (such as the traditional dam configuration) may be operated as base load or peaking plants.

Grids can contract for [[Battery storage power station|grid battery plants]], which provide immediately available power for an hour or so, which gives time for other generators to be started up in the event of a failure, and greatly reduces the amount of spinning reserve required.<ref>{{Cite web|url=https://www.solarpowerportal.co.uk/blogs/uk_battery_storage_capacity_could_reach_70_growth_in_2019_as_business_model|title=UK battery storage capacity could reach 70% growth in 2019 as business models evolve|website=Solar Power Portal|date=13 June 2019 }}</ref><ref>{{Cite web|url=https://www.solarpowerportal.co.uk/blogs/uk_battery_storage_market_reaches_1gw_landmark_as_new_applications_continue|title=UK battery storage market reaches 1GW landmark as new applications continues to grow|website=Solar Power Portal|date=2 April 2020 }}</ref>

=== Demand response ===
[[Demand response]] is a change in consumption of energy to better align with supply. It can take the form of switching off loads, or absorb additional energy to correct supply/demand imbalances. Incentives have been widely created in the American, British and French systems for the use of these systems, such as favorable rates or capital cost assistance, encouraging consumers with large loads to take them offline whenever there is a shortage of capacity, or conversely to increase load when there is a surplus.

Certain types of load control allow the power company to turn loads off remotely if insufficient power is available. In France large users such as CERN cut power usage as required by the System Operator - EDF under the encouragement of the EJP tariff.<ref>{{cite web|title=How CERN is encouraged to not do atom or quark smashing, during periods of high demand and low power station {{sic|nolink=y|reason=error in source|availablity}}, by means of the EJP tarrif |url=http://www.claverton-energy.com/how-cern-is-encouraged-to-not-do-atom-or-quark-smashing-during-periods-of-high-demand-and-low-power-station-availablity-by-means-of-the-ejp-tarrif.html|date=May 24, 2009 |first1= Dave|last1= Andrews}} - Extract from CERN newsletter indication when to switch of loads [http://bulletin.cern.ch/eng/articles.php?bullno=46/2005&base=art bulletin 46]</ref><ref>http://www.claverton-energy.com/download/42/ description of EJP tariff {{webarchive |url=https://web.archive.org/web/20081208120416/http://www.claverton-energy.com/download/42/ |date=December 8, 2008 }}</ref>

[[Energy demand management]] refers to incentives to adjust use of electricity, such as higher rates during peak hours. Real-time variable electricity pricing can encourage users to adjust usage to take advantage of periods when power is cheaply available and avoid periods when it is more scarce and expensive.<ref name="CaliPolicy">{{cite web| url=http://www.energy.ca.gov/2005_energypolicy/index.html| title=2005 Integrated Energy Policy Report| date=November 21, 2005| publisher=California Energy Commission| access-date=2006-04-21| archive-date=2019-06-01| archive-url=https://web.archive.org/web/20190601145045/https://www.energy.ca.gov/2005_energypolicy/index.html| url-status=dead}}</ref> Some loads such as desalination plants, electric boilers and industrial refrigeration units, are able to store their output (water and heat). Several papers also concluded that [[Bitcoin]] mining loads would reduce [[Curtailment (electricity)|curtailment]], [[Hedge (finance)|hedge]] [[Electricity market#Risk management|electricity price risk]], stabilize the grid, increase the [[Energy return on investment|profitability of renewable energy]] power stations and therefore accelerate [[Renewable energy transition|transition to sustainable energy]].<ref name="Fridgen2021">{{Cite journal|last1=Fridgen|first1=Gilbert|last2=Körner|first2=Marc-Fabian|last3=Walters|first3=Steffen|last4=Weibelzahl|first4=Martin|date=2021-03-09|title=Not All Doom and Gloom: How Energy-Intensive and Temporally Flexible Data Center Applications May Actually Promote Renewable Energy Sources|journal=Business & Information Systems Engineering|language=en|volume=63|issue=3|pages=243–256|doi=10.1007/s12599-021-00686-z|s2cid=233664180|issn=2363-7005|quote=To gain applicable knowledge, this paper evaluates the developed model by means of two use-cases with real-world data, namely AWS computing instances for training Machine Learning algorithms and Bitcoin mining as relevant DC applications. The results illustrate that for both cases the NPV of the IES compared to a stand-alone RES-plant increases, which may lead to a promotion of RES-plants.|doi-access=free}}</ref><ref name="Rhodes2021">{{Cite web|last=Rhodes|first=Joshua|title=Is Bitcoin Inherently Bad For The Environment?|url=https://www.forbes.com/sites/joshuarhodes/2021/10/08/is-bitcoin-inherently-bad-for-the-environment/|access-date=2022-01-16|website=Forbes|language=en|quote=Mining and transacting cryptocurrencies, such as bitcoin, do present energy and emissions challenges, but new research shows that there are possible pathways to mitigate some of these issues if cryptocurrency miners are willing to operate in a way to compliment the deployment of more low-carbon energy.}}</ref><ref>{{Cite news|title=Green Bitcoin Does Not Have to Be an Oxymoron|url=https://news.bloomberglaw.com/environment-and-energy/green-bitcoin-does-not-have-to-be-an-oxymoron|access-date=2022-01-16|website=news.bloomberglaw.com|language=en|quote=One way to invest in Bitcoin that has a positive effect on renewable energy is to encourage mining operations near wind or solar sites. This provides a customer for power that might otherwise need to be transmitted or stored, saving money as well as carbon.}}</ref><ref>{{Cite journal|last=Moffit|first=Tim|date=2021-06-01|title=Beyond Boom and Bust: An emerging clean energy economy in Wyoming|url=https://escholarship.org/uc/item/0zp872rt|language=en|quote=Currently, projects are under development, but the issue of overgenerated wind continues to exist. By harnessing the overgenerated wind for Bitcoin mining, Wyoming has the opportunity to redistribute the global hashrate, incentivize Bitcoin miners to move their operations to Wyoming, and stimulate job growth as a result.}}</ref><ref>{{Cite journal|last=Rennie|first=Ellie|date=2021-11-07|title=Climate change and the legitimacy of Bitcoin|url=https://papers.ssrn.com/abstract=3961105|language=en|location=Rochester, NY|doi=10.2139/ssrn.3961105 |ssrn=3961105|s2cid=244155800 |quote=In responding to these pressures and events, some miners are providing services and innovations that may help the viability of clean energy infrastructures for energy providers and beyond, including the data and computing industry. The paper finds that if Bitcoin loses legitimacy as a store of value, then it may result in lost opportunities to accelerate sustainable energy infrastructures and markets.}}</ref><ref>{{Cite journal|last1=Eid|first1=Bilal|last2=Islam|first2=Md Rabiul|last3=Shah|first3=Rakibuzzaman|last4=Nahid|first4=Abdullah-Al|last5=Kouzani|first5=Abbas Z.|last6=Mahmud|first6=M. A. Parvez|date=2021-11-01|title=Enhanced Profitability of Photovoltaic Plants By Utilizing Cryptocurrency-Based Mining Load|url=https://ieeexplore.ieee.org/document/9483629|journal=IEEE Transactions on Applied Superconductivity|volume=31|issue=8|pages=1–5|doi=10.1109/TASC.2021.3096503|bibcode=2021ITAS...3196503E |hdl=20.500.11782/2513|s2cid=237245955|issn=1558-2515|quote=The grid connected photovoltaic (PV) power plants (PVPPs) are booming nowadays. The main problem facing the PV power plants deployment is the intermittency which leads to instability of the grid. [...] This paper investigating the usage of a customized load - cryptocurrency mining rig - to create an added value for the owner of the plant and increase the ROI of the project. [...] The developed strategy is able to keep the profitability as high as possible during the fluctuation of the mining network.|hdl-access=free}}</ref><ref>{{Cite journal|last1=Bastian-Pinto|first1=Carlos L.|last2=Araujo|first2=Felipe V. de S.|last3=Brandão|first3=Luiz E.|last4=Gomes|first4=Leonardo L.|date=2021-03-01|title=Hedging renewable energy investments with Bitcoin mining|url=https://www.sciencedirect.com/science/article/pii/S1364032120308054|journal=Renewable and Sustainable Energy Reviews|language=en|volume=138|pages=110520|doi=10.1016/j.rser.2020.110520|s2cid=228861639|issn=1364-0321|quote=Windfarms can hedge electricity price risk by investing in Bitcoin mining. [...] These findings, which can also be applied to other renewable energy sources, may be of interest to both the energy generator as well as the system regulator as it creates an incentive for early investment in sustainable and renewable energy sources.}}</ref><ref>{{Cite journal|last1=Shan|first1=Rui|last2=Sun|first2=Yaojin|date=2019-08-07|title=Bitcoin Mining to Reduce the Renewable Curtailment: A Case Study of Caiso|url=https://papers.ssrn.com/abstract=3436872|language=en|location=Rochester, NY|doi=10.2139/ssrn.3436872 |ssrn=3436872|s2cid=219382864 |quote=The enormous energy demand from Bitcoin mining is a considerable burden to achieve the climate agenda and the energy cost is the major operation cost. On the other side, with high penetration of renewable resources, the grid makes curtailment for reliability reasons, which reduces both economic and environment benefits from renewable energy. Deploying the Bitcoin mining machines at renewable power plants can mitigate both problems.}}</ref> But others argue that Bitcoin mining can never be sustainable.<ref>{{Cite web |title=Can renewable energy make crypto mining greener? {{!}} Sifted |url=https://sifted.eu/articles/renewable-energy-crypto-mining/ |access-date=2022-06-27 |website=sifted.eu|date=16 June 2022 }}</ref>

Instantaneous demand reduction. Most large systems also have a category of loads which instantly disconnect when there is a generation shortage, under some mutually beneficial contract. This can give instant load reductions (or increases).

===Storage===
{{Main|Grid energy storage}}

[[File:Abengoa Solar (7336087392).jpg|thumbnail| Construction of the Salt Tanks which provide efficient [[thermal energy storage]]<ref>Wright, matthew; Hearps, Patrick; et al. [http://media.bze.org.au/ZCA2020_Stationary_Energy_Report_v1.pdf Australian Sustainable Energy: Zero Carbon Australia Stationary Energy Plan], Energy Research Institute, [[University of Melbourne]], October 2010, p. 33. Retrieved from BeyondZeroEmissions.org website.</ref> so that output can be provided after the sun goes down, and output can be scheduled to meet demand requirements.<ref>[http://www.renewableenergyfocus.com/view/3272/innovation-in-concentrating-thermal-solar-power-csp/ Innovation in Concentrating Thermal Solar Power (CSP)], RenewableEnergyFocus.com website.</ref> The 280&nbsp;MW [[Solana Generating Station]] is designed to provide six hours of energy storage. This allows the plant to generate about 38 percent of its rated capacity over the course of a year.<ref>[http://blogs.phoenixnewtimes.com/valleyfever/2013/10/solana_10_facts_you_didnt_know.php Solana: 10 Facts You Didn't Know About the Concentrated Solar Power Plant Near Gila Bend]</ref>]]
[[File:Battery-cost-learning-curve.png|thumb| [[Learning curve]] of lithium-ion batteries: the price of batteries declined by 97% in three decades.]]
At times of low load where non-dispatchable output from wind and solar may be high, grid stability requires lowering the output of various dispatchable generating sources or even increasing controllable loads, possibly by using energy storage to time-shift output to times of higher demand. Such mechanisms can include:

[[Pumped-storage hydroelectricity|Pumped storage hydropower]] is the most prevalent existing technology used, and can substantially improve the economics of wind power. The availability of hydropower sites suitable for storage will vary from grid to grid.  Typical round trip efficiency is 80%.<ref name="IEA2005WindVar" /><ref name="REPAWindEconomics">{{cite journal|url=http://ideas.repec.org/p/rep/wpaper/2006-02.html |title=The Economics of Wind Power with Energy Storage |access-date=2008-10-20 |last=Benitez |first=Pablo C. |author2=Lilianna E. Dragulescu |author3=G. Cornelis Van Kooten  |date = February 2006|journal=Resource Economics and Policy Analysis (REPA) Research Group |publisher=Department of Economics, University of Victoria }}</ref>

Traditional lithium-ion is the most common type used for grid-scale battery storage {{As of|2020||lc=y}}.<ref>{{Cite web|title=Grid-Scale Battery Storage Frequently Asked Questions|url=https://www.nrel.gov/docs/fy19osti/74426.pdf}}</ref> Rechargeable [[Flow battery|flow batteries]] can serve as a large capacity, rapid-response storage medium.<ref name="MEFlowBattery" /> [[Hydrogen]] can be created through [[electrolysis]] and stored for later use.<ref>{{Cite news|date=2021-02-12|title=The global race to produce hydrogen offshore|language=en-GB|work=BBC News|url=https://www.bbc.com/news/business-55763356|access-date=2021-02-12}}</ref>

[[Flywheel energy storage|Flywheel energy storage systems]] have some advantages over chemical batteries. Along with substantial durability which allows them to be cycled frequently without noticeable life reduction, they also have very fast response and ramp rates. They can go from full discharge to full charge within a few seconds.<ref>{{Cite web|title=Mechanical energy storage|url=https://energystorage.org/why-energy-storage/technologies/mechanical-energy-storage/|access-date=2022-02-19|archive-date=2022-02-19|archive-url=https://web.archive.org/web/20220219164627/https://energystorage.org/why-energy-storage/technologies/mechanical-energy-storage/|url-status=dead}}</ref> They can be manufactured using non-toxic and environmentally friendly materials, easily recyclable once the service life is over.<ref>{{Cite web|title=Kinetic energy storage|url=https://en.kest.energy/tech|access-date=2022-02-19|archive-date=2022-02-19|archive-url=https://web.archive.org/web/20220219163121/https://en.kest.energy/tech|url-status=dead}}</ref>

[[Thermal energy storage]] stores heat. Stored heat can be used directly for heating needs or converted into electricity. In the context of a CHP plant a heat storage can serve as a functional electricity storage at comparably low costs. [[Ice storage air conditioning]] Ice can be stored inter seasonally and can be used as a source of air-conditioning during periods of high demand. Present systems only need to store ice for a few hours but are well developed.

Storage of electrical energy results in some lost energy because storage and retrieval are not perfectly efficient. Storage also requires capital investment and space for storage facilities.

===Geographic diversity and complementing technologies===
[[File:Ontariowindfarmshourlyoutputover5days.gif|thumb|Five days of hourly output of five wind farms in Ontario|330x330px]]
The variability of production from a single wind turbine can be high. Combining any additional number of turbines (for example, in a wind farm) results in lower statistical variation, as long as the [[correlation]] between the output of each turbine is imperfect, and the correlations are always imperfect due to the distance between each turbine. Similarly, geographically distant wind turbines or wind farms have lower correlations, reducing overall variability. Since wind power is dependent on weather systems, there is a limit to the benefit of this geographic diversity for any power system.<ref name="sciencedirect.com">{{cite journal |author1=Junling Huang |author2=Michael B. McElroy |title=Meteorologically defined limits to reduction in the variability of outputs from a coupled wind farm system in the Central US|journal=Renewable Energy|volume=62 |pages=331–340 |year=2014 |doi=10.1016/j.renene.2013.07.022|s2cid=3527948 |url=https://dash.harvard.edu/bitstream/1/10981611/6/Meteorologically%20defined%20limits%20to%20reduction%20in%20the%20variability%20of%20outputs%20from%20a%20coupled%20wind%20farm%20system%20in%20the%20Central%20US_1.pdf }}</ref>

Multiple wind farms spread over a wide geographic area and gridded together produce power more constantly and with less variability than smaller installations. [[Wind power forecasting|Wind output can be predicted]] with some degree of confidence using weather forecasts, especially from large numbers of turbines/farms. The ability to predict wind output is expected to increase over time as data is collected, especially from newer facilities.<ref name="sciencedirect.com"/>

Electricity produced from solar energy tends to counterbalance the fluctuating supplies generated from wind.  Normally it is windiest at night and during cloudy or stormy weather, and there is more sunshine on clear days with less wind.<ref name="AtlanticFragility">{{cite web|url=http://www.rmi.org/images/other/EnergySecurity/S83-08_FragileDomEnergy.pdf |title=The Fragility of Domestic Energy |access-date=2008-10-20 |last=Lovins |first=Amory |author2=L. Hunter Lovins |date = November 1983|work=The Atlantic |archive-url = https://web.archive.org/web/20080625205941/http://www.rmi.org/images/other/EnergySecurity/S83-08_FragileDomEnergy.pdf |archive-date = June 25, 2008}}</ref> Besides, wind energy has often a peak in the winter season, whereas solar energy has a peak in the summer season; the combination of wind and solar reduces the need for dispatchable backup power.<ref>{{Cite journal |title=Pieces of a puzzle: solar-wind power synergies on seasonal and diurnal timescales tend to be excellent worldwide |year=2022 |doi=10.1088/2515-7620/ac71fb |last1=Nyenah |first1=Emmanuel |last2=Sterl |first2=Sebastian |last3=Thiery |first3=Wim |journal=Environmental Research Communications |volume=4 |issue=5 |page=055011 |bibcode=2022ERCom...4e5011N |s2cid=249227821 |doi-access=free }}</ref>

* In some locations, electricity demand may have a high correlation with wind output, {{Citation needed|date=September 2020}}particularly in locations where cold temperatures drive electric consumption (as cold air is denser and carries more energy).
* The allowable penetration may be increased with further investment in standby generation. For instance some days could produce 80% intermittent wind and on the many windless days substitute 80% dispatchable power like natural gas, biomass and Hydro. 
* Areas with existing high levels of [[hydroelectric]] generation may ramp up or down to incorporate substantial amounts of wind. [[Norway]], [[Brazil]], and [[Manitoba]] all have high levels of hydroelectric generation, Quebec produces over 90% of its electricity from hydropower, and [[Hydro-Québec]] is the largest hydropower producer in the world. The U.S. Pacific Northwest has been identified as another region where wind energy is complemented well by existing hydropower.<ref>{{Cite news |last=Harden |first=Blaine |date=2007-03-21 |title=Air, Water Powerful Partners in Northwest |newspaper=[[The Washington Post]] |language=en-US |url=http://www.washingtonpost.com/wp-dyn/content/article/2007/03/20/AR2007032001634.html |access-date=2023-08-08 |issn=0190-8286}}</ref> Storage capacity in hydropower facilities will be limited by size of reservoir, and environmental and other considerations.

===Connecting grid internationally===
{{See also|HVDC|super grid}}It is often feasible to export energy to neighboring grids at times of surplus, and import energy when needed. This practice is common in Europe<ref>{{Cite web |last=JUNE |first=WE |date=2022-01-27 |title=The European Super Grid : A solution to the EU's energy problems • Eyes on Europe |url=https://www.eyes-on-europe.eu/the-european-super-grid-a-solution-to-the-eus-energy-problems/ |access-date=2022-03-31 |website=Eyes on Europe |language=fr-FR}}</ref> and between the US and Canada.<ref>{{Cite web |date=2021-06-30 |title=US, Canada expand clean energy cooperation |url=https://cleanenergynews.ihsmarkit.com/research-analysis/us-canada-expand-clean-energy-cooperation.html |access-date=2022-03-31 |website=IHS Markit}}</ref> Integration with other grids can lower the effective concentration of variable power: for instance, Denmark's high penetration of VRE, in the context of the German/Dutch/[[Governance of hydropower in Scandinavia|Scandinavian grids]] with which it has interconnections, is considerably lower as a proportion of the total system. Hydroelectricity that compensates for variability can be used across countries.<ref>{{Cite web |date=2021-04-19 |title=How Norway became Europe's biggest power exporter |url=https://www.power-technology.com/features/how-norway-became-europes-biggest-power-exporter/ |access-date=2022-03-31 |website=Power Technology |language=en-US}}</ref>

The capacity of power transmission infrastructure may have to be substantially upgraded to support export/import plans. Some energy is lost in transmission. The economic value of exporting variable power depends in part on the ability of the exporting grid to provide the importing grid with useful power at useful times for an attractive price.

=== Sector coupling ===
Demand and generation can be better matched when sectors such as mobility, heat and gas are coupled with the power system. The electric vehicle market is for instance expected to become the largest source of storage capacity. This may be a more expensive option appropriate for high penetration of variable renewables, compared to other sources of flexibility.<ref>{{Cite book|author=IRENA|url=https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2018/Nov/IRENA_Power_system_flexibility_1_2018.pdf?la=en&hash=72EC26336F127C7D51DF798CE19F477557CE9A82|title=Power System Flexibility for the Energy Transition, Part 1: Overview for policy makers|publisher=International Renewable Energy Agency|year=2018|isbn=978-92-9260-089-1|location=Abu Dhabi|pages=25, 42}}</ref> The International Energy Agency says that sector coupling is needed to compensate for the mismatch between seasonal demand and supply.<ref>{{Cite web|title=System integration of renewables – Topics|url=https://www.iea.org/topics/system-integration-of-renewables|access-date=2021-05-21|website=IEA|language=en-GB}}</ref>

Electric vehicles can be charged during periods of low demand and high production, and in some places send power back from the [[vehicle-to-grid]].<ref>{{Cite web|date=2020-11-09|title=Is Vehicle-to-Grid Technology the Key to Accelerating the Clean Energy Revolution?|url=https://www.powermag.com/is-vehicle-to-grid-technology-the-key-to-accelerating-the-clean-energy-revolution/|access-date=2021-02-12|website=POWER Magazine|language=en-US}}</ref><ref>{{Cite web|date=2021-01-18|title=UK city of Nottingham uses vehicle-to-grid (V2G) and IoT to optimise EV fleet charging|url=https://www.traffictechnologytoday.com/news/electric-vehicles-ev-infrastructure/uk-city-of-nottingham-uses-vehicle-to-grid-v2g-and-iot-to-optimise-ev-fleet-charging.html|access-date=2021-02-12|website=Traffic Technology Today|language=en-GB}}</ref>

==Penetration==
Penetration refers to the proportion of a [[primary energy]] (PE) source in an electric power system, expressed as a percentage.<ref name="ieawind.org"/> There are several methods of calculation yielding different penetrations. The penetration can be calculated either as:<ref name="ukerc2006tpa"/>

# the nominal capacity (installed power) of a PE source divided by the peak load within an electric power system; or
# the nominal capacity (installed power) of a PE source divided by the total capacity of the electric power system; or
# the electrical energy generated by a PE source in a given period, divided by the demand of the electric power system in this period.

The level of penetration of intermittent variable sources is significant for the following reasons:

* Power grids with significant amounts of dispatchable pumped storage, [[hydropower]] with [[reservoir]] or [[pondage]] or other peaking power plants such as natural gas-fired power plants are capable of accommodating fluctuations from intermittent power more easily.<ref>http://repa.econ.uvic.ca/publications/Working%20Paper%202006-02.pdf{{dead link|date=November 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>
* Relatively small electric power systems without strong interconnection (such as remote islands) may retain some existing diesel generators but consuming less fuel,<ref>{{Cite web|last1=Shumais|first1=Mohamed|last2=Mohamed|first2=Ibrahim|title=DIMENSIONS OF ENERGY INSECURITY ON SMALL ISLANDS: THE CASE OF THE MALDIVES|url=https://www.adb.org/sites/default/files/publication/543261/adbi-wp1049.pdf}}</ref> for flexibility<ref>{{Cite web|title=Transforming small-island power systems|url=https://www.irena.org/publications/2019/Jan/Transforming-small-island-power-systems|access-date=2020-09-08|website=/publications/2019/Jan/Transforming-small-island-power-systems|date=27 January 2019 |language=en}}</ref> until cleaner energy sources or storage such as pumped hydro or batteries become cost-effective.<ref>{{Cite web|title=Shining a light on a smart island|url=https://www.man-es.com/discover/shining-a-light-on-a-smart-island|access-date=2020-09-08|website=MAN Energy Solutions|language=en}}</ref>

In the early 2020s wind and solar produce 10% of the world's electricity,<ref>{{Cite web|title=Wind and solar produce record 10% of world's electricity, but faster change needed, scientists warn|url=https://www.independent.co.uk/environment/renewable-energy-wind-solar-coal-how-much-electricity-climate-change-a9669651.html |archive-url=https://ghostarchive.org/archive/20220811/https://www.independent.co.uk/environment/renewable-energy-wind-solar-coal-how-much-electricity-climate-change-a9669651.html |archive-date=2022-08-11 |url-access=subscription |url-status=live|access-date=2020-09-08|website=www.independent.co.uk|date=13 August 2020|language=en}}</ref> but supply in the 40-55% penetration range has already been implemented in several systems,<ref name=":0" /> with over 65% planned for the UK by 2030.<ref>{{Cite web|last=Ltd|first=Renews|date=2020-08-11|title=Britain urged to hit 65% renewables by 2030|url=https://renews.biz/62362/britain-urged-to-hit-65-renewables-by-2030/|access-date=2020-09-08|website=reNEWS - Renewable Energy News|language=en-US}}</ref><ref>{{Cite web |title=UK Looks To Triple Solar And More Than Quadruple Offshore Wind Power |url=https://oilprice.com/Latest-Energy-News/World-News/UK-Looks-To-Triple-Solar-And-More-Than-Quadruple-Offshore-Wind-Power.html |access-date=2022-03-31 |website=OilPrice.com |language=en}}</ref>

There is no generally accepted maximum level of penetration, as each system's capacity to compensate for intermittency differs, and the systems themselves will change over time. Discussion of acceptable or unacceptable penetration figures should be treated and used with caution, as the relevance or significance will be highly dependent on local factors, grid structure and management, and existing generation capacity.

For most systems worldwide, existing penetration levels are significantly lower than practical or theoretical maximums.<ref name="ukerc2006tpa">{{cite book
 |url=http://www.ukerc.ac.uk/Downloads/PDF/06/0604Intermittency/0604IntermittencyReport.pdf 
 |title=The Costs and Impacts of Intermittency 
 |publisher=UK Energy Research Council 
 |isbn=978-1-903144-04-6 
 |last1=Gross 
 |first1=Robert 
 |last2=Heptonstall 
 |first2=Philip 
 |last3=Anderson 
 |first3=Dennis 
 |last4=Green 
 |first4=Tim 
 |last5=Leach 
 |first5=Matthew 
 |last6=Skea 
 |first6=Jim 
 |date=March 2006 
 |access-date=2010-07-22 
 |url-status=dead 
 |archive-url=https://web.archive.org/web/20090318231423/http://www.ukerc.ac.uk/Downloads/PDF/06/0604Intermittency/0604IntermittencyReport.pdf 
 |archive-date=2009-03-18 
}}</ref>

===Maximum penetration limits===
Maximum penetration of combined wind and solar is estimated at around 70% to 90% without regional aggregation, demand management or storage; and up to 94% with 12 hours of storage.<ref>{{Cite journal|last1=Tong|first1=Dan|last2=Farnham|first2=David J.|last3=Duan|first3=Lei|last4=Zhang|first4=Qiang|last5=Lewis|first5=Nathan S.|last6=Caldeira|first6=Ken|last7=Davis|first7=Steven J.|date=2021-10-22|title=Geophysical constraints on the reliability of solar and wind power worldwide|journal=Nature Communications|language=en|volume=12|issue=1|pages=6146|doi=10.1038/s41467-021-26355-z|pmid=34686663|pmc=8536784|bibcode=2021NatCo..12.6146T |issn=2041-1723}}</ref> Economic efficiency and cost considerations are more likely to dominate as critical factors; technical solutions may allow higher penetration levels to be considered in future, particularly if cost considerations are secondary.

===Economic impacts of variability===
{{update section|reason=no info on costs of seasonal variability e.g. of solar and seasonal demand variability|date=September 2019}}
Estimates of the cost of wind and solar energy may include estimates of the "external" costs of wind and solar variability, or be limited to the cost of production. All electrical plant has costs that are separate from the cost of production, including, for example, the cost of any necessary transmission capacity or reserve capacity in case of loss of generating capacity. Many types of generation, particularly fossil fuel derived, will also have cost [[externality|externalities]] such as pollution, [[greenhouse gas emissions|greenhouse gas emission]], and habitat destruction which are generally not directly accounted for. The magnitude of the economic impacts is debated and will vary by location, but is expected to rise with higher penetration levels. At low penetration levels, costs such as [[operating reserve]] and balancing costs are believed to be insignificant.

Intermittency may introduce additional costs that are distinct from or of a different magnitude than for traditional generation types. These may include:

* Transmission capacity: transmission capacity may be more expensive than for nuclear and coal generating capacity due to lower load factors. Transmission capacity will generally be sized to projected peak output, but average capacity for wind will be significantly lower, raising cost per unit of energy actually transmitted. However transmission costs are a low fraction of total energy costs.<ref>http://www.claverton-energy.com/what-is-the-cost-per-kwh-of-bulk-transmission-national-grid-in-the-uk-note-this-excludes-distribution-costs.html Electric power transmission costs per kWh transmission / National Grid in the UK (note this excludes distribution costs)</ref>
* Additional operating reserve: if additional wind and solar does not correspond to demand patterns, additional operating reserve may be required compared to other generating types, however this does not result in higher capital costs for additional plants since this is merely existing plants running at low output - spinning reserve. Contrary to statements that all wind must be backed by an equal amount of "back-up capacity", intermittent generators contribute to base capacity "as long as there is some probability of output during peak periods". Back-up capacity is not attributed to individual generators, as back-up or operating reserve "only have meaning at the system level".<ref>http://www.ukerc.ac.uk/component/option,com_docman/task,doc_download/gid,550/ {{webarchive|url=https://web.archive.org/web/20070706221755/http://www.ukerc.ac.uk/component/option%2Ccom_docman/task%2Cdoc_download/gid%2C550/ |date=2007-07-06 }} The Costs and Impacts of Intermittency, UK Energy Research Council, March 2006</ref>
* Balancing costs: to maintain grid stability, some additional costs may be incurred for balancing of load with demand. Although improvements to grid balancing can be costly, they can lead to long term savings.<ref>{{Cite web |last=Welle (www.dw.com) |first=Deutsche |title=Will war fast-track the energy transition? {{!}} DW {{!}} 04.03.2022 |url=https://www.dw.com/en/will-war-fast-track-the-energy-transition/a-61021440 |access-date=2022-03-31 |website=DW.COM |language=en-GB}}</ref><ref>{{Cite journal |last1=Morse |first1=Richard |last2=Salvatore |first2=Sarah |last3=Slusarewicz |first3=Joanna H. |last4=Cohan |first4=Daniel S. |date=2022-03-14 |title=Can wind and solar replace coal in Texas? |journal=Renewables: Wind, Water, and Solar |volume=9 |issue=1 |pages=1 |doi=10.1186/s40807-022-00069-2 |s2cid=247454828 |issn=2198-994X|doi-access=free |bibcode=2022RWWS....9....1M }}</ref><ref>{{Cite web |last=Vetter |first=David |title=5 New Reports Show Wind And Solar Power Can Cripple Putin, Secure Climate Goals |url=https://www.forbes.com/sites/davidrvetter/2022/03/30/5-new-reports-show-wind-and-solar-power-can-cripple-putin-secure-climate-goals/ |access-date=2022-03-31 |website=Forbes |language=en}}</ref><ref>{{Cite web |date=2022-02-17 |title=Accelerating Grid Integration |url=https://www.usaid.gov/energy/sure/grid-integration |access-date=2022-03-31 |website=www.usaid.gov |language=en |quote=Grid modernization reduces medium- to long-term curtailment, stagnation of large-scale renewable energy deployment, reduces long-term costs, and enables new business models, such as electric vehicles (EVs), aggregation, demand-side management, and distributed energy resources. It also promotes regional market coordination and power system integration which can unlock billions of dollars in electricity revenue through cross-border trade.}}</ref>
In many countries for many types of variable renewable energy, from time to time the government invites companies to tender sealed bids to construct a certain capacity of solar power to connect to certain electricity substations. By accepting the lowest bid the government commits to buy at that price per kWh for a fixed number of years, or up to a certain total amount of power. This provides certainty for investors against highly volatile wholesale electricity prices.<ref>{{Cite web |last1=ES |first1=Tetra Tech |last2=order |first2=Inc under USAID’s Scaling Up Renewable Energy task |date=2021-07-28 |title=Renewable Energy Auctions Toolkit {{!}} Energy {{!}} U.S. Agency for International Development |url=https://www.usaid.gov/energy/auctions |access-date=2022-05-19 |website=www.usaid.gov |language=en}}</ref><ref>{{Cite web |date=2021-11-10 |title=Feed-In Tariffs vs Reverse Auctions: Setting the Right Subsidy Rates for Solar |url=https://development.asia/insight/feed-tariffs-vs-reverse-auctions-setting-right-subsidy-rates-solar |access-date=2022-05-19 |website=Development Asia |language=en}}</ref><ref>{{Cite web |title=Government hits accelerator on low-cost renewable power |url=https://www.gov.uk/government/news/government-hits-accelerator-on-low-cost-renewable-power |access-date=2022-05-19 |website=GOV.UK |language=en}}</ref> However they may still risk exchange rate volatility if they borrowed in foreign currency.<ref>{{Cite web |title=Currency Risk Is the Hidden Solar Project Deal Breaker |url=https://www.greentechmedia.com/articles/read/currency-risk-is-the-hidden-solar-project-deal-breaker |access-date=2022-05-19 |website=www.greentechmedia.com}}</ref>

== Examples by country ==

=== Great Britain ===
The operator of the British electricity system has said that it will be capable of operating [[Low-carbon economy|zero-carbon]] by 2025, whenever there is enough renewable generation, and may be [[carbon negative]] by 2033.<ref>{{Cite news|last=Ambrose|first=Jillian|date=2020-07-27|title=UK electricity grid's carbon emissions could turn negative by 2033, says National Grid|language=en-GB|work=The Guardian|url=https://www.theguardian.com/environment/2020/jul/27/uk-electricity-grids-carbon-emissions-could-turn-negative-by-2033-says-national-grid|access-date=2020-11-03|issn=0261-3077}}</ref> The company, National Grid Electricity System Operator, states that new products and services will help reduce the overall cost of operating the system.<ref>{{Cite web|url=https://www.nationalgrideso.com/news/zero-carbon-operation-great-britains-electricity-system-2025|title=Zero carbon operation of Great Britain's electricity system by 2025 {{!}} National Grid ESO|website=www.nationalgrideso.com|access-date=2019-07-09}}</ref>

=== Germany ===
In countries with a considerable amount of renewable energy, solar energy causes price drops around noon every day. PV production follows the higher demand during these hours. The images below show two weeks in 2022 in Germany, where renewable energy has a share of over 40%.<ref>{{cite web |title=Renewable energies in figures |date=11 June 2013 |url=https://www.umweltbundesamt.de/en/topics/climate-energy/renewable-energies/renewable-energies-in-figures |access-date=25 October 2022 |publisher=Umweltbundesamt (German environmental agency)}}</ref> Prices also drop every night and weekend due to low demand. In hours without PV and wind power, electricity prices rise. This can lead to demand side adjustments. While industry is dependent on the hourly prices, most private households still pay a fixed tariff. With [[Smart meter|smart meters]], private consomers can also be motivated i.e. to load an electric car when enough renewable energy is available and prices are cheap.

Steerable flexibility in electricity production is essential to back up variable energy sources. The German example shows that pumped hydro storage, gas plants and hard coal jump in fast. Lignite varies on a daily basis. Nuclear power and biomass can theoretically adjust to a certain extent. However, in this case incentives still seem not be high enough.

{{multiple image|
| total_width       = 700
| align             = center
| direction         = vertical
| image1            = Variable_renewable_energy.svg
| caption1          = Renewable and conventional energy production in Germany over two weeks in 2022. In hours with low wind and PV production, hard coal and gas fill the gap. Nuclear and biomass show almost no flexibility. PV follows the increased consumption during daytime hours but varies seasonally.
| image2            = Electricity price variation Germany.svg
| caption2          = Electricity market in Germany over two weeks in 2022. With the integration of solar PV, prices drop around noon every day in spite of a higher demand. Prices also drop every night and weekend due to low demand. A high availability of wind power causes the low prices on the right half. In hours without PV and wind power, the most expensive substitute set the price. The extreme levels for natural gas in 2022 were caused by Russian's invasion in Ukraine. The low availability of French nuclear power during these months increased the demand for exports.
}}

==See also==
{{Portal|Energy}}
* [[Combined cycle hydrogen power plant]]
* [[Energy security and renewable technology]]
* [[Ground source heat pump]]
* [[Cost of electricity by source]]
* [[Spark spread]]: calculating the cost of back up
* [[List of energy storage power plants]]

== Further reading ==

*{{cite book
|last=Sivaram
|first=Varun
|title=Taming the Sun: Innovation to Harness Solar Energy and Power the Planet
|location=Cambridge, MA
|publisher=MIT Press
|year=2018
|isbn=978-0-262-03768-6}}

==References==
{{Reflist|2}}

==External links==
* [http://www.wind-works.org/articles/GridIntegrationofWindEnergy.html Grid Integration of Wind Energy] {{Webarchive|url=https://web.archive.org/web/20120510064213/http://wind-works.org/articles/GridIntegrationofWindEnergy.html |date=2012-05-10 }}

{{Electricity generation}}

[[Category:Power engineering| ]]
[[Category:Electric power distribution]]
[[Category:Energy storage]]
[[Category:Renewable energy]]