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Day-ahead operation schedules the generation units that can be called upon to provide the electricity on the next day ('''unit commitment'''). The [[dispatchable generation]] units can produce electricity on demand and thus can be scheduled with accuracy. The production of the weather-dependent [[variable renewable energy]] for the next day is not certain, its sources are thus non-dispatchable. This variability, coupled with uncertain future power demand and the need to accommodate possible generation and [[electricity transmission|transmission]] failures requires scheduling of [[operating reserve]]s that are not expected to produce electricity, but can be dispatched on a very short notice.{{sfn|Conejo|Baringo|2017|p=9}}
Some units have unique features that require their commitment much earlier: for example, the [[nuclear power station]]s take a very long time to start, while hydroelectric plants require planning of water resources usage way in advance, therefore commitment decisions for these are made weeks or even months before prior to the delivery.<ref name=psu>{{cite web
For a "traditional" [[vertical integration#Electric utilities|vertically integrated electric utility]] the main goal of the unit commitment is to minimize both the [[marginal cost]] of producing the unit electricity and the (quite significant for fossil fuel generation) start-up costs. In a "restructured" [[electricity market]] a [[market clearing]] algorithm is utilized, frequently in a form of an [[auction]]; the [[merit order]] is sometimes defined not just by the monetary costs, but also by the environmental concerns.{{sfn|Conejo|Baringo|2017|p=9}}
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At the expected demand is 150 MW (a vertical line on the graph), unit A will be engaged at full 120 MW power, unit B will run at the '''dispatch level''' of 30 MW, unit C will be kept in reserve. The area under the dispatch curve to the left of this line represents the cost per hour of operation (ignoring the startup costs, $30 * 120 + $60 * 30 = $5,400 per hour), the incremental cost of the next MWh of electricity ($60 in the example, represented by a horizontal line on the graph) is called '''system lambda''' (thus another name for the curve, ''system lambda curve'').
In real systems the cost per MWh usually is not constant, and the lines of the dispatch curve are therefore not horizontal (typically the marginal cost of power increases with the dispatch level, although for the [[combined cycle power plant]]s there are multiple cost curves depending on the mode of operation, so the power-cost relationship is not necessarily [[Monotonic function|monotonic]]).<ref name="BayónGarcía NietoGrau2013">{{cite journal | last1 = Bayón | first1 = L. | last2 = García Nieto | first2 = P. J. | last3 = Grau | first3 = J. M. | last4 = Ruiz | first4 = M. M. | last5 = Suárez | first5 = P. M. | title = An economic dispatch algorithm of combined cycle units | journal = International Journal of Computer Mathematics | date = 19 March 2013 | volume = 91 | issue = 2 | pages = 269–277 | issn = 0020-7160 | eissn = 1029-0265 | doi = 10.1080/00207160.2013.770482 | pmid = | s2cid = 5930756 | url = https://www.unioviedo.es/bayon/osh/41.pdf}}</ref>
[[File:Hypothetical dispatch curve, USA, Summer 2011.png|thumb|500px|Hypothetical dispatch curve (USA, summer 2011)<ref>{{cite web |title=Electric generator dispatch depends on system demand and the relative cost of operation |url=https://www.eia.gov/todayinenergy/detail.php?id=7590# |website=eia.gov |access-date=30 May 2022 |date=17 August 2012}}</ref>]]
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== Sources ==
* {{cite book | first1 = Antonio J. |last1 = Conejo | first2 = Luis | last2 = Baringo | date = 5 December 2017 | title = Power System Operations | publisher = Springer | pages = | isbn = 978-3-319-69407-8 | oclc = 1015677828 |
* {{cite book | first1 = Kankar | last1 = Bhattacharya | first2 = Math H.J. | last2 = Bollen | first3 = Jaap E. | last3 = Daalder | date = 6 December 2012 | title = Operation of Restructured Power Systems | publisher = Springer Science & Business Media | pages = | isbn = 978-1-4615-1465-7 | oclc = 852788650 | url = https://books.google.com/books?id=PtLgBwAAQBAJ}}
* {{cite web |last1=McCalley |first1=James D. |title=Introduction to System Operation, Optimization, and Control |url=https://home.engineering.iastate.edu/~jdm/ee553/Intro.pdf |website=iastate.edu |publisher=[[Iowa State University]] |access-date=30 May 2022}}
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