Capacity credit

Capacity credit (CC, also capacity value[1]) is a fraction of the installed capacity of a power plant which can be relied upon at a given time (typically during the system stress),[2] frequently expressed as a percentage of the nameplate capacity. A conventional (dispatchable) power plant can typically provide the electricity at full power as long as it has a sufficient amount of fuel and is operational,[1] therefore the capacity credit of such a plant is close to 100%; it is exactly 100% for some definitions of the capacity credit (see below).[3] The output of a variable renewable energy (VRE) plant depends on the state of an uncontrolled natural resource (for example, wind, solar, or waves), therefore a mechanically and electrically sound VRE plant might not be able to generate at the rated capacity (neither at the nameplate, nor at the capacity factor level) when needed,[1] so its CC is much lower than 100%. The capacity credit is useful for a rough estimate of the firm power a system with weather-dependent generation can reliably provide.[4] For example, with a low, but realistic (cf. Ensslin et al.[5]) wind power capacity credit of 5%, 20 gigawatts (GW) worth of wind power needs to be added to the system in order to permanently retire a 1 GW fossil fuel plant while keeping the electrical grid reliability at the same level.

Not to be confused with Capacity factor.
"ELCC" redirects here. For a Canadian church, see Evangelical Lutheran Church of Canada.

Definitions

Dent et al.[1] review few similar definitions of the capacity credit:

  • effective load carrying capability (ELCC) defines the capacity value as the extra power the plant can add to the system without degrading a chosen reliability index (like the loss of load probability);
  • comparison to the capacity of a conventional power plant represents the amount of the conventional generating capacity which can be replaced by a VRE plant while keeping the value of the risk index. A similar metrics, comparing the plant contribution to that of a perfect always-available-at-full-capacity plant is called an equivalent firm capacity or EFC;[6]
  • percentile of peak-period availability defines the capacity value by calculating the capacity at chosen worst-case percentile (say, 5th lowest) of the power distribution during the times of the peak demand.

Values

The capacity credit can be much lower than the capacity factor (CF): in a not very probable scenario, if the riskiest time for the power system is at night, the capacity credit for the solar power without the coupled energy storage is zero regardless of its CF[2] (i. e, under this scenario all existing conventional power plants will have to be retained after the solar installation is added). More generally, the CC is low when the times of the day (or seasons) for the peak load do not correlate well with the times for high energy production.[7] Ensslin et al.[5] report wind CC values ranging from 40% down to 5%, with values dropping off with the increased wind power penetration.

For very low penetrations (few percent), when the chance of the system actually forced to rely on the VRE at peak times is negligible, the CC of a VRE plant is close to its capacity factor.[5] For high penetrations, due to the fact that the weather tends to affect all plants of the similar type at the same time and in the same way - and the chance of a system stress during low wind condition increases,[8] the capacity credit of a VRE plant decreases. Greater geographical diversity of the VRE installations improves the capacity credit value, assuming a grid that can carry all necessary load.[5]

For example, in Texas the peak demand is driven by the air conditioning and occurs on summer afternoons and evenings.[7], while the wind is strongest at night, with offshore wind strongest in the winter.[9] This has an effect of a relatively low CC for most potential wind power locations: a predicted average for the onshore wind is 12.7% and for the offshore wind is 6.7%.[10]

In Great Britain, the solar contribution to the system adequacy is small and is primarily due to scenarios when the use of solar allows to keep the battery storage fully charged until later in the evening.[11] The National Grid ESO in 2019 suggested planning for the following EFC-based de-rating:[12]

Indicative de-rating factors in Great Britain
YearOnshore windOffshore windSolar PV
2020/20218.98%14.65%1.17%
2022/20238.40%12.89%1.76%
2023/20248.20%12.11%1.56%

References
  1. Dent, Keane & Bialek 2010.
  2. Jorgenson et al. 2021, p. 1.
  3. Brand, Stambouli & Zejli 2012.
  4. Jorgenson et al. 2021, pp. 1–2.
  5. Ensslin et al. 2008, p. 3.
  6. National Grid 2019, p. 4.
  7. Jorgenson et al. 2021, p. 6.
  8. National Grid 2019, p. 16.
  9. Jorgenson et al. 2021, p. 7.
  10. Jorgenson et al. 2021, p. 21.
  11. National Grid 2019, p. 6.
  12. National Grid 2019, p. 3.

Sources
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