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Despite the variations in wind conditions and system characteristics among the European countries and regions, capacity credit calculations are fairly similar.
For low wind energy penetrations levels, the relative capacity credit of wind power (that is ‘firm’ capacity as a fraction of total installed wind power capacity) will be equal or close to the average production (load factor) during the period under consideration, which is usually the time of highest demand. For North European countries, this is winter time and the load factor is typically 25–30% onshore and up to 50% offshore. The load factor determining the capacity credit in general is higher than the average yearly load factor.
With increasing penetration levels of wind energy in the system, its relative capacity credit reduces. However, this does not mean that less conventional capacity can be replaced, but rather that a new wind plant on a system with high wind power penetration levels will substitute less than the first wind plants in the system. This is illustrated in Figure 6.2, where the relative capacity credit tails off, towards a value depending mainly on the minimum load factor.
Table 6.1 summarises the factors leading to higher or lower levels of capacity credit.
Figure 6.2, which is based on calculations in the DENA 1 study, shows the effect on capacity credit of expected improved load factors in Germany, resulting from improved wind power technology (more efficient rotors) and the use of sites with higher wind speeds (offshore).
Higher capacity credit (%) Lower capacity credit (%) Low penetration of wind power High penetration of wind Higher average wind speed, high wind season when demand peaks Lower average wind speeds Lower degree of system security High system degree of security Higher wind power plant (aggregated) capacity factor or load factor (determined by wind climate, plant efficiency and specific rated power per m2) Lower aggregated capacity factor (or load factor) of wind power Demand and wind are correlated Demand and wind uncorrelated Low correlation of wind speeds at the wind farm sites, (often related to large size of area considered) Higher correlation of wind speeds at wind farm sites; smaller areas considered Good wind power exchange through interconnection Poor wind power exchange between systems
Table 6.1 Factors affecting positively and negatively the value of the capacity credit of a certain amount of wind power in the system.
Figure 6.2 Relationship of installed wind power and capacity credit in Germany, Source: DENA 2005
Wind power thus displaces conventional capacity in the system. The fraction of wind power that displaces conventional may be limited, but the corresponding absolute capacities are significant. The aggregated capacity credit of the wind power plants in a system depends on many factors. Major decisive factors depend on the power system in question (reliability level and flexibility of the generation mix) and the penetration level of wind power in the system. Other factors are related to wind and wind technology, such as the average capacity factor and the geographical dispersion of wind plants in the system. The relative capacity credit decreases from a value approximately equal to the load factor at high load (25-35%) for low penetrations, to approximately 10-15 % at high penetrations.
Although physically and technically, wind power has a capacity credit, this characteristic currently has no value in the power market, as wind power producers are not generally rewarded for providing firm capacity in the system.
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