Environmental Engineering Reference
being reduced, and large industrial consumers with interruptible tariffs being
disconnected. On both days, the availability of wind generation meant that power
cuts were not required, and a partial blackout was avoided.
At some time during the year, the peak demand on a power system will occur.
In north-western Europe this is likely to occur on a winter weekday evening.
In warmer climates, the peak demand is more likely to happen during the summer
time, coincident with high air-conditioning load. Since electrical energy cannot be
conveniently stored, it follows that there must be sufficient generation plant capa-
city installed and available to meet the peak annual demand. Underutilised plant is
a natural consequence, and system capacity factors (calculated as the ratio of the
average system demand to the peak system demand) of 55-60 per cent are typical
for many utilities. The additional need for spinning and back-up reserve, and the
possibility that individual units may be out of service when the peak occurs, leads
to a requirement for additional capacity beyond the forecast peak.
Given that it may take several years to obtain planning permission and com-
plete plant construction, the electrical utility must predict the peak system demand
10-20 years into the future and ensure that sufficient plant capacity exists when
required. A question that then arises is - how should the (anticipated) growth of
wind farms be factored into determining the required future conventional plant
capacity? In other words, does wind energy have a capacity credit?
With a fuel saver strategy, the task is straightforward - wind forecasts do not
form part of the unit commitment process, and so the question is redundant.
If a wind forecast strategy is adopted, however, the question is more challenging.
In parts of the world where peak annual load occurs during the summer (driven by
air-conditioning load) the capacity credit of wind generation is likely to be reduced
(Piwko et al. , 2005). However, in locations where maximum demand falls during
the winter (increased heating demand) the capacity benefits of wind generation
should be more apparent. Taking Ireland as an example, and by inference northwest
Europe, it was shown in Section 5.3.2 (Figures 5.9 and 5.10) that there is a weak
correlation between both the daily and annual variation in system demand and wind
generation. Thus, it tends to be windier during the day rather than at night
(more noticeably in the summer), and during the winter rather than the summer.
Consequently, wind generation will tend to cause a reduction in both the system
peak and the minimum load. In the United Kingdom a positive correlation has been
observed between the average hourly capacity factor and the electrical demand
(ECI, 2005), where the average energy provided from wind farms during peak
demand periods (winter evenings) is around 2.5 times that produced at minimum
demand (summer nights). However, it does not automatically follow that it will be
windy during the periods of peak demand on the system, although a wind chill
factor could impact on the heating load component of the system demand and
hence influence when the peak actually occurs (Hor et al. , 2005). Recognition that
wind energy has a capacity credit should ultimately result in the avoidance, or at
least delay, in the construction of additional conventional generation. Unlike
hydroelectric generation, which can be subject to significant annual variation in
available production, long-term analysis of wind speed records suggests that the