Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 189

0.5

0.4

0.3

0.2

0.1

0 1,000 2,000 3,000 4,000

Wind capacity (MW)

Figure 5.29 Wind capacity credit for 2020 Ireland system

A significant number of studies for northwest Europe have concluded that
wind does have a capacity credit, roughly equal to the capacity factor of wind
energy during the winter period (Milborrow, 2004). The validity of this approach is
likely to lessen with increased wind penetration. A more conservative calculation
would be based upon the annual, rather than winter period, capacity factor. Hence,
for example, considering 1,000 MW of wind generation, and assuming an annual
capacity factor of 0.30, the capacity credit would be 300 MW, which could be
offset against equivalent conventional generation. Again, these results are only
suitable for low wind penetration levels. Alternatively, the capacity credit can be
approximated as the wind capacity factor divided by the average reliability of
the remaining power system. So, for example, choosing a system reliability of
90 per cent would indicate a wind capacity credit, in this case, of 330 MW
(assuming a significantly large power system). At higher wind penetrations (greater
than 1 per cent of peak load), the capacity credit decreases asymptotically to a level
dependent on the system reliability.
In the United Kingdom the annual capacity factor for onshore wind has varied
between 24 and 31 per cent since 1992, with a long-term average of 27 per cent
(DTI, 2005). During the top 10 per cent periods of demand the capacity credit
exceeds 36 per cent (ECI, 2005). With increased development of offshore sites, and
improving turbine technology, these figures may rise higher. In the United States,
various utilities, regional transmission organisations, public utility commissions, etc.
have estimated annual capacity factors ranging from 2 per cent up to 40 per cent.
The observed variation arises from a multitude of analytical/ad hoc techniques,
varying confidence in wind availability, and, of course, consideration of differing
(climatic) regions of the United States with greatly differing penetrations of wind