Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 209

during peak demand periods, even though wind generation may be high. Instead, to
be economically and operationally viable, both energy storage and load manage-
ment should be seen as system resources utilised for global benefit. Distributed
storage, coupled with wind generation, will also lead to lower distribution network
losses, and probably lower transmission losses as well. The above argument can be
affected by electricity market participation rules (see Chapter 7 - Electricity Markets).
The nature of any diurnal variation in wind generation, and the correlation with
system demand, can influence the role that energy storage will play: greatest benefit is
likely to be achieved in parts of the world where there is a counter-cyclical relation-
ship between wind speed and demand consumption, i.e. peaks and troughs in one tend
not to coincide with peaks and troughs in the other.
New York State, for example, is a case where the seasonal and daily patterns of
wind generation are largely out of phase with the load demand (Piwko et al. , 2005).
Onshore wind production tends to be high at night, low during the day, ramping
down during the morning demand rise and ramping up during the evening fall -
accentuating the load following requirements of conventional plant. Wind pro-
duction also tends to be lowest during the June-August period when the system
load is greatest. Consequently, although the average capacity factor for onshore
wind is approximately 30 per cent, the capacity credit is only 10 per cent, that is,
the wind tends not to blow during peak demand periods. Interestingly, however,
offshore wind generation (sited near Long Island) has a capacity credit approaching
40 per cent, similar to its capacity factor. Here, the wind strength tends to peak
several hours later in the day, as compared with the onshore sites, so production is
more in phase with the demand pattern. As illustrated earlier in Figures 5.9 and
5.10, depicting a typical electrical demand profile and wind generation profile for
Ireland, the same is not true for north-western Europe. Here, a weak correlation
between wind speed and electrical demand exists, suggesting that the benefits of
energy storage are less compelling.
5.5.1 Conventional energy storage
A wide variety of energy storage technologies are commercially available and
include pumped storage, rechargeable batteries, flow batteries and compressed
air. Potential benefits, of relevance here, include capacity reduction, frequency
support, standing reserve provision and blackstart capability. Depending on
technical requirements and geographical settings, a particular utility may avail of
one or more of these technologies. Research effort has also focussed on ultra-
capacitors, high speed flywheels and superconducting magnetic energy storage
(SMES). While these are highly responsive, their energy storage capabilities are
limited, making such approaches more suitable for power quality applications
and for improving system reliability. Of course, even here some niche applica-
tions of interest do exist: Palmdale water district, California employs a 450 kW
supercapacitor to regulate the output of a 950 kW wind turbine attached to the
treatment plant microgrid (Gyuk et al. , 2005). This arrangement helps to reduce
network congestion in the area, while providing reliable supply to critical loads
in the microgrid.