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Appendix C
Comparison of Different Power-Generation Methods 265

cost of any wind energy stored, but it is not envisaged that this would
apply to a large proportion of wind energy generated. Thus, the 2 GW
Dinorwig pumped storage plant adds costs to nuclear energy in the UK
for which it was built, but not to all the power produced from the 30 or
so GW of nuclear plants in the UK.
Peak wind speeds might not coincide with peak demand for electrical
power. In California and Texas, for example, hot days in summer can
have low wind speed and high electrical demand due to air conditioning.
In the UK, however, winter demand is higher than summer demand, and
so are wind speeds. Solar power tends to be complementary to wind
because on most days, with no wind, there is sun, and on most days with
no sun, there is wind. A demonstration project at the Massachusetts
Maritime Academy shows the effect. A combined power plant linking
solar, wind, bio-gas, and hydrostorage is proposed as a way to provide
100 percent renewable power. The 2006 Energy in Scotland Inquiry
report expressed concern that wind power cannot be a sole source of sup-
ply and recommends diverse sources of electric energy.
A report from Denmark noted that its wind power network was with-
ptg
out power for 54 days during 2002. Wind power advocates argue that
these periods of low wind can be dealt with by simply restarting existing
power stations that have been held in readiness. The cost of keeping a
power station idle is, in fact, quite low because the main cost of running
a power station is the fuel.
Wind energy “penetration” refers to the fraction of energy produced
by wind compared with the total available generation capacity. There is
no generally accepted “maximum” level of wind penetration. The limit
for a particular grid depends on the existing generating plants, pricing
mechanisms, capacity for storage or demand management, and other fac-
tors. An interconnected electricity grid already includes reserve generat-
ing and transmission capacity to allow for equipment failures; this
reserve capacity can also serve to regulate for the varying power genera-
tion by wind plants. Studies have indicated that 20 percent of the total
electrical energy consumption can be incorporated with minimal diffi-
culty. These studies have been for locations with geographically dis-
persed wind farms, some degree of dispatchable energy, or hydropower
with storage capacity, demand management, and interconnection to a
large grid area export of electricity when needed. Beyond this level,
there are few technical limits, but the economic implications become
more significant.