Environmental Engineering Reference
In-Depth Information
stations
with hundreds of smaller turbines in highly-visible locations have stimulated
considerable public opposition.
Two additional factors clearly improve the degree of public acceptance. The irst factor
is the perceived energy beneit to the community, including lower cost and higher avail-
ability. The second factor is the degree of careful planning and open communication with
the community and its leaders well before construction starts. However, one can recall that
European and American windmills, which society now views with nostalgic fondness as
romantic artifacts from our past, were work-a-day machines in their time, no different from
our tractors and factory smokestacks. Only history will determine whether the same will
be said in the future about today's modern wind turbines.
System Coniguration Tradeoffs
A wind turbine, like an aircraft, is a complex system which is the result of many
tradeoff decisions made in the search for optimum overall performance and economy. In
the evolution of modern wind turbines, there are a number of major coniguration variables
whose trends over time (and the reasons for those trends) are fairly evident. In other cases,
there is still doubt as to the “best” or optimum approach. In some cases the “optimum” ap-
proach changes with time as technological developments in one subsystem affect the overall
design.
Turbine Size
The question of optimum size of wind turbines for large-scale electrical generation is
probably the most controversial one in the ield of wind power, and there is yet no clear
answer. From a irst-order standpoint, there are two countervailing forces: Rotor
swept
area
and power increase as the square of the rotor diameter, but weight and cost would be
expected to increase roughly as the cube, for geometrically similar structures. This
square-
cube relationship
should favor small systems and limit the maximum turbine size.
However, a converse effect occurs for several reasons. Most sites have a
positive wind
shear
, so wind speed increases with elevation. Input wind energy increases as the cube of
the wind speed, so a larger wind turbine, being taller, should capture more energy per unit
of swept area. This additional energy capture also implies reduced
land requirements
for
a wind power station of a given rating composed of larger turbines, when the turbine
separation is a ixed number of diameters. In addition, the size and cost of many compo-
nents (
e.g.
the control system) do not increase at a cubic rate with increasing rotor diameter,
and sometimes increase very little. Clearly then, there are some
economies of scale
to
offset the square-cube relationship. Thus, there is some optimum size at which these two
countervailing forces are economically balanced.
Several studies in the 1970s attempted to determine optimum size by examining
families of hypothetical wind turbines, estimating scaling laws for individual subsystems,
and calculating
cost-of-energy
(COE, in cents per kilowatt-hour)
vs
turbine size. Many in
the ield concluded that there were, in fact, two minimum energy cost points. Very small,
simple turbines would be expected to be relatively expensive because of the threshold cost
of many components and services. A site maintenance call, for example, could cost the
equivalent of a month's electrical output. As this small turbine is made larger, then, energy
costs would be expected to decrease. However, this downward trend would rapidly reverse
and energy costs would increase again, because simple “brute force” components become
very heavy as dynamic loads increase with size (
e.g.
, a tail in for yaw control of a
HAWT). This reversal establishes the
irst minimum of energy cost
.
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