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October 21, 2000 10:48 Annual Reviews AR118-06
Annu. Rev. Energy Environ. 2000. 25:147–97
Copyright
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 2000 by Annual Reviews. All rights reserved
WINDPOWER: A Turn of the Century Review
1
Jon G. McGowan and
2
Stephen R. Connors
1
Department of Mechanical and Industrial Engineering, University of Massachusetts,
Amherst, Massachusetts 01003; e-mail: ,
2
The Energy Laboratory, Massachusetts Institute of Technology, Cambridge,
Massachusetts 02137-4307; e-mail:
Key Words wind energy, renewable energy, offshore, electricity,
electricity competition
■ Abstract The 1990s saw a resurgence in the windpower industry, with installed
grid-connected capacity expanding more than five-fold between 1990 and 2000. Most
of this increase occurred in Europe, where governmental policies aimed at developing
domestic energy supplies and reducing pollutant emissions provided a sheltered mar-
ket for renewable energy generation. The 1990s were also marked by a return to large,
megawatt-sized wind turbines, a reduction and consolidation of wind turbine manufac-
turers, and increased interest in offshore windpower. This article reviews recent trends
in the windpower industry, including some of the fundamental engineering principles
of wind turbine design. Technological impediments and advances are discussed in the
context of changes in the global electricity markets and environmental performance.
CONTENTS
INTRODUCTION 148

RECENT TRENDS
149
WIND ENERGY APPLICATIONS AND ECONOMICS
151
WIND TURBINE DESIGN CONSIDERATIONS
155
Rotor Axis
156
Orientation
157
Rotational Speed
158
Rotor Characteristics
158
Aerodynamic Power Control
159
Dynamic Load Management at the Hub
160
Tower Structure
160
Other Design Constraints
161
Maintenance Issues
162
Standards and Certification
163
ENVIRONMENTAL DESIGN CONSIDERATIONS
165
Land Use
165

Avian Interaction
166
Local Opposition
167
1056-3466/00/1129-0147$14.00
147
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WIND RESOURCE CONSIDERATIONS 170
RECENT ADVANCES IN WIND TECHNOLOGY
173
Rotor and Blades: Aerodynamics
174
Blades: Materials and Testing
178
Drive Train and Generators
178
Controls and Conditioning
179
Towers and Construction-Erection Issues
181
Resource Trends
181
FUTURE WINDPOWER APPLICATIONS AND DEPLOYMENT
182
Development of Large Wind Turbines
183

Offshore Windpower
183
Small Wind Turbine Systems
187
INDUSTRY TRENDS
187
CONCLUSIONS
191
INTRODUCTION
In 1990 there were roughly 2200 MW of grid-connected wind generating capacity
in the world, mostly in California (1). After the end of the OPEC oil shock, and the
end of U.S. investment tax credits for wind, the industry entered a period of slow
growth. In the early1990s, withconcernsoverclimatechangeand an over-reliance
on fossil fuels reemerging, governmental policies in Europe, the United States,
and elsewhere were re-instituted to help renewable power generation. This, along
with technology improvements and lower installed costs, has led to a remarkable
resurgence in the industry. Denmark and Germany introduced rules that ensured
that wind farms received payments of up to 85% to 90% of the retail price of
electricity (2). In the United States, the Energy Policy Act of 1992 instituted a
production tax credit for wind and other renewables of 1.5¢ per kWh. However,
with the introduction of competition for electricity in nearly every industrialized
country, the long-term planning function of vertically integrated electric utilities
has all but disappeared. In the place of utilities’ integrated resource planning has
arisen renewable portfolio standards and the potential to sell “value priced” green
power. Against this background of liberalized electricity markets, wind turbine
developers have continued to work, improving the technology and bringing out
bigger and bigger machines. In Europe especially, issues regarding land use have
wind farm developers looking to the sea, a very suitable place for large wind
turbines and smoother, faster winds.
To bring the reader up to date, this article covers three main topics. First are the

recent changesinthe wind industry itself, withparticularattentionpaid to therange
and types of wind turbines—or wind energy conversion systems (WECS)—that
are now being installed in onshore and offshore wind farms. Second is a review of
the key wind turbine design issues upon which the continued development of the
wind industry depends. Third is a discussion of where the industry is going. Of
particular interest is how increased competition, or liberalization, in the electric
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WINDPOWER 149
sector will effect the market for windpower and, of course, how this impacts the
increasing need to reduce pollutant emissions and mitigate global climate change.
RECENT TRENDS
At the end of 1999, it was estimated that there was more than 12 GW of grid-
connected windpower in the world. This is more than five and a half times the
amount of installed capacity in 1990 (1, 3). Figure 1 (see color insert) shows how
installed capacity has grown from 1995 through 1999, broken down by geographic
region (4–6). Here the influence of European renewable energy policies is appar-
ent. Table 1 provides details for 1996, 1998, and 2000. In the mid-1990s, North
America and Europe had roughly the same amount of installed capacity (at 46%
of the world’s total each). However, by the end of 1999 Europe’s share of total
installed capacity had risen to over two thirds. From 1997 to 2000, Europe installed
new wind generating capacity at the rate of 1600 MW per year, and from 1995 to
2000 wind generating capacity grew at an average annual rate of 37%. Over the
sameperiod, windcapacityin Asia hasquintupled,primarilydue to efforts inIndia.
By the late 1980s, commercial grid-connected wind turbines were in the 150 to
450 kW range. By the late 1990s, most manufacturers had roughly doubled the size
of wind turbines, offering600to750kW machines. 1000 to 1600 kW machines are
now commercially available. The latest models being developed range well above
2 MW, primarily for offshore applications. Figure 2 shows the comparative height
andsweptareaforvariousmachines. Althoughrotor diameterand tower/hub height

varies among manufactures, the variation is not overly large. Tower height is the
most variable, as site characteristics such as uniformity of the wind’s flow field,
surface roughness, and visual impacts must be considered. However, towers are
commonly one to one-and-a-half the rotor’s diameter in height. A good overview
TABLE 1 Installed wind generating capacity (4, 5, 6)
Region Jan. 1996 Jan. 1998 Jan. 2000
Europe 2518 52.0 46.1 4766 62.8 35.9 8349 67.0 29.1
North America 1676 34.6 −2.7 1615 21.3 0.2 2617 21.0 30.2
Asia and Pacific 626 12.9 157.6 1149 15.1 24.5 1363 10.9 8.4
Latin America 7 0.1 −30.0 34 0.4 21.4 87 0.7 67.3
Middle East 12 0.2 −50.0 21 0.3 0.0 36 0.3 38.5
Africa 0 0.0 3 0.0 0.0 3 0.0 0.0
Total 4839 30.0 7588 24.5 12455 26.9
(MW) (%) (D%) (MW) (%) (D%) (MW) (%) (D%)
(D%—Percent change from previous year)
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5000 kW
112 m
100 m
: Capacity
: Rotor Dia.
: Tower Hgt.
750 kW
48 m
60 m
50 kW

15 m
25 m
1000 kW
60 m
70 m
300 kW
34 m
40 m
2000 kW
72 m
80 m
160 m
120 m
80 m
40 m
Figure 2 Representative size, height and diameter of wind turbines.
of how the size and performance of Danish wind turbines has changed over time
can be found in References 7–9.
Whereas most new wind farm installations remain onshore, The Netherlands,
Denmark, and Sweden have begun to develop their expertise in offshore appli-
cations. Table 2 lists current offshore wind farms (10). Most of these represent
near-shore, sea floor mounted WECS installations. As is discussed below, off-
shore applications present a tradeoff between installed costs and maintenance and
superiorwind resourcesandlowerland-useandcommunity acceptanceconstraints.
TABLE 2 Existing offshore wind installations (10)
Location Country Year Capacity No. Size Manufacturer
Vindeby Denmark 1991 4.95 11 450 Bonus
Lely (Ijsselmeer) Netherlands 1994 2.00 4 500 NedWind
Tunø Knob Denmark 1995 5.00 10 500 Vestas
Dronten I (Ijsselmeer) Netherlands 1996 11.40 19 600 Nordtank

Bockstigen Sweden 1997 2.75 5 550 Wind World
Total 26.10 49
(MW) (kW)
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WINDPOWER 151
WIND ENERGY APPLICATIONS AND ECONOMICS
How individual wind turbines are bundled into wind farms depends upon the wind
resource, topography, economics, and the sensitivity of local populations. Figure 3
(see color insert) shows some of the potential configurations a wind farm can take,
includingsome prospectivearrangements foroffshorepower. Thelargewindfarms
in California range from ridge-top arrays in the Altamont pass to large rectilinear
arrays near Palm Springs. Europe, due in part to population density, has deployed
its wind turbines in smaller groupings, as linear arrays or clusters of perhaps a
dozen machines each (7). Another important factor is the regulatory treatment of
grid interconnections. At what voltage level is the local utility comfortable with
the insertion of a variable power source? Furthermore, there may be economies
of scale for larger wind farms, especially if they are connecting to higher voltage
transmission lines for delivery to distant population centers. Interest in offshore
applications has increased because large high quality wind regimes are relatively
close to population and load centers.
As maintenance requirements drop and remote control and operation capabili-
ties expand, the economics of co-location will diminish. In areas where there are
more people, or existing agricultural land-uses, the “European model” of smaller
groups of WECS allows better integration and synergy of windpower generation
with existing land uses. Although the close proximity of wind may invite local
opposition, if there is good community buy-in, owing in part to local economic
and employment benefits, wind deployment can continue (11).
Of course, interest in windpower is not limited to grid-onnected power. As
illustrated in Figure 4 (see color insert), large- and smaller-scale grid-connected

windpoweris only part of the picture. For veryrural areas, including village power
in developing countries, there is considerable interest in hybrid systems, or mini-
grids. Recent experience in wind-diesel applications in Alaska and Canada focus
on the delivery of reliable power, especially when already expensive fuel deliveries
are interrupted for part of the year due to harsh weather (12, 13). These smaller
kW systems are driven by a different economic equation. Rather than competing
against the grid price of power, they are measured by the value of the service
they provide. In developing countries this can be measured in terms of improved
medical services, equivalent cents per lumen from a kerosene lamp, or clean and
reliablewatersupplies.Onthefarendofthisspectrumaresmall single-usesystems,
generally associated withtelecommunicationsandnavigational applications. Here
the electricaldemandis generally for acontinuouspowersource, ratherthana large
demand for electrical energy. Such battery-charging systems are rarely judged on
a cost per unit power basis.
The primary tradeoff effecting the economics of windpower is the capital cost
of the machine or farm and the quality of the wind resource. Currently, to be
cost-competitive, wind farms must be sited in high quality wind regimes, nor-
mally a Wind Power Class of 4 or higher, preferably 5 or higher. Figure 5 shows
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0
10
20
30
40
50
45678910

Capacity Factor (%)
0
200
300
400
500
600
800
200
300
400
500
600
800
2000
-
-
-
-
-
-
-
Wind Power
Density (W m-1)
5.6
6.4
7.0
7.5
8.0
8.8

11.9
0.0
5.6
6.4
7.0
7.5
8.0
8.8
-
-
-
-
-
-
-
Wind Speed
Range (m s-1)
( 1 )
( 2 )
( 3 )
( 4 )
( 5 )
( 6 )
( 7 )
Wind Power
Class
(at 50m height)
(1) (2) (3) (4) (5) (6) (7)
Average Wind Speed (m s
-1

at hub height)
Figure 5 Comparison of average wind speed and wind power class to capacity factor (14, 15).
a plot of the annual generation from a Vestas 600 kW machine, expressed as a
capacity factor—the percent of a year it would need to run at rated power to pro-
duce its annual output (14). For reference purposes the equivalent Wind Power
Classes have been included on the graph (15). As power output, and therefore
generation, is related to the cube of the wind speed, slightly higher average wind
speeds, or wind regimes with a higher variability in the high velocity range, can
produce significantly more power. The very best wind sites tend to be Class 6.
A Class 4 site is considered marginal by economic standards, especially when
the wake effects of other wind turbines within a wind farm are taken into ac-
count. Therefore, in today’s market, a capacity factor of about 25% can be con-
sidered a lower bound, unless the combined capital and operating costs of wind
turbines drop.
Thecostofwind-generatedelectricityisinfluencedbynumerousfactors.Table 3
shows how the cost of windpower changes as assumptions regarding capacity fac-
tor, capital cost, financing, and operation and maintenance change. Using conser-
vative mid-range assumptions for costs and performance, six cents per kWh is in
line with recent experiences. Costsarecontinuing to drop for windpower, and with
turbine costs approaching $800 per kW, wind generated electricity costs of 4–5¢
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WINDPOWER 153
TABLE 3 Parametric evaluation of electricity cost from wind
Best Mid- Worst
range range range Unit
Capacity/plant factor 40.0% 25.0% 20.0% % of year at rated output
Greenfield overnight cost
$750 $1,000 $1,500 $/kW
Fixed O&M costs

$10.00 $15.00 $30.00 $/kW-yr
Variable O&M costs
$2.00 $8.00 $12.00 $/MWh (mils/kWh)
Cost of generation 2.63 6.05 11.47
¢/kWh
All three calculations use a levelized carrying charge of 10%
per kWh are expected in the near future. The cents per kWh number is a simple
calculation of annual fixed and variable costs divided by the expected generation
suppliedto thegrid. The“BestRange” and“WorstRange”columns inTable3show
how this number changes if the combined best/optimistic and worst/pessimistic
assumptions are used from a recent literature review (16). Greenfield overnight
costs represent the “all-in” cost of the generation facility including grid inter-
connections and access roads, as well as wind turbine costs. Fixed operation and
maintenance costs (O&M) refer to regularly scheduled servicing, while variable
O&M includes utilization-based service and repairs. Payments to landowners and
taxes can be either fixed or variable O&M, based upon contractual and other legal
arrangements.
The four to six cents number is currently one and a half to three times the
average spot price of electricity in the United States, depending on region, so to
be competitive on a head-to-head basis with other sources of wholesale electricity
these factors have to change, or some sort of subsidization or credit calculation
must occur. By comparison, the total cost of generation for a new natural gas–fired
unit can range from two to four cents per kWh, based upon technology and fuel
cost assumptions (16).
In the last Annual Reviews chapter on windpower, Sørensen (17) discussed
the avoided environmental costs of choosing windpower over other options with
pollutant emissions, as well as the life cycle impacts associated with mining,
refining, fuel transportation, and combustion. Also important in addressing the
social costs of various generation alternatives are the risks of severe accidents
and longer-term fuel and solid waste issues. The external costs of windpower

are not included in Table 3’s calculations. Nor are there any credits given in the
calculationforsubsidiessuch asthe ProductionTaxCredit, oravoidedexpenditures
such as the cost of sulfur emissions allowances that U.S. fossil units must now
consider. Portfolio benefits, as demonstrated in Reference 18, can reduce system-
wide variability in costs and emissions, and have some synergistic benefits when
coupled with end-use efficiency efforts. Such estimates of avoided environmental
and other costs are difficult to make without detailed analyses that incorporate the
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●
●
●
2.0
4.0
6.0
8.0
10.0
12.0
14.0
0.0 0.5 1.0 1.5 2.0
Cents per kWh
Ratio to Mid-Range Assumptions
Best Range
Worst Range
Var. O&M
Fixed O&M
Carrying

Charge
Overnight
Cost
Mid-Range
Capacity Factor
(Site Wind Speed)
Figure 6 Parametric evaluation of cost of electricity from wind.
composition of the regional power system, as well as other regional demographics,
air quality, and other environmental criteria. With these factors in mind, what
opportunities are there to bring down the cost of wind-generated electricity?
Figure 6 shows how the cost of wind generated electricity changes as cost and
performance assumptions are varied about the mid-range assumptions in Table 3.
The lines showing variations in fixed and variable O&M and the lines showing
capital costs and carrying charges overlay one another. Although still significant,
changesin O&Massumptionsdonoteffecttheresultingcostas muchasdocarrying
charge, capital cost, or capacity factor. As with other large capital projects, project
finance (represented by the levelized carrying charge) can be as important to the
success of the project as the technology cost itself. Not surprisingly, capacity
factor also plays a large role. It must also be recognized that capacity factor is not
just the wind resource alone. The amount of scheduled maintenance a particular
technology requires and the amount of time a unit is unavailable due to unforeseen
outages also effects capacity factor.
How will the costs of windpower technologies change in the coming decade
and beyond? Arecentstudyexamining the possible impacts of introducing 10 GW
of windpower in the United States by 2006 assumed installed windpower capital
costs would drop to $600 per kW in 2006, owing largely to the economies of mass
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WINDPOWER 155
production (19). A U.S. Department of Energy (DOE)/Electric Power Research

Institute report the previous year had costs dropping to $740 per kW by 2005 (20).
The factors at work in these anticipated reductions are discussed by Neij (21), and
include not only the economies of mass production, but the increasing expertise
of the industry as it designs, builds, installs, and operates greater numbers of wind
turbines. Using data from Denmark, Neij calculated experience curves and rates
of technology improvement, and predicted that if a growth rate of 15%–20% can
be maintained, the cost of wind-generated electricity can drop by 45% over the
next 2 decades (21). For such significant cost reductions to occur, the application
of experience will certainly be needed, not only in the installation and operation
of wind turbines, but also in their design, materials selection, construction, and
siting.
Although such technology forecasting is a tricky business, it remains a valuable
exercise. Anotherfactor toconsiderin estimating thefuturecost ofwind-generated
electricity is the availablewind resource. Thereisa finite amount of land with high
quality Class 5 and 6 winds. How much of this land can be used for windpower,
owing to ecological, local acceptance, and other factors such as access to the high
voltage grid are always a topic of debate. Therefore, over the long run, capital
costs must drop such that more readily available Class 4 wind regimes can be
utilized. It is estimated that in the United States alone there are 232,000 km
2
of
Class 4 land within 10 miles of transmission facilities, nearly 8 times more land
area than there is for Class 5 and 6 wind regimes combined (20). Therefore, a
combination of capital cost drops and operating performance improvements are
required if the predicted cost of wind-generated electricity predictions are to occur
assuming Class 4 wind regimes. Larger land area Class 4 wind regimes also allow
greater siting flexibility, and may avoid some of the siting problems past wind
projects have experienced because they required wind ridge sites in order to be
economically viable.
With this ultimate tradeoff between cost reductions and the finite nature of high

quality onshore wind regimes, the following sections look at some of the more
fundamental aspects of wind energy engineering, beginning with wind turbine de-
sign and environmental considerations, the effects of site selection (offshore versus
onshore in particular), and recent technical advances and how they are effecting
the industry in the development and deployment of windpower.
WIND TURBINE DESIGN CONSIDERATIONS
The design of a wind turbine involves the integration of a large number of mechan-
ical and electrical systems. This process is subject to a variety of constraints that
directly effect the performance and economic viability of wind-generated elec-
tricity. As discussed above, the cost of electrical energy from a wind turbine is a
function of many factors, but the primary ones are the cost of turbine itself and its
annual energy productivity or capacity factor. These and other factors are directly
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influenced by turbine design and necessarily must be considered in the design.
The productivity of the turbine is a function both of the turbine’s design and the
wind resource. Whereas designers cannot control the wind resource, development
of wind turbines that maximize performance given the variability of the wind and
other meteorological factors is of paramount importance. Therefore, a fundamen-
tal tradeoff exists between low capital costs and robust operating performance.
Minimizing initial capital costs has far-reaching implications. It compels the
designer to minimize the cost of the individual components, which in turn pushes
him to consider the use of inexpensive materials. The impetus is also to keep the
weight of the components low, for a variety of reasons. On the other hand, the
resulting turbine must be strong enough to survive any likely extreme events and
operate reliably with a minimum of maintenance for a long time. Wind turbine
components, because they are kept light and flexible, tend to experience relatively

high, variable stresses. These periodic stresses result in fatigue damage, which
eventually leads to failure of the component, requiring its repair or replacement.
The need to balance the cost of the wind turbine with the requirement that the
turbine have a long, fatigue-resistant life is therefore a fundamental concern of the
designer.
Over the past decade, the general design of larger grid-connected machines
has converged, at least to some degree. The overwhelming majority are horizontal
axis machines, usually with three blades. Nearly all now utilize asynchronous
generators that, although they require power conditioning to match the generator’s
output to the grid, provide greater operational flexibility and energy capture from
the wind. Asynchronous generators are now even employed on fixed-speed wind
turbines.
It should be noted that within the wind community there are proponents of par-
ticular aspects of design, such as rotor orientation, number of blades, etc. A good
overview of these disparate design philosophies can be found in Doerner (22).
This debate is centered around the issue of how light a wind turbine can be and
still withstand operational and environmental stresses it will experience during its
intended service life. Similar issues are also discussed by Geraets et al (23). As
such, there are a wide variety of possible layouts or “topologies” for a wind tur-
bine. Most of these relate to the rotor. Below we discuss the design considerations
related to rotoraxis, orientation, rotational speed, and other general characteristics,
as well as aerodynamic power control and load management. Design considera-
tions regarding choice of tower structure, meteorological and other environmen-
tal factors, and issues related to maintenance and design certification are also
addressed.
Rotor Axis
A fundamental decision in the design of a wind turbine is the orientation of the
rotor axis—horizontal or vertical. In most modern wind turbines the rotor axis is
horizontal (parallel to the ground), or nearly so. The turbine is then referred to as
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WINDPOWER 157
a horizontal axis wind turbine (HAWT). There are two main advantages to having
the rotor axis horizontal. First, the rotor solidity of a HAWT (the total blade area
relative to its swept blade area) is lower when the rotor axis is horizontal (at a
given design tip speed ratio). This reduces capital costs on a per kilowatt basis.
Second, the rotor of a HAWT is more easily mounted on top of a tower, increasing
the average wind speed it is exposed to, therefore increasing productivity and
reducing costs on a per installed kilowatt basis.
The major advantage of a vertical axis rotor (resulting in a vertical axis wind
turbine, or VAWT)is that there is no need for a yaw system, which keeps the blades
pointed into the wind. The rotor can accept wind from any direction, at all times.
Another advantage is that in most VAWTs, the blades can have a constant chord
or cross-section, and no twist. These characteristics should enable the blades to
be manufactured relatively simply and cheaply (e.g. by aluminum extrusion). A
third advantage is that much of the drive train (gearbox, generator, brake) can be
located on a stationary tower, relatively close to the ground.
In spite of some promising advantages of the vertical axis rotor, the design has
not met with widespread acceptance. Many machines built in the 1970s and 1980s
suffered fatigue damage of the blades, especially at connection points to the rest
of the rotor. This was an outcome of the cyclic aerodynamic stresses on the blades
as they rotate and the fatigue properties of the aluminum from which the blades
were commonly made.
Currently, horizontal axis designs dominate the market. There are enough ad-
vantages, however, to the vertical axis rotor that it may be worth considering for
some future applications. In these cases, however, the designer should have a clear
understanding of what the limitations are, and should also have some plausible
options in mind for addressing those limitations.
Orientation
Therotorina HAWT maybe eitherupwindordownwindofthetower. Adownwind

rotor allows the turbine to have free yaw, like a weathervane, which is simpler and
therefore cheaper to implement than the active yaw control of upwind machines.
For free yaw to work effectively, the blades are typically coned a few degrees in
the downwind direction. However, with downwind machines the tower produces
a wake, or zone of lower wind speed, which introduces a cyclic variation in the
loading of blades as they pass behind the tower. This introduces additional fatigue
on the blades and drive train, and potentially fluctuations in the electrical power
generated by the turbine. Blade passage through the wake is also a source of noise.
The effects of the wake (also known as tower shadow) may be mitigated to some
extent via tower design. Upwind turbines normally have some type of active yaw
control. This usually includes a yaw motor, gears, and a brake to keep the turbine
stationary when it is properly aligned. Towers supporting turbines with active yaw
must be capable of resisting the torsional loads that will result from use of the yaw
system.
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Rotational Speed
Most rotors on grid connected wind turbines operate at nearly constant or fixed
rotational speeds, determined primarily by the requirements of the electrical gen-
erator and the gearbox. In some turbines, however, the rotor speed is allowed to
varyso that more energy from the wind can be captured (this is a result of matching
the aerodynamics of the rotor with the varying ambient airspeed). Another benefit
of variable speed operation is the potential to reduce loading on the wind turbine
rotor and drive train components. The choice of whether the rotor speed is fixed or
variable may have some impact on the overall turbine design, although generally
in a secondary way. For example, nearly all modern variable speed wind turbines
incorporate power electronic converters to ensure that the resulting electric power

is of the desired frequency and voltage. The presence of such a converter intro-
duces some flexibility in the choice of the generator. Using a low speed generator
can eliminate the need for a gearbox and have a dramatic effect on the layout of
the entire machine. The possible effects of electrical noise due to the power elec-
tronics in a variable speed turbine must also be taken into account in the detailed
design.
Rotor Characteristics
A rotor’s design tip speed ratio (the ratio of blade tip velocity to wind velocity)
is selected such that aerodynamic power coefficient is maximized. The longer a
wind turbine’s blades, the faster their tip speed, and the higher its tip speed ratio.
Selection of this value has a major impact on the design of the entire turbine. First
of all, there is a direct relation between the design tip speed ratio and the rotor’s
solidity (the area of the blades relative to the swept area of the rotor). A higher
speed rotor with longer blades will have less blade area, or solidity, than the rotor
of a slower machine. For a constant number of blades, the chord and thickness
of the blades will decrease as the turbine’s solidity decreases. Owing to structural
limitations, there is a lower limit to how thin the blades may be. Thus, as the
solidity decreases, the number of blades usually decreases as well.
There are a number of incentives for using higher tip speed ratios. First of all,
reducing the number of blades or their weight reduces the cost. Second, higher
rotational speeds imply lower torques for a given power level, and therefore less
stress on the turbine’s drive train and gearbox. This allows the drive train to be
lighter as well, further reducing costs. However, there are some drawbacks to high
tip speed ratios. For one thing, higher speed rotors tend to be noisier than slower
ones. Also, the performance degradation of higher speed airfoils from fouling (the
build-up of dirt and insects on blade surface’s leading edges) tends to be greater.
Most commercially available wind turbines used have three blades, although
some have two. Three or more blades have the advantage that the polar moment
of inertia with respect to yawing is constant, and is independent of the azimuth, or
vertical position, of the rotor. This characteristic contributes to relatively smooth

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WINDPOWER 159
operation even while yawing, or moving side-to-side. Machines with more than
three blades are rare, as the cost of additional blades is not offset by reductions
in these operational stresses. Two bladed rotors, on the other hand, have a lower
moment of inertia when the blades are vertical than when they are horizontal. This
causes additional cyclical stresses, similar to the effects of tower shadow. This
“imbalance” is one of the reasons that most two bladed wind turbines use a rotor
hinged at the hub (teetered rotor) to reduce stress (see 24).
Turbines with lower design tip speed ratios and higher solidities tend to be
relatively stiff. Lighter, faster turbines are more flexible. Flexibility may have
some additional advantages in relieving stresses, although this is less predictable.
Care must be taken when selecting the flexibility of blades for upwind machines,
as high loadings may cause the blade to flex back and strike the tower. Care
must also be taken to ensure that the natural frequency of flexible components
such as blades and towers are not in the range of the machines intended operating
environment.
Aerodynamic Power Control
There are a number of options for controlling power aerodynamically. In the event
of high winds, there must be a way to modify the aerodynamic power, or lift, such
that electricity generation can be maintained at desired levels, and the wind turbine
itself doesn’t sustain damage from excess mechanical loads. Common approaches
include stall, variable blade pitch, aerodynamic, and yaw control. The method of
aerodynamic power control will influence the overall design in a variety of ways.
Stall control reduces the aerodynamic lift caused by higher than desired wind
speeds by altering the wind speed’s angle of attack to the blades. To reduce this
additional lift and torque at high wind speeds it is necessary that the rotor’s speed
be controlled separately, such that the wind’s angles of attack can be altered.
The most common method of stall control is achieved via an induction generator

connected directly to the electrical grid. For this to work, blades in stall-controlled
machines are normally fastened rigidly to the rotor’s hub, a relatively simple
mechanical connection that can reduce costs. The nature of stall control, however,
is such that maximum power is reached at relatively high wind speeds, thereby
losing some of the energy available at lower wind speeds. Also, the drive train
must be designed to accommodate torque encountered under those conditions,
even though such winds may be relatively infrequent. Stall controlled machines
invariably incorporate separate braking systems to ensure that the turbine can be
shut down in extreme circumstances.
Variable pitch control machines have the ability to change the angle of the
bladesrelativetothe windbyrotatingthebladesabouttheirlongaxis.Variablepitch
providesmore control options than stall control, which assists electrical generation
performance. However, the hub mechanism is considerably more complicated, as
pitch bearings need to be incorporated. Some form of pitch actuation system must
also be included. In some wind turbines, only the outer part of the blades may be
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pitched. This is known as partial span pitch control. Another option, defined as
active stall control, is being used on an increasing number of large wind turbines
(greater than 1 MW). Active stall control combines the stall and pitch control
options (25).
Some wind turbines utilize aerodynamic surfaces on the blades to control or
modify power, similar to ailerons on a plane’s wings. These surfaces can take a
varietyof forms, suchas thetipbrakes onsome verysmall windturbines. However,
this can increase the structural and mechanical complexity of the blade, as means
must be found to not only incorporate the stall control device, but operate it as
well. In most cases aerodynamic surfaces are used for braking the turbine.

Another option for controlling power is yaw control. In this arrangement, the
rotoris turnedawayfromthewind, reducingpower.Thismethodofcontrolrequires
a robust yaw system. The hub must be able to withstand gyroscopic loads due to
yawing motion, but can otherwise be relatively simple. In general, this technique
is, in practice, used only for verysmallwindturbinesthat are designed to withstand
the cyclic varying stresses.
Dynamic Load Management at the Hub
As implied above, the hub of a horizontal axis wind turbine is an important com-
ponent of the overall design. The main options are rigid, teetered, or hinged. Most
wind turbines employ rigid rotors. This means that the blades are rigidly fixed
to the hub and cannot move in the flap-wise and lead-lag directions. The term
rigid rotor includes turbines with variable pitch blades. The rotors in two-bladed
turbines are usually teetered. That means the hub is mounted on bearings, and can
teeter back and forth, in and out of the plane of rotation. The blades in turn are
rigidly connected to the hub, so during teetering one blade moves in the upwind
direction, while the other moves downwind. An advantage of teetered rotors is
that the bending moments in the blades can be very low during normal operation,
extending their life. Some two-bladed wind turbines use hinges on the hub that
allow the blades to move into and out of the plane of rotation independently of
each other. Because the blade weights may not balance each other, other provi-
sions must be made to keep them in the proper position when the turbine is not
running, or is being stopped or started. One design variant is known as a gimballed
turbine. It uses a rigid hub, but the entire turbine, including gearbox and generator,
is mounted on horizontal bearings so that the machine can tilt up or down from
horizontal. This motion can help to relieve imbalances in aerodynamic forces.
Tower Structure
The tower of a wind turbine serves to elevate the main part of the machine up into
the air. For a horizontal axis machine the tower must be at least high enough to
keep the blade tips from touching the ground as they rotate. In practice, towers are
usually much higher than that. Winds are nearly always much stronger and less

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turbulent as elevation increases. All other things being equal, the tower should be
as high as practical. Choice of tower height is based on an economic tradeoff of
increased energy capture versus increased cost, including maintenance.
The principal options in towers are tubular and pipe type structures or trusses
(typically bolted). One of the primary considerations is the overall tower stiffness,
which also has a direct effect on its natural frequency. Stiff towers are those whose
fundamental natural frequency is higher than that of the blade passing frequency
(rotor’s rotational speed times the number of blades). They have the advantage of
being relatively insensitive to motions of the machine itself, but tend to be heavy
and therefore more expensive. Soft towers are those whose fundamental natural
frequency is lower than the blade passing frequency. A further distinction is com-
monly made, so that a soft tower’s natural frequency is above the rotor frequency
as well as being below the blade passing frequency. A soft-soft tower is one whose
natural frequency is below both the rotor frequency and blade passing frequency.
These towers are generally less expensive than stiffer ones, since they are lighter.
On the other hand, particularly careful analysis of the entire system is required to
ensure that no resonances are excited by any motions in the rest of the turbine.
Other factors in tower selection include the mode and cost of erection and
aesthetics. If a turbine is erected by tilting it up, there is a benefit to keeping the
tower as light as possible. If a crane is used, attention must be given to the sizes
of cranes expected to be available. If the tower is going to incorporate a lifting
capability, which would obviate the need for a crane, planning for that would be
needed early in the design process. In terms of aesthetics, it should be noted that
preference seems to lie with tubular designs. It should also be noted that tubular
towers appear to be preferable for minimizing impact on avian populations.
Other Design Constraints
There are a number of other factors that influence the design of wind turbines.

Some of these, as mentioned below, include environmental factors, the expected
wind regime, general climate, site accessibility, and availability of expertise and
equipment for installation and operation. Others include the need to withstand
extreme meteorological and other conditions, such as cold temperatures, icing,
extreme wind speeds, or turbulence, and salt spray.
For lower wind-speed sites, larger rotors can be used to increase power produc-
tivity. On the other hand, turbines designed for more energetic or turbulent sites
need to be stronger than those in more conventional sites. Expected conditions at
such sites must be considered if turbines are to meet international standards (26).
General climate can affect turbine design in a number of ways. Turbines in-
tended for use in marine climates need protection from salt, and should be built of
corrosion-resistant materials whenever possible. Turbines for use in hot climates
may need provisions for extra cooling, whereas turbines for cold climates may
require heaters, special lubricants, or even different structural materials.
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Turbines intendedforrelatively inaccessible siteshavetheirdesigns constrained
in a number of ways. For example, they might need to be self erecting. Difficulty
in transport could also limit the size or weight of any one component. Limited
availability of expertise and equipment for installation and operation would be of
particular importance for machines intended to operate singly or in small groups.
This wouldbeparticularly important for applications in remoteareasordeveloping
countries. Inthiscaseitwould be especially important to keep the machine simple,
modular, anddesignedtorequireonlycommonlyavailablemechanicalskills, tools,
and equipment.
Wind turbine proponents inevitably extolthe environmental benefits that accrue
to society through the use of wind-generated electricity. On the other hand, there

will always be some impacts on the immediate environment, and not all of those
impacts may be appreciated by its neighbors. Careful design, however, can min-
imize many of them. Three of the most commonly noted environmental impacts
of wind turbines are noise, visual appearance, and electromagnetic interference.
Some of these issues affecting overall wind turbine design will be discussed below
under environmental factors.
Maintenance Issues
An estimation of the O&M costs for new wind turbines is an important factor
in the determination of the energy production costs and the economic lifetime for
the wind energy system. Recent Danish research on the subject (27) has shown
that O&M costs constitute a sizeable amount of the total annual costs for a wind
turbine. For example, for a new wind turbine, O&M costs might represent 10% to
15% of the unit energy cost, but this cost increases to 20% to 30% towards the end
of the turbine’s life. Thus, once a wind farm has been installed at a given site, it is
important to know (and reduce, if possible) its operating costs.
Until recently, the prediction of O&M costs has been somewhat speculative.
Today, based on experience from the California wind farms and studies in the
United States and Europe, the determination of such costs can be carried out with
more confidence. For example, the most recent U.S. studies (19, 20), estimate that
O&M costs range from 1.0¢ per kWh in 1997 to 0.5¢ per kWh in 2005. At the
same time, the Danish Wind Turbine Manufacturers Association (28) note that
most people use 1.0¢ per kWh for O&M costs estimates.
More detailed O&M cost estimates can also be made using recent European
experience, specifically, work sponsored by the Danish Energy Agency. Lemming
et al (27) have shown that wind energy systems costs vary with turbine size and
age. Table 4 summarizes this work and givesestimatedO&Mcostsasapercentage
of investmentcosts(which include the costofthe turbine, control system, electrical
installation, and grid connection). Again, one should note the predicted increase
in costs as the age of the turbine increases.
It is also of interest to consider which components of the wind energy system

require repair or maintenance during the operating lifetime of the system. Figure 7
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TABLE 4 Comparison of total O&M costs as a function
of size and age of turbine. O&M Costs are expressed as a
percent of total wind farm installation costs (constant dollars)
Years from installation
Turbine
size 1–2 3–5 6–10 11–15 16–20
150 kW 1.2 2.8 3.3 6.1 7.0
300 kW 1.0 2.2 2.6 4.0 6.0
600 kW 1.0 1.9 2.2 3.5 4.5
(see color insert) shows the results of a German study (29) based on data from
about 1500 large scale wind turbines. More than 50% of the sources of equipment
failure are associated with “non-loaded” components; improved engineering and
higher specification standards should readily reduce these types of failures.
The estimation of O&M costs for offshore installations is a topic that is ex-
pected to be the subject of much future investigation. Here, due to the relative
inaccessibility of the site, it is important to reduce the incidence of unscheduled
maintenance problems.
Standards and Certification
Certification is a procedure by which an independent party gives written assu-
rance that a product, process, or service conforms to specified requirements. For
the case of wind turbines, the third party certification body develops standards to
specify the requirements that must be met. For example, this can include stan-
dards for safety and loads, quality assurance systems for wind turbine produc-
tion and installation, quality systems for certification bodies, and quality sys-
tems for measurement bodies. Of the several standards available for wind turbine
technology, the Wind Turbine Type Certificate is the most commonly sought by

manufacturers.
In Europe, certification is normally regulated by national authorities, and stan-
dardization is driven by governmental institutions and research centers in cooper-
ation with industry. This type of certification started in Europe in the 1980s, and
it is generally acknowledged (9) that the European type approval and certification
systems have helped the European wind industry develop a reliable and com-
petitive technology. In the United States certification is less used by authorities,
and standardization is primarily driven by industry. However, there is movement
in many countries outside Europe (including the United States) for wind turbine
certification, led by project financiers, insurance companies, and local building
authorities (30).
Under the guidance of the International Electrotechnical Commission (IEC),
significant work towards the international standardization of wind energy technol-
ogy has occurred over the past 10 years (31). The development of international
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Design Evaluation
Conformity
Statement
Evaluation of Control
and Protection
Systems
Evaluation of
Loads and Load
Cases
Evaluation of
Structural

Components
Evaluation of
Mech. and Electrical
Components
Evaluation of
Component
Tests
Evaluation of
Foundation Design
Requirements
Evaluation of
Design
Control
Evaluation of
Manufacturing
Plan
Evaluation of
Installation
Plan
Evaluation of
Maintenance
Plan
Evaluation of
Personnel
Safety
Figure8 EvaluationcomponentsoftheInternational ElectrotechnicalCommission“Wind
Turbine Type Certificate” (26).
standards, such as IEC 1400 (26), which was completed in 1998, has been a
slow and detailed process. To illustrate the details of this process, Figure 8 sum-
marizes the elements of the design evaluation module of the type certification

process.
Design evaluation does not necessarily require that a prototype of the wind
turbine type be manufactured and tested, as the certification documentation only
consists of drawings, analysis, descriptions, specifications, and schematics. How-
ever, the process does include an evaluation of manufacturing and an installation
and maintenance plan. During the process the certification body should evaluate
plans to verify that the requirements for manufacture, installation, commission-
ing, and maintenance are in accordance with the quality requirements in the design
documentation.
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It should be noted that there are two downsides to the certification of wind
turbines: the high cost and the potential for constrained or noninnovative wind
turbine designs. The first problem is especially important for the manufacturers
of small wind turbines, who may require some type of government assistance in
order to get certified (30). The second point is discussed by Garrad (32), who notes
that whereas certification may certainly help development, it constrains design. He
notes that that a continuous review of standards involving suppliers and customers
is needed to prevent both stagnation of the technology and unnecessary risks.
ENVIRONMENTAL DESIGN CONSIDERATIONS
Wind energy development has both positive and negative environmental impacts.
As discussed in Reference 18, the environmental benefits of windpower are cal-
culated not so much by the windpower itself, but by the avoided emissions from
other alternative sources. As such the environmental benefits of windpower are
stricter if it displaces the emissions from older, less efficient, dirtier generation
units, rather than newer units with greater emission controls. Most environmental
benefits come from the displacement of generation (megawatt-hour) as opposed
to capacity (megawatt). However, the displacement of capacity does have its ben-
efits, particularly if extension of the fuel and water supply infrastructure can be

avoided.
Although theseindirectenvironmentalbenefits are significant, wind farmdevel-
opers are all too aware that local acceptance and permitting is the first and highest
hurdle for windpower to jump. As more wind turbines and wind farms are intro-
duced into the United States, Europe, and elsewhere, their direct environmental
impacts have become a more significant issue. Whereas many publications have
focused on the positive environmental aspects such as reduced or displaced pol-
lutant emissions, with larger scale machines and wind farms under consideration,
local communities have become sensitized to many of the local environmental
issues faced when hosting wind energy generation. Failure to address these envi-
ronmental concerns can lead to projects being delayed or denied.
The following sections discuss some of the primary environmental design con-
siderations that windpower must address if it is to become a commonplace solution
in our long-term energy future. The potential negative impacts of wind energy can
be divided into the following major categories: land use impacts, avian interac-
tion, and local opposition. The last category includes three major factors: visual
impact, noise, and electromagnetic interference. A short review of each of these
potentially negative impacts of wind energy systems follows.
Land Use
In addition to local land-use regulations—such as zoning—one must consider
other land use impacts such as actual land area required per unit of generation, the
amount of land disturbed by a wind farm, nonexclusive land use and compatibility
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with existinguses, ruralpreservation, turbine density, and the need for access roads
and related erosion and/or dust impacts. As compared to other power plants, wind
generation systems are sometimes considered to be more land intrusive rather than

land intensive. On the other hand, while wind energy system facilities may extend
over a large geographic area, the physical “footprint” of the actual wind turbine
and supporting equipment only utilizes a small portion of the land.
In the United States wind farm facilities may occupy only 3% to 5% of the
wind farm’s total acreage, leaving the rest available for other uses. In Europe it
has been found that the percentage of land use by actual facilities is even less
than the California wind farms. For example, U.K. wind farm developers have
found that typically only 1% of the land covered by a wind farm is occupied by
the turbines, substations, and access roads. Also, in numerous European projects,
farm land is cultivated up to the base of the tower, and when access is needed
for heavy equipment, temporary roads are placed over tilled soil. Thus, European
wind farms only occupy from 1% to 3% of the available land.
One important factor in this is wind turbine spacing and placement. Wind farms
canoccupy from10to80acres (4to 32hectares)permegawattof installedcapacity.
The dense arrays of the California wind farms have occupied from about 15 to 18
acres (6 to 7 hectares) per megawatt of installed capacity. Typical European wind
farms have the wind turbines spread out more and generally occupy 30 to 50 acres
(13 to 20 hectares) per megawatt of installed capacity (7).
Because wind generation is limited to areas where weather patterns provide
consistent wind resources over a long season, the development of windpower
in the United States has occurred primarily in rural and relatively open areas.
These lands are often used for agriculture, grazing, recreation, open space, scenic
areas, wildlife habitat, and forest management. Wind development is generally
compatible with the agricultural or grazing use of a site. Although these areas
may be interrupted during construction, only intensive agricultural uses may be
reduced or modified during the project’s operation (33). In Europe, due to higher
population densities, there are many competing demands for land, and wind farms
have tended to be of a smaller total size.
The development of a wind farm may affect other uses on or adjacent to a
site. For example, some parks and recreational uses that emphasize wilderness

values and reserves dedicated to the protection of wildlife (e.g. birds) may not be
compatible with wind farm development. Other uses, such as open space preser-
vation, growth management, or nonwilderness recreation facilities may be com-
patible depending on set-backs, the nature of on-site development, and the effect
on resources of regional importance (33).
Avian Interaction
Environmental problems associated with avian interaction and wind systems sur-
faced in the United States in the late 1980s. It was discovered that birds, especially
federally protected golden eagles and red-tailed hawks, were being killed by wind
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turbines and transmission lines in wind farms in California’s Altamont Pass. This
information caused opposition to the Altamont Pass project among many environ-
mental activists and aroused the concern of the U.S. Fish and Wildlife Service,
which is responsible for enforcing federal species protection laws.
There are two primary concerns related to this environmental issue: (a) the
effects on bird populations from the deaths caused by wind turbines and (b) viola-
tions of the Migratory Bird Treaty Act, and/or the Endangered Species Act (even
if only one bird from a protected species is killed). This problem, however, is not
confined to the United States. For example, in Europe, major bird kills have been
reported inTarifa, Spain (a major point forbirdmigrationacross the Mediterranean
Sea) and at some wind plants in Northern Europe.
Wind energy development can affect birds in the following ways: bird elec-
trocution and collision mortality, altered foraging habits, altered migration habits,
reduction in habitat, and disturbed breeding and nesting (34). It should also be
pointed out that the same author states that wind energy development has the fol-
lowingbeneficial effects on birds: It protects land from more dramatic habitat loss,
provides perch sites for roosting and hunting, provides and protects nest sites on
towers and ancillary facilities, protects or expands prey base, and protects birds

from indiscriminate harassment (34).
Improved knowledge of bird behavior, habitat use, and migratory patterns can
help mitigate the adverse impacts of wind farms. However, some traits that char-
acterize a good wind site also attract birds. For example, mountain passes are
frequently windy because they provide a wind channel through a mountain range,
but for the same reason represent a seasonal flyway for migratory birds.
The risk of collision is the most obvious and direct effect, and many studies
have examineda broadrangeof mitigationoptions(34, 35). Theseincludeavoiding
migration corridors, installing fewer but larger turbines, avoiding micro habitats
(especially nesting sites), alternate tower designs, burying electric lines, and re-
moving nests from structures. Less direct options include prey base management
and the development and conservation of alternate habitats.
It should be pointed out that even if the initial research indicates that a wind
energy project is unlikely to seriously affect bird populations, further studies may
be needed to verify this conclusion. These could include monitoring baseline bird
populations and behavior before the project begins, then simultaneously observing
both a control area and the wind site during construction and initial operation.
In certain cases, operational monitoring might have to continue for years. With
respect to these considerations, a summary of the status of the U.S. DOE/National
Renewable Energy Laboratory (NREL) avian research program has been recently
presented by Sinclair (36).
Local Opposition
Visual Impacts One of windpower’s primary adverse environmental impacts,
and a major concern of the public, is its visibility (7). Unfortunately, compared
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to the other environmental impacts, visual impacts are the least quantifiable. For

example, the public’s perceptions may change with knowledge of the technology,
location of wind turbines, and many other factors. Although the assessment of a
landscape is somewhat subjective, professionals working in this area are trained
to make judgments on visual impact based on their knowledge of the properties
of visual composition and by identifying elements such as visual clarity, harmony,
balance, focus, order, and hierarchy (37).
Wind turbines generally need to be sited in well-exposed sites in order to be
cost-effective. It is also important for a wind engineer to realize that the visual
appearance of a wind turbine or a wind farm must be considered at an early stage
in the design process. For example, the degree of visual impact is influenced by
such factorsasthetype of landscape, the number anddesignofturbines,thepattern
of their arrangement, their color, and the number of blades.
Visual or aesthetic resources refer to the natural and cultural features of an
environmental setting that are of visual interest to the public. An assessment of
a wind project’s visual compatibility with the character of the project setting is
based on a comparison of the setting and surrounding features with simulated
views of the proposed project. To address the potential impacts, the National
Wind Coordinating Committee developed a list of questions to assess a project’s
potential impact on a “viewshed.” These include viewshed alteration, consistency,
and degradation, in addition to a project’s overall synergy with local preferences
on land use, aesthetics, and environmental resource use (33).
Itturnsoutthatthenumberandarrangementof windturbines canbe asignificant
factor. That is, a single wind turbine has only a visual relationship between itself
and the landscape, but a wind farm has a visual relationship between each turbine
as well as with the landscape (37). Overall, visual impact is a highly subjective
topic, and alternative approaches to mitigating visual impacts have been explored
(see several papers in Ratto & Solari—38). Mitigation options may include us-
ing local terrain to mask service roads and reduce erosion, use of low-profile
and unobtrusive buildings and electrical connections, and use of uniform color,
structure types, and surface finishes for wind turbines to minimize project visi-

bility in sensitive areas with large open spaces. (Note, however, that the use of
nonobtrusive designs and colors may conflict with efforts to reduce avian colli-
sions and may be in direct conflict with aeronautical requirements for distinctive
markings.) Additional approaches include modifying the relative location of dif-
ferent turbine types, densities, and layout geometry to minimize visualimpactsand
conflicts. Different turbine types and those with opposing rotation can be segre-
gated by buffer zones. Mixing of types should generally be avoided or minimized.
Mitigation steps related to wind farm installation and maintenance may also be
important (33).
In Europe, a number of investigators have developed some very sophisti-
cated and useful techniques that can be used to illustrate the visual intrusion
of a potential wind farm installation. Many of these approaches have employed
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computer graphics including 3-D modeling, and have been successfully used in
actual wind farm development projects (38). With these methods, “zones of vi-
sual impact” can be identified and avoided. An example of the use of this type
of technique, using geographical information systems is given by Kidner (39). Of
course, if a wind farm needs to install additional transmission corridors to deliver
its power to the grid, this adds another element to the visual impact, as well as
cost, equations. Therefore, proximity to population centers has both positives
and negatives, which in part explains the resurgence of interest in offshore wind
applications.
Noise Problems associated with wind turbine noise is one area where wind en-
ergyengineering can bedirectlyemployed.Althoughnoise levels canbemeasured,
the public’s perception of noise impacts remain somewhat subjective. Noise is de-
finedasanyunwanted sound.Concernsabout noisedependon thelevelof intensity,
frequency, frequency distribution, and patterns of the noise source; background
noise levels; terrain between emitter and receptor; and the nature of the noise

receptor. The effects of noise on people can be classified into three general cat-
egories: subjective effects—including annoyance, nuisance, and dissatisfaction;
interference with activities such as speech, sleep, and learning; and physiological
effects such as anxiety, tinnitus, or hearing loss (33). In almost all cases, the sound
levels associated with environmental noise produce effects only in the first two cat-
egories. The third is more associated with occupational safety. Whether a noise is
objectionable will depend on the type of noise (tonal, broadband, low frequency,
or impulsive) and the circumstances and sensitivity of the person (or receptor)
who hears it (33). Operating noise produced from wind turbines is considerably
different in level and nature than most large scale power plants, which can be
classified as industrial sources. Wind turbines are often sited in rural or remote
areas that have a corresponding ambient noise character. Furthermore, whereas
noise may be a concern to the public living near wind turbines, much of the noise
emitted from the turbines is masked by ambient or the background noise of the
wind itself.
The noise produced by wind turbines has diminished as the technology has
improved. That is, as blade airfoils have become more efficient, more of the wind
energy is converted into rotational energy, and less into acoustic noise. However,
even with this reduction in aerodynamic noise, there remains some mechanical
noise, arising from the wind turbine’s internal components. Wagner et al (40)
discusses the relative contribution of these various noise sources, including air-
borne and structure-borne sources and their relative decibel levels from 115 m
downwind ofa2MWwind turbine.
An appropriate noise assessment study should contain three major components:
a surveyof the existingambient background noise levels, a prediction (or measure-
ment) of noise levels from the turbine(s) at and near the site, and an assessment of
the acceptability of turbine noise levels (7).
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Electromagnetic Interference When a wind turbine is placed between a radio,
television, or microwave transmitter and receiver, it can sometimes reflect portions
of the electromagnetic radiation in such a way that the reflected wave interferes
with the originalsignalarriving at thereceiver. Some keyparametersthatinfluence
the extent of electromagnetic interference caused by wind turbines include type
of wind turbine (HAWT or VAWT), wind turbine dimensions, turbine rotational
speed, blade construction material, blade angle and geometry, and tower geometry.
Inpractice,theblade constructionmaterial androtationalspeed arekeyparameters.
Todayelectromagneticinterferencefromwind turbines is less likely because many
of the components that were previously made from metal are now made from
composite materials, although metallic lighting protection on some blade surfaces
still increases electromagnetic interference.
WIND RESOURCE CONSIDERATIONS
As discussed above, the cost-effectiveness of wind-generated electricity is com-
prised primarily of its capital and operating costs divided by its potential annual
generation. Modern wind turbines have improved to the point that their avail-
ability (the percentage of time a wind turbine is available to produce energy,
regardless of wind conditions) is 98% or better (20). As they can be expected
to be operational whenever it is windy, the primary determinant of their annual
generation will be the wind resource where they are sited. In Figure 5 the Ves-
tas wind turbine had a cut-in wind speed of 4 m per second, reached its maxi-
mum output of 600 kW at about 17 m per second, and a protective cut-out wind
speed of between 20 and 30 m per second depending on choice of rotor diameter
(14). These performance characteristics are representative of most large wind tur-
bines. However, average wind speeds are only one characteristic of a wind regime,
and the variability of that wind regime impacts not only the annual output of the
wind turbine or wind farm, but its earning potential in the competitive electric
market.

Although the average wind speed is the first best performance criterion for
a wind resource, there are many other important aspects of a wind regime that
should be considered when evaluating potential sites. The power in the wind, and
therefore the generation potential by WECS, is related to the cube of the wind
speed. Therefore, the distribution of the wind speed around—and particularly
above—its average is very important. Wind shear and the gustiness or turbulence
of the wind also impact a turbine’s generation potential. Topography, vegetation,
and other features, such as the closeness of neighboring wind turbines affect these
factors as well, as discussed by Frost & Asplinden (41). Such aspects as surface
roughness determine not only the generation potential, but the spacing and tower
height wind turbines require. The greater the surface roughness, the higher the
tower needs to be in order for it to tap the highest available local wind speeds.
Furthermore, altitude and temperature affects air density, and therefore the overall
generation potential of wind turbines.
P1: NRM
October 21, 2000 10:48 Annual Reviews AR118-06
WINDPOWER 171
TABLE 5 Comparative onshore and offshore wind resources
Month of Boston Cape Cod Difference in monthly
year airport Bay windspeed (Airport->Bay)
Jan. 5.57 7.73 2.15 38.6%
Feb. 6.05 7.77 1.72 28.4%
Mar. 6.21 7.74 1.53 24.6%
Apr. 5.73 6.36 0.63 11.0%
May. 5.21 5.52 0.31 6.0%
Jun. 5.33 5.64 0.31 5.9%
Jul. 4.91 4.70 −0.20 −4.1%
Aug. 4.96 4.91 −0.05 −1.0%
Sep. 4.97 5.74 0.77 15.5%
Oct. 5.28 6.54 1.27 24.0%

Nov. 5.22 6.59 1.37 26.2%
Dec. 6.12 8.34 2.22 36.3%
Ave. 5.46 6.47 1.00 18.4%
(m/s) (D m/s) (D% m/s)
Often overlooked in the discussion of windpower’s potential is the annual, sea-
sonal, and daily variability in wind. Like hydropower, there can be good and bad
wind years, particularly if you are relying on high wind periods for a large propor-
tion of your annual generation. Seasonal and daily distributions are also important.
Figure 9 (see color insert) shows the average hourly wind speed (1991–1993) by
month fortwosites near Boston, Massachusetts. Data for the topgraphisfor Logan
International Airport, Boston’s major airport located alongside the city’s harbor
(42). The bottom graph shows the wind speed distribution for an offshore site in
Cape Cod Bay, roughly 30 km due east of Boston (43). Measurement heights for
Logan Airport and Cape Cod Bay were 10 and 5 m, respectively. These are func-
tionally equivalent, owing to surface roughness considerations, and underestimate
the wind resources to some degree as they are well below common wind turbine
hub heights.
Inspection of Figure 9 shows some marked contrasts between the two wind
sites. First, the wind speeds of the Cape Cod Bay site are significantly higher,
and show less daily variation than the airport data. Table 5 shows that they are
18% higher on an annual basis, and over 35% higher in January and February. Of
courseairportsare notgenerally sitedwherewinds aretoo highorgusty, sowhereas
the comparison is valid, it should be noted that it is not between two candidate
windpower sites. In both cases, it is much windier in winter than summer months.
Also, the diurnal variation in wind speeds is less for the offshore site. The larger
diurnal fluctuations of the airport site are due in part to sea breezes and other local
phenomena.