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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.

W INDPOWER : A Turn of the Century Review

1Jon G McGowan and2Stephen R Connors

1 Department of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, Massachusetts 01003; e-mail: jgmcgowa@ecs.umass.edu, jgmcgowa@aol.com

2 The Energy Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02137-4307; e-mail: connorsr@mit.edu

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

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 early 1990s, with concerns over climate change and 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 changes in the wind industry itself, with particular attention paid to the range 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

sector will effect the market for windpower and, of course, how this impacts theincreasing 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 connected windpower in the world This is more than five and a half times theamount of installed capacity in 1990 (1, 3) Figure 1 (see color insert) shows howinstalled capacity has grown from 1995 through 1999, broken down by geographicregion (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, NorthAmerica and Europe had roughly the same amount of installed capacity (at 46%

grid-of the world’s total each) However, by the end grid-of 1999 Europe’s share grid-of totalinstalled capacity had risen to over two thirds From 1997 to 2000, Europe installednew 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 thesame period, wind capacity in Asia has quintupled, primarily due to efforts in India

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, offering 600 to 750 kW machines 1000 to 1600 kW machines arenow commercially available The latest models being developed range well above

2 MW, primarily for offshore applications Figure 2 shows the comparative heightand swept area for various machines Although rotor diameter and tower/hub heightvaries among manufactures, the variation is not overly large Tower height is themost variable, as site characteristics such as uniformity of the wind’s flow field,surface roughness, and visual impacts must be considered However, towers arecommonly one to one-and-a-half the rotor’s diameter in height A good overview

TABLE 1 Installed wind generating capacity (4, 5, 6)

Europe 2518 52.0 46.1 4766 62.8 35.9 8349 67.0 29.1North America 1676 34.6 −2.7 1615 21.3 0.2 2617 21.0 30.2Asia and Pacific 626 12.9 157.6 1149 15.1 24.5 1363 10.9 8.4

5000 kW

112 m

100 m

: Capacity: Rotor Dia.: Tower Hgt

Figure 2 Representative size, height and diameter of wind turbines

of how the size and performance of Danish wind turbines has changed over timecan 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 representnear-shore, sea floor mounted WECS installations As is discussed below, off-shore applications present a tradeoff between installed costs and maintenance andsuperior wind resources and lower land-use and community acceptance constraints

TABLE 2 Existing offshore wind installations (10)

Location Country Year Capacity No Size Manufacturer

Lely (Ijsselmeer) Netherlands 1994 2.00 4 500 NedWind

Dronten I (Ijsselmeer) Netherlands 1996 11.40 19 600 Nordtank

Total 26.10 49

WIND ENERGY APPLICATIONS AND ECONOMICS

How individual wind turbines are bundled into wind farms depends upon the windresource, topography, economics, and the sensitivity of local populations Figure 3(see color insert) shows some of the potential configurations a wind farm can take,including some prospective arrangements for offshore power The large wind farms

in California range from ridge-top arrays in the Altamont pass to large rectilineararrays near Palm Springs Europe, due in part to population density, has deployedits wind turbines in smaller groupings, as linear arrays or clusters of perhaps adozen machines each (7) Another important factor is the regulatory treatment ofgrid interconnections At what voltage level is the local utility comfortable withthe insertion of a variable power source? Furthermore, there may be economies

of scale for larger wind farms, especially if they are connecting to higher voltagetransmission lines for delivery to distant population centers Interest in offshoreapplications has increased because large high quality wind regimes are relativelyclose to population and load centers

As maintenance requirements drop and remote control and operation ties expand, the economics of co-location will diminish In areas where there aremore people, or existing agricultural land-uses, the “European model” of smallergroups of WECS allows better integration and synergy of windpower generationwith existing land uses Although the close proximity of wind may invite localopposition, if there is good community buy-in, owing in part to local economicand employment benefits, wind deployment can continue (11)

capabili-Of course, interest in windpower is not limited to grid-onnected power Asillustrated in Figure 4 (see color insert), large- and smaller-scale grid-connectedwindpower is only part of the picture For very rural areas, including village power

in developing countries, there is considerable interest in hybrid systems, or grids Recent experience in wind-diesel applications in Alaska and Canada focus

mini-on the delivery of reliable power, especially when already expensive fuel deliveriesare 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 competingagainst the grid price of power, they are measured by the value of the servicethey provide In developing countries this can be measured in terms of improvedmedical services, equivalent cents per lumen from a kerosene lamp, or clean andreliable water supplies On the far end of this spectrum are small single-use systems,generally associated with telecommunications and navigational applications Herethe electrical demand is generally for a continuous power source, rather than a largedemand 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 becost-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

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)

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 acapacity 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 PowerClasses have been included on the graph (15) As power output, and thereforegeneration, is related to the cube of the wind speed, slightly higher average windspeeds, or wind regimes with a higher variability in the high velocity range, canproduce significantly more power The very best wind sites tend to be Class 6

A Class 4 site is considered marginal by economic standards, especially whenthe 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 windturbines drop

The cost of wind-generated electricity is influenced by numerous factors Table 3shows 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 inline with recent experiences Costs are continuing to drop for windpower, and withturbine costs approaching $800 per kW, wind generated electricity costs of 4–5¢

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 outputGreenfield 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)

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 simplecalculation of annual fixed and variable costs divided by the expected generationsupplied to the grid The “Best Range” and “Worst Range” columns in Table 3 showhow this number changes if the combined best/optimistic and worst/pessimisticassumptions are used from a recent literature review (16) Greenfield overnightcosts represent the “all-in” cost of the generation facility including grid inter-connections and access roads, as well as wind turbine costs Fixed operation andmaintenance costs (O&M) refer to regularly scheduled servicing, while variableO&M includes utilization-based service and repairs Payments to landowners andtaxes can be either fixed or variable O&M, based upon contractual and other legalarrangements

The four to six cents number is currently one and a half to three times theaverage 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 electricitythese factors have to change, or some sort of subsidization or credit calculationmust occur By comparison, the total cost of generation for a new natural gas–firedunit can range from two to four cents per kWh, based upon technology and fuelcost assumptions (16)

In the last Annual Reviews chapter on windpower, Sørensen (17) discussedthe avoided environmental costs of choosing windpower over other options withpollutant emissions, as well as the life cycle impacts associated with mining,refining, fuel transportation, and combustion Also important in addressing thesocial costs of various generation alternatives are the risks of severe accidentsand longer-term fuel and solid waste issues The external costs of windpowerare not included in Table 3’s calculations Nor are there any credits given in thecalculation for subsidies such as the Production Tax Credit, or avoided expendituressuch as the cost of sulfur emissions allowances that U.S fossil units must nowconsider Portfolio benefits, as demonstrated in Reference 18, can reduce system-wide variability in costs and emissions, and have some synergistic benefits whencoupled with end-use efficiency efforts Such estimates of avoided environmentaland other costs are difficult to make without detailed analyses that incorporate the

●

●2.0

4.06.08.010.0

OvernightCostMid-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, whatopportunities are there to bring down the cost of wind-generated electricity?Figure 6 shows how the cost of wind generated electricity changes as cost andperformance assumptions are varied about the mid-range assumptions in Table 3.The lines showing variations in fixed and variable O&M and the lines showingcapital costs and carrying charges overlay one another Although still significant,changes in O&M assumptions do not effect the resulting cost as much as do carryingcharge, capital cost, or capacity factor As with other large capital projects, projectfinance (represented by the levelized carrying charge) can be as important to thesuccess of the project as the technology cost itself Not surprisingly, capacityfactor also plays a large role It must also be recognized that capacity factor is notjust the wind resource alone The amount of scheduled maintenance a particulartechnology requires and the amount of time a unit is unavailable due to unforeseenoutages also effects capacity factor

How will the costs of windpower technologies change in the coming decadeand beyond? A recent study examining the possible impacts of introducing 10 GW

of windpower in the United States by 2006 assumed installed windpower capitalcosts would drop to $600 per kW in 2006, owing largely to the economies of mass

production (19) A U.S Department of Energy (DOE)/Electric Power ResearchInstitute 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), andinclude not only the economies of mass production, but the increasing expertise

of the industry as it designs, builds, installs, and operates greater numbers of windturbines 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 thenext 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, andsiting

Although such technology forecasting is a tricky business, it remains a valuableexercise Another factor to consider in estimating the future cost of wind-generatedelectricity is the available wind resource There is a finite amount of land with highquality 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 highvoltage grid are always a topic of debate Therefore, over the long run, capitalcosts must drop such that more readily available Class 4 wind regimes can beutilized It is estimated that in the United States alone there are 232,000 km2ofClass 4 land within 10 miles of transmission facilities, nearly 8 times more landarea than there is for Class 5 and 6 wind regimes combined (20) Therefore, acombination of capital cost drops and operating performance improvements arerequired if the predicted cost of wind-generated electricity predictions are to occurassuming Class 4 wind regimes Larger land area Class 4 wind regimes also allowgreater siting flexibility, and may avoid some of the siting problems past windprojects have experienced because they required wind ridge sites in order to beeconomically viable

With this ultimate tradeoff between cost reductions and the finite nature of highquality onshore wind regimes, the following sections look at some of the morefundamental aspects of wind energy engineering, beginning with wind turbine de-sign and environmental considerations, the effects of site selection (offshore versusonshore in particular), and recent technical advances and how they are effectingthe 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 ical and electrical systems This process is subject to a variety of constraints thatdirectly effect the performance and economic viability of wind-generated elec-tricity As discussed above, the cost of electrical energy from a wind turbine is afunction of many factors, but the primary ones are the cost of turbine itself and itsannual energy productivity or capacity factor These and other factors are directly

mechan-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 thewind resource Whereas designers cannot control the wind resource, development

of wind turbines that maximize performance given the variability of the wind andother 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 thedesigner to minimize the cost of the individual components, which in turn pusheshim to consider the use of inexpensive materials The impetus is also to keep theweight of the components low, for a variety of reasons On the other hand, theresulting turbine must be strong enough to survive any likely extreme events andoperate reliably with a minimum of maintenance for a long time Wind turbinecomponents, because they are kept light and flexible, tend to experience relativelyhigh, variable stresses These periodic stresses result in fatigue damage, whicheventually 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 theturbine have a long, fatigue-resistant life is therefore a fundamental concern of thedesigner

Over the past decade, the general design of larger grid-connected machineshas converged, at least to some degree The overwhelming majority are horizontalaxis machines, usually with three blades Nearly all now utilize asynchronousgenerators that, although they require power conditioning to match the generator’soutput to the grid, provide greater operational flexibility and energy capture fromthe wind Asynchronous generators are now even employed on fixed-speed windturbines

It should be noted that within the wind community there are proponents of ticular aspects of design, such as rotor orientation, number of blades, etc A goodoverview 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 andstill withstand operational and environmental stresses it will experience during itsintended service life Similar issues are also discussed by Geraets et al (23) Assuch, 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 considerationsrelated to rotor axis, orientation, rotational speed, and other general characteristics,

par-as well par-as aerodynamic power control and load management Design tions regarding choice of tower structure, meteorological and other environmen-tal factors, and issues related to maintenance and design certification are alsoaddressed

considera-Rotor Axis

A fundamental decision in the design of a wind turbine is the orientation of therotor axis—horizontal or vertical In most modern wind turbines the rotor axis ishorizontal (parallel to the ground), or nearly so The turbine is then referred to as

a horizontal axis wind turbine (HAWT) There are two main advantages to havingthe rotor axis horizontal First, the rotor solidity of a HAWT (the total blade arearelative to its swept blade area) is lower when the rotor axis is horizontal (at agiven 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, increasingthe average wind speed it is exposed to, therefore increasing productivity andreducing costs on a per installed kilowatt basis.

The major advantage of a vertical axis rotor (resulting in a vertical axis windturbine, or VAWT) is that there is no need for a yaw system, which keeps the bladespointed 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) Athird advantage is that much of the drive train (gearbox, generator, brake) can belocated on a stationary tower, relatively close to the ground

In spite of some promising advantages of the vertical axis rotor, the design hasnot met with widespread acceptance Many machines built in the 1970s and 1980ssuffered 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 bladeswere commonly made

Currently, horizontal axis designs dominate the market There are enough vantages, however, to the vertical axis rotor that it may be worth considering forsome future applications In these cases, however, the designer should have a clearunderstanding of what the limitations are, and should also have some plausibleoptions in mind for addressing those limitations

ad-Orientation

The rotor in a HAWT may be either upwind or downwind of the tower A downwindrotor allows the turbine to have free yaw, like a weathervane, which is simpler andtherefore 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 inthe downwind direction However, with downwind machines the tower produces

a wake, or zone of lower wind speed, which introduces a cyclic variation in theloading of blades as they pass behind the tower This introduces additional fatigue

on the blades and drive train, and potentially fluctuations in the electrical powergenerated 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 someextent via tower design Upwind turbines normally have some type of active yawcontrol This usually includes a yaw motor, gears, and a brake to keep the turbinestationary when it is properly aligned Towers supporting turbines with active yawmust be capable of resisting the torsional loads that will result from use of the yawsystem

Rotational Speed

Most rotors on grid connected wind turbines operate at nearly constant or fixedrotational speeds, determined primarily by the requirements of the electrical gen-erator and the gearbox In some turbines, however, the rotor speed is allowed tovary so that more energy from the wind can be captured (this is a result of matchingthe aerodynamics of the rotor with the varying ambient airspeed) Another benefit

of variable speed operation is the potential to reduce loading on the wind turbinerotor and drive train components The choice of whether the rotor speed is fixed orvariable may have some impact on the overall turbine design, although generally

in a secondary way For example, nearly all modern variable speed wind turbinesincorporate power electronic converters to ensure that the resulting electric power

is of the desired frequency and voltage The presence of such a converter duces some flexibility in the choice of the generator Using a low speed generatorcan eliminate the need for a gearbox and have a dramatic effect on the layout ofthe 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 detaileddesign

intro-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 awind 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’ssolidity (the area of the blades relative to the swept area of the rotor) A higherspeed 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 structurallimitations, there is a lower limit to how thin the blades may be Thus, as thesolidity 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, higherrotational speeds imply lower torques for a given power level, and therefore lessstress on the turbine’s drive train and gearbox This allows the drive train to belighter as well, further reducing costs However, there are some drawbacks to hightip speed ratios For one thing, higher speed rotors tend to be noisier than slowerones Also, the performance degradation of higher speed airfoils from fouling (thebuild-up of dirt and insects on blade surface’s leading edges) tends to be greater.Most commercially available wind turbines used have three blades, althoughsome 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, orvertical position, of the rotor This characteristic contributes to relatively smooth

operation even while yawing, or moving side-to-side Machines with more thanthree 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 lowermoment of inertia when the blades are vertical than when they are horizontal Thiscauses 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 rotorhinged at the hub (teetered rotor) to reduce stress (see 24)

Turbines with lower design tip speed ratios and higher solidities tend to berelatively stiff Lighter, faster turbines are more flexible Flexibility may havesome 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 Caremust also be taken to ensure that the natural frequency of flexible componentssuch as blades and towers are not in the range of the machines intended operatingenvironment

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, suchthat electricity generation can be maintained at desired levels, and the wind turbineitself doesn’t sustain damage from excess mechanical loads Common approachesinclude stall, variable blade pitch, aerodynamic, and yaw control The method ofaerodynamic power control will influence the overall design in a variety of ways.Stall control reduces the aerodynamic lift caused by higher than desired windspeeds by altering the wind speed’s angle of attack to the blades To reduce thisadditional 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 generatorconnected directly to the electrical grid For this to work, blades in stall-controlledmachines are normally fastened rigidly to the rotor’s hub, a relatively simplemechanical connection that can reduce costs The nature of stall control, however,

is such that maximum power is reached at relatively high wind speeds, therebylosing some of the energy available at lower wind speeds Also, the drive trainmust be designed to accommodate torque encountered under those conditions,even though such winds may be relatively infrequent Stall controlled machinesinvariably incorporate separate braking systems to ensure that the turbine can beshut down in extreme circumstances

Variable pitch control machines have the ability to change the angle of theblades relative to the wind by rotating the blades about their long axis Variable pitchprovides more control options than stall control, which assists electrical generationperformance However, the hub mechanism is considerably more complicated, aspitch bearings need to be incorporated Some form of pitch actuation system mustalso be included In some wind turbines, only the outer part of the blades may be

pitched This is known as partial span pitch control Another option, defined asactive 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 controloptions (25).

Some wind turbines utilize aerodynamic surfaces on the blades to control ormodify power, similar to ailerons on a plane’s wings These surfaces can take avariety of forms, such as the tip brakes on some very small wind turbines However,this can increase the structural and mechanical complexity of the blade, as meansmust be found to not only incorporate the stall control device, but operate it aswell In most cases aerodynamic surfaces are used for braking the turbine.Another option for controlling power is yaw control In this arrangement, therotor is turned away from the wind, reducing power This method of control requires

a robust yaw system The hub must be able to withstand gyroscopic loads due toyawing motion, but can otherwise be relatively simple In general, this technique

is, in practice, used only for very small wind turbines that are designed to withstandthe cyclic varying stresses

Dynamic Load Management at the Hub

As implied above, the hub of a horizontal axis wind turbine is an important ponent of the overall design The main options are rigid, teetered, or hinged Mostwind turbines employ rigid rotors This means that the blades are rigidly fixed

com-to the hub and cannot move in the flap-wise and lead-lag directions The termrigid rotor includes turbines with variable pitch blades The rotors in two-bladedturbines are usually teetered That means the hub is mounted on bearings, and canteeter back and forth, in and out of the plane of rotation The blades in turn arerigidly connected to the hub, so during teetering one blade moves in the upwinddirection, while the other moves downwind An advantage of teetered rotors isthat 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 thatallow the blades to move into and out of the plane of rotation independently ofeach 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 notrunning, or is being stopped or started One design variant is known as a gimballedturbine 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 fromhorizontal 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 intothe air For a horizontal axis machine the tower must be at least high enough tokeep the blade tips from touching the ground as they rotate In practice, towers areusually much higher than that Winds are nearly always much stronger and less

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 ofincreased 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 whosefundamental natural frequency is higher than that of the blade passing frequency(rotor’s rotational speed times the number of blades) They have the advantage ofbeing relatively insensitive to motions of the machine itself, but tend to be heavyand therefore more expensive Soft towers are those whose fundamental naturalfrequency 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 whosenatural 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 toensure 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 andaesthetics If a turbine is erected by tilting it up, there is a benefit to keeping thetower 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 liftingcapability, which would obviate the need for a crane, planning for that would beneeded early in the design process In terms of aesthetics, it should be noted thatpreference seems to lie with tubular designs It should also be noted that tubulartowers 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 expectedwind regime, general climate, site accessibility, and availability of expertise andequipment for installation and operation Others include the need to withstandextreme 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 tivity On the other hand, turbines designed for more energetic or turbulent sitesneed to be stronger than those in more conventional sites Expected conditions atsuch 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 ofcorrosion-resistant materials whenever possible Turbines for use in hot climatesmay need provisions for extra cooling, whereas turbines for cold climates mayrequire heaters, special lubricants, or even different structural materials

produc-Turbines intended for relatively inaccessible sites have their designs 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 Limitedavailability of expertise and equipment for installation and operation would be ofparticular importance for machines intended to operate singly or in small groups.This would be particularly important for applications in remote areas or developingcountries In this case it would be especially important to keep the machine simple,modular, and designed to require only commonly available mechanical skills, tools,and equipment

Wind turbine proponents inevitably extol the environmental benefits that accrue

to society through the use of wind-generated electricity On the other hand, therewill always be some impacts on the immediate environment, and not all of thoseimpacts 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 belowunder 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 forthe wind energy system Recent Danish research on the subject (27) has shownthat O&M costs constitute a sizeable amount of the total annual costs for a windturbine For example, for a new wind turbine, O&M costs might represent 10% to15% 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 isimportant 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 theUnited States and Europe, the determination of such costs can be carried out withmore confidence For example, the most recent U.S studies (19, 20), estimate thatO&M costs range from 1.0¢ per kWh in 1997 to 0.5¢ per kWh in 2005 At thesame time, the Danish Wind Turbine Manufacturers Association (28) note thatmost people use 1.0¢ per kWh for O&M costs estimates

More detailed O&M cost estimates can also be made using recent Europeanexperience, specifically, work sponsored by the Danish Energy Agency Lemming

et al (27) have shown that wind energy systems costs vary with turbine size andage Table 4 summarizes this work and gives estimated O&M costs as a percentage

of investment costs (which include the cost of the turbine, control system, electricalinstallation, 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 systemrequire repair or maintenance during the operating lifetime of the system Figure 7

TABLE 4 Comparison of total O&M costs as a function

of size and age of turbine O&M Costs are expressed as apercent of total wind farm installation costs (constant dollars)

Years from installation Turbine

Standards and Certification

Certification is a procedure by which an independent party gives written rance that a product, process, or service conforms to specified requirements Forthe case of wind turbines, the third party certification body develops standards tospecify 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 turbinetechnology, the Wind Turbine Type Certificate is the most commonly sought bymanufacturers

assu-In Europe, certification is normally regulated by national authorities, and 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

stan-it is generally acknowledged (9) that the European type approval and certificationsystems 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 turbinecertification, led by project financiers, insurance companies, and local buildingauthorities (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

Design EvaluationConformity Statement

Evaluation of Control and Protection Systems

Evaluation ofLoads and Load CasesEvaluation of StructuralComponents

Evaluation ofMech and Electrical ComponentsEvaluation of ComponentTests

Evaluation of Foundation Design Requirements

Evaluation ofDesignControl

Evaluation of ManufacturingPlanEvaluation of InstallationPlan

Evaluation of MaintenancePlanEvaluation ofPersonnelSafety

Figure 8 Evaluation components of the International Electrotechnical Commission “WindTurbine Type Certificate” (26)

standards, such as IEC 1400 (26), which was completed in 1998, has been aslow and detailed process To illustrate the details of this process, Figure 8 sum-marizes the elements of the design evaluation module of the type certificationprocess

Design evaluation does not necessarily require that a prototype of the windturbine type be manufactured and tested, as the certification documentation onlyconsists of drawings, analysis, descriptions, specifications, and schematics How-ever, the process does include an evaluation of manufacturing and an installationand maintenance plan During the process the certification body should evaluateplans to verify that the requirements for manufacture, installation, commission-ing, and maintenance are in accordance with the quality requirements in the designdocumentation

It should be noted that there are two downsides to the certification of windturbines: the high cost and the potential for constrained or noninnovative windturbine designs The first problem is especially important for the manufacturers

of small wind turbines, who may require some type of government assistance inorder to get certified (30) The second point is discussed by Garrad (32), who notesthat whereas certification may certainly help development, it constrains design Henotes 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 culated not so much by the windpower itself, but by the avoided emissions fromother alternative sources As such the environmental benefits of windpower arestricter if it displaces the emissions from older, less efficient, dirtier generationunits, rather than newer units with greater emission controls Most environmentalbenefits come from the displacement of generation (megawatt-hour) as opposed

cal-to capacity (megawatt) However, the displacement of capacity does have its efits, particularly if extension of the fuel and water supply infrastructure can beavoided

ben-Although these indirect environmental benefits are significant, wind farm opers are all too aware that local acceptance and permitting is the first and highesthurdle for windpower to jump As more wind turbines and wind farms are intro-duced into the United States, Europe, and elsewhere, their direct environmentalimpacts have become a more significant issue Whereas many publications havefocused 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 environmentalissues faced when hosting wind energy generation Failure to address these envi-ronmental concerns can lead to projects being delayed or denied

devel-The following sections discuss some of the primary environmental design siderations that windpower must address if it is to become a commonplace solution

con-in our long-term energy future The potential negative impacts of wcon-ind energy can

be divided into the following major categories: land use impacts, avian tion, and local opposition The last category includes three major factors: visualimpact, noise, and electromagnetic interference A short review of each of thesepotentially negative impacts of wind energy systems follows

interac-Land Use

In addition to local land-use regulations—such as zoning—one must considerother land use impacts such as actual land area required per unit of generation, theamount of land disturbed by a wind farm, nonexclusive land use and compatibility

with existing uses, rural preservation, turbine density, and the need for access roadsand related erosion and/or dust impacts As compared to other power plants, windgeneration systems are sometimes considered to be more land intrusive rather thanland intensive On the other hand, while wind energy system facilities may extendover a large geographic area, the physical “footprint” of the actual wind turbineand supporting equipment only utilizes a small portion of the land.

In the United States wind farm facilities may occupy only 3% to 5% of thewind farm’s total acreage, leaving the rest available for other uses In Europe ithas been found that the percentage of land use by actual facilities is even lessthan the California wind farms For example, U.K wind farm developers havefound that typically only 1% of the land covered by a wind farm is occupied bythe 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 neededfor heavy equipment, temporary roads are placed over tilled soil Thus, Europeanwind farms only occupy from 1% to 3% of the available land

One important factor in this is wind turbine spacing and placement Wind farmscan occupy from 10 to 80 acres (4 to 32 hectares) per megawatt of installed capacity.The dense arrays of the California wind farms have occupied from about 15 to 18acres (6 to 7 hectares) per megawatt of installed capacity Typical European windfarms 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 provideconsistent 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, scenicareas, wildlife habitat, and forest management Wind development is generallycompatible with the agricultural or grazing use of a site Although these areasmay be interrupted during construction, only intensive agricultural uses may bereduced or modified during the project’s operation (33) In Europe, due to higherpopulation densities, there are many competing demands for land, and wind farmshave tended to be of a smaller total size

The development of a wind farm may affect other uses on or adjacent to asite For example, some parks and recreational uses that emphasize wildernessvalues and reserves dedicated to the protection of wildlife (e.g birds) may not becompatible 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 faced in the United States in the late 1980s It was discovered that birds, especiallyfederally protected golden eagles and red-tailed hawks, were being killed by wind

sur-turbines and transmission lines in wind farms in California’s Altamont Pass Thisinformation 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 notconfined to the United States For example, in Europe, major bird kills have beenreported in Tarifa, Spain (a major point for bird migration across the MediterraneanSea) and at some wind plants in Northern Europe

Wind energy development can affect birds in the following ways: bird trocution and collision mortality, altered foraging habits, altered migration habits,reduction in habitat, and disturbed breeding and nesting (34) It should also bepointed out that the same author states that wind energy development has the fol-lowing beneficial effects on birds: It protects land from more dramatic habitat loss,provides perch sites for roosting and hunting, provides and protects nest sites ontowers and ancillary facilities, protects or expands prey base, and protects birdsfrom indiscriminate harassment (34)

elec-Improved knowledge of bird behavior, habitat use, and migratory patterns canhelp mitigate the adverse impacts of wind farms However, some traits that char-acterize a good wind site also attract birds For example, mountain passes arefrequently 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 studieshave examined a broad range of mitigation options (34, 35) These include avoidingmigration 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 managementand the development and conservation of alternate habitats

It should be pointed out that even if the initial research indicates that a windenergy project is unlikely to seriously affect bird populations, further studies may

be needed to verify this conclusion These could include monitoring baseline birdpopulations and behavior before the project begins, then simultaneously observingboth a control area and the wind site during construction and initial operation

In certain cases, operational monitoring might have to continue for years Withrespect to these considerations, a summary of the status of the U.S DOE/NationalRenewable Energy Laboratory (NREL) avian research program has been recentlypresented 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

to the other environmental impacts, visual impacts are the least quantifiable Forexample, the public’s perceptions may change with knowledge of the technology,location of wind turbines, and many other factors Although the assessment of alandscape 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 becost-effective It is also important for a wind engineer to realize that the visualappearance 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 bysuch factors as the type of landscape, the number and design of turbines, the pattern

of their arrangement, their color, and the number of blades

Visual or aesthetic resources refer to the natural and cultural features of anenvironmental 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 isbased on a comparison of the setting and surrounding features with simulatedviews of the proposed project To address the potential impacts, the NationalWind Coordinating Committee developed a list of questions to assess a project’spotential 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)

It turns out that the number and arrangement of wind turbines can be a significantfactor That is, a single wind turbine has only a visual relationship between itselfand 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 subjectivetopic, 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-profileand 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 ofnonobtrusive designs and colors may conflict with efforts to reduce avian colli-sions and may be in direct conflict with aeronautical requirements for distinctivemarkings.) Additional approaches include modifying the relative location of dif-ferent turbine types, densities, and layout geometry to minimize visual impacts andconflicts 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 beimportant (33)

In Europe, a number of investigators have developed some very cated and useful techniques that can be used to illustrate the visual intrusion

sophisti-of a potential wind farm installation Many sophisti-of these approaches have employed

computer graphics including 3-D modeling, and have been successfully used inactual 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) Ofcourse, if a wind farm needs to install additional transmission corridors to deliverits power to the grid, this adds another element to the visual impact, as well ascost, equations Therefore, proximity to population centers has both positivesand negatives, which in part explains the resurgence of interest in offshore windapplications

Noise Problems associated with wind turbine noise is one area where wind ergy engineering can be directly employed Although noise levels can be measured,the public’s perception of noise impacts remain somewhat subjective Noise is de-fined as any unwanted sound Concerns about noise depend on the level of intensity,frequency, frequency distribution, and patterns of the noise source; backgroundnoise levels; terrain between emitter and receptor; and the nature of the noisereceptor 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 physiologicaleffects such as anxiety, tinnitus, or hearing loss (33) In almost all cases, the soundlevels associated with environmental noise produce effects only in the first two cat-egories The third is more associated with occupational safety Whether a noise isobjectionable will depend on the type of noise (tonal, broadband, low frequency,

en-or impulsive) and the circumstances and sensitivity of the person (en-or recepten-or)who hears it (33) Operating noise produced from wind turbines is considerablydifferent in level and nature than most large scale power plants, which can beclassified as industrial sources Wind turbines are often sited in rural or remoteareas that have a corresponding ambient noise character Furthermore, whereasnoise may be a concern to the public living near wind turbines, much of the noiseemitted from the turbines is masked by ambient or the background noise of thewind itself

The noise produced by wind turbines has diminished as the technology hasimproved That is, as blade airfoils have become more efficient, more of the windenergy is converted into rotational energy, and less into acoustic noise However,even with this reduction in aerodynamic noise, there remains some mechanicalnoise, 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 mdownwind of a 2 MW wind turbine

An appropriate noise assessment study should contain three major components:

a survey of the existing ambient background noise levels, a prediction (or ment) of noise levels from the turbine(s) at and near the site, and an assessment ofthe acceptability of turbine noise levels (7)

measure-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 interfereswith the original signal arriving at the receiver Some key parameters that influencethe extent of electromagnetic interference caused by wind turbines include type

of wind turbine (HAWT or VAWT), wind turbine dimensions, turbine rotationalspeed, blade construction material, blade angle and geometry, and tower geometry

In practice, the blade construction material and rotational speed are key parameters.Today electromagnetic interference from wind turbines is less likely because many

of the components that were previously made from metal are now made fromcomposite materials, although metallic lighting protection on some blade surfacesstill increases electromagnetic interference

WIND RESOURCE CONSIDERATIONS

As discussed above, the cost-effectiveness of wind-generated electricity is prised primarily of its capital and operating costs divided by its potential annualgeneration 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

com-to be operational whenever it is windy, the primary determinant of their annualgeneration 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 windspeed 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 thewind turbine or wind farm, but its earning potential in the competitive electricmarket

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 thatshould be considered when evaluating potential sites The power in the wind, andtherefore the generation potential by WECS, is related to the cube of the windspeed Therefore, the distribution of the wind speed around—and particularlyabove—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 thesefactors as well, as discussed by Frost & Asplinden (41) Such aspects as surfaceroughness determine not only the generation potential, but the spacing and towerheight wind turbines require The greater the surface roughness, the higher thetower 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 overallgeneration potential of wind turbines

TABLE 5 Comparative onshore and offshore wind resources

Month of Boston Cape Cod Difference in monthly year airport Bay windspeed (Airport->Bay)

sea-Inspection of Figure 9 shows some marked contrasts between the two windsites 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 are18% higher on an annual basis, and over 35% higher in January and February Ofcourse airports are not generally sited where winds are too high or gusty, so whereasthe comparison is valid, it should be noted that it is not between two candidatewindpower 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 largerdiurnal fluctuations of the airport site are due in part to sea breezes and other localphenomena

The data from Cape Cod Bay are equivalent to some of the best wind regimesfor New England, most situated in the mountains near the Canadian border Unlikethese sites, however, offshore wind farms are at sea level and generally much closer

to load centers It is expected that even with transmission system upgrades, therewould still be significantly higher transmission losses, on the order of 15% to 25%,

to export wind generation from rural, northern New England to the more heavilypopulated southern New England (18) Advances in high voltage transmissiontechnologies may reduce these line losses, but they add to the cost of deliveredwindpower, especially if the cost of the transmission upgrade cannot be sharedwith other uses Mountainous sites face other drawbacks, such as greater windshear along ridges, the need for higher towers to get WECS above surroundingtrees and forests, and opposition from environmental and nature groups who object

to their close proximity to protected wilderness areas and recreational uses such

as The Appalachian Trail

Seasonal and daily distributions impact both the cost and environmental fectiveness of windpower In New England for example, the need for power isgreatest in the summer, when electricity demand for air conditioning in south-ern New England is highest Not only is this true when generating capacity isneeded most, but when revenues from the sales of electricity are highest Simi-larly, baseload power in the early morning has a significantly lower market pricethan peak-coincident power on summer afternoons or winter evenings In theNew England example, we see that wind-generated electricity is not likely to be

ef-in synch with regional electricity demands, and that New England wef-ind resourcesare not likely to displace the need for additional generating capacity needed duringperiods of peak demand in the summer

These temporal and spatial characteristics are becoming much more importantfor developers in places such as North America and Europe, who intend to sell theirelectricity in the competitive market place Of course, alternatives exist, as will bediscussed below Although some area’s wind resources are well matched with localneeds, such as the Altamont Pass in California—where afternoon sea breezes matchelectricity demands, this is not true everywhere However, the greater the annualwind generation available from a site, the less binding are the temporal synergies It

is therefore no surprise that interest in offshore windpower has increased in recentyears, with its superior winds, in both quantity and quality and lower competitionwith land uses, as well as its proximity to coastal cities Of course, the development

of offshore resources does have some drawbacks, such as the need for materials thatwithstand salt spray, increased installation and maintenance costs, and navigationalconsiderations, but this again speaks to the fundamental tradeoff between cost andgeneration performance

As mentioned above, the environmental performance of windpower is bestcharacterized by the generation it displaces The temporal aspects of the windregime come into play here as well While greenhouse gases, and to some ex-tent sulfur emissions, have little in the way of seasonal impacts, nitrogen oxides(NO) emissions—a major precursor to ozone and smog formation—do In the

Northeastern United States strict emissions controls on NOxemissions and otherozone precursors are only active during summer months, thereby reducing wind-power’s environmental performance, at least with respect to smog Therefore, abetter understanding of wind regimes, not only of how they impact wind turbineand wind farm economics and performance but also of regional needs for electricpower and emissions reductions, is required.

RECENT ADVANCES IN WIND TECHNOLOGY

Advances in wind energy system technology during the 1990s have producedmajor successes in the following three areas (44):

1 Cost of delivered energy This success has occurred as a result ofcontinued technology improvements, increased size and number of sales,and increased financial confidence

2 Flexibility of wind technology Because wind energy systems represent amodular technology, it can be added in relatively small steps, making iteasier to speed up or slow down introductions to meet immediate economiccircumstances Also, wind technology is relatively easy to transfer, making

it attractive to developers in expanding international markets

3 Availability The availability, or fraction of time that a wind turbine isavailable to produce power has increased to the point where values of 98%

to 99% are typical for established wind farms This high level ofavailability represents values that are higher than many conventional utilityscale power generation systems

Furthermore, it has been pointed out (44) that a great deal of credit for thesuccess of the wind industry has gone to the movement to integrated design andmanufacture of wind turbines during the 1990s A major factor here has been thedevelopment of mathematical models for wind turbine components, wind turbines,

and wind farms that have resulted in (a) the development of analytical tools that allow detailed modeling of the behavior of wind turbines and (b) the development

of design tools that can be used to predict the behavior of wind turbines in a windfarm, and to predict the long-term wind resource at any site

Numerous computer modeling and analysis codes are used today by a ety of users and for many purposes The codes have been developed by industry,industry consultants, national wind research programs, and university researchprograms Furthermore, these codes are used by wind turbine manufacturers, uni-versity researchers, windpower educational programs, wind energy system projectdevelopers, industry analysts, and certification agencies

vari-These computational codes can be divided into three categories: codes for

(a) modeling, (b) data collection and analysis, and (c) operation and control In

general, the modeling codes are digital models of physical systems The modeling

codes can be subdivided into programs that are used for machine design (the design

of turbine components and complete turbines) and those that are used for turbinesystem design (the design of wind farms) Data collection and analysis codesare used to collect and analyze data from wind resource measurement studiesand hardware testing Operational and control codes are used for the control ofindividual wind turbines and the control of wind farms

Figure 10 illustrates the relationships between machine design and turbine tem design codes (45) Each of the codes applies the expertise gained in a specificdiscipline to the larger task of designing and operating a wind energy system Asshown in the figure, each of the codes works in conjunction with other codes used

sys-in the design process Often the output of one computer modelsys-ing code can beused as input for another In the process of designing a wind turbine, the short-term turbulence wind modeling codes are used to provide realistic wind inputs forthe aerodynamic and turbine design codes The turbine design codes also use theresults of component design codes to determine expected turbine performance.This performance data is used with long-term wind projections and topographicdata to optimize power plant performance

A detailed review of available codes is beyond the scope of this review ever, recent work at the University of Massachusetts has addressed this subject (45).The following discussion will concentrate on a select group of technical compo-nents and systems where advances in wind energy systems are now occurring

How-Rotor and Blades: Aerodynamics

The aerodynamic design of wind turbines continues to be a major factor in thedevelopment of wind turbines As discussed in a detailed review of this subject(46), significant work has been carried out in the design of specialized and tailoredairfoils for horizontal axis wind turbine rotor blades

In the United States, this work has been carried out by a joint effort betweenthe National Renewable Energy Laboratory (NREL) and Airfoils, Inc Their effort(47) has resulted in the development of more than 10 airfoil families designedspecifically for wind turbines A characteristic of these airfoils is that their max-imum lift coefficient is insensitive to airfoil surface roughness as compared toairfoils that were derived from aircraft applications Also, these airfoil familieswere designed to address the needs of stall regulated, variable pitch, and variablerotational speed wind turbines An application of the use of this family of air-foils was recently conducted by Giguere et al (48), who carried out a systematicblade design study using blade design tradeoffs for a 750 kW stall-regulated windturbine

In Europe, a number of researchers have also designed airfoils specificallyfor stall- or pitch-regulated wind turbine applications (49, 50) In the most recentexample of this work, researchers from Risø National Laboratory in Denmark (51)have designed an airfoil family (six airfoils) that were applied to the design of a

600 kW rotor They obtained the following characteristics for this airfoil family: