Forces Behind Wind Power

Physical and Operational Characteristics of Wind Farm Turbines

Figure 1. Wind Energy System Schematic

To understand the advances in wind farm technology, general knowledge of a wind turbine and its components is essential. Recent advances in component design in addition to site-specific optimization have been instrumental in improving energy output and reducing operation and maintenance costs. The text box that follows below provides a brief summary of the components in a wind turbine (see also Figure 1). Physical Characteristics During the past quarter century, extensive public- and private-sector efforts were made to optimize wind turbine design, including development of advanced rotor blade materials, design concepts, advanced turbine designs, and other wind energy conversion systems (WECS) components, such as towers.

Turbine Component	Function
Nacelle	Contains the key components of the wind turbine, including the gearbox, yaw system, and electrical generator.
Rotor blades	Captures the wind and transfers its power to the rotor hub.
Hub	Attaches the rotor to the low-speed shaft of the wind turbine.
Low speed shaft	Connects the rotor hub to the gearbox.
Gear box	Connects to the low-speed shaft and turns the high-speed shaft at a ratio several times (approximately 50 for a 600 kW turbine) faster than the low-speed shaft.
High-speed shaft with mechanical brake	Drives the electrical generator by rotating at approximately 1,500 revolutions per minute (RPM). The mechanical brake is used as backup to the aerodynamic brake, or when the turbine is being serviced.
Electric generator	Usually an induction generator or asynchronous generator with a maximum electric power of 500 to 1,500 kilowatts (kW) on a modern wind turbine.
Yaw mechanism	Turns the nacelle with the rotor into the wind using electrical or other motors.
Electronic controller	Continuously monitors the condition of the wind turbine. Controls pitch and yaw mechanisms. In case of any malfunction (e.g., overheating of the gearbox or the generator), it automatically stops the wind turbine and may also be designed to signal the turbine operator's computer via a modem link.
Hydraulic system	Resets the aerodynamic brakes of the wind turbine. May also perform other functions.
Cooling system	Cools the electrical generator using an electric fan or liquid cooling system. In addition, the system may contain an oil cooling unit used to cool the oil in the gearbox.
Tower	Carries the nacelle and the rotor. Generally, it is advantageous to have a high tower, as wind speeds increase farther away from the ground.
Anemometer and wind vane	Measures the speed and the direction of the wind while sending signals to the controller to start or stop the turbine.

This section discusses the results of these efforts and their impact on enabling wind farm developers to optimize WECS design based on site requirements. Information focuses on technology deployed by Enron/Zond, Vestas, and NEG Micon, the current major wind farm developers in the United States.

Technology Advances for Improved Wind Farm Performance and Reliability. The current generation of utility-scale wind turbines uses technology developed over the past 20 years. Advances in technology have resulted in lower installed cost per kilowatt of a wind turbine, improved turbine performance, and improved turbine reliability and reduced maintenance cost.

Following are some of the major improvements that have made these benefits possible:

• Airfoil Design. Over the past 20 years, international research efforts have led to new airfoils designed specifically for horizontal axis wind turbines. In the United States, the Zond Energy Systems Z-750 series utility-scale turbines use airfoil designs developed at the National Renewable Energy Laboratory (NREL). The results of similar research by European manufacturers are incorporated into the blade design of European turbines. NREL's airfoils, when used with stall-regulated turbines, have produced 23 percent to 30 percent more electricity annually in the field.
  
  
• Structural Testing Improvements. Structural test bed  facilities  have  been constructed for full-scale testing of turbines. Tests are performed on prototypes to validate design assumptions, test materials, and make corrections. Testing includes fatigue testing, strength static testing, and non-destructive analysis such as photoelastic stress analysis. International efforts have resulted in safety and performance certification standards for wind turbines. In the United States, the Underwriters Laboratories, Incorporated (UL), certifies turbines using international standards issued by the International Electrotechnical Commission (IEC). The NREL National Wind Technology Center has developed test procedures to assess compliance with standards. For instance, their test procedures to assess compliance with power quality, structural load, blade structural load, power performance, and noise standards have been accepted by the American Association of Laboratory Accreditors and by certifying parties throughout the world. Additionally, NREL has developed a wind turbine design evaluation quality system to enable design certification by international organizations.
  
  
• Power Electronics Advances. Power electronics enable variable speed operation of the Zond Z-750 turbine, improving electricity generation efficiency and reducing structural loads by allowing a lightweight, low-cost configuration. In both the United States and Europe, improvements in inverter design⁽²¹⁾ and smart controls and reduction of the cost of such components has contributed to addressing power quality more cost-effectively. Remote access and control of wind systems via modem or satellite has also become common place in most sites.
  
  
• Smart Aerodynamic Control Devices. Smart, reliable controls reduce the likelihood that high winds and generator load loss will cause significant damage to turbines. In addition, such controls enable turbine operation to adapt to natural wind speed variations, insect-impact accumulations, and minor blade damage, which cause inefficient rotor output.
  
  
• Modeling and Wind Characterization Capabilities. New computer simulation codes allow a wide array of system architectures to be designed for various applications, while simulating results using local wind regimes for particular sites. Wind characterization has reached a greater degree of accuracy through the use of sophisticated instrumentation and monitoring capabilities.
  
  
• Capability to Optimize WECS Design. Currently, European turbine manufacturers supply the majority of the world market for utility-scale wind turbines.⁽²²⁾ Enron Wind Corporation's Zond Energy Systems subsidiary was the fifth largest manufacturer worldwide in 1999 with 9 percent of market. Zond is the only U.S. manufacturer presently manufacturing utility-scale turbines. Zond's Z-750 turbine is the first U.S. machine in several years to be installed in large numbers in wind power plants owned by independent power producers. Enron, which purchased Zond Energy Systems in California in 1996 and German manufacturer Tacke in 1997, has plans to develop a 1 MW next-generation turbine by 2002. In addition, another U.S. company, The Wind Turbine Company, has announced similar plans for a 1 MW machine. Both companies are developing their 1 MW-scale machines under DOE's Next-Generation Turbine Development Program.

The general trend is toward wind turbines with maximum power output of 1 MW or more. European firms--such as Danish companies Vestas and NEG Micon--currently have more than 10 turbine designs in the megawatt range with commercial sales. Due to decreasing wind development sites with adequate wind regimes on the landmass, Europe has recently focused on developing larger-than-megawatt turbines for offshore wind farms. Because expensive foundations are required for offshore applications, the cost of such wind plants can be up to 30 percent higher. However, due to stronger winds offshore (as well as the water's smoother surface than land), the higher production will offset the higher installation costs over the life of the facility. Aside from this, Vestas and Micon still lead the markets in manufacturing advanced, land-based, utility-scale turbines. In 1999, Micon and Vestas were the number one and number two wind turbine manufacturers worldwide, sharing about 40 percent of the global market.⁽²³⁾

Wind turbine design is dictated by a combination of technology, prevailing wind regime, and economics. Wind turbine manufacturers optimize machines to deliver electricity at the lowest possible cost per kilowatthour (kWh) of energy. Design efforts benefit from knowledge of the wind speed distribution and wind energy content corresponding to the different speeds and the comparative costs of different systems to arrive at the optimal rotor/generator combination. Optimizing for the lowest overall cost considers design factors such as relative sizes of rotor, generator, and tower height. For example, small generators (i.e., a generator with low rated power output in kW) require less force to turn than larger ones. Therefore, fitting a large wind turbine rotor with a small generator will produce electricity during many hours of the year (harvesting energy at lower wind speeds), but will capture only a small portion of high-speed wind energy. Conversely, a large generator will be efficient at high wind speeds, but unable to turn at low wind speeds. For a given turbine rated output (e.g., 750 kW), rotor diameter can be a design variable, specifying a smaller rotor diameter for turbines that will operate at sites with high wind speeds. In addition, system design can be optimized further and performance efficiency can be increased with innovative tower design, increased tower height to 50-70 meters (which increases energy output), and lighter weight turbines.

In general, most utility-scale wind turbines on the market today are three-bladed systems that use asynchronous generators and sophisticated controls to monitor and regulate turbine operation in different conditions and the quality of power delivered to the grid. The following synopses provide a general overview of the current technologies utilized by the three major utility-scale wind turbine manufacturers to optimize design.⁽²⁴⁾ NEG Micon has the simplest design while Zond the most complex design:

• NEG Micon. This design approach is the simplest of the three major manufacturers; the basic design is about 20 years old. The blades have a fixed pitch and rotate at a constant speed (fixed rpm). Parts are bolted to the frame in a way that makes it easy to remove and replace a part. The turbine is connected directly to the electricity grid. The power flowing through the grid is used to maintain a constant turbine speed through electromechanical means.
  
  
• Vestas. This turbine has a variable pitch design; a computer system controls blade pitch. Like the NEG Micon machine, the turbine operates at a constant speed. The Opti-Slip technology incorporated into the design allows slight speed variation to relieve stress on the turbine.⁽²⁵⁾ The Opti-Slip technology acts like a spring, allowing an increase in speed to relieve stress, then returning to a rated speed. Like the NEG Micon turbine, the Vestas machine is connected directly to the grid without power electronics; speed is controlled electromechanically by the grid.
  
  
• Zond. The Zond turbine has both a variable pitch blade design and a variable speed rotor and electric generator design. Together, these design elements enable the turbine to convert wind energy to rated turbine power output over a broader range of wind speeds than possible with the constant speed generator design employed in the NEG Micon and Vestas turbines. Because of the variable speed design, electricity from this turbine must flow through power conditioning equipment prior to entering the grid. The power conditioning equipment converts the variable frequency AC from the generator into DC, then (via an inverter) to 60 cycle AC that is also synchronous with the grid.

Operational Characteristics

Wind turbine manufacturers have developed basic wind turbine designs that can be modified to optimize the turbine for reliable operation at a specific site. The wind farm developer provides the manufacturer with site characteristics that will have an impact on the turbine's capacity factor and on the reliability of turbine operation. Factors include annual distribution of wind speed, annual variation in site temperature, frequency of lightning, and salty air in coastal regions. Modifications to enable operation in climates that are hotter or colder than the design temperature operating range, operation in coastal environments with salty air, and enhanced lightning protection will add to the cost of the turbine system. The following discussion covers some of these modifications.⁽²⁶⁾

Ability to Operate Over a Range of Wind Speeds. Currently available wind turbine designs enable reliable operation over a range of wind speeds. Rotor diameter can be modified from a standard diameter to one slightly larger for sites with low wind speeds or one slightly smaller for sites with high wind speeds.

Protecting Turbines in High Winds. Wind turbines are designed to operate up to a certain wind speed. Winds above this speed could damage the turbine, so all turbines are designed with a cut-off or shutdown mechanism. The following examples discuss such mechanisms for each major manufacturer:

• NEG Micon. The turbine operates at a fixed rotation per minute (rpm). Its blade is shaped so that the energy conversion efficiency of the turbine drops at high speeds and the turbine stalls. The turbine has two braking systems. The tip of each blade turns 90 degrees at high centrifugal force to exert drag that stops the blade. A disk brake system exerts hydraulic pressure to release the brake as long as electricity is available.
  
  
• Vestas. Blade pitch control is used to stall the turbine. Pitch control is achieved by feathering the blades. Disk brakes also can stop the machine.
  
  
• Zond. The blades have variable pitch control to enable feathering at wind speeds above the rated 50 to 60 mph range.

Ability to Operate in Hot or Cold Climates. In hot climates, the transmission cooling system is upgraded, and blades are made with epoxy resins that withstand heat and ultraviolet light. In cold climates, a heater is added to ensure that generator oil, transmission fluid, and hydraulic systems are maintained at adequate operating temperatures. Black blades are advantageous as a deicing mechanism in cold climates because they absorb heat. For example, the NEG Micon turbine operates optimally in the -20ºC to 35ºC range.⁽²⁷⁾ Below -20ºC, a cold weather package is installed; above 35ºC, a hot weather package is installed.

Ability to Operate in Coastal Salty Air. Paint sealants and nacelle designs that inhibit penetration of salty air are used to protect the turbine, generator, blades, and tower from corrosion. The sealant is baked on at the factory.

Lightning Protection. Lightning is attracted to the tallest structure in an area, making wind turbines a prime target. Turbines are designed with a lightning protection system, and lightning damage may be included in the warranty. For instance, Vestas offers "Total Lightning Protection" in its 600 kW and 1.65 MW turbines, providing a route for the lightning to travel through the turbine to the ground.⁽²⁸⁾ Vestas blades are protected by a 50 mm² copper conductor, enabling lightning to travel along the blade without a significant increase in temperature. The lightning travels from the blade to the blade hub into the nacelle. The rear of the nacelle is protected by a lightning conductor. Lightning protection in the nacelle protects the wind vane and anemometer. Lightning is carried down the tower to the earthing system through two parallel copper conductors. The earthing system, which provides grounding for the turbine, consists of a thick copper ring conductor placed one meter below the surface and one meter from the turbine's concrete foundation. The copper ring is attached to two diametrically opposed points on the tower and to two copper-coated earthing rods on either side of the foundation. Additionally, the turbine transformer is also protected.

Compatibility with Grid Power Quality. "Power quality" refers to voltage stability, frequency stability, and absence of various forms of electrical noise (e.g., flicker or harmonic distortion) on the electrical grid. Power companies deliver three phases of alternating current and power, each with a smooth sinusoidal shape, with few jags, breaks, or surges in any phase (less than 9 percent harmonic distortion). Once the wind is strong enough to turn the rotor and generator, the turbine connects and is synchronized to the grid's phase. Lack of synchronization may lead to rotor overspeeding and overtaxing of equipment components. The impact on the turbine could be costly equipment wear and tear.

Wind turbine designs and balance of system components are available currently that enable grid-connected wind farms to provide electric power in a form compatible with grid power quality. Different manufacturers have different solutions, as seen in the following examples:

• NEG Micon and Vestas. The design does not require power electronics to maintain power quality. The grid electromechanically controls the turbine to keep blade rotation speed at a fixed rotation rate (e.g., rpm). This control solves the power conditioning problem but captures less wind energy than do other solutions.
  
  
• Zond. Because the turbine design incorporates a generator that is variable speed rather than constant speed, power electronics are required in the design to maintain power quality. While power electronics add to system cost, they enable the turbine to convert more wind energy into electricity.

Electronic controllers in modern wind turbines prevent damage from power surges by constantly monitoring grid voltage and frequency. For example, disturbances in the grid may lead to "islanding," which refers to a power outage in one part of the grid while the wind-connected section of the grid is still supplied with power. If disturbances are large enough to cause islanding, electronic controllers automatically disconnect the turbines from the grid, and aerodynamic brakes are used to stop the rotor. As connection to the grid is re-established, electronic controllers protect the turbine from power surges.

An asynchronous or induction generator, which generates alternating current, is presently used for wind farm applications. These inexpensive generators may be described as an electric motor that operates in reverse, generating rather than consuming electricity. Wind cranks the rotor, which creates an electromagnetic force in the generator. The faster the rotor moves (greater than the generator stator's rotating magnetic field), the more current is induced in the generator and converted to electricity, which is fed into the grid. One of the most important properties of an induction motor is that it will reduce its speed, as increases in wind speed lead to an increase in torque on the motor, leading to less wear and tear on the gearbox. Another beneficial feature is that the generator must be magnetized by power from the grid before it works, facilitating its synchronization with grid power.

Current Federal R&D To Improve WECS Performance and Reliability

The objective of the U.S. Department of Energy (DOE) Wind Energy Program is to enable the U.S. wind industry to complete the research, testing, and field verification needed to fully develop cost-effective and reliable advanced wind technology.⁽²⁹⁾ Activities are classified under one of three research areas: applied research, turbine research (which includes large, utility-scale turbines), and cooperative research and testing. The cooperative research and testing activity offers the wind industry the facilities to test their turbines and turbine components and provides a turbine certification test program. This activity helps the industry control costs by limiting the extent to which turbine manufacturers in the United States need to invest in and staff such facilities.

Applied Research.⁽³⁰⁾ The Applied Research Program seeks to understand the basic scientific and engineering principles that govern wind technology and underlie the aerodynamics and mechanical performance of wind turbines. The program also seeks to improve the cost and reliability of different wind turbines by conducting applied research in the following areas:

• Aerodynamics and Structural Dynamics. The objective is to lower turbine cost and increase turbine life, possibly by developing lighter, more flexible turbines. Such turbines may be made possible through an understanding of complex wind/wind turbine interactions and using such information to improve design codes. Data for such analyses come from both highly instrumented experimental wind turbines and turbine testing in the NASA Ames Research Center low turbulence wind tunnel. The advantage of the low turbulence wind tunnel is that it enables three-dimensional testing of the dynamic response of full-scale wind turbines to steady wind inflow, as the tunnel eliminates normal atmospheric turbulence.
  
  
• Systems and Components. The objective of this research is to advance the design of wind turbine components and subsystems beyond the current generation. The Advanced Research Turbine (ART) Test Bed tests innovative approaches to component design. The highly instrumented ART turbines also support testing of large-scale turbine components such as generators, rotors, data acquisition systems, and controls. The ART Test Bed is being used in FY 2000 for the Long-Term Inflow and Structural Testing Program (LIST), which aims to understand inflow and resulting loads on turbines.
  
  
• Materials, Manufacturing, and Fatigue. This research aims to reduce capital and maintenance costs by improving blade strength and reliability during the manufacturing process. Activity areas include the development of advanced manufacturing techniques and blade fabrication and testing.
  
  
• Avian Research. This research uses analyses of bird deaths at current wind turbine sites to develop strategies to avoid bird fatalities. Research has addressed impacts of wind turbines on individual birds protected under legislation such as the Migratory Bird Treaty Act of the Endangered Species Act, as well as impacts on specific species. Research has been conducted to survey what species use a wind resource area, what part of the site they use, and when they use it. Research also focuses on studies of factors that may affect the impact of wind turbines on birds. Factors include analyses of the impact of topography, weather, habitat fragmentation, urban encroachment, habitat loss, species abundance, distribution, bird behavior, and turbine type and location. Preliminary results of survey and factors research indicate that wind turbines can be installed without causing any biologically significant impacts on bird species.

Turbine Research.⁽³¹⁾ The objective of this research is to assist the U.S. wind power industry in developing competitive, high-performance, reliable wind turbine technology for global energy markets. The program funds competitively selected industry partners in their development of advanced technologies. Wind turbines in various sizes from 10 kW to more than 1 MW are constructed and tested.

Currently, some of the research projects include: a Near-Term Research and Testing contract with Zond Energy Systems; Next-Generation Turbine Development contracts with the Wind Turbine Company and Zond Energy Systems; Small Wind Turbine Projects with Bergey Windpower Company, WindLite Corporation, and World Power Technology; and a cold weather turbine development contract with Northern Power Systems.

Cooperative Research and Testing. The Federal Government, through the National Wind Technology Center at the National Renewable Energy Laboratory, offers cooperative research, testing and certification, and standards programs to wind turbine manufacturers.⁽³²⁾ Without these programs, the industry would bear the costs, which would be reflected in a higher wind turbine cost. Cooperative research enables turbine manufacturers to leverage their R&D efforts with related Federal efforts and ensures, through commitment of manufacturer resources, that R&D worthwhile to them is pursued. Wind turbine blade testing includes three types of tests--ultimate static strength, fatigue, and nondestructive--to identify and correct problems before going into full-scale production. Modal testing provides useful information about the structural dynamic characteristics of a wind turbine system. This information is used to avoid designs that are susceptible to fatigue-related failure and excessive vibrations. Testing of full-scale wind turbine drivetrains on a 2.5 MW Dynamometer Test Stand located at NREL was initiated in mid-1999. The dynamometer can test turbine drivetrains as large as 2 MW both to identify weak points in the design and to measure the lifetime of systems. Receipt of certification services enable U.S. manufacturers to show that their turbines meet international standards; such certification is needed for U.S.-made turbines to sell in many foreign markets.

Operation and Maintenance for Wind Farm Turbines

Modern wind turbines are designed for about 120,000 hours of operation over a 20-year lifetime.⁽³³⁾ During this period, planned preventive maintenance and breakdown maintenance are performed. Additionally, system components may be replaced as their performance degrades; such replacements also are performed to extend the operating life of the turbine. Generally, maintenance costs are low for new turbines and increase as the turbine ages. Failure of wind turbine system components can be characterized by a relatively higher initial rate of failure followed by a lower failure rate through most of the turbine's design life until components begin to wear. During the initial period, assembly defects are detected and rectified. Commonly, wind turbines are sold with a 2- to 5-year manufacturer warrantee covering the cost to repair these design-related breakdowns.⁽³⁴⁾ Wind turbine models are available today for which minimal initial failure rate problems may be expected because the current turbine design is (1) a variation of past designs that have proven successful in the field and (2) manufactured with adequate quality assurance procedures. The reliability of new turbine designs improves over time as field experience enables resolution of technical problems. Field experience is particularly important for more complex designs, including those that deviate more from past design generations.

The average annual maintenance cost for newer turbines is approximately 1.5 percent to 2.0 percent of the cost of the machine.⁽³⁵⁾ Most of the maintenance expenses are associated with the routine service of turbines. Wind turbine manufacturers and service contractors certified to perform maintenance on a manufacturer's turbines can be contracted on an annual basis to perform planned preventive maintenance. For example, the cost of a preventive maintenance contract for a 750 kW turbine ranges from $12,000 to $14,000 per year, per turbine.⁽³⁶⁾ Maintenance on a 600 kW or 660 kW turbine can be performed for a comparable cost, $12,500 per year, per turbine.⁽³⁷⁾ Comparable maintenance on a 1.65 MW turbine would increase to $18,000 per year, per turbine.⁽³⁸⁾ Some analyses state the cost of preventive maintenance in terms of dollars per kilowatthour of electricity output. When expressed in these units, turbines with higher annual kilowatthours of electricity output have lower per-kilowatthour maintenance cost. A turbine with higher electricity output either has a higher maximum kilowatt output rating or a higher capacity factor. Such analyses have stated a maintenance cost of around $0.01 per kWh.⁽³⁹⁾ Larger generation capacity turbines are serviced at the same frequency and cost as smaller ones, which results in a lower maintenance cost per installed kW; however, over time stresses and strains inherent in operation of larger capacity turbines cause more wear and tear on system components, leading to accelerated component replacement.

Additionally, wind farms benefit from the economy of scale related to semi-annual maintenance visits, administration, and inspection. Wind farm operators increase the life of a turbine by replacing certain components, such as rotor blades, generators, and gearboxes, which are subject to more wear before the end of the turbine's design life. The price of replacement components is usually 15 percent to 20 percent of the price of the turbine and can extend the life of the turbine by the same or longer amount.⁽⁴⁰⁾

Planned Maintenance

Planned maintenance covers all preventive maintenance, including routine checks, periodic maintenance, periodic testing, blade cleaning, and high voltage equipment maintenance. Routine checks are performed monthly for every machine using a checklist that includes inspection of the gearbox and oil levels, inspection for oil leaks, observation of the running machine for unusual drive train vibrations, brake disc inspection, and inspection of all emergency escape equipment.

Periodic maintenance takes place approximately every 6 months and includes checking the security of all supports and attachments, high-speed shaft alignment, brake adjustment and pad wear, and yaw mechanism performance; greasing bearings; inspecting cable terminations; and replacing oil filters. For pitch-regulated machines, the pitch calibration is also checked. In addition, this may be the time to replace components that are known to fail after a few years of operation, such as anemometers, wind vanes, and batteries.

Periodic testing of the overspeed protection system should be conducted to ensure proper operation of this feature. Blade cleaning should be a maintenance consideration when the performance of the turbine is affected due to dirt buildup; however, because of the high cost of equipment for accessing the blades, this task should be evaluated for cost-effectiveness. High voltage equipment maintenance is usually contracted to the utility company.

Electrical Safety Maintenance

Regular maintenance of the turbine's electrical systems and a complete set of replacement parts minimize downtimes caused by electrical faults and ensure operational efficiency. A maintenance program may consist of monthly inspection of breakers, security, and battery voltages; annual checks of relay settings, oil levels, ground connections, and corrosion; 2-year interval testing of protection mechanisms, oil quality and levels, and high voltage circuit breakers; and 4-year inspections of all the switchgear, the grid transformer, and all wiring.

In addition, since some components need to be ordered, carrying a comprehensive set of replacement parts may be the difference between minor downtime or shutdown of the entire wind farm to await delivery. For this reason, a full set of protection relays, transformer windings, bushings, moving contacts, fuses, and gaskets must be stocked on-site.

Breakdown Maintenance

The frequency of wind turbine shutdowns or breakdowns is affected by operational factors and design complexity. More major system faults are generally categorized as human error, "acts of God," design faults, or system component wear and tear. Operational factors that affect breakdown frequency include overspeeding, excessive vibration, low gearbox oil pressure, yaw error, pitch error, unprompted braking, synchronization failure, loss of grid, and loss of batteries. A significant portion of wind turbine maintenance events can be detected by wind turbine system controllers, which can sense problems such as loose connections due to vibration or defective sensors.

Wind turbine designs, evolving with new research and development breakthroughs, have in some cases become more complex. Initially, a turbine that incorporates several new design concepts may experience an increase in breakdown frequency when compared to older proven turbine designs. Breakdowns may be caused by the design of a specific part or by problems that arise when parts incorporated into the new design do not function together as a system. Field experience enables technical problems to be detected, facilitating their resolution through additional development.

Beyond the initial period of resolving technical problems in a new turbine design, more complex machines may experience higher expenditures on periodic planned maintenance and higher replacement part costs. Expected higher expenditures do not necessarily reflect on the reliability of the turbine; they reflect more on the cost of maintaining and replacing complex parts. The cost-effectiveness of the turbine depends on such costs being covered by the incremental electricity production benefit that rationalizes the new design.

In Europe, gradual changes in wind turbine design during the past 20 years have been accompanied by testing and certification and by the hours of field experience needed to demonstrate wind turbine reliability. This process of turbine design, testing, certification, and field experience has resulted in the NEG Micon and Vestas wind turbines deployed in wind farms currently being developed in the United States and worldwide. In the United States, the U.S. Department of Energy, the National Renewable Energy Laboratory, and Underwriters Laboratories, Inc., have worked together to provide comparable turbine testing and certification for U.S. wind turbine companies.⁽⁴¹⁾

Summary

Research and development throughout the past 20 years has resulted in a current generation of utility-scale wind turbines, with maximum electricity generating capacity often exceeding 500 kW per turbine, designed for about 120,000 hours of operation over a 20-year lifetime. In the United States, wind farm development activity in 1999 was motivated by the June 1999 expiration of the Federal production tax credit, and dominated by installation of utility-scale turbines manufactured by NEG Micon and Vestas, both Danish firms, and by Zond Energy Systems, a subsidiary of Enron Wind Corporation, a U.S. firm. Research and development for utility-scale turbines has been directed toward increasing the amount of wind energy that a turbine can convert into electricity for the lowest amount of capital investment and the lowest on-going operating cost. Following are examples of the R&D efforts that have contributed to current utility-scale turbine technology:

• Improvements in the aerodynamics of wind turbine blades, resulting in higher capacity factors and an increase in the watts per square meter of swept area performance factor.
  
  
• Development of variable speed generators to improve conversion of wind power to electricity over a range of wind speeds.
  
  
• Development of gearless turbines that reduce the on going operating cost of the turbine.
  
  
• Development of lighter tower structures. A by-product of advances in aerodynamics and in generator design is reduction or better distribution of the stresses and strains in the wind turbine. Lighter tower structures, which are also less expensive because of material cost savings, may be used because of such advances.
  
  
• Smart controls and power electronics have enabled remote operation and monitoring of wind turbines. Some systems enable remote corrective action in response to system operational problems. The cost of such components has decreased. Turbine designs where power electronics are needed to maintain power quality also have benefitted from a reduction in component costs.

In the United States, the Zond Z-750 series turbine represents a very innovative but less gradual design change. Enron Wind Corporation wind farms, which use the Zond Z-750 technology, address the risk of the design innovation with performance contracts that guarantee turbine electricity production, in addition to power curve and reliability guarantees normally included in wind turbine performance contracts. The results of R&D have been incorporated into utility-scale wind turbine design more gradually in Europe, followed by operation in wind farms to assess reliability over time.

Near-term R&D efforts are expected to continue in directions that increase the efficiency with which wind turbines convert wind energy to electricity. For instance, researchers report that further optimization of blade design is possible.⁽⁴²⁾ Taller towers and rotor/generator systems with maximum power ratings exceeding 1 MW will continue to be improved. Other areas of development that affect turbine cost include advanced manufacturing methods and use of alternative, more cost-effective materials for turbine system, and tower fabrication.

The result of turbine R&D has been a reliable utility-scale wind turbine generator that can be optimized for operation in a variety of wind farm locations. For example, annual wind farm capacity factors of 28.5 percent to 32 percent have been achieved in Denmark and the United States, respectively, and capacity factors of 35 percent to 38 percent are projected for wind farm capacity that was recently installed in Minnesota and Iowa, respectively (Table 4).

42. U.S. Department of Energy and National Renewable Energy Laboratory, Wind Power Today, DOE/GO-102000-0966 (Washington, DC, April 2000), p. 31-32.

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Table of Contents

Incentives, Mandates, and Government Programs for Promoting Renewable Energy

"Technology, Manufacturing, and Market Trends in the U.S. and International Photovoltaics Industry"

"The Impact of Environmental Regulation on Capital Costs of Municipal Waste Combustion Facilities: 1960-1998"

"Forces Behind Wind Power" Page 1, Page 3, Appendix A

Glossary