The U.S. market fluctuates from year to year depending on the status of the federal production tax credit (PTC). The federal PTC expired on December 31, 2003 and was extended to December 2005 by tax legislation passed by Congress in September 2004. Since then the PTC has been extended three times to December 2007, 2008 and 2009 by Congress. As a result of the PTC extensions, wind capacity additions have soared in the United States.
The power in the wind varies proportionally with the cube of the wind speed, which has important bearing on the design and citing of wind turbines. As a result, even a small increase in wind speed can substantially boost the power available in the wind. For example, a 25% increase in wind speed approximately corresponds to a doubling in the power contained in the wind, which illustrates the importance of accurate resource assessment to a project’s success.
Accurate assessment of the quality of the wind resource at a proposed project site is a critical first step to the success of that project. Quality can vary significantly from site to site. Obviously, some locations are windier than others; and even within a known wind resource area, the wind resource can vary with location and terrain.
Evaluating wind resource quality is further complicated by the fact that for a given site, wind resources generally exhibit seasonal, diurnal, and hourly variations. Wind resource quality is characterized by wind speed and direction, the wind shear or variation of wind speed with elevation, and the intensity of turbulence.
Prior to final site selection, the wind resource is measured for an extended period of time, usually two to three years, to statistically quantify the resource. A meteorological tower or mast is erected at one or more locations to continuously measure wind speed, direction, temperature, and sometimes other weather parameters. The measurements are made at multiple elevations above the ground (typically 10, 30, and 60 meters) to allow the wind shear to be estimated. The resulting data are stored onsite by a data logger and periodically downloaded onsite or remotely by modem. Data are analyzed to resolve erroneous values and calculate average wind speeds, directions, and temperatures over annual, seasonal, monthly, and hourly time intervals. The information is often expressed in wind speed frequency distributions and wind roses, which graphically show the relative frequency of wind speed and direction and wind energy.
Wind energy is divided into seven classes based on the wind speed measured at a height of 50 m (164 ft) above grade. The wind power is classified from Class 1 to Class 7 with a classification of one being a low wind speed at less than 5.6 m/s (18.4 ft/s) and seven being wind with a speed greater than 8.8 m/s (28.9 ft/s). As would be expected, strong, frequent winds are the best for generating electricity. Currently, areas with wind speeds of Class 5 and higher are being used with large wind turbines with the future goal of utilizing Class 4 sites.
Over the years, many improvements have been made in wind resource assessment, significantly expanding the size and nature of wind energy resource knowledge. Because techniques of wind resource assessment have improved greatly, more detailed high-resolution wind resource maps have been developed. Wind is distributed unevenly around the country.
The average wind resource potential is the most in the Midwest such as North Dakota, South Dakota, Texas, Kansas, and Montana, as well as parts of Idaho, Wyoming, and Colorado. Wind speeds increase at greater heights and winds are generally stronger at sea than on land.
In addition, the wind is more uniform at sea than on land. Therefore, offshore wind farms are being constructed to take advantage of this weather phenomenon. However, offshore plants must account for factors such as wave and ice loading. One advantage of offshore wind turbines sited along the U.S. coastline is that the load centers would be close to the offshore sites compared to the inland Class 4 or greater wind sites, due to the fact that the coastal areas tend to have a higher population concentration per square mile.
The major wind turbine components are considered to be mature commercial technology. Over the last 20 years, numerous wind turbine design configurations have been proposed, including vertical axis and horizontal axis with upwind and downwind rotors. Rotors have been designed with one, two, and three blades to drive fixed-speed, two-speed, and variable-speed generators.
Today, the most common configuration utilizes the “Danish concept”: a three-blade, upwind, horizontal-axis design. Failures of gearboxes, blades, and other components continue to reduce the productivity of wind power plants. To address the gearbox reliability problem, several new technologies are being developed and applied to improve the reliability of the gearbox or eliminate the gearbox entirely.
Wind turbines are designed to function within a wind speed window, which is defined by the “cut-in” and “cut-out” wind speeds. Below the cut-in wind speed, the energy in the wind is too low to be of use; once the wind reaches the cut-in speed, the wind turbine comes online and power output increases with wind speed up to the speed for which it is rated. The turbine produces its rated output at speeds between the rated wind speed and the cut-out speed—the speed at which the turbine shuts down to prevent mechanical damage.
Power output and stress on mechanical components at high wind speeds are controlled through active or passive yawing to track wind direction and stall or blade pitch regulation to control power output. Stall-regulated airfoils are designed to lose their lift at high wind speeds and are, therefore, self-regulating. Pitch-regulated turbines vary the pitch of the blade to reduce lift and shave off power in high winds. If the wind speed rises to a cut-out value, the blade feathers and the turbine stops turning to avoid excess loads on the rotor and other mechanical components.
Pitch-regulated blades also provide a means for optimizing the power output at lower wind speeds. Other power-reducing alternatives that have been employed include pitching only the blade tips, tip brakes, and ailerons.
The nameplate capacity of a wind turbine is determined by the manufacturer, but it can be approximated by the size of the generators being used. Individual designs range from less than 1 kW for remote sites with low power needs to machines up to 3 MW in size. Average turbine size has steadily increased with technological advances such as improved blade manufacturing technology, more sophisticated controls, and power electronics. Globally, the average size of individual wind turbines installed in 2007 was 1.5 MW.
The overall size of wind power plants, or wind farms, have also increased; the average size of wind plants installed in 2007 was 120 MW, roughly double that during the 2004 to 2005 period. The largest wind plant in operation is the 735 MW Horse Hollow plant in Texas, and a number of GW-scale plants are under development.
A wind farm consists of one or more wind turbines arranged in rows or grids, with the longest dimension arranged perpendicular to the prevailing wind direction. Individual turbines are generally separated by five to nine rotor diameters downwind and three to five rotor diameters in the direction perpendicular to the prevailing wind. Wind turbines must be arranged so that the turbines do not shadow each other.
As a result, the amount of land that is actually utilized by the wind turbines is only 5–10% of the total land area upon which the units are located. Large wind farms consisting of more than five to 10 machines are typically connected to the transmission grid through a substation. Smaller distributed wind plants with fewer than five to 10 machines are often connected directly to the distribution grid without a substation.
Wind power plants typically operate unattended and are monitored and controlled via a supervisory control and data acquisition (SCADA) system, which communicates with a remote terminal at the utility control facility or other location via a telecommunications link. Under the control of onboard computers, wind turbines automatically start up when the wind speed reaches the cut-in velocity, shut down when the wind speed drops below the cut-in speed or exceeds the top speed, and yaw into the wind as it changes direction.
The control system also is designed to shut down the turbine when a mechanical or electrical fault is detected, such as excess speed
operation, loss of hydraulic pressure, or excessive vibration. The operational status of each wind turbine in the wind farm is monitored continuously and can be controlled from a remote location to respond to changing operating conditions. Maintenance crews are dispatched only on an asneeded basis when alarms occur and indicate mechanical or electrical problems.
Though there are variations in the system design, a wind turbine can basically be broken into the following subsystems:
1. tower and foundation,
2. rotor (the blades and the center hub that the blades are attached to),
3. drive train, and
4. electrical controls and cabling
Current and Projected Technology Performance and Costs
The cost for a Wind unit varies widely depending on the resource type (wind class), regional considerations, site specific conditions, owner design philosophy etc. EPRI TAG® presents cost data by six NERC regions, by different resource types and includes generic site specific costs such as substation etc.
The major capital, operation and maintenance cost influencers for a given site are:
1. Site Location—Regional labor cost differences, labor productivity, climate requirements on design, site specific requirements on design etc
2. Construction techniques and requirements based on code
3. Owner Design and Operating philosophy
4. Technology supplier (vendor) design offering
Leading Vendors: General Electric, Enercon, Vestas.
Large wind turbine manufacturers: GE Energy, Clipper Windpower – U.S., Nordex, Vestas – Denmark, Mitsubishi – Japan, Suzlon – India, Acciona Windpower, Ecotècnia, Gamesa– Spain, Fuhrländer, Multibrid Enercon, Nordex, REpower, Siemens, VENSYS Energiesysteme – Germany, LEITNER – Italy, WinWinD – Finland, Nordic Windpower AB – Sweden, AAER – Canada, Lagerwey Wind – TheNetherlands, Goldwind Science & Technology – China, Sinovel-China.
Global installed capacity has increased by a factor of 12 in the past decade due to dramatic cost reductions, renewable energy requirements, and in the U.S. the federal wind production tax credit (PTC).
Developing low wind speed turbines to reduce electricity cost from 5 – 6 ¢/kWh to 3 ¢/kWh. Construction of offshore wind farms.