There are many different wind turbine designs, but all of them have things in common. The main component that transforms the wind energy into mechanical energy is the rotor, which includes the blades. Based on this commonality, wind turbines are classified by the structure of the rotor and its location in the airflow. The two main types of wind turbine are horizontal axis and vertical axis, referring to the axis of the blade rotation. At this time, the only type of configuration commercially available for medium and large installations is the horizontal axis turbine, so it is the only one considered here.
The rotor of a horizontal axis wind turbine rotates around a horizontal axis, parallel to the wind direction. The blades of the rotor are arranged rigidly in a plane, that is always oriented perpendicular to the wind. These turbines generally have an enclosed part or nacelle that houses all of the wind turbine’s mechanical infrastructure such as the generator, gearbox, break, and power electronics. While most smaller wind turbines have a tail, tails are not common on larger turbines. All horizontal axis wind turbines are mounted on top of a tower, which is either tubular or lattice frame in design.
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Wind Turbine Performance
For any location with a known wind resource, there are several factors that can be used to predict electrical generation from a wind turbine. The most important include the power curve, the cut-in wind speed, the rated wind speed and power output, and the cut-out wind speed. These factors along with turbine availability all contribute to the capacity factor of a turbine. All of these terms are explained in more details as follows.
Wind Turbine Power Curve
The main way to assess the performance of a wind turbine is through the examination of a wind turbine power curve. On the vertical axis, the power curve depicts the expected electrical output of the turbine, at specific wind speeds, which are shown on the horizontal axis. Wind turbine power curves can be calculated either based on the design of the turbine, or measured from actual turbine operation. For smaller permanent magnet generators it is especially important to get a measured power curve from a manufacturer since this curve can be different from the calculated version. Furthermore, a power curve is not universally valid. It depends on turbulence, atmospheric pressure, and ambient temperature at the measuring location. A power curve is usually corrected to sea level and 68°F ambient temperature.
Cut-in Wind Speed
The lowest wind speed at which the turbine will generate power is called the cut-in wind speed. Although at face value this parameter should be clear, there are several nuances. Because of the mass of the rotor, a spinning turbine will produce power at a lower wind speed than a turbine starting from a standstill. If the power curve is calculated based on the properties of the turbine, the cut-in wind speed tends to be lower, as it does not account for the rotational mass of the rotor. A low cut-in wind speed is generally desired, since this translates to more time when the turbine is producing at least some power.
Cut-out Wind Speed
Cut-out wind speed defines the speed at which the wind turbine is designed to be shut down to prevent damage to the wind turbine. The wind speed is usually monitored by the turbine control system, and if the cut-out wind speed is reached, the turbine breaking system is applied and the turbine will not operate. Typical cut-out wind speeds are around 25 m/s (56 mph). By over-sizing specific components, wind turbines can be designed to have higher cutout wind speeds. Small wind turbines with furling mechanisms do not have a cut-out wind speed. Instead, as the wind speed increases, the furling mechanism engages, turning the turbine rotor out of the wind, and thus reducing the turbine strain and power output.
Rated Wind Speed and Rated Output Power
The rated wind speed and output power are relative values that give an indication of wind speeds required for the turbine to produce large amounts of power. If the rated wind speed for the turbine is much higher than the typical wind speed for the site, it is probably not a good turbine to use. There will be little time when the turbine is producing significant amounts of power. Usually, a generator with lower rated wind speed is better than a similar one with a higher rated wind speed. This is because the turbine with a lower-rated wind-speed will reach fall rated output under more likely wind speeds.
Survival Or Maximum Wind Speed
The survival wind speed is the maximum wind speed that the wind turbine is designed to withstand safely. Most wind turbines have a specified survival wind speed of 50 m/s - 65 m/s (112 mph - 145 mph), and in many cases this value is regulated by national standards. Wind turbines can also be specified to have higher survival wind speeds for installations in unusual or special environments. Small wind turbines with furling mechanisms will still be generating power up to the survival wind speed, while non-furling turbines will not be operating at wind speeds higher than the cut-out wind speed. The survival wind speed is really more of an insurance or a safety consideration, as wind turbines typically do not suffer any damage from winds higher than the stated survival speeds.
Availability describes the amount of time that a wind turbine is ready to produce energy. It is defined as the ratio between the number of hours the wind turbine operates divided by the number of windy hours over the same time period. A high availability describes a turbine that is producing power whenever the wind is blowing. Availability is a term used to describe the operational and maintenance condition of the wind turbine. In modern wind turbines, availabilities over 95% are expected. For small wind turbines, availability over 99% is not unusual.
Wind turbine capacity factor describes the amount of energy that the wind turbine produces compared with theoretical production if it were running at full, rated power. The capacity factor is reported over a fixed time period, usually a month or a year, and is calculated by dividing the turbine’s energy production over that time by the energy production if the turbine were running at rated power over the same time period. Capacity factor describes the power production expectations of the wind turbine. It is most strongly related to the wind resource at the site. Capacity factors of 25%-40% are typical, while values up to 60% have been reported.
Wind Turbine Types
As would be expected with any power generation technology, not all wind turbines are created equal. Additionally, specific design features make some turbines more appropriate for remote or Alaskan installations. One of the primary problems with wind turbines installed in rural Alaska during the 1980s, is that little thought was given to appropriate application of the turbines or the long-term sustainability of the projects.
Wind Turbine Class And Certification
The International Electrotechnical Commission (IEC), an international standards development organization, has developed a classification system for wind turbine systems. It specifies the design conditions for particular wind turbines. Class I, II, and III specify the design wind speeds for a specific turbine product. Manufacturers who are certifying wind turbines must pick one of these classes.
Class I turbines are designed to operate in the harshest climates, with strong annual average wind speeds and turbulent wind. Class II turbines are designed for most typical sites and Class III turbines are designed for low wind resource sites. Typically Class II and III turbines have a larger turbine rotor (longer blades) to capture more of the wind energy at lower wind speeds. They may look more appealing from an energy capture point of view, even at high wind speed sites; but this should not encourage people to install higher class turbines for lower class sites. The class of wind turbine should be selected based on the conditions at a particular site.
The IEC has also developed standards for many other parameters, such as power performance, noise, and electrical characteristics. Most large wind turbines have been certified to IEC standards; however, this is not as common for medium wind turbines, due in large part to the cost. Turbine Class and certification should be considered when selecting a turbine.
Turbine Design Types
Interconnecting a wind turbine into a remote or weak grid network can be complicated, and specific wind turbine design characteristics play a key role in determining just how hard the job will be. Traditional wind turbines with a synchronous generator, stall regulated control, and no power electronics can cause large power spikes and/or power variability depending on the wind conditions or during start-up. Turbines using synchronous generators but active pitch control allow better or smoother power quality. Variable speed wind turbine technology with active pitch control can actually allow the control system to specify a desired power output from the turbine, as opposed to being limited to accepting whatever energy the turbine produces. Additional devices may also be purchased to smooth out power fluctuations from the wind turbines, such as capacitor banks, turbine soft starts, and variable motor drives. In any case, the turbine selection process should consider the level of turbine and power quality control depending on the application and system requirements.
Other Design Selection Criteria
A multitude of other selection or design criteria should also be considered when determining the turbine model for a particular application. Turbine weight and installation height will be determined by the equipment available to move and install the turbine. Tower type (lattice or tubular, tilt-up or crane-installed) will depend on the site conditions and manufacturer’s options. Some turbine manufacturers have cold weather packages that allow turbines to operate at lower temperatures and in icing conditions. Finally, there are applications where it makes sense to install an older turbine, which may have lower performance and limited control options, but which can be maintained more easily in rural areas rather than to purchase a modern turbine, which will have better specific performance and advanced control, but may be more difficult and costly to service.