Wind turbine design is the process of defining the form and configuration of a wind turbine to extract energy from the wind.[1] An installation consists of the systems needed to capture the wind's energy, point the turbine into the wind, convert mechanical rotation into electrical power, and Contact online >>
Wind turbine design is the process of defining the form and configuration of a wind turbine to extract energy from the wind.[1] An installation consists of the systems needed to capture the wind''s energy, point the turbine into the wind, convert mechanical rotation into electrical power, and other systems to start, stop, and control the turbine.
In 1919, German physicist Albert Betz showed that for a hypothetical ideal wind-energy extraction machine, the fundamental laws of conservation of mass and energy allowed no more than 16/27 (59.3%) of the wind''s kinetic energy to be captured. This Betz'' law limit can be approached by modern turbine designs which reach 70 to 80% of this theoretical limit.
In addition to the blades, design of a complete wind power system must also address the hub, controls, generator, supporting structure and foundation. Turbines must also be integrated into power grids.
Blade shape and dimension are determined by the aerodynamic performance required to efficiently extract energy, and by the strength required to resist forces on the blade.
The aerodynamics of a horizontal-axis wind turbine are not straightforward. The air flow at the blades is not the same as that away from the turbine. The way that energy is extracted from the air also causes air to be deflected by the turbine. Wind turbine aerodynamics at the rotor surface exhibit phenomena that are rarely seen in other aerodynamic fields.
Rotation speed must be controlled for efficient power generation and to keep the turbine components within speed and torque limits. The centrifugal force on the blades increases as the square of the rotation speed, which makes this structure sensitive to overspeed. Because power increases as the cube of the wind speed, turbines have must survive much higher wind loads (such as gusts of wind) than those loads from which they generate power.
A wind turbine must produce power over a range of wind speeds. The cut-in speed is around 3–4 m/s for most turbines, and cut-out at 25 m/s.[2] If the rated wind speed is exceeded the power has to be limited.
A control system involves three basic elements: sensors to measure process variables, actuators to manipulate energy capture and component loading, and control algorithms that apply information gathered by the sensors to coordinate the actuators.[3]
Any wind blowing above the survival speed damages the turbine. The survival speed of commercial wind turbines ranges from 40 m/s (144 km/h, 89 MPH) to 72 m/s (259 km/h, 161 MPH), typically around 60 m/s (216 km/h, 134 MPH). Some turbines can survive 80 metres per second (290 km/h; 180 mph).[4]
A stall on an airfoil occurs when air passes over it in such a way that the generation of lift rapidly decreases. Usually this is due to a high angle of attack (AOA), but can also result from dynamic effects. The blades of a fixed pitch turbine can be designed to stall in high wind speeds, slowing rotation.[5] This is a simple fail-safe mechanism to help prevent damage. However, other than systems with dynamically controlled pitch, it cannot produce a constant power output over a large range of wind speeds, which makes it less suitable for large scale, power grid applications.[6]
A fixed-speed HAWT (Horizontal Axis Wind Turbine) inherently increases its angle of attack at higher wind speed as the blades speed up. A natural strategy, then, is to allow the blade to stall when the wind speed increases. This technique was successfully used on many early HAWTs. However, the degree of blade pitch tended to increase noise levels.
Vortex generators may be used to control blade lift characteristics. VGs are placed on the airfoil to enhance the lift if they are placed on the lower (flatter) surface or limit the maximum lift if placed on the upper (higher camber) surface.[7]
Furling works by decreasing the angle of attack, which reduces drag and blade cross-section. One major problem is getting the blades to stall or furl quickly enough in a wind gust. A fully furled turbine blade, when stopped, faces the edge of the blade into the wind.
Loads can be reduced by making a structural system softer or more flexible.[3] This can be accomplished with downwind rotors or with curved blades that twist naturally to reduce angle of attack at higher wind speeds. These systems are nonlinear and couple the structure to the flow field - requiring design tools to evolve to model these nonlinearities.
Standard turbines all furl in high winds. Since furling requires acting against the torque on the blade, it requires some form of pitch angle control, which is achieved with a slewing drive. This drive precisely angles the blade while withstanding high torque loads. In addition, many turbines use hydraulic systems. These systems are usually spring-loaded, so that if hydraulic power fails, the blades automatically furl. Other turbines use an electric servomotor for every blade. They have a battery-reserve in case of grid failure. Small wind turbines (under 50 kW) with variable-pitching generally use systems operated by centrifugal force, either by flyweights or geometric design, and avoid electric or hydraulic controls.
Fundamental gaps exist in pitch control, limiting the reduction of energy costs, according to a report funded by the Atkinson Center for a Sustainable Future. Load reduction is currently focused on full-span blade pitch control, since individual pitch motors are the actuators on commercial turbines. Significant load mitigation has been demonstrated in simulations for blades, tower, and drive train. However, further research is needed to increase energy capture and mitigate fatigue loads.
A control technique applied to the pitch angle is done by comparing the power output with the power value at the rated engine speed (power reference, Ps reference). Pitch control is done with PI controller. In order to adjust pitch rapidly enough, the actuator uses the time constant Tservo, an integrator and limiters. The pitch angle remains from 0° to 30° with a change rate of 10°/second.
As in the figure at the right, the reference pitch angle is compared with the actual pitch angle b and then the difference is corrected by the actuator. The reference pitch angle, which comes from the PI controller, goes through a limiter. Restrictions are important to maintain the pitch angle in real terms. Limiting the change rate is especially important during network faults. The importance is due to the fact that the controller decides how quickly it can reduce the aerodynamic energy to avoid acceleration during errors.[3]
One technique to control a permanent magnet synchronous motor is field-oriented control. Field-oriented control is a closed loop strategy composed of two current controllers (an inner loop and cascading outer loop) necessary for controlling the torque, and one speed controller.
In this control strategy the d axis current is kept at zero, while the vector current aligns with the q axis in order to maintain the torque angle at 90o. This is a common control strategy because only the Iqs current must be controlled. The torque equation of the generator is a linear equation dependent only on the Iqs current.
So, the electromagnetic torque for Ids = 0 (we can achieve that with the d-axis controller) is now:
Thus, the complete system of the machine side converter and the cascaded PI controller loops is given by the figure. The control inputs are the duty rations mds and mqs, of the PWM-regulated converter. It displays the control scheme for the wind turbine in the machine side and simultaneously how the Ids to zero (the torque equation is linear).
Large turbines are typically actively controlled to face the wind direction measured by a wind vane situated on the back of the nacelle. By minimizing the yaw angle (the misalignment between wind and turbine pointing direction), power output is maximized and non-symmetrical loads minimized. However, since wind direction varies, the turbine does not strictly follow the wind and experiences a small yaw angle on average. The power output losses can be approximated to fall with (cos(yaw angle))3. Particularly at low-to-medium wind speeds, yawing can significantly reduce output, with wind common variations reaching 30°. At high wind speeds, wind direction is less variable.
Cyclically braking slows the blades, which increases the stalling effect and reducing efficiency. Rotation can be kept at a safe speed in faster winds while maintaining (nominal) power output. This method is usually not applied on large, grid-connected wind turbines.
A mechanical drum brake or disc brake stops rotation in emergency situations such as extreme gust events. The brake is a secondary means to hold the turbine at rest for maintenance, with a rotor lock system as primary means. Such brakes are usually applied only after blade furling and electromagnetic braking have reduced the turbine speed because mechanical brakes can ignite a fire inside the nacelle if used at full speed. Turbine load increases if the brake is applied at rated RPM.
Turbines come in size classes. The smallest, with power less than 10 kW are used in homes, farms and remote applications whereas intermediate wind turbines (10-250 kW ) are useful for village power, hybrid systems and distributed power. The world''s largest wind turbine as of 2021 was Vestas'' V236-15.0 MW turbine. The new design''s blades offer the largest swept area in the world with three 115.5 metres (379 ft) blades giving a rotor diameter of 236 metres (774 ft). Ming Yang in China have announced a larger 16 MW design.[8][9]
For a given wind speed, turbine mass is approximately proportional to the cube of its blade-length. Wind power intercepted is proportional to the square of blade-length.[10] The maximum blade-length of a turbine is limited by strength, stiffness, and transport considerations.
Labor and maintenance costs increase slower than turbine size, so to minimize costs, wind farm turbines are basically limited by the strength of materials, and siting requirements.
The nacelle houses the gearbox and generator connecting the tower and rotor. Sensors detect the wind speed and direction, and motors turn the nacelle into the wind to maximize output.
For large horizontal-axis wind turbines (HAWT), the generator[14] is mounted in a nacelle at the top of a tower, behind the rotor hub. Older wind turbines generate electricity through asynchronous machines directly connected to the grid. The gearbox reduces generator cost and weight. Commercial generators have a rotor carrying a winding so that a rotating magnetic field is produced inside a set of windings called the stator. While the rotating winding consumes a fraction of a percent of the generator output, adjustment of the field current allows good control over the output voltage.
The rotor''s varying output frequency and voltage can be matched to the fixed values of the grid using multiple technologies such as doubly fed induction generators or full-effect converters, which converts the variable frequency current to DC and then back to AC using inverters. Although such alternatives require costly equipment and cost power, the turbine can capture a significantly larger fraction of the wind energy. Most are low voltage 660 Volt, but some offshore turbines (several MW) are 3.3 kV medium voltage.[15]
In some cases, especially when offshore, a large collector transformer converts the wind farm''s medium-voltage AC grid to DC and transmits the energy through a power cable to an onshore HVDC converter station.
Hydraulic wind turbines perform the frequency and torque adjustments of gearboxes via a pressurized hydraulic fluid. Typically, the action of the turbine pressurizes the fluid with a hydraulic pump at the nacelle. Meanwhile, components on the ground can transform this pressure into energy, and recirculate the working fluid. Typically, the working fluid used in this kind of hydrostatic transmission is oil, which serves as a lubricant, reducing losses due to friction in the hydraulic units and allowing for a broad range of operating temperatures. However, other concepts are currently under study, which involve using water as the working fluid because it is abundant and eco-friendly.[16]
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