1 Overview
Wind power is a widely accepted green technology. In some places where wind resources are abundant, in addition to environmental benefits, wind power is also cost-competitive with low-cost fossil energy power generation. At present, the capacity of wind turbines connected to the grid in the world is huge, and the industrial ambitions of many European governments have also promoted the rapid development of this industry. Australia’s early Mandatory Renewable Energy Target (MRET) program encouraged the construction of wind farms in the country, particularly in South Australia and Victoria.
- Types of wind turbines
Horizontal axis wind turbines, ie the axis around which the wind turbine blades rotate, are placed horizontally.
Blades of various types of wind turbines can be divided into two types: traction type and lift type. Traction blades are mostly used in old wind turbines, and are driven by wind resistance on the blades to rotate, so the wind speed range for working conditions is limited. Lift blades are mostly used in modern wind turbines. They have an airfoil appearance, and the wind passing through the blades generates different pressures on them, thereby pushing the blades to rotate. As long as there is air flow through the blades, thrust will be generated to make the blades rotate. The starting wind speed required for lift blades is very low, and its rotation speed can be several times the wind speed.
There are not many applications of vertical axis wind turbines, especially Savonius wind turbines require large starting torque, and their rotational speed is limited by wind speed. The Darius motor rotor also requires a high starting torque, but once it exceeds a certain speed, it can spin at several times the wind speed. In large wind farms, when the wind speed sensor shows that the current wind speed is suitable, the rotor of the wind turbine is generally driven by a small motor to a certain speed to make it work normally, and then the small motor is out of operation, and the small motor is mostly powered by wind power. Would you like to learn more about the power supply from batteries? go now). The windward horizontal axis wind turbine is the most widely used wind turbine in the current independent power supply system.
- Wind turbine output power
The power produced by a wind turbine depends on the following:
(1) The length of the wind turbine blade. The area of the circle formed by the wind turbine blade as the radius is called the swept area. The output power of the wind turbine is related to the swept area. Since the calculation formula of the rotating area is xr2 (r is the radius of the circle), the rotating area is proportional to the square of the blade length. For example, if the blade length is doubled, the output power will be increased by four times.
(2) Wind speed. The output power of the wind turbine is proportional to the third power of the wind speed. For example, if the wind speed is increased by a factor of 2, the output power will be increased by a factor of 8.
(3) Wind energy conversion coefficient of wind turbine. That is, the ability of a wind turbine to convert wind energy into rotational kinetic energy of the wind turbine.
The formula for the output power of the wind turbine is as follows:
where P is the output power of the wind turbine;
——Air density (the density of dry air at a temperature of 20°C and a pressure of 101kPa is 1.205kg/m³);
d——Blade diameter, m;
v——wind speed, m/s;
Co – wind energy conversion coefficient of wind turbine [theoretical maximum value is 0.59, typical average value is 0.25 (small unit) to 0.35 (large unit)].
According to this formula, as the wind speed increases, the output power increases, so the wind turbine should be able to work at all wind speeds, but this is not the case. Mechanical damage to wind turbine components can occur if the wind turbine rotates too quickly. The output efficiency of a wind turbine is related to its rotational frequency, and the wind turbine should work as close to the optimal rotational frequency as possible. Many large wind turbines are equipped with a variety of blades so that they can operate efficiently over a wide range of wind speeds.
The blade angle of the new downwind wind turbine can be adjusted inward, so that the swept area can be adjusted, so it can work normally under extremely high wind speeds. The purpose of this design is to maintain the maximum speed of the blade at a stable 200r/min, while also limiting the possible blade noise. This is different from the design of windward wind turbines. The windward wind turbines will be out of operation at ultra-high wind speeds to achieve self-protection of the wind turbine.
The electrical power output by the wind turbine is usually smaller than the value calculated by the wind energy conversion factor. For example, at low wind speeds, the wind turbine cannot output any power because the wind turbine needs to reach the minimum wind speed before it can start. The minimum wind speed required for this start-up is called the cut-in wind speed. Models with low cut-in wind speed should be installed in low wind speed areas. Similarly, when the output power of the wind turbine reaches the rated value, in order to protect the wind turbine from overload, it is necessary to artificially reduce the efficiency and limit the output near the rated power value. When the wind speed is very high, the wind turbine will stop completely (cut out wind speed).
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