1) Power in the wind
Wind turbines for electricity generation emerged at the end of nineteenth centry. The technology grew and became mature for industrial applications since the 1980s. The typical size of wind turbines has been growing steadily in terms of rotor diameter and rated power over the past two to three decades.
In the early 2000s, the most cost-effective size range was 600-750 kW with rotor diameter in the range of 40-47 m. These turbines have been manufactured in substantial quantities by all manufacturers and are the baseline technology from which a new generation of megawatt-scale turbines have recently been developed.
As of early 2007, some manufacturers have started producing turbines with rated power of several MW and with rotor diameter of around 90 metres (e.g. Vestas V90 3.0 MW machine, Nordex N90 2.5MW machine), or even around 100 metres(e.g GE 3.6MW machine). These are primarily for the European market where good sites are at a premium and there is much pressure to make the most energy out of each available site.
Another class of even larger wind turbines are designed or being prototyped primarily for offshore applications, such as the 5MW machine of RE Power with rotor diameter of 126 metres.
Energy in the wind is the kinetic energy it contains. For certain mass of air, the kinetic energy can be calculated by the formula below.
It can be easily shown that the power of the wind flowing through a certain area is given by
At sea level and at 15 degrees Celsius, dry air has a density of approximately 1.225 kg/m3. varying with pressure and temperature. Air density decreases with increasing altitude.
It can be seen that the power in the wind is proportional to the cube of the velocity, and is proportional to the swept area of the turbine rotor. However, only a fraction of this power can actually be extracted by the rotor.
Modern wind turbines work on aerodynamic lift principle, just like the wings of an aeroplane. The wind does not "push" the turbine blades, but instead when the wind flows across and past a turbine blade, the difference in the pressure on either sides of the blade produces a lifting force, causing the rotor to rotate and cut across the wind.
Not all the power in the wind can be extracted by the turbine rotor. Theoretically, the maximum amount of power that can be extracted by a wind turbine, according to the Betz Law, is 59.6% of the power in the wind. Most wind turbines can extract about 40% or less of the power in the wind.
A wind turbine mainly comprises of three major parts - a rotor, a nacelle, and a tower. The horizontal axis, three-blade turbine on a free-standing tubular tower is the predominant configuration for large grid-connected wind turbines.
The rotor blades are made of composite materials. Unlike small wind turbines, the rotors of large wind turbines rotate rather slowly. Simpler wind turbines are fixed speed machines, often with two speeds - a lower speed for weaker wind conditions and a higher speed for stronger wind conditions. For fixed speed machines, the induction generator directly produces alternating current at grid frequency.
Newer designs are variable speed ones (for example the speed range is 14 to 31.4 rpm for Vestas V52-850kW machine). Under variable speed operation, rotor aerodynamic efficiency is improved, leading to better energy capture and bringing about additional benefits such as lower noise in light winds. Variable speed has been gaining ground over fixed speed design.
Sensors mounted on the nacelle detect the wind direction, and a yawing mechanism automatically orientates the nacelle and rotor to face the wind.
The rotational motion of the rotor is transmitted via a gearbox to the electric generator inside the nacelle (or in the case of gearless machine, is transmitted directly to the electric generator). Wind turbines with gearboxes are the common type in the industry. However, the use of purpose-built multi-pole direct-drive generators is also gaining notable development.
A transformer at the base of the tower (or for some designs inside the nacelle) steps up the generator voltage to the grid voltage (11kV in the case of Hong Kong).
All turbines produce a varying power output dependent on the wind speed. The two most common means of limiting the power output (and hence the stress on the rotor) in high winds are "stall regulation" and "pitch regulation". With stall regulation, an increase in wind speed beyond the rated wind speed causes progressive stalling of the air flow over the rotor. Tip brakes are used to brake the wind turbine when the wind gets excessively strong. In the case of pitch-regulated machines, each turbine blade can rotate about its own length-wise axis. The "pitch angle" of the turbine blade varies with the wind speed, altering the aerodynamic performance of the rotor. When the wind gets too strong, the leading edge of the blade actually faces the wind so that the wind turbine brakes.
Lightning protection stripes are embedded inside the turbine blade, to bring the lightning charges to ground when a lightning stroke hits the blade.
At very low wind speeds, the rotor of a wind turbine will stay still. At the cut-in wind speed (typically 3 to 4 m/s), the rotor starts to rotate and the wind turbine will start generating power. As the wind gets stronger and stronger, the power output will increase and when the rated wind speed (or "nominal wind speed") is reached, the wind turbine will deliver its rated output power (or "nominal output"). After that, the output power will more or less flat off and at the cut-out speed, the wind turbine will brake and deliver no output at all so as to protect itself from possible damage.
The performance of a wind turbine is expressed by its power curve which shows the relationship between the power output of the turbine at different wind speeds, from cut-in to cut-off.
In order to select a suitable wind turbine for a specific site, a general approach is to make use of the power curves of individual wind turbines and the wind data for the site to conduct energy yield prediction. (More information on this aspect is given in the Large Wind Turbine - Resource Potential section.)
The rated output power (rated power, power rating, nonimal output) of a wind turbine is declared for a particular rated wind speed (nominal wind speed). As power varies directly with the cube of wind speed, the rated power of a wind turbine varies significaltly with the wind speed at which the rate power is stated.
A larger or a smaller generator can be fitted to turbine rotors of the same construction and diameter. Therefore two wind turbines of the same rotor construction and diameter may have rather different rated power values, depending on whether the turbine is designed for higher wind speed locations (fitted with a larger generator) or for lower wind speed locations (fitted with a smaller generator).
Horizontal axis wind turbine VS vertical axis wind turbine
Based on the orientation of the shaft about which the aerofoil blades are configured, wind turbines can be divided into horizontal axis and vertical axis wind turbines. The rotation axes of horizontal axis wind turbines operate in line with the wind while the rotation axes of vertical axis turbines are at right angle to the wind.
Horizontal axis wind turbines are continuously realigned to keep the rotor in line with wind direction. But this is not required for vertical axis wind turbines as they can harness wind from all directions.
Horizontal axis turbines are the dominant turbine type all over the world.
Upwind wind turbine VS downwind wind turbine
An upwind wind turbines is a horizontal axis wind turbine with the rotor facing the wind while a downwind turbine is one with the wind blowing from the back of the rotor. The majority of modern large wind turbines are up-wind machines.
Single blade, two blade, and three blade machines
The number of blades of a wind turbine is determined by various factors including aerodynamic efficiency, complexity, cost, noise and aesthetics. Large wind turbines have been built with single blade, two blades or three blades.
Wind turbine of fewer blades need to rotate faster to get more interaction with the wind so as to extract more energy from it, and hence they are noisier. When the number of blades are too many, they can interact with each other and hence reduce the overall efficiency of the turbine. Nowadays, turbines with three blades are the dominant turbine type. Aesthetically, a three blade turbine looks more balanced and pleasing.
On-shore wind energy systems may be either individual machines, or multiple machines arranged as a wind farm, in a linear arrangement or in an array arrangement.
Wind turbines in a wind farm should be installed with certain separation distance from each other, to avoid causing excessive turbulence on each other, or causing a significant reduction in the power output of the back-row wind turbines due to "wake effect" caused by the front-row turbines.
Roads need to be constructed for transporting the large components of the turbines (in particular the blades) to the installation sites. Power lines carry the generated power from the wind farm to the grid connection point.
Up to the end of 2005, world total installed capacity for wind enrgy reached 58 GW. Germany, Spain, the USA, India and Denmark are the leading countries in terms of installed wind power capacity. In the case of Denmark, wind power provides nearly 20% of the country's electricity. In Hong Kong, the first large wind turbine was installed on Lamma Island in late 2005 and inaugaurated in February 2006 by The Hongkong Electric Company Limited. Its rated output power is 800 kW.
Wind energy is the fastest growing sector in renewable energy. The average annual growth rate of wind energy in the year between 1995 - 2005 was 28.5%. The annual growth rate of wind power will continue to be high and it is estimated by DEWI, the German Wind Energy Institute, that the global wind power capacity will probably reach 150 GW by 2012.
This flash illustrates wind turbine installations around the world. The paragraph above describes wind turbine installations around the world.