A wind energy system transforms the kinetic energy of the wind into mechanical or electrical energy that can be harnessed for practical use. Mechanical energy is most commonly used for pumping water in rural or remote locations – the "farm windmill" still seen in many rural areas of the U.S. is a mechanical wind pumper – but it can also be used for many other purposes (grinding grain, sawing, pushing a sailboat, etc.). Wind electric turbines generate electricity for homes and businesses and for sale to utilities.

There are two basic designs of wind electric turbines: vertical-axis, or "egg-beater" style, and horizontal-axis (propeller-style) machines. Horizontal-axis wind turbines are most common today, constituting nearly all of the "utility-scale" (100 kilowatts, kW, capacity and larger) turbines in the global market. [American Wind Energy Association]

Almost all wind turbines producing electricity for the national grid consist of rotor blades which rotate around a horizontal hub. The hub is connected to a gearbox and generator, which are located inside the nacelle. The nacelle houses the electrical components and is mounted at the top of the tower. This type of turbine is referred to as a 'horizontal axis' machine.

 

Rotor diameters range up to 80 metres, smaller machines (around 30 meters) are typical in developing countries

Wind turbines can have three, two or just one rotor blades. Most have three.

Blades are made of fibreglass-reinforced polyester or wood-epoxy.

The blades rotate at 10-30 revolutions per minute at constant speed, although an increasing number of machines operate at a variable speed.

Power is controlled automatically as wind speed varies and machines are stopped at very high wind speeds to protect them from damage.

Most have gearboxes although there are increasing numbers with direct drives.

The yaw mechanism turns the turbine so that it faces the wind. Sensors are used to monitor wind direction and the tower head is turned to line up with the wind.

Towers are mostly cylindrical and made of steel, generally painted light grey. Lattice towers are used in some locations. Towers range from 25 to 75 meters in height.

 

 

 

 

Commercial turbines range in capacity from a few hundred kilowatts to over 2 megawatts. The crucial parameter is the diameter of the rotor blades - the longer the blades, the larger the area 'swept' by the rotor and the greater the energy output. At present the average size of new machines being installed is now super megawatt, 1.3-1.85MW, and there are larger machines on the market. The trend is towards moving to these larger machines as they can produce electricity at a lower price.

There are many different turbine designs, with plenty of scope for innovation and technological development. The dominant wind turbine design is the up-wind, three bladed, stall controlled, constant speed machine. The next most common design is similar, but is pitch controlled. Gearless and variable speed machines follow, again with three blades. A smaller number of turbines have 2 blades, or use other concepts, such as a vertical axis.

Most turbines are upwind of the tower - they face into the wind with the nacelle and tower behind. However, there are also downwind designs, where the wind passes the tower before reaching the blades.

 

Stall and pitch control

There are two main methods of controlling the power output from the rotor blades. The angle of the rotor blades can be actively adjusted by the machine control system. This is known as pitch control. This system has built-in braking, as the blades become stationary when they are fully 'feathered'.

The other method is known as stall control. This is sometimes known as passive control, since it is the inherent aerodynamic properties of the blade which determine power output; there are no moving parts to adjust. The twist and thickness of the rotor blade vary along its length in such a way that turbulence occurs behind the blade whenever the wind speed becomes too high. This turbulence means that less of the energy in the air is transfered, minimising power output at higher speeds. Stall control machines also have brakes on the blade tips to bring the rotor to a standstill, if the turbine needs to be stopped for any reason.

Most wind turbines start operating at a speed of 4-5 metres per second and reach maximum power at about 15 m/s.

 

Factors affecting performance

Most important is the windiness of the site. The power available from the wind is a function of the cube of the wind speed. Therefore a doubling of the wind speed gives eight times the power output from the turbine. All other things being equal, a turbine at a site with an average wind speed of 5 meters per second (m/s) will produce nearly twice as much power as a turbine at a location where the wind averages 4 m/s.

Second is the availability of the equipment. This is the capability to operate when the wind is available - an indication of the turbine's reliability. This is typically over 98% for modern machines. Last is turbine arrangement. Turbines in wind farms must be carefully arranged to gain the maximum energy from the wind - this means that they should shelter each other as little as possible from the prevailing wind.

 

Benefits of wind power

Apart from generating electricity without causing pollution, wind energy has numerous other advantages.

It is widely distributed - more countries have sizeable wind power potential than have large resources of hydro-power or fossil fuel reserves.

It is ideal for generating electricity at a local level - European wind schemes are typically clusters of around 10 - 40 turbines, providing enough electricity for 4,000 to 16,000 households. Some countries such as Denmark and Germany also have a high proportion of single turbines. The electricity can be fed directly into the distribution network, reducing electricity distribution and transmission losses. By contrast, electricity from larger power stations has to be transmitted on high voltage power lines and travel long distances before it gets to the point of use.

Wind energy is good for island communities - the supply can be connected to diesel or solar systems to provide back-up when the wind is not blowing.

Wind energy is low risk - the relatively small unit size of each individual wind turbine (or wind scheme) also reduces the risk of technical failure or industrial action compared with larger generating units.

[RenewableUK]