Wind energy is the kinetic energy of moving air. Wind energy comes from the uneven heating of the atmosphere by the sun, irregularities in the earth’s surface and the earth’s rotation. The speed of the wind determines the amount of kinetic energy that can be converted into mechanical energy or electricity. Mechanical energy can be used, for example, to grind grain and pump water. Mechanical energy can also be used to run turbines that produce electricity. This work focuses specifically on wind power rather than other non-electric forms of wind energy.
There are two main ways in which wind energy can be converted (for both mechanical and electrical purposes): using either ‘aerodynamic drag’ or ‘lift’ forces. The aerodynamic drag method means simply placing one side of the surface against the wind while the other side is to the leeward side. The movement due to aerodynamic drag is in the same direction as the wind blows. The upwind method slightly changes the wind direction and creates a force perpendicular to the wind direction. The drag method is less effective than the lift method.
The concentration of wind energy ranges widely, from 10 W/m-2 (in a light breeze of 2.5 m/s) to 41,000 W/m-2, during a hurricane with a wind speed of 40 metres per second (m/s) or 144 km/h. In general, wind power is proportional to the cube of the wind speed. This means that electrical power is extremely sensitive to wind speed (when wind speed doubles, power increases by a factor of eight).
Global trends
Wind energy, with its beginnings in the late 1970s, has become a global industry involving energy giants. In 2008, new investments in wind power reached USD 51.8 billion (EUR 35.2 billion) (UNEP, 2009).
According to statistics published by the European Wind Energy Association (EWEA, 2011), prosperous markets exist in locations with the right conditions. In 2008, wind power installations generated around 20% of all electricity in Denmark, more than 11% in Portugal and Spain, 9% in Ireland and almost 7% in Germany, more than 4% of all electricity in the European Union (EU) and almost 2% in the USA (IEA Wind Energy, 2009).
Since 2000, total installed capacity has grown at an average annual rate of 30% (see figure). In 2008, more than 27 GW of electric capacity was installed in more than 50 countries, bringing the global onshore and offshore potential to 121 GW. In 2008, the World Wind Energy Council estimated that some 260 million megawatt hours (260 terawatt hours) of electricity were generated.
Wind turbine technology
The ability to generate electricity is determined by the design of wind turbines. All wind turbines consist of blades that rotate an axis connected to a generator, which generates an electric current.
Wind turbines can be located almost anywhere where there is wind, such as at sea, on land and in built-up areas.
Wind turbines come in a variety of sizes and power ratings. The largest turbine has blades with a span greater than the length of a football field, the height of a 20-storey building and generates enough electricity to power 1,400 buildings. Conversely, a wind turbine the size of a small house has blades 8 to 25 feet in diameter, over 30 feet tall, and can power a fully electrified building or small business.
The size and capacity of wind turbines varies widely. There are three main types of wind turbine: horizontal axis, vertical axis and ducted.
Horizontal axis turbines (Propeller wind turbines)
Propeller wind turbines (abbreviated PVT) are currently dominant. This type is similar to a windmill with propeller-shaped blades that rotate around a horizontal axis.
Propeller wind turbines have a main rotor axis and an electric generator at the top of the mast. The rotor axis must face the wind. Small turbines are guided downwind by simple guides mounted perpendicular to the rotor blades, while larger turbines usually have a wind sensor controlling a turning motor. Most large wind turbines have a gearbox, which converts the slow rotation of the rotor into a fast rotation of the generator, which is important for power generation.
The blades of wind turbines are made rigid to prevent the blades from hitting the mast in high winds. In addition, the blades are positioned at a considerable distance from the mast and sometimes slightly inclined.
Because turbulence is created behind the mast, turbines are usually placed on the side where the wind blows from. Otherwise, turbulence can lead to accidents from fatigue stresses, reducing the reliability of the plant. Nevertheless, despite the problems of turbulence, units have been built with the turbine pointing downwind as they do not need an additional mechanism to orient them downwind and, during high winds, their blades can bend, which reduces the sliding area and thus the wind resistance.
Vertical Axis Wind Turbines (Windrotor Wind Turbines)
Wind turbines (WT) come in different types, but all have one thing in common: the main rotor shaft is vertical (not horizontal).
The different models (see below) are designed specifically for locations where wind direction is very variable or turbulent. WWTs are generally considered easier to install and maintain as the generator and other main components can be placed close to the ground (there is no need for a mast to hold the turbine components and the components become more accessible).
WWTs tend to be less efficient than PVTs for the following reasons:
They often create drag when rotating.
Often installed at a lower height (ground or roof of a building) where wind speeds are lower.
The presence of vibration related problems such as noise and faster wear and tear on the supporting structure (as the airflow has more turbulence at low height).
Darier wind turbine
Patented in 1931 by the French aeronautical engineer Georges Jean-Marie Darier, the Darier wind turbine is often called the “egg beater” because of its appearance. It consists of several vertically directed blades that rotate around a central axis.
The difference between a PVT and a Darier VVT is that the axis of a propeller turbine always faces the wind, whereas a Darier turbine is a cylinder perpendicular to the airflow. Thus, part of the turbine works and the other part just spins in a circle.
The difference between a PWT and a Darier PWT is that the axis of the propeller turbine always faces the wind, while the Darier turbine is a cylinder perpendicular to the airflow. In this way, part of the turbine works and the other part simply spins in a circle.
The blades allow the turbine to reach speeds that are higher than the actual wind speed, making it suitable for generating electricity rather than pumping water, for example. The Darier turbine can operate at wind speeds of up to 220 km/h and in any wind direction.
The main disadvantage of the Darje turbine is that it cannot be switched on by itself. An external drive (e.g. a small engine or a set of small Savonius turbines) is required to start the turbine. At sufficient speed, the wind generates sufficient torque and the rotor starts to rotate around the axis with the help of the wind.
The Darier type turbine is theoretically as efficient as the propeller type if the wind speed is constant, but in practice this efficiency is rarely realised due to the physical stresses involved, the design features and the variability of the wind speed.
A special type of Darier turbine is the ‘Type H’ (or ‘Gyromill’). It works on the same principle as the Darier wind turbine to generate wind power, but instead of curved blades, 2 or 3 straight blades individually attached to a vertical axis are used.
The Savonius turbine
The Savonius turbine is a simple type of turbine that was invented in its modern form by the Finnish engineer Sigurd Johannes Savonius in 1922. It is usually used in applications requiring high reliability rather than high efficiency (e.g. ventilation, anemometers, indoor micro-production).
Savonius turbines are much less efficient than PVT and WWT Darier (about 15%, see calculation of wind energy below), but unlike the former, they work well in turbulent winds and, unlike the latter, they are self-starting. Structurally, they are stable, can withstand strong winds well and remain undamaged and operate more quietly than other types.
Unlike the Darier turbine, which is driven by a ‘lift’ force, the Savonius turbine works behind the principle of ‘aerodynamic drag’. It consists of 2-3 “buckets”: the curved elements experience less resistance when running upwind than when running downwind because of the curved shape of the buckets. In aerodynamic terms, it is this differential drag that makes the Savonius turbine spin.
Calculation of wind energy
The wind power output (P in watts) at a known wind speed is calculated using the following formula:
P = ½ x “air density” x “coverage area” x (“wind speed”)3
Above sea level the “air density” is about 1.2 kg/m3 , “wind speed” is the wind speed (m/sec) and “coverage area” refers to the area of space covered by the wind turbine rotor. It can be calculated from the length of the turbine blade:
A = π x (‘blade length’)2
However, once important technical requirements for wind turbines are taken into account (e.g. strength and wear resistance, gear ratio, bearing requirements, generator), the limit of the amount of energy which can be obtained from wind energy is reduced to 10-30% of the actual wind energy. This limit is called the ‘power factor’, which is unique for each type of wind turbine. To calculate the amount of energy extracted, this power factor (“Cp”) must be entered into the formula above:
P available = ½ x “air density” x “coverage area” x (“wind speed”)3x Cp
The power factor Cp depends on the type of wind turbine, and varies from 0.05 to 0.45.