Wind Power

Wind Power

Wind is a form of solar energy. Winds are caused by the uneven heating of the atmosphere by the sun, the irregularities of the earth’s surface, and rotation of the earth. Wind flow patterns are modified by the earth’s terrain, bodies of water, and vegetative cover. Wind energy is the kinetic energy of air in motion. This wind energy can be harvested. Wind power is the conversion of this wind energy into a useful form of energy, such as electrical power by using wind turbines, mechanical power by using wind mills, pumping or drainage of water by wind pumps and as sails to propel ships. Wind energy is a renewable or non-conventional source of energy. This is clean and non polluting energy source. It is available in large amounts in many parts of the world. It does not generate any greenhouse gasses during the production of electricity.

The total amount of economically extractable power available from the wind is very high.  Axel Kleidon of the Max Planck Institute in Germany carried out a ‘top down’ calculation on how much wind energy there is, starting with the incoming solar radiation that drives the winds by creating temperature differences in the atmosphere. He concluded that somewhere between 18 TW and 68 TW (Terawatt which is one trillion watts) could be extracted. Cristina Archer and Mark Z. Jacobson presented a ‘bottom-up’ estimate based on actual measurements of wind speeds. As per this estimate there is 1700 TW of wind power available at an altitude of 100 meters over land and sea. Out of this available power, between 72 and 170 TW could be extracted in a practical and cost competitive manner. They later estimated it to be 80 TW. However research at Harvard university estimates 1 Watt/Sq m on an average and 2 to10 MW/Sq km capacities for large scale wind farms, suggesting that these estimates of total global wind resources are too high by a factor of around 4.

The generation of electric power from wind energy takes place with the help of wind turbines. Each wind turbine is coupled with a generator (alternator). Simply stated, a wind turbine is the opposite of a fan. Instead of using electricity to make wind, like a fan, wind turbines use wind to make electricity. The wind turns the blades, which spin a shaft that connects to a generator to produce electricity. A number of turbines are connected together to get the desired output. This assembly of large number of wind turbines is called a wind farm.  A wind farm is normally constructed where the wind speed is sufficient to move the blade of turbine.

Power from wind energy

The use of wind energy for generating power is growing at a very fast rate. Wind power uses wind turbines to harvest the energy of the moving air and convert that energy into electricity. The principle of generating wind power from wind energy is as follows.

Total wind energy flowing through an imaginary area A during the time t is given by following equation

Equation 1Where ? is the density of air, v is the wind velocity, Avt is the volume of air passing through A (which is considered perpendicular to the direction of the wind), Avt? is therefore the mass m passing per unit time. Note that ½ ?v2 is the kinetic energy of the moving air per unit volume.

Power is energy per unit time, so the wind power incident on A (e.g. equal to the rotor area of a wind turbine) is:

Equation 2

From the above equation the following are inferred.

  • Power is directly proportional to air density ?. As air density increases, the power of the turbine increases.
  • Power is directly proportional to swept area of the turbine blades. If the length of the blade is increased, the radius of the swept area increases accordingly so turbine power increases.
  • Wind power also varies with velocity and in an open air stream it is proportional to the third power of the wind velocity (v). The available power increases eight folds when the velocity of wind is doubled. Wind turbines for grid electricity therefore need to be especially efficient at higher wind speeds.

Wind turbines

In wind power plant wind turbine uses kinetic energy, presents in wind to rotate prime mover of alternator (generator) to make electricity. When a sufficient wind hits the blades of the turbine, they rotate. Blades are coupled with a rotor. So when blades move, rotor also moves. In a wind turbine, pitch system control the speed of the rotor. Rotor is connected to low speed shaft. This low speed shaft is connected to the high speed shaft of the generator by means of a gear system. The gear system raises the rotational speed of generator shaft to the normal speed of a common generator. This high speed generator produces electricity.

Wind turbines also consist of a controller when to start or stop the machine. Normally wind turbines are operated within a range of wind velocities. When the wind velocity crosses the lower limit, the turbine is started and the turbine is automatically stopped when the wind velocity reaches the upper limit also known as survival wind velocity. All wind turbines are designed for this maximum wind velocity (survival wind velocity)

Wind turbines have an anemometer which determines the wind velocity and sends regular information to controller whether the wind velocity is high or not. Brake works in emergencies to stop the rotor mechanically, electrically, or hydraulically. Wind turbine also contains wind vane, yaw drive and yaw motor. Their functions are to measure the wind direction, and to adjust up wind turbines to stay in front of wind when the direction of wind changes.

There are generally two kinds of wind turbines. Horizontal axis wind turbine (HAWT) and vertical axis wind turbine (VAWT). Horizontal axis is divided as upwind and downwind whereas vertical axis is divided as a drag based and lift based.

In HAWT upwind turbine, the shaft of turbine and alternator both are aligned horizontally and the turbine blades are placed at the front of the turbine that means air strikes the turbine blades before the tower. In the case of HAWT downwind turbine the shafts of the rotor and generator are also placed horizontally but turbine blades are placed after the turbine that means the wind strikes the tower before the blades.

If we observe VAWT drag based turbine, the generator shaft is located vertically with the blades positioning up and the turbines are normally mounted on the ground or on a tiny tower. In the case of VAWT lift based turbine, the generator shaft is placed vertically with the blade’s position is up. Most large modern wind turbines are horizontal axis turbines because of their high efficiency. Since the blades always move perpendicularly to the wind, and receive power through the whole rotation. The turbine has the following main components.

  • Blade or rotor which converts the energy in the wind to rotational shaft energy
  • Drive train including a gearbox and a generator
  • Tower that supports the rotor and drive train
  • Balance equipments include controls, electrical cables, ground support equipment, and interconnection equipment.

The components of a wind turbine are shown in Fig 1

Components of wind turbine

 Fig 1 Components of a wind turbine

 A wind farm is a group of wind turbines in the same location used for production of electricity. A large wind farm may consist of several hundred individual wind turbines distributed over an extended area, but the land between the turbines may be used for agricultural or other purposes. A wind farm may also be located offshore.

Almost all large wind turbines have usually the same design which consists of a horizontal axis wind turbine having an upwind rotor with three blades, attached to a nacelle on top of a tall tubular tower. In a wind farm, individual turbines are interconnected with a medium voltage (around 33 kV), power collection system and communications network. At a substation, this medium voltage electric current is increased in voltage with a transformer for connection to the high voltage electric power transmission system.

The design specification for a wind turbine is usually based on a power curve and guaranteed availability. The typical operating temperature range is ?20 to 40 deg C. In the areas with extreme climate and hot weather versions are required. Wind turbines can be designed and validated according to IEC 61400 standards.

The aerodynamics of a HAWT is a bit complicated. The air flow at the blades is not the same as the airflow far away from the turbine. The aerodynamics at the rotor surface exhibits phenomena that are rarely seen in other aerodynamic fields. In 1919 the physicist Albert Betz showed that for an ideal wind energy extraction machine, the fundamental laws of conservation of mass and energy allowed no more than 59.3 % of the kinetic energy of the wind to be captured. Modern turbines approach this Betz law limit and can reach 60 % to 70 % of this theoretical limit

Wind turbines are normally designed to produce a maximum of power at wide range of wind velocities. All wind turbines are usually designed for a maximum wind velocity. Wind turbines have the following three modes of operation.

  • below rated wind velocity operation
  • around rated wind velocity operation (usually at nameplate capacity)
  • above rated wind velocity operation

If the rated wind velocity of wind is exceeded the power has to be limited. This is done in different ways. A control system involves three basic elements consisting of sensors to measure process variables, actuators to manipulate energy capture and component loading, and control algorithms to coordinate the actuators based on information gathered by the sensors.

Other issues of wind power

  • Since the velocity of wind not constant, a wind farm’s annual energy production is never as much as the sum of the generator nameplate ratings multiplied by the total hours in a year. The ratio of actual productivity in a year to this theoretical maximum is called the plant load factor (PLF). Typical PLF achieved are 15 % to 40 %. Higher PLF values at the upper end of the range are achieved in favorable sites and are due to wind turbine design improvements.
  • Wind power hardly ever suffers major technical failures, since failures of individual wind turbines have hardly any effect on overall power, so that the distributed wind power is highly reliable and predictable.
  • Although wind power plants have relatively little impact on the environment compared to fossil fuel power plants, there is some concern over the noise produced by the rotor blades, aesthetic (visual) impacts, and birds having been killed by flying into the rotors.
  • The major challenge to using wind as a source of power is that it is intermittent since wind does not always blow when electricity is needed. Wind cannot be stored (although wind-generated electricity can be stored, if batteries are used), and not all winds can be harnessed to meet the timing of electricity demands. Further, good wind sites are often located in remote locations far from areas of electric power demand.
  • Wind resource development is to compete with other uses for the land, and those alternative uses may be more highly valued than electricity generation. However, wind turbines can be located on land that is also used for grazing or even farming.