Wind power

WIND POWER

 
Wind energy is the kinetic energy of air masses in motion all around the globe.
Wind energy is an indirect form of solar energy: solar radiations absorbed in the atmosphere lead to differences in temperature and pressure. 
It's a renewable energy, which produces no greenhouse gas emissions during the operating phase.
Thus air masses are set in motion and accumulate kinetic energy. This can be transformed and used for several purposes: 

• The transformation into mechanical energy: the wind is used to move a vehicle (sail-boat or sand yachting) for pumping water (wind turbine pumping for irrigation or livestock watering) or to rotate the wheel of a mill;

• The production of electrical energy: the turbine is coupled to an electric generator to produce direct or alternating current. The generator is connected to a power grid or works in an "autonomous" system with an extra generator (e.g. a generator set), a batteries park or other energy storage device.

Operating modes of the wind energy:
 
Onshore wind turbines: They are installed on the hearth
    There are two ways of using onshore wind energy:

• Industrial use as part of a wind farm (a set of wind turbines are grouped on the same site which is connected to the high voltage electrical grid).



Row of wind turbines in Alberta, Canada (© 2013)

• Domestic use under the “small wind energy”(some onshore wind turbines, of smaller wingspan, can also meet the needs of individuals or small farmers). The turbines used in this case operate on the same principles of the conversion of kinetic energy of wind. Their power varies between 0.1 and 36 kW up to their mast is less than 35 m. These turbines can power individual buildings not connected to the grid or be connected to the network to sell the production.

(For more information, see “Onshore wind turbines”)

Offshore wind turbines: They are installed in the sea where the wind is stronger and more constant.
  There are two ways of using offshore wind energy:

• Until now, offshore wind projects include installing wind turbines in shallow water or moderately deep, 5 to 40 meters deep. Beyond it is difficult and very expensive to plant the wind turbine in the seabed or to deposit its base.
• New projects of floating wind turbines "far shore", located several kilometres from the sides and at over 50 meters deep, offer interesting perspectives.They will only be anchored to the seabed by means of cables.

(For more information, see “Offshore wind turbines”

Operation of a conventional wind turbine (three-bladed horizontal axis):
Due to its high efficiency, the three-bladed horizontal axis wind turbine design dominates, today, massively the onshore wind market. The transformation of the kinetic energy of wind into mechanical energy and then into electrical energy is made in four steps:

1. The transformation of the wind kinetic energy into mechanical energy. The blades function according to the same principle as the wings of an air-plane: the pressure difference between both sides of the blade creates an aerodynamic force, setting in motion the rotor.
2. The acceleration of the rotational movement through the multiplier. The blades of a large wind turbine rotate at a speed between 5 and 15 revolutions per minute (more greater is the wind-turbine, slower is the rotational speed). However, most wind turbine generators must rotate at high speed (1 000 - 2 000 rpm) in order to produce electricity. This increased speed is carried out by using of the multiplier, also called gearbox. The multiplier is a heavy and expensive work piece. This is why some large wind turbines work with some low-speed generators said "direct attack” in which the rotor directly drives a generator specially designed without intermediate gear set.
3. The production of electricity by the generator. The generator located in the nacelle of the wind turbine is driven by a mechanical shaft. The transmitted mechanical energy is converted into electrical energy by the generator at a voltage of 600 to 1 000 volts.
4. Treatment of electricity by the converter and the transformer. The electricity produced by a wind turbine is then processed through an electronic converter. It adjusts the current frequency produced by the wind turbine to the electrical network frequency to which it is connected (50 Hz in Europe). The voltage is also increased by a transformer up to 20 000 or 30 000 volts. Electricity is then sent through a cable to be injected into the electric network.

Calculation of the wind turbine power:
The electrical or mechanical energy produced by a wind turbine depends on three parameters: the shape and length of the blades, wind speed and, finally, temperature which influences the density of the air.
The wind velocity, and, therefore, the mechanical energy stored by the wind turbine rise with altitude. (When altitude doubles, the wind speed increases from 10%).
The disruption of air currents is less important in height. 

The recoverable energy corresponds to the kinetic energy that it is possible to extract. It is proportional to the area swept by the rotor and to the cube of the wind speed surface.

The maximum recoverable power (P) is given by the Betz Law:

 P = 0.37 x S x V3

where 0.37 is the air constant at standard atmospheric pressure   (1 013 hPa), S is the swept surface and V is the wind speed.

In practice, a wind turbine produces four times more energy if the blade is twice as large and eight times more power if the wind velocity doubles. The density of the air also comes into play: A wind turbine produces 3% more electricity if, for a given wind speed, the air is colder than 10°C.
The set blade/rotor is oriented upwind by a rudder system. Most wind turbines start up when the wind speed reaches about 3 m/s and stops when the speed reaches 25 m/s. Generally, wind turbines are configured to best exploit the winds of intermediate power. 

 The strengths of wind energy:

• Wind energy is renewable and clean.
• In its operational phase, this energy is almost completely independent of fossil fuels.

The land where wind turbines are installed remains usable for industrial and agricultural activities. The system can be dismantled relatively easily.

• Their offshore development has a significant potential.
• Implanted locally, wind turbines can be used to meet electrical needs large-scale just like to limited domestic needs, depending on the size of the turbine.

The cons:

• Wind power depends on the strength and regularity of the wind.
• This is a source of intermittent energy.
• The development areas are limited.
• Wind turbines can generate conflicts about environmental issues such as visual and noise nuisances.
• There may be territorial conflicts with other users (farmers or fishermen).

Integration of wind turbines according to the energy structure of countries:
Wind energy can integrate beneficially into the countries whose structure of energy production is based on fossil fuels. The variable character of wind power is offset by the great availability of gas, oil or coal power stations (Average time to start up production 45 minutes). This is particularly the case of Germany.

In contrast wind energy is less suitable in countries where the structure of energy production is based on sources less easy to control, and therefore less able to respond to sudden changes in demand for electricity. This is particularly the case of France and its nuclear reactors (Average time to start up production: 15 days).

Return to "Renewable energies"