The most important way to use wind power is to produce electric power through wind generators, namely aerogenerators. Electric power is obtained by exploiting wind kinetic energy: airflows move at more than 10 km/h speed making the blades of a propeller turn. They are connected to a generator that transforms mechanic power into electric power.
There are different types of aerogenerators, that differ in shape and dimension. They can have one, two or three blades, of different length. Those with 50 cm length are used as battery chargers, while those with 30 cm blades can supply 1,500 kW power, managing to satisfy the daily power need of around 1000 households.
The most popular aerogenerator is made up of a steel tower of 60-100 meter height, with two or three blades that are around 20 metres long. It generates a power of 600 kW, which equals the daily power need of 500 families.
The blades of the wind generator are fixed on a mechanical element called hub and form the rotor. According to the position of the axis, it is possible to distinguish between horizontal and vertical axis rotors. The first ones are the most common and popular; while the second ones have been used since ancient times but only recently they have been subject to studies and researches to improve their efficiency (the main advantages of vertical axes are: their constant functioning regardless of the wind direction, a better resistance even when the wind is strong and turbulent).
The structure of a wind generator with horizontal axis is simple: a support (foundations and tower) with a gondola or nacelle on the top. Inside there is a slow-driving shaft, as well as a turn multiplier, the fast shaft, the power generator and auxiliary devices (braking system and control system).
The rotor (consisting of the hub, on which the blades are mounted) is fixed at the extremity of the slow shaft.
The shape of the blades is designed in such a way that the incoming airflow activates the rotor.
From the rotor, the wind kinetic energy is transmitted to a power generator. The wind generator works according to the strength of the wind. Under 4/5 metres per second it cannot start. The minimum speed allowing the device to provide power is 10/12 metres per second to produce a few hundred kilowatts. When the speed is high (20/25 m/s) the generator is switched off for safety reasons.
The progress made in the design of aeolian rotors in the last 10 years allow them to work at lower wind speed, catching a higher quantity of energy also at higher levels, increasing the quantity of wind power that can be exploited.
Rotors with “mobile” blades have been created: by changing the blade inclination with a different wind speed it is possible to keep the quantity of power produced by the aerogenerator constant.
Onshore wind farms
Several wind turbines connected together form wind farms, which are veritable power plants. A wind farm consists of a group of wind turbines located in the same place, interconnected by a medium voltage connection network, which collects the energy produced by each turbine and conveys it to a collecting station where a transformer converts the medium voltage current into high voltage current and feeds it into the transmission and distribution system.
Offshore wind farms
The most recent wind farms are usually placed offshore, on the sea, far from the coasts, where it is possible to exploit the strong winds not delayed by obstacles. This happens on the sea surface, but also on the great lakes.
In order to produce enough electric power the place where the aerogenerator is installed has to be very windy. The assessment of the output potential of a wind power plant is a difficult and complex operation, depending on the characteristics of the winds that blow in the area where the plant is to be created.
Types of wind plants
Electric power can be used through two types of plants: plants for isolated users and plants to be connected to already-existing electric networks.
The first type of plant is the one to produce “utility” electric power supplied by small aerogenerators with less than 1 kilowatt power (1-2 metre rotor) to feed equipment in isolated areas, like radio relay stations, detectors, signalling systems, etc. these systems often compete or are used together with photovoltaic systems.
Mini wind power plants
Generally, using nominal power as a criterion for classification, we speak of mini wind power plants when the power ranges from 20 kW to 200 kW (plants with powers lower than this are considered micro-wind power plants). In the case of larger amounts, the power plants are classified as large-scale wind power plants.
Wind is abundant, economical, inexhaustible, widely distributed, and wind energy is a renewable source that does not produce any polluting emissions. Therefore, especially in the sector of electric energy production, a greater diffusion of this energy can contribute significantly to a decrease in the so-called “greenhouse gas” emissions. Furthermore, compared to solar energy and geothermal energy, wind energy has the advantage of being available in a mechanical form and is therefore easy to transform into electricity. Wind generators, furthermore, do not produce polluting chemical or radioactive substances as these consist of metal and plastic materials only.
It must also be considered that the energy produced by a wind-generator during the course of its mean life span (approximately 20 years for onshore plants and over 25 years for the offshore plants) is about 80 times greater than the amount required for its construction, maintenance, operation, dismantling and disposal. It has been calculated that two or three months are sufficient to recover all the energy required to build the wind generator and to keep it operational.
Wind energy also has some disadvantages. First of all it is an intermittent source, it varies with the seasons and on a daily basis. For this reason installing 100MW wind turbines does not mean having 100MW power continuously, but a lesser amount. The actual annual capacity is equal to 45% of the nominal capacity in the more windy zones, and an average of 30% of the nominal capacity on a global scale. In other words, in order to obtain an actual capacity of 100MW the installed power must be 250MW. Another problem that must be faced is related to the energy-transmission and distribution networks that the wind power plants are connected to, which must be predisposed to receive electricity with an intermittent flow, and an average tension. The distribution networks that are currently present in the industrialized countries are designed for opposite characteristics, as they are connected to few very large power plants with a very high tension and a controlled and predictable flow of energy. Passing to a production of energy coming from many small-sized wind-energy or other plants, requires adequate and costly changes in the electricity distribution network.
The visual impact of an aerogenerator of a 40-60 metre tall wind power plant is obvious, but it can be downsized by building plants at a certain distance from the nearest urban center. Today the visual impact is reduced by positioning the machines on a single row and using neutral colours (like white).
The potential acoustic pollution caused by aerogenerators determines two types of noise: mechanical noise that comes from the generator and aerodynamic noise caused by the rotor blades.
With regard to noise, in terms of decibels, the humming of aerogenerators is far lower than town noise.
Effects on plant and animal life
With regard to possible changes in plant and animal life, based on the available information, it has been reported that possible relevant interferences have been noted only with regard to the birds’ impact with the machine rotors. Generally, collisions are rare, and mostly limited to birds of prey. Migratory birds instead seem to adapt to the presence of these obstacles.