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Wind energy: horizontal axis wind turbine technology Foreword Wind energy can be exploited, converting it into mechanical or electrical energy, thanks to special machines, i.e. wind turbines. There are several types of wind machines, which differ in terms of system configuration (horizontal or vertical axis), energy production (large and small wind turbines) and site location (at sea, on land, on buildings). In this article we will focus on large horizontal axis wind turbines.

The advantages and disadvantages of the technology Wind energy is one of the most important sources of renewable energy, able to harness the solar energy that on Earth is transformed into wind. Wind turbines are based on exploiting the kinetic energy of wind to generate mechanical energy, in turn converted into electrical energy thanks to the use of machines called wind turbines. A set of individual wind turbines connected to each other constitutes a wind farm.

An example of a wind farm, situated in Montemurro (PZ) at approx. 1000 m above sea level, consisting of 36 wind turbines for a total nominal power of 29 MW, able to generate electricity for the annual requirement of approx. 50,000 people (source: Vento di Montemurro Srl). Photo: Dario Colucci

The main advantages of this alternative energy source can be summarised as: no pollutant emissions during electricity generation and selling price of electricity generated very competitive compared to other renewable sources. The negative aspects are mainly: non-constant generation of electricity (wind speed is in fact variable over time and depends on the installation site), possibility of a significant environmental impact, mostly visual and high implementation costs. The cost of installation of a system depends on several factors. For example, for on-shore (on land) installations, the cost is approx. one million euros per installed MW, but if the system is installed in a sufficiently windy place, the construction costs can be recovered in just a very few years.

Where and how to install wind turbines The choice of the site for a wind farm is very important. First of all, it is necessary to assess the productivity of the site itself and then define the continuity and intensity of the wind in the area of interest. A reliable assessment of the

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productivity of a wind farm must therefore be based on anemometric sampling, performed directly at the site where the farm is to be built. Using anemometers (instruments capable of measuring wind characteristics), at a height of at least 2/3 that of the future turbine, wind speed and direction are measured over time. The duration of these measurements should not be less than 12 months, and a good anemometric sampling should last at least a couple of years. Generally, the best sites to build a wind farm are offshore, but they are also installed on mountains and hills. The main reason why these areas are preferred is the absence of obstacles that could cause turbulence and thus negatively affect the wind speed and direction at the expense of electricity generation.

400 MW off-shore wind farm in Denmark (Source: siemens.com)

Type of installations for off-shore wind turbines (Source: boem.gov)

Turbulence can be generated by an infinite number of factors: it may develop due to the presence of trees, of vegetation therefore, or may be linked to anthropogenic factors, such as buildings, houses or other structures. Several factors are directly related to the topography of the land, while another very important aspect to take into consideration during the design phase is the evaluation of the distance between the various wind turbines. Having these wind turbines too close to each other can easily create significant disturbance that can affect the wind intensity. In general, the distance between two adjacent towers must be approx. 2 twice the diameter of the rotor, while the distance between two towers arranged

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"one behind the other" should be approx. 4 times the diameter of the rotor. Consequently, the position of the wind turbines is of primary importance and their positioning can follow different patterns: square or rectangular lattice, rectilinear arrangement, in parallel or criss-crossed rows, or can simply follow the topography of the terrain.

Example of a wind farm with rectilinear layout of the machines (Photograph by Bertrand Rieger, Hemis/Corbis. nationalgeographic.com)

Example of a configuration with dense criss-cross rows (on the plains) and according to the topography of the terrain (on the crest of a mountain).

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The components of a wind turbine The main components of a wind turbine are: the foundations, the tower, the nacelle and the rotor.

The main components of a wind tower.

The foundations The foundations are very impressive, often completely underground, and are built using reinforced concrete. They support the entire machine and help it withstand the numerous oscillations and vibrations caused by the kinetic energy of the wind. The foundation plinths and pillars reach considerable depths, on average approx. 20-25 metres into the ground.

Construction of the foundations of a 2.5 MW wind tower (wattsupwiththat.com)

The tower The tower supports the nacelle and the rotor. It can be a steel pylon (like the high voltage power pylons) or in tubular

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steel due to its better stress resistance. The latter are characterised by several segments that are assembled on site. Inside is a steel ladder that leads to the nacelle, electrical cables and the computer that manages the entire machine.

The interior of a small tubular steel (47 metre) Vestas V47 - 660 kW wind tower situated at the "Vento di Montemurro Srl" wind farm in Montemurro - Basilicata. Photo: Dario Colucci.

Example of wind farms with pylon tower (Ph. Jan Tångring) in California.

The rotor and the blades The rotor consists of a hub on which the blades are fixed. There are several rotor configurations with one, two and three blades. The performance is very similar but with the configurations with one and two blades vibration and noise increase. The blades are made of steel and fibre glass, even if the most modern are made of carbon fibre. They are in fact much lighter, but with prices that rise sharply. The blades are flexible and have a so-called "warped" shape that gives them better aerodynamic characteristics and thus allows them to optimally exploit the wind's kinetic force, "capturing" more energy. Generally the height of the tower is approximately equal to the diameter of the blades.

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Transporting a blade (https://tallbloke.wordpress.com/category/wind/).

Section of a B75 blade during the construction phase (Source: siemens.com).

Transporting a blade (Source: technologijos.lt).

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The nacelle The nacelle is positioned on the tower. Its main characteristic is rotation on its axis according to the wind direction. Inside the nacelle there are a number of components that convert the kinetic energy of the wind into electricity: the output shaft, the gearbox, the input shaft, a brake system and the electricity generator. The output shaft is directly connected to the hub and its rotation speed (15-30 rpm) is directly related to that of the wind. The gearbox has the function of converting the low rotation speed of the output shaft into the higher speed of the input shaft, up to 2000-3000 rpm. This value allows the proper functioning of the electricity generator, which reaches the frequency value of the desired voltage, so it can be sent directly to the national grid. The electricity generator consists of a constant frequency alternator that produces alternating current with a frequency of approx. 50Hz, i.e. a rotation speed of the rotor, connected to the input shaft, equal to approx.50 revolutions per second, approx. 3000 rpm. This rotation speed of the rotor generates a magnetic field that produces an electromotive force induced in the stator (i.e. Faraday) windings, thus converting the kinetic energy into electricity. The brake system controls the power of the wind turbine, i.e. slows or stops the rotor in the event of excessive wind speed or following malfunctions. Many wind turbines also have a remote generation or fault control device. Monitoring, control and statistical analysis can be viewed locally or remotely. For more details on the construction procedures of the various parts of a wind turbine, please refer to the Gamesa website: (http://www.gamesacorp.com/en/products-and-services/wind-turbines/design-andmanufacture/manufacturing-and-assembly-process.html).

Components housed in the nacelle of a standard wind turbine. Diagram of a Vestas V47 - 660 kW (Source: vestas.com)

by Dario Colucci and Marco Suanno