One of the most important renewable energies iswind power. Their operating principle seems simple: the wind turns the blades, and a system transforms this movement into electricity. However, as we take a look at the various components, we’ll see that behind this apparent simplicity lies an unsuspected complexity. We’ll see :
- The rotor, consisting of the blades and the hub
- The mechanical transmission: main shaft and gearbox
- The electrical generation system, comprising a generator and an electrical conversion system
- The structure: the tower and its foundations
- The nacelle
- And other important devices, such as the anemometer, wind vane and brake system
I. Wind turbine rotor
The rotor of a wind turbine is an essential component that captures the wind’s kinetic energy and converts it into mechanical energy. It consists of two main parts: the blades and the hub.
Blades
Blades are aerodynamic structures that act like airplane wings, capturing the wind’s energy and transforming it into rotational motion. Blade shape and size are optimized to maximize the capture of wind energy. The main aspects of the blades are
- Aerodynamic profile: Blades are designed with an aerodynamic profile that generates a pressure difference between the front face (top surface) and the rear face (bottom surface) of the blade. This pressure difference creates a lift force, which causes the rotor to rotate.
- Blade length: Blade length has a significant impact on the power generated by a wind turbine. This is because the power captured by the rotor is proportional to the area swept by the blades, which in turn is proportional to the square of the blade length. Longer blades can therefore capture more energy, increasing the power generated. However, longer blades also increase mechanical stress and manufacturing costs, necessitating an optimum compromise between performance and feasibility. Their length is in the order of 1 to 5 meters for domestic wind turbines (0.5 to 10 kW), 5 to 20 meters for medium-sized turbines (10 to 250 kW) and 20 to 80 meters or more for large turbines (250 kW to 15 MW).
- Angle of incidence: The angle of incidence, or angle of attack, of the blades is adjustable to optimize the capture of wind energy. By modifying the angle of incidence, lift can be maximized and drag minimized, increasing rotor efficiency. On some wind turbines, the angle of incidence can be automatically adjusted according to wind speed, known as variable blade pitch.
The role of blade length in power calculation is linked to the area swept by the blades. The power captured by the rotor depends on the amount of wind kinetic energy intercepted by the blades, and this amount is proportional to the swept area. The area swept by the blades (A) is given by the following formula: A = πR² where R is the radius of the circle formed by the rotation of the blades.
Wind power (P_wind) is given by the following formula: P_wind = 0.5 * ρ * A * V³ where ρ is the air density (approx. 1.225 kg/m³ at 15°C and sea level) and V is the wind speed. The electrical power generated by the wind turbine (P_elec) is equal to the wind power multiplied by the turbine’s power coefficient (Cp): P_elec = Cp * P_wind. The power coefficient (Cp) is the wind turbine’s efficiency in converting the wind’s kinetic energy into mechanical energy. It depends on the design of the turbine and varies between 0 and the Betz limit (around 0.59).
What we’ve just said applies mainly to horizontal-axis wind turbines. These calculations are more complex for vertical-axis wind turbines, and do not apply to bladeless wind turbines or some flying wind turbines.
Hub
The hub is the central part of the rotor that connects the blades to the main shaft. It is responsible for transmitting the mechanical energy captured by the blades to the whole turbine. The hub is generally made of steel or composite materials to withstand the forces and mechanical stresses imposed by the movement of the blades.
Hub design must be optimized to ensure efficient power transmission and minimize mechanical losses. Engineers must also take into account factors such as weight, load resistance and maintenance when designing the hub.
II. Wind turbine mechanical transmission
The mechanical transmission of a wind turbine, consisting of the main shaft and gearbox, is responsible for transmitting mechanical energy from the rotor to the generator. The main shaft supports the rotational forces generated by the blades, while the gearbox adapts the rotational speed to the optimum range for the generator.
Main shaft
The main shaft is a solid mechanical element that connects the rotor hub to the gearbox. Its role is to transmit the rotational force (or torque) generated by the movement of the blades to the gearbox. The main shaft must be designed to withstand the forces and mechanical stresses imposed by blade rotation and load variations.
- a) Materials: The main shaft is generally made of high-strength alloy steel to withstand mechanical stresses and dynamic loads.
- b) Design: The design of the main shaft must take into account torsional forces, bending forces and axial forces, as well as fatigue stresses caused by load variations and operating cycles.
- c) Lubrication and cooling : To reduce wear and friction, the main shaft is usually fitted with a lubrication system. A cooling system may also be required to prevent overheating and ensure optimum operation.
Gearbox
The gearbox is an essential component that increases the speed of the main shaft before transmitting it to the generator. Most generators operate at much higher rotational speeds than wind turbine rotors, so a gearbox is needed to adapt the rotational speed to the generator’s optimum operating range. Some wind turbines, known as direct-drive wind turbines, do not use a gearbox and connect the main shaft directly to the generator.
- a) Transmission ratio: The gearbox is designed to provide a specific transmission ratio, which is the ratio between the rotational speed of the input shaft (linked to the main shaft) and the rotational speed of the output shaft (linked to the generator). This ratio depends on the characteristics of the rotor and generator, and is optimized to ensure efficient operation of the wind turbine.
- b) Gearbox types: There are different types of gearbox used in wind turbines, such as parallel gearboxes, planetary gearboxes and helical gearboxes. Each type has its own advantages and disadvantages in terms of efficiency, reliability and manufacturing costs.
- c) Lubrication and cooling: As with the main shaft, the gearbox requires a lubrication system to reduce wear and friction between the gears. A cooling system
- may also be required to prevent overheating and ensure optimum gearbox operation.
- d) Maintenance and monitoring: Gearboxes are subject to high mechanical stress and can be prone to failure. Consequently, maintenance and condition monitoring of gearboxes are essential to ensure reliable, long-lasting operation of wind turbines. Condition monitoring techniques include vibration measurement, lubricating oil analysis and visual inspection.
III. Wind turbine electrical generation
The electrical generation system of a wind turbine is responsible for converting themechanical energy captured by the rotor into electricity that can be fed into the power grid. This system comprises the generator, which produces the electricity, and the electrical conversion system, which makes it suitable for transfer to the grid.
Generator
The generator is the key component that converts mechanical energy into electrical energy. There are several types of generator used in wind turbines, such as induction generators and synchronous generators. Scientific aspects of the generator include:
- a) Principle of operation: Generators operate on the principle of electromagnetic induction, where a current is induced in copper coils in the presence of a variable magnetic field created by the rotation of permanent magnets or electromagnets.
- b) Generator types: Induction and synchronous generators are the two main types used in wind turbines. Induction generators are simpler and less expensive, while synchronous generators offer better voltage and frequency control.
- c) Cooling and insulation: Generators produce heat due to electrical and mechanical losses. A cooling system, such as air or water, is required to keep the generator temperature within an acceptable range. Insulating materials are also used to prevent short circuits between the coils and the metal parts of the generator.
Electrical conversion system
The electrical conversion system is responsible for transforming the electrical energy generated by the generator into a form compatible with the electrical grid. This system includes the inverter, transformers and protection devices:
- The inverter is an electronic device that converts the direct current (DC) generated by the generator into alternating current (AC) compatible with the electrical grid. Inverters use pulse-width modulation techniques to generate alternating current of the appropriate voltage and frequency.
- Transformers are used to increase or decrease the voltage of the alternating current generated by the wind turbine to adapt it to the requirements of the electrical grid. They operate on the principle of electromagnetic induction, and consist of copper coils wound around a magnetic core.
- Protective devices, such as circuit breakers and fuses, are used to protect the wind turbine and the electrical grid in the event of overvoltage, overcurrent or short-circuit. They detect electrical anomalies and automatically disconnect the wind turbine from the grid to prevent damage.
IV. Wind turbine tower and foundations
The tower and foundations of a wind turbine are essential elements in ensuring the stability and durability of the installation. They are designed to support the structure and withstand the forces and loads imposed by weather conditions and the vibrations generated by wind turbine operation.
Tower
The tower is the vertical structure on which the nacelle containing the generator, rotor and other wind turbine components is mounted. Scientific aspects of wind turbine tower design include:
- a) Materials and design: Wind turbine towers are generally made of tubular steel, prestressed concrete or wire mesh. The choice of material depends on factors such as turbine size, site conditions and economic constraints. Towers are designed to withstand the bending and torsional forces generated by wind and mechanical vibration.
- b) Height and diameter : Tower height directly influences the amount of energy captured by the wind turbine, as wind speed generally increases with height. However, higher towers also mean higher manufacturing and installation costs. The diameter of the tower must be adapted to the load it has to support, taking into account structural constraints and local climatic conditions.
Here’s a comparison of the different construction materials:
- Tubular steel towers are commonly used in the wind power industry. They are made of tubular steel sections welded together. They offer good load-bearing capacity, are easy to install and require little maintenance. Their main drawback is their weight, which increases transport and installation costs.
- Lattice towers consist of a metal structure in the form of a grid. They are generally lighter than tubular steel towers and can be dismantled and reassembled, making them easier to transport and install. However, they require more maintenance due to their open design, and are less aesthetically pleasing than tubular steel towers.
- Concrete towers are an alternative to steel towers. They are made of prefabricated concrete segments or cast in situ. Concrete towers are robust, durable and corrosion-resistant. However, they are heavier and more complex to install than steel towers.
Note that towers do not apply to flying wind turbines, which by definition have none.
Wind turbine foundations
The foundation is the base on which the wind turbine rests, and is a crucial element in ensuring its stability and durability. Scientific aspects of wind turbine foundations include:
- a) Foundation types: There are different types of foundation for wind turbines, such as solid concrete foundations, pile foundations or caisson foundations. The choice of foundation type depends on soil characteristics, turbine size and economic constraints.
- b) Design and sizing : Wind turbine foundations are designed to withstand the forces and moments generated by wind, mechanical vibrations and the weight of the structure. Engineers must take into account the bearing capacity of the soil, hydrological conditions, seismic forces and dynamic loads when designing and sizing foundations.
Here’s a comparison of different construction materials:
- Concrete foundations are the most common type of foundation for wind turbines. They consist of reinforced concrete blocks buried in the ground, on which the wind turbine tower rests. They offer excellent load-bearing capacity and durability. Their main drawback is their weight, which may require soil reinforcement prior to installation.
- Steel foundations are an alternative to concrete foundations. They consist of steel plates fixed to piles driven into the ground. They are lighter than concrete foundations, but may be less resistant to loads and require more maintenance due to corrosion.
- Pile foundations are used when the ground is too soft or unstable to support concrete or steel foundations. They consist of piles driven deep into the ground, on which the wind turbine tower rests. Piles can be made of steel, concrete or wood. Pile foundations offer good load-bearing capacity, but are more complex and costly to install than concrete or steel foundations.
This is particularly true of floating marine wind turbines, which are usually attached to the seabed by cables.
V. Wind turbine nacelle
The wind turbine nacelle is a crucial element which houses the main components of the wind turbine, such as the generator, gearbox and control system. The scientific aspects of the nacelle and its components, including the control, cooling and lubrication system, are essential to ensure the safe and efficient operation of the wind turbine.
Control system
The control system manages and monitors the wind turbine’s performance. It controls rotational speed, wind orientation and other parameters to optimize energy production and protect the turbine in extreme weather conditions.
The control system is responsible for monitoring and managing the operation of the wind turbine. It comprises sensors, actuators and control software that regulate energy production according to environmental conditions and power grid requirements. The scientific aspects of the control system include:
- a) Sensors: Sensors measure various parameters such as wind speed and direction, temperature, vibrations and grid voltage. This data is used to adjust turbine operation in real time.
- b) Actuators: Actuators are devices used to control the mechanical elements of the wind turbine, such as the orientation of the nacelle (wind tracking) and the angle of attack of the blades (power regulation).
Cooling system
The cooling system is essential for maintaining the optimum temperature of the wind turbine’s internal components, such as the generator, gearbox and electronic systems. Scientific aspects of the cooling system include:
a) Types of cooling : Wind turbines can use air, liquid or a combination of both cooling systems. The choice of cooling type depends on turbine size, component design and climatic conditions.
b) Heat exchangers: Heat exchangers are devices that transfer heat from internal components to the outside of the nacelle, thereby regulating component temperature.
Lubrication system
The lubrication system reduces friction and wear on moving parts such as gearbox gears and bearings. Scientific aspects of the lubrication system include:
a) Types of lubricants : Lubricants can be mineral oils, synthetic oils or greases. The choice of lubricant type depends on wind turbine design, environmental conditions and maintenance constraints.
b) Distribution system: The lubricant distribution system transports lubricant to moving parts and returns used lubricant to a reservoir for filtering and recycling.
VI. Other devices
The anemometer measures wind speed, and the wind vane measures wind direction. This information is used by the control system to steer the turbine in the optimum direction and regulate rotation speed.
Anemometer
The anemometer is an instrument that measures wind speed. It is usually located at the top of the wind turbine tower, near the nacelle. The data collected by the anemometer is essential for controlling and optimizing wind turbine operation. Scientific aspects of the anemometer include:
a) Anemometer types: There are several types of anemometer, such as cup anemometers, ultrasonic anemometers and hot-wire anemometers. Each type has its advantages and disadvantages in terms of accuracy, reliability and cost.
b) Wind speed measurement : The anemometer measures wind speed at a specific height, usually close to the height of the turbine hub. This data is used to determine whether conditions are favorable for energy production, and to regulate blade orientation.
Wind vane
The weathervane is a device that measures wind direction. It is also located at the top of the wind turbine tower. The data collected by the weathervane is used by the control system to orient the nacelle and turbine blades to the wind. Scientific aspects of the wind vane include:
a) Types of wind vanes: Wind vanes can be mechanical or electronic. Mechanical weathervanes use vanes or arrows that align with the wind direction, while electronic weathervanes use sensors, such as magnetometers, to measure wind direction.
b) Measuring wind direction : The wind vane measures wind direction at a specific height, usually close to the height of the turbine hub. This data is used to optimize the turbine’s orientation in relation to the wind.
Wind turbine brakes
The function of a wind turbine’s brake system is to slow down or stop rotor rotation in specific situations, such as excessively high wind speeds, maintenance operations or component failures. Here are the main features:
- Brake types: Wind turbines can use mechanical, hydraulic, electric or aerodynamic brakes. Mechanical and hydraulic brakes are generally located on the wind turbine’s main shaft and are actuated by actuators. Electric brakes use generator systems to create resistive torque. Aerodynamic brakes are integrated into the blade design and are activated by altering the blade angle of attack.
- Brake operation: The control system activates the brakes according to wind conditions, maintenance requirements or component failure. An effective braking system is crucial to the safety and durability of the wind turbine.
- Design and maintenance: Engineers need to design reliable, efficient braking systems that can withstand the high stresses to which they are subjected. Friction materials, brake dimensions and clamping force are just some of the elements to be considered in the design. Regular maintenance is also essential to ensure the smooth operation and long life of braking systems.