In a world where the urgent need for sustainable energy solutions is ever-present, wind energy is one of the renewable sources that is paving the way for sustainable power generation and reducing greenhouse gas emissions. And wind turbines are at the forefront of technologies helping to harness wind energy.

Wind turbines operate by converting the kinetic energy of the wind into rotational energy, which is then used to generate electricity through a generator. These magnificent structures not only captivate the eye (see Figure 1), but they also hold the key to unlocking a vast source of clean and renewable power. For instance, Global Wind Energy Council statistics show that the total installed wind power capacity is 837 GW, avoiding over 1.2 billion tons of carbon dioxide (CO₂) emissions annually.

Figure 1: Wind turbines operate by converting the kinetic energy of the wind into rotational energy, which is then used to generate electricity through a generator. Source: mimadeo/Adobe Stock

However, understanding the fundamental workings of wind turbines and the basic calculations governing their operation is essential for anyone looking to design (or specify) them for an application.

Operating principle of a wind turbine

Figure 2 shows the schematic of a simple wind turbine with its key components. It features a tower, rotor blades, a generator, and other essential components like the gearbox and control system. These components operate to produce electricity using the rotor blades’ aerodynamic force.

Figure 2: Schematic of a wind turbine showing the rotor blade, gearbox, generator, tower, and power cables. Source: Bomazi/Public domain

The rotor blades of a wind turbine are designed to have a curved shape, similar to an airplane’s wings. When the wind blows over these rotor blades, it causes a decrease in air pressure on one side of the blade, resulting in a difference in air pressure across the blade. As a result, lift and drag forces are generated, which the turbines utilize to cause the rotor blades to spin.

The gearbox acts as an intermediary between the rotor blades and the generator, increasing the rotor blade’s rotational speed and allowing a smaller generator to be used. Meanwhile, the control system continuously monitors essential parameters like wind speed, rotor speed and power output. Based on this information, the control system regulates the angle of the rotor blades and other parameters to ensure stable and efficient operation.

[Learn more about wind turbines on GlobalSpec]

The power extracted from the motion of air

To understand the parameters that affect the maximum power wind turbines can extract, consider the wind passing perpendicularly through the cross-section area (A) of a small volume. In such a scenario, the maximum power (or theoretical power) available from the motion of the wind through this volume is given by:

Where:

p is the air density (kg/m³)

A is the cross-section area (m²)

V is the upstream wind speed (in m/s).

The actual power extracted by the wind turbine

In reality, the maximum power that wind turbines can extract from the motion of the wind is usually a fraction of the theoretical power due to air drag and friction of the air on the rotor blades. Wind turbine manufacturers usually account for these losses by modifying the theoretical power equation to include a power coefficient (C_p). Therefore, the extractable power by a wind turbine experiencing an upstream air velocity of V is given by:

Where:

A is the rotor blade’s swept area (m²)

The power coefficient typically ranges between 0 and 0.40 in practical wind turbine technologies. Therefore, for a fixed power coefficient, the maximum power that wind turbines can extract depends on the air density, rotor blade swept area and the upstream wind speed. For instance, consider a simple case of a wind turbine design with a swept area of 2000 m² and a power coefficient of 0.40. If this turbine is subjected to an upstream wind speed of 13 m/s with an air density of 1.29 kg/m³, the extracted power by the wind turbine would be 1.13 MW.

However, let’s consider a scenario where the wind speed increases to 13.5 m/s. Then, the new extracted power would be 1.27 MW, representing about a 12% increase in the extracted power. This is one of the reasons engineers consider wind speed as one of the key factors affecting the overall performance of wind turbines. As such, when finding an ideal location to mount wind turbines, the wind speed of the region must be considered.

Other considerations for wind turbine design

The equation for the maximum power shows how different basic parameters affect the overall performance of wind turbines. However, there is more to designing wind turbines than just specifying these parameters.

For instance, engineers also consider the aerodynamic performance of the system by designing blades to have the optimal airfoil shape, which affects the lift and drag characteristics. Computational fluid dynamics (CFD) simulations are usually used to simulate airflow over the blade and perform parametric studies to determine the optimal rotor blade design.

Since wind turbines are huge systems mounted several feet above the ground and with rotor diameters of up to 120 m, they must be designed to withstand the structural requirements of the materials and forces generated by the wind. For instance, engineers design rotor blades to withstand dynamic loads generated by the wind and centrifugal forces generated due to their rotation.

Conclusion

Wind turbines are efficient technologies that are paving the way for harnessing wind energy. While this article presents useful information about wind turbine designs, many more considerations and calculations go into the design of this complex system. Therefore, it is recommended to work with wind turbine manufacturers to discuss application requirements.