Investigating the Effective Design of the Modern Wind Turbine

Wind turbines — there are over 300,000 of them around the world generating 600 gigawatts of energy, enough to continuously power almost 400 million homes. You may have seen them through your car window across a windy landscape and admired their astounding stature and sleek design. Every 24 hours, the kinetic energy of wind produces over 30 times as much electricity as humanity uses each day, and unlike coal or oil, this source is completely renewable every day. To understand the simple yet incredibly effective contraption of the wind turbine, we must reverse-engineer the fundamental aspects of the everyday generator.

How Does a Wind Turbine Work?

Before we delve into the physics behind the wind turbine’s design, we must understand how motion energy from the rotor blades is converted into power. The essential electrical conversion occurs inside the nacelle, the hub at the top of the tower that stores the generator.

Under operating circumstances, the blade rotor rotates a long shaft inside the nacelle; here, a ring of magnets is turned around a metal coil connected to an external circuit. When there is a relative motion between a conductor (such as metal coil) and a magnetic field (created by magnets), an electromotive force is induced in the conductor. This induced force can cause an electric current to flow if the conductor forms a closed loop.

This effect can be justified under Faraday’s law of electromagnetic induction, which states that whenever a conductor is placed in a varying magnetic field, an electromotive force is induced. In fact, the curl, the cross-product of the del operator and the electric field that quantifies the circulation of a field at a certain point, is equal to the rate of change of the magnetic field with respect to time. In simpler terms, when the rate of change in the magnetic field caused by spinning magnets varies around a coil, it induces an electric current in the wire, leading to electricity generation.

The type of current generated depends on the nature of the motion and the design of the system. If the magnet rotates continuously in one direction relative to the coil, it will produce direct current (DC). However, if the direction of rotation changes periodically (as in a typical wind turbine design), it will produce alternating current (AC). This principle is fundamental to how many generators and alternators work, converting mechanical energy from the motion of the magnets into electrical energy.

Optimizing Its Size

The first step of an effective wind turbine design is establishing its overall size and proportions. The height of the tower is perhaps the most self-explanatory element: the higher up you go, the less the input wind is impeded by ground objects like trees and mountains, and therefore the more wind energy it can successfully capture.

When deciding how large the blade radii should be, we acknowledge that larger blades generally capture more energy from the wind, but they may also introduce more drag due to the overbearing weight. Optimal efficiency is crucial; hence, the blade size should balance between maximizing energy capture and minimizing drag. With that being said, the bigger the area of the rotation, the more wind we can use — therefore the more wind energy is captured. Accordingly, for a wind turbine to capture optimal wind force, it must be both giant and tall; today, most wind turbines stand at over 100 meters tall with 50-meter blades.

Shape of Blades

A common paradox of wind turbines is that they must capture energy from the wind while letting the wind pass itself. If a wind turbine took 100% of the wind’s energy, the air would come to an abrupt halt right past the blades, and this stationary air would block any further air from blowing through the system — thus we must let a certain amount of wind through.

In 1919, German physicist Albert Betz concluded that the theoretical maximum efficiency for a wind turbine is 59.3%, meaning that at most only 59.3% of the kinetic energy from wind can be successfully converted to spin the turbine and generate electricity. This value, or the Betz Limit, was derived by isolating the ratio of the downstream velocity (Uₔ) to the upstream velocity (Uᵤ), which maximizes the efficiency equation at a ratio of 1:3 (Uₔ/Uᵤ = 1/3); and substituting this ratio back into the equation, we conclude that the maximum efficiency of the system is 0.593, or 59.3%.

Since wind turbines can not block too much of the incoming wind force, they are faced with a tradeoff of either having fast-moving blades that cover small areas or slow-moving blades that cover large areas. This is because the higher the acceleration of the blades, the more wind it catches; therefore blades need to be relatively thin and few so as to not slow the wind too much. On the contrary, a wind turbine that employs slow-moving blades can have wide and large quantities of blades. From commonplace observations of modern wind turbines, we understand that engineers today have opted for more fast-moving blades rather than traditionally wide, slow-moving blades.

The reason behind this change lies in Newton’s Third Law: with every action comes an equal and opposite reaction. As the wind pushes the blades sideways, the blades push back on the wind, creating a force that gives the subsequent wind a reverse twist and hence losing out on a quantity of rotational kinetic energy. The most efficient wind turbine would give the air as little rotational twist as possible — and this can be achieved by utilizing fast-moving blades.

The nature behind this phenomenon can be explained through a simple analogy made by Minutephysics. A similar occurrence happens when a ball falls and bounces off of an angled block. If the block is not moving, conservation of momentum and energy means that the ball bounces left and the block gets pushed right. However, if the block starts off moving to the right, it is able to absorb more of the ball’s energy when it accelerates; the faster the block moves, the more energy it extracts from the ball.

The design of the wind turbine blade employs the same idea: the faster the blades, the more energy they can extract from the moving wind. For decent efficiency, a blade should move at a velocity of at least five times faster than that of the incoming wind. In summary, the ideal wind turbine blade is one that maximizes speed through a thin shape and relatively light materials.

Engineers persevere to achieve as close to the Betz Limit as physically possible, and the best current solution is through the use of airfoils. Similarly to the shape of a plane wing, airfoils consist of a curved teardrop shape where the curved top surface is longer than the flat bottom surface, creating a “camber”. This shape causes the air to move faster over the curved top surface compared to the flat bottom surface. According to Bernoulli’s principle, faster-moving air has lower pressure than slower-moving air — therefore, the pressure above the airfoil decreases more than the pressure below it, creating a pressure difference. The pressure difference between the top and bottom surfaces of the airfoil results in a net upward (perpendicular) force called lift; this lift force is what allows airplanes to fly and wind turbines to rotate. With this mechanism, wind turbines can capture up to 50% of the wind’s energy — and they are still improving.

Moreover, the angle at which the airfoil meets the oncoming airflow, known as the angle of attack, is crucial to implement. This is why modern wind turbines frequently incorporate a twist to optimize how much of the blade can cut through the wind. At low angles, the airfoil generates little lift, and as the angle of attack increases, the lift also increases until reaching a critical angle where the flow can become turbulent, leading to a subsequent decrease in lift. The critical angle of attack is typically around 15°-20° for most airfoils.

This critical angle of attack is also what causes stalling on planes, experiencing a sudden drop in lift. To prevent stalling, planes are equipped with high-lift devices like flaps and slats. By deploying these devices, pilots can modify the wing’s shape, increasing its camber and therefore its ability to generate lift even at higher angles of attack.

Another important aspect to consider is the angle the wind turbine is facing. Modern horizontal wind turbines optimize energy generation by turning to directly face the movement of the wind perpendicularly. This turning process is called “yawning” and used to be completed through manual monitoring. Today, wind sensors and computer programs are used to automatically adjust the blades to precisely face the direction of the wind.

Number of Blades

According to Siemens in 2007, modern three-blade wind turbines have combined intelligent blade design and a well-chosen rotational speed of up to 80% of the Betz limit (approximately 51% efficiency). The more blades that a wind turbine has, the more torque it produces and the slower the rotation speed, due to the increased drag caused by resistance to wind flow. Therefore, as demonstrated through Newton’s Third Law, greater power generation results from a smaller number of blades.

Due to its reduced drag, a one-blade design is the optimal number for maximum efficiency; however, a single blade causes imbalance and, hence, is not practical. On the other hand, two-blade wind turbines endure an unbalanced torsional force acting at the center of the blade, causing them to vibrate and lose kinetic energy.

Three-bladed turbines can achieve a great compromise between capturing wind energy efficiently by having enough blade area (torque) and minimizing aerodynamic losses due to drag. Furthermore, they are less exposed to vibration and fatigue compared to fewer blades, which can extend the lifespan of the turbine and reduce maintenance costs.

Material of Wind Turbines

A final element of the intriguing design of the modern wind turbine is the materials used to optimize its performance. Most blades are made from fiberglass-reinforced plastics and resin layers. These composites offer a combination of strength, flexibility, and light weight, allowing the blades to efficiently capture wind energy while enduring the stresses of heavy rain, critical lightning, and blistering sunlight for over 20 years straight.

Wind turbine towers are typically made of steel or concrete. Some designs or installations might even use hybrid or modular structures combining steel, concrete, and other materials. Steel offers strength and durability, supporting the weight of the turbine components and withstanding dynamically changing forces. Concrete towers can be an alternative in certain situations, especially for onshore installations, providing more stability and durability.

In Conclusion

Understanding the sophisticated intricacies of modern wind turbine design offers a meticulous balance of engineering, physics, and material science. The focus on blade shape, size, and material selection highlights the goal of optimizing energy capture while ensuring durability against environmental factors. With three-bladed, airfoil configurations emerging as a preferred choice for their balance between efficiency and stability, and materials like fiberglass-reinforced plastics and steel offering strength and longevity, today’s wind turbines represent a culmination of decades of research and innovation. As we continue to harness wind energy on an unprecedented scale, these technological inventions highlight our commitment to sustainable energy solutions and emphasize the importance of ongoing advancements in turbine design and materials.