Three is the Magic Number: A Look at Wind Turbines

Wind energy has long been one of the most widely used forms of renewable energy and wind turbines, the method by which we capture the wind, are now seemingly ubiquitous - a design that we just accept. But have you ever thought of why they are designed the way they are? 

Wind turbines having three blades may seem like an arbitrary choice but in reality it is the culmination of centuries of experimentation and modern aerodynamics. You may think that adding more blades would harness more of the wind’s energy, or that fewer blades would bring down costs, but the three-blade design represents a fine balance between efficiency, stability, and practicality [1].

Some of humanity's earliest attempts to capture the power of the wind were in Persia, where their windmills often had four or more blades. These designs, although effective at low wind speeds created significant drag especially at higher wind speeds, limiting their efficiency [2]. As wind technology progressed, similar wind mill designs were used in Europe for grinding grain and pumping water. With the rise of modern engineering, experimental methods demonstrated that fewer blades could achieve higher rotational speeds and greater efficiency. In the 20th century, NASA extensively tested two-blade turbines [3]. Unfortunately, they suffered from vibrations, mechanical stress, and noise - in part due to their lack of rotational symmetry. Eventually engineers discovered that in order to maximise efficiency and stability they needed three blades which provided the best balance - fast enough to generate power efficiently, but stable enough to avoid destructive wobbles at high wind speeds [1].

The physics behind this choice is based on fluid dynamics. Each blade acts like an air foil, generating lift as wind flows across it causing the turbine to spin. The power extracted from the wind can be expressed as:

Power (W) = 1/2 x ρ x A x Cp x v^3 [2]

• Power = Watts

• ρ (rho, a Greek letter) = density of the air in kg/m3

• A = cross-sectional area of the wind in m2

• Cp  = Power Coefficient (the ratio of the total power extracted by turbine to the total power in the wind)

• v = velocity of the wind in m/s

Of these variables  the one that changes the most is wind speed; however it is also the one that has the largest impact on the power generated since it is a cubed term.

In the design of the three blade turbine, each blade is spaced 120° apart to ensure rotational symmetry, reducing wobble and distributing aerodynamic forces evenly across the hub and gearbox [1]. An even number of blades results in uneven loading and having four or more blades add unnecessary weight and cost to the turbine. Noise is also a factor to consider: two-blade turbines make a pulsing sound as each blade cuts through the air, whilst three blade turbines rotate more smoothly and quietly, allowing them to be more suitable for use in urban environments. Aesthetic appeal plays a role too, three blades are perceived as being balanced and elegant; reducing their visual impact on their environments helping the public to accept wind farms [4].

Defining all of this is the Betz limit, the theoretical maximum efficiency (59.3%) for extracting energy from the wind [5]. Regardless of blade number, no turbine can capture all of the wind’s energy. This is because for a turbine to be 100% efficient the downstream wind speed must be 0m/s (all energy has been taken out of the wind) but this is physically impossible given that there must be wind flowing through the wind turbine for it to spin. However, having three blades approaches this limit closely enough to make them the standard [2]. Modern computational modelling has confirmed that three blades deliver optimal efficiency without excessive cost, and now almost all modern turbines rely on this configuration to generate electricity reliably and in an efficient manner[3].

The three-blade wind turbine is not simply just a random design. It is the product of centuries of refinement, from Persian windmills to NASA’s experiments, from aerodynamic theory to modern computational fluid dynamics. It is a compromise between the competing forces of: efficiency, stability, cost, and noise. Two blades are unstable, four blades are inefficient, but three blades are just right. Thus, this configuration has become the worldwide standard for harnessing one of the most important renewable energy sources, ensuring reliable electricity generation for our future[1].

References:

1. Javier B. (2017) "Organoids : A new window into Disease, Development and Discovery,” Harvard Stem Institute. Available at https://www.hsci.harvard.edu/organoids

2.  Debomita C. PhD (2024) “An Introduction to Organoids, Organoid Creation, Culture and Applications,” Technology Networks. Available at https://www.technologynetworks.com/cell-science/articles/an-introduction-to-organoids-organoid-creation-culture-and-applications-369090

3. Madeline L. (2024) “Creating brain organoids to uncover what makes us human,” Cambridge Society for the Application of Research. Available at https://www.youtube.com/watch?v=x1Pg56WWm5U&t=2207s

4. Qinying W., Fanying G., Yanlei M. (2022) “Applications of human organoids in the personalized treatment for digestive diseases,” Nature. Available at https://www.nature.com/articles/s41392-022-01194-6#Abs1

5. Ruth Lehmann a,*, Connie M Lee b, Erika C Shugart c, Marta Benedetti d, R Alta Charo e, Zev Gartner f, Brigid Hogan g, Jürgen Knoblich h, Celeste M Nelson i, Kevin M Wilson. (2019) “Human organoids: a new dimension in cell biology,” PubMed Central. Available at ​​https://pmc.ncbi.nlm.nih.gov/articles/PMC6724519/

6. Helen Shen, (2018) “Organoids have opened avenues into investigating numerous diseases. But how well do they mimic the real thing?” PNAS. Available at https://www.pnas.org/doi/10.1073/pnas.1803647115

7. Image: https://commons.wikimedia.org/wiki/File:Wind_turbines_on_blue_sky_with_clouds.jpg