One common design element among horizontal-axis wind turbines is that they virtually always have three blades. But how do wind turbine engineers decide to use three blades, and not two or even four or even five?
This is because designers weigh various factors in developing the optimum design. Some of the most important considerations are energy production, structural design, cost, and noise.
The majority of the world’s wind turbines have three blades because they are more balanced. Two-bladed wind turbines suffer from a phenomenon called ‘gyroscopic precession’, and a single blade wind turbine would need a counter-balance and therefore be impractical and inefficient.
Typically, a three-bladed turbine is best when considering numerous factors, but this isn’t a given. Moreover, as the wind energy industry matures and advances, the number of blades on new turbines could also shift.
Structural Design & Balance
Wind turbines must be balanced and stable to promote longevity and prevent equipment failure. However, having a wind turbine with one or two blades can create structural issues.
If a turbine has just one blade, it isn’t balanced on the rotor as it spins. Also, the blade would get stuck in the 6 o’clock position, and it would be difficult to get it to start rotating. In fact, some manufacturers have made wind turbines with just one blade, but then they need a counterbalance to help resolve balance issues. Unfortunately, adding the counterweight can cancel out the advantages of lighter-weight turbine blades, so this design is uncommon.
In theory, some of the advantages of a one-bladed turbine are the lighter weight and lower materials cost, but they are typically considered impractical.
So if one blade isn’t practical, why not have 2-bladed wind turbines? Having two turbine blades would balance themselves out without needing a counterbalance. Unfortunately, if a wind turbine has two blades, it is prone to gyroscopic precession, which could cause wobbling, especially as it turns to face the wind.
Unfortunately, this wobbling puts stress on the wind turbine, causing wear and tear, impacting wind farm durability and even safety. Therefore, a wind turbine with three or more blades are more balanced, avoiding this phenomenon.
However, some wind energy engineers have attempted to overcome this stability issue with a teetering rotor by placing a hinge between the hub and main shaft. Although numerous 2-bladed wind turbines have been developed over the last 50 years, they have not achieved widespread commercial adoption.
These 2-bladed wind turbines typically have limited outputs under 1 MW, so they aren’t well-suited for utility-scale wind farms.
For example, Vergnet produces a 200 to 275 kW, 2-blade turbine. In addition, Envision Energy has created a 3.6 MW 2-bladed offshore wind turbine with a direct drive design (with no gearbox).
By contrast, turbines with three or more blades are more stable, and the angular momentum stays more constant. In addition, when one blade is pointing up, others are pointing at an angle, causing the turbine to rotate in the wind more smoothly.
Wind Turbine Drag
Air resistance or drag is a force caused by air. As the wind turbine rotates, the air particles produce drag. Although this is unavoidable, it is critical to minimise drag for optimum wind energy production. However, if there isn’t enough drag, the blades will rotate too fast, creating too much noise.
In addition, rotating very quickly doesn’t necessarily mean more energy production. In fact, wind turbines are designed to shut down at very high wind speeds to protect the equipment.
Wind turbines have both a cut-in speed when they start producing power and a cut-out speed where the turbine shuts down. The cut-out speed is often around 25 meters/second for a utility-scale wind turbine.
Wind turbines are equipped with anemometers to measure wind speed and automatically shut down when reaching these wind speeds. This is a safety feature to protect wind energy equipment during storms and extreme weather events and to prevent unnecessary strain on the rotor. Once the anemometers register wind speeds below the cut-out speed, they will resume operation.
When a wind turbine reaches its rated speed, it produces its rated power. Therefore, there is a range where the wind turbine maxes out on energy production and higher wind speeds wouldn’t result in more electricity production. In the diagram below, this is with wind speeds between about 15 and 25 m/s.
Yet, if a wind turbine cuts out, it stops producing energy. Therefore, it is counterproductive to have a wind turbine that can easily reach its cut-out speed. These speeds are determined by the wind turbine manufacturer to prevent damage to the equipment, and stability and balance are critical for durable equipment.
Wind turbine noise
As the blades on a turbine spin, they make a swishing sound. This happens as the wind turbine blades pass through the air. Therefore, wind turbine manufacturers must limit the amount of noise that turbines make to within an acceptable level to avoid creating a disturbance.
If wind farms are too noisy, residents wouldn’t want them constructed nearby, and then siting would be much more difficult. Likewise, many people find one and even two-bladed wind turbines more visually intrusive.
So, wind turbines are designed to mitigate the creation of noise through blade design and by limiting rotational speed.
Unfortunately, the faster the blades move through the air, the more noise they make, so noise can be more problematic for wind turbines with fewer blades because more blades slow down the rotation of the turbine both from the additional weight and the wind resistance.
Even though it would seem much better to allow a turbine to rotate very quickly to produce wind electricity, that isn’t necessarily the case.
The speed that the blades rotate is also a balancing act of considerations. If they turn too quickly, it can cause damage to the equipment and create too much noise. On the other hand, if it rotates too slowly, the wind farm won’t produce an adequate amount of power.
So, three blades are usually just right. They prevent the turbine from spinning so quickly that it’s excessively noisy but not so slowly that it doesn’t produce much power.
Aerodynamic design
Wind turbine engineers can control the width of the wind turbine blades to have an aerodynamic design. Typically, with fewer blades, each one is also wider. However, this can be problematic because a manufacturing facility needs high ceilings to create turbine blades that can be more than 5 meters in width. From a transportation perspective, it is also harder to ship large things.
Thus, there are also some advantages to using more wind turbine blades because to optimise the aerodynamic design, each blade is narrower. In fact, the more blades on a turbine, the more slender they should be.
However, manufacturing slender blades has its own issues too, and having more than three blades can also be problematic for other reasons. For example, manufacturing a narrower blade requires a sufficiently rigid material for the blade not to bend. However, it is challenging to make blades economical, lightweight, slim, and rigid enough to be practical, but the material selection can make a difference.
Using carbon fibre instead of fibreglass is helpful because it is stiffer, but it’s also more expensive. Again, it becomes a tradeoff, causing engineers to weigh the various pros and cons — and typically favouring a 3-blade design.
Wind Farm Costs
Because wind energy developers want profitable projects, financial considerations are also critical. In theory, manufacturing a wind turbine with less than three blades is more economical because there are fewer components to manufacture and transport. In addition, it is cheaper and easier to assemble wind turbines with just one or two blades than three or four, leading to lower labour and construction costs.
Although their length varies, blades on utility-scale wind farms are often 52 meters long or more. However, GE manufactures the longest wind turbine blade for offshore use, which is 107 meters long. To put this in perspective, it’s about as long as a football field.
In addition to manufacturing costs, the blades also have to be transported to the job site, so transportation costs and challenges are a consideration. Often, rural roads are not equipped to handle such large equipment and many wind farms are located in rural areas.
Unlike the wind turbine tower, which can be assembled from several pieces on-site, wind turbine blades are transported as one single piece. Unfortunately, this can be very expensive and cumbersome and hinders design options.
Are two-bladed wind turbines more efficient?
Yes, a wind turbine with two blades can be more efficient than a unit with three blades and have a higher energy yield. Because blades are heavy and create drag, a two-bladed turbine weighs less and is more efficient at rotating. This means two-bladed turbines can turn at a higher speed, creating more torque and increasing the thrust and centrifugal force on the turbine.
In fact, some smaller wind turbines have just two blades, which can be assembled in one continuous piece. This helps reduce material costs and installation labour. However, this isn’t the only critical consideration. Durability, cost, stability, and noise are also important factors.
A big issue with 2-bladed turbines is structural balance due to the configuration of the blades. Often, wind energy developers want to reduce risk by using more proven wind turbine designs because structural imbalances could create premature wear and tear if not adequately addressed.
Will wind turbines with 2 or 4 blades be more common in the future?
It is difficult to predict this, but it is certainly possible. Choosing the optimum number of blades is a tradeoff involving various factors. However, if any of those factors change, including design requirements, it can make a different design more or less advantageous.
For example, rising gasoline costs are also increasing transportation expenses. Another way that design requirements are changing is with the rise in offshore wind farms. However, offshore wind turbines are often bigger than land-based wind turbines, but they are also further out and harder to access.
As a result, off-shore wind farms have high expenses for transportation and installation, so the priorities are a bit different than land-based wind farms. Therefore, these factors might make 2-bladed wind turbines more appealing for offshore wind projects if engineers overcome structural design factors.
Likewise, advances in material science could help make more rigid, lightweight wind turbine blades possible. If a rigid and cost-effective material were available, it would help make wind turbines with four blades more attractive and financially viable because it would help overcome the challenge of making thinner wind turbine blades that are aerodynamic.
In addition, innovations in on-site manufacturing could also have implications. For example, if there is a way to manufacture wind turbines on-site, then transportation isn’t such an important consideration, and then larger components may be possible.
Currently, the National Renewable Energy Laboratory (NREL) is researching next-generation wind turbine blade manufacturing techniques that incorporate 3D printing, material science, and automation. NREL is at the forefront of research into thermoplastic resins for wind turbine blades, which is an advanced composite material.
This material seems especially promising because it allows for lightweight and longer blades that are recyclable. The result is a lower-cost material with a high stiffness-to-weight and strength-to-weight ratio. In addition, this material is also truly recyclable, which most blades are not. This is an appealing quality when considering the entire lifecycle of a wind farm and not just construction and operation.
On the other hand, some wind energy experts expect bladeless wind turbines to become popular. These wind turbines require less maintenance and have advantages in manufacturing, transportation, and installing the equipment. They also produce less noise and are not as much of a threat to birds and bats. They also cost 40% less than three-bladed wind turbines.
Unfortunately, these turbines are less efficient at generating energy, but the technology is still in its infancy. Bladeless wind turbines are a reminder of how the wind energy industry is still rapidly evolving as technology advances. Maturing wind energy technology helps promote renewable energy production at a lower cost while helping to reduce greenhouse gas emissions across the globe.