Basics of Wind Turbine Design - Why do Wind Turbines Have Three Blades?

Updated: Mar 9, 2018

A typical full size wind turbine in operation.

A wind turbine is a mechanical machine that converts kinetic wind energy into electrical energy. It is a very clean method of producing electricity. There are various types of wind turbines that include horizontal axis wind turbines which are most commonly used and known as a modern HAWT (Horizontal Axis Wind Turbine) and then there are 2 other types; a vertical axis wind turbine (rotates about a vertical axis with a large surface area aerofoil or multiple large surface area aerofoil in all directions), that means its rotation is not dependent on wind direction called a Savonius VAWT and a another vertical axis wind turbine that has singular aerofoil blades that are smaller but optimised to obtain the greatest amount of speed due to the wind force called a Darrieus VAWT. These 3 types of wind turbines can be seen below. These can then placed on land or offshore depending on the most suitable conditions and surrounding environment. Many people object to having wind farms on land as they are not aesthetically pleasing (apparently) and often winds are much greater offshore, meaning more electricity production. However, the initial build costs are very high due to difficulty creating foundations under water and building the structure off land.

The three types of wind turbines.

The first question we must understand is which of the 3 is most efficient under any conditions. One design feature is that a modern HAWT has less moving components and subsequently maintenance. This also drives a large cost of manufacturing components that are dynamic and run the length of the turbine blades. VAWT’s also have relatively heavy components away from the center of rotation. This means that there is large bending moment force at the base of the tower and subsequently structural foundations need to be much stronger to allow for this reaction force. As well as this wind typically only blows from one direction so the loads are much greater at the point when the rotation (360 degrees) meets the force of wind. This can cause a highly dynamic load at a singular point in the rotation about the vertical axis and consequential vibration forces. All these factors added up mean that these products are not economical to be a self sustaining method of clean energy generation. However, they are still used in certain circumstances and for smaller businesses or as back-up power on farms. They are quite simple in terms of continuous power generation without having to be pointed in a particular direction and due to their aerodynamic footprint size and ultimately total size, have a relatively low cost and are not overly large structures that are difficult to construct.

This leaves us with HAWT’s, but how are they designed to their optimum efficiency. HAWT’s rely on the wind direction being the method of generation to electrical energy. Their turbine blade utilise two primary aerodynamic forces: lift and drag. This is achieved by shaping the blades into an aerofoil. The aerofoil is then twisted to obtain optimal efficiency throughout the whole 360 degrees of rotation about the horizontal axis. As with an aircraft, the lift and drag of the aerofoil propeller creates thrust. This thrust, rather than allowing the structure to take off and by fixing the position of the turbine, can be harnessed into electrical energy.

How energy is created through using a typical aerofoil propeller set-up utilising wind energy that is then converted into electricity. Source: How Stuff Works

Now that we understand the most efficient method of harnessing wind energy, what is the most efficient method of obtaining that energy for an appropriate cost and ability to construct.

The underlying principle of harnessing the most wind energy requires the direction of the force vector to be normal to the wind. This means that not only do the blades themselves have to move in a 360 degree circular motion but also about the axis of the vertical tower structure of 360 degrees. This requires an additional level of complexity when designing the mechanism for a wind turbine.

An important equation is the coefficient of power which is effectively the efficiency of the wind turbine.

As you can see power is effectively proportional to the efficiency of the wind turbine. It should be notes that the maximum theoretical value of efficiency from a wind turbine is around 60% and not what can be derived from the above equation. This is mainly down to the fact that not all the air passing through is captured on the surface area of the blades and the efficiency of the energy conversion itself. This efficiency value is known as Betz Law and states that Cp is max. 0.59.

The simplest option of improving the out power is to increase the rotational speed of the turbine but this creates an issue: noise. Any onshore farms cannot exhibit this attribute so increasing the rotational speed is not the best option. For this reason 2 blade turbines rotate at a much greater rate than 3 blades due to a reduction in drag but again they will experience too much sound. This prevents people from living or working near wind farms but also reduces performance as sound is a type of energy and reduces turbine output efficiency. Additionally, increased rotational speed also causes blade erosion and extra stresses on the mechanisms due to an increased weight due to the centrifugal force. The next option is to increase turbine length. Longer turbine blades means more surface area for the wind to push on and a greater rotational force. By doubling the length of the blade an estimated 4 fold increase can be obtained in power generated. This can be derived by using the following equation:

This equation can be derived from using the equation of power coefficient of turbine efficiency, Newton’s 2nd Law of Motion, the rate of change of energy and the mass flow rate equation.

Through increasing the length of the turbine blade, the structure subsequently also has to increase and allow for the additional weight of the blade. It also increases manufacturing costs significantly. You may also notice from the above equation that the velocity of the blade is cubed, giving an 8 fold increase in power but again we run into the same issues as mentioned previously. One key variable in this equation that may be overlooked is the air density. Air density can be roughly given as 1.23kg/m^3 however, air density can vary with air temperature and elevation. The optimal place to increase air density is at sea level with a very low temperature.

So, why are 3 blade designs commonly chosen? The answer is quite complicated and we must compare it to its direct competitors, 2 and 4 blade wind turbines. As stated previously to obtain the same amount of power output from a 2 blade design, the velocity must be very high, which ultimately cannot be achieved through simply using the above equation. But what about 4 blade designs? In fact why not go for as many blades as possible to increase the total surface area. For that we have to consider the cost of manufacturing an extra blade. Each blade costs many thousands of dollars to manufacture and the subsequent addition of weight makes the structure much more expensive to manufacture as well.

A typical full size single blade ready to be installed with its farm substation in the background.

For that reason, we are only consider 2, 3 and 4 blade turbines. We have proven above, that the efficiency increase from having 3 blades over 2 is far more important than the relative cost of only manufacturing 2. Looking at the figures of output power efficiency we can calculate that going from 2 to 3 will give you an increase of 3% whereas 3 to 4 only gives you an efficiency of 0.5%. This is because a reduction in blades allows for an increase in speed. Therefore the marginal increase in performance does not justify the extra blade cost, internal gearing mechanism strength, total foundations strength and structure stiffness. This is why 3 blade turbines are the most commonly used design. Ideally, an infinite number of blades would provide the most efficient wind turbine but in reality the cost of this does not just the benefits. Also, a reduction in efficient is also related to the turbulence created by each blade. Below, is a rough estimation for change in efficiency between 2, 3 and 4 blade wind turbines.

Diagram showing the change increases in efficiency for wind turbines with 2, 3 and 4 blades.

In the future, I certainly see wind as a viable source of clean energy. I believe as manufacturing technology in metals and composites increases, ultimately decreasing cost, larger 3 blade wind turbines can be created. However, wind cannot be the only source of energy. As with solar where the sun isn’t always shining, wind isn’t always blowing so battery technology and integration into the grid is critical. Elon and Musk and Tesla have made some remarkable advancements in battery technology and recently installed the world's largest battery for a wind farm in South Australia. This is only the beginning and I foresee many improvements in large scale battery being made as many nations attempt to comply with the strict cleaner energy regulations to prevent global warming.

How wind energy is converted into usable energy within the grid. Source: International Journal of Scientific and Research Publications

I hope this has given you a good overview into wind turbine design and wind energy as a whole. It is certainly an interesting subject and we only discussed the high level concepts. There are many other factors including aerofoil optimisation and other aerodynamic improvements that can be made however this requires extensive research, often to PhD level. There are also many other interesting facts which we will address in the future for example why they are painted white - easier visibility for aircraft and many other questions you may have. If you do however have any specific questions, please don’t hesitate to contact us or interact via our forums in the link below:

Additionally, check out our types of engineering page on wind turbine related fields such as power systems engineering, offshore engineering or even structural engineering:

Luke T Seal Engineering