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What is TSR ?

TSR is known as the Tip Speed Ratio (TSR) and it is the ratio of the blade's tip speed to the actual wind speed. We emphasize the word blade because TSR is applied to lift type turbines, such as a three bladed turbine or Darrius VAWT. TSR is not applicable for a drag type turbines such as the Savonius and the Flap-Turbine. The TSR of a Savonius Turbine may be around one (1) and the Flap-Turbine you see on my site may have a TSR of 0.7, however I can assure you that the Flap-Turbine is much more efficient than any Savonius Turbine. To understand why Savonius Turbines are so inefficient please refer to this link (coming soon), but I will state that these inefficiencies have nothing to with the turbines TSR.

As an engineer I am aware that efficiency is measured by the ratio of extracted energy from the wind to the available energy in the wind. But what does TSR have to do with this efficiency? A lot! TSR and energy efficiency are inseparable, directly related. Let me explain.We know that the wind energy is transformed from the air particle's kinetic energy to the rotating blades on the turbine. In order for a 3 bladed turbine to be efficient, during it's rotation, the blades should be in contact with as many air particles as possible. How can we achieve higher contact rate of air particles with blade? By designing blades such that they rotate faster for a given wind speed. So the secret of a three bladed turbine is to make their rotation faster for the given wind speed. Therefore, the larger the TSR the more efficient a three (3) blade turbine becomes. To understand this concept please watch the following flash simulation.

This flash animation demonstrates that higher the TSR more air particles interact with turbine blade in a given time interval. Thus, the bladed turbines with high TSR becomes more efficient.

Please drag the slider in the animation to increase TSR and see how the number of bright red spots (blade air particle contact) increases for higher TSR ratio.

Notice that the air particles are shown as square grid of faded red dots, when an air particle hits a blade it become bright red particle. As you can see most of the air particles pass through the turbine without ever touching a blade; these are the faded red dots, while the particles in contact with blade become bright red. The reason that most of the air particles pass through the turbine without ever touching a blade is because initially the TSR is set to one (1.) Once you begin to increase the TSR more air particles will come into contact with the blades, thus increasing the efficiency of the turbine. By increasing the TSR for a three (3) bladed turbine you will come closer to the Betz Limit (59.3% efficiency.) However, no matter how fast the blades rotate there will always be some air particles that pass through without every coming into contact with a blade. Therefore, the turbine will never reach its optimum efficiency of 59.3% (but it is likely that it will pass 50% with large turbines.) Are the air particles that never touch the blade bad for the wind turbine? This depends on the TSR of the blade. If the TSR is low then it is bad because the turbine becomes inefficient, because a lot of air particle pass through turbine without loosing their kinetic energy. If the TSR is high then it is good because the air particles that do not touch any blades will sweep away the air particles that have lost most of their kinetic energy to the turbine and this will increase the overall efficiency of the turbine.

Now we know that the TSR is a very important factor for calculating the efficiency of bladed turbines, however is this concept applicable to drag type turbines? ABSOLUTELY NOT! In any given instance the Flap-Turbine interacts with almost 50% of the air particles passing through its projected area, no matter what the rotation speed of the turbine is. Therefore, the Flap-Turbine gathers power from almost 50% of the air particles passing through its projected area while the three (3) bladed turbine's may not be gathering any (which is zero for when the blades are tilted and not rotating; TSR of 0) to a maximum number which depends on the turbines TSR.

Above I have tried to explain why the TSR is important for a three bladed (this also aplicable two or one bladed) horizontal axis wind turbine and not for the drag type turbines (such as the Savonius and the Flap-Turbine.) To further convince you, I will give another example, however before this next example we must agree that a turbine is a turbine no matter what medium they operate in. That is why you see so many people trying to place a three bladed turbine into the water and generate electricity from tidal currents.

At this point, you may or may not know that the Pelton turbine is an action (drag) type turbine which works much like a Savonius turbine and its TSR is 0.5, however it is considered to be one of the most efficient water turbines. To read more about the Pelton turbine please click here. As I stated before the Savonius turbines are the least efficient turbines and that the Pelton Turbine (which is like a Savonius Turbine) is the most efficient water turbine. My statements sounds like a contradiction, however this is not the case. The reason the Pelton turbine is so efficient is because the high speed water jet pushes (or acts) on the downward moving blades while the upward moving blades are moving in the air, not effected by the water. On the other hand, the Savonius turbine's blades are all affected by the air stream with the upwind moving blade creating a lot of negative drag. If the Pelton turbine's upward moving blade were effected by the high speed water, it would be much like the Savonius turbine and inefficient. The Flap-Turbine however is much like the Pelton turbine, by opening its flaps on the upwind direction, therefore it has a higher efficiency than the Savonius turbine.

I also recommend that you read about the Kaplan turbine on this wiki page. Note that Kaplan turbines are just like the three bladed turbines but operating in water. These are also efficient turbines, the only difference between these is that the Pelton turbine works for high head (water height) and the low volume of water, while the Kaplan turbine works for low head and high volume of water. My point is that, you cannot state that if a turbine has a low TSR it is inefficient. Many factors have to be looked at before a statement like this can be made and apparently this argument is not working for the Pelton turbine, which is an impulse (drag) type turbine with a TSR of 0.5 and considered one of most efficient water turbines. The Savonius turbine is inefficient, but this does not mean that all drag type turbines are inefficient.

The fact of the matter is, you use the best turbine for the given condition. The Pelton turbine cannot be efficient where the water head is low but this does not mean it is an inefficient turbine it only means that it is not suitable for low head applications.

In conclusion Tip Speed Ratio (often known as the TSR) is of vital importance in the design of the bladed wind turbine generators. If the rotor of the bladed wind turbine turns too slowly, most of the wind will pass undisturbed through the gap between the rotor blades. Alternatively if the rotor turns too quickly, the blurring blades will appear like a solid wall to the wind. Therefore, bladed wind turbines are designed with optimal tip speed ratios to extract as much power out of the wind as possible. On the other hand drag type generators always use %50 of their projected area to the wind and TSR plays no role in these turbines.

There will always be people who argue that a drag type turbine is inefficient because of a low TSR, but I can assure you that a drag type turbine can't have TSR much bigger than 1, this is theoretically and practically not possible.