What's Tip Speed Ratio (TSR)?

Curtis Riley
October 18, 2011 — 3,730 views  

I've had a couple people ask me what tip speed ratio is and why it's important in wind turbines. The tip speed ratio is simply a number from 0 - (theoretically a huge number, but usually 8 - 10) which tells you how fast the blades are moving in relation to the wind speed.

The formula for tip speed ratio is as follows:

TSR = Blade Tip Speed / Wind Speed

For example, if we have blades whose tips are moving at 80MPH and a wind speed of 10MPH that would give us a tip speed of 8 (which is what the windmax blades give).

In wind generators a higher tip speed ratio is better and here is why. We normally have 3 blades which are travelling through the air. A low TSR means that the blades aren't moving very fast and there is quite a bit of air that passes through the wind generator without even touching the blades. With a higher TSR the blades are moving faster which means that they 'touch' more wind during each rotation. This means that they are able to extract more energy from the wind which means they are more efficient. The wind generator blades we use have a TSR of 8 and an efficiency of about 49% (maximum efficiency is around 60%).

There is a downside to having that increased efficiency though... a higher TSR means that the blades are rotating faster which means that your wind generator will produce more noise. This could be a problem for you in a (sub)urban neighborhood. We've had people email us and say that they were forced to take down their turbine because people complained.

When trying to figure out what TSR you should use for your wind generator make sure to think about noise. When the wind really gusts you'll hear some chopping.

So, what should you look for? Well, the higher the TSR the more efficient the blades are so I'd recommend looking for blades with a TSR of 6-8. The windmax blades that we recommend have a TSR of 8 and work great.

There is a way to determine the optimal TSR based upon the number of blades you have. We don't personally use it and I'll explain why later. Here's the formula to determine the optimum TSR for the number of blades that you have.

Max Power = (4 x π) / n
n = number of blades.

Why are you telling me to use 3 blades with TSR's of 8! Well, that's easy. The best motor we have found produces 12 volts at 150 rpm. Now, that's not enough to even start charging your battery bank. If we use blades with a TSR of 8 than our blades move twice as fast as they would with the optimum TSR. While we may be sacrificing a little efficiency these blades are 49% efficient which is one of the highest we've seen. It's a good trade off to get the higher RPM's and lose a teeny bit of efficiency.

For PVC blades there's not much you can do to improve the TSR. TSR is a hard thing to predict when building your own blades and can vary quite a bit just by how much sanding you do to make them more aerodynamic. If you find that at high wind speeds the blades slow down this means your TSR is too low and they're actually stalling (similar to a plane... no lift is being created).

There are also dangers of having to high of a TSR. Let's say that you have the amazing blades whose TSR is 10. That means in 10MPH winds the blades are travelling at 100MPH. At 20MPH they're travelling at 200MPH. What happens in a big storm and you get a gust of just 30MPH wind? The blades are moving at 300MPH and snapped blades are likely to result. This is an unlikely scenario as I haven't seen blades with a TSR higher than 8, but you should know that as TSR increase the blades need to be made stronger and stronger to withstand the forces being applied to them.


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Curtis Riley

I built my own solar panels and a wind generator and saved well over $30,000. I get a cheque from the power company because I produce more energy than I use. If you'd like to learn how you can do the same check out Build Wind Generator.