Science Buddies: "Ask an Expert"

I'm in the middle of my research on a project that tests how differently shaped winglets affect wind turbine efficiency, and I'm a bit confused. I must've read at least ten different papers, and all of them cite different methods of how they measured their turbine efficiency. I'm (obviously) a student and therefore unable to buy any expensive software, so how do you suggest I do the experiment so as to be able to accurately measure the differences between the turbine designs' movement (and therefore energy output)?

You chose a complex area of recent interest. Efficiency over a wide range of operating conditions is always a difficult thing to define. In the case of wind turbines, the problem of propeller efficiency is not independent of turbine efficiency which will vary with rotational speed and electrical load which will be seen by the propeller as a torque that must be overcome in order to turn. These modern wind turbine generators often have variable speed transmisions to better match propeller speed and torque to turbine speed and torque to achieve a better overall efficiency.

From a theoretical standpoint, if you take the area that the tips of the propeller inscribe and the velocity of the wind and for a reasonably long a period of time you can theoretically calculate the energy it took to move the air through that inscribed area in that time. If you measure the torque and rotational speed of the propeller for that same period of time, you can theoretically determine the mechanical energy derived from the propeller. The ratio of the two would be the efficiency. Unfortunately, the simple fact that there is a turning propeller in that space means that the air flow pattern will not be undisturbed. It will be somewhat harder for the air to flow through the inscribed area than it will be for the air to flow around the propeller. This makes the air speed measurements harder to estimate. The shape of the ground, any buildings, trees, etc. near the windmill will also affect the airflow patterns. For your experiment, you will want to figure out ways of eliminating these kinds of factors from your measurements.

Given that you are trying to conduct a scientific experiment on a smaller scale, you are going to have to come up with your own measurement system appropriate to the scale of your equipment. In other words, just like the various papers you read defined things the way they wanted to, you get to do the same as long as there is a scientific basis for your choice. In this case, scientific basis means that you can convince others that you have a fundamentally scientifically sound approach.

Hope this helps you start thinking about breaking down and simplfying things instead of complicating them. Some things you might want to consider are what you can use as a tachometer and how you can measure torque or work on a small scale. You might also think about how the Wright brothers measured lift and how they applied wing design to propeller design. They certainly worked in an era where there weren't all kinds of sophisticated electrical measurement devices and were able to accurately evaluate things with some rather simple methods.