# Small Wind Turbine Power Measurement

Description of method: The rotor's mass is constant so when it accelerates in a given wind, it does so with an acceleration = torque/mass. This acceleration will depend essentially on wind speed. If tests are carried out in the open with non-turbulent winds it is possible by using a tachometer to determine what is the final rps for the rotor and also it is possible to measure the time necessary for the rotor to go from rest to final speed. Now knowing the moment of inertia of the rotor it is a simple step to calculate the energy stored in it. If you have the number of joules stored and the number of seconds necessary to do the job, it is easy to calculate the average power produced by the rotor.
Of course that average power is rather different than peak power, so, knowing that, we're building some electronic data logger that will let us know rps (and thus acceleration) every "n" miliseconds, and with that information, we'll be able to calculate values nearly equal to instantaneous power for every aforementioned "n"

I like this method mainly because it is non-invasive, it can be done as many times as one feels necessary and because it costs nothing.

Now, my question is : Do you see anything in this procedure that is not acceptable from a physical point of view? Is there anything else we might do to improve our methodology? Thanks in advance,

## Answers and Replies

russ_watters
Mentor
You're not including power lost due to aerodynamic drag, mechanical friction, or an electric generator, nor does it include the fact that the lift changes as the turbine speeds up. I don't think this method will give you anything close to the real power output of the turbine.

You're not including power lost due to aerodynamic drag, mechanical friction, or an electric generator, nor does it include the fact that the lift changes as the turbine speeds up. I don't think this method will give you anything close to the real power output of the turbine.

I'm in fact including it (lost due to aero drag), I'm just measuring how much power does the rotor takes out of the wind, and that's my main objective. I dont know why you mention the change on lift related to the wind speed change. Again, I want to know how much power does the rotor takes out of wind. I dont care about generator eficiency and the like... Mine is a mechanical determination

berkeman
Mentor
I'm in fact including it (lost due to aero drag), I'm just measuring how much power does the rotor takes out of the wind, and that's my main objective. I dont know why you mention the change on lift related to the wind speed change. Again, I want to know how much power does the rotor takes out of wind. I dont care about generator eficiency and the like... Mine is a mechanical determination

Maybe you could say more about what the ultimate point of your experiment is. After all, a freely-spinning rotor has no practical applications, except maybe for measuring wind speed (where you would not care what the energy transfer was).

Using the "Dyno-Jet" method of measuring accelerating torque is perhaps reasonable to a point, if you plot the data similarly to a Dyno-Jet horsepower and torque curves. But performing the experiment on a freely-rotating rotor would not seem to have any practical applications. You need a load on the rotor to be emulating any practical real-world situation that I can think of.

The method will probably work for ballpark numbers, maybe in the area of +/- 25%. A turbines power output is directly related to the tip speed ratio in which your method is constantly changing and is constantly in a transient state. In order for any kind of accurate measurement you will need to measure torque and speed at a steady state condition.

In order for any kind of accurate measurement you will need to measure torque and speed at a steady state condition.

That's an interesting point, in fact, in the university where we proposed our method we've been told exactly the same as you've said, but, the curious thing is that there is not any kind of "steady state" in the wind field, I mean, windmills work with constantly changing winds (in terms of both velocity and direction - velocity vector). Why proving them under conditions they are not gonna face?

Our method use just the Newton Second Law (in this case rotational movement and acceleration), and we still haven't had any reason which could eventually make us think we're wrong.

We often get the kind of responses you gave us (we appreciate your time of course), but they dont show *where* we're wrong if we are...

Maybe you could say more about what the ultimate point of your experiment is. After all, a freely-spinning rotor has no practical applications, except maybe for measuring wind speed (where you would not care what the energy transfer was).

The ultimate (and unique) point is to know how much torque the rotor has on every angular velocity (given a certain wind velocity). Remember that if there is an acceleration, there is a force (in this case a torque). It doesn't care if you use such a force to accelerate the rotor itself or to move a generator. This has nothing to do with a freely-spinning rotor.

Was I able to explain myself or not? Please tell me cause I'm sure I can be wrong, but I need facts

kind regards