# Alternative method for power assessment of small wind turbines

1. Oct 13, 2008

Hello, I am Daniel from La Plata, Argentina and would like some help on a complex (for me) question. I am working with some friends on the development of an innovative wind turbine. We have built several prototypes with diameters of 0.5 m and 2 m. We have tested these prototypes on trucks with the assistance of people from the Fluid Dynamics Lab from the Department of Aeronautical Engineering, La Plata National University. In order to measure torque, a 0.3 m wheel was braked by means of a metal strip wound around said wheel and at the end of which different weights were hung. By means of a tachometer the rotor speeds were measured and after many readings we got a clear picture of what kinds of power our turbine was producing. The report from the lab gave a Cp of 0.53 which is very high for a two-meter rotor. However, we were not pleased with some aspects of the procedure and we firmly believe this Cp should have been a bit higher. Now, if the lab had the right device for measuring torque (i.e.:torque transducer) I would not be asking for help. The fact is, we don't have it nor can we buy one, as they are very expensive (something like U$6000). In the meantime I have been trying an alternative way to do this which seems to give good results but that is systematically rejected by the people who specialize in wind turbines. 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 to make accurate measurements by means of a digital camera. Basically we are interested to know how long it takes for the rotor to go from rest to final speed (to measure angular speed we take a video and then process it frame by frame - it lasts about 40 seconds when wind speed is about 5 m/s). 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. By means of some maths, it is possible to calculate peak power based on average power (This conversion needs a factor which is dependent on the beta angle which is the angle between chord and rotation plane). 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. My question is then: 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, Daniel 2. Oct 29, 2008 ### danielhugo Hi everybody! You may watch my turbine in action following this link: Believe me, you'll be surprised! Last edited by a moderator: Sep 25, 2014 3. Oct 31, 2008 ### abelanger Wow very nice!! Keep it going :D! 4. Nov 2, 2008 ### makethings Nice vid, How fast is the wind and how much power are you producing? 5. Nov 2, 2008 ### danielhugo Hi, makethings! Wind speed was just below 5 m/s (average 4.5 m/s) and average power output was close to 300 W. The rotor diameter is 2 m. Starting torque at this wind speed is 12.5 Nm. In the video the rotor accelerates from 0 to 6.2 rps in just 35 s (the first five seconds are not in the vid, but it is easy to verify that the rotor increases its speed (omega) 6.28 radians every five seconds and its moment of inertia is 10 kg(m)m2. Thanks for your interest. 6. Nov 11, 2008 ### Enthalpy Hello Daniel and everybody! Found a clear picture of Daniel's turbine here: http://www.theinquirer.net/en/inqui...vents-simpler-way-to-see-inside-metal-objects at the section "Wind Power galore". Daniel and I already had a discussion elsewhere, but the site is "closed for maintenance", which might well last for a looooong time... http://www.physforum.com/index.php?showtopic=23494&st=0 where I proposed for this turbine a gearless generator that looked quite good to my eyes. Good opportunity to switch to a forum with a different style. The text is still available - though in "low-fidelity format" - here: http://lofi.forum.physorg.com/Small-Wind-Turbine-Power-Measurement_23494.html just increase the text size in your browser to read the end of the discussion. I will come back with a simpler and cheaper variant. See you! 7. Nov 11, 2008 ### makethings That's cool.. I cannot tell at what altitude in the video you were testing your turbine, but if you were even higher up in the earth's boundary layer you can get faster wind speeds. I am curious how high wind speed can your wind turbine handle? And what would the maximum power you can get from that? Do you have a cut-off wind speed and brake? Or have you tested it to failure in a wind tunnel? 8. Nov 11, 2008 ### danielhugo Hi Enthalpy! I'm very glad to hear from you, after the sudden loss of contact because of the closing of the other site. Totally unexpected! The link to the inquirer shows a nice picture of an earlier version of our turbine. It has 54 blades and solidity within blade area is 100%. At this point we were researching the validity of the theory that says that three is the best number of blades due to the 'fact' that as solidity increases lift goes down. With this turbine we measured a starting torque of 160 Nm at 10 m/s wind speed. Torque at peak power with same wind went down to 110 Nm (peak power was 1600 W). This prototype was built with a very low budget (it cost U$120 in materials and three months' time of hard work to finish it. Aerodynamically it is not as efficient as the model on the video, but it helped us learn a lot and led the way to the new version. Regarding your advice as to the generator, our team is trying to use it on their prototype design. They are doing all they can but we are still running on low budgets. Haven't sold a single turbine yet! Now we have some people from:

http://www.aldeaschweitzer.com.ar

who would like to have a number of our turbines generating power in the province of Neuquén (not like Chubut but close).

This forum gives you the possibility of sending private messages; as I am new to it, I don't know how to go about it. As soon as I figure out how to do it, I'll send you a couple of lines.

9. Nov 11, 2008

### danielhugo

Hi makethings,
On this video, we are 2 km southwest from the River Plate coastline close to the city of Berisso. The wind is just a summer breeze of 16 km/h. The turbine is mounted on a truck and we were making ready to carry out some tests. Normal winds in this part of the Buenos Aires province are rarely higher than twenty km/h, which explains why we need a truck to generate relative winds up to 50-60 km/h. Out of curiosity we subjected the turbine to a 80 km/h wind and it run fantastically! I was below the turbine and I can tell you. It's an incredible experience and I was alone down there because the others in the team were afraid it would explode. We did not measure power at this speed because we would have blown up the generator. Power output at this kind of wind speed is over 10 kw! Max power for our generator is slightly higher than 600W. We are right now preparing a new series of tests. I'll make sure someone shoots a few videos which I'll certainly post here.
My regards

Last edited: Nov 11, 2008
10. Nov 11, 2008

### Enthalpy

I haven't computed the improved design today. I hope to remove completely the iron core at the stator and let the copper wires alone - with a lower induction then. This would make the prototype easier, as it avoids any exotic material. Maybe tomorrow.

The closure of the other forum was unexpected... But I heard "anticiper" before - the French code for such covert actions. No idea what happened to them, nor if it was related to that forum. The forum was under heavy trolling attack for several weeks.

11. Nov 25, 2008

### Enthalpy

12. Nov 26, 2008

### Q_Goest

Hi Daniel,
Welcome to the board.
You say this method has been “systematically rejected by the people who specialize in wind turbines.” Can you explain why?

I’m not an expert in this field, but if I had to guess, I’d say there are 2 basic problems. The first is trying to determine the rate of change (ie: acceleration) of rotational velocity of the wind turbine. If you can determine that (ex: using fast acting photography) and if you know the rotational inertia, then yes, you can calculate the torque. That should be quite simple really. But the second problem is a bit more difficult to define and defend. The other issue is how instantaneous torque might be affected by a change in air dynamics between a constant rotational turbine and an accelerating one. This is essentially saying that the turbine undergoing an ACCELERATING RPM may have a torque exerted on it that is DIFFERENT than the torque exerted on the same turbine which is rotating at a CONSTANT RPM. I don’t buy this second argument, but I won’t go into detail. You could counter this argument easily by adding lots of rotational inertia to your turbine, such that the rotational acceleration is small. You might for example, run a belt off the turbine to a wheel that carries a ‘flywheel’ to increase the inertia.

Regarding the use of torque meters, I find it surprising that a transducer can’t be purchased for only a few hundred dollars. Where have you looked? If you wanted to make your own, that would be fairly simple. The idea you have of a belt over the pully which you then attach ‘fish scales’ to, will do the trick nicely. You’ll need to compensate for the weight of the belt, as well as whatever braking blocks you may need, but in principal, the idea of braking the turbine to determine torque is perfectly valid and easily accomplished. Having these two different techniques and showing they match should go a long way in proving the amount of torque and thus the amount of power your wind turbine is capable of producing.

13. Nov 28, 2008

### danielhugo

Hi Q Goest: Thanks for your welcome! I really like the site. Your question about the people who systematically ... We (TEUSA) are working with the help of a team who call themselves Fluid dynamics and Boundary Layer Lab, (La Plata University, Argentina). Their argument is exactly what you pinpoint as your second basic problem, i.e.: steady state versus accelerating rotational movement. Now, my own view is that the wind is never in a steady state as normal turbulence implies a constant change of wind speed. In this constant change of wind speed, a rotor in a conventional installation goes through all sorts of states, where at least one of them must necessarily be equivalent to the accelerating rotor while it is being tested. This implies, as far as I can see, that steady state torque can be no different than accelerating torque because if it were smaller, the rotor would slow down instead of keeing up its speed. The solution you suggest for measuring rotational velocity, is the one we are using and its really fantastic. You can't beat the speed of light! And on top of this, you are not drawing energy (almost) from the rotor, so Heinsenberg must be very worried about it.
Finally, the torque -transducer problem. We checked with different companies, and the cost was always about the same - around U$5000. This is FOB. We have to pay an additional cost (mostly taxes) of around U$3500. This is way too much. We are now trying to design one using two metal discs that by compressing four or six springs mounted on the edge and by using a pen nd some kind of paper roll we may get some approximate numbers. I want to thank you again , this time for you support and interest. I wonder whether you have seen our unique rotor. In case you haven't, here's the link where you can see it in action:

http://es.youtube.com/watch?v=I0wqQl527sY

Bye for now

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