# shaft power versus RMP curve in wind turbine simulation

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 August 25, 2012, 14:04 shaft power versus RMP curve in wind turbine simulation #1 New Member   michael Join Date: Aug 2012 Posts: 13 Rep Power: 13 Hi, Dear friends, I am simulating HAWT now. My purpose is to obtain the curve between shaft power and RMP(or wind speed , TSR). I used the way ghorrocks mentioned in http://www.cfd-online.com/Forums/cfx...on-baldes.html "Far easier, but a little less elegant is to sweep through a range of rotation speeds in a series of steady state runs. This will give you a torque/power versus speed curve. Then interpolate on this curve your intended torque/power and there you have it. If you like then run this point as well to improve the accuracy of the interpolation" The weird thing is, when I fixed wind velocity, I increased rotational speed and torque increase too. The result is Cp is more than 0.59 (betz limit). For example, wind speed=8m/s, angular velocity are 20 rad/s, 40 rad/s and 50 rad/s, respectively. The corresponding torque are 8.5, 22.35, 34.8 N.m. I think it is wrong. Because when wind speed is constant, the total wind power in the rot domain of the blades is fixed. Higher rotating speed means lower torque. (my analysis type:steady) Hence, I want to add CEL to my cfx-pre. This link gives me some clues: http://www.cfd-online.com/Forums/cfx...imulation.html from his CEL: he defined wind power as an expression. But I don't understand where he used it. in my mind, I think the wind power and Betz limit should be as the constraint conditions. Otherwise, the results will be wrong. These are my CFX pictures: Any help would be greatly appreciated.

 August 25, 2012, 20:22 #2 Member   Paulo Join Date: Jun 2011 Posts: 34 Rep Power: 14 I'm having this problem aswell. I'm using this expression to cp: P_dot= torque_z()@blades*2*pi*Angular velocity [rev s^-1] P= 0.5*ave(Density)@inlet*ave(Velocity)@inlet^3*area( )@in_rotor cp=P_dot/P But it results in cp>1.

 August 25, 2012, 20:56 #3 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,727 Rep Power: 143 Before worrying about getting the speed/torque correct, have you done all the basic checks to ensure your simulation is accurate? http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

August 25, 2012, 21:38
#4
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michael
Join Date: Aug 2012
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Quote:
 Originally Posted by ghorrocks Before worrying about getting the speed/torque correct, have you done all the basic checks to ensure your simulation is accurate? http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F
Hi, ghorrocks, I think I can't include all relevant physics conditions in my simulation. I wonder I should set wind power in the rot domain as a constrain condition. But I don't know where can I add it and how to use it.

By the way, my blades' TSR=7.2. According to the rated wind speed, the corresponding rotating speed can be obtained. In that case, I can obtain the reasonable shaft power and Cp.

August 26, 2012, 07:11
#5
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Glenn Horrocks
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Quote:
 I wonder I should set wind power in the rot domain as a constrain condition.
I have no idea what you mean by that.

The method I recommend is quite simple - do a series of steady state (well, frozen rotor or other simple RFR model) at various rotation speeds and get the torque versus rotation speed line. You can then match that to the load point (or line) to get the operating point.

I am not a wind turbine person so I have no idea what TSR=7.2 means. But you seem to imply that you already know the operating speed of this turbine.

 August 26, 2012, 08:27 #6 New Member   michael Join Date: Aug 2012 Posts: 13 Rep Power: 13 Hi, ghorrocks,Thank you for your reply. Actually, I know all the information of the blades in my wind turbine. So, I just use it to verify my simulation. In fact, I found some guys have the same problem: http://www.cfd-online.com/Forums/mai...orque-cfd.html http://www.cfd-online.com/Forums/flu...imulation.html Anyway, I will try to figure it out. After that, I will respond my method.

August 26, 2012, 08:29
#7
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michael
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Quote:
 Originally Posted by strobel I'm having this problem aswell. I'm using this expression to cp: P_dot= torque_z()@blades*2*pi*Angular velocity [rev s^-1] P= 0.5*ave(Density)@inlet*ave(Velocity)@inlet^3*area( )@in_rotor cp=P_dot/P But it results in cp>1.
Hi, strobel, did you resolve it?

August 26, 2012, 20:17
#8
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Paulo
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Quote:
 Originally Posted by zhenduan306 Hi, strobel, did you resolve it?
No, my problem is quite strange, because when i set a lower rotation i get cp<0.59, but if i increase the rotation, cp increase too and i get cp >1. I gonna try that way Glenn said, if it fix i post here the results.

 August 26, 2012, 21:00 #9 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,727 Rep Power: 143 If you already know the operating point, then you not simulate that point, and check the torque it produces is correct? No need to generate an operating curve if you do not need it.

August 26, 2012, 21:24
#10
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michael
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Quote:
 Originally Posted by strobel No, my problem is quite strange, because when i set a lower rotation i get cp<0.59, but if i increase the rotation, cp increase too and i get cp >1. I gonna try that way Glenn said, if it fix i post here the results.
Good luck to you!

Last edited by zhenduan306; August 26, 2012 at 22:19.

August 26, 2012, 21:27
#11
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michael
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Quote:
 Originally Posted by ghorrocks If you already know the operating point, then you not simulate that point, and check the torque it produces is correct? No need to generate an operating curve if you do not need it.
I just know the information of the blades. I am designing a new wind turbine now. Hence, I need this curve to choose a generator.

August 27, 2012, 06:24
#12
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Glenn Horrocks
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Quote:
 Hence, I need this curve to choose a generator.
Then you need to generate the turbine operating curve.

 August 27, 2012, 07:18 #13 Member   DB Join Date: Apr 2011 Posts: 87 Rep Power: 15 Try areaAve or VolAve and see if it works out __________________ -D.B

August 27, 2012, 08:02
#14
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michael
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Quote:
 Originally Posted by D.B Try areaAve or VolAve and see if it works out
Hi DB, can you tell me more details?

Thank you very much!

 August 28, 2012, 01:23 #15 Member   Paulo Join Date: Jun 2011 Posts: 34 Rep Power: 14 Zhenduan, maybe when you increase too much your rotation your turbine don't work freely in this rotation and doesn't work anymore as Turbine. So, in these conditions, it is no longer more a turbine and become a pump.

 August 28, 2012, 07:25 #16 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,727 Rep Power: 143 I presume you are looking for the steady state operating condition - in that case, if the net torque is negative it simply means the rotor will decelerate, so the steady state speed is slower than the current speed.

August 28, 2012, 09:44
#17
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michael
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Quote:
 Originally Posted by strobel Zhenduan, maybe when you increase too much your rotation your turbine don't work freely in this rotation and doesn't work anymore as Turbine. So, in these conditions, it is no longer more a turbine and become a pump.
Yes, you are right! CFX is just a tool. it can't identify the minimum or maximum condition the wind turbine is able to run.

August 28, 2012, 09:48
#18
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michael
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Quote:
 Originally Posted by ghorrocks I presume you are looking for the steady state operating condition - in that case, if the net torque is negative it simply means the rotor will decelerate, so the steady state speed is slower than the current speed.
OK, I will check it. Thank you very much!

October 31, 2012, 15:33
#19
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Paulo
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Quote:
 Originally Posted by zhenduan306 OK, I will check it. Thank you very much!
i still getting high values of coefficient of shaft power(Cp). Have been strange, because in low rotation, i get Cp near of experimental values, but when i increase rotation Cp increase so fas and i get Cp higher than Beltz limit.

November 1, 2012, 14:24
#20
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michael
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Quote:
 Originally Posted by strobel did you solve your problem? i still getting high values of coefficient of shaft power(Cp). Have been strange, because in low rotation, i get Cp near of experimental values, but when i increase rotation Cp increase so fas and i get Cp higher than Beltz limit.
Unfortunately, I don't have time to continue it. If you just want to get Cp and Tip Speed Ratio curve, I suggest you use Qblade. It is a very good software. You can download from websit. Just google it.