Wind Turbine Blade Tip Deflection/Bending, Good or Bad?

Is bending of wind turbine blade tips at moderate/higher wind speeds good or bad compared to a very stiff blade? especially for small wind turbines.
Does it affect the power output? Is there a maximum ratio for tip deflection?

Nacelle Tilting and Rotor Conning are simple solutions to get extra clearance from the tower.
but does these solutions have any negative effect on the power output and the overall performance, especially the tilted rotor.

Although some concepts do exist for flexible/aeroelastic blades, but I can’t find any solid references.

Dear Haseeb,

Blades are becoming more flexible due to the drive to reduce cost through reductions in weight. In most turbines, the blade deflection does not have a sizeable impact on the power output. You would have to get to very sizeable blade deflections (very uncommon) before a reduction of swept area (and hence power) would be a problem.

For a rigid rotor, the swept area of the rotor is proportional to the cosine of the shaft tilt and cosine of the precone. You would have to get to a sizeable tilt or cone angle before cos(angle) = 1 assumption is invalid, which is uncommon. For a flexible upwind rotor, the blade deflection actually increases the swept area of the rotor, counteracting some of the small effect of the precone.

Best regards,

Dear Dr.Jason,
Thank for your reply,
for tilted rotor- bendable blades (to some extent) advantage can be taken from aerodynamics as air gets slides over the blade leading to increase area of contact and hence more force can be exerted on blades by the wind, which may increase in power output, how practical is this?

Dear Haseeb,

I have not heard of that, but perhaps a more experienced aerodynamicist can respond.

Best regards,

Dear Jason,

I have a question concerning the maximum out-of-plane blade tip deflection for the upwind onshore 5MW NREL wind turbine. The blade tip can deflect from its nominal position only with certain range, determined by the presence of the tower behind it.

From my rough calculations where I included OverHang, ShftTilt, Precone, blade radius, and tower diameter (at the level of the blade tip when pointing downward - @BTL) I estimated the maximum OoP blade deflection to be within the 5.05 - 5.15 meters.

Equation: (OverHang + R*sin(Precone))*cos(ShftTilt) - D_Tower@BTL/2

D_Tower@BTL was calculated from upper and lower diameter of the tower and it total height.

However, results from FAST for a steady wind speed of 11.4 m/s yield maximum blade deflection of ~5.25m when the blade is pointing downwards (corresponding to the azimuth angle of 180 degrees). In this case, the blade would hit the tower. But instead, 11.4 m/s is it Rated wind speed. It reaches even greater values (around 6m) during Extreme Wind Models. How is that possible?? Does the FAST sensitive for exceeding maximum blade deflection and inform about it?

I need this knowledge to finish my Bachelor Thesis so thank you in advance for your help

Regards,
Sebastian Dziomdziora

Dear Sebastian,

Your equation for the tip-to-tower clearance is not correct. The precone and shaft tilt angles should sum. As discussed in my Jan 18, 2013 post in the following forum topic: http://forums.nrel.gov/t/tower-blade-tip-clearance/630/1, the clearance for the undeflected NREL 5-MW wind turbine (not including subtracting the tower radius) equals: (5.019 m)*COS(5 deg) + (63 m)*SIN(5+2.5 deg) = 13.223 m.

Best regards,

I am having a tower strike warning but I am a little confused about what the value I obtain for tower clearance is actually giving me. I am using FASTv8

I am running the turbine pitch-to-stall so expect larger deflections.

In one simulation I get no warning and my minimum value of TipClrnc1 is 3.21m. From what I have read I thought the tower radius at the tip was 4.5m and that this needed to be subtracted from the TipClrnc value which would mean this simulation should also give the tower strike warning.

In other conditions I get the Tower Strike warning and my minimum value of TipClrnc1 is 2.96m (actually 3.3m after transients have died away).

Has the value been updated in FAST v8 such that we do not need the tower radius subtracting too or am I missing something else here?

Many thanks for your time,
Dawn

Dear Dawn,

The calculation of ElastoDyn outputs TipClrnc1-3 in FAST v8 has not changed from earlier versions of FAST. From the OutListParaeters.xlsx spreadsheet included with FAST v8:

So, you must still subtract the local tower radius from this output to get the actual tower clearance. This calculation is limited because ElastoDyn does not know the tower radius (which is not an input to the ElastoDyn module).

The tower strike warning/error is triggered by the tower-influence model in AeroDyn (tower strike is an error in AeroDyn v15 and a warning in AeroDyn v14), whereby the blade analysis nodes are checked to be within the volume occupied by the tower, including the effects of the local tower radius (which is an input to the AeroDyn module).

I’m not sure what turbine you are referring to that has a tower-top radius of 4.5 m.

I hope that helps.

Best regards,

Thanks, that was just what I didn’t want to hear unfortunately!

The 4.5m was for the radius of the tower of the 5MW NREL turbine, at the blade tip when pointing straight down as given by Marshall.Buhl in the post shown below

Dear Dawn,

I’m not sure what wind turbine Marshall was referring to in that post, but the NREL 5-MW turbine has a tower diameter that ranges from 6 m at the base to 3.87 m at the top (the tower radius would be half these values). These values come from section 6 of the NREL 5-MW specifications report: nrel.gov/docs/fy09osti/38060.pdf.

Best regards,

Jason, did you have a problem with the tower clearance when you did the analysis on the barge operating in stall?

I can’t remember any mention of changes other than the smoothed airfoils which I am also utilising, but I may have missed changes made for that study to cone or shaft angles or other blade properties perhaps?

Hi Dawn,

I don’t recall assessing the tip-to-tower clearance for the NREL 5-MW turbine atop the ITI Energy barge during active pitch-to-stall control studies. I would not be surprised, though, if there is a problem given the large resonant-frequency excitation of the barge. In general, I would only recommend using the ITI Energy barge at sheltered sites where the sea states do not get extreme.

Best regards,

Many thanks Jason.