1. The generics of wind turbine nacelle anemometry
Sten Frandsen
Jens N. Sørensen
Robert Mikkelsen
Troels F.
Pedersen
Ioannis Antoniou
Kurt
Hansen
DTU
Siemens Windpower

2. Assessment of productive capacity
Nacelle anemometry serves multiple purposes and it is frequently used for verifying productive capacity of wind projects. Thus, erection of expensive met masts is saved
Question: is it meaningful to use the nacelle anemometer?

3. The nacelle anemometer is meant to measure the free wind … but the flow around the nacelle is complicated – exposed to numerous sources of disturbances

4. Major influence parameters
Sketch of major influence parameters on nacelle anemometry. Parameters regarding instrumentation to consider are: the type and class of anemometer, the mounting position and height, the mounting structure, safety railings and aviation light. Induced flow parameters to consider are: the upstream rotor induced flow, the profiled blade induced flow, the blade root vortex, the swirl induced flow, the cylindrical blade wake and the nacelle induced flow. The operational modes of the turbine to consider are: control system software version, control parameters, operational mode (rpm & pitch) and yaw error characteristics. Environmental influential parameters to consider are: other nearby wind turbines, terrain roughness, terrain obstacles, RIX number, turbulence, inflow inclination angle and air density (reference: IEC CD and present analysis).

5. Specific problem
When we are doing performance verification, what are we actually looking for? Answer: a possible change in power relative to a warranted power curve
Generics: while the reading of the nacelle anemometer is disturbed by a number of effects, some disturbances are related to the wind turbine rotor’s action on the flow
“Calibration” components of the nacelle anemometer:

6. Betz 1D rotor model

7. Calibration of nacelle anemometer

8. Nacelle anemometry: what is the generic problem?
Measuring the reference PC,
and calibrating the nacelle anemometer:
Measuring the PC in the field,
but the real PC is:

9. For the 1-D rotor – relation between dp and dv
For the Betz rotor, we have:
Assume a change in power relative to warranted power, at free wind speed u:
Measured nacelle wind speed v1:
However, presumed free wind speed:

10. The ”1-D” generic error of nacelle anemometry
Resulting change and error in power at wind speed u:
The error relative to the actual change in power:

11. The relative generic error for 1-D rotor
Approx. state of art:
CP,max~0.5

12. Illustration of generic error

13. Siemens, CFD 3-D RANS rotor flow

14. DTU: CFD 3-D RANS rotor flow

15. 3-D induction

16. Computed and measured performance characteristics

17. Comparison of Cp and CT with 1-D ideal Betz distrib.

18. Comparison of Cp and CT with 1-D ideal Betz distrib.

19. Conclusions
For 1D Betz rotor: Nacelle anemometry introduces an error which is of the order 75%. The error is one-sided and may in principle be corrected for
For 3D rotor: the induction close to the rotor center and the nacelle is typically small and the error is in general terms less. However, the gradients is the induction factor are here large, which makes the uncertainty of the error large
These wind speed gradients close to the nacelle make it an attraktive option to measure the flow speed, say, 10-20m in front of the spinner