The generics ofwind turbine nacelle anemometry

Sten Frandsen

Jens N. Sørensen

Robert Mikkelsen

Troels F. Pedersen

Ioannis AntoniouKurt

Hansen

DTU DTU DTU DTUSiemens

WindpowerDTU

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?

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

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: IEC61400-12-2 CD and present analysis).

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:

generictindependenpower

),()( auvuvu ji

Betz 1D rotor model

v=(1-a)u u

(1-2a)u

Calibration of nacelle anemometer

a

vu

10

Nacelle anemometry: what is the generic problem?

P0

vcal=(1-acal)u0

u0

u0

vfield=(1-afield)u0

P0+P

Measuring the reference PC,

000 ;1

);( Pa

vPu

cal

cal

and calibrating the nacelle anemometer:

calcal

va

u

cal.cont.

0 1

1

Measuring the PC in the field,

PP

a

v

cal

field 0;1

but the real PC is:

PP

a

vPPu

field

field 000 ;1

);(

For the 1-D rotor – relation between dp and dv

For the Betz rotor, we have:

2)1(4 aaCP 23 )1(4½ aaAup R

Assume a change in power relative to warranted power, at free wind speed u:

pupup )()( 0

u

vaauav aa

111 1)(1

Measured nacelle wind speed v1:

However, presumed free wind speed:

a

vu

11

1

The ”1-D” generic error of nacelle anemometry

Resulting change and error in power at wind speed u:

errorpperrorp pupup ,0,111 )()(

aR

aR

p

errorperrorp

aaAu

aaAur

)1)((6

)1(6

313

3,

,

a

ar errorp

31,

The error relative to the actual change in power:

The relative generic error for 1-D rotor

1

10

100

1000

10000

0 0.1 0.2 0.3 0.4

Induction a

Err

or

in d

P (

%)

Approx. state of art:CP,max~0.5

Illustration of generic error

0

500

1000

1500

2000

2500

0 5 10 15 20

Wind speed [m/s]

Po

we

r

P

P_1

0.00

0.04

0.08

0.12

0.16

0.20

0 5 10 15 20

Wind speed [m/s]

Indu

ctio

n fa

ctor

a

a_1

0.0

0.1

0.2

0.3

0.4

0.5

0 5 10 15 20

Wind speed [m/s]

Po

we

r co

effi

cie

nt

C_P

C_P_1

0

5

10

15

20

0 5 10 15 20

Wind speed [m/s]

Ch

an

ge

P %

Actual,WSM

Ch withWSN

Siemens, CFD 3-D RANS rotor flow

DTU: CFD 3-D RANS rotor flow

3-D induction

Computed and measured performance characteristics

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

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

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