The stable atmospheric boundary layer a challenge for wind turbine operations, agu fall meeting, dec 5, 2008, san francisco

NREL is a national laboratory of the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy operated by the Alliance for Sustainable Energy, LLC

American Geophysical Union Fall Meeting San Francisco Neil D. Kelley Bonnie J. Jonkman National Wind Technology Center December 15, 2008

The Stable Atmospheric Boundary Layer: A Challenge for Wind Turbine Operations

AGU Fall Meeting, San Francisco December 15, 2008 2 National Renewable Energy Laboratory Innovation for Our Energy Future

Outline • Background

• Field experiments using full-size wind turbines to provide an understanding of the role of atmospheric turbulence in inducing damaging fatigue loads on modern wind turbines

• Turbulence characteristics associated with increased turbine dynamic loads and component wear and tear (fatigue)

• Stable atmospheric boundary layer structures that are responsible for these characteristics

• Conclusions

• What are we doing about this?


Background • More than 21,000 MW of wind energy

will be installed in the US by the end of 2008 with over 1300 MW added from January to September alone

• Turbine capacities now being installed generally exceed 2 MW with hub heights in the 80-120 m range and rotor diameters of 100 m or more

• Large capacity turbines are critical in meeting the national goal of 20% of electrical power from wind by 2030

• An industry-wide systemic power underproduction in the range of 10-15% now exists as well as incurring much higher than expected maintenance and repair costs

Source: American Wind Energy Association


Wind Turbines and Turbulence • Wind turbine structures have

become much more flexible as the power generating capacity has steadily increased into the multi-megawatt range

• This has resulted in a greater number of high frequency vibrations becoming active under the dynamic loading induced by atmospheric turbulent structures embedded in the turbine inflow

• Like aircraft, wind turbines are considered fatigue critical structures

• The service lifetime of a typical wind turbine component is heavily influenced by the rate at which cumulative fatigue damage occurs


Determining the Effects of Turbulence

• Experiments have been performed within a multi-row wind farm in California’s San Gorgonio Pass and at NREL’s National Wind Technology Center in Colorado

• Detailed, simultaneous measurements were made of both the turbulent inflow characteristics and the resulting aeroelastic response of the wind turbine(s)

• A range of analysis techniques including time-frequency (wavelets) and statistical correlation have been used to establish the turbulence characteristics that have the greatest influence on turbine fatigue loads


The San Gorgonio Pass Experiment

Pacific Ocean Salton

Sea

wind farms (152 m, 500 ft)

(−65 m, −220 ft)

(793 m, 2600 ft) Mt. Jacinto (

row 37

(3166 m, 10834 ft)

Wind Farm


The Experiment at Row 37 of San Gorgonio Pass Wind Farm

Hub-height sonic

anemometer

AeroStar Rotor

SERI S-Series Rotor

30 m turbine inflow Tower


The Experiment at the NWTC

NWTC (1841 m – 6040 ft)

NWTC

Great Plains

Terrain Profile Near NWTC in Direction of Prevailing Wind Direction

Denver

Boulder


Measuring the Spatial Effects of Turbulence on Turbine Response

Continental Divide3960 m

EldoradoCanyon

EldoradoCanyon

1765 m1765 m

NWTC600 KW ART

Turbine

NWTC600 KW ART

Turbine43-m diameterplanar sonicanemometer

array

43-m diameterplanar sonicanemometer

array

NCARMesa Lab

1885 m


Diurnal Variation of High Blade Fatigue Loads

Local standard time (h)2 4 6 8 10 12 14 16 18 20 22 24

Prob

abili

ty (%

)

0

2

4

6

8

10

12

14sunrise sunrset

Local standard time (h)

0 2 4 6 8 10 12 14 16 18 20 22 24Pr

obab

ility

(%)

0

2

4

6

8OctMay Oct May

San Gorgonio Wind Farm NWTC

Missing data: Turbine not able to run due to very turbulent conditions

Diurnal periods that favor encountering high blade loads


Variation of High Blade Fatigue Loads with Vertical Stability

Turbine Layer Vertical Stability, Ri-0.06 -0.04 -0.02 0.00 0.02 0.04 0.06 0.08 0.10

Prob

abili

ty (%

)

0

10

20

30

40

50

60

Turbine Layer Vertical Stability, Ri

-0.08 -0.06 -0.04 -0.02 0.00 0.02 0.04

Prob

abili

ty (%

)

0

5

10

15

20

25

Both peaks occur in weakly or slightly stable conditions!

San Gorgonio Wind Farm NWTC


Example of Turbine Rotor Encountering a Coherent Turbulent Structure – Aeroelastic Response

In-plane root bending moment (kNm)

AeroStar rotor out-of-plane (flapping) root bending moment (kNm)

SERI blade AeroStar blade

Blade 1 Blade 2

Blade 3

Significant stress reversals = fatigue cycles


Stable Boundary Layer 3-D Coherent Turbulent Structure at Hub Height Responsible for Observed Rotor Dynamic Response

Instantaneous local Reynolds stresses (m/s)2

Estimated local vorticity components (sec-1)

ωy

ωz

v’w’

u’v’

u’v’ v’w’

ωx ωy ωz


SBL Coherent Structures: Important Source of Turbine Fatigue Loading

• We have defined a fluid dynamics parameter that correlates well with observed turbine fatigue loads: Coherent TKE

• Ingesting coherent turbulent structure can induce strong oscillatory responses (stress reversals) in turbine components

Upwind arrayinflow CTKE

m2 /s

2

0

20

40

60

80

100

120

0

20

40

60

80

100

120rotor top (58m)rotor hub (37m)rotor left (37m)rotor right (37m)rotor bottom (15m)

IMU velocity components

0 2 4 6 8 10 12

mm

/s

-20

-10

0

10

20

-20

-10

0

10

20

Time (s)

492 494 496 498 500 502 504

vertical (Z)side-to-side (Y)fore-aft (X)

zero-meanroot flapbendingmoment

kNm

-400

-300

-200

-100

0

100

200

300

400

-400

-300

-200

-100

0

100

200

300

400

Blade 1Blade 2

Loads at Blade Roots

Drivetrain X,Y,Z velocities

ART Turbine Coherent Turbulence Response

Coherent TKE from Array

2 2 2 1 21 2 // [( ' ') ( ' ') ( ' ') ]= + +CTKE u w u v v w


Coherent Structures in Sub-jet Shear Layer

NOAA LIDAR TKE

Tower Sonics CTKE

SBL Vertical Structures: Low-Level Jets

• The West and Central Great Plains contain the best wind resource in the US

• Nocturnal low-level jets and the accompanying strong shear layers are ubiquitous during the warmer months (Apr-Sep)

• Kelvin-Helmholtz Instability (KHI) frequently occurs in these layers and is responsible for generating coherent structures such as KH billows that form, break, and decay

June 16-17, 2002

10-min mean wind speed (m/s)

2 4 6 8 10 12 14 16 18 20 22

Hei

ght a

bove

gro

und

leve

l (m

)

0

50

100

150

200

250

300

350

400

450

0

50

100

150

200

250

300

350

400

450

17:0019:0023:0000:0001:0002:0003:0004:00

Evolution of Low-Level Jet at Lamar Site


Conclusions

• Turbulent processes associated with shear flow instabilities in the stable boundary layer are likely one of the major sources of fatigue damage and the less than design lifetimes being observed of components used with modern, multi-megawatt wind turbines

• At most locations with a usable wind resource, the boundary layer is typically stable about 2/3 of the time an thus increasing the exposure to the turbulent structures generated under such conditions

• Evaluations of potential wind energy sites should identify any regional and local conditions that enhance the development of organized, nocturnal turbulence and match the design of the turbine to be installed accordingly


What Are We Doing About This?

• We have developed a stochastic turbulence inflow simulator (TurbSim) that replicates flow fields experienced at the NWTC, in the western Great Plains, and in and near a multi-row wind farm

• This simulator is used to drive the dynamics of numerical simulations of wind turbine designs and is used as a critical element of the design process

• We are studying methods employing remote-sensing technology to support the development of real-time turbine control schemes to reduce fatigue loads and to increase turbine production efficiency