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.

Wind Energy Aerodynamics – Rotor, Wake, 

and Wind Plant

Stanford Seminar for 

Faculty and Students

Stanford, CA

October 12, 2010

Scott Schreck, PhD

NREL’s National Wind 

Technology Center

NREL/PR-5000-49705
National Wind Technology Center

NPS 1.5MW Direct Drive 

Generator

• Turbine technology 

since 1977 (SERI)

• Development of design 

and analysis codes

• Pioneers in component 

and field testing

• Unique test facilities

• Blade Testing

• Dynamometer

• CART turbines

• Modern utility-scale 

turbines

• Approx. 160 staff on-site

• Budget approx. $35M

• Many CRADAs with 

industry

• Leadership roles for 

international standards

Blade Test Facility

PIX #15847

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.
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.

Critical Elements for 20% Scenario 

300 GW by 2030

• 80% Land 20% Offshore

• Improved Performance

– 10% reduction in capital cost

– 15% increase in capacity factor

– Address Wind Farm 

underperformance

• Mitigate Risk

– Reduce O&M costs by 35%

– Foster the confidence to 

support continued 20% per 

year growth in installation 

rates from now until 2018

• Enhanced Transmission System 

(AEP)

– $60 billion cost estimate over 

20 yrs

– 19,000 mi of line

– Supports 200-400 GW addition

• Policy, Communication & 

Outreach

• Infrastructure Development
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.

Electrical Power Generation by Source

Source: Electric Power Monthly, March 15, 2010 

http://www.eia.doe.gov/cneaf/electricity/epm/epm_sum.html

3700 TWh Annually

(0.42 TW Continuous energy use)
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.

20% Requires 300 GW - Land & Offshore

0

50

100

150

200

250

300

2000 2006 2012 2018 2024 2030

Cumulative Installed Capacity (GW)

Offshore

Land-based

Actual

Projected
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.

Wind Resource Distribution
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.

How Much Wind is Available ... Really?

Excludes PTC, includes transmission costs to access 10% existing electric 

transmission capacity within 500 miles of wind resource. Source: Black & Veatch/NREL

50% of US 

Electricity
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.

Technology Evolution 

Credit: NREL Historical Photos
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.

• 2.5 MW - typical commercial 

turbine Installation

• 5.0 MW prototypes being 

installed for testing in Europe

• Clipper Wind Power developing 

an 8.5 MW turbine

• Most manufacturers have a 10 

MW machine in design

• Large turbine development 

programs targeting offshore 

markets

• Development Outpacing Test & 

Validation Capability

Wind Turbine Scale – Present and Future

Boeing 747- 400

Clipper 2.5 MW, 99 m rotor

PIX #14647
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.

2.5 MW

10 MW

Stanford stadium overhead image from web address below. Use rectangle above to scale.

http://msrmaps.com/image.aspx?T=4&S=10&Z=10&X=2871&Y=20716&W=1&qs=Nelson+Road%

7cStanford%7cCA&Addr=Nelson+Rd%2c+Stanford%2c+CA+94305&ALon=-

122.1601053&ALat=37.4320765
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.

WindPACT Baseline Design:

y = 0.1452x2.9158

LM Advanced Blade Design:

y = 0.4948x2.53

WindPACT Static Load Design:

y = 0.2113x2.8833

WindPACT Final Design:

y = 0.1527x2.6921

0

5000

10000

15000

20000

25000

30000

35000

0 10 20 30 40 50 60 70

Rotor Radius (m)

Mass (kg)

WindPACT - Static load design

TPI - baseline design

WindPACT - Baseline design

WindPACT - Final design

LM Glasfiber Blades

Offshore 5 MW Turbines

WindPACT - Commercial Data

TPI Innovative Concept Blades

Offshore 5 MW models

Gamesa

Baseline blade mass curve = WindPACT baseline

Advanced blade mass curve = LM advanced design

Structure Size and Weight – Implications
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.

Technology Challenges Remain

Wind plant energy production

– Example

• 200 wind turbines @ 2 MW

• 36% cap factor→1.26x109 kWh/yr

• 5 ¢/kWh, 1% AEP underproduction

• $630K/year = $12.6M/plant lifetime

– 1% - 10% underproduction common

Turbine O&M cost prediction

– Blade delamination, cracking

– Gear, bearing failures

– Unanticipated fatigue loading
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.

• Overall cost

– Aircraft: 600 USD/lb

– Wind turbine: <6 USD/lb

• Wing/blade cost

– Aircraft: >600 USD/lb

– Wind turbine: <9 USD/lb

• Lifetime fatigue cycles

– Aircraft: 106

– Wind turbine: 108

• Inspection/maintenance

– Aircraft: Daily/weekly

– Wind turbine: Six months/one year

Wind Turbines vs. Aircraft 
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.

14

Fundamental Challenges

Current engineering approach

– Linearized and reduced order

– Partitioned for tractability

– Limited scale range and interaction

Physics and numerics

– Coupled and nonlinear

– Broad scale range 

– Multiple physics
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.

NREL UAE Phase VI Turbine in NASA Ames 80’x120’

PIX #09112
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.

Zero Yaw – Rotational Augmentation

Max rotational augmentation of 3X parked 

Deviation same as 15 – 20 % TI, ~10 Hz

0.30R

0.0

1.0

2.0

3.0

0.0 20.0 40.0 60.0

LFA (deg)

C

n

0.30R

ROTATING

PARKED
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.

Flow Field Topology

Off-surface structures 3-D and complex

Topology responsive to operating condition

(CFD courtesy of N. Sørensen, Risø National Laboratory)

U∞= 15 m/s
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.

Nonzero Yaw – Dynamic Stall

Mean Cn maxima = 1.5X - 3X static stall levels

Rise times = 0.1 - 0.2 sec (1/8 - 1/4 cycle)

0.0

1.0

2.0

3.0

4.0

-180.0 -90.0 0.0 90.0 180.0

BLADE AZIMUTH (deg)

MEAN C

n

0.30R

0.47R

0.63R

0.80R

U∞= 13 m/s

Yaw = 40°

Static

Stall
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.

Flow Field Topology

Vortex convection not radially uniform

Operating condition drives 3-D deformation

U∞= 13 m/s

Yaw = 30°

U∞= 15 m/s

Yaw = 40°
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.

•Test results for advanced MIMO control

• Integrated blade pitch-generator torque control

0

0.2

0.4

0.6

0.8

1

1.2

Tower s-s

bending DEL

(kNm)

Tower f-a

bending DEL

(kNm)

Blade flapbending DEL

(kNm)

Blade edgebending DEL

(kNm)

Normalized Value

Baseline

State-space

Control Effectiveness
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.

•Real-time sensing of inflow velocity field

•Independent blade pitch actuation

Advanced Control – Sensors/Actuators
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.

Implications of Blade Flow Physics

Blade flow fields

– Amplified loads

– Highly unsteady

– Large bandwidth

– Energetic vortices

– 3-D flow fields

– Commonly occur

Control impacts

– Bandwidth

– Nonlinearity

– Sensors/actuators

– State identification

– Actuator authority

– Feasibility

Difficult to understand & predict

Challenging to control

Drive COE
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.

: Axial velocity : Turbulence intensity

Near wake

Far wake

?

Sørensen, EWEC 2007

Wake Structure Development
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.

Wake Dynamics – Meandering

Graphic courtesy of J. Mann, Risoe-DTU

LIDAR (gray scale) tracks wake velocity

Model (red) assumes passive transport
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.

25
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.

26

N. Troldborg, VindKraftNet, 2008

Complex Topography Effects
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.

Turbine, wind farm, PBL; 

similar dimensional scales 

Farm / inflow interactions not 

quantified

Characterization & prediction 

remain an issue

Detailed inflow information 

required for turbine design and 

optimized control 

Diurnal variation

Growing concerns include:

– Quality of the downwind 

resource

– Microclimatology changes

– Agriculture impacts

– Permitting

sunset

midnight

Planetary Boundary Layer (PBL)
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.

Unification Across Models & Scales

Treating Multi-Scale Flow Interactions Among Models

Mesoscale, LES and CFD models normally run separately

LES and CFD models use constant or periodic boundary conditions

Some work already underway in linking these types of models

Include terrain and variable land-use into LES models

Mesoscale Models LES Models CFD Models

temporal and spatially varying BCs

parameterizations
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.

PBL Parameterizations

“No mans land”

∆x

sub-grid 

scale

1 m 10 m 100 m 1 km 10 km 100 km

Large Eddy 

Simulation (LES) 

Model

Mesoscale 

Model

Models cover multiple spatial scales, but not all encompassing

For ∆x < 1 km, but problems exist – e.g., “double booking” turbulence

sub-grid 

scale
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.

Climate effects

– Understand and predict wind resource variability

– Wind plant and local/regional/global climate

Risø-DTU

12km

4km

4km

NCAR

30% difference

BLUE = lower later

RED = higher later

Energy 

Density Δ

(91-05)–

(73-87)

Climate Effects
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.

Micrositing Turbine

&Array

Mesoscale

Processes

Climate

Effects

Domain

Size (m)

10-5

101

103

104

101

103

105

107

Cell

Size (m)

Computational Modeling Scales
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.

Deep Water Modeling Requirements

Fully coupled aero- hydro-servo-elastic 

interaction

Wind-Inflow:

–discrete events

–turbulence

Waves:

–regular

–irregular

Aerodynamics:

–induction

–rotational augmentation

–skewed wake

–dynamic stall

Hydrodynamics:

–scattering

–radiation

–hydrostatics

Structural dynamics:

–gravity / inertia

–elasticity

–foundations / moorings

Control system:

–yaw, torque, pitch
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.

Large Facility Requirements

A 45-meter wind turbine blade 

undergoing fatigue testing at the 

NWTC, July 2004.

New Large Blade Test Facilities:

• Boston, MA with Massachusetts 

Technology Collaborative 

• Corpus Christy, TX with University of 

Houston

DOE NOI for 5-15 MW Dynamometer

PIX #13889

PIX #12414
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.

DOE 1.5 MW GE Turbine:

• Model: GE 1.5SLE

• Tower Height: 80 m

• Rotor Diameter: 77 m

• DOE owned; used for research and 

education

Siemens 2.3 MW Turbine:

• Model: SWT-2.3-101

• Tower Height: 80 m

• Rotor Diameter: 101 m

• Siemens owned and operated

• Multi-year R&D CRADA; aerodynamics 

and rotor performance

Multi-MW Turbines at NWTC

DOE 1.5 MW

Siemens 2.3 MW

PIX #17694

PIX #17720
Scott Schreck, PhD

NREL’s National Wind Technology 

Center

Phone: (303) 384-7102

Email: scott.schreck@nrel.gov

Questions?

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.