Sandia’s Wind Energy Technology Department seeks opportunities to advance the current state of the art in rotor technology such that rotors can capture more energy, more reliably, with relatively lower system loads – all at a lower end cost. The blades make up about 14% of the capital cost of a full turbine (Tegen, 2013), but they are responsible for practically 100% of the energy capture in a wind plant. They are responsible for all the steady and dynamic loads which drive the design and cost for the remainder of the turbine system – hub, shafts, bearings, gearbox, nacelle bedplate, tower and foundation. Optimal relationships between rotor structures, rotor aerodynamics, and the rest of the turbine system are complex. The wind industry has evolved over time to provide competitively low cost of electricity using increasingly optimized systems.rotorinnovation1_SAND_2014_0991P

Sandia rotor innovation activities are directed primarily toward two approaches

1) Quantitative evaluation and reporting of rotor innovation concepts using numerical studies. Examples include the Sandia 100m blade design progression and the Sandia investigation of the effects of increasing maximum tip velocity on optimum rotor designs. Complete and accurate numerical design methods are critical to our work in this area. Projects result in public, detailed models and tools that are beneficial to the entire wind energy research community.

2) Design and field test of rotor hardware for validation of rotor innovation concepts or hardware created to support goals of larger experimental campaigns, such as the DOE Atmosphere-to-electrons (A2e) Initiative. Work in this area leads to public field test data which is used to improve important simulation and analysis tools, enabling effective evaluation of future innovations.

Related Topics:

Sandia’s Wind Energy Technologies Department creates and evaluates innovative large-blade concepts for horizontal-axis wind turbines (HAWTs) to promote designs that are more efficient aerodynamically, structurally, and economically. Recent work has focused on developing a 100 m blade for a 13.2 MW HAWT, a blade significantly longer than the largest commercial blades that existed at the beginning of this project (~60 m long).

Sandia’s Wind Energy Technologies Department creates and evaluates innovative large-blade concepts for horizontal-axis wind turbines (HAWTs) to promote designs that are more efficient aerodynamically, structurally, and economically. Recent work has focused on developing a 100 m blade for a 13.2 MW HAWT, a blade significantly longer than the largest commercial blades that existed at the beginning of this project (~60 m long).

Selected design studies:

  1. Sandia 100m blade design progression
  2. Sandia investigation into tradeoffs in rotor designs due to increasing tip velocity

External Reference

Tegen, S., Lantz, E., Hand, M., Maples, B., Smith, A., and Schwabe, P. “2011 Cost of Wind Energy Review,” National Renewable Energy Laboratory: NREL/TP-5000-56266, 2013.

Rotor Innovation Projects

Aerodynamic Design Overview

rotor velocity

Induced velocity in the rotor wake 0.1-3 diameters downstream of the rotor plane of the NREL 5MW wind turbine reference model, as predicted by a free-wake vortex method. Velocity vectors are projected in the planes shown.

The aerodynamic design of a wind turbine rotor is performed using a combination of experimental data and a variety of different design and analysis tools. Blade element momentum theory (BEMT) has been a staple of rotor design. It is still used heavily for rotor design work, but is now commonly utilized with optimization methods, where thousands of cases can be analyzed in less than an hour on modern desktop computers.

Blade element vortex methods (BEVM) have been available for several decades, but are less commonly used for rotor design work. Vortex method based rotor design tools capture more of the physics of the interaction between the atmosphere, rotor, and the rotor wake, but at a higher computational cost than BEMT. For example, a free-wake BEVM can produce three-dimensional predictions of the rotor wake as well as wake velocity deficits, as shown in the two attached images.

velocity magnitude
Instantaneous induced velocity magnitude in the rotor wake from 0.1 to 3 rotor diameters downstream of the rotor plane.

Integrating Wake Effects into the Design Process

Models of various fidelities are being utilized to quantify the effects of blade designs on wake characteristics. The near-wake structure is created by and dependent upon the blade circulation distribution. A free-wake, blade element vortex method is being used for most of the analysis cases, as the method allows for the relatively fast computation of near-wake induced velocities as a function rotor blade circulation. High fidelity large-eddy simulations (LES) of the same rotor wake are also being performed for direct comparison to the wake deficit predicted by the free-wake vortex method. These simulations will define constraints on the parameters used for scaled rotor design work.
Utilizing Multiple Model Fidelities

Reynolds averaged solutions of the Navier-Stokes equations (RANS) are being used for full rotor simulations for research purposes and are gradually being adopted for design work. These solutions can capture more complicated flow details and interactions, but at a much higher computational cost than BEMT and BEVT. Large eddy simulations (LES) are even more expensive, and are being used with actuator line rotor definitions to analyze entire wind farms, including complex terrain and atmospheric stability and turbulence.

Blade designers are often limited to using airfoils with which they have familiarity. The designer’s trust of an airfoil is driven by available wind tunnel data and proven field experience with its performance in relevant operating regimes. Sandia is currently using computational analyses of multiple fidelities, such as RANS and LES, to enhance the applicable design space of specific airfoils. LES has also been used in high fidelity predictions of airfoil noise.Velocity magnitude around a DU97-W-300 at α = 6° analyzed using a Sandia CFD code. High fidelity computational analyses are used to expand the design space beyond the limitations of experimental data.
velocity magnitude at 6 degrees
Velocity magnitude around a DU97-W-300 at α = 6° analyzed using a Sandia CFD code. High fidelity computational analyses are used to expand the design space beyond the limitations of experimental data.

Why Rotor Instrumentation?

During a wind turbine’s operational lifetime, its rotor blades endure a wide variety of wind loading. The blades directly capture all of the force applied to the entire wind-turbine system.

A turbine’s exact wind loading is fundamental to overall system design. Due to the large potential for variability in
•atmospheric conditions,

•terrain topology, and

•turbine placement with respect to other turbines,

exact loading is difficult to model accurately. To overcome the wind-loading unknowns the structure will experience during its lifetime—to ensure that the unit operates to the end of its design/warrantied lifetime (usually 20 years)—additional safety factors are engineered into the design, usually requiring heavier and more costly components.

By implementing a set of sensors to measure wind-loading forces on the rotor blades during actual operation, we can shine light on the unknowns of how the wind acts directly on the wind-turbine system.

This work has been funded by the Department of Energy’s Wind and Water Power Program.
Fiber Gragg Grating sensors installation
Sandia staff installing Fiber Bragg Grating sensors in the roots of 13 m wind-turbine blades used at the Scaled Wind Farm Technology (SWiFT) facility.

Sensor Capabilities

Attaining a better understanding of the fundamental physics behind the loading that occurs during wind-turbine operation requires many different sensor types. Sandia engineers work with a wide array of transducers to characterize the surrounding environment as well as the turbines’ aerodynamic and structural responses. We measure

•atmospheric pressure, relative humidity, and temperature to determine local air density;

•incoming wind speed and direction with state-of-the-art sonic anemometers, as well as with International Electrotechnical Commission-standard calibrated cup anemometers and wind vanes;

•structural response on the turbine tower, including acceleration and strain, in order to determine rotor thrust loading—and detect any vibration that may be out of the ordinary;

•rotor-blade strain, acceleration, and surface pressure via sensors mounted on the rotor itself—in order to quantify direct loading during turbine operation from within the rotating reference frame.

SMART rotor
A SMART rotor with embedded sensors.

Recent Projects

The SMART active aerodynamics rotor was recently flown to assess the control authority of trailing edge flaps during operation. Many structural sensors monitored rotor loading to positively confirm the ability of the flaps to regulate blade wind loading.

Sandia develops computer-aided engineering (CAE) tools with capabilities driven by the needs of current research projects. Rotor design is supported by a collection of tools which enable design and optimization of the rotor aerodynamics and structural performance. This unique and customizable toolset enables innovative designs and research on rotors, often including unconventional design objectives.

Numerical Manufacturing And Design Tool (NuMAD) is an example of a design tool we have developed to simplify the process of creating structural models and calculating blade properties for aero-elastic simulation. NuMAD is open-source and freely available for download.
numad
NuMAD© is a research code with capabilities that have been driven directly by Sandia/DOE research projects. The current version of NuMAD© (v2.0), released April 2013, may be obtained by returning the completed NuMAD Request form (as per instructions on the form).

Learn More