Jonathan Charles Berg

Scaled Wind Farm Technology Facility Overview.

In the past decade wind energy installations have increased exponentially driven by reducing cost from technology innovation and favorable governmental policy. Modern wind turbines are highly efficient, capturing close to the theoretical limit of energy available in the rotor diameter. Therefore, to continue to reduce the cost of wind energy through technology innovation a broadening of scope from individual wind turbines to the complex interaction within a wind farm is needed. Some estimates show that 10 40% of wind energy is lost within a wind farm due to underperformance and turbine-turbine interaction. The US Department of Energy has recently announced an initiative to reshape the national research focus around this priority. DOE, in recognizing a testing facility gap, has commissioned Sandia National Laboratories with the design, construction and operation of a facility to perform research in turbine-turbine interaction and wind plant underperformance. Completed in 2013, the DOE/SNL Scaled Wind Farm Technology Facility has been constructed to perform early-stage high-risk cost-efficient testing and development in the areas of turbine-turbine interaction, wind plant underperformance, wind plant control, advanced rotors, and fundamental studies in aero-elasticity, aero-acoustics and aerodynamics. This paper will cover unique aspects of the construction of the facility to support these objectives, testing performed to create a validated model, and an overview of research projects that will use the facility.

Design, Fabrication, Assembly and Initial Testing of a SMART Rotor 1

Sandia National Laboratories has designed and built a full set of three 9m blades (based on the Sandia CX-100 blade design) equipped with active aerodynamic blade load control surfaces on the outboard trailing edges. The design and fabrication of the blades and active aerodynamic control hardware and the instrumentation are discussed and the plans for control development are presented. , Albuquerque, NM 87185-1124

Impact of Higher Fidelity Models on Simulation of Active Aerodynamic Load Control For Fatigue Damage Reduction

Active aerodynamic load control of wind turbine blades is being investigated by the wind energy research community and shows great promise, especially for reduction of turbine fatigue damage in blades and nearby components. For much of this work, full system aeroelastic codes have been used to simulate the operation of the activel y controlled rotors. Research activities in this area continually push the limits of the models and assumptions within the codes. This paper demonstrates capabilities of a full system aeroelastic code recently developed by researchers at the Delft Universi ty Wind Energy Research Institute with the intent to provide a capability to serve the active aerodynamic control research effort, The code, called DU_SWAMP, includes higher fidelity structural models and unsteady aerodynamics effects which represent improvement over capabilities used previously by researchers at Sandia National Laboratories. The work represented by this paper includes model verification comparisons between a standard wind industry code, FAST, and DU_SWAMP. Finally, two different types of a ctive aerodynamic control approaches are implemented in order to demonstrate the fidelity simulation capability of the new code.

Fabrication Integration and Initial Testing of a SMART Rotor.

EDUCING ultimate and oscillating (or fatigue) loads on the wind turbine rotor can lead to reductions in loads on other turbine components such as the drive train, gearbox, and generator. This, in turn, is expected to reduce maintenance costs and may allow a given turbine to user longer blades to capture more energy. In both cases, the ultimate impact is reduced cost of wind energy. With the ever increasing size of wind turbine blades and the corresponding increase in non-uniform loads along the span of those blades, the need for more sophisticated load control techniques has produced great interest in the use of aerodynamic control devices (with associated sensors and control systems) distributed along each blade to provide feedback load control (often referred to in popular terms as „smart structures‟ or „smart rotor control‟). A recent review of concepts and feasibility and an inventory of design options for such systems have been performed by Barlas and van Kuik at Delft University of Technology (TUDelft) 1 . Active load control utilizing trailing edge flaps or deformable trailing edge geometries (referred to here as Active Aerodynamic Load Control or AALC) is receiving significant attention, because of the direct lift control capability of such devices and recent advances in smart material actuator technology. Researchers at TUDelft 2-3 , Riso/Danish Technical University Laboratory for Sustainable Energy (Riso/DTU) 4-10 and Sandia National Laboratories (SNL) 11-17 have been very active in this area over the past few years. The SNL work has focused on performing extensive simulations of AALC on several turbine configurations and has analyzed the simulation results to estimate the fatigue damage reduction on the rotor and gearbox and the costof-energy benefits of integrating trailing edge technology into the tip region of the turbine blades. These simulation results show the potential for significant impacts on fatigue damage and cost of energy, but experimental data is badly needed to confirm the simulation-based analyses. To the best of our knowledge, no research group has yet built and field tested a rotor with a full smart blade set. SNL has built a set of blades for its 100 kW test turbine in order to test AALC concepts. The main thrusts of this effort are to develop and validate a highly accurate structural dynamics model of the operating rotor, to work through the implementation details involved with developing appropriate control algorithms for such a rotor, and to obtain experimental verification of simulation runs; we are not attempting to design the optimal rotor for integration of AALC control capability or to develop the optimal control strategy. The design of the blade set is covered briefly in this paper and the reader is directed to a previous AIAA paper 18 for additional information. The fabrication, integration, and test results to date for this smart blade set are the subjects of this paper.

Mapping of 1D Beam Loads to the 3D Wind Blade for Buckling Analysis.

This paper discusses the development of a consisten t methodology for mapping onedimensional distributed beam loads to a three-dimensional shell structure. The resultant force distribution is a linear approximation to the actual aerodynamic pressure distribution but is sufficient to obtain accurate strain and dis placement results. The purpose of the mapping technique is to apply more realistic wind l oads to the shell model of a wind turbine blade without the need to set up and run expensive computational fluid dynamics or fluid structure interaction problems. Subsequent buckling and stress analysis reveal how this approach compares to other simplified methods of defining the loads.

Effects of increasing tip velocity on wind turbine rotor design.

A reduction in cost of energy from wind is anticipated when maximum allowable tip velocity is allowed to increase. Rotor torque decreases as tip velocity increases and rotor size and power rating are held constant. Reduction in rotor torque yields a lighter weight gearbox, a decrease in the turbine cost, and an increase in the capacity for the turbine to deliver cost competitive electricity. The high speed rotor incurs costs attributable to rotor aero-acoustics and system loads. The increased loads of high speed rotors drive the sizing and cost of other components in the system. Rotor, drivetrain, and tower designs at 80 m/s maximum tip velocity and 100 m/s maximum tip velocity are created to quantify these effects. Component costs, annualized energy production, and cost of energy are computed for each design to quantify the change in overall cost of energy resulting from the increase in turbine tip velocity. High fidelity physics based models rather than cost and scaling models are used to perform the work. Results provide a quantitative assessment of anticipated costs and benefits for high speed rotors. Finally, important lessons regarding full system optimization of wind turbines are documented.

Field Test Results from the Sandia SMART Rotor

*† ‡ Sandia National Laboratories has concluded field testing of its wind turbine rotor equipped with trailing-edge flaps. The blade design, fabrication, and integration which have been described in previous papers are briefly reviewed and then a portion of the data is presented and analyzed. Time delays observed in the time-averaged response to stepwise flap motions are consistent with the expected time scales of the structural and aerodynamic phenomena involved. Control authority of the flaps is clearly seen in the blade strain data and in hub-mounted video of the blade tip movement.