Complex Flow Workshop Assesses Future R&D Needs

This is an excerpt from the Second Quarter 2012 edition of the Wind Program R&D Newsletter.

The Energy Department's Wind Program occasionally convenes expert workshops to help identify and assess promising new areas of research. One such area is the study of multiscale complex aerodynamics, which spans everything from large-scale national weather patterns all the way down to millimeter-scale wind interactions with individual wind turbine blades. Substantial gains in understanding these truly complex flows represent one of the largest remaining opportunities to reduce the levelized cost of energy for land-based wind power. Researchers have identified the following opportunities for cost reduction:

• Power losses can be as high as 20%–30% in operating wind farms, due solely to complex wake interactions occurring in wind farm arrays.
• Forecasting accuracy improvements of as little as 10%–20% could result in hundreds of millions of dollars (estimated at $140-260 M) in annual operating cost savings for the U.S. wind industry.
• Drivetrain components such as gearboxes are failing significantly earlier than their 20-year design life. These failures are caused in large part by uncertainty in aerodynamic loading conditions.

To obtain input on existing gaps and future opportunities for research in complex flow modeling and experimental validation, the Wind Program held a 2-day workshop in January in Boulder, Colorado. The meeting provided participants with an opportunity to share information and facts regarding this topic based on individual experience.

The public meeting consisted of an initial plenary session in which invited speakers surveyed available information and needs for various applications related to complex flow modeling and validation. For the remainder of the meeting, break-out groups provided participants an opportunity to present Energy Department representatives with information on specific areas. Groups were organized around the following topics:

• Mesoscale Modeling and Validation: Participants examined the meteorological effects at the regional, multi-wind-farm-scale. This exploration of atmospheric science topics included model nesting (examining turbine-scale within wind-farm-scale within regional-scale models), long-term data collection requirements, and downwind effects of wind farms.
• Wind-Farm-Scale Modeling: Participants examined complex aerodynamic phenomena in, around, and through wind farms, including turbine-wake interaction, wake-wake interaction, complex terrain, and turbulence effects.
• Wind-Turbine-Scale Modeling: Participants examined inflow and outflow characteristics in the vicinity of individual wind turbines, as well as the implications for aerodynamic loading of the rotor and overall structure.
• Experimental Data and Validation Requirements: Participants examined the requirements for, as well as the feasibility and efficacy of, existing and future experimental techniques for cost-effective, high-fidelity data collection.

While the detailed findings from each break-out group were distinct in terms of scale and scope, there was considerable overlap. Participants from multiple groups noted that future complex flow research and development will require substantial improvements to meteorological and engineering models, as well as high-fidelity measurements that cross spatial and temporal scales simultaneously. Phenomena of different spatial and temporal scales need to be predicted and measured using different modeling and observational techniques. This need for various techniques provides a challenge when modeling because the techniques need to be coupled and this requires that an experimental research program be well planned. For example, national weather forecast models must interface with wind farm models, which in turn must interface with individual wind turbine models. This represents a range of scales from hundreds of kilometers down to millimeters. Further complicating the matter, each model will require validation data to be collected at each of these scales. Once validated, these improved simulations, which may require significant high-performance computing resources, must eventually be reflected in the tools used by industry to plan, manufacture, and operate wind farms.

Download the full report: U.S. Department of Energy Complex Flow Workshop Report. Read more about current complex flow research: A Large-Eddy Simulation of Wind-Plant Aerodynamics.