Documenting Complex Flows in Wind Farms using the TTUKa Mobile Research Radars

An increased understanding of the complex flow fields through wind farm systems is necessary to fully optimize their power output capabilities. Power output models do not accurately describe the flow through a wind farm, including the impact of turbine wakes. The modulated flow fields through wind farms are poorly understood leading to uncertainty in optimal wind farm design and layout, and an a general power output underperformance. Full-scale observations of the flow through a wind farm are limited both in quantity and spatial coverage. The analyses presented here utilize research grade mobile Doppler radar observations coupled with innovative analyses methodologies to assess the structure and evolution of the wind flow through wind farms.

Texas Tech University has developed and maintains two mobile Ka-band (35 GHz) Doppler radar systems (TTUKa) equipped with pulse compression capabilities. These radar systems have been deployed within multiple wind farms providing high spatial and temporal resolution data in differing wind speed regimes for a variety of turbine types and spacing. In each deployment, wake structure from multiple turbines is assessed including turbine-to-turbine interactions. The effects of heterogeneous local terrain and surface roughness are also evident and contribute to the observed flow fields. Single-Doppler radial velocity and spectrum width fields are effective tools for tracking the variability in turbine wake length and meandering character. Using repetitive range-height indicator and plan-position indicator scanning, modulation of the flow field and associated turbulence through several rows of turbines is observed with scan revisit times of only a few seconds.

Coordinated volumetric scanning from both TTUKa radars within a wind farm allows for the construction of dual-Doppler synthesis of the full horizontal wind speed and wind direction. The resulting gridded wind fields allow for a three dimensional flow-field “map” with a horizontal footprint of tens of square kilometers. Dual-Doppler analyses from multiple horizontal and vertical planes allow for investigation of wind speed reductions within multiple turbine wakes; including flow variability across the rotor sweep at various distances downwind of a given turbine. Transient flow features translating through a given wind farm are frequently observed and can significantly alter wake structure and orientation on small time scales. Dual-Doppler wind speed and wind direction analyses are presented for multiple hours of scanning.

The full-scale measurements shown in this work can be integrated into modeling efforts to improve expected power output projections for a given wind farm. The construction of real-time wind field maps can be used by turbine control systems for anticipatory control. Real-time, full-scale observations coupled with turbine power performance data can lead to enhanced power output predictions, including assistance in resource assessment. The presented technology and analyses methodologies offer the potential to impact the cost of energy.