NextEra's Cottonwood wind project (113.6 MW) proposes to site all of its 52 turbines within 2 to 7 miles of the Blue Hill, Nebraska NEXRAD radar. This will be the closest of any operating turbines in the U.S. to weather stations in Tornado Alley. The National Weather Service meteorologist-in-charge at the Grand Island/Hastings Weather Forecast Office in Hastings, NE, agreed to sign this letter of intent addressing the times when operational curtailment of the turbines would be required in order to evaluate a storm within the range of the radar. The agreement is in effect for five years from the date it was signed (November 24, 2014). The agreement is entirely voluntary and non-binding on any party. It can be terminated at any time upon notice of just one party. The introduction of the agreement is provided below. The full agreement can be accessed by clicking the link(s) on this page.
NextEra's Cottonwood wind project (113.6 MW) proposes to site all of its 52 turbines within 2 to 7 miles of the Blue Hill, Nebraska NEXRAD radar. This will be the closest of any operating turbines in the U.S. to weather stations in Tornado Alley. The National Weather Service meteorologist-in-charge at the Grand Island/Hastings Weather Forecast Office in Hastings, NE, agreed to sign this letter of intent addressing the times when operational curtailment of the turbines would be required in order to evaluate a storm within the range of the radar. The agreement is in effect for five years from the date it was signed (November 24, 2014). The agreement is entirely voluntary and non-binding on any party. It can be terminated at any time upon notice of just one party. The introduction of the agreement is provided below. The full agreement can be accessed by clicking the link(s) on this page.
INTRODUCTION
The National Oceanic and Atmospheric Administration’s (NOAA), National Weather Service (NWS), and others have analyzed the real and potential impacts of wind farms on weather radars. The analysis has revealed that the chief impact of wind turbines comes from the rotating blades, which produce a radar detectable signal very similar to that produced by weather. Currently, the NWS’ radar-algorithm for removing clutter (e.g. buildings, towers) is dependent upon an object having zero, or near zero, velocity and thus cannot remove an operating turbine’s signal. There is no known signal-processing algorithm that can remove wind turbine clutter while completely preserving the weather signal. This wind turbine-induced clutter contaminates the radar’s base data (reflectivity, velocity, spectrum width) and internal algorithms, which in-turn can impact alerts and derived products (e.g. estimated precipitation). As a result, critical radar data over wind farm areas could be lost and distract forecasters as they conduct severe weather warning operations. NWS-funded studies to develop an algorithm that can automatically identify wind turbine corrupted signal data is in its initial stages and does not currently provide a mitigation option. Until a radar-based solution is developed, one available mitigation option is limited operational curtailment of turbines during severe weather events (e.g., tornadoes, severe thunderstorms).
The purpose of curtailing turbine operations is to allow weather forecasters to view radar data uncontaminated by wind turbine clutter. During operational curtailment, the wind farm operator would feather the turbine blades bringing them to a stop, or near stop, for short time periods (on the order of 15 to 60 minutes). This allows the radar to filter out any signals returned from the wind farm. Rotation in a feathered condition can occur, but is very slow (up to 4 rpm) compared to operational speeds. Turbine clutter will be significantly reduced or completely eliminated in most circumstances, hence data contamination and algorithm errors are greatly reduced. Example scenarios for the implementation of operation curtailment are presented at Appendix A.
