Uncertainty in wind: How data extrapolation impacts your project

Wind data is traditionally collected with cup anemometers, a thermometer, and a barometer mounted on steel mast, all at no more than 60 meters off the ground. This device is known as a meteorological mast, meteorological tower, measurement mast, measurement tower, or met mast for short. Modern wind turbine hub heights regularly exceed 100 meters, which requires wind data to be extrapolated an additional 2/3 of the original measurement height above the ground. Mathematically and statistically, uncertainty increases as you extrapolate data farther away from your measurement height. Wind data collection should occur for the life of the project to assist with performance monitoring and turbine maintenance.

Collecting wind data for the lifetime of a project

Wind turbine project performance estimates are based on wind data collected. Industry researchers at Vaisala found that in a study of 111 data sets, uncertainty in the extrapolated data resulted in a 3%-4% uncertainty in annual energy production. Variations of this magnitude can result in turbine performance losses of tens of thousands of dollars per turbine.

Met masts are not only for preliminary data collection but can also provide insights into wind project performance during the lifetime of the project if permanently installed and maintained during project operation. Met masts can be critical resources in assisting the analysis of underperformance, turbine failure, etc.

Profit margins are squeezed in every aspect of the wind turbine industry. One part of your profit margin that may be improved before you develop the wind project is understanding suitable wind turbine hub height and rotor diameter. Wind data provides the basis for wind project performance, and you want it to provide the least amount of uncertainty and the best representation of wind characteristics for each turbine location within your wind project. If you site your turbine in complex terrain, it is important to understand what unique wind characteristics may influence your turbine maintenance schedule and how your project may perform over time.

We recommend collecting data for no less than 12 consecutive months, as some wind turbine project investors want to see as much wind data as possible. Also, some turbine suppliers recommend no less than one year of wind data be collected before providing a wind resource assessment and turbine suitability report.

Met mast positioning and data frequency

Met mast siting should be designed to reduce your spacing variation—the measurement location versus the turbine location. You should site your met masts to provide the best estimate of wind conditions at your project, as you will finance it for 20 or more years based on the information collected in the met mast campaign. You want that data to best represent the expected performance of the wind project for wind turbine hub height, rotor diameter, and proximity to the actual turbine locations.

Your measurement campaign should provide the ability to average the data in less than 10-minute intervals. One-second data collection may provide insight into wind anomalies such as eddies or low-level jets. Both anomalies may be averaged out of view when looking at 10-minute averaged data. Knowing that these phenomena may be present during your met mast campaign will be beneficial in determining the suitability of wind turbine models. Knowing what anomalies exist also helps in predicting operating and maintenance costs in the future. Try to collect data in shorter-than-10-minute intervals. One-second data collection is ideal—your goal should be to capture any potential wind phenomena given the terrain and know weather conditions near the wind turbine project.

Turbine supply contracts

Your wind turbine supply contract should include a performance guarantee along with a minimum number of turbines needed to be tested. Test turbine locations should be carefully selected, as the performance test met mast locations are critical to the uncertainty of the wind data collected during the test. You must agree with the wind turbine supplier in selecting the test locations, met mast locations, and the third-party performance testing company. If data filtering is discussed in the contract, know when testing is allowed, when it is not, and when your test criteria are met to allow the test to be deemed complete.

Wind data collection is the most important pre-construction activity in a wind turbine project. It is the basis for performance pro-formas and provides a basis for the turbine suitability analysis. Additionally, it is used in the selection of wind turbines for power performance testing. Proper met mast placement, including measurement heights for data collection, will minimize uncertainty between the collection point and the actual turbine location. Installing a permanent met mast for the life of your project provides actual wind conditions experienced by the turbine. This data can be used for a variety of purposes, including estimating lifecycle, understanding component failures, and validating performance. The importance of establishing a well-structured measurement campaign could significantly impact the annual energy generation of a wind project. We work together with clients to develop a solution for site-specific needs.

Our technical advisory team can help determine the optimal configuration for implementing a measurement campaign based on site-specific characteristics.