LIDAR USE IN WIND ENERGY APPLICATIONS

General Use in Wind Energy Yield Assessment and Other Applications

Initial applications of coherent Doppler lidar methods for wind energy analysis focused on vertical profiling, to obtain measurements of wind speed and direction with height above ground reliably up to near 100 meters. Today, the most prevalent form of lidar use focuses on vertical profiling to heights well above 150 meters with sufficient data recovery, and on a less frequent basis, scanning Doppler lidar to obtain volumetric measurements of wind speed, virtual vertical profiles or point measurements at hub height, at times using two or more scanning lidars, depending on the scan strategy employed. 

Our experience with coherent Doppler lidar measurements is unequivocal; they are a valuable measurement system, of more than sufficient accuracy and reliability for standalone onshore and offshore wind energy resource assessment, power performance testing, validating wind flow model performance, and volumetric or virtual met mast studies of wind and wakes, or for other bespoke wind-energy related purposes.

Meteorological towers involve substantial costs for materials and permitting, as well as long lead times, which can strain project budgets and schedules. These challenges are amplified as turbines advance toward taller hub heights and larger rotor diameters.  Vertically-profiling, or suitably deployed dual, Doppler lidars can be a suitable substitute for measurement towers at wind turbine hub heights or taller. Lidar deployment for wind energy applications, given the sensitivity of wind turbines and their energy production to wind characteristics, must be accompanied by careful technical guidance for deployment, adequate maintenance, and careful attention to data quality requirements and diurnal and seasonal data recovery with height.

Like all measurement techniques used for wind energy analysis, the coherent Doppler lidar system must be calibrated, validated, and have a sufficiently robust chain of evidence of such to be utilized effectively during robust data analysis and for finance-grade wind energy yield assessment. Without proper operations and maintenance, any measurement system can fail to meet data analysis standards, or less significantly, limit the usefulness of data records due to poor data recovery in certain atmospheric conditions, operations, or maintenance use cases. International Electrotechnical Commission (IEC) standards bodies in which we participate have recently published international standards (e.g., the IEC -50 and -12 series) that codify the use of lidar equipment, the German FGW TR6 guideline, and an emerging (not yet published) IEC 61400-15 standard further support their standalone use for finance-grade wind energy analysis. The use of lidar for wind energy measurement and data analysis purposes does not eliminate the need for the measurement of other atmospheric variables (e.g., temperature, pressure, precipitation, etc.) and as necessary, transfer functions for certain wind variables (e.g., TI) to other forms, for wind turbine site suitability and other calculations important to wind energy analysis.

Use In Power Performance Testing

Bureau Veritas Power Performance Testing (PPT) is accredited to ISO/IEC 17025 for performance testing to IEC 61400-12/50 standards, by A2LA through its original approval as ArcVera Renewables. The PPT team is IECRE approved, holds MEASNET-accepted proficiency, and is authorized by all major turbine manufacturers to conduct performance warranty testing. The global team capabilities include onshore and offshore testing with nacelle-mounted horizontal-profiling lidars, ground-based vertical-profiling lidars, and meteorological mast-based wind measurement systems, and have executed lidar verifications across multiple markets internationally, demonstrating consistent quality through effective execution. Building on this experience, Bureau Veritas offers practical insight into the applications, advantages, and limitations of different wind measurement technologies, and works with clients to identify the most effective strategies for tests worldwide. 
 

Lidar Usage in Complex Terrain and Flow Curvature Correction

Various solutions exist to correct for the influence of complex terrain on lidar measurements using numerical wind flow modeling techniques; this correction process is known generally as flow curvature correction (FCC). Computational Fluid Dynamics (CFD) model-based approaches have proven to be the most popular, where modeling the influence of static terrain to a steady state is sufficient to estimate the lidar data corrections in complex terrain. The ability to model atmospheric dynamics and thermodynamics in time series, such as may be completed with a suitably high-resolution (small grid spacing) large-eddy simulation numerical weather prediction model, is also a viable and accurate alternative for flow curvature correction.

Wind flow curvature, caused frequently in complex terrain, can induce significant errors in the measured lidar wind speed reconstruction algorithm. The need for flow curvature correction in these circumstances is due to the horizontal flow homogeneity assumption that is used to calculate wind speed and direction across the lidar volume; see the Figure below.

Figure: Diagram of one form of common vertically profiling lidar measurement system used in wind energy applications, demonstrating the horizontally homogenous assumption applied in its wind analysis algorithm.

In general, the flow curvature correction model process utilizes a simplified three-dimensional model domain centered around the lidar location using publicly available high resolution digital elevation model topography and land use/land cover data (mapped to a given surface roughness). The CFD or numerical weather prediction model is then run to produce a result for each of the wind direction sectors, typically in 15-20° sectors. The results are post-processed to compile a matrix of correction factors, one for each wind direction sector and measurement height combination. The correction factors are then applied to the time series of measured data using the correction factor matrix as a lookup table.

We have seen increases in measurement accuracy for every case where flow curvature correction has been applied, even sites with semi-complex terrain. We frequently recommend that customers plan to execute flow curvature correction for a complex terrain site in advance, and the expectation should be that flow curvature correction should be required in such circumstances. In some cases, data analysis may reveal unexpected discrepancies between the lidar and met mast wind speeds and/or wind shear profiles, even in terrain that we would not have considered particularly challenging and flow curvature correction frequently improves accuracy in this instance.

Conclusion

Bureau Veritas’ Technical Center of Excellence in Renewable Technical Advisory endorses the use of wind measurement data from Doppler lidar (light detection and ranging) for wind energy applications, subject to the technical considerations outlined above. Building on decades of experience, Bureau Veritas is recognized as a leader in the application of lidar for wind measurements, having advanced its use across diverse markets and project types. Through active participation in international standards development, continuous refinement of best practices, and implementation of advanced methodologies, we remain at the forefront of lidar technology, ensuring its effective deployment for finance-grade analysis, power performance testing, and emerging applications in the progressing wind energy industry.