Vaisala, a provider of environmental and industrial measurement, recently collaborated with WindSim, a provider of computational fluid dynamics (CFD) modeling, on a study that demonstrates how to mitigate the effects of complex terrain on wind measurements collected by remote sensing units.
For wind developers prospecting at more challenging, off-grid sites – such as on ridgelines, hillsides and heavily forested areas – the versatility, maneuverability and size of remote sensing devices offer numerous potential advantages over met towers, according to Vaisala. Validating the accuracy of a remote sensing system in complex terrain removes one of the few remaining barriers to the widespread adoption of the technology across the industry, the company says.
“While remote sensing devices are increasingly supplanting met towers as the preferred technology for wind measurement campaigns, concerns over the way both SoDAR and LiDAR technologies respond to complex air flows have posed an obstacle,” says Pascal Storck, director of renewable energy at Vaisala. “Our study with WindSim shows how topographical factors can reliably be accounted for.”
Vaisala says independent R&D efforts continue to set the standard for wind measurement practices worldwide. Increasingly, advanced CFD methodologies are being used by companies such as WindSim to evaluate how remote sensing units respond to wind flow in complex terrain in order to determine where corrections to wind estimates can be made.
This is not required in areas where the ground is relatively flat and the flow field is uniform, but in complex terrain, this flow field can be disrupted, introducing a bias into the measured wind speeds, according to Vaisala.
The validation study spanned 20 individual sites worldwide, each with a collocated Triton Wind Profiler and met mast, collecting simultaneous measurements for a period of one to four months.
Using publicly available elevation and land cover data, WindSim configured CFD simulation domains surrounding each collocation site and conducted simulations for a variety of inflow conditions. From these simulations, Vaisala developed correction factors to account for wind flow curvature for each measurement height and wind direction sector.
By applying these CFD-based corrections to Triton data, Vaisala found a reduction in the mean wind speed difference between the Tritons and met towers – from -1.7% to -0.1% – and in the standard deviation, from 2.5% to 2.1%. The correction, therefore, not only eliminated the overall bias but also reduced the spread of error among the sites, says Vaisala.
“The accuracy of remote sensing data has widely been recognized as on a par with that of met towers in all aspects – except when it comes to operating in complex terrain,” comments Dr. Arne Gravdahl, WindSim’s founder and chief technical officer. “Using CFD simulations to adjust Triton data removes this concern, leveling the playing field in terms of data quality and allowing developers to take full advantage of the Triton’s versatility.
“Our study shows that it’s a reliable, repeatable methodology,” Gravdahl continues, “and we’re looking forward to seeing the impact it has for Vaisala’s clients as they pioneer wind development in those parts of the world where the best resource isn’t always on flat terrain.”
The full results of the study can be downloaded here.