In December 2024, Vaisala hosted a webinar with DNV and AES Clean Energy to talk about how Lidar helps achieve lower measurement uncertainties and guarantees higher bankability in a wind-measurement campaign. Wind Systems was given permission to publish some of the highlights of the program. It has been edited for the magazine. For the complete transcript, go to: www.vaisala.com/en/dnv-and-aes-clean-energy-q-and-a.

Is it expected for Lidar to be bankable as a stand-alone measurement device in complex terrains?
Vaisala: WindCube has been used and validated in complex terrain for many years despite the absence of industry’s consensus with regard to standalone Lidar usage in complex terrain. When using a Lidar for vertical and horizontal extrapolations in complex terrain, Flow Complexity Recognition (FCR) or CFD post-processing should be applied. The closer the measurement point is to the prospective turbine location the better, because the uncertainty increases with distance from a measurement. In complex terrain, it is usually recommended to have one measurement point for each 5 to 7 turbines.
DNV: In order for a Lidar to be used as a wind measurement on a complex site, an on-site verification has to be performed along with a Complex Flow Correction using an advanced CFD model. DNV recommends that after the onsite verification, a Lidar be located for a minimum of three to six months at a stand-alone location to start introducing spatial extrapolation benefit. However, the most certain approach is to measure at a location for at least one year to correctly capture the seasonal variation in both wind speed and direction. It should be noted that short campaigns less than 12 months provide limited value, and when this approach is taken, there should be at least one other measurement on site with a full year of data. Otherwise, the project uncertainties would be very large, and the analysis will be considered preliminary.
Do European turbine manufacturers accept standalone onshore Lidars for site suitability?
Vaisala: For full acceptance of standalone Lidar for site suitability, the industry (including the turbine OEMs) has to come to a consensus with regard to the Lidar-based Turbulence Intensity (TI) measurements. There are ongoing industry working groups such as DNV-JIP and CFARS dedicated to the advancement of Lidar TI measurements. Vaisala is also working on this topic.
What do the IEs believe needs to change with the financiers to get RSD-only campaigns accepted in the U.S.?
DNV: In order for lenders to accept a wind-measurement campaign based on remote sensing, the uncertainties must be low enough for the lender to be comfortable with the level of risk. Having international standards in place will help ease insecurities regarding the technology. The spectrum of remote sensing acceptance among lenders is variable, and it is recommended to approach your lender in the early phase of project to ensure they are comfortable with your wind-measurement campaign plan.

What do you see as the greatest challenge prohibiting widespread adoption of stand-alone Lidar usage without met masts for WRA and Power Performance Testing? Is there any campaign, study or effort from Vaisala and/or DNV to combat this challenge?
Vaisala: Lidar has been used for many years for various applications by many industry stakeholders. There exist numerous publications and validations of Lidar performances in a variety of environmental conditions. Vaisala is the member of industry consortiums and working group dedicated to the advancement of Lidar technology. One of the notable organizations is the mission of IEA Task 52 dedicated to make wind Lidar the best and preferred wind measurement tool for wind energy applications.
DNV: For power performance measurement (PPM), Annex L of IEC 61400-12-1 requires that a Lidar be monitored with a short mast, but if all parties agree to a deviation from the IEC standard, then a monitoring mast can be excluded. In these cases, it should be noted that a pre- and post-deployment verification of the Lidar is required. The main challenge to excluding a monitoring mast in a PPM is not having a density sensor near hub height. Though pressure can be accurately extrapolated from near surface measurements to hub height, temperature and humidity cannot be accurately extrapolated from the surface. DNV has completed a few studies using different approaches that have provided acceptable air density measurements when a monitoring mast is unavailable. Depending on the site and approach, the magnitude of additional uncertainty may be negligible to small.
The use of standalone Lidars for WRA has been accepted by the industry for years as the first order quantities (wind speed and wind direction) from the Lidar correlate very well with met mast data.
However, extreme wind gusts and turbulence intensity (TI) from the Lidar is not yet widely accepted. Turbine site condition studies, such as turbine load modeling, require input from cup anemometers as TI Lidar measurements are fundamentally different. As a result, most projects will have at least one meteorological mast. Once a better understanding of second order quantities are understood, the adoption of Lidar-only WRA will likely gain wider acceptance in simple-to-moderately-complex terrain. For complex-terrain sites, a meteorological mast is required to allow for complex flow corrections using CFD.
DNV is involved in the development of IEC Standards that directly affect the use of remote sensing in the industry, We are an active member of IEA Task 52; we participate in the development of remote sensing best practices and, historically and currently, are involved in a number of studies using remote sensing to advance their adoption in the industry.

Who takes the reins to get banks to accept Lidar data?
Vaisala: This should be a joint industry effort. At Vaisala, we are committed to developing the best Lidar technology to meet rigorous market needs and working with the industry’s opinion leaders and stakeholders to develop best practices and guidelines on the use of Lidar technology for bankable wind measurements.
We also continuously strive to educate the market and contribute to the discussion with banks and financiers whenever needed.
How do weather conditions affect Lidar measurements?
Vaisala: According to the WindCube v2.1 Classification report issued by Deutsche WindGuard, only three environmental variables affect WindCube measurements (it is typically more for other Lidars): wind direction, turbulence intensity, and wind-shear coefficient.
Pulsed Lidar technology accuracy is not affected by cloud. A comprehensive journal article by Fraunhofer IEE and University of Marburg shows no fog impact below 80 meters. Higher measurements range can be affected by fog below 100 meters, lowering data availability during fog events.
Has there been any work done to explain why data recovery is better in some regions more than others or in different climate conditions or different terrain? Data recovery varies between types of Lidars and very little is understood about what effects the data recovery.
Vaisala: Lidar availability can be reduced by a number of atmospheric conditions such as fog, rain, clear sky, or cloud. This is the same on- and offshore and will vary strongly depending on local climate at the measurement site. As long as the campaign availability is within the IEC stated limits, the impact on resource assessment should be small. For pulsed Lidar technology, weather condition does not impact measurement accuracy. Data availability increases again as soon as heavy rain, low fog (less than 100 meters) or clear sky (low level of particles) conditions are released.
What are the differences in treatment of Lidar and uncertainties in an offshore setting?
DNV: Both onshore and offshore uncertainties are defined by IEC 61400-12-1 for vertically facing fixed Lidars (IEC 61400-50-2) and nacelle mounted Lidars (IEC 61400-50-3). Later this year, IEC 61400-50-4 Use of floating Lidar systems for wind measurements will be released that provides guidance on device use cases and uncertainties for floating Lidars. This standard will include the requirements of current industry best practice documents — The Carbon Trust Offshore Wind Accelerator (OWA) Roadmap for the Commercial Acceptance of Floating Lidar Technologyand the IEA expert group study on recommended practices for floating Lidar systems.
Is Lidar easier to permit than met masts?
Vaisala: Yes, no permit is required to install a Lidar itself if the land owner gives their approval. Met masts, on the other hand, require a construction permit that can take up to several months to acquire. Additionally, Lidars are easy to move and leverage for additional projects over time.
DNV: Some coastal and offshore areas may need permitting.

What is the ROM cost difference between use of short met mast and WindCube? If the cost is similar, why not use met masts for every project?
Vaisala: WindCube can be re-used for multiple projects and enables measurements at different locations at a given site with no additional cost (it is easily moved across the site). In most regions, WindCube is also cheaper than a met mast. For example, in the U.S., a met mast costs about $280,000 over its 20-year lifetime (including maintenance and instrument replacement).
DNV: The use of Lidars, masts, or a combination of both technologies will depend on the terrain, project measurement requirements, and budget. For a preconstruction energy assessment, the aim is to lower the temporal, vertical, and horizontal extrapolation uncertainties while managing the project’s budget. In many cases, this balance is achieved with the use of both meteorological masts and remote sensing technology to measure winds across the site up to hub height.
The cost of a meteorological mast will depend on the structure type, height, the number of instruments, and if it is leased or purchased. For example, the purchase price or one-year lease of a short mast may be similar to a Lidar, but a hub height mast greater or equal to 100 meters would be more expensive than a Lidar.
How trusted is Lidar in the case of rain (even if the Lidar contains a wiper)?
Vaisala: Rain does not affect measurement accuracy. Data availability is not affected by medium rain but is reduced during heavy rain (monsoon) conditions, which can be detected thanks to the provided PTH sensor, which includes rainfall measurements. Measurement is back to normal as soon as the rain flow decreases.
In order to carry out a bankable wind resource campaign, do you calibrate and verify each Lidar as per IEC (against a met mast), or would these two procedures only apply for power curve measurements? If you do, how frequently do you perform these procedures?
Vaisala: Yes, IEC 61400-50-2 clause 7 requires verification against IEC compliant met mast before (less than 1 year) and after campaign. This is also described in IEC 61400-12-1. As an alternative, an in-situ test of the RSD against a met mast present at the site can be performed.
DNV: For a Wind Resource Assessment, it is recommended to verify your Lidar on-site (or off-site against an IEC compliant mast) before the wind-resource campaign. Verification against a mast will significantly reduce project uncertainties. For moderately complex and complex terrain types, an on-site verification allows for complex flow correction with CFD, thereby decreasing the measurement uncertainty. It is further recommended to verify the Lidar again after maintenance or change in firmware. Location changes on-site do not necessarily require re-verification.
A post verification is also recommended if there are any concerns about the long-term consistency of the wind-measurement campaign or if the measurement campaign has been greater than a year.
Is additional uncertainty applied to Lidars in complex terrain?
DNV: The measurement uncertainty of a Lidar is comprised of the verification uncertainty, device type classification uncertainty, mounting uncertainty, and non-homogenous flow uncertainty. In simple terrain, the flow is assumed to be homogenous across the Lidar measurement volume, and, therefore, this uncertainty is zero. However, as the terrain complexity increases, the heterogeneity of the flow across the volume increases, thereby increasing the non-homogenous flow uncertainty. Assuming all other factors are equal, a Lidar will have a greater measurement uncertainty at a complex terrain site. This uncertainty can be reduced at complex terrain sites by implementing a complex flow correction using CFD.
Should the Lidar be validated in flat terrain before installing it in complex terrain and then validated in flat terrain after the campaign?
Vaisala: Yes, IEC 61400-50-2 clause 7 requires verification against an IEC compliant met mast before (less than 1 year) and after campaign. This is also described in IEC 61400-12-1. As an alternative, an in-situ test of the RSD against a met mast present at the site can be performed.
DNV: For a Wind Resource Assessment, it is recommended to verify your Lidar before the wind resource campaign in terrain similar to the point of interest. Therefore, a verification on a simple terrain will add no or limited value for a complex terrain project.
A post verification at a similar complexity location is also recommended if there are any concerns about the long-term consistency of the wind-measurement campaign or if the measurement campaign has been greater than a year.
What are Vaisala’s specific recommendations for the co-location of Lidar and met mast?
Vaisala: If your goal is to compare Lidar measurements to mast measurements for validation or calibration, it is best to locate the Lidar as close to the met mast as possible to ensure consistency in data analysis. In this scenario, precautions should be taken on Lidar orientation so that none of the beams intersect with the tower or guy wires. WindCube can be installed adjacent to measurement towers.
In simple terrain and where space allows, we recommend locating the Lidar outside of the guy wire footprint and at a slight offset (or directional rotation) to avoid guy wires as well as the tower itself. For best performance, locate the Lidar and its beam paths to avoid any wakes from the measurement tower or its guy wires.
In general, a minimum distance of 5 meters is acceptable for 80-meter masts, but if the tower structure is taller, then a larger distance is required. Note this recommendation applies for the measurement heights of the vertical profiling Lidar (below 40-meter Lidar can be installed closer to the met mast).
More info www.vaisala.com/en