LiveCast Q&A: Achieving bankability with wind lidar – answers to your questions

Weather & Environment

Wind and Solar Energy

In December 2024, we hosted a webinar with DNV and AES Clean Energy to talk about how lidar helps achieve lower measurement uncertainties and guarantees higher bankability in wind measurement campaigns There were many questions we didn’t have time to answer during the webinar, and are pleased to now answer them here.

Lidar acceptance and bankability

Question: Is it expected for lidar to be bankable as a stand-alone measurement device in complex terrains as well?

Vaisala response:
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. Get familiar with WindCube usage in complex terrain.

DNV response:
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 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 < 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.

Question: Do European turbine manufacturers accept stand alone onshore lidars for site suitability?

Vaisala response:
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; here is one of our recent works dedicated to TI.

Question: What do the IEs believe needs to change with the financiers to get RSD-only campaigns accepted in the U.S.?

DNV response:
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.

Question: 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 response:
Lidar has been used for many years for various applications by many industry stakeholders. There exist numerous publications and validations of lidar performances in 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 response:
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 of 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 maybe negligible to small.

The use of stand-alone 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 modelling, require inputs 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 impact 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.

Question: Who takes the reins to get banks to accept lidar data?

Vaisala responds:
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.

Question: And for floating offshore? How we should proceed to achieve the bankability?

Vaisala:
Carbon Trust OWA provides guidelines on the usage of FLS for offshore measurements. There is also ongoing work to release the IEC 61400-50-4 standard dedicated to the floating lidar measurements for bankable WRA. Lidar bankability in offshore campaigns has been practically achieved thanks to the windspread usage of floating lidars and Dual Scanning Lidars.

Question: I came across the DNV document on LiDAR maturity stages (i.e. Stage 1 to 3). Correct me if I am wrong, but I understand this is superseded nowadays with a different classification? Where to access this information?

DNV response:
As the industry’s understanding of RSDs has matured, DNV moved away from a closed guidance of position statements several years ago, which told the developers how to use a device, to an open system that evaluates each device and measurement campaign on its own merits. The adoption of RSD data will depend on the level of uncertainty that it brings to a project; so, for example, an unverified RSD in complex terrain will have an unacceptably high level of uncertainty for a bankable assessment and a verified device in simple terrain may have a measurement uncertainty equivalent to a cup anemometer. The measurement uncertainty of the device will be guided by industry standards and best practices.

It is noted that DNV will continue to adopt use case restrictions outlined in standards such as the IEC 61400-12-1:2022 and the Offshore Wind Accelerator roadmap, where applicable.

Question: What are your thoughts on the use of lidar in flat, but forested, terrain?

DNV Response:
A lidar can be used in a forested area provided that manufacturer's best practices for installation are followed.

Vaisala response:
Refer to the WindCube Siting guidelines' chapter 2 (Figure 2) for installations next to obstacle or forested areas.

Question: What's your opinion about validating lidar data against met mast? Most of the customers prefer to do that, but unfortunately lidar is failing most of the time.

Vaisala response:
Verification against a met mast is required by IEC 61400-50-2. WindCube has been extensively validated through numerous verification campaigns that show repeatable and reliable measurements. If you experience issues with your lidar systems, don't hesitate to get in touch with our Technical Support through MyVaisala or at [email protected].

DNV Response:
A lidar should be verified against anemometry on a meteorological mast that is compliant with IEC 61400-50-1. All comparisons should be completed in wake free wind direction sectors with high quality calibrated cup anemometers that are free from degradation or malfunction. DNV has completed hundreds of lidar verifications against IEC complaint masts that show in most cases industry accepted lidars are able to reproduce cup anemometer wind speeds and wind directions at an accurate and acceptable level.

Lidar universality & usage

Question: How does Vaisala plan to tackle sites with low aerosol count in the absence of a met mast?

Vaisala response:
Vaisala has developed data availability simulators and novel techniques for areas with low aerosol presence. You can read more about these advancements in our scientific paper.

Question: How does Vaisala plan to tackle measuring TI with lidars in the absence of a met mast?

Vaisala response:
Vaisala is a member of industry's working groups and research projects dedicated to the topic of lidar-based TI measurements (mentioned above). For vertical profiling lidars, there are ongoing projects with some results published here (for onshore). For the offshore TI project, please refer to the presentation of Maxime Thiébaut from France Energies Marines. Also, Vaisala has developed a new TI algorithm for WindCube Nacelle lidar.

Question: How weather conditions affect lidar measurements?

Vaisala response:
According to the WindCube v2.1 Classification report issued by Deutsche WindGuard, only 3 environmental variables impact WindCube measurements (it is typically more for other lidars): Wind direction, Turbulence Intensity, and Wind shear coeficient.

Pulsed lidar technology accuracy is not impacted by cloud. A comprehensive journal article by Fraunhofer IEE and University of Marburg shows no fog impact below 80m. Higher measurements range can be affected by fog below 100m, lowering data availability during fog events.

Question: What is the difference between Vertical and Nacelle lidar working prensib? What are the laser specifications that are used in both models?

Vaisala response:
Both are Dopler pulsed lidar technologies, using invisible infrared laser pulses, measuring at 10 to 20 users configurable distances. Both can be used for Power Performance Testing according to the IEC standards. Signals processing principles are similar, although nacelle-mounted lidar offers wind field preview for turbine applications.

Question: What is the difference between using continuous or pulsed laser?

Vaisala response:
WindCube is a Doppler lidar that uses pulsed laser technology to measure ambient wind conditions. The lidar sends invisible, infrared laser pulses into the atmosphere. Four beams are sent successively in four cardinal directions along a 28° scanning cone angle, followed by a fifth, vertical beam. Laser pulses are backscattered by aerosols in the air (dust, water droplets, pollution, etc.) that move at the speed and direction of the wind. The collected backscattered light allows for the calculation of wind speed and direction by measuring and comparing the Doppler-induced laser wavelength shift. Up to 20 different range gates can simultaneously be measured using the laser pulse time of flight allowing for measurement of the wind speed at 20 different heights. Measurement probe is constant over height. A dedicated signal processing algorithm computes wind vector component from the five consecutive line-of-sight measurements.

In contrast, continuous wave (CW) lidars continuously send laser beams, measuring in "a cone mode" over time. Therefore, an obstacle like a guy wire, a tree or a turbine blade, even at a far distance, may impact measurement at any height. Hence more restrictions in locating a CW Lidar. (in contrast to 4 beams in 4 direction for pused lidar). Internal signal processing enables to reconstructs wind speed, with a special sensor to distinguish wind direction.

CW lidars need to change beam focusing to change measurement heights: each height is captured at a different time, hence shear profile accuracy is lower. More heights implies more beam focusing changes: impacts temporal resolution. CW lidars have a varying probe volume with measurement height: measurement quality varies along the wind profile. Measuring probe varies with measurement height : While big probe volume at higher heights are more sensitive to the shear profile. Depending on internal filtering algorithm, for a CW Lidar, Laser lights backscattered from clouds, fog, or layers of dense dust and sand particles, of any altitude, may impact not only data availability but also measurement quality of the focused height.

Question: 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 response:
Lidar availability can be reduced by a number of atmospheric conditions such as fog, rain, clear sky, 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 (<100m) or clear sky (low level of particles) conditions are released.

Question: Can you speak about how our understanding of lidar measurements and our treatment of them changed in the last few years?

DNV response:
In the last few years the largest change is the inclusion of lidar measurements in International IEC standards that has lead to wider adoption of the technology.

Question: What are the differences in treatment of lidar and uncertainties in an offshore setting?

DNV response:
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 Technology and the IEA expert group study on recommended practices for floating lidar systems.

Question: What do you think of Turbulence Intensity? Is this important as both met mast and lidar sensors provide this. Which is better?

DNV response:
Lidars and cup anemometers measure TI fundamentally different for a number of reasons including volume averaging and response to atmospheric stability. Therefore, TI data from lidars cannot directly replace TI measurements from cup anemometers. Methods for wake and turbine loads modelling as well as performance calculations have been understood from cup TI data, and these methodologies are found in current standards, e.g. IEC 61400-1 and 61400-13, which assumes the application of cup anemometers.

DNV’s joint-industry recommended practice for the use of TI measured by ground-based, vertical-profiling lidars has defined acceptance criteria that provide clear limits when lidar TI can be used as an alternative to a cup or sonic anemometer TI. The recommended practice is limited to simple terrain and a copy can by obtained here.

Question: Is lidar easier to permit than met masts?

Vaisala response:
Yes, no permit is required to install a lidar itself if 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 response:
To add to Vaisala's response, some coastal and offshore areas may need permitting.

Question: What 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 response:
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 US, a met mast costs about $280,000 over its 20-year lifetime (including maintenance and instrument replacement).

DNV Response:
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 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 depends on the structure type, height, the number of instruments, and if it is leased or purchased. For example, the purchase price or 1 year lease of a short mast may be similar to a lidar, but a hub height mast ≥100 m would be more expensive than a lidar.

Question: How trusted is lidar in the case of rain (even if the lidar contains a wiper)? In India we are facing 30-40% time we can expect rain.

Vaisala response:
Rain does not impact measurement accuracy. Data availability is not impacted 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.

Slide/Project specific questions

Question: Is it only lidar based measurement for all the sites?

DNV response:
The figures in slide 8, showing the use of remote sensing devices in North American in wind resource assessments include projects that have used lidar and sodars as a primary or suplemental wind measurement.

Number of DNV energy assessment projects using remote sensing devices in North America over the past decade

Question: Reesa, can you please report on verification height (mast top) in relation to planned hub height.

DNV response:
A reference meteorological mast should have at least three measurement levels above the ground to compare with a lidar. Sometimes, there are only two heights available for verification. Though not ideal, DNV accepts this deviation without adding additional uncertainty provided that the verification results at both heights are similar and fulfill key performance indicators or acceptance criteria.

For power curve testing, the three concurrent heights must be compliant with 61400-12-1:2022, which states that two of the mast measurement heights shall be either ±25% of lower blade tip and ±25% of hub height. For preconstruction energy assessments this is not a requirement, but it is noteworthy that the closer the reference verification mast is to the proposed hub height, the lower the hub height measurement uncertainty at the lidar.

Question: Does it apply only to hub heights above 100m or it can also be deployed to hub heights below 100m?

Vaisala response:
There is no restriction for hub height. WindCube can measure at 20 user configurable height between 40 and 300m.

DNV response:
Lidars can be used to decrease vertical extrapolation uncertainty when co-located against any mast measuring below hub height. The uncertainty benefits are typically larger when the extrapolation distances are larger.

Are the results presented based onshore?

Vaisala response:
Yes, all results presented in the webinar are from onshore campaigns.

Question: RSD complex terrain correction from DNV: which were the heights of the measurements for comparisons?

DNV response:
The results of the RSD complex terrain correction were based on mast-RSD pairs between 20 m and 160 m.

Standardization and verification

Question: What is the exact definition of a "Golden LiDAR?"

Vaisala response:
Golden Lidar is a system which is regularly validated by a third party against a met mast. Vaisala owns several Golden Lidars to ensure speed and continuity of operations. Our Golden Lidars are certified every 2 years across more than 20 key performance indicators, and this process is applied for more than 10 years. Vaisala's internal quality control process includes lidar validations against Golden Lidars. In 2021, Vaisala partnered with DNV to improve transparency in the wind speed data validation study of each WindCube going out of factory. Thanks to this process, each WindCube receives its Golden Validation report issued by DNV. Please contact Vaisala to learn more about this process.

Question: 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 these two procedures would only apply for power curve measurements? If you do, how frequently do you perform these procedures? Before each location change? Appreciate it if you can comment on what IEC-15 standard would require.

Vaisala response:
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 response:
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 there a recommended minimum distance from a 60m mast for lidar validation in simple terrain (such that met mast wakes won't impact lidar data)?

Question: Is additional uncertainty applied to lidars in complex terrain?

DNV response:
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.

Question: Should the lidar be validated in flat terrain before installing it complex terrain and then validated in flat terrain after the campaign?

DNV response:
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.

Question: When verifying lidar to a met mast, have you made findings on potential short comings of the cup/met mast behavior/accuracy, from the comparisons to the touchless/wake-free lidar measurements?

DNV response:
A lidar shall always be verified against high quality calibrated cup anemometers in wind direction sectors that are free from mast wakes or obstructions from nearby obstacles. It is best practice to plot the difference in lidar and cup anemometer wind speeds against wind direction to ensure that only free steam wind speeds are used for the verification. On a number of occasions, DNV has identified cup anemometer drift and/or degradation on poorly instrumented masts. Mast wakes and distortion from short booms can also be visualized when comparing cup anemometers to lidars.

Vaisala response:
IEC standard defines uncertainty class for each lidar type, considering cup anemometer reference uncertainty.

Question: What site do you use for the WindCube verifications against the Golden Lidar?

Vaisala response:
This verification is done for all newly produced WindCube lidars at end of the production in Vaisala factories.

Question: Can you quantify the difference in uncertainty (%) by using: a mast and co-locating a lidar with a mast?

DNV response:
The difference in uncertainty will depend on a number of factors which include the mast height, terrain complexity, turbine hub height, and duration of colocation. Using the example in the DNV presentation of a 60m mast and hub height of 100 m; including a collocated lidar can decrease the 1-year P99/P50 ratio by up to 1%.

Vaisala response:
Uncertainty calculation method is defined in IEC standard 61400-12-1. Verification against met mast before and after campaign is required as per IEC 61400-50-2.

Question: What are Vaisala's specific recommendations for the co-location of lidar and met mast?

Vaisala response:
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 5m is acceptable for 80m 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 40m lidar can be installed closer to the met mast). To learn more on the WindCube siting guidelines, please get in touch with Vaisala.