You asked, we answered – Lidar for WRA webinar Q&A

Lidar for WRA webinar Q&A
Weather & Environment
Wind and Solar Energy 

In our recent webinar, experts from Vaisala and Deutsche WindGuard® discussed the major milestones and the current status on lidar acceptance for Wind Resource Assessment (WRA), as well as remaining challenges for lidar to fully replace meteorological masts (met masts) for onshore measurements. Vaisala also demonstrated bankable use cases across different countries, terrains, and measurement scenarios. Today’s blog covers questions we didn’t cover during the webinar. If you haven’t seen it yet, you can watch now.

Met masts have been the primary measurement technology for wind applications for many years. According to Vaisala estimations, there will be a need for 4,000 wind measurement devices yearly across the globe for WRA. IEA Task 52 (Large-Scale Deployment of Wind Lidar) estimates that today, vertical profiling lidars are replacing 10% of met masts in the onshore market for WRA and operational plants. Offshore, floating lidar systems and fixed lidar installations have almost completely replaced met masts. Lidar is expected to fully replace met masts in onshore WRA campaigns in the next two decades.

When considering a WRA campaign using lidar measurements, there are several aspects to keep in mind. We will look at these aspects by topic.

Verification/Validation and Calibration

The standard practices for validation and calibration of lidar unites before/during/after a measurement campaign can vary from one region to another. For example, in Germany, TR6 advises for pre- and post-verification of lidars. In many other regions, pre-verification or no verification can be sufficient (e.g., Korea, Japan, Australia).

Two methods exist for lidar validation: lidar-to-lidar validation (in factory) or a full verification against a met mast anemometry (in the field). In some cases, lidar-to-lidar validation is considered sufficient. We perform lidar-to-lidar validation at Vaisala, the so-called Golden Validation, when one WindCube® lidar is compared to another WindCube previously verified against a met mast (Golden WindCube). The Golden Validation provides the traceability back to the cup anemometers but does not replace a full verification (lidar to met mast). Local verification of the lidar in the field can also be conducted — on the customer’s site if there is already an existing met mast, or at another location as long as the terrain is flat and simple.

Measurement campaign

Lidar is considered an acceptable means to establish a bankable Energy Yield Assessment (EYA) report. MEASNET Site Assessment Guideline and German Technical Guideline 6 allow standalone lidar for WRA applications. Standalone nacelle-mounted lidar measurements are also allowed for wind turbine Power Performance Testing according to IEC 61400-50-3 (you can read more about this here). Pre-calibration and post-calibration of the lidar against a met mast also help to convince financial institutions on the bankability of lidar campaigns. Lidar classification takes care of uncertainties evolving from differences of influential parameters during the calibration and application of the lidar. The IEC 61400-15 standard for standalone ground-based lidars in expected to be released in 2023. WindCube as a standalone measurement device has been already successfully used in several bankable WRA campaigns across the world.

Lidars bring many advantages to wind measurement campaigns, both in simplex and complex terrain. We estimate that, on average, a 110 m met mast has the same cost as a lidar. However, lidar will measure beyond the met mast limit, and it can be easily moved around the site and repurposed for other projects or applications. Additionally, lidar doesn’t require expensive and time-consuming building permits needed for met masts, which makes lidar deployment straightforward.

WindCube lidar measures up to 300 m with 20 simultaneous, user-defined measurement heights. Other advantages of pulsed lidar include constant probe volume (better accuracy at high altitudes), better time resolution (measurement of all altitudes simultaneously), and reduced power consumption.

The lidar provides two types of data files: 1 Hz data (per second data) and 10-minute average data. For each measurement height, the following data is provided: wind speed, direction, standard deviation, data availability, CNR levels.

WindCube is IEC classified using 80% data availability threshold. We believe a monthly 80% - 85% data availability rate is a good level for a valid WRA campaign. To filter lidar measurements for highest precision during the measurement campaign, 90% filter with >-20dB CNR is a good filtering strategy. For highest campaign data availability, using a 50% threshold still meets the WindCube wind speed measurement specifications.

If there is poor visibility on site, the data availability will be impacted. The advantage of the pulsed technology used by WindCube, however, is that only the data availability will be affected and not the wind speed measurement. At Vaisala, we use a Data Availability and Range Estimator (DARE) tool to estimate the probability of low aerosol content on a site. You can read more about this in our scientific poster: Pre-campaign tools for use of lidars in EYA.

For the post-measurement quality check, we recommend checking data availability levels and CNR for rain periods, low aerosol content, or other environmental conditions. Lidar accuracy is not generally affected by rain. When considering forestry, it will depend on the terrain slopes and CFD might be required to correct the data.

Complex terrain specifics

For projects in complex terrain when lidar is often co-located with a met mast, we recommend placing the lidar between the predominant wind direction and the met mast to avoid having the met mast’s antenna and guy wires near the lidar. Also, lidar measures in a cone shape and if the met mast is "centered" it usually has a limited impact on wind flow.

For a year of wind measurements where lidar is co-located with a met mast in complex terrain, we recommend for lidar to remain at the same location for 3 - 4 months. For horizontal uncertainty reduction, the lidar can be moved 2-3 times.

Although the standalone lidar use in complex terrain hasn’t yet achieved its maturity, Deutsche WindGuard has already conducted successful WRA campaigns in Germany using standalone WindCube measurements coupled with a CFD solution.

Want to discuss with us? Contact us, and we’ll be in touch as soon as possible.