May 19, 2014 | by NRG Team Voices | Engineering + Technology, Remote Sensing, Wind Plant Optimization

Do you suspect that you have underperforming wind turbines? Are you familiar with the causes and consequences of yaw misalignment?

In a recent webinar, we discussed one of wind farm owners’ biggest problems, turbine underperformance, and one of the industry’s most promising tools, nacelle-mounted Lidar. Using the results of a Lidar installation on a First Wind turbine, we explored how Lidar can be used to improve the profitability of wind projects.

Missed the webinar? You’re in luck—you can view a recording of the presentation here:

In addition, here are some of the questions that participants asked:

Q: I'm already using SCADA data to determine which turbines are underperforming. How would the Wind Iris benefit me?

Evan: Yaw misalignment is one cause of turbine underperformance in particular, and it's difficult to quantify without actually measuring free stream wind speed and wind direction. SCADA data is helpful for quantifying relative performance, but not for absolute performance or absolute turbine alignment.

Q: What is the status of using nacelle-mounted Lidar for IEC power performance testing?

Evan: Currently, no Lidars or other remote sensors are approved for use per the IEC 61400-12-1 standard. The next iteration of that standard will allow for ground-based remote sensing to be incorporated into power curve testing. It is unlikely to include nacelle-mounted Lidars, for now. That said, the IEA Task 32 Group is developing pre-normative standards for power curve testing with nacelle Lidars.

Furthermore, an EUDP-funded project led by Danish Technical University (DTU)—in partnership with Siemens Wind Power A/S, DONG Energy, and Avent Lidar Technology—has resulted in an independent set of guidelines that can currently be used to help inform operational or non-contractual power performance testing with a Wind Iris Lidar.

Q: Why is SCADA data necessary to calculate misalignment in complex terrain?

Evan: SCADA data provides absolute turbine yaw position, which when used in conjunction with relative wind direction data measured by a Wind Iris, can confirm whether an individual turbine’s misalignment has a directional component to it. It is not uncommon for misalignment to vary depending on incoming wind direction, as upstream terrain complexities (and inflow conditions) vary as well. In such a scenario, the optimal correction to the misalignment could depend on the site-specific wind rose and/or site-specific wind speed distribution.

Q: What is the data availability of the Wind Iris during the measurement campaign?

Evan: Availability is primarily affected by atmospheric conditions (the concentration of aerosols) and mounting position, as the Lidar beams cannot see through the wind turbine blades themselves. The first variable can potentially affect the duration of measurements required to gain confidence in any conclusions reached, but it will not affect the quality of the measurements themselves. The second variable is largely a question of mounting position, and it can be mitigated through proper positioning of the Lidar atop the nacelle. It is normal to lose some percentage of data to blade blockage, but over a longer averaging interval (10 minutes, for instance), those losses become insignificant for this particular application.

Q: How easily can the Wind Iris be moved from turbine to turbine?

Evan: With a trained crew, the Wind Iris is fairly easy to handle and install. Typically, installers—whether direct employees of a wind farm operator or an independent service provider—should be directly trained by Renewable NRG Systems.

Q: Does the Wind Iris require any calibration or recalibration?

Evan: The Wind Iris is factory-calibrated before it is shipped for the first time. Recalibration typically isn't required. Because Lidar measurements are based on the speed of light (a constant) and the Wind Iris features fixed optics, there is unlikely to be any drift in the accuracy of the device. RNRG will recalibrate following any major upgrade to the system where the laser alignment might change, but otherwise the accuracy should not change.

Q: When installing a Wind Iris, how do you ensure alignment to avoid errors caused by the Lidar itself?

Evan: The results are only as accurate as the installation of the Wind Iris. If you don't align the device properly, your results will be biased as well. Fortunately, we have developed tools to mitigate the likelihood of such errors.

The Wind Iris has two small LEDs that are projected onto the roof of the nacelle. They are used to align the turbine centerline during the commissioning process. This technique is repeatable and highly accurate. Finding the centerline can be done in a number of ways, and we have developed different techniques for different turbine models in some cases. Many operators also have their own methodology to determine the centerline.

Q: What maintenance needs to be performed on a Wind Iris?

Evan: Typically, very little maintenance is required. Every 24 months, we recommend returning the system to our facility in Vermont to have its components checked. More frequent maintenance typically isn’t necessary, beyond cleaning the windows on the front of the Lidar between deployments. As long as a Wind Iris is properly handled, it should last quite a long time. There are no moving parts on the device.

Q: You spoke about measuring power curves with the Wind Iris. Can you describe how this is done?

Evan: As I mentioned, DTU Wind Energy has issued a set of guidelines that are available for download. As for the process itself, it is somewhat similar to the one that would be undertaken with a calibrated power performance mast. The most obvious difference is that, instead of having a met tower with multiple vertical levels of measurement, the Wind Iris provides multiple horizontal distances of measurement from in front of the turbine.

It is important to use a distant-enough range gate, or measurement distance, to mitigate upwind blocking effects while still measuring a representative wind condition. We recently presented a poster at AWEA WINDPOWER 2014 regarding Wind Iris power curve measurements in complex terrain. This poster, in addition to providing a new method for filtering blocking effects and terrain effects from the Wind Iris data, also illustrates the bias in power that can result from calculating a power curve based on wind speeds measured too closely to the turbine. Download the poster.

Q: What is the averaging period for wind direction?

Evan: Typically, the Wind Iris is set to 10 minute averaging. That can be changed by the user, bearing in mind that there is a limit to how fast and how often the yaw motors are going to work. For that reason it isn’t necessarily important to capture incoming wind direction at a much faster, higher resolution. That data is available and retrievable from the Wind Iris processing unit, but it is typically not useful for the misalignment calculation.

If you have any further questions about the Wind Iris, contact Evan Osler at

Blog ImagesArtboard 1
NRG Team Voices



View more posts by NRG Team Voices