Q & A – Webinar: Driving the Standard: Wind Testing, Solar Trackers, and Peer Review

By on December 18, 2019

Watch the webinar on Greentech Media here. To download the webinar slides directly, please click here

For those who submitted questions during the webinar, thank you. We’ve compiled your questions and answered them below.

Topics Covered:

  • Understanding the importance of the peer review process in providing independent verification of wind tunnel testing methods and results
  • Validated conclusions from Dr. David Banks and Dr. Girma Bitsuamlak around proper aeroelastic wind tunnel testing methodology
  • Examination of major aeroelastic effects that impact tracker structures
  • Wind risk mitigation factors for robust 2-in-portrait tracker design

[NEXTracker] Q: How often would you estimate that the code-required peer review is ignored by jurisdictions?

The code does not specifically address single-axis trackers (SATs), and the code for sloped roofs and bridge decks should not be applied to SATs for the reasons outlined in the webinar. Wind tunnel testing is required to address structures not covered by code. It is our recommendation as a best practice that IEs and AHJs look for wind tunnel analysis from a reputable wind engineering firm, plus a third-party peer review to remove bias. Not following this diligence step can lead to wide variations in results for similar structures, and risk of under-calculating wind loads.

[NEXTracker] Q: How are dynamic loading factors/coefficients taken into account in the design loading of a solar array? Can you provide an example of how this load determination is conducted?

In its simplest form, dynamic amplification factors (DAF) need to be incorporated along with static coefficients to the structural calculations of the tracker structure and components. Furthermore, since the tracker can twist along its length, different coefficients and factors need to be applied along the span to ensure design compliance. Lastly, trackers will undergo some torsional deflection, no matter what, due to the fluctuating nature of the wind that will result in additional torque demand. This is estimated from the accelerations measured during an aeroelastic wind tunnel test and should be added on to the static load demand.

[NEXTracker] Q: What is a ballpark cost per watt for wind damage annually for the average worst- and best-case scenarios?

Properly designed trackers should experience zero damage from wind annually, providing the wind is within design wind speed conditions. NEXTracker’s NX Horizon Gen 2, with over 23 GW operating / under construction, has had no significant failures or damage over hundreds of sites. Both NX Horizon and NX Gemini were developed using the same wind engineering best-practices, specifically aeroelastic wind tunnel testing with thorough consideration of all potential dynamic effects, including torsional divergence, torsional galloping, flutter and vortex lock-in.

For trackers that do not take dynamic wind and other factors into consideration, failures can range widely in costs.

[NEXTracker] Q: What are the increased risks for installing modules during the construction phase prior to the installation of damping components?

It is recommended to install dampers first since modules are the driving force of wind loads on a tracker structure. Best practices suggest that they be placed only after all components have been installed. Dampers stiffen the structure and will help prevent failures even at low wind speeds. In addition, as the plant will not be commissioned, alternate stow practices should be followed prior to the plant becoming operational.

[Dr. Banks] Q: Can measured wind speeds at a PV site, performed over a long period of time, replace the wind speed requirements in the codes?

Generally, no, because so many years of data are needed to determine a 50 or 300-year design speed. However, measurements from a site can be used to determine if there is anything unusual about winds at the site that might not appear in local airport data, such as katabatic (downslope) winds.

[Dr. Banks/Dr. Bitsuamlak] Q: What is the border between a low and high tilt angle?

The aerodynamic forces acting on a solar panel are dictated by attached and separated flow structures and the associated pressure around it, at low and high tilt angles respectively. The boundary between these regimes is defined by stall angle, which can be obtained from wind tunnel measurements of the aerodynamic torque versus wind angle of incidence.  For the low tilt angle range, the aerodynamic torque increases with the angle of wind incidence. The specific value for the boundary (i.e. stall angle) depends on the geometric details of the tracker.

[Dr. Banks] Q: How can long assemblies be adequately tested in the limited width of wind tunnels?

If the assembly span is too great, a wider tunnel may be needed because the trackers cannot be made too small, or important details will be missed, and/or similarity parameters (such as mass moment of inertia) can become impossible to match.

[Dr. Bitsuamlak] Q: For the aeroelastic simulation, do you treat the material as purely elastic? That is, are you making a small-deformation assumption but allowing for large rotational displacements or are you accounting for the possibility of large deformation? Cauchy or Green strain?

For aeroelastic simulations, the stiffness of the structure is estimated assuming linear elastic behavior of materials. The nonlinearly is predominantly coming from aerodynamic hysteresis.