For solar mounting systems on flat roofs, aerodynamics help to optimize ballast needed. For solar fixed-tilt or tracking ground mount systems, static and dynamic wind loads need to be determined for the design. Aerodynamic instabilities such as torsional galloping are known to occur on tracking systems with single or multiple axes.

Design ballast for solar PV racking systems mounted on flat roofs

Task

Racking systems mounted on flat roofs are mostly secured by ballast to withstand uplift and sliding forces due to wind actions. In this context, flat roofs are defined as roofs having a slope of less than ± 7° (cf. ASCE/SEI 7-10) so that with view to the wind flow over them it can be assumed that the flow separates on the roof edge or parapet.

Typical for the flow over flat roofs is the forming of edge vortices due to cornering flow. These forms of flow also called "delta wing vortices" present high local rotational speeds and create correspondingly high suction effects on the roof, especially in the corner and edge zones.

 

Solution

Wind tunnel studies conducted in the I.F.I. wind tunnel laboratory have shown that many parameters affect the wind loads on racking systems mounted on flat roofs. Parameters such as tilt angles of modules and deflectors, venting gaps, row spacing, parapets, PV array setback from the roof edges play a major role in the wind load calculation.

Typically, design pressure coefficients are given separately for different array and roof zones. Furthermore, the necessary ballast for the securement of ballasted roof mount solar systems depends on the stiffness of connecting members. If wind forces on highly loaded zones of arrays can be largely redistributed by the interconnected substructure, the benefits of load sharing are applicable. The building itself also needs to be stiff which may be generally assumed for low-rise buildings not higher than 60 ft.

 

Advantage

The above brief summary of parameters dictating the wind loads on roof-mounted solar PV arrays shows that simplified assumptions have to be made regarding wind tunnel studies from which design wind loads for a wide range of building and array dimensions are to be determined.

According to SEAOC-PV2-2012 wind tunnel reports on solar roof mounted structures need to be peer-reviewed by an independent expert. In this way I.F.I. wind tunnel reports have been approved several times. I.F.I. Institute for Industrial Aerodynamics is also a City of LA Department of Building and Safety (LADBS) approved laboratory for wind tunnel testing of buildings and structures, Testing Agency License Number TA 24830.

Wind loads on roof-parallel (“flush-mounted“) solar PV systems

It is tempting to calculate wind loads on flush-mounted solar PV systems simply by applying Figures 30.4-2A through 30.4-2C of ASCE 7-10 which give external pressure coefficients for gable and hip roofs. However, simple approaches are often conservative. As wind tunnel studies conducted in the large I.F.I. boundary layer wind tunnel have shown, reductions in the range of 30% to 50% of ASCE 7-10 code values may be easily achieved.

The reason is that in the wind tunnel the favorable effect of pressure equalization between upper and lower module sides is modeled, whereas code values simply represent external pressure coefficients.

Wind loads on single-axis and double-axis solar PV tracking systems

Single-axis and double-axis tracking systems may be subjected to wind dynamic effects such as buffeting forces resulting from the wakes of upstream rows. Thus dynamic amplification of the mode shapes may occur below or at typical design wind speeds if the vortex shedding frequency matches the natural frequency of the structure. Structural damping of the tracking system acts to mitigate the structure’s response.

There are two ways to handle trackers.

A) Time domain analysis: 
From the wind tunnel testing pressure-time-series corresponding to global structural load effects such as lift forces, drag forces, hinge moment or bending moments can be identified. These pressure-time-series may be fed into structural analysis software to calculate peak stresses or displacements, foundation loads etc. due to resonant effects.

B) Frequency domain analysis: 
A structural modal analysis has to be conducted first. The results of the modal analysis (mode shapes, natural frequencies, critical damping, mass distribution, stiffness) have to be provided to I.F.I. for calculation of equivalent static load distributions. This analysis has the advantage of being in a format that structural engineers are used to work with. Furthermore, equivalent static wind loads can be easily combined with other loads such as snow or earthquake by applying the combination rules of wind loading codes such as ASCE 7-10.

Wind loads on fixed-tilt, ground-mounted solar PV racking systems

Wind loads on fixed-tilt, ground-mounted solar PV racking systems depend mainly on parameters such as tilt angle and row spacing. Perimeter rows and array columns are generally subjected to higher wind loads than panels in the array interior which benefit from sheltering effects.

Resonant dynamic effects due to buffeting are also observed in fixed-tilt, ground-mounted PV arrays. The effect generally decreases with increasing downwind distance from the first row. Due to higher design wind speeds the dynamic self-excitation is generally less pronounced than for photovoltaic tracking systems, but it also depends on natural frequency and damping ratio.

European wind loading standards

The adoption of National Annexes to EN 1991-1-4 of the CEN member states (“National members” and “Affiliates”) has evolved remarkably during the past years.

“National members” are regular members of the Comité Européen de Normalisation (CEN). Regular members are national standardization bodies of the EU and EFTA member states. Additionally, the general assembly may offer a regular membership to candidates for an EU or EFTA membership.

“Affiliates” may be national standardization bodies that reside in the “European neighborhood” and are regular or corresponding members of the International Organization of Standardization (ISO). Historically, the term “European neighborhood” is interpreted quite widely. Besides Eastern Europe and the Caucasus, it includes larger parts of the Mediterranean and of the Middle East. Originally, this status was supposed to be a probationary membership for EU- and EFTA-candidates, but today it is also offered to other interested countries.

Current regular CEN members are: 
Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Macedonia, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom.

“Affiliates“ within Europe are: 
Albania, Belarus, Bosnia and Herzegovina, Moldova, Montenegro, Serbia and Ukraine.

“Affiliates” outside Europe are: 
Armenia, Azerbaijan, Egypt, Georgia, Israel, Jordan, Lebanon, Libya, Morocco and Tunisia.

Many national standardization bodies have webpages or even offer online shops where standards may be ordered. But in several cases requests via e-mail are necessary, even though these are not always answered or answered inadequately.

The current state shows an advanced state of adoption of National Annexes among the regular CEN members.

The following groups of countries may be distinguished:

  • The current 33 regular CEN members with very few exceptions have adopted the EN 1991-1-4 and have published National Annexes. Exceptions are Macedonia, Malta, Spain, Switzerland and Turkey. For Malta the estimated date of publication for the National Annex is unknown. For Macedonia no information was available. The Swiss standard SiA 261 has been partially revised and was republished in July 2014. In Spain and Turkey valid national standards are known, however, no National Annexes to EN 1991-1-4 are published.
  • Among the “Affiliates” only Belarus has published a National Annex to EN 1991-1-4. Serbia, Bosnia and Herzegovina and Montenegro are currently developing National Annexes, but have not yet decided when these will be published.
  • Armenia has no national wind loading standard and does not consider adopting the Eurocode.
  • Ukraine has not published a National Annex. The valid Ukrainian standard shows a lot of similarities to the Russian standard, even though both codes are not identical.
  • Russia and Azerbaijan are still using the standard of the former USSR.
  • Israel does have an independent wind loading standard.

No official wind loading standards were found by means of the web search neither for Albania and Moldova, nor for Egypt, Jordan, Lebanon, Libya, Morocco and Tunisia. Yet, unofficial wind loading recommendations exist in many if not in all of these countries.

Standards related to wind loading in the US

Wind load related standards in the US are ASCE 7-05 and ASCE 7-10. These standards also briefly define the requirements for wind tunnel testing. A more thorough description of how to conduct wind tunnel tests in boundary layer wind tunnels is given in ASCE 49-12. This standard also defines appropriate methods for post-processing of wind tunnel data.

Moreover, wind tunnel testing of solar roof mount systems was specified in the SEAOC-PV2-2012 report which was published by the Structural Engineers Association of California.

Available Downloads

Testing Agency Certificate of Approval

Zertifikat „Department of Building and Safety“, City of Los Angeles, USA

Gültig bis: 01. September 2020
Ausstellungsdatum: 01. September 2019

International wind loading codes

Besides EN 1991-1-4 and ASCE 7-10, other advanced wind loading codes may be found all around the world. Examples are AS/NZS 1170.2 in Australia and New Zealand, the AIJ Recommendations on Loads for Buildings and the Building Standard Law of Japan, the 2015 edition of the International Building Code or ISO 4354:2009.

The aforementioned codes differ in averaging periods for design wind speeds, pressure coefficients for various structures and wind dynamic effects.

Torsional galloping / torsional instability of single-axis tracking systems

Torsional galloping / torsional instability of single-axis tracking systems

In the past, galloping-like instabilities (a mix of vortex lock-in and torsional galloping) have been observed on single axis trackers in the field at tilt angles close to 0°. This happens due to a sudden excitation of the first mode of vibration. This mode is a helical twisting which increases with distance from the torque motor. The instability is a result of vortices forming on, and then shedding from, the leading edge as it twists up and down. The sudden release of torque as the vortex is shed lead to torsional galloping, but only for trackers initially positioned roughly parallel to the ground. The instability is quite robust with a strong dependence on torsional stiffness and a weak dependence on damping.

To suppress self-induced oscillations which may seriously damage the structure, stowing at a tilt angle above 20deg is recommended. Proof of torsional stability is possible by conducting simplified two-dimensional Computational Fluid Dynamics (CFD) simulations with varying wind speed neglecting any three-dimensional helical deformation of the torsional mode.

VideoMitWing_V3U15_timeStep0p1_15s

Another way to eliminate the torsional instability, when stowing for survival would be to lock the torque tube position. This could be realized by using a rotational stop (torsional safety valve, torsional relief system) or by undertaking other counteracting measures. It is also important to monitor the health of the dampers.

It is equally important to keep an order of installation of tracking system components which ensures that torsional instability cannot occur prior to commissioning of a solar plant. With regard to this, it is highly recommended that dampers be installed ahead of modules.

Your contact

Thorsten Kray, PhD

Head of Building Aerodynamics

Your contact for:
Wind loading on buildings and solar structures

Language:
German, English, French
+49.241.879708-12 | +49.241.879708-10 | kray@ifi-ac.com

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