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Although most large format modules undergo laboratory testing for certification, the laboratory is not the real world. The field load applied to a solar module depends on the structure on which it is mounted and the project terrain.
At the RE+ 2023 conference in Las Vegas, suppliers from around the world showcased their largest and thinnest bifacial solar modules, showcasing their achievements in PV cost efficiency. With previously unthinkable powers, the solar energy cost-cutting giant has moved forward.
For those of us who have designed a solar module and carried out mechanical load tests, there is one detail that draws attention and asks to be explored further. These huge modules come equipped with some of the smallest racks ever seen.
The previously ubiquitous 2 by 1 m module with a 50 mm high frame is now approximately 55% larger in area with frames up to 30 mm high. How is this possible when mechanical load capacities have been kept constant and the height of a beam is of vital importance to its strength? This physics applies to bridges, buildings, and even the frame of a solar module. Wind and snow loading increases proportionally with increasing surface area, but the frames of the newer and longer modules have their height reduced by ~40%, severely reducing their load capacity.
The modules undergo various standard mechanical load tests for certification. These tests apply loads to the front and rear of the module to evaluate its resistance to real-world environmental conditions. Current industry standards (UL 61730-2, IEC 61730, IEC 61215-2) generally agree on mechanical load testing procedures. Many of the modules present in the conference room advertise compliance with these standards and these certification tests are carried out with the utmost care and diligence by the industrys leading testing laboratories.
Although large format modules meet these standards in the laboratory, the laboratory is not the real world. The field load applied to a solar module depends on the structure on which it is mounted and the project terrain. The larger the wind zone, the greater the load on the module.
Less obvious is that larger tilt angles also tend to increase the wind load on the modules and that this varies between locations throughout the assembly. Imagine a ship with its sails raised or lowered during a storm. Which one has more strength to project its ship forward?
Snow can often have the opposite effect. Panels with a higher pitch angle will often shed more snow than panels with a lower pitch and will therefore be more favorable to loading the module by snow. Any house roof at a northern latitude will show this phenomenon. Project designers must carefully check that the selected modules work with the mounting structure at each location on the project site.
Therefore, to understand the engineering gap at hand, it is key to combine the design of large format module racks and the structural design of shelving systems. Because module loading depends on the supporting structure (e.g., tilt angle, among other variables), structural suppliers typically specify the anticipated module loading in the project design. Many framework providers are good at validating that the module itself falls within the certification classification. However, is it possible that some suppliers are still overlooking maximum module loads in windy conditions?
A SETO-funded research project being carried out through a joint venture of Lawrence Berkeley National Laboratory and the University of Berkeley has determined that suppliers must consider effective wind areas smaller than spans between foundations ( not what is shown in Figure 1 A) when estimating the loading of the individual modules. PV modules can break if attributable areas as small as a quarter of the module are overloaded (individual fixation level loading – D in Figure 1) and this can be shown to occur at maximum project design conditions for many projects that They are being installed today. Although the assessment typically carried out revolves around a maximum design load, the SETO-funded research team is currently studying how a lower, uneven cyclic loading can also lead to structural failure.
If the maximum wind load of modules has been underestimated in project design over the last 15 years, then module failures should be rampant, right? In practice, the frames of the older modules have served a double function by hiding this oversight. Some of those racks were designed with safety factors of 3. Today, large format modules appear to be designed with safety factors of 1.5, according to some manufacturers data sheets and industry standards. This allows the modules to be competitive in the downward march of costs.
When a certification laboratory tests a module with an actual load of 2,400 Pa on the back face, the maximum design pressure for which it is certified is 1,600 Pa. It is essential to check whether the advertised module rating is what was tested ( including safety factors) or whether it is the maximum allowable design pressure (without safety factors). A pressure of 1,600 Pa on a module is approximately equivalent to a 72 mph wind gust for a module pressure coefficient of 3. The LBNL/UC Berkeley research team has determined that this coefficient is achievable at the ends of the row for module inclinations greater than 15 degrees. This is hardly a sufficient design for any project in the US based on the latest ASCE 7-22 wind maps. If a designer were to mistakenly use 2,400Pa as the design pressure, this would increase the allowable wind gust to 88 mph. Therefore, it is important to understand what module classification includes.
Load capacity
The market has taken the load capacity of the modules to the limit. This seems to be especially true in the case of rear loading (wind lifting). The combination of legacy engineering assumptions, larger module footprints, lower module rack heights, and unclear manufacturer classifications results in a recipe for failure. The goal is not to blame anyone, but to understand the technical issues and provide guidance on what stakeholders can do.
Below are tangible ways in which developers, financiers, insurers, owners, asset managers, structure manufacturers and module manufacturers can manage these risks:
1. Ensure that sufficient budget and independent engineer (IE) time is allocated per project (particularly on smaller projects) so that key details about module loading can be checked not only by project, but at each location of the project (for example, outside rows, corners, fixings).
2. The structure manufacturers due diligence must confirm that:
Clip and bolt loads for module retention use “module clip loads” (D in Figure 1) instead of row mean areas (A in Figure 1) or even module-level areas (B in Figure 1). For more details, see the wind tunnel test coefficients.
It should not be assumed that the wind load on the modules is the same throughout the assembly. The wind loads on the modules at the ends of the rows are usually higher than those inside. This applies to both tracking and fixed tilt systems. [See the latest version of SEAOC PV2 Wind Design for Loading Arrays].
It should not be assumed that the clip/bolt load is the same at all module locations. The load on one half of the module is usually quite different from the other. The fasteners may end up being of the same design, but they must be designed to support the highest load and not a lower average load distributed among the four fasteners.
Module rails should also be sized accordingly, with particular emphasis on the outer module rails and their appropriate rail-level area loading (C in Figure 1) and with assumptions for uneven module loading.
3. Module due diligence should confirm:
If the front/rear side mechanical load rating of the module data sheet includes the test safety factor (typically 1.5). If not, reduce the load rating by the appropriate safety factor and confirm that the structural load demand does not exceed that new lower rating based on the module (follower) wind/snow retraction angle or tilt angle. of the installation (fixed inclination).
That the module frame is designed to withstand the additional forces resulting from unequal loading for the wind/snow retraction angle (follower) or installation tilt angle (fixed tilt) of the system.
The mounting method exactly matches the module certification mounting method and is listed in the module installation manual. Otherwise, the module manufacturer must be requested to issue a letter stating that the non-approved mounting method will maintain the warranty under the project conditions. Testing may be necessary. |
