AE 868
Commercial Solar Electric Systems

Mechanical BOS Components

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After reviewing different types of PV mounting structures, it is time to discuss the components that form these mounting structures. In this section, we will focus on the mechanical BOS components that help assemble the PV system components. These mechanical BOS components include fasteners, brackets, enclosures, racks, and other components that support the structure of the PV system.

Starting at the PV Array

PV modules need to be securely fastened to the racking structure, and that is done through rails, splices and clamps.

Rails

PV modules cannot be directly fastened to the roof of the building, for example. They require a structure that holds modules together that is later fastened to the roof or the ground. This structure is made of what is referred to as rails, which are rated to withstand heavy weights. Rails come in different shapes, profiles, lengths, and materials. Since rails manufacturers don’t make every desired possible length, the rails are tailored to fit each specific PV system.

Splices

When the PV system requires longer spans than the default length of the rails, splices are used to extend and connect rails to each other. Splices are usually provided by the same rail manufacturers.

Clamps

In order for the PV modules to be fastened to the rails, clamps are usually used to secure the connection between the rails and the modules. PV modules are usually installed adjacent to one another. As a result, some modules will be located in the middle and other modules will be installed on the edges. In this case, the middle modules will require what is referred to as mid-clamps to fasten the modules to each other and to the rails and end-clamps at the edge modules to lock the ends of the array.

Moving Away From the Rails

The rails need to be fastened to the mounting structure. This usually depends on the mounting type that can be roof mount, ground mount, or pole mount structures.

Roof mount (rooftop with penetration)

In case the PV system is installed on rooftops, designers should consider elevating the rails a couple of inches against the roof to allow for ventilation and lower temperature operating ranges. For that reason, there are multiple solutions in the solar industry to achieve that clearance. The rails can be fastened to the roof using one of the following options:

L-foot

L-foot utilizes screws that are fastened to the rafters or trusses from one side, and the other side will be attached to the rails. L-foot can be directly used to allow some roof ventilation; however, when the L-foot is not long enough to satisfy the ventilation requirements, Stand-offs are usually used to elevate the rails above the roof to allow proper ventilation to meet the design criteria. Stand-offs come in different lengths, and designers choose the best length based on the application and ambient temperature.

Tile Hooks

Roofs come in different shapes and materials. In the case of ceramic tile roofs, roof tile hooks can be directly used as roof attachment equipment that allows the rails to be fastened to the roof by going underneath the tiles. Tile hooks provide roof ventilation, as they are designed to be elevated from the tiles.

Roof Mount (without roof penetration)

PV systems can use S-5 clips or applied weight (i.e. sand or concrete) to fasten the rails to the roof. Let's elaborate on each type:

S-5 clips

In order for a PV array to be fastened to a metal roof (flat or tilted), manufacturers have developed solutions to allow rooftop installation without roof penetration. This solution includes an innovative clip that is called "S-5" and bites from one side to the metal roof. The other side will be fastened to the rails of the PV array. This solution minimizes the use of equipment grounding since the frame metals are directly connected to a metal structure. Designers should consult with the manufacturer's recommendations for grounding requirements when using these clips.

Applied Weight

Another solution to allow rooftop installation without roof penetration and is done using weight that holds the array to the roof. The applied weight can utilize sand or concrete, based on the design. Designers should take into account the roof age and the maximum allowed weight of the roof in order to accommodate the additional weight of the PV array. In some cases, the PV array's structural design needs to be reviewed by Professional Structural Engineers to ensure the structure can handle the additional weight.

Ground mount/Pole Mount

Reinforced concrete bases are usually used for ground and pole mount systems with steel structure attached to the rails. In some cases, the PV array's structural design needs to be reviewed by Professional Engineers to ensure the structure can withstand dynamic loads based on the location and wind speed.

Note:

Racking structure load calculations are usually provided by the manufacturers in the specification datasheets. There are main parameters such as dead load, live load, wind load, and snow loads that contribute to the total weight of the PV array. These loads should be taken into account when calculating the weight per unit area that is usually referred to as Pound per Square Foot (PSF), or the concentrated Point PSF, and finally, the pullout force (uplift) that is directly related to the wind factor and how they might affect the panels.

For further reading:

For more information about the mechanical and structural load calculations, please refer to your Required Reading: Chapter 10 from the Dunlop text.