We talked briefly about tracking systems and some design considerations for the tilt angles in Lesson 2. However, we didn’t elaborate on the mounting types available for PV systems. Each PV system is different from the rest in terms of racking structure and mounting types. This is due to the fact that PV systems are space constrained and the installation requirements vary from one place to another. For that reason, many structural holding solutions have been developed to accommodate different needs such as ground mount, pole mount, and roof mount. Each type comes with advantages and disadvantages that allow a good designer to make the optimal decision for each specific PV system.
Ground Mount
This type of mounting structure allows multiple rows of modules to be installed on the ground, as shown in Figure 1.
PV modules can get extremely hot during daytime due to the excessive heat generated from the modules and the ambient temperature. The best way to cool down the modules is natural convection using air, which is easily done when modules are elevated from the ground or roof so that air can carry out the heat and cool down the modules.
Safe installation (no climbing required)
Easy access for maintenance/troubleshooting
No roof penetration/liability
Flexible positioning for max production (potential use for single axis tracking). Ground mount is usually not restricted to be positioned at a certain tilt angle. That said, it is easier to choose the optimum tilt for maximum production.
Disadvantages
Easy access (theft, vandalism, damage)
Requires earth construction work (concrete, foundation, trenching)
Space requirement
Inter-row shading considerations
PV modules can shade one another when installed in rows. For example, the first row can shade the second row located behind it. For more information, please refer to the video (6:09) explanation below from EME 810[3].
Pole Mount
This structure is common in public areas where the system is space constrained, as seen in Figure 2.
PV modules can get extremely hot during daytime due to the excessive heat generated from the modules and the ambient temperature. The best way to cool down the modules is the natural convection using air, which is easily done when modules are elevated from the ground or roof so that air can carry out the heat and cool down the modules.
Easy access for maintenance/troubleshooting
No roof penetration/liability
Flexible positioning for max production
Easy to adjust the tilt angle (potential use for dual axis tracking)
Small footprint (one pole)
Disadvantages
Easy access (theft, vandalism, damage)
Requires construction (concrete, steel, foundation, trenching)
There are various types of roof mounts that don’t require roof penetration especially when the roof is flat, as illustrated in Figure 3. Of these types, ballasted roof mount is one of the most used racking structure for PV systems installed on flat roofs. It utilizes the weight of concrete or sand to ensure the system stays still to stand all kinds of external forces such as pullout wind forces.
In most buildings, the roof is considered unused space. PV array can occupy these spaces and transfer them to useful spaces that can generate electricity for the building.
Provides secure access for authorized individuals only
Disadvantages
Requires roof penetration (roof leak liability or roof damage)
Requires professional installers
Higher module temperature (poor ventilation)
PV modules can get extremely hot during daytime due to the excessive heat generated from the modules and the ambient temperature. Since PV modules cannot be elevated from the roof for more than a couple of inches, air movement is restricted from carrying out the heat to cool down the modules.
Difficult conduit runs
Most roofs have different gable shapes and sizes. In addition, the main electrical panel is usually located at the bottom of the building. That said, the PV array conduits have to run through the roof to the main panel and that route can be wiggly and require more attention to the details when installing the system.
Fixed title and orientation
Most roofs come with single pitch that cannot be changed. When a PV array is installed on the roof, the orientation and tilt are restricted by the roof pitch and orientation, and that might affect the production of the PV system.
Pullout forces
Wind speed flowing against the PV modules on the roof can cause forces that act similarly to pullout forces that try to remove the modules from the roof. That said, PV designers should ensure that the PV array is securely fastened to the roof using the right attachment mechanisms, such as screws and hooks.
Considerations
Roof age
Roof age is a critical factor to installing a PV array. A weak and old roof may not be able to withstand the added weight from the PV array.
Snow and wind loading
Snow and wind can add weight to the roof. If the roof is not designed to carry that additional weight, the roof might collapse.
Fire issues
When PV arrays are installed on roofs, installers should work with the local fire department to ensure that there are no fire hazards or accessibility issues if fire occurs.
There are other mounting types that are beyond our discussion for this class and students are encouraged to look for different racking structures for their own benefit.
For Further Reading
For more information about the mounting structures of the PV system, please refer to your Required Reading: Chapter 10 from the Dunlop text. Please note that some information in the reading chapter is not directly related to this topic; however, students are encouraged to read through the chapter for their own benefit.