Determining how and where to mount a PV array are crucial steps in the PV system design process. Will it be on the ground, a pole, or the roof? And what mounting methods will be used? Although everyone prefers a shade-free site, we are limited to what the situation offers us (like if only part of the yard has good solar access). Finally, each mounting method has its pros and cons that are important to understand before settling on a design.
The ideal is to have a shade-free array throughout the entire day, from dawn until dusk. However, obstructions such as trees, buildings, and mountains are common. Early morning and late afternoon sunlight isn’t as powerful as those midday hours, so aiming for a shade-free “solar” window from 9 a.m. to 3 p.m. will give you most of the energy available at the site.
Predicting where shade will be cast throughout the day (and throughout the year) by obstructions is difficult, so system designers use site analysis tools, such as the Solar Pathfinder or Solmetric SunEye, to compare different mounting sites. These tools also establish a shade factor that can be used in system sizing calculations.
A PV system’s output depends on the angle of sunlight striking the array’s surface, with the highest output achieved when solar rays are perpendicular to the array. Unless you’re planning to use a dual-axis tracker (see “Trackers” sidebar), the sun’s position relative to the array surface is constantly changing. For a fixed PV array, we can determine an orientation and tilt to capture the maximum amount of sunlight throughout the day and year. While we may not actually mount the modules at the best orientation or tilt (for example, mounting an array flush to a roof for aesthetic reasons), we can establish a baseline for maximum system production and increase the array size to account for non-optimal tilt and orientation (see “PV Array Output at Various Tilts and Orientations”).
Ground mounting is most common in rural areas, where there’s often more wide-open, shade-free space. This method can be appealing, since installation and maintenance take place at ground level rather than on a high roof. Ground mounting also provides ample airflow behind the modules, helping to preserve efficiency, since PV array voltage decreases as PV cell temperature increases. Other advantages include more space to accommodate a larger array, and the ease of mounting modules at their optimal tilt and orientation.
Conversely, ground mounts often require excavation and concrete footings for the attachment points, which can increase installation time and cost. Because the array is accessible to animals and humans, precautions must be taken to protect any exposed wiring, per NEC 690.31(A). Vegetation around the array needs to be managed to avoid shading. In cold climates, the array must be high enough to keep it out of the snow. For multiple rows of modules, interrow shading must be avoided by keeping enough space between module rows (see “Interrow Shading” in HP151). And not to be ignored, the accessibility of ground-mounted systems means a larger risk of theft or vandalism, which may or may not be an issue at your site.