The above graphs are based on two different arrays: one in Bend, Oregon, mounted parallel to the roof plane and 4.25 inches above the roof surface, and another in Eugene, Oregon, mounted at a 30-degree angle to a flat roof. For the comparison, only data obtained with incident solar radiation between 900 and 1,000 watts per square meter was used. Data was grouped into four "bins" with ambient temperature between 20 and 24°C, 24 to 28°C, 28 to 32°C, and 32°C and above. The data is represented by trend lines. Exact system performance will vary with local conditions.
The Module Temperature & Performance graph shows the approximate 0.5% decrease in output per 1°C increase in module temperature at each site. The Rack Style & Module Temperature graph illustrates the temperature advantage to mounting PV modules away from the roof plane to increase air circulation and cooling. The modules mounted at a 30-degree angle to the roof stayed about 12°C cooler and performed about 6% better than those flat-mounted 4.25 inches above the roof.
Pole mounts allow easy tilt adjustment and snow clearing, and ample air circulation means cooler modules for more power output.
The top-of-pole mounting solution is a favorite among many installers for a variety of reasons. The ability to locate an array far away from shading objects, to tilt and orient the array in an ideal position, and to avoid punching a bunch of holes in a customer's roof are all positives. With the advent of highvoltage string inverters, and MPPT controllers that can step down higher-voltage PV arrays to a lower battery charging voltage, pole mounts can be located up to a few hundred feet from the charge controller or inverter. Top-of-pole arrays are viable for locations with enough land space and where possible aesthetic concerns are not an issue.
Depending on the size of the array, the support pole can be as small as 2-inch-diameter schedule 40 steel pipe to 8-inch-diameter schedule 80 for large arrays. The footing for the pole is encased in concrete according to manufacturer's specifications (or local engineering) for the array size and the site's soil and wind-loading conditions. In these setups, the top of the array is generally too high to be easily accessible and a ladder or scaffolding system will be required during installation.
With the exception of the actual pole, which is purchased locally, the mount manufacturer provides all the necessary components and hardware to mount the array. Included are the mounting sleeve, which slips on top of the pole, and all necessary bracing and cross members, as well as module mounting hardware. (See "How to Install a Pole-Mounted Solar-Electric Array: Part 1 & Part 2" in HP108 & HP109 for pole-mount installation specifics.)
The ability to adjust the array tilt seasonally is a natural function of any top-of-pole mount. This can be of particular interest for off-gridders who rely on every KWH of electricity produced by their PV systems. In cold climates, top-of-pole mounts are one of the most convenient racking options if snow needs to be periodically cleared from the array. Top-of-pole arrays can also be used with tracker systems to help boost PV production even more (see "Tracker Types & Features" sidebar).
Because the array sits several feet from the ground, allowing for the greatest amount of airflow, top-of-pole mounted arrays operate at lower temperatures than roof- and ground-mounted arrays. This reduces the amount of power lost when ambient temperatures are high.
Top-of-pole mounts generally are not viable options in urban or suburban areas due to the yard space required. And the additional excavation required to place a pole and trench to the electrical distribution can make top-of-pole mounts more costly in certain situations. Finally, side-of-pole mounts, which are popular for small stand-alone outdoor lighting systems, are also available.
The proliferation of commercial PV systems has resulted in the advent of a number of different racking approaches for large arrays and installations on flat roofs. These solutions include custom designed and fabricated mounting structures, integrating the PV array into the roofing material, and using a nonpenetrating ballast system for flat-roof applications.
The most common type of commercial racking system is the ballast rack, which uses the weight of the modules and rack in conjunction with ballast to securely keep the arrays in place. Masonry blocks are placed in ballast pans that are located either directly under, or in front of and behind,
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Ballast mounts rely on the weight of the ballast, modules, and racking—rather than fasteners and roof penetrations— to secure the array.
Tracker Types & Features
The sun's path through the sky changes throughout the day: In the morning, it's low on the eastern horizon; at noon, it's high in the sky; and at sunset, it's low again, but on the western horizon. Because a PV array generates the most energy when its modules are directly facing the sun, those interested in getting every last watt-hour from their PV modules often investigate tracking systems.
Trackers are available for almost all sizes of PV arrays. For smaller residential applications, tracking systems are commonly mounted on top of an appropriately sized steel pole. Larger commercial systems will often employ long rows of PV modules set on single-axis trackers.
Fixed vs. Tracked Arrays
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