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PV Array Mounting Options by Ryan Mayfield

Photovoltaic module mounting systems are one piece of a solar-electric project that generally does not receive a lot of attention in press releases or during solar home tours. But a high-quality racking system is an important component that shouldn't be overlooked.

For most PV applications, gone are the days of having to custom-manufacture a mounting system. Many commercially available solutions exist—from pole-top module mounts to rooftop rails. This guide will help you navigate the variety of options available today and their associated advantages to fit your specific application.

Top-Down Rail Systems

One of the most common—and very popular—PV module mounting methods is the "top-down" rail system, since modules attach from their upper side to the rails with specified clamps. This versatile system can be used with almost all roofing types. Top-down mounting systems consist of four main components:

• Feet or posts (also called "footings" or "stand-offs") that are typically secured to the roof's rafter system;

• Extruded aluminum rails fastened to the feet or posts and the array;

• End-clips that secure the ends of the PV array to the rails;

• Mid-clips that hold the junction of two modules to a mounting rail.

Top-down rail mounts are popular in both home- and business-scale installations.

Top-down rail mounts are popular in both home- and business-scale installations.

Mounting clips attach PV modules to the rails from the top, allowing quicker installations.

mounting PVs

Manufacturers of top-down rail systems each take their own approach, with slight nuances on the same basic mounting principles. Mounting rails are extruded lengths of aluminum which allow installers to use standard or mount-specific stainless-steel bolts and mounting clips to attach the PV modules to the rails.

Top-down rail systems offer several advantages for roof-mounted PV arrays. The first is that the array is mounted parallel to the roof plane, which minimizes the array's visual impact compared to other mounting options. Second, the PV modules are attached to the rails from the front side, instead of the back, which decreases installation time. Many top-down rail manufacturers have also developed integral array equipment-grounding options.

The adjustable spacing of the footings and rails also offers a flexible design element. Since the feet can be

Conergy's SunTop rail system offers a flexible, top-down mounting approach for a wide range of roofing materials.

Sharp's top-down mounting rails are one component of their SRS system kits.

Sharp's top-down mounting rails are one component of their SRS system kits.

attached to the rails at any place along their length, the exact footing location is inconsequential as long as the spacing between each does not exceed the manufacturer's recommendations. In a top-down system, PV modules can either be placed in a portrait or landscape configuration to accommodate a particular roof's characteristics. Although the rails are generally mounted perpendicular to the rafters, when necessary the rails can be run parallel to the rafters.

The downsides? Rail systems typically result in only 3 to 6 inches of space between the back of the modules and the roof surface. Although this space does allow for some airflow underneath the array, modules mounted this

UniRac's module clips used in conjunction with S-5! clamps to mount an array directly to standing-seam roofing.

mounting PVs

A DP&W rack sets an angle steeper than the roof pitch. Fixed-pitch and adjustable racks are available.

close to the roof surface tend to reach higher temperatures (more so compared to most other mounting options), which diminishes the amount of power delivered from the array (see "Module Mounts & PV Performance" sidebar). Besides the negative effects of temperature on the array, access to the back of the modules is greatly reduced. However, since the majority of modules are prewired with quick connects and inaccessible junction boxes, access to the backs of the modules will only be necessary if troubleshooting is required. Finally, when the PV array is mounted parallel to the roof's surface, the roof dictates the array's tilt angle. Less-than-optimal tilt will result in less-than-optimal performance from your PV array.

These racks offer a lot of versatility: The same rack can be used for a ground-or roof-mounted array, or even in an awning configuration on the side of a building. The footing attachments also vary, although aluminium angle L-feet and post-type mounting feet are the two most popular options. Some rack mounts are designed for top-down module installation, while others require the modules to be secured from the back. In the latter case, the predrilled holes on the back of a module's frame are used to fasten the PV modules to the rails.

Rack-type mounts have a few disadvantages. Many designs require ordering a specific rack with mounting-hole spacing that matches the PV module's mounting holes. In roof-mounted systems, rack mounts have less layout flexibility than top-down rail systems. Most rack mounts have a fixed distance between the mounting feet based on standard rafter spacing of 24 inches. If the rafter spacing was poorly laid out or based on a nonstandard pattern, adding blocking against the underside of the roof sheathing between the rafters may be required. Finally, rack mounts tilted to angles that significantly differ from a building's roof pitch tend to have a greater aesthetic impact on a building than arrays that are mounted parallel to the roof surface.

(continued on page 62)

Rack Mounts: Adjustable- & Fixed-Tilt

Rack mounts can be tailored to fit a variety of situations, accommodating both ground-mount and roof-mount applications. Perhaps the biggest benefit to racks is that they can allow for a variety of specific tilt angles. The PV array can be set at an optimal tilt angle based on the site's latitude or, if adjustable racks are chosen, repositioned seasonally to optimize energy output.

The back-leg assembly of adjustable rack mounts can either be set to hold the array at a fixed tilt or to be manually adjusted. When an adjustable tilt is required, the tilting legs are manufactured with predrilled holes or slots that correspond to different tilt angles, or with telescoping legs that have specific tilt-angle attachment points or, in some cases, are infinitely adjustable.

Since these mounts tilt the array away from the mounting surface, the backs of the modules can usually be conveniently accessed to get to the wiring, junction boxes, and grounding points, making installation and maintenance easier. The increased distance from the mounting surface also facilitates greater airflow along the back of the modules and results in a lower array temperatures compared to the parallel-to-roof method.

A rack-style mount used in an awning configuration.

A rack-style mount used in an awning configuration.

mounting PVs

Module Mounts & PV Performance

PV system owners get very excited about the number of kilowatt-hours produced by their systems. And for good reason—this electricity is helping to offset their utility bills or, in off-grid scenarios, providing most or all of their electricity. Anything that can help or hinder production should be investigated, and racking methods are no exception.

Frank Vignola and his team at the University of Oregon's Solar Radiation Monitoring Laboratory have been collecting environmental and PV system performance data across the Pacific Northwest (see http://solardat.uoregon.edu). This data has enabled PV designers to accurately predict PV array output for a variety of installation conditions—including mounting methods.

Because high temperatures adversely affect a PV system's performance, it's particularly important to try to implement best practices when feasible. In general, arrays that are mounted parallel to the roof surface with less than 6 inches of space between the array and roof will experience cell temperatures of about 35°C (63°F) above ambient temperatures. For rackmounted arrays, where the back of the array is tilted off the roof surface greater than 6 inches, cell temperatures are estimated to be about 30°C (54°F) above ambient. Top-of-pole mounted arrays will operate at approximately 25°C (45°F) above ambient temperatures. In general, PV array output takes a 0.5% hit for every 1°C rise in temperature.

Two other critical array mounting concerns for optimizing system production are the array's orientation (relationship to true south) and the tilt (the array's angle off of horizontal). As a general rule, for a fixed array to produce the maximum amount of energy annually, it should be oriented toward true south (after correcting for magnetic declination), with its tilt angle fixed to correspond to the site's latitude.

However, this general approach can be fine-tuned based on site specifics such as what time of year the most potential solar energy is available, or if the interconnection agreement with the utility is based on annual, rather than monthly, system output. For locations in western Oregon, for example, an array will produce the most energy when oriented slightly west of south at a tilt angle of approximately 30 degrees (approximate latitude minus 15 degrees). In the eastern half of the state, where there is a greater solar resource, the ideal array position is approximately a 35-degree tilt (approximate latitude minus 10 degrees) and either a true south or slightly east-of-south orientation.

Rack Style

& Module Temperature

Module Temperature & Performance o o d

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