5
Installing Mechanical Components
PV modules and array mounting systems are installed in accordance with manufacturer’s instructions. Not following these instructions may void product warranties or listings. Specialized training and experience may be required to install certain products and systems, such as large area modules, building-integrated products or large tracking arrays.
5.1
Install Mounting System
PV arrays are constructed from building blocks of individual PV modules, panels and sub arrays that form a mechanically and electrically integrated dc power generation unit. The mechanical and electrical layout and installation of PV arrays involves many interrelated considerations and tradeoffs. Some of the many factors to consider include: • • • • • • • • • •
Module physical and electrical characteristics Array electrical design and output requirements Mounting location, orientation and shading Type of mounting surface (roof or ground mount) Access and pathways for installation, maintenance and fire codes Structural loads on modules, mounting structures and attachments Thermal characteristics of modules and effects of mounting system Weathersealing of building penetrations and attachments Materials and hardware compatibilities with the application environment Aesthetics
Mounting system designs have a strong effect on average and peak array operating temperatures. Higher operating temperatures reduce array voltage, power output and energy production, and accelerate degradation of modules and their performance over many years. Rack mounted arrays have the greatest passive cooling and lowest operating temperatures, with temperature rise from 15°C to 25°C above ambient temperatures under solar irradiance levels of 1000 W/m2. Direct mounts have the highest operating temperatures, with temperature rise coefficients of 35 to 40°C/kW/m2. Standoff mounts have moderate operating temperatures, depending on the standoff height. Maximum passive cooling gains are generally achieved with the tops of PV modules 3 to 6 inches above the roof surface. Common standoff PV arrays are mounted slightly above and parallel to rooftops. PV modules are typically bolted or clamped with their long dimension across two structural rails or beams for support. The rails are then fastened and weathersealed to the building structure at defined points along the rails with special brackets designed for a specific type of roof. PV arrays installed in higher wind regions require stronger rails, or smaller spans between rail attachments (more attachment points) to avoid excessive rail and module deflections. These brackets support the entire structural loads on the PV array at
104 • NABCEP PV Installation Professional Resource Guide
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