Unit 4 Adapting the mechanical design
PV modules and array mounting systems are installed in accordance with manufacturer’s instructions. Not following these instructions may avoid product warranties or listings. Specialized training and experience may be required for certain products and systems, such as large area modules, building-integrated products or large tracking arrays.
4.1 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 the attachment points, which are either screwed of bolted to the roof trusses or structure. Lag screws are commonly used for screwed attachments to residential roof types.
The point attachments must be installed properly to structural members.
Where lag screws are used, they must be centered into a truss generally only 1-1/2 inches in width. To find the exact center of trusses, special deep-penetrating stud finders can be used. With practice, the trusses on a shingled residential roof can usually be located by hitting the roof with a hammer. The center of the truss can be located by driving a small nail through the roof covering, deck and into the truss, then moving over 1/8 inch or so at a time until the nail does not penetrate into the truss, locating the truss edge. Backing up ¾ in then defines the center of the truss. A drill alignment tool can help center the appropriate size pilot holes prior to screw installation.
When structural members are not present or cannot be located for array attachment points, the installer may be required to add additional blocking in the attic between the roof trusses. This is commonly required toward the edges of hip roofs. Typically a solid anchor between trusses can use pairs of 2x6 boards that are attached between rafters or trusses. The 2x6 pairs provide three inches of wood into which a lag screw can penetrate, as well as a relatively large area for mounting the bracket on top of the roof. In order to provide proper support for the array, the boards must be nailed or screwed securely onto the rafters or trusses with at least two fasteners on each side of each board.
PV array mounting system designs and all components must be able to withstand the maximum forces expected in any given application. Oftentimes, independent engineering or test results may be required to certify PV array structural designs for local building code compliance. The critical design area is usually the point attachments of the array mounting system to a structure.
A number of pre-engineered standoff mounts are available commercially. When installed according the manufacturer’s directions, engineers or test laboratories certify these mounts to be capable of withstanding specified wind loads. If engineered mounting systems are used, it is necessary follow the instructions to ensure that the system is installed properly to address the design wind load requirements. During inspection, it should be pointed out that the directions were followed to meet the loading requirements.
4.2 Install PV Modules
Most standard flat-plate PV modules are glass laminates enclosed in an aluminum frame. The frame provides mechanical support for the laminate, and a means to structurally attach the module to a mounting system and for electrical grounding. PV modules are either bolted with fasteners or clamped to supporting rails or beams.
In common sloped rooftop applications, the rails are usually laid out with the length in an east west direction across the roof, which permits variable width attachments to the underlying roof structural members, such as rafters or trusses. As the spacing between rafters or trusses is usually fixed, this may constrain the installation of rails up and down the roof slope (in a north-south direction). This is because PV modules require the support rails to be located at certain points on the module frame to support the specified mechanical loads. Refer to mounting hardware manufacturer’s data on maximum allowable loads and deflection on module support beams.
Manufacturer’s instructions should be carefully followed whenever handling or installing PV modules.
Although PV modules are designed to withstand environmental extremes for many years, they can be damaged if improperly stored, handled or installed. Some modules are more durable than others, but care should be taken to ensure that the module edges are not chipped or impacted. Unframed laminates are particularly susceptible to edge damage and require significantly more care in construction handling. Small chips or nicks in the glass result in high stress points that become cracks that destroy the module. Since clamps are commonly used to fasten PV modules, it is important to install the proper clamps for the modules used, and torque to the proper values so that the clamps stay firmly in place but do not crush the module frame. Follow the PV module manufacturer’s installation instructions for the allowable mounting points to meet the maximum design loads.
Working safely with PV modules involves taking precautions to avoid electrical shock from potentially high dc voltages, especially when several modules are connected in series. Wiring faults may also lead to hazardous conditions and high voltages on metal components.
Care in handling, transporting, storing and installing PV modules includes the following:
· Leave modules in packaging until they are to be installed.
· Carry modules with both hands, do not use connectors as a handle.
· Do not stand modules on hard ground or on their corners.
· Do not place modules on top of each other or stand on them.
· Do not mark or work on them with sharp objects.
· Keep all electrical contacts clean and dry.
· Do not install modules in high winds.
General safety precautions for installing PV modules include the following:
· Use the appropriate safety equipment (insulated tools/gloves, fall protection, etc.).
· Never insert electrically conducting parts into the plugs or sockets.
· Never connect non-load break connectors under load or if dirty or wet.
· Never use damaged modules.
· Do not dismantle modules.
· Do not remove any part or label fitted by the manufacturer.
· Never treat the rear of the laminate with paint, adhesives or mark it using sharp objects.
· Do not artificially concentrate sunlight on modules.
/ Photovoltaic Solar Panel Installation1
EE02-Photovoltaic System Installations / Unit 4