References · Photovoltaic Systems, p. 260-267 10 - 12 Fixed-tilt PV arrays are non-movable...

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The mechanical integration of photovoltaic arrays requires an understanding of the site conditions, the physical and electrical characteristics of PV modules chosen, the desired electrical output for the array, and the mounting system and structural attachments. It also involves considerations for the installation, maintenance and accessibility of equipment, and architectural integration. The objective is to produce the least-cost mechanical installation that is safe, secure, appealing and appropriate for the application.

References:Photovoltaic Systems, Chap. 10Minimum Design Loads for Buildings and Other Structures, ASCE 7Wind Load Calculations for PV Arrays; Stephen Barkaszi, FSEC & Colleen O’Brien, BEW Engineering: www.solarabcs.org/wind/

Mounting hardware manufacturers websites:Unirac: www.unirac.comProfessional Solar Products: www.prosolar.comIron Ridge: www.ironridge.comDirect Power & Water: www.dpwsolar.com

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Reference: Photovoltaic Systems, p. 255

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PV arrays are constructed from building blocks of individual PV modules, panels and subarrays 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 that are affected by the system design, the equipment used and the site conditions.

Reference: Photovoltaic Systems, p. 255-260

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Higher operating temperature reduce array voltage, power output and energy production, and accelerate degradation of modules and their performance over many years. Mounting system designs have a strong effect on average and peak array operating temperatures.

Rack mounted arrays have the greatest passive cooling and lowest operating temperatures, with temperature rise coefficients from 15 to 25 °C/kW/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.


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Preferably, PV modules in source circuits are installed in a single row or rack, with each module adjacent to another, with the module junction boxes aligned on the same sides to facilitate wiring connections. Note that PV module connector leads are only a certain length and additional cabling and connectors may be required for non-standard installations.


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Reference: Photovoltaic Systems, p. 268, 279-280

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Building codes require all roofing system and building penetrations to be properly flashed and weathersealed. There are a variety of weathersealing methods used, depending on the type of penetrations or attachments used, with some requiring sealants and others not. Weather sealants, where required, should maintain flexibility over 30-50 year life and expected temperature extremes, readily adhere to roofing and construction materials, and have UV inhibitors to resist degradation from sunlight. Butyl rubber, elastomeric and polyurethane based sealants have long life and readily adhere to dry roofing materials, including metal, wood and asphalt shingles. Silicone, latex and acrylic sealants are generally not suitable for the application.


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PV arrays can be mounted on the ground or attached to buildings or other structures using a variety of methods. PV array mounting orientations can also be classified as fixed-tilt, adjustable or sun-tracking mounts. Ground-mounted designs include racks, poles mounts and sun-tracking arrays. Common building mounts include standoff mounts and rack mounts that can be retrofitted to existing rooftops. Building-integrated PV (BIPV) arrays, including direct mounts and integral mounts are integrated with building components and cladding materials such as windows, awnings and roofing tiles.


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Fixed-tilt PV arrays are non-movable structures that position the PV array in a constant orientation. Fixed PV arrays installed in northern latitudes are tilted up from the horizontal and oriented toward the south, and away from shading obstructions to maximize the solar energy received. Most PV arrays installed on buildings and ground mounts are fixed-tilt PV arrays.


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Adjustable-tilt PV arrays use mounting structures with removable fasteners, telescoping legs or other manual means to allow for seasonal adjustments of the array tilt angle. Adjusting the tilt angle of PV arrays twice per year, around the time of the equinoxes in the spring and fall, can marginally improve system output, but is generally not practiced for most installations.

Suggested Exercise: Compare the differences for your location for the average peak sun hours on a fixed south-facing latitude tilt surface compared to an adjustable south-facing surface adjusted to latitude - 15° tilt on April 1st, and adjusted to latitude + 15° tilt on October 1st.


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Sun-tracking arrays use mounting structures that automatically and continually move the array surface array to follow the sun’s position throughout the day. Sun-tracking arrays are characterized by their tracking mode and whether they track the sun on one or two axes. Most single-axis trackers are designed to move the array surface east to west on a north-south tracking axis that is tilted from the horizontal.

Sun-tracking arrays can receive up to 20-30% more solar radiation than fixed south-facing arrays. Single-axis trackers do not point exactly to the sun at all times, but generally receive 20% or more solar radiation than received on south-facing fixed-tilt surfaces. Marginal benefits are achieved in going from single to two-axis tracking. Point-focus concentrating PV modules require two-axis sun-tracking to capture the direct beam solar radiation component, while linear-focus concentrators can use single-axis tracking.


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Sun-tracking arrays can be controlled by passive of active means. Passive means use solar heating of working fluids in the tracker internal structure to create weigh shift and move the tracker, or to pressurize piston actuators to move the structure. Active tracking methods use electromechanical drives or stepper motors that are controlled by smaller PV modules attached to the array, or by external power sources and microprocessors.


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The tradeoff for sun-tracking arrays involves considering the initial costs and recurring maintenance, versus the cost of additional modules on a fixed array to achieve similar energy performance. Tracking PV arrays are usually installed on the ground as opposed to on top of buildings due the large structural loads at the foundations. Sun tracking arrays also require larger surface areas and sufficient spacing between individual trackers to avoid one tracker shading an adjacent one. Some tracker designs use a backtracking approach to limit the tracker movement early in the morning and late in the afternoon and sacrifice some solar energy gain, in order to permit closer spacing of individual trackers and avoid shading.


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Active sun-tracking arrays use hydraulic pistons or motors to drive the tracking mechanism.


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Ground-mounted arrays are detached from buildings, and usually permit the greatest flexibility in mounting and orienting the array. Types of ground mounts include racks, poles and sun-tracking mounts. Ground mounts require anchoring to foundations such as concrete, setting poles directly in the soil, or by self-ballasting. The site conditions and the methods and materials specified by the mounting system designer manufacturer dictate the best installation practices.

Since ground-mounted arrays are typically at lower elevations, shading from nearby trees, fences, buildings and other obstructions may be a concern. Ground-mounted PV arrays generally require restricted access by fencing or elevating the array to reduce safety hazards.


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Rack-mounted arrays are commonly used on the ground, buildings and other structures, and offer the greatest flexibility in mounting the array at specific tilt angles. Small rack-mounted arrays can be installed on poles, and larger racks can be installed in multiple rows for larger arrays.


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Some array structures may be installed as a monolithic unit by a crane.


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Rack-mounted arrays can be elevated on poles above harms way for safety and protection.


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Pole-mounted arrays employ either fixed, adjustable, or sun-tracking arrays installed on a rigid metal pipe or wooden pole. Pole-mounted designs allow the arrays to be elevated to protect from harm and to avoid shading, and most allow the array azimuth angle to be rotated for optimal orientation. Due to the large foundation loads, pole mounts are usually installed on the ground and not on buildings, and have limitations on the size of array they can support based on he size of the pole and foundation.


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Depending on the site soil conditions, poles may be set directly into holes and compacted, or may require reinforced concrete foundations. Many types of standard utility and light poles are rated for wind loads, and their ability to support heavy equipment. Foundations used for pole mounts must have sufficient mass and any attachments must have appropriate strength to counter the forces attempting to blow the pole over during high wind load conditions.


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Roof-mounted PV arrays may use standoff mounts, rack mounts and building-integrated PV arrays. Rooftops often have large areas of unused space, and are popular locations for installing PV arrays. Rooftop locations provide higher elevations that help avoid shading and offer additional protection and safety for the array. Most roof-mounted PV arrays use fixed-tilt support structures that are retrofitted to existing rooftops. Roof mounts may also be classified according to the type of roof structure or roof covering the array attaches to, such as sloped or flat roofs, or asphalt shingle, metal, tiles or composition roofing materials. For practical and structural considerations, roof-mounts generally do not use movable sun tracking arrays or pole mounts.


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Standoff-mounted arrays are the most common way PV arrays are attached to sloped rooftops. Standoff mounts typically locate the PV modules 3 to 5 inches above and parallel to the roof plane. They are not usually tilted at a different angle than the roof surface. This is because the added complexity and costs required to install mounting structures obliquely with respect to the roof surface do not usually justify the marginal increases in solar energy gain and system performance. Several manufacturers provide standard mounting hardware for standoff arrays that meet the structural loads for most applications.


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A self-ballasted PV array is a type of rack mount that relies on the weight of a the PV modules, support structure and additional ballast material to secure the array. Self-ballasted arrays are intended to reduce or eliminate direct structural connections to a building or foundation, thereby avoiding additional labor and weathersealing concerns. Typical ballast materials include sand and concrete blocks installed in trays at the bottom of the racks. Self-ballasted arrays usually require additional restraints in seismic and high wind load regions.


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Building-integrated PV (BIPV) arrays replace conventional building cladding, where PV modules are integrated into roofing materials, glazing, awnings and other architectural features. Integral mounts and direct mounts are types of BIPV arrays. The advantage of BIPV arrays is that the PV array replaces conventional building materials, saving on materials and construction costs. Most BIPV arrays use custom designed modules and require special installation procedures.


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A direct mount is a fixed array mounting system where the PV modules are attached flush to an existing roof surface or decking. Special PV modules and custom array designs are typically required for direct mount applications. Direct-mounted arrays also experience high operating temperatures.


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Integral-mounted arrays are a type of BIPV array where PV modules replace conventional building cladding, such as roofing and window systems. Integral mounts are custom designs using special installation and weathersealing procedures.


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Mobile PV arrays require custom designs and special attachments to protect the array from shock and vibrations.


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How not to install PV arrays!

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The attachment of PV arrays to buildings and other structures is governed by the standard ASCE 7, Minimum Design Loads for Buildings and Other Structures, which is adopted into most building codes throughout the U.S. However, it does not specifically address the installation of roof-mounted arrays. PV system installers and designers are responsible for ensuring that the design and structural attachments of PV arrays meet all anticipated loads, and that allowable loads on existing structures and mounting systems are not exceeded.

References:Photovoltaic Systems, p. 268-279Minimum Design Loads for Buildings and Other Structures, ASCE 7

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PV arrays must be designed and secured to withstand the maximum possible mechanical loads.

Dead loads (D) are static loads due to the weight of the array and mounting hardware, typically about 5 pounds per square foot (psf) for most PV arrays. Self-ballasted arrays can have substantially higher dead loads.Live loads (L) are loads from temporary equipment and personnel during maintenance activities. Generally these loads are small for PV installations, on the order of 3 psf. Typical flat and pitched roofs must be designed for a minimum uniformly distributed live load of 20 psf. All roofs subject to maintenance workers must be designed for a minimum concentrated point load of 300 lbs.Wind loads (W) are typically the highest of all loads experienced by PV arrays, and the only uplifting force. Snow loads (S) can be up to and greater than 20 psf in northern climates.Hydrostatic Loads (H) are due to the lateral pressure of the earth (soil) or ground water pressure on a structure.Seismic (earthquake) loads (E) are based on region and seismic design category.

References:Photovoltaic Systems, p. 268-279Minimum Design Loads for Buildings and Other Structures, ASCE 7

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Allowable Stress Design is method to determine the design loads for structural materials based on the maximum allowable elastic stress limits for the structural materials used. Consequently, it includes a factor of safety for unfactored loads. Allowable stress design considers various load combinations, and the most unfavorable loading condition is used for structural design.

Reference: Minimum Design Loads for Buildings and Other Structures, ASCE 7, Chap. 2

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For common standoff-mounted PV arrays installed just above and parallel to sloped roofs, the mounting methods are quite similar among different PV modules and mounting system suppliers.

Key points of the structural evaluation include:•PV module allowable loads and required position of deflection support and attachments.•PV module attachments to underlying beams or rails (machine screws or clamps).•Allowable deflections in beams or rails•Point attachments to structure


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The Main Wind-Force Resisting System (MWFRS) consists of structural elements that provide support and stability for the structure, like walls. beams, trusses, roof deck, etc. The PV array attachments to a structure may be considered part of the MWFRS.

Components and cladding are building envelope materials such as roof coverings and glazing that are not part of the MWFRS. PV modules and attachments to the immediate support structure are considered components and cladding. Different calculations also apply to wind load calculations for the MWFRS and for components and cladding. Higher wind loads generally apply to components and cladding where the load is distributed over a smaller area than for the MWFRS. A conservative approach evaluates all PV array elements as components and cladding.

Reference: Minimum Design Loads for Buildings and Other Structures, ASCE 7, Chap. 6

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Wind loads are by far the most significant concern for mounting PV arrays for most applications. Most PV modules are listed to handle wind loads of 2400 Pa (50 psf), and some are tested for loads up to 5400 Pa (112 psf). Generally, PV modules must be supported in certain positions to achieve maximum load capability. Refer to specific PV module manufacturer’s installation instruction for allowable mounting configurations and maximum loads.

Three methods can be used to determine the design wind loads for buildings and other structures; a simplified procedure, an analytical procedure, and a wind tunnel procedure. The simplified procedure applies to many residential and commercial buildings.

References:Photovoltaic Systems, p. 268-279ASCE 7, Chap. 6

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The design loads for components and cladding can be computed using the simplified method if certain conditions are met. The building must be enclosed and regular-shaped, and must have a flat roof or a gable roof with slope no more than 45 degrees, or a hip roof sloped no more than 27 degrees. The mean building height must be no more than 60 ft and the building or site must not have unusual characteristics or wind response.

References:Photovoltaic Systems, p. 268-279ASCE 7, Chap. 6

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Reference: ASCE 7, Chap. 6

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Basic wind speed maps show the maximum design wind speed by location.

References: Photovoltaic Systems, p. 272ASCE 7, Chap. 6

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Buildings and other structures are classified in categories based on their consequences of failure. Buildings and structures classified in higher categories must be designed for greater loads.

Reference: ASCE 7-10, Table 1-1

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Most buildings in urban and suburban areas fall under exposure B.


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Adjustments factors increase wind loads for building heights above 30 feet and for exposure category.

Reference: ASCE 7, Figure 6-3

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Pressure coefficient zones define areas of a roof with higher wind loads. The width of the pressure coefficient zone is a minimum of 3 feet, or 10% of the smallest horizontal dimension or 0.4h, whichever is greatest.


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Net Design Wind Pressures for components and cladding are determined from the basic wind speed, roof type and slope, pressure zone and effective wind area. These net pressures are defined for Exposure B, for a mean roof height of 30 feet, for importance factor = 1 and for topographic factor = 1. The effective wind area is the area tributary to a single set of supports. Higher wind pressures are given for smaller effective wind areas. The given pressure coefficients are applied as downforce (positive) and uplift (negative) forces perpendicular to the building surfaces. The positive and negative design wind pressures used for load calculations must not be less than +10 psf and -10 psf, respectively.


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Standoff PV arrays attached close to and parallel with sloped roof surfaces can be considered components and cladding, and design loads can usually be evaluated using the simplified method.


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Most PV modules and pre-engineered mounting systems are designed to support the loads determined in the example.


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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.


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PV module specifications give the maximum mechanical loads that the module can support using specified supports and attachments.


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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. Follow the PV module manufacturer’s installation instructions for the allowable mounting points to meet the maximum loads.

Suggested Exercise: Review PV module installation instructions for approved mounting and attachment methods.


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Commonly, PV modules are installed across two aluminum rails that act as structural beams. The rails are then bolted to the underlying point attachments at specific locations along the rail that define the rail spans. PV arrays installed in higher wind regions require stronger rails, or smaller spacing between rail attachments to avoid excessive rail and module deflections.

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.

Suggested Exercise: Review some mounting system manufacturer’s websites and tools to determine the appropriate mounting hardware and support structures using any PV module for any size PV array for various applications.

References: Photovoltaic Systems, p. 268-279

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Point attachments connect the array assembly to the underlying structure (building or ground) at specified intervals. Point attachments produce concentrated loads on a structure or foundation.


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Point attachments are usually made directly to structural members like trusses, not to the roof decking. Additional blocking between trusses may be required for some installations.


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Array installations on metal roofs use special clamps that crimp to the metal roof seams, and rely on adequate attachment of the metal roofing to the underlying structure. These attachments do not penetrate the roof surface, and therefore avoid problems with leakage. Installations on slate or tile roofs use special attachments that direct the mechanical loads on the array directly to the roof structure, not on the tiles.


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Lag screws are commonly used to secure point attachments directly to the tops of trusses or other structural members. The screw diameter, thread embedment and species of lumber determine the allowable withdrawal loads. A proper size pilot hole should be drilled 60-70% of the screw shank diameter, unless self-drilling SPAX lag screws are used.

The allowable withdrawal loads in the table are calculated for lag screws in the side grain of seasoned wood, using the empirical formula: P = 1800G3/2D3/4, where G is the specific gravity, D is the screw diameter in inches, and P is the allowable withdrawal load in pounds per inches of thread penetration depth, and includes a factor of safety of 4.

References:Photovoltaic Systems, p. 274-275Marks’ Standard Handbook for Mechanical Engineers, 8th Ed.

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Lag screw load calculations are based on point loads, diameter of the screw, thread embedment depth and type of lumber. Note that lag screws longer than 1 inch are not threaded the full length of the shank.


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Many manufacturers offer universal hardware for installing all types and sizes of flat-plate PV modules on racks, poles, trackers, and on many types of roofs and other building surfaces. This greatly improves standardization, and reduces design, materials, and labor costs associated with installing PV arrays.

Reference: www.unirac.com

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Reference: www.prosolar.com

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References · Photovoltaic Systems, p. 260-267 10 - 12 Fixed-tilt PV arrays are non-movable...

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