Photovoltaic Mechanical Integration

Objectives:

• Identify the key considerations for integrating arrays on buildings and other structures.

• Understand the key factors involved in choosing a mounting system.

• Differentiate between the various types of mounting configurations and their features.

• Differentiate between the various types of attachment methods.

• Compare the various types of structural loads on arrays and the factors that affect each type.

Ill. 1. Aerial lifts are sometimes required to reach roofs or areas with poor accessibility.

MECHANICAL CONSIDERATIONS:

There are a number of factors to consider in the mounting of PV arrays and other system equipment, and they often depend on the specific application, site conditions, components, and the priorities of the owner. These criteria identify areas of concern, opportunities for enhancement, and possibilities for various products, materials, and installation techniques, helping the designer or installer select the best type of installation for an application.

Physical Characteristics

Some of the first considerations in the mechanical design of an array are the physical characteristics of the modules, such as size, weight, laminate composition, frame type, and mechanical load ratings. The electrical characteristics of modules also influence the array's physical size and mechanical configuration, which affect the choice of the mounting system as well.

Smaller modules with lower output volt age require more connections and hardware to achieve a given nominal power output. Larger modules may require more sophisticated procedures or tools, such as lifting equipment, but they typically involve less installation cost, time, and labor, especially for larger systems. Using larger modules also reduces the number of modules needed for a given power requirement, which reduces the number of connections and , consequently, the number of junction boxes, conduits, conductors, and other balance-of- system (BOS) components. However, there are practical size limitations based on the manufacturing process, effort required for installation, and the mechanical loads permitted on the array structure and attachment points.

Structural Support

Regardless of the type of structure used to sup port an array, such as a roof, wall, foundation, or even the ground, the strength and rigidity of the structure must be evaluated. This is especially important when installing a PV system as a retrofit, since the structure was not originally designed to support such a system. Most modem buildings are designed and constructed with a significant margin of safety that easily encompasses the relatively small additional loads from a PV system. However, structural evaluation may be required for permit applications, older buildings, or those with nonstandard construction. Analysis of structural loads and fastener strengths also helps determine the best attachment method to use when installing an array.

Accessibility

Accessibility to all parts of a PV system is an important consideration in system planning and design. The selection of an appropriate array mounting system, the location of equipment, and the overall layout of mechanical and electrical components all affect accessibility.

Arrays are most often installed on roofs. These installations frequently involve climbing ladders, operating lifts and buckets, or working at heights where fall protection is required. See Ill. 1. The safest and most practical means to access the array and other equipment should be identified and , for larger commercial installations, provisions such as permanent ladders or stairways, access covers, tie-off points, and other safety features should be considered.

Arrays mounted on roofs may extend all the way to the roof edges and have little or no space between modules. While this may improve the appearance of arrays on some buildings, it compromises accessibility and may result in higher wind loads. Adequate space to safely install, inspect, clean, remove, replace, or maintain the array should be considered.

Access to attics or other spaces directly beneath roof surfaces must also be considered. Some mounting systems do not require access to these spaces for attachment, but it may be necessary to route electrical conductors through these areas. Attic spaces can be difficult to work in because of high temperatures and lack of working space, so this work should be carefully planned.

Also, if an array mounting system is de signed for seasonal tilt adjustment, it is desirable to have convenient access to adjustment points. Owners may be less inclined to make tilt adjustments if access is limited.

Thermal Effects

Temperature affects the electrical performance and lifetime of arrays. Additionally, arrays mounted on a building may impact the building's thermal loads, which should be considered when mechanically integrating PV systems.

Array Performance. Temperature is an important consideration in array mechanical design. High temperatures reduce the power output of crystalline silicon modules. Less is known about thermal effects on module lifespan or on thin-film PV cells, particularly after several years of exposure, but cooler arrays generally are more reliable, last longer, operate with greater efficiency, and produce more power.

For these reasons, array temperatures should be minimized wherever possible. Active cooling means, such as fans and water-circulating pumps, may be used with some concentrating arrays, but are not practical for flat-plate modules. Only passive cooling means are employed for flat-plate modules, such as mounting the array in a way that allows air circulation around the modules. Mounting system design and installation that allows natural cooling is the principal means of preventing the array from operating at excessively high temperatures.

Keeping modules and arrays clear of obstructions is the easiest way to promote natural cooling. This may include trimming nearby vegetation and avoiding blocking or restricting the open spaces underneath modules with flashings, plates, or other barriers to airflow. Aligning module support channels with the slope of the array or roof surface is a simple way to channel natural airflow, which helps remove heat from the array. Alternately, arranging modules so that the array's longer dimension is lateral (landscape layout) results in lower temperatures because the heat has a shorter distance to escape from under the array. See Ill. 2. These recommendations may not be feasible for some installations, such as building- integrated designs, but tradeoffs between installation design and thermal performance must be considered.

Passive-Array Cooling: Ill. 2. Several passive techniques can be used to keep arrays cool, which improves array performance. UNOBSTRUCTED SPACE BETWEEN MODULES and ROOF SURFACE ALLOWS AIRFLOW; LANDSCAPE LAYOUT FACILITATES HEAT ESCAPE

The temperature-rise coefficient specifies how the tempera turn of PV cells in an installation increases with ambient temperature and irradiance. The installed nominal operating cell temperature (INOCT) is an estimate of the normal temperature in these situations

The installed nominal operating cell temperature (INOCT) is the estimated temperature of a module operating in a specific mounting system design. This value is based on the module manufacturer's rated nominal operating cell temperature (NOCT) for a particular module, but also takes into account how the mounting method affects module temperature. Some mounting systems affect cell temperature more than other systems. System designers may use a temperature-rise coefficient or INOCT reference values in array calculations. In the field, installers can directly measure cell temperature.

Building Thermal Loads. The impact that an array can have on the heating and cooling loads of a building is an often-overlooked aspect of array mechanical design. Arrays may affect heat transfer into conditioned spaces, depending on the roof type, mounting system, and other factors.

Some arrays radiate additional heat into a building. The array absorbs heat energy from direct radiation and conducts it through the roofing materials to the underside of the roof surface. There, the energy heats the interior spaces of the building. This can be a benefit in cold climates, but is a disadvantage in warm climates with heavy air-conditioning loads. Large, well-ventilated attic spaces with adequate insulation can moderate the heat gain into conditioned spaces. Radiant barrier materials applied to the underside of the roof surface can also reduce the heating effect.

Conversely, arrays mounted above building surfaces have little effect on building heat gain, or may even reduce interior building temperatures. These modules shade part of the roof from direct radiation, while the space between the modules and the roof surface keeps the modules from transferring much heat to the roof and allows wind to cool the roof surface. Roofs in these installations are cooler, which reduces heat transfer into conditioned spaces.

Electrical Equipment. Temperature effects must also be considered in the installation and mechanical integration of electrical equipment, including inverters, batteries, and other BUS hardware. Equipment temperatures are usually managed by selecting the best locations for installing the equipment.

Most inverter designs use passive heat sinks or fans to protect critical components. Manufacturers provide specific recommendations for mounting and locating inverters, including instructions for minimizing operating temperatures by avoiding other heat sources, areas where air circulation is obstructed, and direct sunlight.

Batteries are also very sensitive to temperature, both heat and cold. Batteries may be buried in underground containers to mitigate temperature extremes, or installed in enclosures that are well insulated, ventilated, shaded, or possibly air-conditioned.

Conductors in conduits exposed to direct sunlight are a particular concern with PV systems, especially conductors used for array and source circuits, because they may experience temperatures exceeding 60°C (140°F). Locating conduit under roof eaves, beneath the array, or inside the building space are ways of reducing conductor temperatures.

Neat and organized installation makes for an aesthetically pleasing array.

Thermal Loads: Ill. 3. Modules mounted directly on the roof surface increase the heat transfer into a building. ATTIC OR CONDITIONED SPACE.

Aesthetics:

While the outward appearance of arrays and overall installations has little to do with system functionality or performance, it has a notable influence on consumer acceptance of PV technology. Because the array is the most visible part of any PV system, unattractive array installations may lead to poor public perception of PV systems and unwillingness of others to implement such systems on their own buildings or properties. In many cases, the appearance of an array presents tradeoffs with other system design and installation issues. Design considerations must be appropriately balanced with respect to safety and performance. A number of architectural principles may be applied to PV system design and installation that can improve appearance and public perception without significantly affecting cost or performance.

For arrays mounted on sloped roofs, the lines and location of the array should be consistent with building features. Arrays should be mounted parallel to the roof surface, centered and square with the rooflines and edges. Modules should be of the same size and shape and aligned in the same direction. Roof obstructions, such as roof vents, chimneys, or air conditioners, that interrupt contiguous groups of modules should be avoided if possible. Flat rooftops are less visible from the ground and often use racks to tilt the array.

Aesthetic Installations: Ill. 4. PV systems that match the shape, color, and /or alignment of the mounting surface produce aesthetically pleasing installations. Some PV modules are actually windows that produce electricity while letting a portion of sunlight into the building A further innovation on this idea uses photochromic technology to change the opacity of the window material and regulate interior natural lighting

Color may be a consideration in choosing and integrating modules. The color of PV cells is generally determined by the manufacturing technology, ranging from dark red to gray to bright blue. Sometimes module manufacturers offer color choices for the module frame. If choosing the color of modules is not an option, the colors of building features maybe adjusted to blend with the array.

Reflections and glare from arrays can cause annoyances or serious safety issues, affecting nearby buildings, people, traffic, and even air craft. Reflections can be especially problematic for highly tilted arrays at northern latitudes, when the sun is low in the sky. However, glare is typically limited to specific points of view and times of the day and year. If reflections are a concern, the installer can evaluate solar incidence angles to identify the locations and times with significant glare issues to determine the best mounting option.

Finally, the quality of workmanship on the overall system installation affects the aesthetic appearance of PV systems. Array support structures, hardware, and electrical wiring, conduit, and junction boxes should be as inconspicuous as possible, neatly gathered or concealed beneath the array. The routing of conduits or conductors from the array to other components should be as inconspicuous as possible, especially in locations where it passes through a roof or eave. Installation of a PV system and related equipment in a neat and professional manner improves the overall appearance of the system.

Costs

The mechanical integration and structural installation of an array is often the largest variable in overall system costs, ranging from as little as 10% to more than 40% of initial system costs. Custom, site-specific designs can incur especially high costs, due to additional engineering and architectural requirements and nonstandard installation. However, the use of standard mounting designs and installation techniques is becoming more common, which helps keep mechanical integration costs down. Mechanical integration costs are approximately proportional to the physical area of the array. Consequently, modules and system designs that are more efficient, and therefore require less area, reduce overall system costs. Other ways to reduce costs for mechanical integration may require a tradeoff with desirable features. For example, alternative materials may cost less than standard materials, but may also be less resistant to corrosion.

Reducing installation time by careful preparation can significantly reduce costs. For ex ample, it is faster and easier to assemble panels or subsystems on the ground before installing them on the roof. Assembling components on the ground is also safer.

Pre-assembly: Ill. 5. Assembling PV subsystems such as panels before lifting them to the roof is often easier and reduces installation time.

ARRAY MOUNT SYSTEMS

Module Mounting Systems: FIXED-TILT MOUNT; ADJUSTABLE-TILT MOUNT. Ill. 6. Mounting systems may hold modules at a fixed tilt, or may allow adjustments to be made to the tilt for greater solar energy gain.

An array mounting system must securely hold modules in the most favorable orientation possible. Several types of mounting systems have been developed to meet these requirements for a variety of applications.

Most flat-plate modules have a similar construction and are secured by their frame, either with fasteners through factory-drilled holes or by clamping the module frame to a structural support. Several manufacturers offer universal hardware for installing most types and sizes of modules onto a variety of mounting systems. This standardization makes PV systems versatile, in creases installation options, and reduces the costs associated with design, materials, and labor.

The simplest and most common type of array mount for modules is the fixed-tilt type. A fixed-tilt mounting system is an array mounting system that permanently secures modules in a non-movable position at a specific tilt angle. An adjustable mounting system is a variation of a fixed-tilt array mounting system that permits manual adjustment of the tilt and /or azimuth angles to increase the array output. See Ill. 6. The tilt angle is decreased for the summer when the sun is higher in the sky, and increased for the winter when the sun is lower. Manual tilt adjustments are usually made monthly or seasonally.

Fixed-tilt and adjustable-tilt mounting systems are further differentiated by whether they mount to a building or the ground, and by the design of the support structure. Each type of mounting system has advantages and disadvantages in terms of cost, performance, ease of installation, and maintenance, and may affect the amount of solar energy received on the array surface.

Building Mounting Systems

Rooftops often offer the best opportunities for the installation of arrays on buildings, because they are usually large unused spaces that are high enough to avoid significant shading from nearby obstructions. Direct mounts, roof rack mounts, and standoff mounts are all designs for mounting arrays on buildings.

Direct Mounts. A direct mount is a type of fixed-tilt array mounting system where modules are affixed directly to an existing finished rooftop or other building surface, with little or no space between a module and the surface. See Ill. 7. Direct mounting of conventional flat-plate modules is not recommended because the lack of cooling from natural airflow (breezes) results in high array operating temperatures. INOCT for direct-mounted modules is the highest among building mounting systems. Temperature-rise coefficients for direct mounts can be as high as 40 to 50°C/kW/m (72 to 90°F/kW/m Accessibility to individual modules for installation and maintenance can also be a problem with direct mounts, though their low profile can produce very attractive installations.

Ill. 7. Direct mounts have little or no space between the modules and the mounting surface.

Roof Rack Mounts. A rack mount is a type of fixed- or adjustable-tilt array mounting system with a triangular-shaped structure to increase the tilt angle of the array. Rack mounts are used on flat or low-pitched roofs, as well as for ground mounts. While rack mounts may be more expensive than other common mounting designs, they are simple to install and are flexible in the module types and tilt angles they can accommodate.

Since rack mounts use an open, trusslike structure, air can circulate freely around the modules and keep them cool. INOCT for rack- mounted arrays is the lowest among building mounting systems, and temperature-rise coefficients are as low as 15 to 20°C/kW/m (27 to 36°F/kW/m This type of structure also allows access to the back surface and electrical terminations of the modules, simplifying installations and maintenance. However, because of their higher profile, rack mounts may experience higher wind loads than do arrays mounted closer to building surfaces.

In some cases, rack mounts may be used on pitched roof surfaces to increase the tilt angle of the array above the roof pitch. If a roof does not face south, rack mounts may be rotated from the east or west to face the sun and increase solar energy received. However, racks mounted obliquely to surfaces should be avoided unless no other options exist.

Standoff Mounts. A standoff mount is a type of fixed-tilt array mounting system where modules are supported by a structure parallel to and slightly above the roof surface. See Ill. 9. Most standoff mounts are designed to hold modules to support rails that are attached to the roof by brackets. Standoff-mounted arrays are by far the most common, preferred, and least- expensive method for installing arrays as a retrofit to existing rooftops, as well as for new construction.

Ill. 8. Roof rack mounts secure modules on a triangular trusslike structure that mounts to flat or low-tilt roofs.

Ill. 9. Standoff mounts allow several inches of space between the modules and the mounting surface.

Standoff mounts allow air to circulate beneath the array, keeping modules cool and reducing heat gain into buildings. INOCT for standoff arrays is a function of the standoff height. Standoff heights from I" to 3" have a high INOCT, standoff heights from 3" to 6" are somewhat cooler, and standoff heights above 6" are the lowest INOCT for standoff arrays. However, increasing standoff height also increases wind loads and may adversely affect the aesthetic appearance. In general, standoff mounts should be installed between 3" and 6" from the top of the module and the roof surface. Temperature-rise coefficients for this height range are around 20 to 30°C/k W/m (36 to 54°F/kW/m Like most types of mounting systems, standoff arrays are built in a modular fashion by assembling portions of the array into electrical and mechanical panel units, and integrating the assemblies together on the rooftop. These designs also provide reason able access to the underside of the array for inspection, troubleshooting, and maintenance, by detaching a few module fasteners.

Standoff mounts are attached directly to building structural members, such as roof trusses or rafters, never to decking. In most cases, standard standoff mounting hardware allows for direct attachments from above the roof surface, but some designs may require access to attics or spaces underneath the roof and array. This ensures a strong and reliable structural connection, but can be more time consuming and costly to install.

Structural attachments for smaller standoff panels may use four attachments at corner points, while larger panels might use six or more attachments distributed across the en tire length of the panel. One disadvantage of standoff mounts is the relatively large number of structural attachments required, typically around 4 to 6 attachments for every 20 ft to 40 ft of array surface area. Each attachment is a roof penetration that must be properly sealed against weather.

Building-Integrated Systems

A building-integrated photovoltaic (BIPV) array is a fixed array that replaces conventional building materials with specially designed modules that perform an architectural function in addition to producing power. BIPV arrays can be integrated into roofing, windows, skylights, curtain wall sections, or complementary architectural features such as awnings, facades, or entranceways. See Ill. 10. Some BIPV systems may replace only the outermost building material, such as roof shingles, while others may replace the entire thickness of a portion of building shell, such as the entire assembly of roof shingles, substrate, and decking. BIPV arrays are most often incorporated into new construction for large commercial buildings, but can be used in residential and small commercial structures as well. Some designs may be available as retrofits.

The cost of the array is partially offset by avoiding the costs of some of the conventional building materials, though high engineering and architectural costs often outweigh the savings. However, BIPV arrays have favorable aesthetic and architectural features because they are so thoroughly integrated into the building's appearance.

Of the many possibilities for BIPV arrays, roofing replacement has been one of the most popular, since roof surfaces are generally well oriented to receive solar energy. Common BIPV roofing products have been designed to replace asphalt shingles, slates, tiles, and metal roofing. In addition, roofing is often replaced at least once during a building's life, making BIPV retrofits an attractive option for reroofing.

Windows, skylights, and other building surfaces are also possible BIPV applications, though they are not as popular because they are often not optimally oriented. Another challenge is that BIPV windows and skylights must have some degree of transparency to retain their dual function. Some use standard cells spaced farther apart in a transparent laminate, utilizing the clear spaces between the cells to allow some light through. However, the low cell density reduces the overall area efficiency. Thin-film products can also be modified to various levels of transparency by changing the backing material. However, since much of the light then passes through, the efficiency of these modules is also reduced. There are many opportunities to improve BIPV module designs, including more efficient modules or new technologies in fully transparent or semi transparent PV materials.

Standoff mounting systems often consist of long rails that hold the array together as a rigid structure and create a gap between the roof and the modules for airflow.

Ground Mounting Systems

PV Arrays: SHINGLES, WINDOWS, SKYLIGHTS, AWNINGS: Ill. 10. PV modules can be integrated into building exteriors as roof shingles, windows, sky lights, awnings, and many other structures.

Arrays are often ground mounted when suit able space on a roof is not available, or for stand-alone remote-power applications where no alternative exists. Arrays can also be in stalled on structures other than buildings, such as navigational aids, communications towers, signs, bridges, trailers, and vessels. Common ground-mounted array designs include rack mounts and pole mounts. Sun-tracking systems are also usually mounted on the ground.

Ground-mounted arrays generally offer more flexibility than building-mounted arrays in the location and orientation of arrays, allowing for optimal solar energy gain. These systems operate at lower temperatures than do most building mounts, due to greater airflow around the array. Ground-mounted modules are also more accessible for adjustments and maintenance. As with any array, ground- mounted arrays must be appropriately secured to resist wind loads and other forces acting on the array and its attachment points.

Ground Rack Mounts. Rack mounts are extremely versatile in securing modules in any orientation and tilt, and are used in both building and nonbuilding mounting systems. Rack- mounted arrays can be installed on poles, walls, directly on the ground, or on an intermediate superstructure between the rack and the ground. They can also be installed in multiple rows for larger arrays. For rack-mounted arrays on the ground, the array support structure is generally anchored to the ground with concrete piers, footers, earth anchors, or a foundation of pres sure-treated timbers buried in the ground. See Ill. 11. Wood construction is generally avoided in PV system installations, but high- quality pressure-treated timbers or railroad ties may provide adequate long-term performance for mounting foundations. However, local fire codes must be consulted in these situations because this practice may not be allowed in some jurisdictions.

Since rack-mounted arrays on the ground are lower than roof-mounted arrays, shading may be more of a concern due to nearby trees, vegetation, fences, buildings, and other obstructions. Also, arrays installed at ground level may present a safety hazard to the public, or may be more vulnerable to incidental physical damage or vandalism. If ground-mounted arrays need protection, chain-link fencing is preferred over solid fencing to maintain ad equate air circulation around the array.

Pole Mounts. A pole mount is a type of array mounting system where modules are installed at an elevation on a pedestal. See Ill. 12. Pole mounts may be fixed tilt, adjustable tilt, or sun tracking, and are typically used for ground mounts or attached to other structures. They are generally not used on buildings. Since pole-mounted modules can be placed at a significant height, they are often protected from harm and positioned above the shadows of nearby obstructions.

Ground Rack Mounts: Ill. 11. Ground rack mounts are versatile designs that can accommodate both large- and small-scale installations.

Ill. 12. Pole-mounted arrays can be used in a variety of applications, such as lighting, communications, water pumping, and signage. Pole mounted arrays can be attached to either the top or the side of a pole Mounting an array at the top of the pole allows more versatility in array orientation

Pole mounting systems often allow changes to the azimuth orientation by rotating either the array or the pole. Care should be taken to ensure that the pole itself, or any other arrays or equipment located on the pole, will not shadow the array at any critical time, especially when the sun is in the northern part of the sky in morning and afternoon in summer.

Disadvantages for pole-mounted arrays include inherent limitations on the size of array that can be installed on a given pole, and that special lifting equipment is often needed to install, inspect, and maintain the array. De pending on the strength of the pole, the size of the array, and its height above the ground, a pole may be set directly into an auger hole and the soil compacted, or it may be necessary to set the pole base on a concrete foundation, and secure it with fasteners to the steel reinforcement in the concrete.

Pole-mounted PV systems may also contain lights, weather stations, communications equipment, security cameras, or other devices, and these may or may not be powered by the array. For example, a stand-alone PV lighting system may include the array, battery, controller, and light fixture all on one pole. Alternatively, a self-contained unit including a small array, battery, and security camera may be installed on an existing commercial light pole.

[The insolation values for both single-axis and two axis sun- tracking surfaces can be found in the solar radiation data set for the nearest location. This makes it very easy to compare the potential gain in power output from sun tracking to fixed orientation surfaces]

Sun-Tracking Systems

Single-Axis Tracking: VERTICAL-AXIS TRACKING; EAST-WEST TRACKING: Ill. 13. Single-axis tracking mounts rotate one axis to approximately follow the sun as it moves across the sky.

Two-Axis Tracking: ALTITUDE-AZIMUTH TRACKING; EQUATORIAL TRACKING. AUTOMATED TILT-AXIS (ALTITUDE) ROTATION; -AUTOMATED VERICAL-AXIS (AZIMUTH) ROTATION Ill. 14. Two-axis tracking mounts rotate two axes to exactly follow the sun as it moves across the sky.

Fixed- and adjustable-tilt mounts are the easiest to install and maintain, but they sacrifice energy gain because the array is not always in the optimal orientation to the sun. Sun-tracking arrays can follow the path of the sun, enhancing the amount of energy collected. However, they are complex systems and require a greater investment in expense and installation time.

A sun-tracking mount is an array mounting system that automatically orients the array to the position of the sun. This can increase annual solar gain by as much as 40% in some areas when compared to a fixed-tilt mounting design. Sun-tracking mounts are classified according to the number and orientation of the axes rotated to track the sun, and by the means of rotation.

A single-axis tracking mount rotates one axis to approximately follow the position of the sun. The mount may rotate the vertical axis, which changes the azimuth angle, or the north-south axis, which allows the array to follow the sun from east to west. See Ill. 13. Most single-axis tracking mounts can also be manually adjusted for tilt. Single-axis tracking mounts can be mounted atop vertical poles or on horizontal supports in long rows.

To exactly follow the sun's position (solar altitude and azimuth), two-axis tracking is required. A two-axis tracking mount rotates two axes independently to exactly follow the position of the sun. See Ill. 14. Two- axis tracking maximizes the amount of solar energy received. Altitude-azimuth tracking uses the vertical and tilt axes. The vertical axis rotates to follow the sun's azimuth angle and the tilt axis rotates to follow the sun's altitude. Equatorial tracking uses the north-south and tilt axes. The north-south axis rotates to follow the sun from east to west in an arc. The tilt axis needs to be activated to adjust the tilt only periodically throughout the year. Two- axis tracking mounts are generally mounted atop vertical poles to allow sufficient room for movement.

The movement in trackers is produced by either active or passive means. An active tracking mount is an array mounting system that uses electric motors and gear drives to automatically direct the array toward the sun. Active tracking mounts may track in either one or two axes. The tracking direction is determined by a computer calculating the sun's position, or with sun-seeking sensors. In order for active tracking mounts to be effective, the solar energy gained by tracking must more than compensate for the higher system cost and additional electrical energy used by active tracking motors.

A passive tracking mount is an array mounting system that uses nonelectrical means to automatically direct an array toward the sun. Refrigerants can be used to move the mount, because they vaporize and expand when heated by the sun. The expanding fluid causes the tracker to pivot toward the sun as the weight of the fluid shifts from one side of the tracker to the other. An alternate design uses refrigerant to operate a hydraulic cylinder and linkage arrangement. The control of the tracker direction for both designs uses sunshades to regulate the heating of the fluid by the sun. Passive tracking mounts typically control only one axis.

The value of a tracking array depends on whether the additional energy produced offsets the added cost and complexity of the equipment. The energy gain must also offset the increased maintenance and troubleshooting likely with these systems. The moving parts may require periodic calibration and lubrication.

Utility-scale systems often use tracking systems to maximize array output. Tracking results in the greatest energy enhancement in the summer, when days are long, particularly in high latitudes. Trackers are less beneficial in the winter, when days and sun paths are short.

MECHANICAL INTEGRATION

Galvanic Corrosion: Ill. 15. Galvanic corrosion can occur when two dissimilar metals are in contact with each other. Sacrificial Anodes: Ill. 16. Sacrificial anodes are more prone to galvanic corrosion than the metal they protect, so they corrode first.

Based on the site survey, structural evaluation, customer requirements, and other considerations, a suitable mounting system is chosen for an array. The installer must then consider several factors in its mechanical integration, including selecting the appropriate materials and attachment methods based on the location and an analysis of the structure or foundation.

Materials

Because PV systems are exposed to outdoor elements, materials for structural supports and other system hardware must be chosen to withstand environmental conditions with out degrading. Degradation can increase maintenance costs, decrease functionality and serviceability, affect overall appearance, and create an unsafe situation. Any materials used should match the expected 20- to 30-year service lifetime of the overall system under the given site conditions. Wooden exterior structures are usually avoided because they may degrade in less time than PV systems are expected to last. However, wood may be used in interior spaces, such as attics, to help sup port arrays mounted on the roof, and pressure- treated wood may be used as primary ground support for stand-alone rack-type arrays.

Corrosion can make servicing and adjustments difficult. Wherever possible, corrosion- resistant materials should be used in all parts of the array structure. Corrosion is most prevalent in hot, humid, and marine climates, and corrosion rates may be 400 times higher in southern coastal areas than in the arid desert locations. Stainless steel alloys 316 and 403 are recommended for most fasteners, particularly in humid and marine climates, while hot-dipped galvanized or coated steel fasteners and structural members are acceptable in drier climates. Aluminum structural alloys 6061 and 6063 are commonly used in array designs because they are particularly corrosion resistant, lightweight, and relatively inexpensive.

Galvanic corrosion results from direct con tact of dissimilar metals. Galvanic corrosion is an electrochemical process that causes electrical current to flow between two dissimilar metals, which eventually corrodes one of the materials (the anode). See Ill. 15. The rate of corrosion depends on the properties of the two metals in contact, as well as temperature and humidity. Aluminum (module frames) and steel (mounting structures) combinations are particularly prone to galvanic corrosion. Galvanic corrosion can be mitigated by electrically insulating the metals with rubber or fiber materials or by adding sacrificial anodes.

A sacrificial anode is a metal part, usually zinc or magnesium, that is more susceptible to galvanic corrosion than the metal structure it is attached to, so that it corrodes, rather than the structure. See Ill. 16. Therefore, sacrificial anodes must be replaced periodically. However, they are typically only needed in the most extreme marine or coastal environments.

Certain types of rubber compounds may degrade quickly under UV exposure, although EPDM or butyl rubber materials offer good performance and can be used to isolate dissimilar metals. Anti-seize compounds applied to threaded fasteners retard corrosion and make the fasteners easier to assemble and remove later. Also, any electrical conductors and other electrical components exposed to sunlight should be explicitly labeled for UV resistance.

Structural Loads

Arrays, modules, mounting systems, fasteners, and buildings must withstand the maximum forces expected from several types of structural loads. Structural loads on arrays are deter mined by the size, weight, and orientation of the modules, prevailing site conditions, and other application-specific issues. The principal types of structural loads are dead loads, live loads, wind loads, snow loads, and vibration loads. These loads are either static (constant) or dynamic (changing).

Depending on the location, some loads may be more significant than others. In Florida, for example, wind loads are most important because the area is prone to hurricanes, while snow loads are nonexistent. In the Midwest, however, snow loads may be significant.

Most structural loads are based on unit area, and are typically represented in pounds per square foot (psf or lb/ft^2). For noncritical applications, the load is generally assumed to act evenly over the entire area. See Ill. 17. When the load pressure must be applied to a set number of points, such as the attachment points of an array, the load is divided between the points.

Arrays and their attachment points must be designed and installed to withstand the forces from a combination of structural loads. A design load is a calculated structural load used to evaluate the strength of a structure to failure. Each type of structural load has a design load, which is a calculated estimate of the load the structure must withstand. Design loads are estimated because the actual load can sometimes only be measured after the structure is built. Some types of design loads, such as wind load, are much greater than the actual average load because the source of the load is highly variable and structures must withstand the worst-case scenarios. Other types of loads, such as dead loads, can be calculated from the actual weight of building materials, which is constant, so the design dead loads are usually only a little higher than actual dead loads.

Local building codes dictate the requirements for addressing structural loads, though most use the same building standards and have similar requirements. The American Society of Civil Engineers (ASCE publishes the governing standard on structural loads used in most building codes throughout the United States. ASCE 7-05 minimum. Design Loads for Buildings and Other Structures.

Many installations take advantage of pre engineered mounting systems or complete PV system packages, which have already been analyzed for potential structural loads. If installed according to the manufacturer's recommendations, these structures are designed to withstand all types of structural loads under common conditions. However, custom or altered designs may require independent engineering or testing to certify compliance to local building codes. In either case, the installer might not be responsible for a detailed structural analysis, but should be familiar with the factors affecting loads and understand when to consult with engineering professionals.

Structural Loads: Ill. 17. Most structural loads are specified as a force per area. When the area attaches to other structures at certain points, the load is divided between the points.

LOAD AT ATTACHMENT POINTS

[Safety Factor: The ratio between an expected service load and the corresponding actual failure load is the safety factor. Many formulas, particularly those having to do with structural loads and occupied buildings, include safety factors in the calculations. A safety factor in structural calculations is a multiplier used to increase the design load far above normally expected loads. This means that a structure built to withstand high design loads will easily withstand smaller loads. This also gives the structural design a large margin of safety that should cover worst-case scenarios.

The safety factor may be explicitly given in the formula, such as with a variable, or may be embedded in the formula in a way that is not easily discernable. For example, the allowable withdrawal load for lag screws includes a safety factor of 4.5. That is, the stated allowable load for the screws is less than one-quarter of the actual failure load. For example, if a lag screw has an allowable withdrawal load of 100 lb/in., it can actually handle loads up to 450 lb/in. before the attachment fails. Safety factors in structural design are always equal to at least 4, and may be higher for applications with more significant consequences of failure.]

Low tilt angles reduce power output at high latitudes, but also reduce wind loads.

Dead Loads and Live Loads: RESULTING FORCES ON ATTACHMENT POINTS; WEIGHT OF ITEMS and PEOPLE TEMPORARILY ON STRUCTURE; Ill. 18. Dead loads result from the weight of arrays and permanent components. Live loads are caused by the weight of people and /or items that are temporarily on the structure.

Ill. 18. Dead loads result from the weight of arrays and permanent components. Live loads are caused by the weight of people and /or items that are temporarily on the structure.

Dead Loads. A dead load is a static structural load due to the weight of permanent building members, supported structure, and attachments. See Ill. 18. In the case of PV systems, the dead load is equal to the combined weight of the modules, mounting structure, and BUS components, divided by the area of the array. Less weight or greater area decreases the dead load. Since it is based on weight, a dead load acts only downward. Dead loads affect the mounting structure of the array and the building or foundation to which it is attached. Dead loads are often the smallest of the structural loads. Based on typical weights of modules and support structures and their installation density, dead loads of PV systems are about 5 psf to 10 psf.

Live Loads. A live load is a dynamic structural load due to the weight of temporary items and people using or occupying the structure. The live load is equal to the combined weight of all temporary objects, divided by the area of the array. Less weight or greater area decreases the live load. A live load also acts only downward. Generally, live loads on arrays are infrequent and minimal, such as from maintenance equipment or a person briefly leaning on a module. Many live loads to mounting systems can be avoided by using scaffolding or bucket lifts to access the array, or installing modules with enough space between them for access. When unavoidable, live loads can be reduced by distributing a weight over a greater area by using temporary platforms across modules. With these practices, live loads are often as low as 5 psf to 10 psf.

Wind Loads. A wind load is a dynamic structural load due to wind, resulting in downward, lateral, or lifting forces. See Ill. 19. Wind loads are typically the most significant of all the types of structural loads, and can range from 25 psf to over 50 psf. Wind loads can be especially large in coastal areas. Local building codes may have a minimum design wind load, often 10 psf, even if calculations indicate lower loads.

Calculating design wind loads involves complex formulas including many variables, some of which are not easily quantified. Mounting system manufacturers usually provide guidance on these calculations with respect to their mounting type and common PV modules. However, for most PV system applications, three primary factors influence wind loads: wind speed, exposure, and array tilt.

The basic wind speed is the maximum value of a 3 sec gust at 33' (10 m) elevation, which is used in wind load calculations. The local building code authority may provide this information, or it can be determined from a map. See Ill. 20. Low wind speeds are typical for interior regions, while wind speeds up to 150 mph are used in calculations for coastal areas prone to hurricanes. Special wind regions in certain mountain areas and other areas prone to high winds require more information from the local AHJ. Higher wind speeds increase the wind load on surfaces.

Ill. 19. The wind-load forces at attachment points can be downward, lifting, or lateral forces, depending on wind direction and the orientation of the array. Wind Loads: WIND EXERTING [ ON STANDOFF-MOUNTED ARRAY; - RESULTING LIFTING FORCES; RESULTING LATERAL FORCES

Exposure is a wind load factor that accounts for the array height and the characteristics of the surrounding terrain. For example, urban or suburban areas, wooded areas, or other areas with closely spaced obstructions reduce the wind forces on surfaces. Conversely, flat unobstructed areas, such as grasslands and water surfaces, increase potential wind loads.

The tilt angle of a surface greatly affects potential wind loads. Wind loads are small for array tilt angles around 20°, and increase with larger tilt angles up to 90° (vertical). Large wind loads also occur at low tilt angles of 10° to 15°. Even though the array may tilt away from the wind direction (like a wedge), an array at low angles may act like an airfoil and experience lifting forces from the wind.

Ill. 20. Basic Wind Speeds: Basic wind speeds are region-specific and are highest in coastal areas prone to hurricanes. NUMBERS INDICATE BASIC WIND SPEED IN MPH.

The location of the array, and therefore the basic wind speed, can't easily be changed to decrease wind loading. However, several other methods can be used to reduce wind loads. Minimizing the profile height for rack- mounted arrays, perhaps by using multiple short racks instead of a single tall rack, and lowering the overall altitude of the array reduces the exposure to the wind. Avoiding tilt angles greater than the roof pitch and locating arrays near the center of roofs, away from the edges, may also help. Arrays with the longer dimension oriented along the wind direction, as opposed to across it, experience lower wind loads. Keeping standoff heights low and undersides clear of obstructions are additional means of minimizing wind loads.

[Wind load calculations include a topographic factor that ac counts for the acceleration of wind over steeply sloping terrain, resulting in increased wind loads This factor may affect PV installations near sharp changes in ground elevation, such as or escarpments]

Snow Loads. A snow load is a static structural load due to the weight of accumulated snow. Snow loads cause forces similar to dead loads, but the magnitude can vary greatly according to amount of snow.

Significant snowfall is expected only at high latitudes, where arrays typically have large tilt angles to maximize solar energy gain, and snow is less likely to accumulate on steeply sloped arrays. However, when the tilt angle is smaller because of roof geometry or for maximum summer energy gain, the weight of snow on an array can be significant. Ground-mounted arrays are also vulnerable because snow can accumulate or drift high enough to load the structure.

Some sunlight can pass through light snow into the modules, warming the cells, which usually melts the snow or causes it to slide off the array. If snow does not melt off in a reasonable amount of time, it should be promptly removed. Besides the structural loads from the weight, snow cover shades the array, severely limiting the electrical output.

Snow loads vary greatly by region. Snow load maps show the approximate loads for various regions of the United States. See Ill. 21.

However, in some areas snowfall is too variable to assign specific snow load values. Snow loads in such areas must be evaluated on a case-by-case basis to determine the most appropriate value for structural calculations.

Vibration Loads. A vibration load is a dynamic structural load due to periodic motion. Vibration loads can produce oscillations of varying magnitudes and frequencies, but the forces are most severe at the resonant frequency. Resonance is the condition when a vibration frequency matches the fundamental frequency of the structure. Structures can sometimes be designed to avoid naturally occurring resonant frequencies. Special shock-absorbing structural members can also be used to dampen vibration.

Vibration loads can be caused by nearby construction or heavy equipment, and , in certain regions, seismic activity. For most installations on permanent structures outside of seismically active regions, vibration loads will be minimal. However, portable arrays mounted onto trailers or vehicles are especially prone to vibration loads. In these cases, mounting systems may require special engineering.

Attachment Methods

A variety of attachment methods may be used to install arrays and other equipment on buildings and other structures. The type of mounting system, mounting surface, and anticipated loads determine the best type of attachment method. Structural attachments made directly to a building or other foundation must be able to withstand the expected mechanical loads that work to pull the mount from its foundation. Most designs use conventional threaded fasteners and screws, while others use special clamping fasteners to hold modules in position on support structures. Some arrays require no attachments at all to a building rooftop or the ground.

[Most modules are designed to be rigid, though loads will deflect the glass and frames slightly For each module model module manufacturers specify the maximum allowable deflection that does not result in damage Mounting system manufacturers use this information to recommend the amount of spacing between attachment points to avoid damage from deflection For a given load, spacing the attachment points closer together leaves less unsupported module area which decreases deflection.]

Snow Loads: Ill. 21. Snow loads cause forces similar to dead loads, but the potential magnitude of a snow load varies greatly among geographic regions. SNOW LOAD (PSF)

Individual modules or panels may be in stalled first on an intermediate support. The connection methods for intermediate supports are manufacturer-specific and those from reputable companies are well engineered for common structural loads. However, a primary connection between a bracket, plate, or structural angle and a building or foundation must be made. This connection method will often be chosen by the installer, though manufacturers may provide recommendations.

Mounting system attachments should be made through the roof cladding and into building structural members, such as rafters. Most mounting systems allow direct attachments from the exterior roof surface, though some methods or situations require access to attics or other under-roof spaces. This may occur when the locations of support brackets are limited and new structural members must be added to accommodate the attachment points. With rare exceptions, arrays should not be attached to plywood decking or other roof covering without also engaging structural members.

Lag Screws. Most mount brackets can be fastened into roof rafters directly from the exterior roof surface. Lag screws are the most common fasteners for attaching array mounts to rooftops since many roofs do not have access to the underside for other mounting methods. See Ill. 22. Lag screws look like very large wood screws with a hex-shaped head. Wrenches are used to drive lag screws into predrilled pilot holes.

Each lag screw must be properly secured into the roof structure. Most rafters are only 1½" wide, and determining the exact center of the rafters from above a roof can be difficult, particularly when rafters are not exactly parallel. The approximate locations can be found by hitting the roof surface with a hammer and listening for a change in sound. The centers of the rafters can usually then be located with a quality stud finder. If a lag screw continues to turn after seating a bracket, it is likely not embedded into the structural member and only sticking through the plywood decking. In this case, the hole must be sealed and the attachment point redone.

Lag screws are generally rated by their allowable withdrawal load. The allowable withdrawal load is the force required to remove a screw from a material by tensile (pulling) force only. Since the required force depends on how deeply the screw is embedded, the allowable withdrawal load is represented in pounds of force per inch of penetration depth. It also increases with the diameter of the screw and the density of the material. See Ill. 23. The penetration depth is the length of threaded portion of fastener embedded in a structural member. Common lag screw sizes are ¼", 3/16", and 3/8" in diameter and 3" to 5" long. Pilot holes are typically 60% to 75% of the screw nominal shank diameter. Larger pilot holes are required for harder woods like common truss lumber, than for softer woods like framing lumber. Tables provide allowable withdrawal loads for various woods and screw sizes.

Ill. 22. Lag screws are the most common type of fastener used to attach array mounting systems to wood structures, usually residential roofs. Lag Screws. THICKNESS OF ROOF DECKING and COVERING/-PENETRATION DEPTH;

[Calculating Allowable Withdrawal Load: For wood types not listed in tables, the allowable withdrawal load can be calculated from the specific gravity of the wood and the nominal screw diameter. Specific gravity is similar to density and relates to the wood's strength and resistance. The specific gravities of various types of woods can be found in engineering reference tables. Allowable withdrawal load can be calculated with the following formula:

p= 1800 where p = allowable withdrawal load (in lb/in.) = specific gravity of wood

D = nominal screw diameter (in in.) ]

Ill. 23. Allowable withdrawal loads for lag screws are greater with larger screw diameter, deeper thread penetration, and higher-density lumber. Allowable Withdrawal Loads*

When a lag screw is used to mount a bracket to a roof, it must pass through the combined thickness of the bracket, shingles or other roof covering, roof membrane, and decking, which can add up to more than 1". This means that roughly I" of the length of the screw is not embedded in the rafter. The thickness of these materials must be accounted for in the length of the screw and the thread penetration depth.

Bolts. Bolts are often used to secure modules to mounts, and mounting system manufacturers specify (or sometimes include) the best sizes and types of bolts for use with their products. Bolts or threaded rods can also be used to attach the mounts to a building or structure. The sizes and types of fasteners for this part of the installation may be left for the installer to choose.

Common bolts have a hex head and are se cured by a hex nut. Threaded rods are used when bolts are not long enough. Threaded rods are cut to the required length from long pieces, and fastened at both ends. Flat washers spread the load from a hex head or nut over a larger area, and lock washers or special lock nuts keep the fastener from loosening. Antiseize compounds are highly recommended for stainless steel lock nuts to prevent galling and cross-threading, and can facilitate servicing and disassembly as required. Bolts and threaded rods are classified by the mechanical loads they can handle. Typical bolt diameters include 1/4", 1/16", and 5/16".

Attaching mounts to a roof with bolts produces a stronger attachment than with lag screws, but requires access to spaces under the roof. Bolts can't be fastened into roof trusses or rafters like lag screws, but also should not be fastened to the roof decking alone. Blocking and spanning are techniques used to strengthen bolt attachments.

Blocking is the addition of lumber under a roof surface and between trusses or rafters as supplemental structural support. See Ill. 24. Boards for blocking are short pieces of 2 x 4 or 2 x 6 lumber that are nailed or screwed into the rafters with at least two fasteners on each side. By stacking two blocking boards, lag screws or bolts can be supported when required attachment points do not match rafter locations.

Spanning is the addition of lumber under a roof surface and across trusses or rafters as supplemental structural support. See Ill. 25. Spanners are only used for bolt-type fasteners, usually long threaded rods, and require intermediate blocking between the spanner and underside of the roof deck to keep the spanner from bowing under applied loads.

[Hand tools are recommended for tightening fasteners since power tools can strip threads. Mounting system manufacturers may also recommend tightening fasteners to certain torque specifications.]

If the mounting system requires future adjustment or disassembly, antiseize paste should be used on the fasteners. This paste also helps during initial assembly.

Ill. 24. Blocking is used to provide a structural member between roof rafters. Bolts with Blocking. ROOF DECKING and COVERING;

Ill. 25. Spanning is used to provide a structural member across roof rafters. Blocking boards are required to support the spanner.

Threaded Rod with Spanning. ROOF DECKING and COVERING

J-Bolts. While not as common as lag screws and bolts, J-bolts may be used to attach mounting brackets. An f-bolt is a fastener that hooks around a secure support structure and has a threaded end that is used with a nut to secure items. See Ill. 26. The use of J-bolts requires access to the underside of a roof, and they must be carefully sized to fit around the roof rafters or trusses. J-bolts are also less flexible in the placement of attachment points because the attachments must be precisely next to the truss or rafter.

Self-Ballasting. Self -ballasting is an attachment method that relies on the weight of the array, support structure, and ballasting material to hold the array in position. Self-ballasted systems do not require direct structural connections with fasteners to a building or foundation. A major advantage of self-ballasting systems is that there are no penetrations into the building surfaces, eliminating concerns about weather sealing of attachment points. They are also installed very quickly and use fewer fasteners. However, self ballasted systems must be installed on level surfaces and may be limited to regions without high basic wind speeds.

Typical self-ballasted systems include containers or trays built into the bottom of the mount to hold a large amount of water, concrete, or sand. A simple yet effective mounting system consists of custom brackets that connect rows of modules and provide spaces in between for ballasting material.

Ground Foundations. Ground-mounted arrays, unless they use a self-ballasted mounting system, require a foundation for support. Foundation designs vary widely, but typically require concrete footers or bases. For ground- mounted racks, they are arranged similarly to roof attachment points, with the rack being secured at the corners and at set intervals along the edges. The concrete foundations may be embedded into the ground, or may rest on the ground surface like a self-ballasted system. See Ill. 28. Wooden structures may be incorporated into ground rack foundations, typically as part of the aboveground support structure.

Ill. 26. J-bolts secure attachment points by hooking around structural members. J-Bolts. MOUNTING BRACKET; ROOF and COVERING

Ill. 28. Ground foundations for rack mounts typically include concrete footers and may use wood as part of the aboveground rack structure.

Ill. 27. Self-ballasting systems rely on the weight of the array, support structure, and ballasting material to secure the array without making roof penetrations. BALLASTING MATERIAL (CONCRETE BLOCK); ,-LEVEL SURFACE; .-CUSTOM BRACKET;

Pole-mounted arrays are secured at only one location, which usually requires a deeper foundation in order to resist the twisting and bending forces from the wind and the weight of the array. The pole is usually embedded in concrete, but in some areas compacted soil alone can support and secure the mount. Another option may be to use screw piles, which are twisted into the ground like giant screws. See Ill. 29.

The type of soil and its strength determine which methods can be used. Local building requirements should be consulted in order to design adequate ground foundations for local soil types.

Weather Sealing

Weather sealing of structural attachments and penetrations is a major concern for arrays mounted on buildings. One small water leak can do considerable damage to a building, both structurally and aesthetically, as well as to consumer confidence. The weather sealing of attachments and penetrations through building surfaces should use accepted roofing industry practices and materials that meet or exceed the lifetime expectations for the PV system.

The number of roof penetrations should be minimized, while still meeting structural support requirements. This saves time, reduces costs, and minimizes the potential for leaks after installation. For example, using fewer strong attachments is generally better than using a greater number of weak attachments.

There are a variety of materials and techniques for weather sealing, and methods of application vary among the types of mounting systems. Caulking or gaskets are often used to seal under and around direct attachments of mounting brackets to building surfaces. Weather sealants must remain flexible over a range of temperatures, maintain adhesion, resist degradation from long-term exposure to UV radiation, dispense easily, and cure in a reasonably short amount of time. Polyurethane, elastomeric, butyl rubber, and asphalt-based compounds are some of the more popular sealants. Alternatively, asphalt or cork gasket tape can be used for sealing. Basic latex, acrylic, or silicon caulks are generally unacceptable, due to their tendency to degrade and lose adhesion to roofing materials.

Ill. 29. Pole foundations may be encased in concrete or compacted soil, depending on local building requirements and the type of soil. Pole Foundations: CONCRETE FOUNDATION; COMPACTED SOIL FOUNDATION; SCREW PILE FOUNDATION

Many mounting systems use simple angle brackets, plates, or other footings that attach the array to a rooftop or other building surface. For example, a bracket maybe placed on top of a shingle and secured through the roof cladding to the structural members. Weather-sealing material is applied between the bracket and the roof surface, around the fastener, and in the pilot hole. See Ill. 30. While this is a quick, simple, and low-cost approach, this method is not the best roofing practice and may require occasional inspection and resealing to maintain reliable, long-term protection.

Weather Sealing with Caulking: Ill. 30. To weather-seal roof penetrations, caulking material is applied liberally around the entire attachment area to form a continuous seal.

[Direct contact between aluminum and concrete causes aluminum to chemically erode because of the alkaline properties of concrete.

Certain weather-sealing techniques, such as the use of rubber gaskets, can be used to isolate the two materials to prevent this type of corrosion]

A better approach uses flashings and rubber boots to seal around roof penetrations. The flashings are installed underneath or in place of shingles, or on top of other roofing systems, and a rubber boot is installed around the footing to provide a weathertight seal. While this method can be used to retrofit existing roofs, it is easiest during initial or reroofing work. See Ill. 31.

Ill. 31. Flashings and rubber boots provide the highest-quality weather seal for attachment penetrations.

Electrical conduits routed through exterior building surfaces are also potential sources of water intrusion. Conductors from an array generally enter the building through the roof, eaves, or walls via conduits or junction boxes. These penetrations require weather sealing, usually with caulking, gaskets, or flashings, though in some cases an inverted entrance conduit may be used to route conductors into the building while preventing water intrusion:

Careful attention should be paid to brackets or other roof-mounted equipment that may dam water and debris above or behind it, which may degrade weather sealing and cause leaks. Small, angled flashings installed above the equipment can deflect water and debris away from a weather-sealed penetration.

Structural Analysis

After evaluating the design loads, the strength of the modules, the mounting system, the structure to which the array is mounted, and the attachment points should be considered. Each part must be able to withstand either the largest load (typically the wind load), or a combination of loads (such as the sum of the dead, live, and snow loads). The local building code will determine how loads must be evaluated.

Module manufacturers will often specify allowable loads and acceptable deflections under various support positions. Mounting system manufacturers will often recommend different configurations, attachment methods, or attachment-point spacing depending on the structural loading.

Evaluating attachment points requires a comparison of the total force at each point to the withdrawal strength of the fastener. Lag screw attachments are relatively simple to evaluate for withdrawal strength against lifting forces from wind loads. For example, consider a lifting wind load of 45 psf acting on a 200 ft array that attaches to the roof with 24 support brackets. Each bracket is fastened through the 1"-thick roof surface into a southern yellow pine truss with a single 5 lag screw. The total force from the wind load on the entire array is 9000 lb (200 ft x 45 psf = 9000 lb). Therefore, each attachment point must resist 375 lb of force (9000 lb -24 attachment points = 375 lb per attachment point).

Since 3/16" lag screws have withdrawal resistance of 332 lb/in, in southern yellow pine, the thread penetration depth must be at least 1.13" (375 lb ÷ 332 lb/in. = 1.13"). Since the roofing thickness is about 1", the lag screw must be at least 2.13" long, 2¼" being the nearest common size.

Summary:

• Existing structures should be evaluated for structural soundness before installing an array.

• Good access to the array mounting location makes installation safer, easier, faster, and less expensive.

• Only passive cooling means are practical for flat-plate modules, such as mounting in a way to allow air circulation around the modules.

• Arrays mounted directly on building surfaces can increase heat transfer into conditioned spaces.

• Arrays mounted above building surfaces have little effect on building heat gain and may even help reduce interior building temperatures.

• Architectural principles that can improve appearance and public perception can be applied to PV system design and installation without significantly affecting cost or performance.

• Array support structures, hardware, electrical wiring, conduit, and junction boxes should be as inconspicuous as possible and neatly gathered or concealed beneath the array.

• Reducing installation time through careful preparation can significantly reduce costs.

• The simplest and most common type of array mount is the fixed-tilt type.

• Rack mounts are simple to install and flexible in the types and sizes of modules they can accommodate.

• Standoff-mounted arrays are by far the most common, preferred, and least-expensive method for installing arrays as a retrofit to existing rooftops, as well as for new construction.

• BIPV arrays can be integrated into roofing, windows, skylights, curtain wall sections, or complementary architectural features such as awnings, facades, or entranceways.

• Ground-mounted arrays are generally more flexible in location and placement, operate at lower temperatures than most building mounts, and are more accessible.

• Sun-tracking arrays can follow the sun, enhancing the amount of energy collected, but are also complex and require more expense and installation time.

• Wood construction is generally avoided in PV system installations.

• Aluminum, stainless steel, and galvanized steel parts and fasteners are commonly used in array designs because they are corrosion resistant, lightweight, and relatively inexpensive.

• Arrays, modules, mounting systems, fasteners, and buildings must withstand the maximum forces expected from structural loads.

• The principal types of structural loads are dead loads, live loads, wind loads, snow loads, and vibration loads.

• Most attachment methods use conventional threaded fasteners and screws.

• Blocking and spanning techniques can be used to support bolted attachment points.

• Self-ballasted systems do not require direct structural connections to a building or foundation.

• Poles are usually embedded in concrete or compacted soil or are twisted into the ground like giant screws.

• Weather sealing of structural attachments and roof penetrations is a major concern for building-mounted arrays.

• The number of roof penetrations should be minimized.

TERMS:

• The installed nominal operating cell temperature (INOCT) is the estimated temperature of a module operating in a specific mounting system design.

• A fixed-tilt mounting system is an array mounting system that permanently secures modules in a non-movable position at a specific tilt angle.

• An adjustable mounting system is a variation of a fixed-tilt array mounting system that permits manual adjustment of the tilt and /or azimuth angles to increase the array output.

• A direct mount is a type of fixed-tilt array mounting system where modules are affixed directly to an existing finished rooftop or other building surface, with little or no space between a module and the surface.

• A rack mount is a type of fixed- or adjustable-tilt array mounting system with a triangular-shaped structure to increase the tilt angle of the array.

• A standoff mount is a type of fixed-tilt array mounting system where modules are supported by a structure parallel to and slightly above the roof surface.

• A building-integrated photovoltaic (BIPV) array is a fixed array that replaces conventional building materials with specially designed modules that perform an architectural function in addition to producing power.

• A pole mount is a type of array mounting system where modules are installed at an elevation on a pedestal.

• A sun-tracking mount is an array mounting system that automatically orients the array to the position of the sun.

• An active tracking mount is an array mounting system that uses electric motors and gear drives to automatically direct the array toward the sun.

• A passive tracking mount is an array mounting system that uses nonelectrical means to automatically direct an array toward the sun.

4. How does the distance between modules and a building surface affect the module's installed nominal operating cell temperature (INOCT) and temperature-rise coefficient?

5. Explain how building-integrated PV (BIPV) arrays offset some building-material costs.

6. How do sun-tracking systems improve the power output of a PV system?

7. What factors influence the added value of a sun-tracking mounting system?

8. Why do the aluminum materials common in module frames present a corrosion problem when installed on steel structures?

9. Explain the difference between static and dynamic structural loads and classify the principal types of structural loads as static or dynamic.

10. Explain the three primary factors that influence wind loads for most PV applications.

11. How can lag-screw attachment points be supported when they do not match rafter locations?

12. Describe the advantages and disadvantages of self-ballasted systems.

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