Photovoltaic Tutorial:
continued - return to tutorial menuBalance of System: Racking and Mounts
Artsign Triangle Racking
While PV modules and inverters are the primary components in a home solar electric unit, the system requires other hardware to make it both user-friendly and compliant with the National Electric Code. These items are collectively known as Balance of System material, or BOS for short.
With solar technology continuously evolving, it helps to understand different approaches to installing PV arrays. There are many brands and models available to get the job done. Some products work better than others, while a few may not even pass the building inspection. That's why it's important to verify the quality and suitability of all your components. Be sure to choose:
- Manufacturers and suppliers with proven track records and multi-year warranties.
- Code-compliant products that are eligible for tax rebates or other incentive programs.
- Mounting hardware that includes engineering specs for snow, hail and wind loads, along with an instruction manual and technical support department that you can contact by phone.
- Electrical wire and components that have the necessary Underwriters Laboratory labeling.
Conergy SunTop
A basic mounting system for a solar array includes racking rails to hold up either side of each row of modules. The rails are mounted to the roof using brackets called "L-feet". Lag screws of the correct length must be fastened through the roof and solidly into the rafters for a secure attachment that will withstand heavy wind loads. The modules themselves are fastened to the rails with small clamps. What's missing from this scheme is a better method for sealing the roof penetrations. Very likely the installers squirted a polyurethane sealant into the drill holes before inserting the lag screws, but flashings are a better choice. (See photos below.)
With these objectives in mind, you can begin perusing what's on the market once you've decided on the array location, system size, modules and inverter. Of course, you'll also need the data collected during the site survey, discussed in more detail in the Steps to Going Solar guide.
Here's an overview of all the mechanical and electrical BOS items that contribute to the function of a grid-tied system:
Rails and Racking
A good mounting system is one that's easy to install after reading the instruction manual. It shouldn't require the purchase of expensive specialty tools and propriety hardware (footings, bolts, clamps, etc.) that cost an arm and a leg per unit. Since there are a wide variety of systems available, it will pay to take time and research the pros and cons before making a selection.
If you hire a solar company to do the installation, your salesperson should be able to describe in detail the racking it prefers, or at least get you the product literature so you can look it over yourself. As a rule, the mounting system should be made of lightweight, corrosion-resistant aluminum. The racks or rails needs to be durable enough to withstand wind, hail, snow, wildfires and corrosion for the life of the solar modules, typically 25-30 years.
The hardware and fasteners, on the other hand, should ideally be made of stainless steel. Galvanized steel is also used in some systems, but stainless steel is the better choice when it comes to durability and corrosion resistance.
As a general rule, you can't mix and match components from different companies, as each manufacturer incorporates its own proprietary technology. But some systems do combine the rails or racks of one manufacturer with the mounts of another. In any case, here's a sample set of desirable specs to look for when shopping around:
Solar Installation Type: Residential, Pitched Roof
Load tested: 50 PSF (pounds per square foot)
Wind Load: Up to 90 mph
Snow Load: Up to 1.5KN/M2 (kilo-newton per square meter)
Applicable module (panel type): Framed or Filmed
Module Orientation: Vertical (aka portrait)
Module Dimension: Applicable to any panel width, length and thickness
Warranty: 10 years
Ordering racks or rails for an array can be an adventure. The product literature online makes you wonder if all the writers have been instructed to ignore the age-old dictum, "Start from the beginning". While it can be very confusing at first, the spec sheets, installation manual, FAQ pages and telephone sales support should enable you to decipher what's going on. But before you start shopping for any BOS items, you'll need to assemble the following information:
- the brand, model #, and quantity of modules in the array(s)
- the number of rows and columns
- the type of roofing surface you have (asphault tiles, curved tiles, gravel, etc.)
- whether the modules will be mounted in a portrait or landscape position
- whether you want to tilt the array, or leave it parallel with the roof
- color requirement, if any
- preferred height of the racks off the roof
Here are a few other things to keep in mind when planning your installation:
- For every row of modules, you'll need two rails to hold them in place.
- If you're tilting the array, a triangular type of rack is needed. Specifically, it will have high, adjustable posts (or legs) on one side to get the desired elevation.
- Since rails can only be shipped in 8-12 foot lengths, you may need to buy splices to fasten them together.
- Accompanying hardware - clamps, feet, bolts, clips, etc. - may be sold either as part of "kit" with the rails, or separately. Either way, be sure you can get all of the hardware you need to complete the installation.
- While racking systems are far more expensive than cobbling together a hundred feet of aluminum channel and a sack of bolts on your own, the pricier material comes with engineering specs and long warranties. These are required by most building inspectors, and are also needed to qualify for tax credits, rebates and other solar incentives.
Panel racking, like this product from Iron Ridge, consists of rails, roof mounts, fasteners and optional hardware. Notice the "tilt leg kits (5 to 45 degrees)" on the right. These attachments allow you to adjust the array tilt manually. DIY enthusiasts will sometimes fabricate their own racking from inexpensive aluminum C-channel, but fastening everything together may take a much longer time. Photo: Iron Ridge XRS Solar Panel Racking
Mounts, Footings and Standoffs
While rails hold your modules in place, footings, standoffs and other types of mounts secure the rails to the roof or other array base. Most of the time, a lag screw drilled down through the opening in the mount and into a rafter is what does the job, but other components are needed for height extensions, corrosion prevention, and to keep water from penetrating beneath your roof. (During your site survey you or your contractor must measure the depth of your roofing material down to the wood frame. Then you'll know the right length of lag screw to order.)
Rails typically require a mount to be installed every 4-6 feet. This distributes the array load evenly. Specifically, a load of approximately 3 - 3.5 ft per square foot is normally mandated by both the racking manufacturer specs and local building codes.
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At top, a composition mount, a flat tile mount with standoff, and curved flashing. Above, an L-foot fastened to a composition mount, and a conduit mount. All products are from Quickmount PV.
Tilt leg attachments allow you to change the altitude angle of your modules. Some racking systems include the option of using adjustable tilt leg attachments. As explained during the Array Placement section of this tutorial, a tilt adjustment enables you to manually change a modules tilt angle in different seasons of the year. Of course, tilting panels means you must plan for sufficient space between each row so that one row of modules won't cast a shadow on the next.
Quickmount PV
Fixed or adjustable height extensions are used to tilt the upper rail of a solar panel.
If you live in an area with high heat in the summer, it's highly recommended that you mount your array several inches off the roof. Heat reduces voltage, and the higher above the roof a rack stands, the better the cooling air flows underneath. Standoffs are small posts that elevate the racks anywhere from two to seven inches. (Inserting gaps between the modules also contributes to faster heat dissipation.) If you plan to use high tilts, you'll have to make sure the extra wind load can be handled by the racks. A building inspector may ask to see the spec sheet, so make sure you do your homework beforehand.
solarpanelsnorthwales.com
Roof hooks are used to mount rails when tiles are especially thick or curved, for roofs with steep slopes, or when panels are tilted. A roof hook may also provide a more substantial base for composition and other mounts, depending on the brand of racking that's used.
When selecting hardware and structural components, be sure to assess the potential for galvanic corrosion. This happens when dissimilar metals touch each other - e.g. aluminum and stainless steel. Stray electricity flows between them, causing one metal to degrade over time.
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At left, a roof hook is hidden inside flashing. To the right, installers clamp a module into place. Notice that it extends a few inches past both rails. Second photo: LEVENTBAS
Flashing
Flashing is a type of sheet metal (aluminum or stainless steel) that surrounds each roof mount where water leakage is possible. In combination with a liberal application of roof sealant, the flashing keeps water from penetrating into your house. Cone mounts, like the one shown in the photo above right, are especially designed with resist water seapage, and can sometimes be used with standoffs in order to raise the height of the array.
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