Roofs aren’t the only places for PV systems—ground-mounted systems offer their own unique advantages.
Choosing the structure to secure your PV array—roof, pole, or ground mount—is a process of weighing the pros and cons of each. Cost or energy-production comparisons might yield an answer, but sometimes the site speaks loud and clear.
Ground mounts, like pole mounts, can often accommodate larger PV arrays, as they are not limited to the size of the roof and can take advantage of the best solar window a site offers. Array installation and maintenance is done on terra firma—no climbing, no safety roping. There are no roof penetrations to leak, and with increased airflow around them, modules can operate at lower temperatures—yielding higher performance. Being able to choose the perfect tilt angle and azimuth also results in optimum performance. And while PV arrays don’t have to be cleaned, spotless modules do produce more energy; ground-mounted modules can be more easily washed of dirt, dust, and bird droppings. So if a site has wide-open, unshaded space to use for a PV array, ground-mounting can be the best option.
Ground-mount racks are made of a mix of aluminum and steel poles, rails, and channel attached to ground-anchoring structures—working together to provide the strength to resist live loads (like wind and snow that exert variable uplift and downward pressure) and dead loads (the weight of the rack and modules). While it’s possible to scratch-build a ground-mount, it is rarely advisable. The engineering and testing that go into manufacturers’ designs take the guesswork out of ground-mounting a PV array, and the end result is likely to be cheaper, sturdier, longer-lasting, better engineered, and quicker to install than a homemade rack.
Ground-mount manufacturers offer versatile designs for PV arrays, from two modules to upwards of 2,000 modules. The manufacturer typically provides a site-specific engineered design, layout, and prefabricated components, along with a list of what needs to be provided locally. The locally sourced parts will vary according to design—some systems come so complete that only concrete needs to be locally sourced. Although some rack manufacturers focus only on larger arrays (20 kW and up), nearly a dozen manufacturers provide smaller array (500 W to 20 kW) solutions, delivering fully engineered mounts with 10- to 15-year warranties.
Ground-mounted PV modules are arranged in one or more rows, in either a landscape or portrait configuration. A common layout for larger arrays is two rows of modules in a portrait configuration, but smaller array layouts vary substantially, depending on the number of modules. Care should be taken when specifying a design to match the number of modules and series strings with the layout to ensure a symmetrical final product with the shortest possible cable and conduit routes.
A variety of methods are used to anchor the rack to the ground: poured footings made with concrete column forms; driven steel piles of beam or pipe (or, rarely, wood); “earth screws” (also called “anchors” or “helical piers”) and even ballast (see “Ballast Mounts” sidebar).
Most smaller ground-mounted arrays use concrete footings because no special equipment (like a pile driver) is necessary. As more manufacturers join the industry and R&D accelerates, ground-mount anchor installation options are expanding. For example, Next Generation Energy’s Helical System uses hot roll steel plate and steel tubing to make a sturdy anchor that quickly “drills” into the soil. The anchors cannot be used in hardpan soils or soils with larger than 6-inch rocks. The depth, diameter, height, and spacing of the ground-mount support anchors, as well as the design of the rack itself, will be specified by the rack manufacturer on a site-by-site basis. The manufacturers engineer the design to the most conservative building code standards for the site specifics, so you must provide accurate inputs for the engineer, or the rack design might not be built to withstand the particular environmental conditions of the site, with potentially catastrophic consequences.
Every site is unique, and every mount design will be, too. For example, an array located in an area that receives heavy snowfall should have ample ground clearance so that snow can slide off, accumulating below without shading the modules.
In addition to site-specific considerations, the tilt angle of the array is a key factor. While adjustable legs are a common option with many ground-mounts, it isn’t nearly as easy as adjusting a pole-mounted array. Most pole mounts are balanced and have a single point of tilt adjustment, which a person can handle alone. Ground-mounted racks have multiple bolts, and the weight of the modules is resting on legs that have to be adjusted simultaneously or each leg a little at a time. This means at least two people can be needed for adjustments, or the process becomes arduous.
For a design starting point for a single-row, grid-tied array—that most commonly has a fixed tilt—use NREL’s online PVWatts calculator to find the angle that gives the highest possible annual energy (kWh) estimate for your site. Deciding on a fixed tilt angle for the array can be tricky if you’re off-grid or have multiple array rows within close proximity.
For setting the tilt of off-grid arrays, consider when loads are heaviest or the fewest sun-hours are available and choose an angle close to perpendicular to the sun’s rays for that time of year. If no set tilt angle can meet the consumption demand year-round, then calculate what fixed tilt will most likely reduce generator run time via a comparison of seasonal production at different tilts versus seasonal loads.
For ground mounts with multiple rows in close proximity, the choices become even more complicated. An array set at lower tilt angles means interrow spacing can be narrower, since the shadow cast by each row is shorter—and more rows can fit into a smaller footprint. The trade-off is that as module tilt decreases below the optimum, array energy production will decrease as well. A good starting point that balances the two goals for a multirow array is to set the tilt at latitude minus 10°. Manufacturers can assist in calculating interrow shading and spacing where necessary.
Positioning the inverter(s) and other balance of system (BOS) components is another critical design decision, especially when the goal is to keep voltage drop to a minimum. Higher voltages with lower currents allow smaller wire sizes, which means money saved. For high-voltage, grid-tied residential arrays using string inverters, the DC voltage will nearly always be higher than or about the same as the inverter AC-side voltage—which means keeping the longer runs of conduit and wire on the DC side of the inverter can be a good design decision. The positioning of array BOS components will directly impact where conduit will be buried and how the array can be accessed for maintenance.
Section 690.31A of the 2008 NEC specifies that PV wiring should not be readily accessible. For ground and pole mounts, that means either containing it in a raceway (which is possible when the modules have junction boxes that accommodate conduit) or by other means—the usual solution being locating the array behind a fence that does not shade the array. A fence can be a good choice for other reasons, such as in high-profile areas where theft or vandalism might be an issue. Several ground mounts have wire-management channels, into which PV wires can be safely tucked. These channels are either integrated into the design or offered as an option. These are not frequently UL-listed as a raceway, so the authority having jurisdiction over the installation might question their use for wire management.
The grounding for ground-mounted arrays also should be carefully considered. NEC Section 690.47D requires grounding electrodes connected directly to the array structure at the location of the ground mount. Exposed, non-current- carrying metal parts that may become energized in a fault situation, like module frames and rack rails, also must be grounded and connected to the grounding-electrode system. It is best to verify your grounding techniques with your inspector beforehand.
One of the biggest challenges for ground-mounted systems is keeping the surrounding area free of array-shading vegetation. Solutions include gravel, landscape fabric, and mowing. Sheep have been successfully used to keep the area around the array clear (but don’t use goats, which will climb on the array and try to chew on conductors)! Mowing or string trimming should be done carefully, as thrown rocks can damage the panels. Another factor to consider is the permanence of the installation—some sites, especially in land trusts or farmland, require a plan for future removal of concrete piers and associated structures.
Some manufacturers design mounting structures only for “larger” arrays or split their product lines into residential, commercial, and utility-scale solutions. If in doubt, check their Web site, and get in touch with them. A manufacturer should be easily accessible by phone or email, so it is worth the trouble to call a few and discuss upcoming projects to get a feel for how responsive they will be.
Rebekah Hren (rebekah.hren at o2energies.com) is a licensed electrical contractor and NABCEP-certified PV installer living in Durham, North Carolina. Rebekah teaches PV system design and installation classes, and co-authored The Carbon-Free Home, a book on residential energy efficiency retrofits. Her newest book, Solar Buyer’s Guide for Home and Office, will be available in October.
AEE Solar • www.aeesolar.com
Applied Energy Technologies • www.aetenergy.com
Conergy • www.conergy.us
DPW Solar • www.power-fab.com
HatiCon Solar • www.haticonsolar.com
Hilti • www.hilti.com
Next Generation Energy • www.zillarac.com
Panel Claw • www.panelclaw.com
Professional Solar Products • www.prosolar.com
Solar Flexrack • www.solarflexrack.com
SunLink • www.sunlink.com
Unirac • www.unirac.com