PV Ground-Mounting

Although ground mounts are typically more expensive than roof mounts, they are an ideal solution when a roof is unavailable or impractical. Plus, you get to avoid climbing ladders and the dangers of clambering around on the roof. Ground mounts also eliminate the need for roof penetrations and keep electrical components off the roof. Ground-mounted systems can operate more efficiently, since there’s usually more airflow underneath the array and thus less energy loss from heat. 

On the downside, ground-mounted systems involve concrete foundations and digging trenches for conduit. Installation is more susceptible to rocky ground and the weather, as rocks, mud, or frozen soil can turn an otherwise easy installation into a serious challenge.

Types

Most ground-mounted arrays are fixed-tilt. And they are often based on a steel pipe substructure, made from schedule 40 pipe, which is available from your local hardware store or pipe supplier. There are tracking arrays available as well, but they are less common and sufficiently different, so we have omitted their discussion here.

Multipost ground mounts provide the most efficient use of structural materials and are the cheapest and most common. The fundamental structure is a set of four posts that can be expanded toward either side. Longer rows of modules can be easily made by adding pairs of posts in the east-west direction to extend the array. On flat ground, the array is extended in the north-south direction by adding rows of modules. If the array site is on a large south-facing slope, you may be able to simply extend the array up the slope by adding posts to create a single, large array all in the same plane. Multipost ground mounts are typically the most adaptable to these types of installations.

Some manufacturers offer single-row multipole mounts. The main difference between this and a more conventional ground mount is that the basic structure for these arrays is cantilevered off of a single row of poles or posts, rather than two rows. As a result, the structure and foundation need to be more robust; larger steel members are usually required for the poles, and larger foundations are needed. The end result is that this style of structure tends to add cost compared to a four-post design, although this may be mitigated by the savings of using fewer poles overall. However, these mounts are much easier to design for seasonal adjustability, since they tend to pivot about a single horizontal pipe rather than being supported by multiple rows of posts. Being able to change the tilt helps maximize the array’s production throughout the year, or more readily shed snow loads in the winter.

Monopole mounts (“top-of-pole” mounts), sometimes referred to as “solar on a stick,” are perhaps the most interesting-looking, but least practical of the ground-mount types. The post and concrete requirements per module area are greater than for single-row multipole mounts. They are usually the easiest to tilt, and can be easily adapted to varied terrain since they are not configured in long rows. The majority of monopole mounts used today are for single- or dual-axis tracking arrays or for small-area arrays like those used in remote solar water pumping or small DC power applications.

Array Design

A layout starts with siting and orientation. This usually means that the array is facing south and has good solar access throughout the year. Since they sit lower than roof-mounted arrays, ground-mounted arrays can be more susceptible to shading, but not being restricted to the roof opens up the possibilities to other places on a property that may have better solar access.

Make a maintenance plan to keep ground cover and vegetation from shading the modules—most ground-mounted arrays are installed 2 to 3 feet off the ground on the south (or short) side but may be higher depending on the average snowfall for the area. Common vegetation control methods include a regular mowing schedule and weed control mats or gravel. If you opt for a goat, make sure you don’t have exposed wires. Be certain to check with your local building department for any special code requirements, such as fencing or a fire break area around the array. 

Foundation work requires basic knowledge of your local soil conditions. Most generic rack engineering specifications assume relatively poor soil conditions, which means that specified piers and footings are deeper/larger than may actually be needed. If your project is large, spending some money for a soil survey first may lead to saving money on the installation.

If the array area is sloped, look for signs of creep or movement in the ground. Foundations on sloped arrays typically need to go deeper than they would if they were installed in a flat area. Look for signs of water drainage, and erosion or ponding, and take these into account as well. If you have rocky soil, be ready for some extra work digging your foundations. Sometimes it’s hard to know what you have until you start digging.

Since a ground-mounted array typically stands alone, you will need a strategy for locating your electrical equipment and running conduit. For modern grid-tied systems, mounting the inverters at the array is common, as this provides a convenient shaded location and minimizes DC runs. Another acceptable option is to mount the inverters on a nearby structure or next to your electrical meter.

Consider your needs for accessing the array. In addition to controlling vegetation, you may want to access the array to clean it, or to view the inverters if they are mounted on the array structure.

As you get into the details of your array design, you will need to know your local wind speed and snow accumulation. The easiest way to get this information is from your local building department.

If your site slopes to the east or west instead of south, you can either install the array at an angle parallel to the ground or level, with occasional “steps” as it goes down the slope. Running your array parallel to the slope lends to an easier installation. Stepped arrays require additional substructure and bracing, and also require gaps in the array to prevent self-shading. In either case, multiple rows of arrays have to be laid out so that one row’s high back edge does not shade the next row’s lower edge. 

Array Layout

Once your array is designed and the building permits are approved, it’s time to start playing in the dirt. First, clear the vegetation from the site. You may also want to grade the site to get it level and down to bare soil.

Always call 811 before you dig (www.call811.com). The site could have a gas line or electric cable buried in the location, which could make working in that area dangerous or costly. Also, some utilities have a right of way on private property, which could prevent you from building on that site. Make certain that your local utility has flagged all underground pipes, lines, and cables before doing any digging—especially if you are using power equipment for this task! Look for common signs of underground utilities and consult past plans for the site. When in doubt, dig by hand and always proceed with caution.

There are many options for array foundations. Concrete piers are the most common due to their basic design and adaptable nature. Digging holes and setting posts in concrete is straightforward, and piers can almost always be installed without rebar reinforcements.

Grade beams are a fantastic variation on concrete piers for flat soil where you can’t dig deep due to rock, shallow water tables, or filled land. A grade beam is a narrow, shallow foundation that usually runs from the north to the south edge of the array. They are typically installed very shallow (18 to 24 inches) or even right on the surface of the grade, avoiding the necessity of digging deep piers. Their only drawbacks are that they tend to require rebar and more concrete, and may also require some additional form work. Other options—like screw-in piers and driven piles—are available, but may be less accessible or practical for the DIYer. The equipment for these more specialized supports tends to be expensive and they may be less robust over the long term than posts in concrete.

Local jurisdictions may require that your concrete go below the frost line, which can mean deeper piers. However, PV array foundations aren’t as sensitive to frost heave as building foundations, so this may not be necessary—check with your local building jurisdiction to be sure.

Once you have your system design and foundation strategy, you can lay out and mark post locations. Marking out an array can be difficult if you haven’t done it before, particularly if you are working on a sloped surface. Common methods for marking out foundation locations include wood stakes, string, and marking paint.

Using a power auger will save lots of work compared with digging post holes by hand. If you are using screw-in or driven piles, you will need the appropriate equipment as specified by the post manufacturer for installation. Most screw-in post companies provide installation service.

Once the holes are dug, it’s time to set the posts. Some installers will use braces made of 2 by 4s or other materials to hold the vertical posts or poles in place while the concrete sets. Others skip this entirely by using quick-setting concrete. Check the structural engineering documents for the system you are using to be sure you are meeting the requirements for the concrete you use. A typical compression strength requirement is 2,500 to 3,000 psi, with higher values being stronger. If installing without the use of forms, the first step is to fill the hole to the top with concrete.

From there, the post is lowered into the hole and positioned in line with the other posts, and checked for vertical alignment with a level. The viscosity of the concrete will hold the post in place as you move on to the next one. Be sure to keep checking your posts as the concrete sets to ensure that they stay lined up and plumb!

Once the posts have set, you can trim them to the right height. Be sure to double check the geometry before trimming posts—measure twice, cut once. Use a laser level or string line to mark the post heights. If you are following the slope of the soil with your array, you may have to get creative to make a laser level work. In these cases, it’s probably easier to stick with the string line.

Most array mounts require cross bracing, which may need to be installed before the top cross members of the array structure go in. PV arrays are large and catch a lot of wind. Follow the guidelines for system bracing specified in the engineering documents. Cross bracing is about creating triangles within the structure, typically between the top of one post and the bottom of the next. Note that most array designs require bracing in both directions.

In most systems, the next steps are to install the horizontal cross members, followed by sets of module rails. Set the rails at the ends of the array and line up the rails in the middle with a string line or laser. Review your trigonometry, as it will come in handy when squaring the rails on most types of arrays. Take the extra time to line up the rails correctly to ensure a good-looking array.

Module Installation

Module installation tends to go pretty quick on ground mounts, but keep these tips in mind:

• Don’t assume your PV modules are square. This can make installation very frustrating!
• Watch for module creep. A common technique is to use a string line for installing the bottom row of modules. Once that first row is in, the rest of the array will “stack up” from there. Take the time to get the first row in straight, and the rest of the installation will go very quickly.
• Strings are good for lining things up but they also tend to blow in the wind. Make sure your string line is tight. Line of sight is a very useful tool for lining things up, and lasers may be an option as well.
• Work as a team. Having a second pair of hands is particularly useful during module installation and alignment.
• As modules get installed, you can wire as you go. However, since ground-mounted arrays often have very good underside access, you can always postpone this task.
• An increasing number of rack systems have wire-management channels for holding module leads and home-run wiring. These are particularly nice for ground arrays given the underside visibility and access. Note that these channels may not be UL-listed as a raceway, so check with your local authority having jurisdiction to make sure this approach complies with their interpretation of NEC 690.31(A), which addresses PV wiring accessibility.

Once the modules are placed, you can think about some of the finishing touches:

• Trim the rails close to the array. Particularly on ground mounts, having rails sticking out of the edge of the array is a hazard. If possible, use a system that has end clamps that allow the rails to be cut flush to the end of the array. Many of these systems also have rubber end caps to make for a safer, more attractive installation.
• If you haven’t already done so, take appropriate steps to mitigate vegetation under the array. A weed control mat, covered with mulch or gravel, is an effective barrier.
• Protection from vandalism is often critical for ground-mounted arrays. Theft-deterrent hardware is available to thwart would-be module thieves.
• Electrical safety is more critical for ground mounts. Access to wiring by children, pets, or varmints should be considered. Some jurisdictions require screening the back of the array to restrict access to wiring and junction boxes, or you might fence around the entire array. (Just make sure the fence doesn’t shade the array.)

Access

Greg McPheeters has been designing solar-electric systems and PV mounting solutions since 2001. Greg is the lead design engineer and innovator for SnapNrack.

Tim Vaughn is a 21-year veteran of the solar and PV industry. Tim is currently the program manager for SnapNrack.

Manufacturers:

Array Technologies • www.wattsun.com

Applied Energy Technologies • www.aetenergy.com

Conergy • www.conergy.us

DPW Solar • www.power-fab.com

HatiCon Solar • www.haticonsolar.com

JAC-Rack • www.jac-rack.com

Next Generation Energy • www.zillarac.com

PV Racking • www.pvracking.net

Professional Solar Products • www.prosolar.com

Schletter • www.schletter-inc.us

SnapNrack • www.snapnrack.com

Solar Flexrack • www.solarflexrack.com

TerraSmart • www.terrasmart.com

Unirac • www.unirac.com

Zep Solar • www.zepsolar.com 

Zomeworks • www.zomeworks.com