Market Driver: When augers, ground screws make economic sense for solar contractors

 

auger-ground-screw

The use of augers and ground screws has been of interest in mounting solar systems for some time, and for the right size job, they offer smaller solar contractors an opportunity to grow their business.

Small site factors

For one, with smaller PV systems, one may not need to spend money on a soil engineering analysis and the cost to permit the design separately. The typical soil type in an area may be known from experience. Perhaps local experience with other construction such as a home foundation or a water line installation can provide clues to the soil type.

A method used by some contractors is to use a hammer drill and ground rod available from an electric supply store and see how easily the rod can be driven into the earth. If the rod hits solid rock 6 inches below the surface, or if the rod is very hard to drive, this could either disqualify the use of ground driven foundations, or in some cases lead to using ground screws rather than augers.

Additionally, many counties and states have published maps showing the soil types for many locations. Other sources of data are well sites where there is often a record by the foot of the surface to depths much greater than one would drive a ground-mount.

Selecting a ground-mount

Once a determination has been made as to the type of soil at a site, the installer should select a ground mount to use at a site. If the soil type is not heavily compacted and not rocky, one can consider the use of augers. Most typically, a ground auger driven 7 to 10 ft. will suffice for most 3- and 4-row landscape arrays.

If the ground is compacted, made up of heavy clay, or has small rocks within the first 10 ft., then a ground screw would probably be a better choice. Ground screws offer lower torque when driving them into the soil and are less likely to break in harder ground. However, in soft, loamy soils a ground screw will not provide big pullout values compared to an auger.

If the ground is too rocky, other options such as post and concrete, ballasted arrays, or rock anchors may be a better alternative. Experience with ground arrays will greatly help in the selection of a ground mounting system.

Driving ground mounts

Some form of tractor or track machine is required to drive ground-driven foundations. These machines are easy to rent and use, and depending on the volume you are doing, worth owning. Small arrays with only 8 or 12 posts are probably not worth the investment, but between that and larger arrays that require a specialized company to drive the mounts, there is a sweet spot that makes financial sense.

The machine used will need some form of rotary head such as the small Bobcats used to dig holes for pole buildings and fence posts. Alternately, some farm tractors have a rear-mounted rotary

driver used for fence posts that may be used.
Most equipment rental yards can supply a small track machine normally used with a hole-digging auger. With the hole-digging auger removed, an adaptor can be used to mate the drive head to fit augers and ground screws. A 2 in. hex adaptor that fits the machine can be purchased by the installer if not available from the equipment rental yard with the machine.

The amount of torque required to drive a ground mount should not be more than a nominal 3,000 lbs. If more torque is required, or if the mounts are breaking, than the wrong mount was selected. If augers break, a ground screw should have been used. If ground screws break, then a non-driven mount should be used.

If occasionally a mount breaks due to an undetected boulder or other issue, a traditional post and concrete mounting should be used. In the case of Groundwater, a 50-kW project in Portland, Ore., where over 400 augers were used, eight anchors broke due to large sporadic rocks and were replaced with eight concrete-mounted posts.

Calculations and measurements

There are many resources available covering the use and calculations for commercial construction using augers and ground screws. These include Chance Hubble manuals, and other commercial suppliers of augers. However, there are some general guidelines one can follow summarized below.

Augers have a pitch determined by the blade angle. Our auger is a 10-to-1 auger. Using a 10-to-1 auger, each ft lb of torque driving the auger provides approximately 10 times the uplift capability when driven to 10 ft of depth. For example, if an auger is driven with 500 lbs of torque to 10 ft. the pullout will be approximately 5,000 pounds. Typically, augers are driven much harder, resulting in tested pullup values of 20,000 to 30,000 lbs. Most often, augers driven in reasonable soil values will dramatically exceed the pullout values actually required to resist pullout or overturn of the array.

In the case of ground screws, they are typically applied to more dense soils and solids with rock intermixed. A ground screw should not be used in solid rock.
Ground screws in hard soils have pullout values of 1,500 to 5,000 lbs at a depth of 5 ft., however this estimate is entirely based upon the soil density. The use of ground screws in soft soils will not provide a satisfactory base for a solar array.

The use of a torque measurement gauge is recommended as an additional check on the drive torque and resulting pullout capability. Some modern machines one can rent or buy have a built-in torque gauge. Additionally, there are devices that can mount between the hydraulic head and the ground mount to measure the torque. However, a careful operator will have some sense of the amount of effort required to drive the ground mounts, and in most cases can successfully install and drive ground arrays without a torque head.

Cliff Schrock is an engineering consultant with SunModo.

 


On the Scene

Ready to rack

AP Alternatives’ Ready Rack mounting hardware is designed for both large utility-scale projects and small commercial projects. The small helical anchors and quick-install cross bracing make the simple system robust even for high wind zones. The mini-tilt brackets are adjustable and allow for quick field alignment of the post height. This allows the anchor posts to be installed rapidly and any terrain variation can be accounted for by simply adjusting the tilt bracket up or down to achieve the best aesthetics on an ungraded site. This system is nimbly installed with an attachment that fits on a skid steer.

— Solar Builder magazine

When augers, ground screws make economic sense for solar contractors

SunModo solar auger

A small Bobcat 331 being used to drive a 10 ft auger using the technique of advancing upon the auger to vertical as it is driven.

The use of augers and ground screws has been of interest in mounting solar systems for some time, and for the right sized job, they offer smaller solar contractors an opportunity to grow their business.

Ground conditions

For one, with smaller PV systems one may not need to spend money on a soil engineering analysis and the cost to permit the design separately. The typical soil type in an area may be known from experience. Perhaps local experience with other construction such as a home foundation or a water line installation can provide clues to the soil type.

A method used by some contractors is to use a hammer drill and ground rod available from an electric supply store and see how easily the rod can be driven into the earth. If the rod hits solid rock 6 inches below the surface, or if the rod is very hard to drive, this could either disqualify the use of ground driven foundations. In some cases this could lead to using ground screws rather than an augers. Additionally, many counties and states have published maps showing the soil types for many locations. Other sources of data are well sites where there is often a record by the foot of the surface to depths much greater than one would drive a ground driven mount.

Selecting a ground mount

Once a determination has been made as to the type of soil at a site, the installer should select a ground mount to use at a site. If the soil type is not heavily compacted and not rocky, one can consider the use of augers. Most typically, a ground auger driven 7 to 10 ft. will suffice for most 3 and 4 row landscape arrays.

If the ground is compacted, made up of heavy clay, or has small rocks within the first 10 ft., then a ground screw would probably be a better choice. Ground screws offer lower torque when driving them into the soil and are less likely to break in harder ground. However, in soft, loamy soils a ground screw will not provide big pullout values compared to an auger.

If the ground is too rocky, other options such as post and concrete, ballasted arrays, or rock anchors may be a better alternative. Experience with ground arrays will greatly help in the selection of a ground mounting system.

SunModo augers

Auger being driven at Groundwater site with Bobcat 337

Driving ground mounts

Some form of tractor or track machine is required to drive ground driven foundations. These machines are easy to rent and use, and depending on the volume you are doing, worth owning. Small arrays with only 8 or 12 posts are probably not worth the investment, but between that and larger arrays that require a specialized company drive the mounts, there is a sweet spot that makes financial sense.

The machine used will need some form of rotary head such as the small Bobcats used to dig holes for pole buildings and fence posts. Alternately, some farm tractors have a rear mounted rotary driver used for fence posts that may be used.

Most equipment rental yards can supply a small track machine normally used with a hole digging auger. With the hole digging auger removed, an adaptor can be used to mate the drive head to fit augers and ground screws. A 2 inch to hex adaptor that fits the machine can be purchased by the installer if not available from the equipment rental yard with the machine.

Figure 4 – Drive adaptor to convert from hydraulic head on rental machine to 2 inch ground mount.

The amount of torque required to drive a ground mount should not be more than a nominal 3000 pounds. If more torque is required, or if the mounts are breaking, than the wrong mount was selected. If augers break, a ground screw should have been used. If ground screws break, then a non-driven mount should be used.

If occasionally a mount breaks due to an undetected boulder or other issue, a traditional post and concrete mounting should be used. In the case of Groundwater a 50 kW project in Portland Oregon, where over 400 augers were used, 8 anchors broke due to large sporadic rocks and were replaced with 8 post and concrete mounted posts.

Auger and Ground Screw Guidelines

This post is an excerpt of this full guideline from SunModo, which clears up the confusion surrounding the use of augers and ground screws. To view the entire white paper, enter the info below.

Calculations and measurements

There are many resources available covering the use and calculations for commercial construction using augers and ground screws. These include Chance Hubble manuals, and other commercial suppliers of augers. However, there are some general guidelines one can follow summarized below.

Augers have a pitch determined by the blade angle. Our auger is a 10 to 1 auger. Using a 10 to 1 auger, each ft. lb. of torque driving the auger provides approximately 10 times the uplift capability when driven to 10 ft of depth. For example, if an auger is driven with 500 pounds of torque to 10 ft. the pullout will be approximately 5000 pounds. Typically augers are driven much harder, resulting in tested pullup values of 20,000 to 30,000 lbs. Most often, augers driven in reasonable soil values will dramatically exceed the pullout values actually required to resist pullout or overturn of the array.

In the case of ground screws, they are typically applied to more dense soils and solids with rock intermixed. A ground screw should not be used in solid rock.

Ground screws in hard soils have pullout values of 1500 to 5000 pounds at a depth of 5 ft., however this estimate is entirely based upon the soil density. The use of ground screws in soft soils will not provide a satisfactory base for a solar array.

The use of a torque measurement gauge is recommended as an additional check on the drive torque and resulting pullout capability. Some modern machines one can rent or buy have a built-in torque gauge. Additionally, there are devices that can mount between the hydraulic head and the ground mount to measure the torque. However, a careful operator will have some sense of the amount of effort required to drive the ground mounts and in most cases can successfully install and driven ground array without a torque head.

Cliff Schrock is an engineering consultant with SunModo.

— Solar Builder magazine

Four energy dense solar mounting systems for C&I rooftops

SunModo SunBeam

sunmodo

As a permanent part of the building and roof structure, the SunBeam system eliminates any abrasions, moss build up and need of system removal for roof repair or re-roofing. In addition, it provides shading of HVAC equipment, increasing efficiency and faster temperature response. Twenty-year warranty.

Material: High-grade aluminum and 304 stainless steel hardware. Anchor-only attachment.

How it maximizes energy density: The SunBeam system elevates above obstructions such as HVAC, pipes and vents. By spanning over roof obstructions such as HVAC, pipes and vents, the system takes full advantage of the available roof surface thereby maximizing the PV system size. The system can be easily adjusted to account for the multiple roof pitches on site.

Everest Solar D Dome R²

everest

The D Dome R² system is an east/west commercial flat roof solution. The third generation of this product is now rail-less with only five major components and minimal hardware. It sits at a fixed 10-degree pitch and allows for 3.5-in. inter-row spacing. Twenty-year warranty.

Material: Aluminum, ballast with attachment optional. The ballast blocks sit under the panels.

How it maximizes energy density: Everest Solar Systems believes east/west systems are more efficient south of the tropic of cancer. First, an east/west system practically eliminates inter-row spacing which allows more modules on the roof, thus increasing module density. On one internal study, Everest compared a the production of a 10-degree south-facing system with its east/west system in southern California at different azimuths. The south-facing fit 88 modules and had a 14 percent decrease in at the 225-degree azimuth. The D Dome R² system reached 108 modules in the same space and had less than 0.1 percent change between all azimuth angles.

Solar Mounting Solutions

sms

SMS Racking consists of only three major parts that arrive with all hardware pre-inserted allowing for quick single tool installation. The THRU-ITT integrated wire management system allows wiring to remain organized and protected by running wires east-west and north-south internally. Since this racking design does not rely on the panel for integrity, installers can complete racking and wire installation independent of the panel. Twenty-year warranty.

Material: G90 coated steel and optional galvanized steel, aluminum, powder-coated. Ballast only.

How it maximizes energy density: SMS developed an Excel spreadsheet that determines the optimum row-to-row spacing based on the selected solar module, optimum tilt angle, solar azimuth angle, and the altitude angle all specific to install location. By optimizing the length of row-to-row spacers, the SMS system can greatly reduce the amount of redundant material, which in turn will reduce racking cost and avoid installing the modules in a high shadow region. The racks are designed with minimal distance between modules in the east/west direction to eliminate unused area.

Ecolibrium EcoFoot5D and EcoFoot2+

ecolibrium

EcoFoot5D 5-Degree and EcoFoot2+ 10-Degree speed installation and simplify logistics for flat-roof installs. Main components are: a base, pre-assembled clamps (integrated bonding without washers) and a wind deflector. The system is black, ASA-PC, UL Listed Resin with a 25-year warranty mounted with ballast, attachments or a mix.

How it maximizes energy density: EcoFoot5D 5-Degree delivers 18.4 percent more power than the 10-degree system and lowers cost per watt. The system maximizes roof density while maintaining the ease and simplicity of EcoFoot. The modular base is small at 7 in. x 16.7 in., and inter-row spacing is a dense 9.9 in., creating a tightly packed array. Stackable bases enable up to 290 kW per pallet, resulting in fewer pallets and minimized shipping, storage and onsite crane use.

— Solar Builder magazine

Degrees of Separation: How to mount commercial rooftop PV systems to maximize energy

Ecolibrium EcoFoot system

Ecolibrium EcoFoot system

On commercial rooftops, design trends are all about maximizing energy density. Module selection is a huge factor there, but so are the layout and tilt decisions — figuring out the perfect shape and tilt to mount as many modules as possible without compromising their performance.

Pairing the right racking system with a flat-roof space opens up a world of possible equations. Use a racking system that will position the panels to maximize the energy output, which includes the tilt angle, inter-row spacing and the direction the panels will face. As always, geography matters. For one, the roof’s azimuth, or the direction the pitch faces. For a perfect south-facing system, the azimuth should be 180.

But new systems are tweaking the traditional. East/west systems are becoming popular below the tropic of cancer. Designers are playing more with tilt angles, with the general trend moving toward 5-degree tilt — likely to reduce inter-row shading without compromising the number of modules used or resulting in too much soiling.

“Rooftop energy density is maximized by fitting more panels on the roof using a 5-degree racking system,” said Jonah Coles, product solutions manager, Ecolibrium Solar. “The key to fitting more panels on the roof is to use racking with a small footprint and narrow inter-row spacing. The combination packs in panels, yet the inter-row spacing is wide enough to allow for the working room needed for ease of installation and post-installation maintenance.”

RELATED: Why energy density matters — and three ways to maximize it

But the tilt decision isn’t one-size-fits all. Everest Solar Systems notes tilt angle efficiency correlates to latitude — the higher the latitude often requires a higher tilt. The latitude in Hawaii, for instance, allows a system to be virtually flat, but there needs to be enough tilt to keep the rain from pooling and to keep dust off the modules. Brandon Gwinner, regional sales manager, SunModo, puts that minimum at a 4-degree tilt.

SunModo Sunbeam

“The tilt degree is dependent on the region/location and optimum output based on TSRF,” he says. “The minimal tilt degree racking systems are typically to maximize the number of modules you can get on a roof without your rear post being 8 ft off the roof and to get the most energy density/power density per the project.”

There are also some wind/snow load considerations that can keep tilt below a certain height/tilt degree, as well as parapet walls and billowing of wind. The installer has to find the balance between production and engineering capabilities.

Also, installers looking to maximize production in summer months should consider using lower tilt angles than installers looking to maximize production in winter months. In snowy northern climates, Everest Solar recommends a 10-degree system tilt angle, which is better for shedding snow, plus the wider inter-row spacing allows more room for snow to land without piling up and casting a shadow or covering the modules.

“If you can hit your power goal with a 10-degree system, then 10-degree would be the system of choice. If not, 5-degree racking can enable a successful system when 10-degree wouldn’t fit enough panels to generate enough power,” Coles said.

Commercial installations have significantly more requirements than residential installations, so understanding jurisdictional requirements at the onset of the project will make the process go smoothly. Some states, like Oregon, do not require extra engineering when the tilt is under 18 in. on the back edge of the array, based on a prescriptive path. So, cost analysis vs. ease of permitting is a factor for tilt decisions too.

The inter-row spacing issue

Tilted PV panels cast shadows on the rows of modules behind them, necessitating a gap between rows to minimize the effects of production loss due to shadows cast on panels in anterior module rows. Here are a few ideas to mitigate the impact of this phenomenon on your PV installation via Peter Abou Chacra, engineering consultant, SunModo.

  • Reduce the tilt of your south-facing array. For peak energy production on a per-module basis, PV modules have an ideal incident angle with solar rays emanating from the sun. For some installations, however, it may make sense to reduce the tilt of the modules to a less optimal incident angle. Though this means less production on a per module-basis, it can mean a significant increase in the daily unshaded collection time for the array. This gain in effective collection time can offset the losses caused by a sub-optimal tilt for the module itself. Using software dedicated to modeling and analyzing a system’s performance at a different tilt angle and inter-row spacing should figure out the best path.
  • Locate your system on a south-facing slope. Even a five-degree inclination can have a marked impact on the amount of inter-row spacing required. This can significantly increase the number of modules you can fit in a given area.
  • Consider 3-in-landscape or 4-in-landscape monoslope installations. Coupled with a low tilt, this strategy can reduce inter-row spacing significantly on a given installation since modules on the same structure and slope don’t require significant spacing between them. This can be particularly effective if you can gradually elevate the anterior monoslope PV structures as you work your way north through the site.

— Solar Builder magazine

This solar mounting system from SunModo anchors entirely above roof tiles

TopTile sunmodo

Professional solar installers no longer need to grimace when considering a risk-laden solar PV installation on tile roofs. Referenced in our latest Mounting Guide issue, SunModo’s TopTile Mount System conveniently anchors solar racking entirely above the tiles, which spares the installer needing to disrupt tiles, remove battens, damage underlayment, or leave leak-prone holes or fasteners.

The TopTile Mount System features three mounting options that secure panels 4-7 inches above the tile surface, using stanchions with water-proof sealing washers and moldable flashing. Installers can choose either SunModo’s patented deck mounting system when anchoring into decks or a rafter mounting system. The system can also be mounted to a flat concrete surface. The system works on concrete, clay, stone, fiberglass or rubber tile roofs, in accordance with ICC building codes.

sunmodo toptile mount

“Our TopTile system makes it easy for installers to pursue tile roof installations because they no longer have to bear the cost and risk of tearing apart tile roofs,” said Stella Sun, Marketing Director of SunModo Corp. “This innovative solution is destined to become an industry standard with application on more than 90 percent of tile roof environments.”

SunModo TopTile Mount Systems are part of a complete line of racking and mounting options available through leading distributors or directly from the company at www.sunmodo.com. Samples are available and will be on display at leading solar shows.

SunModo is a private U.S. company headquartered in Vancouver, Wash., that is focused on innovative racking and mounting solutions for professional installers in the fast-growing solar industry. Energized by a desire to make solar installation easier, more reliable and more affordable, its solar racking solutions are at work in 15 countries, and from Hawaii to Connecticut.

sunmodo top tile specs

— Solar Builder magazine