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

RoboSolar

Alion Energy

Alion Energy uses robot technology to drive down the cost of brownfield solar installations.

Robots in the solar industry are still a bit of a novelty out in the field, but Alion Energy of Richmond, Calif., has just rolled out its second generation robotic panel placement unit — the Rover 500 — that will help cut the cost of brownfield installations by around 20 cents per installed watt.

“We can do a brownfield installation with our technology for a few cents less than what a standard greenfield, fixed-tilt ground-mount system would cost,” says Jesse Atkinson, vice president of marketing and business development at the company. The new Rover 500 is capable of installing 500 kW per day, based on a two-shift operating day, he says. The old generation Rover 200 was only capable of 200 kW per day.

“Because we have a fully ballasted system, we are finding strong interest for brownfields and other sites where penetrating the ground is either prohibited or problematic, such as rocky and corrosive soils,” says Atkinson. “It’s not just that we avoid posts penetrating 10 to 12 ft under ground, it’s also the consideration of total weight and load spreading that are key considerations at brownfields.”

Alion Energy

Alion Energy builds its first utility-scale project near Lancaster, Calif., with its Rover robot.

Brownfields may refer to old EPA sites, less toxic industrial waste sites, municipal dumps or unused and untillable land. The EPA estimates there are 400,000 brownfield sites in the United States with “the presence or potential presence of a hazardous substance, pollutant or contaminant.” PV penetration of this niche seems only to be beginning. The number of municipal landfills in each state is more difficult to count, since many have been closed. New Jersey, for example, has more than 800 closed landfills and an estimated 10,000 brownfields, according to a local utility.

Installation and Maintenance
The Alion system involves a field-extruded 3,000 psi concrete rail system onto which standard PV modules with fold-down legs are placed by the new second generation robot into epoxied slots.

“Our concrete extrusion process and automated installation machines eliminate the cost penalties typically associated with building ballasted systems,” Atkinson says. That ballast element of a standard installation can cost up to $0.20 per installed watt, he notes. “The solar modules with the new system install two-and-a-half times faster and with a 20% reduction in concrete volume.”

Alion's new Rover 500

Alion’s new Rover 500 panel placement robot carefully positions a panel into place.

Much of the cost savings of the Alion system is in the reduction of installation hours achieved by robotic labor. “Normally we operate with about a dozen people to pour, run wires and install the panels; a typical installation would require three or four times as many people. So we can achieve about a 20% cost savings in BOS labor cost,” Atkinson says. “And we are faster than traditional installs, with the capability of building 50 MW in 13 weeks, versus an estimated 20 weeks for a standard installation workforce.”

Once the system is in place, a separate robot, named Spot, rolls along the rail section cleaning the panels at an annual service cost of $2,000 to $3,000 per megawatt installed. In a basic block of eight or more rows, a single Spot can be repositioned to adjacent rows through a screw-drive mechanism mounted perpendicular to the end of the rows. “Our automated cleaning vehicles eliminate any added soiling losses resulting from locating projects in industrial areas,” notes Atkinson.

Partnerships and Projects
Other functional areas of building a solar power plant are being conducted through partners like S&C Electric, based in Chicago. S&C and Alion agreed in February “to mutually construct ground-mounted solar power plants incorporating their respective industry leading technologies. This combination of proven EPC bankability, best-in-class electric power switching, protection and storage technologies with disruptive robotic installation and cleaning efficiencies, further enhances the cost competitiveness of ground-mounted solar generation,” the companies announced at the time.

The company’s first large-scale project, a 3.8-MWdc array, was recently installed near Lancaster, Calif., with close to 13,000 Trina Solar modules on a 20-acre field. The energy is being provided to Southern California Edison under a pair of PPAs at a nominal rate of $94/MWh. Because the time-of-day PPA considerations were more important than levelized cost of energy (LCOE), the tilt was set at about 8° off of an optimal LCOE angle.
“We now are looking at a number of brownfield sites on the East Coast, and in other states, like Michigan,” says Atkinson.

Built at a cost of about $500,000, pairs of the robots will only need to be stationed regionally for the company to provide national-scope services. “We can make a robot in about a month, but we don’t want to have to make too many of them,” Atkinson says.

“We are also looking at international markets including Chile and Saudi Arabia,” Atkinson says. To streamline the planning and execution stages, Alion is seeking to develop strategic partnerships with EPCs or large construction companies specialized in brownfields or sites with similar special requirements. The concrete rail system can climb an 8% grade in order to compensate for irregularities in the soil topography and even inject additional concrete to fill in shallow spots under the rail level.

Charles W. Thurston is a freelance writer who covers solar energy from Northern California.

— Solar Builder magazine