Solar Builder

SEP-OCT 2018

Solar Builder focuses on the installation/construction of solar PV systems. We cover the latest PV technology (modules, mounting, inverters, storage, BOS) and equip installers/contractors with tips and tools to make informed purchasing decisions.

Issue link: http://digital.solarbuildermag.com/i/1019375

Contents of this Issue

Navigation

Page 17 of 55

18 S E P T E M B E R / O C TO B E R 2 0 1 8 If the ground is too rocky, other options such as post and con- crete, 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 special- ized 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 calcula- tions for commercial construction using augers and ground screws. These include Chance Hubble manuals, and other commercial sup- pliers 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 reason- able 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 pro- vide 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 capabil- ity. 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. AP Alternatives' Ready Rack mounting hardware is designed for both large utility-scale projects and small com- mercial 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. SCENE Ready to rack MOUNTING NEW IN ON THE

Articles in this issue

Archives of this issue

view archives of Solar Builder - SEP-OCT 2018