Featured in Solar Power World Online – November 19, 2018
There’s an inherent long-term difficulty to installing solar arrays in regions that experience winter weather. During those months overcast skies minimize solar exposure to diffused light seeping through clouds; snow can cover panels, blocking out light entirely, and that’s just on the surface. Below the soil, pockets of moisture wait, capable of freezing, expanding, moving—and possibly damaging—modules and mounts if ground temperatures drop low enough.
The phenomenon is known as frost heave, and it could mean extra operating fees, maintenance and repairs for mounting companies and solar installers.
“[Frost heave] is basically underground in the pore space in your soil,” said Mike Faraone, director of engineering at TerraSmart. He is authoring a white paper on frost heave that he expects to complete by late 2018 or early 2019. “You have this formation of these ice lenses, and if you have a water table nearby, those ice lenses grow. They start sucking up more and more water, and then they heave the soil up. You can see a foot of deformation of the soil actually coming up out of the ground.”
There are telltale signs of frost heave affecting a solar array. Ice lenses jack array foundations up, causing bows in columns, and, in extreme cases, damages panels. Solar installations are susceptible in states with temperatures that drop below the freezing point.
Previously, the solution to frost heave prevention can come at an added cost, and more steps for installers; array sites have been cleared several feet down to frost depth, lined with PVC sleeve and filled with gravel or frost-resistant soil; cement foundations can crack in freeze-thaw weather; and too much surface area on a driven pile is ideal for frost jacking. However, three solar companies have engineered and manufactured solutions to frost heave, each cutting down install times, unnecessary excavations and material costs for solar projects.
TerraSmart Ground screw
Frost heave isn’t an issue exclusive to solar. Any industry that mounts structures atop or within soil needs to be aware of frost depth if they plan to keep everything in one piece. TerraSmart is a turnkey solar company with headquarters in sunny Fort Myers, Florida, a manufacturing facility in Columbus, Ohio, and a new operational facility in Selkirk, New York. The company specializes in ground mounting, and its solution to frost heave is its proprietary ground screw.
“Our advantage at TerraSmart is that we can work in a lot of rocky terrains, along with undulating terrains and high slopes,” Faraone said. “We mitigate a lot of the earth work even there.” The ground screws have proven to work in less-than-ideal soil and can even install into buried rock if need be.
To install a mounted panel, four ground screws are drilled into soil, and the rack is set atop them. The 24-inch threaded section of the TerraSmart ground screw is driven below the frost depth line—usually designated by a geotechnical engineer—that leads to frost-free soil below. When uplift force like frost heave is applied to the ground screw, its threads resist being pulled out. They create a cone of soil that further secures the ground screw into place.
“There’s no extra material length required, no additional coatings needed and no extra construction issues in the field,” he said. “It’s just the ground screw ready to go.”
A TerraSmart ground screw can be installed in about a minute.
“When frost heave was first becoming an issue brought up to TerraSmart, we were building some of these arrays in northern regions that we were actually seeing on some project sites—they thought our racks were sinking, and when we came out there and resurveyed. We saw that it didn’t move. We finally realized that actually the soil was heaving up so much that it was raising up around our ground screw and then settling back down.”
The ground screw was holding the array in place. In this case, the soil moving.