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.
Nuance Energy’s Osprey Power Platform
It’s ironic that a mounting system is named after a bird of prey, but Nuance Energy Group’s set out to make the Osprey Power Platform a feasible option for installers big and small. But what keeps it in place is Nuance’s patent-pending earth anchor.
The Osprey Power Platform is secured using anchors that are driven into soil vertically using a pneumatic hammer or by hand with a sledgehammer. Once two-to-four feet in, a wench is attached to the anchor’s steel cable and hand-cranked, so it’s pulled horizontally in place while submerged. This creates ground tension, making the soil above it a weight, and keeping the anchor in place.
“That is fundamentally different than how other ground mount systems are installed,” said Brian Boguess, CFO and co-founder of Nuance Energy. “We borrowed technology from other industries and applied it to solar.”
According to a Nuance white paper, “essentially, earth anchors work the same way as helical piles and ground screws but with much less steel, greater adjustability and without the need for the specialized installation equipment.” The anchor is held to the rack with a steel cable instead of a pile.
Nuance developed the Osprey PowerPlatform for ease-of-use, making the entire mounting system, racking and module installable with small power tools. As a testament to its effectiveness in cold conditions, the ground mount system is used in a 940-kW solar installation in Old Crow, Yukon, Canada–one of the northernmost solar arrays in the world. Its anchors were driven through 2-foot layers of permafrost with the same tools.
Like the ground screw, the earth anchor is installed below frost depth, lined to the surface with a steel cable. Which means there’s less surface area for frost heave to act on than a traditional driven pile.
“It reduces damage to equipment and modules and reduces the amount of material, whether it be steel or concrete in the ground, to essentially overcome the heaving effects of typical systems,” said Jono Stevens, VP of Nuance Energy. “Whether it be a pile driven system or concrete–those systems sort of require that they don’t move. So in order to combat the effects of the heave, installers have to go very deep, again depending on the soil conditions, and put either a lot of concrete in the ground or a lot of steel.”
AP Alternatives’ Helical Anchor
An inch of surface area on a helical anchor can mean the difference between an attached array staying securely fastened or being uprooted by frost heave. The greater the surface area of a pile, the easier it is for frost heave to jack mounts out of place. AP Alternatives, a mounting and racking solar company from Ridgeville Corners, Ohio, has a 2-inch helical pile that has proven effective in frost-laden soils.
“The number one thing is the 2-inch post. You calculate the outside surface area of that post, and that surface area is what the frost can grab on and pull it out of the ground,” said Josh Von Deylen, CEO of AP Alternatives. “So, using a 2-inch post allows us to have a very small surface area for frost to grab ahold of.”
AP Alternatives was founded 10 years ago in Northwest Ohio, where installers have always faced frost heave. Von Deylen said a lot of progress has been made the past two years for combating frost heave, but they’ve been aware of the phenomenon since the start.
“We really started in a frost zone from day one 10 years ago and understanding the frost jacking and the calculations that are going with it, plus having all that experience, you really are first,” he said. “Back then 40 or 50-plus MW was all there was in the local Ohio-Michigan area, but we had a good test well, as 50 to 60,000 anchors were installed in these frost zones with never any issue. So, we knew that our calculations and the way we were installing and designing is correct–not only from a mathematical standpoint, but we also have proof of time.”
The 2-inch helical anchor has variable helix attachments for use in different soils. Like the previous mounts mentioned, it too is driven below the frost line, wedging the earth above it when pulled upward.
“Other companies you hear put coatings on their anchors; put sleeves on the anchors; drill out their anchors and then they put sand in the hole. Maybe other companies go twice as deep, have massive calculations. We do not do any of that with a helical anchor. As long as we get the helix underneath the frost line, it’ll stay.”
AP Alternatives’ helical anchor has been installed more than 500,000 times, “and a large portion of those are in 3-foot or deeper frost,” he added.