By Hitesh Patel, manager of project engineering, PCL Construction
Solar energy is now a proven, reliable and competitive energy solution and has gained viability in colder environments due to government policies, incentives and Renewable Portfolio Standards set by states on utilities. Solar PV systems are typically designed for a 20- to 25-year lifespan and provide an energy source at a set competitive rate for its lifetime. However, solar projects can be susceptible to environmental conditions such as frost heave, which inflict significant maintenance costs and drastically shorten the facility’s lifespan.
Frost heave is a geotechnical phenomenon that occurs when water freezes in subsurface soils and creates ice lenses that shift and move upward. If the frozen layer is well bonded to the solar rack foundations, the upward movement of the soil may also move the foundation. The rate of frost heave is not uniform and can occur at more than 1 in. per day when extreme conditions exist, depending on local frost depth and the presence of subsurface water. When frost heave occurs, it impacts the foundations and racking systems resulting in structural deflections with costly consequences including cracked modules, and damaged conductor and grounding. These components all require extensive replacement and repairs to bring the system back into operation, which is not affordable to any project owner.
Companies like PCL Construction that work in frigid climates like the Great Lakes Region of North America are all too familiar with the frost heave phenomenon. Unfortunately, there are several projects across North America that continue to experience technical, economical and schedule impacts caused by frost heave. Significant stresses can develop due to imposed frost heave conditions, causing racking system structures and connection failures as well as PV module failures due to broken glass, microfractures and seal failures, and mechanical damage to electrical terminations and cable trenches underneath the frozen soil.
However, these impacts can be avoided if project teams implement a cold weather mitigation strategy from the start of the initial design phase.
The use of particular types of foundation methods can prevent the adverse effects of frost heave and help EPC companies avoid costly repairs and lost revenue. Previous experience with ground-mount PV foundations in cold climates has demonstrated that bearing-type footings, such as helical piles and concrete spread footings, perform well in frost-susceptible soils when local recommendations are followed. Allowable deflections and differential movements limits are provided by the racking manufacturer or the project’s contract documents. Owners concerned about frost heave for their projects should consider hiring a local firm that is experienced with the issue to perform a geotechnical study to optimize parameters for foundations and help ensure the project achieves useful operation throughout the project lifecycle.
Protection and Testing Strategies
Projects with a target lifespan over 20 years require additional measures to prevent the effects of frost heave, which can be exacerbated by corroded steel piles. Galvanizing the steel’s surface is a common method to protect against corrosion and frost heave, as corroded steel tends to have higher frost adhesion values. All racking attached to the fixed piles must have adequate ground clearance to allow movement of the ground without catching braces or other components that can then be damaged or cause damage elsewhere. Local weather reports or the National Weather Service can provide soil temperature depth maps to determine anticipated frost depth, which varies based on soil type, water content, snow and vegetation cover on the surface.
A pile verification program is progressively more important in areas where frost heave is prevalent, as it can impact the final design. EPC companies should implement a design verification testing program to test and verify the pile capacity and optimize the pile size and embedment depth to account for frost heave loading conditions. Foundations with a wider width below the frost line may help the pile resist frost uplift. Using probing as part of the program can help EPC companies identify areas of potential concerns, such as soft or obstructed areas.
It’s imperative to install foundations under a quality control program that includes visual inspection, torque measuring and production proof testing that tests 100% of the anticipated frost load and deflections. By testing foundations in advance of installation, teams can correct issues early in construction. If the team notices excessive deflections, additional proof testing is needed to specify the problem area. Pile remediation can be performed to include deeper embedment with longer piles or spliced piles, larger or multiple helix piles or insulation. Examining each case individually will determine the best remediation type as conditions can vary dramatically between sites.
EPC companies should also account for the stress frost heave has on the equipment concrete pad and ancillary equipment, including the transformer medium-voltage terminations. Excessive ground movement can be mitigated by allowing for additional loops in the cable and installing horizontal runs in the PVC conduits. The additional cable will prevent the termination devices from experiencing direct stress which in turn will prevent any electrical fault or shock hazards. Conduits installed underground should be equipped with expansion coupling in horizontal runs so it can absorb the frost heave horizontal movement in the soil. Combiner box output and inverter direct-current recombiner box connections can be protected by expansion couplings in vertical conduit runs. Conduits installed as a means of mechanical protection to fix electrical enclosures should have expansion joints based on ground heaving conditions at the time of installation. Cable terminations subject to movement by frost heave should have slack between the termination and the top of the heaving cable trench.
If not appropriately designed, cable trench heaving and combiner box foundation pile movement can cause damage to wire strings as well. The heaving can cause stress in the string wiring and DC feeder running back to the inverter. Proper slack and support to these cables should be allowed to alleviate the movement stress on them. Inverter connections in DC and AC termination box should be protected in the event of frost heave by applying these preventative measures. The combiner box and its foundation, trench design, frost penetration depth, vertical cable stub up and conduit protections should be reviewed as a system in summer and winter months to ensure differential movements are understood and the system performs as required.
When a pile moves, it creates stress on all aspects of the solar PV system, from electrical infrastructure to foundations. If owners want their projects to meet their target energy outputs for the project’s contracted life cycle, they should utilize an EPC company that understands frost heave and its potentially serious ramifications. EPC companies deploying solar energy solutions in ice-prone areas must develop a solution to prevent frost heave, and ensure the processes are in place to guarantee success.