Ground-Mounted Racking Considerations: Page 4 of 7
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Fencing. Many existing landfills are already fenced, and some of these fences require only minor modifications to comply with National Electrical Code requirements. Unfenced landfills can present significant design and budgetary challenges. The landfill membrane often extends for many acres beyond the usable array area. Fencing this long perimeter is a costly and time-consuming proposition. More often than not, project developers need to contend with wetlands located at the perimeter of the landfill that could require additional permits through local conservation committees, extending the project mobilization timeline. If it is not practical to build a conventional fence, system designers can consider adding a ballasted fence at the array boundary, provided they factor the fence location into the array layout.
PV SYSTEM CONSIDERATIONS
After taking landfill design into account, project developers need to adapt their PV system design and installation practices to the site.
Build from the vegetation up. It is a good idea to hire a site contractor who has experience with landfill closures or making landfill repairs. Given the minimal amount of material above the geomembrane liner, the most productive design and construction strategy is to build the PV system up from the vegetation layer. This means that the array foundations, equipment pads, electrical infrastructure and access roads are all installed above grade. The goal of this design approach is to maintain the existing protection of the cap and to preserve the functionality of existing storm water controls.
Prior to commencing construction activities, an authorized agent should inspect the existing landfill conditions to ensure that both the gas vents and storm water controls are intact and that no depressions due to settlement exist. The developer needs to remedy any depressions under the supervision of the AHJ prior to construction. During construction, crews need to immediately remediate any ruts or divots caused by construction traffic and weather events. The contractor can alternate construction activities among different areas or project phases to allow the protective layers of the landfill to dry out between weather events or while the crew is repairing them in an effort to minimize project delay.
Road layout. The first step in the design process is to determine the proper location of the construction road. Placing the access road along the spine (the highest points) of the landfill reduces or minimizes impacts to existing storm water controls. If space and landfill topography permits, it is ideal to have a road entering one side of a landfill and exiting the other, as this is beneficial for project logistics and project velocity. At a minimum, PV designers need to provide a sufficient number of turnarounds to accommodate construction traffic and material delivery.
Array layout. Designers determine the usable array area flanking the access road based on the slope of the terrain and the mechanical tolerance of the mounting system. Since ballasted mounting systems are susceptible to sliding forces and constrained by the allowable bearing pressure on the landfill cap, these systems are commonly limited to a 5°–7° slope. Once designers have identified existing landfill features and areas with acceptable slopes, they can begin to lay out the array, keeping in mind maintenance activities and cap protection.
Mounting system selection. Developers also must consider differential settlement when designing this type of system. As the landfill decomposes, the surface naturally settles differentially due to the varying types of material decomposing below. Landfills will settle 10–20 feet over their lifetime, and settlement of 1 foot per year in the first years of closure is common.
To tolerate this settlement, designers should use a statically determinate mounting structure that they can analyze using static equations of equilibrium to determine reaction forces. A structural design with two foundations per array table that allows for future in-field adjustment is generally best for withstanding settlement over the life of a system. By comparison, a structural design with three or more foundations may be subject to intolerable stresses that result in module deflection and possible destruction.
Cable tray. Landfill projects have unique electrical infrastructure requirements in two ways: The design needs to account for settlement, and the site has insufficient cover material to allow burial of conduit or cables. Since landfill applications require an above-grade solution, designers typically specify cable tray. Cable tray comes in 10- or 12-foot sections that installers do not have to physically connect to one another, so it can tolerate differential settlement, provided the conductors offer adequate slack.
Each cable tray section generally has four independent height-adjustable feet, allowing for installation on uneven ground. The base area for these feet is often adequate to disperse the weight of the cable tray and conductors, as well as keep the point loads below the maximum allowable bearing pressure. As an added benefit, cable tray remains visible to the landscaping crew even when the grass grows tall between mowings, and physically protects the electrical cables from both mowers and string trimmers.