Ground-Mounted Racking Considerations
Inside this Article
Part 1: Site Grading and Design Optimization
By Samuel Laughlin and Bill Reaugh
When developers consider a piece of land for a large-scale ground-mounted PV power generation asset, the costs for grading and earthwork can be significant drivers of project viability. The project developer’s or owner’s goals determine in part when, where and how much to grade. These decisions must also fit the requirements of the site with respect to AHJ controls and the mechanical limitations of the equipment the developer may use.
The best approach to this puzzle is one that integrates a holistic view for these requirements across multiple engineering disciplines, including civil, structural, electrical, mechanical and geotechnical, and water resources. A few of the first questions to ask are: How does the topography behave? Is the proposed equipment capable of dealing with the site in its current state or are modifications required? If the requisite site modifications are significant, are there other solutions that align better with the project’s financial goals?
To Grade or Not to Grade
Grading typically includes two major activities: cutting, the process of removing quantities of soil, and filling, the process of building up quantities of soil. Ideally, the amount of soil imported to or exported from the site is near zero to reduce both costs and environmental impacts.
The needs and conditions of the land underlying the solar array structure make each project unique. Depending on site conditions and construction requirements, a piece of land may need no preparation, minor surface clearing and grubbing of subsurface plant roots, smooth grading or full grading. Generally speaking, developers perform grading because the site requires it or because doing so will support plant optimization.
In certain circumstances grading is unavoidable. The most common reasons for grading are to meet AHJ requirements or best practices for access roads or storm water management. Grading may also be required to conform to vendor-specified mechanical tolerances for the mounting system.
On-site access roads. AHJs and industry best practices dictate minimum and maximum slopes for access roads, as well as compaction and surface maintenance requirements. Grading is required where the existing topography does not meet these longitudinal or cross-slope requirements. While the local fire department typically has the final say on access roads, the local building department or the site owner may also have applicable requirements.
Storm water management. Site-specific hydrologic characteristics are a critical factor in determining grading requirements and plant design. If a site lies in a flood zone, for example, the flood depth determines the minimum height for electrical equipment. Storm-induced runoff and scour affect minimum pile embedment depths. The contributing watersheds and historical water flows may dictate detention basins or improvements to the existing storm water channels. Local environmental agencies may have requirements related to dust or water quality that impact these grading activities.
Mechanical tolerances. In general, fixed-tilt mounting systems are more capable of dealing with topographical changes across a site than are single-axis trackers. However, many large-scale ground-mounted systems use trackers to maximize energy production from the available land area. Trackers have a maximum slope (% grade) associated with the north-south axis of the torque tube and, if applicable, the east-west elevation of the driveline. Should existing slopes fail to accommodate the maximum slopes of the torque tubes or drivelines, grading is one possible solution.
While tracker equipment manufacturers specify maximum slope values, installation guidelines or product datasheets do not always provide these design criteria. Instead, some manufacturers provide a grading requirements document or similar reference upon request that details civil engineering needs for sites. This document provides information about mechanical tolerances to help engineers determine project-specific design criteria, including maximum slopes and maximum and minimum pile elevations above grade.