Designing Solar Projects on Non-Flat Terrain

There are many moving parts during the early stages of designing a solar project site. From site feasibility to offtake strategy, every phase of the development is connected and critical to delivering a successful project. As the project enters product selection and layout of the arrays, developers lean on racking suppliers for design support. The site layout and tracker technology can often influence other areas of the design and project budget, aside from just the dollar value of the equipment. Systems that are capable of following the natural terrain can add value through increased use of land, as well as savings on time and costs of development. Therefore, involvement of racking solution suppliers in early-stage design is imperative for full project optimization. 

To begin the design process, an equipment-specific 2D layout should be created. The developer or EPC design teams can then incorporate inputs and boundary conditions for the full site design and proceed with the 3D layout. After the 3D layout is complete, an X/Y/Z point file of the foundation locations can be generated. From there, tracker equipment providers can identify the appropriate technology to accommodate the various angle changes and produce an accurate bill of materials. 

As designers work through the initial site layout, they should understand how various inputs impact the equipment bill of material, and subsequently the cost. Typically, if space allows, designers should avoid a one string row configuration, as these are often not cost effective. Sticking with two and three string rows for standard ~500W modules produces an economy of scale by increasing the number of modules allocated to a single motor and controller unit.

Another major impact to the site design cost comes from the ratio of exterior to interior rows. Exterior rows or bays are required when the row is fully or partially exposed to wind. These are typically the three outer-most east and west rows of each block. However, the criteria for this classification may vary slightly depending on each racking provider’s independent wind tunnel studies. Exterior rows require thicker torque tubes and often other alterations, such as limiting bay lengths, additional drive units, or adding additional mechanisms to withstand torsional loads. Effort should be taken to limit the use of exterior rows throughout a given site. To do this, some racking suppliers have taken innovative steps to allow for longer, or combined, rows. But even with these options available for the structure, the overall site boundaries will largely dictate the exterior/interior row ratio of the site. Unique site features including environmental areas of concern such as wetlands, soil conditions, and wildlife, can further break up blocks.  With more distributed boundaries and blocks, generally the site becomes more fractured, making it difficult to limit exterior rows.
 

In addition to environmental site limitations, there are also constructability constraints or preferences that contribute to the design process. Site layouts need to consider civil strategy, including roadway access and install capabilities. Roadways are placed to support installation and O&M, but they can also be strategically placed to minimize exterior row quantities by avoiding splitting blocks and positioning in terrain areas where installation could be more challenging. Areas with extreme terrain, or cliff like drops, are not realistic building locations. Even if the tracker manufacturer can accommodate the slopes, driving foundations and completing install may be infeasible or dangerous for the equipment and laborers. Some tracker companies claim slope allowances of up to 37%, which is well beyond what would likely be feasible. Aside from civil criteria, developers or EPC’s may have other requests to follow for the design such as allocating a specific quantity of modules to certain areas throughout the site to accommodate inverters or interconnection strategy; or lining up motor piles in the block East-West to optimize electrical & cabling.

Of course, designing to maximize production is a key consideration. Backtracking is a common strategy used to eliminate inter-row shadowing and has proven effective with the potentially devastating energy loss from inter-row module shading. Although plane-of-array losses can increase on non-flat terrain, they are still typically less than 1%. If seen as necessary, designers could look at ways to separate the site to best match the terrain and decrease the plane of array loss or increase the row-to-row spacing.

Another layer of complexity on non-flat terrain is the incidence angle from the sun. Energy generation on north-facing slopes is lower than on flat or south-facing slopes, so north-facing slope limits should be defined early on to understand which areas of the site are actually usable. A good rule of thumb is that as north-facing slopes start to approach 5 degrees, the energy losses become material to the project.  It is important that designers understand the trade-off between complete optimization and system cost before any final design is determined.

Developing layout design software can be a real asset in improving upfront design speed and accuracy. Based on key predetermined variables such as boundaries, civil and electrical structures, string size, module type, target capacity, etc., design software can be used to generate and optimize multiple layouts in a matter of minutes. This will allow the racking company to identify the solution option best suited and respond quickly to requests.

In addition to tools for completing conceptual layout design, the site-specific layout with corresponding point file can be used to optimize each of the variable structural components within the site. With tools to properly analyze the site topography, the net angle change at each foundation can be identified allowing for the selection of appropriate bearing types or foundation reveal requirement. Similarly, torque tube lengths and sizes can be automatically chosen based on their location and loading within each block. Lastly, foundation placement along the length of each row should be carefully considered on non-flat terrain. Northing distances decrease with increasing slope, which can be accounted for in 3D Civil CAD programs. Once this analysis is complete, a detailed visual of the layout can be created to display each of the individual components used at each point of interest. These outputs are useful not only for the preliminary design work, but also for install teams to effectively plan and stage the site.

Lastly, in this current climate of uncertainty, it is more important than ever to have system flexibility and reliable partnerships with equipment manufacturers.  Key component changes throughout the design process, or following award, are not uncommon, and tracker solutions need to be able to adapt to these changes with ease. With changes to modules for example, a shared, standardized or easily swapped module attachment bracket reduces project delays and costs.

The design process for solar projects can span years and the earlier tracker manufacturers can get involved and share design expertise, the better optimized the site will be. Nevados’ innovative All-Terrain Tracker and supporting controls and software solutions make challenging terrain sites feasible and as simple as flat land throughout the entire design process.