If you’re a designer of ground-mounted PV arrays, you may notice that only rarely will a project with a perfectly rectangular and level site come across your desk. Instead, you are likely seeing many sites with challenges, such as irregular boundaries, varying slopes, wetland zones and utility easements (which inevitably run diagonally through the array area). And that perfect rectangle? Well, chances are it’s neither the ideal length nor the ideal width to seamlessly accommodate the mounting system specified for the project.
Taylor Davis & Samantha Beaulac
Although solar is a modern-day technology, an old adage still rules the day: “Knowledge is power” (pun intended). In-depth knowledge of the site, the mounting system, and the project owner’s requirements and expectations will enable you to deliver solutions that reduce field issues, lower costs, and ensure customer satisfaction.
Know your project
Maintaining open communication within the project team is essential to successful project design. Among the project owner, civil engineer, electrical engineer, mounting system provider and construction team, is everyone on the same page? Consider the following important design-impacting factors:
- Does everyone understand how the choice of mounting system impacts tilt (or tilt range for trackers), ground coverage ratio, aisle width, and maximum system size?
- Is much site grading planned, and what slopes and grade changes need to be accommodated by the mounting system?
- Are access road and inverter pad locations already set in stone?
- Are wind or snow loads such that non-standard mounting system configurations are required? (This is also affected by height, module choice and other factors.)
A handful of discussions are all it takes to address these issues and make sure there is alignment before the project is so far along that changes could have a major impact on the design (or relationship). Any available survey data, a geotechnical report or other pertinent site information is critical in informing this conversation, as well.
Know your site
Consider first the geometry of the site boundary, as that is typically difficult or impossible to modify to suit your chosen racking and mounting system. Survey data showing site boundaries is generally easy to acquire and, in many cases, is required for the sale or subdivision of land.
However, the boundaries are not the only landmarks that will govern the array layout. The site could have existing structures or utilities that need to be protected, remain accessible, or have minimum offsets to any new construction. There might also be an area where bedrock is visible, which could present problems during foundation installation. Maybe part of the site is a protected wetland and cannot be disturbed. There are many site realities that have the potential to substantially impact both the area of the array and the cost to construct it.
In the face of that reality, how can you help to mitigate risk? When it comes to topography, it is crucial to know both the magnitude and the direction of the sloped areas:
- Do they fall within the capabilities of the mounting system’s standard configuration, or are additional grade-following measures required?
- Is the terrain fairly consistent across the site? If so, it may be most efficient to design all posts for the worst case. If not, different areas of the site will need to be designated as using different posts, which saves steel but adds complexity in design, delivery and construction.
- How does the direction of the slope compare to the orientation of the array rows? Slopes that are parallel to a row will have different impacts on an array than slopes that are perpendicular to the row, especially in the case of trackers. For sites with highly varied slope conditions, designers should consider mixing fixed-tilt systems and trackers in order to optimize the array area.
Once the general site topography is known and accounted for, make sure you review any localized variations or unique conditions. Look for terrain features such as depressions, knolls, gullies or ruts that significantly change grade elevations in localized areas of the site. Assuming that such conditions will arise on-site, it is wise to include an appropriate allowance for post height in the design of the array.
When reviewing site data, consider both the level of accuracy and the reliability of the information. For instance, a topographic map with a large contour interval will not allow you to confidently determine how the grade changes between closely spaced array posts. Aerial imagery will not be as accurate as a traditional site survey, so additional allowances are important when accounting for possible variations. Older data may also have limited utility, thanks to erosion or earthwork in the interim. It can still be useful during early stages in the design but should not be relied upon for construction drawings.
Special consideration should be given to site-specific geotechnical data. The geotechnical report can confirm whether sufficient exploratory drilling or pull testing was conducted to represent the whole site. If a report is not available or field testing has not been completed, you should discuss options for obtaining more information with the project team. In many cases, the upfront costs of a thorough geotechnical evaluation are more than offset by the significant labor and material savings that stem from an optimized foundation design that anticipates ways to address variations in soil conditions across the site.
Know your mounting systems
It’s easy to copy/paste a CAD block of solar trackers or fixed-tilt mounting systems across a site in an array layout drawing, but there may be more to dealing with irregular site boundaries than simply snipping off the modules that don’t fit.
Different mounting systems are broken down into smaller blocks in different ways, and it may take a conversation with your mounting system provider to understand the subtle nuances of your chosen product’s best practices. Datasheets and other product literature will not typically provide the technical details essential in understanding how to best optimize your project.
For example, some trackers are only available in specific block sizes, while others can only be adjusted by subtracting whole rows. These limitations minimize engineering and supply chain complexity, both for the mounting system vendor and for the installer. Other products are much more flexible, but expect the cost per watt of your system to increase if customized configurations are used extensively.
There may be more creative ways to fit awkward sites, such as “stair-stepping” rows of trackers or using multiple ground coverage ratios or azimuths in one site. However, any deviation from the standard configuration could impact the structural components or the electronics and software that control the system.
Fixed-tilt systems, while typically simpler than trackers and made of smaller assemblies, can have many of the same challenges. Arm yourself with a palette of engineered blocks before you paint yourself into a corner by promising more power production than can cost-effectively fit on your site. Once you have these building blocks, set aside some time to experiment with filling in your array area in different ways. Like painting a house, it’s all in the prep work.
It’s also critical that you understand the grade-following characteristics of the chosen tracker or fixed-tilt mounting system early in the array layout process in order to ensure the system is suited to the site’s terrain. The mounting system must be engineered to handle not only the absolute slope, but also the variation in terrain that occurs underneath it.
Some trackers require an exceedingly planar, if not level, installation, while others may tolerate significant slope and ground undulation. The former type of system requires naturally flat land, grading work or extra foundation height adjustability, while the latter may involve higher component costs or installation complexity. Grade-following capability may be difficult to communicate with only a few lines in a product spec sheet, so be sure to have an in-depth discussion with your mounting system provider, preferably with a topographic map in hand, as you develop the site strategy.
Be ready for installation
Designing a cost-effective, elegant PV array on a challenging site is an accomplishment, but if all the installation details aren’t communicated effectively to the field, the project won’t be a success.
Generic construction drawings may only detail the typical (largest) configuration, which leaves field crews to use a calculator (or their imagination) to build the smaller tracker blocks and fixed-tilt tables. Also, construction drawings that only show how to build the system on a flat site might lead to mechanical interferences when the crews follow the same instructions on a steep site. Does your installer know how to field verify that the system is within spec? A picture is worth a thousand words, and providing a set of diagrams drawn ahead of time will save at least a dozen frantic e-mail chains.
Preparing for the unexpected underground can also save many headaches. Having a plan for refusals ahead of time can avoid costly delays for pile drivers, and having a solution at the ready for damaged foundations will keep installers moving, instead of waiting for the engineers to calculate, draw and approve a detail.
Intelligent tech for solar’s future
Advancements in the ability to optimize array designs for challenging sites will rely heavily on improvements in the acquisition and manipulation of site information.
Fortunately, the revolution in geographic information system technology over the past 20 years has set the stage for these changes to occur. The development of commercially available GPS equipment, coupled with advances in optics, computer processing, graphics and 3D modeling software, has culminated in detailed and accurate light detection and ranging (LiDAR) mapping capabilities. For topographic mapping applications, the preferred aerial LiDAR systems using readily available drone technology are gaining popularity. Top it all off with new software purpose-built for the solar industry, and array designers now have the potential to obtain and use high-resolution 3D site data with ease, at a relatively low cost.
The key in today’s solar environment is to leverage this rich data with installation methods that provide the same degree of accuracy. The construction industry has the answer with today’s generation of “smart” equipment. For example, pile-driving machines now have options for GPS integration, which allows an operator to program project plans and watch as the machine automatically installs piles at the correct location and depth while logging drive times and other QA/QC data. When these tools are able to also understand product capabilities and tolerances, they will calculate pile heights on the fly to ensure that the mounting system can be installed properly.
Whether it’s through the latest data-driven technology or the most time-tested communication practices, using information to inform project decision-making and processes yields successful projects that are on time, within budget and on behalf of a winning team.
Taylor Davis is a project engineering manager and Samantha Beaulac is a structural design engineer at SunLink Corp., a full-scope solar energy solutions provider.
