On commercial rooftops, design trends are all about maximizing energy density. Module selection is a huge factor there, but so are the layout and tilt decisions — figuring out the perfect shape and tilt to mount as many modules as possible without compromising their performance.
Pairing the right racking system with a flat-roof space opens up a world of possible equations. Use a racking system that will position the panels to maximize the energy output, which includes the tilt angle, inter-row spacing and the direction the panels will face. As always, geography matters. For one, the roof’s azimuth, or the direction the pitch faces. For a perfect south-facing system, the azimuth should be 180.
But new systems are tweaking the traditional. East/west systems are becoming popular below the tropic of cancer. Designers are playing more with tilt angles, with the general trend moving toward 5-degree tilt — likely to reduce inter-row shading without compromising the number of modules used or resulting in too much soiling.
“Rooftop energy density is maximized by fitting more panels on the roof using a 5-degree racking system,” said Jonah Coles, product solutions manager, Ecolibrium Solar. “The key to fitting more panels on the roof is to use racking with a small footprint and narrow inter-row spacing. The combination packs in panels, yet the inter-row spacing is wide enough to allow for the working room needed for ease of installation and post-installation maintenance.”
But the tilt decision isn’t one-size-fits all. Everest Solar Systems notes tilt angle efficiency correlates to latitude — the higher the latitude often requires a higher tilt. The latitude in Hawaii, for instance, allows a system to be virtually flat, but there needs to be enough tilt to keep the rain from pooling and to keep dust off the modules. Brandon Gwinner, regional sales manager, SunModo, puts that minimum at a 4-degree tilt.
“The tilt degree is dependent on the region/location and optimum output based on TSRF,” he says. “The minimal tilt degree racking systems are typically to maximize the number of modules you can get on a roof without your rear post being 8 ft off the roof and to get the most energy density/power density per the project.”
There are also some wind/snow load considerations that can keep tilt below a certain height/tilt degree, as well as parapet walls and billowing of wind. The installer has to find the balance between production and engineering capabilities.
Also, installers looking to maximize production in summer months should consider using lower tilt angles than installers looking to maximize production in winter months. In snowy northern climates, Everest Solar recommends a 10-degree system tilt angle, which is better for shedding snow, plus the wider inter-row spacing allows more room for snow to land without piling up and casting a shadow or covering the modules.
“If you can hit your power goal with a 10-degree system, then 10-degree would be the system of choice. If not, 5-degree racking can enable a successful system when 10-degree wouldn’t fit enough panels to generate enough power,” Coles said.
Commercial installations have significantly more requirements than residential installations, so understanding jurisdictional requirements at the onset of the project will make the process go smoothly. Some states, like Oregon, do not require extra engineering when the tilt is under 18 in. on the back edge of the array, based on a prescriptive path. So, cost analysis vs. ease of permitting is a factor for tilt decisions too.
The inter-row spacing issue
Tilted PV panels cast shadows on the rows of modules behind them, necessitating a gap between rows to minimize the effects of production loss due to shadows cast on panels in anterior module rows. Here are a few ideas to mitigate the impact of this phenomenon on your PV installation via Peter Abou Chacra, engineering consultant, SunModo.
- Reduce the tilt of your south-facing array. For peak energy production on a per-module basis, PV modules have an ideal incident angle with solar rays emanating from the sun. For some installations, however, it may make sense to reduce the tilt of the modules to a less optimal incident angle. Though this means less production on a per module-basis, it can mean a significant increase in the daily unshaded collection time for the array. This gain in effective collection time can offset the losses caused by a sub-optimal tilt for the module itself. Using software dedicated to modeling and analyzing a system’s performance at a different tilt angle and inter-row spacing should figure out the best path.
- Locate your system on a south-facing slope. Even a five-degree inclination can have a marked impact on the amount of inter-row spacing required. This can significantly increase the number of modules you can fit in a given area.
- Consider 3-in-landscape or 4-in-landscape monoslope installations. Coupled with a low tilt, this strategy can reduce inter-row spacing significantly on a given installation since modules on the same structure and slope don’t require significant spacing between them. This can be particularly effective if you can gradually elevate the anterior monoslope PV structures as you work your way north through the site.
— Solar Builder magazine