Degrees of Separation: How to mount commercial rooftop PV systems to maximize energy

Ecolibrium EcoFoot system

Ecolibrium EcoFoot system

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.”

RELATED: Why energy density matters — and three ways to maximize it

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.

SunModo Sunbeam

“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

Why energy density matters — and three ways to maximize It

as much energy for the space as possible

System designs like this squeeze as much energy out of your available space as possible.

In previous generations of solar installations, the primary cost-driver was the electricity-generating modules. But in the past five years, as costs for the modules themselves have fallen, myriad other costs have become far more central to making the economics of projects work.

The increasing financial burden of fixed-cost items like permitting, interconnection and customer acquisition — just to name a few — forces project designers to evaluate closely whether it’s worth paying a premium for the most efficient solutions. When comparing options, engineers need to look at costs over the project’s lifetime, not just the dollars‑per‑peak‑watt cost of respective modules.

Solar installers generally have less space than needed to generate all the electricity used by a building. Even for distributed-generation ground mounts, where the generation is co-located with the load, developers have space limitations. All of this explains why energy density has become an area of concentration for new product development by manufacturers as of late.

Energy Density Explained

I use the term energy density in this case to indicate the amount of energy that can be generated by a PV system per unit area in a year. Energy density is something that system designers can leverage to achieve the best levelized cost of energy (LCOE) for all solar projects today, whether it’s in rural, suburban or urban areas. A more efficient solar panel or more panels squeezed into the same area will produce more kilowatt hours per square foot.

In recent years, the race to maximize energy density has moved into high gear. From May 2016 to December 2016, four companies broke panel efficiency records (Hanwha set a record in June that it topped in December). Researchers in Australia set a new record at more than 35 percent efficiency, and NREL recommitted itself to the race to find new materials for solar cells that could break the theoretical 29-percent maximum efficiency of traditional silicon solar panels.

The most popular candidates to replace traditional silicon cells are perovskite (which are not yet commercially available) and CIGS thin-film cells like those produced by First Solar. Thin-film panels can not only push efficiencies higher, but they have an added well known benefit of half the temperature coefficient of traditional silicon PV cells.

LCOE Heroes: How inverters drive down PV levelized cost of energy

Three Technologies

But those aren’t the only technologies that can help solar installers produce the best energy density for their clients’ investment. Here are three technologies that will make the most of the space you have, no matter where your project is located:

1) Bifacial solar modules: Bifacial modules capture the sun’s energy on the front and the backs of the panels, significantly increasing the amount of sunlight each module can convert to electricity. This technology is inspiring new thinking on installation, including the idea of installing bifacial modules at unusual orientations, like west-facing vertically, which exposes both sides to sunlight at the same time. The technology is still evolving, but for roof-space-intensive projects, I believe bifacial modules hold a lot of promise.

2) Creative racking solutions: When space is sparse, creativity is crucial. For example, when Standard Solar won a DC Department of General Services (DC DGS) contract to install solar arrays on 30 buildings in the densely populated Washington, D.C., area, the engineering team realized quickly that we had to figure out how to maximize the energy density on such tight roofs. The solution, as it turned out, was a high-density racking solution — double-sided with limited row spacing — that allowed us to pack more panels into the same space. As a result, the project was able to realize more kilowatts on each roof because the racking systems allowed an array design that was as aggressive as possible to produce a higher power output from the space.

3) MLPEs: Module-level power electronics provide high granularity for module power output control, shade tolerance and data monitoring for the asset managers. MLPEs have been mostly relegated to residential applications where significant shading issues are common. However, MPLEs can deliver better yield by reducing losses even when no shade is present. MLPEs also open wider options for array design and placement — an advantage for engineers looking to maximize the power production of any given space.

As the costs of modules continue to plummet and the share of other costs increase as part of the financial calculations involved in a solar project, installers and engineers must perform a detailed cost-benefit analysis of how to increase the amount of power a project can deliver from any given space to maximize project economics.

And as the space for arrays becomes increasingly restricted, engineers and installers will have to use all the equipment and innovation at their disposal to increase energy density — and keep the solar revolution growing.

C.J. Colavito is director of engineering at Standard Solar.

— Solar Builder magazine