Solar Design Choices

In the past, design options were based on a project’s latitude. Solar engineers say that a module’s tilt had to be equal to the latitude of the location. In addition, the row spacing was measured by the shadows from the rows on the winter solstice.

With innovative technology, economical hardware, and improved software tools, the design is changing.

Background of Design Rules

Modules were tilted at or near the latitude of the project’s location. For example, this means a 25° tilt in Miami and a 35° tilt in Raleigh. This orientation maximizes the sun angles yearly. This points the modules as close as possible to the sun’s position in the sky.

The row spacing was calculated by a rule of not permitting inter-row shade on the winter solstice from 10 a.m. to 2 p.m. Modules tilted more heavily at higher latitudes, but since the sun is lower in the horizon in northern locations, the shadows were longer.

The row spacing were extra big. Such rules sustained a high energy productivity per watt, yet resulted in smaller systems on space limited rooftops. When modules were pricey, the rules made more sense. But as prices plummeted, the high-tilt approach became less practical.

With cheaper modules, a lower tilt is optimal. This squeezes more watts into a unit of area, sacrificing yield per peak-watt to optimize total energy yield. Most were installed at low tilts, but the row spacing rule of avoiding shade on the winter solstice from 10 a.m. to 2 p.m. remained.

This improved alignment between the systems. While the sun angles on the winter solstice differ based on latitude, the net impact on row spacing is modest. This results in system sizes that’s around 10% different between Miami and Seattle.

Solar comes with Software

As software tools enabled system designers to review different combinations of row spacing and tilt, the final stage of design is made on iteration and optimization. It’s the most northern locations, Spokane and Boston, where a higher tilt and wider spacing are ideal.

As an outcome, a rooftop in Miami versus Seattle vary in size by 14%. Ideal designs for different locations diverge based on various aspects of each site. Locations have contrasting weather patterns, temperatures and sun angles. The designs don’t alter too much. Optimized systems are trending toward looking more similar.

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