With grid-tied PV systems becoming more and more popular, it is important for RE professionals and system owners alike to have realistic expectations of their systems’ performance. Solar-electric power production can be affected by several factors. In this article, we discuss many of those factors and offer helpful tips for maximizing system performance.
One of the most important considerations is locating the PV array to maximize solar exposure. Industry experts recommend siting your array in an unobstructed solar window from 9 a.m. to 3 p.m. You can use a solar site analysis tool (Solar Pathfinder, Solmetric Suneye, or Acme Solar Site Evaluation Tool) or a trusty compass and a sun-path chart for your latitude, to determine true south and the solar window at your site.
Shading. Many people—installers and homeowners alike—fail to consider the impact of even negligible shade caused by overhangs, second stories, trees, exhaust vents, and chimneys. Each PV module consists of dozens of cells that, when even partially shaded, will result in decreased performance, which is like throwing KWH and money down the drain. Lower performance means more electricity purchased from the utility, and less financial return on your solar investment. For those systems that qualify for performance-based incentives, even more revenue is lost with poor performance.
Most PV modules today incorporate bypass diodes that can route power around a shaded portion of the module, thus minimizing power losses from localized shading. Because of the resistance caused by an inactive portion of a series circuit, the impact of shading across a series of cells can be severe. Shading is the number-one system performance problem and should be avoided.
Arrays installed in rows and tilted up from the roof plane require special attention to avoid one row shading the next. Calculating the distance needed between the rows can be complex, but at least for flat roofs, there is a simple design rule—the space between a row of modules should be at least three times the height of the row in front of it. For example, if a south-facing array is mounted on a flat roof and stands 2 feet tall, each row would start 6 feet behind the row in front of it. This will provide a clear solar window from at least 9 a.m to 3 p.m., even as far north as 45 degrees latitude. In the southern half of the U.S., closer spacing may be possible, but minimum spacing should not be less than two times the height of the adjacent row. Those are minimums—wider spacing may be used to squeeze out a bit more energy production in the early morning and late afternoon.
Another method is to set up a first row, and then move behind it at roof level with a Solar Pathfinder until the no-shade spot is reached—that is where the next row would begin.
In snowy regions, drifts or accumulated snow can further complicate row spacing and array placement. Be sure to provide ample clearance under and around the array to help keep it clear.
Proper Array Orientation & Tilt. An array’s orientation and