With PV modules, energy production is golden—as long as sunshine is hitting them. But shade (and even partial shade) can make your modules as useful as garden gnomes.
A PV module’s output is directly proportional to the amount of sunlight hitting it. Modules produce electricity when photons—little packets of energy contained in sunlight—hit the cells and knock available electrons loose and into motion. When fewer photons hit the PV cell, for example due to haze or poor orientation, fewer electrons are put into motion, and less electricity is produced. But shade—even a small amount—can in some cases shut down production completely.
Most modules are manufactured with built-in bypass diodes that can help mitigate the effects of partial shading (see “Bypassing Shade” sidebar), but even a shaded row of cells can disable the module. Shading’s impact necessitates careful site planning and design considerations for solar-electric arrays.
Most sites will have at least some shade to consider, whether it is a neighbor’s multistory home or trees on your property. And while wide-open, dawn-to-dusk exposure is ideal, PV system designers generally shoot for a shade-free solar window from 9 a.m. to 3 p.m. (“solar time” for all days/months of the year). These are the hours the majority of solar radiation is available. However, local climate variations may affect that. For example, in some areas, early morning fog can shift the “prime” solar window toward sunnier afternoon hours.
Predicting shading throughout the year from various obstructions—such as tall trees, nearby buildings, roof dormers, and even chimneys—can be challenging if done by sight alone, requiring many observations over the course of the year. But several tools, including the Solar Pathfinder, the Acme Solar Site Evaluation Tool, and the SunEye, can help you quickly assess shading on your site throughout the year—with one site visit. Although these tools differ in technique and price, all will get the job done. All require that they be used at a proposed array location to evaluate it.
Solar Pathfinder ($299; with case and tripod) • www.solarpathfinder.com
With its easy-to-tote case and simple instructions, the Solar Pathfinder has been a common solar site analysis tool since the late 1970s. It uses a clear plastic dome over a sun-path diagram to show surrounding obstructions.
This device is first oriented and leveled, then users can take a digital photograph of the setup for use in the software program, or trace the superimposed reflections onto the black chart with a white pencil, which reveals which months and times of day objects may shade the array (see photos, opposite page).