Whether it’s hitting the road in sleek speedsters (electric, of course), surfing the powder on snowboards, or pounding the trails on mountain bikes, most of us love being in motion. Even those who consider themselves untethered to mainstream thrills are often mesmerized by the spinning blades of a wind generator or the swiftly moving waterways powering a microhydro turbine. But there’s not much excitement in a fixed photovoltaic array in action—beyond watching the electrical meter spin backward.
Because they don’t rely on moving parts to produce electricity, most PV arrays do their job without much fanfare or fuss. However, PV modules produce the most power when they are aligned perpendicular to the sun’s rays, and since the turning Earth moves the sun across the sky, the only way to do this is to put the array in motion by using a tracking device. Under the right circumstances, tracked PV arrays can intercept much more sunlight than stationary PV arrays. While exact performance numbers will vary depending on the site location and system specifics, trackers typically can increase overall energy production by 25% to 40%.
Trackers are divided into two groups: active and passive. Most active trackers use an optical sensor to determine the sun’s position, and an electronic control and one or more motors to position the array. Single-axis active trackers are set to a fixed tilt angle (which can be adjusted seasonally) and then track the azimuth angle of the sun across the sky. Dual-axis devices track both the sun’s azimuth and altitude angle (its angle to the horizon), and offer the most accurate tracking, but cost more (see “Cost of Tracking” table on page 54). Active trackers are susceptible to damage from nearby lightning strikes, which can damage the motor and controls.
Passive trackers use the heat from sunlight to vaporize liquid Freon contained in canisters mounted on the tracker. As the gas expands, it forces the liquid to the canister on the other side of the tracker, and the change in weight causes the rack to move. Passive trackers are single-axis and will follow the sun from east to west. These trackers will not track the altitude angle of the sun but can be tilt-adjusted seasonally. The primary advantage to passive trackers is that they do not depend on electric motors or controls to function. However, because they are thermally controlled, and dependant on the intensity of incoming sunlight, these trackers may have difficulties in extremely cold climates, and/or in hazy conditions, when delayed heating can reduce overall energy production.
Because tracking arrays receive more peak sun-hours, energy production is improved. This allows a smaller array to be installed, shrinking the required array footprint—a benefit for sites with limited space.
Sites with wide-open solar access (shade-free from dawn to dusk with low horizons) can benefit the most from a tracking array, wringing every last electron from the sun over the seasons. The energy gained during early morning and late evening hours can be significant—especially during long summer days when, at many northern latitudes, the sun rises in the northeast and sets in the northwest.