The sun’s path through the sky changes throughout the day: In the morning, it’s low on the eastern horizon; at noon, it’s high in the sky; and at sunset, it’s low again, but on the western horizon. Because a PV array generates the most energy when its modules are directly facing the sun, those interested in getting every last watt-hour from their PV modules often investigate tracking systems.
Trackers are available for almost all sizes of PV arrays. For smaller residential applications, tracking systems are commonly mounted on top of an appropriately sized steel pole. Larger commercial systems will often employ long rows of PV modules set on single-axis trackers.
A good site for a tracked array receives dawn-to-dusk sun. There’s no point in buying a tracker if your site doesn’t begin receiving sunlight until 10 in the morning, or loses it at 2 in the afternoon due to shading from hillsides, trees, or buildings. When located at a site with a wide-open solar window, a tracked array will generate approximately 25% to 40% more energy annually than a static array.
There are two forms of PV tracking: single axis and dual axis. The single-axis method follows the sun’s path from east to west, with the array tilted at a fixed or manually adjustable angle off the horizon. This approach is common in large-scale, commercial installations but can be used in residential applications as well. Dual-axis trackers adjust the PV array to track the sun’s path from east to west and adjust the array’s tilt to account for the change in the sun’s altitude.
Trackers come in two basic types: electrically operated and thermally operated (referred to as “active” and “passive”). Common electrically operated trackers for residential systems typically rely on photosensors that signal motors (via a controller) to orient the array toward the sun as its position changes throughout the day. Thermally operated trackers use the transfer of mass (weight) to follow the sun. In a passive system, a canister and shading fin is attached to the east and west sides of the tracker. These tubes are filled with a material—usually Freon—that vaporizes (becomes a gas) at relatively low temperatures. As the sun hits one canister, the warmed Freon vaporizes and pushes some of the cooler liquid Freon to the other side. This process transfers weight from the one side of the tracker to the other and orients the array toward the sun.
Both electrical and thermal trackers have associated advantages and disadvantages. The big advantage of electrical trackers is that they are extremely precise and will generate more energy for a given period than passive trackers. But electrical trackers are not without their weak points. Because they rely on electronics and electric motors, their reliability is lower than thermally operated trackers. Electrical trackers are also sensitive to damage from lightning. The manufacturers of these trackers have made great strides in making their products resistant to lightning damage, but in the event of a close or direct strike, damage still may occur.
Due to their simplicity, thermal trackers have proven to be very durable in the field. On the downside, because they rely on solar heat, these trackers can be slow to respond. At night, they remain facing west and rely on early morning sunlight to return to the east. This process may take an hour or more depending on ambient temperature. In winter weather, thermal trackers can be somewhat sluggish and imprecise in performance because they are dependent on building up enough heat to vaporize the Freon.
Whether a tracker will be cost effective for your application will depend in part on your seasonal electrical use patterns. Trackers give you more gain in the summer when the days are longer, with somewhat less improvement in the winter. For grid-tied systems with annual net metering, this can be a bonus because the tracker’s excess summer production will help offset your winter utility bill. In PV-direct (batteryless) water-pumping systems, trackers with as few as two PV modules can be cost-effective compared to buying additional modules to pump the amount of water required.