Given the right circumstances, tracked PV arrays can be a good design strategy, but at what cost? The answer to this question depends on several factors, including system size, solar access, and system type. See the “Cost” table (above) for a cost comparison of fixed versus tracking PV systems. Although the table shows that up-front system costs can be less expensive for a tracked system (even in areas like Seattle), this cost comparison does not include additional maintenance costs or savings from incentive programs. As noted, if the rebates are based on installed watts, this could make a fixed PV array less expensive.
Tracking arrays, like all pole-mounted arrays, are like big sails: Set several feet above the ground, they can experience significant wind-loading. To keep them in place, arrays are usually mounted on large poles (6 to 8 inches in diameter, or even bigger for taller poles), which must be set in large concrete footers.
Tracker components, especially the drive assembly for electronically controlled trackers, can be heavy, tipping the scales at more than 200 pounds. Often, they’ll need to be placed with a crane, backhoe bucket, or some other mechanical means.
When installing module home-run wiring, leave lots of wire length from the modules to the junction box on the pole. This will ensure that when the tracker is in its farthest positions the cables are not pulled tight. (Bundle these cables together with zip ties to keep the installation as tidy as possible.)
Electronically controlled trackers have an optical sensor that needs to be mounted at the top of the array. Also, a tracker controller needs to be mounted on the drive and powered either by the battery bank or AC power, if available. In the latter case, a switching power supply, another option at purchase, converts AC to DC to power the DC tracker controller. In either case, a power line needs to be appropriately planned for and run from the power source to the controller. Although the controller can be powered directly from the PV array, this option is discouraged—because there’s no power available from the PV array after sunset, the tracker must wait until the next morning to turn back to face the sun. During the summer months, the optical sensor can get confused as to which direction it should return, since it will end up pointing northwest in the evening. Tracker rotation must be limited to 180° to avoid this confusion. However, this limits the energy gain the tracker can yield in the summer months, since the sun’s azimuth angle exceeds 180°.
The decision to invest in a tracking PV array is a personal one based on budget and your willingness to deal with possible maintenance (or even repair) issues, since moving parts can break down. In the right circumstances, though, tracking systems can save money up-front and keep you on track to greater energy production.
Justine Sanchez is a NABCEP-certified PV installer, Home Power Technical Editor, and Solar Energy International instructor. Justine dreams of one day installing her own tracking PV array, but for now lives, works, and teaches from an on-grid, fixed-mounted, PV-powered home in Paonia, Colorado.
Ed Marue holds a bachelor’s degree in physics. He’s the principal of Solar Lava Development Corp., a sustainable architecture company, and the former owner of Tri-Ex Tower Corp. and chipmaker High Voltage Devices.
Array Technologies • www.wattsun.com
Small Power Systems • www.smallpowersystems.com
Zomeworks Corp. • www.zomeworks.com