Manufacturers are constantly making incremental improvements to their solar panels to create a higher energy yield per unit than previous and competing models. Another proven way to increase system output is by using trackers, which, unlike fixed-tilt ground-mount systems, make solar panels follow the sun’s path throughout the day.
There are two main types of solar trackers available on the market: single- and dual-axis.
Single-axis solar trackers track the sun east to west, rotating on a single point, moving either in unison, by panel row or by section. Dual-axis trackers rotate on both the X and Y axes, making panels track the sun directly.
“Solar trackers make financial sense when the yield gain over fixed-tilt applications outweighs the capital expenditure of the system,” said Alex Au, chief technical officer at NEXTracker. “In the past decade, the cost of solar trackers has come down considerably with [levelized cost of energy] value engineering and overall demand for these systems, given a 15 to 30% production gain over fixed-tilt systems on the same size array.”
Tracking technology is not new to the solar market, but single-axis solutions have recently become a standard in utility-scale applications. Berkeley Lab found that 70% of utility-scale solar installed in 2018 used tracking systems.
“I’d say even eight years ago, trackers were either nonexistent in a utility-scale market or were just a small fraction of the share relative to plants being built in the U.S. right now,” said Jeff Krantz, CCO at Array Technologies.
Single-axis trackers will gather less energy per unit compared to dual-axis trackers, but with shorter racking heights, they require less space to install, creating a more concentrated system footprint and an easier model for operations and maintenance.
Single-axis trackers are split into centralized and decentralized tracker types. Centralized or distributed trackers use a single motor to power a driveline between rows that will move an entire segment of panels. Decentralized systems have one motor per tracking row. There are also instances of trackers with motors present on every set of racking, making rows more adjustable during installation and in some cases allowing them to track independently of neighboring modules.
“In recent years, with rapid reduction in global solar module pricing, banks and developers are favoring single-axis tracking systems’ power capacity (number of modules) to achieve PPA targets over other such systems like complicated dual-axis applications that improve efficiency,” Au said.
Single-axis solar trackers were initially designed to work like Venetian blinds with panel rows moving in unison throughout the day. Now, considerations are being built into tracking software to have panel rows compensate for diffuse light, adverse wind conditions and row and horizon shading.
On cloud-covered days, sunlight doesn’t reach the Earth’s surface with direct beams — it is received as diffuse light — which means a panel facing directly at the sun won’t necessarily have the most generation. It could mean panels will stow horizontally to catch the diffuse light.
“In areas where you’ll have a significant amount of cloud cover over the course of the year, there’s some benefit of having the tracker at a flatter angle than actually following exactly where the sun should be,” Krantz said
Similarly, with harsh wind conditions, trackers are programmed to stow panel rows horizontally. Newer models can compensate for higher wind loads, and, in certain cases, system-level monitors can determine which panel rows are unaffected by wind events and allow those rows to continue tracking.
Linked-row single-axis solar trackers historically don’t lend as easily to installation on steep, undulating topographies, where grading might be required. However, certain models can now compensate for uneven landscapes. The technology, which hasn’t had a large presence in colder climates, is starting to venture further into those regions.
“Many colder weather, higher altitude climates are seeing the adoption of solar trackers — and as such, controller technologies and stow strategies have advanced to withstand colder temperatures and snow loads such as NEXTracker,” Au said.
Certain racks and tracking algorithms are designed to account for bifacial panels and their dual-sided solar production. Other single-axis models are designed to fit modules in two-in-portrait orientation.
“It’s a relatively heavy engineering lift,” Krantz said. “Having a metal infrastructure for these millions of panels that can sustain all the different elements that it’s going to experience over the course of that 25 or 30 years and then also to have it be moving because it inherently needs to follow the sun from east to west. There were some naysayers that believed it wouldn’t be feasible in the early stages, but at this point, that’s really not the case for most companies.”
Since the Earth’s rotation relative to the sun is not the same all year, with an arc that will vary by season, a dual-axis tracking system will consistently experience greater energy yield than its single-axis counterpart since it can follow that path directly. A dual-axis tracker produces 30 to 45% more energy yield than fixed-tilt solar systems.
Dual-axis trackers are used more in residential and smaller commercial applications but are beginning to see utility-scale deployment. Each of these trackers is installed atop a single elevated post to account for the greater range of angles the installed panels will reach. Dual-axis trackers can hold upwards of 20 panels per unit.
With higher elevation, panels aren’t as easily accessible for cleaning. However, with more headspace, dual-axis trackers leave the ground underneath open for other purposes, such as agriculture or even carports.
It’s a largely unexplored market segment, but tracking solutions for rooftops are starting to surface as well. These roof trackers operate somewhat like dual-axis trackers, rotating on a carousel-style track at the bottom of their racking, and adjust panel angle to follow the sun more accurately than single-axis trackers.
Following the sun
Solar trackers use different drivers, software and physics to track the sun’s location. Active trackers use drivers, which are motors linked to sensors reacting to light from the sun or following GPS coordinates of its position. Some tracker models have separate, smaller PV panels specifically to power the driving system.
Passive trackers rotate using compressed gas and liquid that passes between channels in the driver from exposure to sunlight.
Most solar trackers have five-year warranties for drives and controls and 10-year warranties for racking. In the last several years, tracking systems companies have made operational improvements to their systems, developing monitoring software for preventive maintenance and optimizing angles for higher energy yield. Tracking technology will continue to evolve, helping installers maximize the power output of solar projects.