Tracking has taken off with developers of solar plants but there is a lot to know about this simple technology.

Solar is simple. Sunshine hits a PV panel and electricity is made. A vast amount of solar energy is needed to help replace coal, however. A simple way to maximise the amount of power conjured from a module is to keep it facing the sun as it loops across the sky. Tracking solutions that tilt arrays from sunup to sundown are becoming standard issue at utility-scale solar farms across the country.

Installers who may not have worked at that scale should know that tracking isn’t as simple as it sounds.

The sites selected for large solar projects are usually big enough for owners not to have to worry about PV arrays being densely arranged. In cases where space is tight, a developer must weigh up the amount of power that can fit on a site against the additional production efficiency gained by using a tracking solution.

If there isn’t much room, a fixed-tilt structure could end up working out better value. More of that land will need to be panel-free if tracking is used, to allow for O&M access and to allow the tracker its full range of motion.

But although tracking requires more space per panel the result is higher yield per watt of peak power installed. “You’re getting the most out of the panels that you install,” says Fotowatio Renewable Ventures technical manager Ronald Maran.

Big shoulders

The Lilyvale Solar Farm in Queensland, a 125MW plant delivered by FRV that relies on single-axis tracking.

The main purpose of tracking is to maximise production at the times of day when a fixed-tilt solution wouldn’t be producing optimally. With fixed-tilt, there will be a period of three to four hours where a panel will be producing at optimal efficiency. “Outside of that, efficiency of generation from that structure is reduced because the array effectively isn’t facing the sun,” Maran tells EcoGeneration.

Tracker manufacturers claim up to 30% additional production over the same module installed on a fixed-tilt structure, and Maran has seen similar results in the right conditions. “It can be a big benefit.”

When assessing a project for tracking, Maran considers the size of the land, the targeted peak power, the power at the point of connection, irradiation (which can vary across the country), soil conditions and topography (how variable the terrain is). The major tracking providers (there are about six) offer solutions at different lengths, different axis heights and with different ranges of motion, so the characteristics of a project may favour a certain technology over another. FRV has used different providers for different jobs.

The systems are sophisticated, with proprietary software determining how an array will turn throughout the day. Owners can usually plug in latitude and longitude for a site and the system will work out its tracking motion through the day.

Over the past few years tracking companies have developed software that will customise the tracking algorithm throughout the day to deal with onsite conditions. If the weather turns overcast, for instance, it might change position to maximise output from diffuse irradiation reaching the module, instead of facing the sun. Another trick is to minimise the amount of “backtracking” occurring, where parts of an array may be partially shaded by a neighbouring array during some of its motion.

In an ideal world a plant would be designed so that arrays are far enough apart to eliminate shading, but it’s not uncommon for rotating arrays to sometimes cast neighbours into shadow. The software might detect when there is shading on panels or measure the amount of irradiation hitting a tracking row and adjust the angle. In another case a system’s software might customise the tracking algorithm by taking the terrain and system design into account, says Maran, whose responsibilities at FRV include selecting EPCs and conducting technical due diligence before construction.  

Both sides of bifacial

Array trackers at a site with 1P orientation.

Bifacial modules have become popular at utility scale as owners calculate that the additional generation from the rear of the panels makes the extra cost worth it. “You want to maximise the amount of irradiation reaching the bottom of the module,” Maran says. The shift to bifacial has had a significant impact on the tracking sector, where taller and more spaced-out trackers have a beneficial effect on the bifacial gain. Any increase in tracker height will incur a greater wind loading on the panels, however, where the relationship between height and wind loading is non-linear.

Larger-format modules are also becoming popular, as manufacturers step up production of models featuring 210mm wafers. These modules are about 400mm longer and 300mm wider than the standard 2m by 1m format that has prevailed for so long. A longer, wider panel means the span of a tracking structure is larger. “You are left with a larger tracker that needs to be higher off the ground,” Maran says. Higher tracking means proportionally greater wind loading, and system designers are having to adapt to the new, larger PV technology.

Tracking with bifacial can provide superior levelized cost of energy compared with an equivalent fixed-tilt array. Module manufacturers are favouring 210mm wafers and the tracking crowd will have to adapt.

2P or not 2P

An example of tracking that uses the 2P orientation, at a solar installation that is part of a hybrid plant owned by a business based in Victoria.

Decisions about which orientation to use on a project with tracking – a single row of vertical panels (1P) or two rows of vertical panels (2P) – are determined by the probability of high wind, a site’s topography, the technology that is being used and the designer’s preference. On a flat site with low wind, Maran says the decision between 1P and 2P comes down to the designer’s preference or which orientation most suits the selected technology.

“Certain sites work better with 1P and certain sites work better with 2P,” he says, pointing out that there isn’t a clear indication that 1P or 2P is better for the majority of applications.

Developers and tracker suppliers must pay careful attention to projects planned for cyclonic regions. It’s not impossible to engineer a 2P tracking solution in such conditions, with the right engineering, but 1P orientation would endure less stress when it’s howling.

The reflective possibilities of ground cover can add another few percentage points in yield from bifacial panels, according to some trials around the world where grass is compared with sand, cement, pale gravel and white-painted surfaces. The light reflected onto the rear of panels from below, called “albedo”, can vary in quality depending on what it has bounced off. Humidity, haze and cloud cover also have an effect. It’s probably impractical to replicate such trial conditions at utility scale, so soil and grass will have to do.

Sheep have been shown to live quite happily under tracking, with the mutual agreement that they’ll control vegetation for the owner in return for shade on hot days. Cattle are not such good tenants; they are too big and can cause damage when they rub up against technical gear. Similarly, goats and solar don’t seem to get along. They can be tempted to climb onto the panels.

Not on the level

Tracking solutions are flexible when it comes to working on a slope, with gradients of 15% or even 20% (about 8.5° or 11°) able to be accommodated by some manufacturers. No site is ever dead flat, and it pays to be careful when it comes to undulations. A typical 1P tracking solution is about 100m in length per unit, or about half that for a 2P unit. The ground it’s mounted on will change in elevation over that stretch and different foundation solutions might need to be used at various points or grading may be required. These small changes in elevation will be a more important consideration when using tracking than the absolute gradient of the site.

The quality of the ground a system is built on also matters. In some cases, expansive clay responds to wet and dry cycles where a drenching after a dry spell can cause clay to expand, pushing the foundation upwards. Over time, these incremental movements can add up, so that a pile might be 10cm, 20cm or more out of the ground. As it is pushed out, the foundation is less effective and the wind loading on the more elevated array is higher. Uneven pile heights can also cause operational problems for a tracker.