Backtracking: The Fast Track To Solar ROI

By Jay Johnson, Vice President of Business Development at Exosun

The 3.8 MW Porette de Nérone power plant in Corsica, France, uses Exosun’s Exotrack HZ solar tracker. Exosun, which is now entering the U.S. market, uses a backtracking algorithm with its entire range of solar trackers. The algorithm adjusts module positions during periods of low solar height, lessening the effect of shadows on solar cells.

The 3.8 MW Porette de Nérone power plant in Corsica, France, uses Exosun’s Exotrack HZ solar tracker. Exosun, which is now entering the U.S. market, uses a backtracking algorithm with its entire range of solar trackers. The algorithm adjusts module positions during periods of low solar height, lessening the effect of shadows on solar cells.

Solar project developers are constantly looking for new strategies to simultaneously increase system performance and drive down the overall cost of system ownership. More and more, focus is shifting to solar trackers as a reliable means to optimize energy generation and accelerate ROI.

Transparency Market Research reports the global solar tracker installed capacity will grow from 1.9 GW in 2010 to approximately 7 GW per year in 2020. Trackers increase system output by tilting modules throughout the day, thereby capturing maximum sunlight. Exosun has shown how trackers and backtracking can boost system performance.

To enhance the performance of trackers, developers can use a backtracking algorithm to fine-tune modules’ positions during periods of low solar height — early morning and late afternoon — as shadows can affect the modules’ production levels. When implemented properly, backtracking will drive modules to avoid table-on-table shading that may occur during these times.

The algorithm that allows the trackers to know their position 365 days a year is programmed in a program logic controller, or PLC. The program can be modified by the tracker manufacturer. Exosun has developed an algorithm specifically for the Exotrack HZ, but other trackers use a similar solution.

Figure One

Figure One

Depending on the project requirements, such as land availability or electrical design, the developers have to make a choice between maximizing the total power installed on the plant and maximizing the yield (specific production).

If the developer chooses to boost yield, it will have a direct influence on the GCR value. Defined as the ratio between the PV modules’ area and the total ground area, GCR determines the appropriate angle of panel orientation, which changes throughout the day as the sun moves across the sky. The greater the GCR, the smaller the tilt required to avoid shading. As an example, Figure 1 shows how a single table is tilted at the beginning of the day, and how that inclination must be modified to avoid shadings if the GCR is higher.

Figure Two

Figure Two

Backtracking algorithms are designed to capture maximum energy harvest during the early morning and late afternoon, when panels are most likely to cast shade on each other. In Figure 2, backtracking positions panels during these times of day to avoid shading, even when this means the optimal incidence angle is slightly compromised. Production improvements from shade avoidance mitigate any small losses incurred when panels aren’t positioned in the best angle of incidence.

As Table 3 shows, Exosun’s simulations and test results show that for a 50% GCR the backtracking impact on the energy output of the plant is at 15.1%. Conversely, for a GRC at 33% the impact is close to 7%.

FIgure Three

FIgure Three

Those differences can be explained by the irradiances and electrical losses due to shaded cells on PV modules, as the string current is determined by the weakest shaded panel. Also, an optimized backtracking strategy smooths the daily production curves, specifically during early mornings and late afternoons.

For utility-scale projects affected by shading, solar tracker systems featuring backtracking algorithms can offer a sound strategy to accelerate ROI. SPW

Jay Johnson is the vice president of business development for Exosun, a developer of solar tracking systems for PV and CPV applications. Johnson has more than 20 years of experience in sales, marketing and management generally involving emerging utility markets.