Scientists identify new loss factor in tracker-based PV plants on slightly undulating terrain

Researchers at Spain’s Polytechnic University of Madrid claim to have identified a new source of underperformance in ground-mounted PV plants utilising solar trackers.

“We initiated this work after receiving several queries at IES-UPM questioning why the irradiance gain of trackers in utility-scale PV plants was lower than that estimated during the design phase,” corresponding author Juan Santamaría Sancho told pv magazine. “To address this issue, we analysed real operational data from more than 7,000 trackers across seven utility-scale PV plants. The results show that tracker angles are overridden to avoid shading effects caused by slightly undulating terrain, which leads to lower-than-expected tracking irradiance gains, as the actual operating angles are more tilted than those assumed in standard simulations.”

“This behaviour reveals a new category of energy losses, defined as suboptimal backtracking losses, which explain part of the gap between simulated and actual energy yield. We quantify these losses using our PV simulation tool SISIFO, finding differences of up to 2% in tracking-related irradiance gains between standard modelled expectations and real operation.”

Backtracking in PV plants is a control strategy used in solar tracking systems to reduce shading between adjacent rows of panels. Instead of always pointing directly at the sun, trackers tilt slightly backward when the sun is low to prevent one row from casting shadows on another. This helps maximise overall energy yield across the plant, especially in dense layouts or early morning and late afternoon conditions.

The scientists explained that under flat terrain assumptions, shading is precisely avoided, and energy yield matches simulation results, while real PV plants often exhibit small height differences between tracker rows due to uneven ground. These deviations break the coplanarity assumption and introduce unexpected shading during backtracking if ideal angles are used.

Pictures taken at a utility-scale PV plant installed on sloping terrain during backtracking periods.
*Image: Polytechnic University of Madrid, Renewable Energy, CC BY 4.0*

To prevent this, tracker controllers apply an overcorrection, slightly reducing tilt to eliminate inter-row shading. This overcorrection leads to visible ground illumination patterns, indicating lost irradiance that is not captured by the solar modules. Such effects are especially evident during backtracking periods in both morning and afternoon transitions, with field observations confirming that this phenomenon is absent during midday when trackers follow the sun directly.

“Standard performance indicators like performance ratio do not capture these losses because they rely on in-plane irradiance assumptions,” the scientists said. “As a result, the discrepancy between simulated and actual energy yield becomes difficult to detect through conventional metrics.”

The researchers compared experimentally measured tracker rotation angles from SCADA systems of the PV plants with theoretical values calculated for perfectly flat terrain under ideal backtracking conditions. All analyzed plants used one-axis tracking systems installed on nominally horizontal sites.

Operational data showed deviations from simulated behaviour. These deviations were determined by unevenness-correction strategies embedded in tracker control algorithms and inherent tracking delays or misalignments. Moreover, time-series data for representative trackers were analyzed and compared against different modeled backtracking correction approaches, with the best-fitting correction model for each PV plant being identified by minimizing deviations between measured and simulated angles.

“Results showed that even plants on flat terrain exhibit systematic angular modifications during backtracking, confirming the presence of hidden correction strategies in real controllers,” Santamaría Sancho said. “Additional simulations using SISIFO indicate that incorporating measured or corrected angles significantly improves agreement with real energy production compared to idealized assumptions.”

The analysis also demonstrated that annual losses attributable to tracking-related effects can exceed 5% when compared to ideal simulations without operational constraints. Overall, the results demonstrated that real tracker behaviour systematically deviates from ideal backtracking models due to practical control adaptations. These deviations have a measurable impact on irradiation capture and help explain part of the gap between simulated and actual PV performance.

“An examination of tracker rotation angle data from systems supplied by various manufacturers and operating under different configurations across seven real PV plants, distributed across five geographic regions with an aggregate capacity of about 1 GW, showed that the issue stems from simulations generally assuming perfectly flat terrain, whereas actual sites exhibit varying levels of ground unevenness,” Santamaría Sancho said.

“Importantly, these losses are particularly valuable from an economic standpoint, as they occur during periods without curtailment and when energy prices are typically higher,” the scientists concluded. “This makes even seemingly small deviations of 1%–3% in captured irradiation significant in terms of revenue”

Their findings are available in the study “Modelling energy losses arising from overridden backtracking in utility-scale photovoltaic plants on slightly undulating terrain: Implementation in the SISIFO tool for performance pre-assessment,” published in Renewable Energy.

From pv magazine Global

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