From pv magazine Global
A research team from Japan’s Ritsumeikan University has investigated hotspot mirroring in commercial 120-half-cell silicon heterojunction (HJT) PV modules under partial shading conditions and has identified the electrical mechanisms that drive the phenomenon and developed a circuit-level model to predict hotspot formation in affected cells.
Hotspot mirroring occurs in half-cell PV modules when partial shading triggers a bypass diode, causing an unshaded cell in the parallel mirror substring to enter reverse bias. Due to current mismatch, the cell can experience localised overheating similar to the shaded cell.
“Previous studies established that hotspot mirroring occurs both under controlled laboratory conditions and in field-relevant outdoor environments,” the researchers said. “However, they did not provide a quantitative, circuit-level attribution of mirror-substring dissipation to its governing parameters, nor has the temperature–power feedback that modulates dissipation under real operating conditions been incorporated into a validated predictive framework.”
The team said its work aims to address these gaps by identifying the conditions under which hotspot mirroring becomes severe and determining cell-level design factors that could help mitigate the effect.
The researchers first tested a 120-half-cell silicon heterojunction module comprising six substrings. Each substring contained 20 series-connected half-cells, with two substrings connected in parallel to form a bypass diode-protected unit. Three such units were connected in series at the module level.
The module was installed outdoors under natural sunlight and connected to an electronic load. The researchers selectively shaded individual half-cells using a light-blocking sheet while monitoring temperature distribution with infrared thermography. Measurements were performed at two fixed operating voltages, 36.5 V and 22.7 V, with the module held at each voltage for 10 minutes before thermal images were captured.
The experiments showed significant heating in an unshaded mirror-substring cell only at the lower operating voltage, where the bypass diode was activated. In one example at 22.7 V, the mirror cell reached 44.4 C, representing a 15.2 C increase above the surrounding temperature. No comparable heating was observed at 36.5 V.
To analyse the underlying mechanism, the researchers developed an equivalent circuit model in LTspice using parameters derived from an encapsulated HJT coupon cell. The model reproduced the measured current-voltage characteristics of the module and was used to simulate voltage distribution and power dissipation at the individual cell level under partial shading conditions.
The simulations examined the influence of short-circuit current mismatch (ΔIsc), shunt resistance, and temperature on mirror-cell hotspot formation.
“Cell-resolved simulations demonstrate that bypass activation is necessary but not sufficient for a localised mirror hotspot: if the mirror-substring cells are identical, the negative unit voltage is distributed broadly,” the researchers said. “In contrast, a slight short-circuit-current mismatch can force one mirror cell into deep reverse bias, making it the dominant dissipation site.”
The parametric analysis showed that ΔIsc has the strongest influence on hotspot power dissipation, while variations in shunt resistance have a secondary effect.
“These results provide outdoor-validated, circuit-level guidance for mitigating hotspots in half-cell modules beyond shaded-cell-only evaluation,” the academics concluded.
Their findings in “Outdoor validated mismatch driven mirror-cell hotspots in half-cell silicon heterojunction modules,” published in Solar Energy.
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