Hotspot formation in polymer-based lightweight Solarge PV modules

Abstract

This thesis presents experimental research on a phenomenon that has been widely observed in photovoltaic systems in recent years. Hot-spots, most commonly reported as areas of extreme temperature within PV modules, have been identified as a critical issue in the industry. This is primarily due to the fact that solar cells are subjected to various mismatches and time-varying stresses in real-world operation, affecting their long-term reliability. Moreover, hot-spots are mostly invisible to the naked eye and require special inspection such as EL and IR imaging for detection, making them challenging to solve. The use of low-temperature lamination techniques on the front sheet and encapsulation increases the risk of hot-spots within Solarge’s polymer-based lightweight modules, with permanent damage being observed after exceeding temperatures beyond 160°C. Therefore, robust design criteria were needed to pass the hot-spot certification test, which was developed keeping in mind the thermal limits of failure for the polymer module. This study provides an overview of contemporary cell technologies, encompassing Passivated Emitter and Rear Cell (PERC) and Interdigitated Back Contact (IBC), in the context of reverse bias characterization. The methodology involves comprehending hot-spot behavior across various configurations, beginning from single-cell laminates to strings, followed by module-level testing. The experiments include the assessment of various commonly used industry wafer sizes to evaluate cell breakdown properties and performance using IV curve analysis. Notably, two electrical processes contribute largely to the power dissipation in a solar cell in reverse: zener and avalanche breakdown. The investigation revealed cell breakdown voltages of -5.2V for IBC cells and -19.6V for PERC cells. These findings were consistent with values provided by manufacturers, reinforcing the widely accepted nature of the cells. The main objective of the investigation of a string or a module of cells in series without bypass diodes was to identify cells with lower performance, which increases the vulnerability to the emergence of hot-spots. The hot-spot endurance test was conducted on a standard polymer module, in accordance with the IEC 61215 guidelines, revealing essential parameters that significantly impact the resulting temperatures. A notable observation that emerged was the linear correlation between string length and power dissipation. This trend became evident when temperature values exceeded 165.84°C, accompanied by power dissipation of 150–200 watts and heat flux (Q˙) ranging from 0.52 to 0.69 W/cm2 for string lengths up to 24 cells in series. This led to irreparable damage to the module and the failure of the endurance test. Therefore, to pass the test, the string length was confined to less than 20 to stay below the thermal threshold. In addition, the findings included broader observations delving into intricate topics of hot-spot characteristics such as shape, frequency of occurrence, location, shading ratios, orientation, and various other relevant parameters. Hot-spot detection can be used to assess module health and enable proactive maintenance strategies. Understanding the phenomena of cell breakdown and power dissipation assists in the development of safer modules. Finally, this study aims to emphasize the importance of design considerations and offers the potential for optimized module architectures for the industry.