Electrical and thermal modeling of junction boxes
The power of photovoltaic modules is the product of single gain and loss factors. These factors influence the cell-to-module (CTM) ratio and final power of modules. We extend an existing CTM-methodology by presenting a complete model to calculate the losses attributed to junction boxes and cabling. We find the total junction box losses to be small (< 1 W) compared to the power of common photovoltaic modules. Electrical losses in cabling are the dominant loss factor (> 80%) for junction boxes. We simulate the thermal behavior of a junction box using the finite element method and analyze the temperatures of bypass diodes. The model is verified using infrared thermography. We find the diode temperature in the analyzed setup to be below critical temperatures for a thermal runaway. From our FEM-model we find that diode temperatures can be reduced by 13K using potting material with increased thermal conductivity (0.8 W/m*K compared to 0.2 W/m*K). Electrical losses of the junction box increase with elevated temperatures but are still comparably low. We perform an economic analysis and consider costs of power loss as well as material costs for cabling. We find the optimal cable cross section to be 4 mm² at Standard Testing Conditions. At irradiations other than 1000 W/m² the optimal cross section is found to be different (6 mm² at 1200 W/m², 2.5 mm² at 600 W/m²).