Setting the series resistance of photovoltaic modules

Series resistance (Rs) in PV modules is a critical parameter affecting efficiency and can be determined through analytical, measurement-based, or modeling approaches.

Understanding Series Resistance

Series resistance in a PV module represents the resistance encountered along the current path, including the internal resistance of cells, contact resistance at interconnections, and wiring resistance within the module. High series resistance reduces the fill factor and overall power output, particularly affecting the slope of the I–V curve near the maximum power point (MPP) and open-circuit voltage (Voc),. It is distinct from shunt resistance, which relates to leakage paths bypassing the cells.

Measurement and Calculation MethodsI–V Curve Analysis: Series resistance can be extracted by fitting the illuminated I–V curve to the single-diode or double-diode model. This involves adjusting Rs along with other diode parameters to match the measured curve. The slope of the I–V curve near Voc can provide an apparent series resistance, but this is not the same as the Rs used in the one-diode model.

Maximum Power Point (MPP) Method: A practical approach is to measure current and voltage near the MPP, which minimally affects power output. This method allows accurate determination of Rs without significantly reducing module performance. Experimental validation has shown that series resistance determined near MPP can differ by less than 2% from values obtained using the full I–V curve.

Analytical and Correction Methods: Advanced methods involve deriving Rs from the derivative of the I–V curve and applying correction techniques to account for environmental conditions, such as irradiance and temperature. This ensures that series resistance values are comparable across different operating conditions and can be used for long-term health monitoring of PV modules.

Practical Considerations

Environmental Effects: Series resistance varies with temperature and irradiance, so measurements should be corrected to standard conditions for accurate comparison. Impact on Module Performance: Lower Rs results in sharper I–V characteristics at high voltages, improving fill factor and efficiency. Software Modeling: In PV simulation tools like PVsyst, Rs is an adjustable parameter in the one-diode model and is not directly measurable.

Its effect is coupled with the diode's exponential slope, so careful calibration is required.

Summary

To set or determine the series resistance of PV modules effectively: Measure voltage and current under controlled or corrected environmental conditions. Use I–V curve fitting or MPP-based methods for accurate extraction. Apply analytical corrections to ensure comparability across different operating conditions.

Consider Rs in modeling and monitoring to optimize module performance and assess aging or degradation. By following these approaches, series resistance can be accurately quantified, enabling better performance evaluation and long-term monitoring of photovoltaic systems.