Scientists have developed a way to compare the data given by PV manufacturers based on standard test conditions (STC) to the real conditions in the field. The proposed technique is based on the single-diode model and the Newton-Raphson algorithm. The maximum relative error was found to be at 1.37%.
Researchers from Egypt’s American University in Cairo have developed a novel methodology to validate measured performance and warranty conditions of PV modules.
“Manufacturers of PV modules have datasheets that provide product-specific data points on the I-V curve at standard test conditions (STC),” said the academics. “PV modules typically have a 10-to-25-year warranty, and the warranty is stated as a guarantee that the panel will function above a certain percentage of the maximum power under STC. As the STC conditions virtually rarely occur during the maintenance time of the panels, a methodology is needed to map the measured I-V curves at the measurement conditions to STC.”
The new approach is based on a single-diode model, which represents solar cells as an electrical circuit and solves its equations with the numerical algorithm Newton-Raphson, which is commonly used for approximation of the roots of the real-valued functions.
The novel methodology starts with field measurements, collecting some I-V points: open-circuit voltage (Voc), short-circuit current (Isc), voltage at maximum power point (Vmp), current at maximum power point (Imp), and two points before and after the maximum power point (MPP). In addition, irradiance and ambient temperature are also taken at the moment of recording.
Then, the Newton-Raphson algorithm solves the single-diode model and yields photocurrent (Ipv), diode saturation current (Is), series resistance (Rs), shunt resistance (Rsh) and the diode ideality factor (n). From all of these values, the I–V and P–V curves can be constructed. Then, the program also calculates the temperature coefficient – seeing how different parameters change when the irradiance stays the same but the temperature changes. This reportedly enables the methodology to convert results to STC terms and compare the results to the datasheet.
“The simulation model is validated with measurement data and accurately represents the PV module’s electrical characteristics with a maximum relative error of 1.37%,” the scientists said. “It enables a seamless comparison of the actual power output degradation with the warranty condition. Hence, identifying any excessive degradation due to aging and speeding up the process of filing for warranty claims in case of a faulty PV module.”
This method was demonstrated on a SunTech monocrystalline half-cell PV module of 550 W. Its datasheet, under STC, mentioned the Voc to be 49.88 V, the Isc as 14.01 A, the Vmp as 42.05 V, and the Imp as 13.08 A. Real measurements of it were taken at a temperature of 44.6 C and an irradiance of 714 W/m2. For the temperature coefficient, performances were taken under 823 W/m2 with cell temperatures of 44.6 C and 55.1 C.
“The results show that the module’s performance is consistent with the datasheet warranty standards, where the PV module under test has experienced an average power degradation of -4.88% after the first two years of operation,” said the team.
The novel methodology was presented in “Methodology to validate measured performance and warranty conditions of PV modules,” published in Solar Energy.