• Energy Research

Abstract Improvement of the quantitative performance characterization of photovoltaic (PV) strings was investigated, based on their monitoring data during maximum power point tracking (MPPT) operation. The maximum power voltage Vmp and current Imp of the PV strings under MPPT were continuously monitored, and corrected to standard temperature of 25 °C by using the temperature correction formulas, which were recently developed for PV modules. The irradiance G was measured by using a PV module irradiance sensor. It was verified that the formulas are applicable to PV strings, enabling reproducible temperature correction of Vmp and Imp with relative standard deviation (σ) of about 0.4–0.9% under various weather conditions. Comparison of the outdoor results with indoor current–voltage curve measurements of the constituent modules in the strings showed that the maximum output power Pmax under the standard test conditions STC, i.e. 25 °C and 1 kWm−2, estimated from the temperature-corrected Vmp and Imp, agreed with the indoor result typically within ±1.5%. The experimental results also showed that other information of the string such as partial shading and activation of bypass diodes can be also sensitively detected by analyzing the temperature-corrected Vmp and Imp. The outdoor measurements and data analysis of this study can be carried out without stopping the MPPT operation of the string. The analysis is performed by using the real-time data. The present method is expected to be applicable to most kinds of crystalline silicon PV strings without modification.

Abstract The temperature and irradiance dependences of the current at maximum power (Imp) and the voltage at maximum power (Vmp) of crystalline silicon photovoltaic (PV) devices are investigated by experiments and numerical simulations based on a single diode model. It is shown that the experimental Imp is nearly constant for temperature variation at fixed irradiances, which agrees with the simulation results being within ±0.02%/K over 273.15–343.15 K (0–70 °C) over the range of parameters represents typical commercial crystalline silicon PV devices. The experimental Imp is nearly proportional to irradiance (G) at fixed temperature, which also agrees with the simulation results that the Imp/G is nearly constant within ±1.3% for the irradiance range between 0.5 and 1.2 kW/m2. Based on these results, a new formula for the temperature correction of Vmp is proposed. It does not require advance information of diode parameters and temperature coefficient. The Imp − Vmp curves which are measured outdoors and corrected to 25 °C by using the formula showed good reproducibility within ±0.13% for many days, which confirms the validity of the formula. The maximum power Pmax can be also precisely estimated by the temperature correction of Vmp. These results are useful for characterizing the performance of crystalline silicon PV devices by using the Imp and Vmp values, which can be measured during their maximum power point tracking (MPPT) operation.

Abstract Estimation of the performance of photovoltaic (PV) module and system by using continuous monitoring data is an important issue, since the output power of PV modules is changeable, affected by the irradiance, temperature and shading effect, as well as degradation. This work proposes a method to detect partial shading on a module during its maximum power point tracking (MPPT) operation. It identifies whether there is shading on a module or not by analyzing data of the voltage at maximum power (Vmp) and the current at maximum power (Imp). Recently developed temperature correction formulas for the Vmp and Imp are used, in order to analyze them under various temperatures and irradiances. The experimental and simulation results show that the shading effect usually results in larger Vmp than the shadeless case, compared at the same Imp. Therefore, the Imp − Vmp curve shifts toward higher voltages by the shading effect, thereby enabling detection of the existence of the shading effect. A method to identify the shading effects on a PV module from the Imp – Vmp curve has been clarified, which does not require I–V curve measurements. Slight partial shading such as a drop in the photocurrent of a single cell in a module by about 5–10% is possibly detected. The present results are also applicable to PV systems that include multiple modules. They are expected to be useful for accurately monitoring the performance of PV modules and systems under operation, since the Vmp and Imp are measurable without breaking the MPPT operation.