Determining the optimal standard test condition correction procedure for high‐throughput field I–V measurements of photovoltaic modules

Abstract

The field-measured current-voltage (I-V) curves of photovoltaic (PV) modules need to be corrected to Standard Test Conditions (STC) in order to estimate the degradation rates. STC correction procedures have various attributes such as accuracy, requirement of minimum number and types of I-V curves, required irradiance range, and the type of correction (specific points or entire I-V curve) that determine their optimality for specific applications. This paper presents the investigation of accuracy and constraints of six different STC correction procedures for high-throughput field I-V measurements through experimental and simulation studies. Following STC correction procedures are considered in this paper: IEC 60891-Procedure 1, IEC 60891-Procedure 2, Modified IEC 60891-Procedure 1, Standard Irradiance and Desired Temperature (SIDT) procedure, Anderson procedure, and Voltage-Dependent Temperature Coefficient (VDTC) Procedure. Eight different simulation models for predicting the performance of PV modules at arbitrary irradiance and temperature are compared, and the simulation model that yields lowest root mean square error and the most accurate estimation of power temperature coefficient is identified. The simulated I-V curves using this model and the experimentally measured I-V curves on a flash tester at different temperatures and irradiances are provided as an input to all of the STC correction procedures. The average percentage errors in correction of maximum power (P max ), open-circuit voltage (V oc ), short-circuit current (I sc ),and fill factor (FF) were determined as a function of irradiance and temperature during measurement. Systematic biases introduced during correction by certain procedures were also identified. Based on the error estimation, constraints of various procedures, and requirements of high-throughput field I-V measurements, the most optimal STC correction procedure was identified. Moreover, the analysis of the root cause of superior performance of this procedure is also presented.

Sensitivity of accuracy of various standard test condition correction procedures to the errors in temperature coefficients of c‐Si PV modules

“…Contour plots of average percentage errors in P max obtained in STC correction by Anderson procedure with ( A E , A s ) −15%, ( B E , B s ) 0%, 9 and ( C E , C s ) +15% errors in all three TC ( α , β , and γ ) for experimental (left) and simulation (right) data…”

“…The I‐V curves of six multi‐c‐Si PV modules (three each from the two major tier‐1 module manufacturers 15 ) were experimentally measured at STC and at other irradiance and temperature conditions using the SPIRE 5600SLP BLUE system. Based on these measurements and the simulation models discussed in previous studies, 9,16–18 the I‐V curves at desired temperature and irradiance values were generated. The irradiance conditions for the experimentally measured I‐V curves range from 200 to 1,100 W/m 2 with a step of 100 W/m 2 , while the temperature conditions range from 25°C to 70°C with a step of 5°C.…”

“…The I‐V curves generated through simulation have the same temperature range and step size; however, they are generated over a slightly broader irradiance range of 200 to 1,300 W/m 2 with the same step size. The Average Root Mean Square Error (ARMSE) was computed by comparing the experimentally measured I‐V curves with those that were generated through simulation 9 . The average percentage errors in all the performance parameters ( P max , V oc , I sc , and FF ) for the entire range of measured and simulated irradiance and temperature conditions for all the six STC correction procedures were predicted and reported 9 .…”

“…The Average Root Mean Square Error (ARMSE) was computed by comparing the experimentally measured I‐V curves with those that were generated through simulation 9 . The average percentage errors in all the performance parameters ( P max , V oc , I sc , and FF ) for the entire range of measured and simulated irradiance and temperature conditions for all the six STC correction procedures were predicted and reported 9 . The reported average percentage errors in STC correction in Golive et al 9 are predicted with an assumption that we know the true values of temperature coefficients.…”

“…The same conclusion was also valid in a broader range of 200–1,300 W/m 2 irradiance. Out of these procedures, the IEC 60891‐Procedure 4 (VDTC) (the authors have used this name throughout this paper for IEC 60891‐Procedure 4, to be consistent with our previous publication 9 ) was deemed most appropriate for field applications due to its high accuracy and the requirement of only one I‐V curve measured in the field. It should be noted that the VDTC procedure described in Hishikawa et al 8 and the IEC 60891‐Procedure 4 described in the standard have a minor difference.…”