IEC 60904-1
Uncertainly analysis for measurement of PV module I-V characteristic in accordance with IEC 60904-1
In view of PV module output power characterization in accordance with IEC 60904-1 [1], the uncertainty analysis is typically performed for the following variables, resulting from I-V curve measurement:
- Short circuit current (Isc)
- Open circuit voltage (Voc)
- Maximum output power (Pmax)
- Fill factor (FF)
For each variable the calculation tables for expanded measurement uncertainty need to be developed independently.
It must be noted that out of the four variables, only three are independent. Therefore, depending on the order of determination, the uncertainty of the fourth variable has to be determined considering the correlation between the other three. For example, if FF uncertainty is to be determined based on equation
[math]\displaystyle{ FF=\frac {P_{MAX}} {I_{SC} \cdot V_{OC}} }[/math]
there will be considerable correlation between the uncertainties of the three other variables. For FF parameter, care must be taken that a double count of uncertainty contributions is excluded.
Figure 1 shows how major uncertainty sources affect the shape of the PV module I-V curve. The I-V curve measurement depends on the ambient test conditions, which are given by the PV module temperature and the irradiance setting of the solar simulator. The uncertainty analysis of these test conditions is performed separately and the resulting expanded combined uncertainties are used as input for further uncertainty calculation of parameters Pmax, Isc and Voc.
Figure 1: Impact of uncertainty sources on I-V measurement of PV modules
Table 3: Calculation spreadsheet for irradiance measurement uncertainty
Table 3 shows the listing of uncertainty sources for irradiance setting of a solar simulator, which is typically used for calibration or performance measurement of PV modules.
The values given in the yellow fields for uncertainty sources are example data. It is the task of the test laboratory to calculate values from measurement series or from the test geometry. Also estimates for best practice can be used. The review of the uncertainty analysis by technical auditors is part of laboratory accreditation in accordance with ISO/IEC 17025.
Remarks:
| 1) | Refer to calibration report of accredited test institute |
| 2) | Historical data of reference cell calibration |
| 3) | Refer to data sheet, no entry if transimpedance amplifier is used |
| 4) | Current impact from voltage drop caused by shunt resistor, refer to I-V curve of reference cell, no entry if transimpedance amplifier is used |
| 5) | Refer to data sheet of instrument, no entry if shunt resistor is used |
| 6) | Refer to reference cell data sheet . The temperature accuracy of sensor depends on the sensor type (i.e. Pt100 or thermocouple) and the temperature set value (i.e. 15°C to 75°C). The ). The “Uncertainty related to REF temperature uncertainty” is given by (temperature accuracy of the sensor) x (temperature coefficient of cell). |
| 7) | Manufacturers' data sheet and verification by annual calibration |
| 8) | Best practice: a) use estimate acc. to lab experience for REF - DUT combinations and irradiance levels, b) set zero if SMM correction is performed. |
| 9) | Best practice: a) Set zero if UC related to SMM is estimated, b) use lab experience if SMM correction is performed
Note: Care should be taken that the entry in lines 8) or 9) are connected. One of the cells is zero and the other has a value |
| 10) | Only applicable for non-simultaneous measurement of irradiance (REF), DUT current and DUT voltage |
| 11) | Text experience and best practice of laboratory |
| 12) | Depends on view angles of REF and DUT related to the optical axis. Impact increases with rising diffuse irradiance in the test area. |
| 13) | Glass thickness or frame design may lead to a shift of the cell to lamp distance. Uncertainty to be calculated according to quadratic distance law. |
| 14) | Irradiance at RC position must correspond to average irradiance in the module area. Uncertainty contribution to this non-uniformity can be reduced if non-uniformity correction is applied. The residual uncertainty is then the reproducability of the non-uniformity i.e. the uncertainty of the non-uniformity correction factor. |
Table 4: Calculation spreadsheet for PV module temperature measurement uncertainty
Table 4 shows the listing of uncertainty sources for measurement of PV module operating temperature. The values given in the yellow fields for uncertainty sources are example data. It is the task of the test laboratory to calculate values from measurement series or to give estimates based on best practice. The review of the uncertainty analysis by technical auditors is part of laboratory accreditation in accordance with ISO/IEC 17025.
Remarks:
| 1) |
|
| 2) | Reference temperature for calculation of sensor uncertainty |
| 3) | No entry if IR sensor is used, refer to DAQ data sheet |
| 4) | No entry if IR sensor is used, best practice of test laboratory |
| 5) | No entry if surface temperature sensor is used, refer to sensor/instrument data sheet |
| 6) | Estimate according to test experience of laboratory |
| 7) | Estimate according to test experience of laboratory |
| 8) | Estimate according to test experience of laboratory |
Uncertainty analysis for maximum output power (PMAX)
Table 5: Calculation spreadsheet for PV module for maximum output power (PMAX) uncertainty
Table 5 shows the listing of uncertainty sources for maximum output power determination of a PV module with single junction solar cells. The values given in the yellow fields for uncertainty sources are example data. It is the task of the test laboratory to calculate values from measurement series or to give estimates based on best practice. The review of the uncertainty analysis by technical auditors is part of laboratory accreditation in accordance with ISO/IEC 17025.
Remarks:
| 1) | Value transferred from spreadsheet “irradiance measurement uncertainty” |
| 2) | Value transferred from spreadsheet “temperature measurement uncertainty” |
| 3) | Refer to lab measurement or PV module data sheet |
| 4) | Refer to manufacturers' data sheet and verification by annual calibration |
| 5) | Refer to data sheet, verification by annual calibration |
| 6) | Xi shall be determined from a minimum 10 successive I-V measurements under the same test conditions (either forward or reverse voltage sweeps) and without electrical disconnection |
| 7) | Xi shall be determined from the spread of a time-series of Pmax measurements of a reference PV module, covering the ranges of ambient conditions in the lab and instrumentation practices from operators that are qualified for this measurement |
| 8) | Xi is the difference between the reported Pmax value by the I-V data acquisition system and the Pmax value resulting from quadratic regression of I-V data points around the maximum power point. This uncertainty source is relevant for a low resolution of I-V data points. |
| 9) | Refer to lab test experience based on error propagation studies for an assumed spread of PV module I-V correction parameters (see section 4) |
| 10) | Lab experience: Analysis of I-V curves recorded for forward and reverse voltage sweeps as a function of I-V data acquisition time, b) Estimate provided by the developer of the test method to compensate transient effects |
| 11) | Refer to lab test experience |
| 12) | Depending on the PV technology |
References
[1] IEC 60904-1:2020 “Photovoltaic devices - Part 1: Measurement of photovoltaic current-voltage characteristics”