Adjustment of PV module output variables with temperature and irradiance measurement

Once the installation stage of the photovoltaic systems is complete, the project commissioning phase begins. At this stage, the system's compliance with regulatory standards, especially ABNT NBR 16274:2014, is inspected and verified, in addition to good engineering practices. The aforementioned standard has three test branches:

• The first, which governs a detailed visual inspection of compliance with the project and good practices;
• The second, which employs electrical tests to ensure that the system operates safely and;
• The third, which presents performance tests.

Due to the fact that performance testing equipment has a relatively high cost, it ends up not being unanimous among professionals, which makes it unfeasible for the data collected in the field to be properly treated and analyzed as stipulated in the booklet. Without a temperature and irradiance sensor, measurement and data analysis gains empiricism and loses precision. The minimum instrument required for measurements in the commissioning stage of a photovoltaic system is a multimeter for measuring the open circuit voltage and short-circuit current of photovoltaic modules or strings. It is possible to obtain precious information even with simple equipment like this. However, the data provided by photovoltaic module manufacturers is based on pre-established climatic conditions (STC or NOCT, as we will explain below). Therefore, the measured voltage and current variables need to be adjusted or normalized to the same conditions specified in the data sheets. The standard test condition STC (standard test condition), in which the electrical parameters of the modules are presented in the data sheets, corresponds to a cell temperature of 25 ºC and irradiance of 1000 W/m2. As the STC condition is not achieved in practice, manufacturers from time to time provide another option so that it is possible to have a different perspective on the electrical parameters of the modules. The NOCT (nominal operating cell temperature) condition corresponds to an ambient temperature of 20 oC, with irradiance of 800 W/m2 and wind speed of 1 m/s. Due to the fact that the main parameters of photovoltaic modules are dependent on the irradiance and operating temperature of the cells, there is no doubt the need for sensors capable of measuring these two variables. In this article, a practical example of normalizing the open circuit voltage and short circuit current of a photovoltaic module will be demonstrated, enabling an effective comparison with the data present in the datasheet provided by the manufacturer.

Experiment carried out in the field

We will be based on a practical experiment to demonstrate the procedures for adjusting the open circuit voltage (VOC) and short circuit current (ISC) of a photovoltaic module, taking as an adjustment basis the cell temperature and irradiance measured in the local. The following instruments were used in this experiment:

• Irradiance and temperature sensors coupled to a display device;
• 320 Wp polycrystalline silicon module, positioned on a roof with a 5º slope;
• A conventional multimeter.

In order to measure the climatic conditions in a situation closer to the NOCT condition, it was decided to carry out the tests at a time around noon, when the irradiance and ambient temperature are highest. The ambient temperature at the time of the tests was 35 ºC. The fact of carrying out the experiment at a time of high irradiance is also useful to improve the analysis of open circuit voltage. The open circuit voltage of the photovoltaic module is directly dependent on temperature and is practically not influenced by irradiance, as long as the irradiance level is high. In other words, the adjustment of the open circuit voltage for one of the standard conditions (STC or NOCT) can be done only as a function of the temperature variable. When setting up this experiment, the temperature sensor was installed in contact with the central region of the back of the module (backsheet). The irradiance sensor was installed coplanar to the surface of the photovoltaic module, as shown in Figure 1. Finally, the sensors are connected to the display terminals, enabling the visualization of the parameters during the tests.

Once the sensors were installed, the multimeter test leads were connected to the open circuit terminals of the photovoltaic module. The photographic record of the results collected in the field is shown in Figure 2 below.

Figure 3 shows the module short-circuit current being measured simultaneously with temperature and solar irradiance. This measure must be carried out cautiously, as closing and opening the circuit can cause a dangerous electric arc. The connection of the photovoltaic module in series with the multimeter and the closing and subsequent opening of the circuit must be done with the aid of a disconnector switch. Direct current photovoltaic circuits must never be sectioned under load (i.e., with the presence of electric current) without a sectioning device capable of extinguishing the electric arc.

STC and NOCT characteristics from the datasheet

To compare the measured results with the characteristics specified by the manufacturer, we need to look for the electrical parameters in the tables presented in the data sheet. In this example, the 320P6K-36 module was evaluated. The parameters we look for are the open circuit voltage (VOC) in STC and NOCT and the thermal coefficients of current and voltage in relation to temperature, normally expressed in %/oC. The %/oC unit indicates that the thermal coefficient determines the percentage variation in voltage or current (in relation to the nominal value in STC) as a function of temperature variation. As can be seen in the data sheet, the current coefficient is positive and the voltage coefficient is negative. A negative coefficient indicates that an increase in temperature causes a reduction in the open-circuit voltage of the photovoltaic module.

Table 1: Electrical parameters of the photovoltaic module in NOCT

Table 2: Photovoltaic module thermal coefficients

Table 3: Electrical characteristics of the photovoltaic module in STC

From the tables we can extract the following parameters of the 320P6K-36 module:

• Open circuit voltage at STC: VOC,STC = 46.39 V;
• Open circuit voltage in NOCT: VOC,NOCT = 42.8 V;
• Short-circuit current in STC: ISC,STC = 9.15 A;
• Short-circuit current in NOCT: ISC,NOCT = 7.42 A;
• Current thermal coefficient ISC: Alpha = 0.07 %/oC;
• VOC voltage thermal coefficient: Beta = -0.31 %/oC.

Open circuit voltage adjustment

After measuring the variables of open circuit voltage, cell temperature and solar irradiance, our objective will be to analyze the voltage value and compare it with the value shown in the module's data sheet. For this comparison to be possible, it is necessary to adjust the value in relation to temperature and irradiance, to obtain a normalized voltage value in the STC condition (25 oC and 1000 W/m2). Alternatively, we can also use the module parameters in NOCT as a basis for our analysis. In other words, we can choose to evaluate the module based on its NOCT condition reported in the data sheet. In this case, we will adjust the VOC value measured under the experimental conditions to the NOCT condition, and then compare the result of this adjustment with the VOC,NOCT value (open circuit voltage in the NOCT condition) presented in the data sheet. In the module used in this experiment we found the open circuit voltage VOC,NOCT = 42.8V and the thermal coefficient Beta equal to -0.31%/ºC, which expresses the variation in the open circuit voltage as a function of temperature. We will show how the measured open circuit voltage is adjusted. With this procedure we will be able to compare the measured voltage with the voltage presented in the manufacturer's data sheet.

Open circuit voltage adjustment for NOCT condition

According to the manufacturer's datasheet, the cell's operating temperature under NOCT conditions is 45 ± 2 ºC. Therefore, we must take this value as a reference to adjust our data. According to the temperature sensor, we have a temperature of 61.3 ºC in the experiment, a value 16.3 ºC above the NOCT standard. The temperature difference must be multiplied by the Beta coefficient, resulting in a correction factor that expresses the percentage variation of the open circuit voltage in relation to the NOCT value. The normalized open circuit voltage is obtained as follows:

DT = TEXPERIMENTAL – TNOCT

fc = Beta x DT

VOC,ADJUSTED = VOC,EXPERIMENTAL – fc * VOC,NOCT

Where:

• DT = temperature interval between the experimental value and the reference value (NOCT) [ºC];
• fc = correction factor calculated over the temperature range;
• Beta = thermal voltage coefficient [%/ºC];
• VOC,EXPERIMENTAL = VOC measured at experimental temperature [V];
• VOC,NOCT = VOC measured at reference temperature [V];
• VOC,ADJUSTED = VOC adjusted for the NOCT [V] condition.

Numerical example, based on the experiment carried out:

DT = 61.3 ºC – 45 ºC = 16.3 ºC

fc = -0.31 x 16.3 = -5.053 %/ºC

fc * VOC,NOCT = -5.053/100 x 42.8 V = -2.16 V

VOC,SET = 40.19 V – (-2.16 V) = 42.35 V

Comparing numerically the values of the VOC,SET voltage (42.35 V) with the VOC,NOCT voltage from the catalog (42.8 V), we see that the values are very similar. This makes it possible to verify that the photovoltaic module, from the point of view of open circuit voltage, is intact. This temperature measurement and adjustment procedure becomes more important the greater the number of modules connected in series. The professional, when measuring the voltage of a string with numerous modules, is not sure of measuring the correct value except by adjusting the measured variable in relation to a reference condition (STC or NOCT), so that the measured value can be compared with the values found in the product data sheet.

Setting the short-circuit current for STC condition

The short-circuit current (ISC) treatment process follows a different methodology. In this case, the adjustment of the measured variable is made based on two variables: temperature and irradiance. In the previous case, we neglected the effect of irradiance, since for a high irradiance value its effect on the open circuit voltage is negligible, and this variable depends directly on the temperature. In the case of short-circuit current, on the other hand, the main influencing variable is irradiance, together with a smaller effect of temperature. For our calculations, we must employ the alpha thermal coefficient of the short-circuit current, which can also be found in the data sheet. We have the following equation to adjust the data collected under experimental conditions for the STC condition [1]:

ISC,ADJUSTED = ISC,EXPERIMENTAL * [ 1 – Alpha x (TSTC – TEXPERIMENTAL) ] * IrradSTC/IrradEXPERIMENTAL

Where:

• ISC,SET = current adjusted for the STC condition [A];
• ISC,EXPERIMENTAL = experimentally measured current [A];
• Alpha = thermal coefficient of the short-circuit current [%/oC];
• TSTC = STC temperature (25 oC);
• TEXPERIMENTAL = experimental temperature [oC];
• IrradSTC = irradiance in STC (1000 W/m2);
• IrradEXPERIMENTAL = experimental irradiance [W/m2].

Numerically, we have:

ISC,ADJUSTED = 8.089 * [ 1 – 0.07/100 x (61.7 – 25) ] * 1000/903 = 9.19 A

Numerically comparing the ISC,ADJUSTED values (9.19 A) with the STC value obtained from the catalog (9.15 A), the test presented an error of 0.04% in relation to the data provided by the manufacturer. The values are very close, showing that the tested module presents a short-circuit current compatible with the manufacturer's specification.

Conclusion

Photovoltaic commissioning tests are essential to validate the correct functioning of a system. The NBR 116274:2014 standard describes several categories of tests. Open circuit voltage and short circuit current tests are part of category 1, which are the minimum requirements of the standard. To correctly evaluate a photovoltaic module – or a string of modules – it is necessary to adjust the measured voltage and current values as a function of temperature and solar irradiance, which need to be measured simultaneously. In this article we show how to adjust the open circuit voltage for the NOCT temperature and the short circuit current for the STC condition. In both cases, the results obtained experimentally were very close to the values obtained in the module manufacturer's data sheet.

References

• [1] Roy, J., Bliss, M., Betts, T. R., & Gottschalg, R. (2010). Effect of IR translations of irradiance-temperature on the energy yield prediction of PV module and spectral changes over irradiance and temperature. Proceedings of the 6th Photovoltaic Science Applications and Technology Conference (PVSAT-6), 149–152. https://dspace.lboro.ac.uk/
• [2] ABNT NBR 16274:2014 – Grid-connected photovoltaic systems – Minimum requirements for documentation, commissioning tests, inspection and performance assessment

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