2. Properties of a Photovoltaic Module
2.3 Limit Values of NAPS NP190GKg PV Module Output
Maximum and minimum values of the current, voltage and power of the PV module output are needed in the design of a PV converter. The converter should be able to control its input voltage between the minimum and maximum MPP voltages of the PV module. Maximum value of the MPP voltage appears when the whole PV module is evenly illuminated and when the temperature is low and irradiance high. Respectively, the minimum value appears when the PV module is partially shaded so that only one group of the series connected cells is not bypassed and when the temperature is high and irradiance low.
The minimum and maximum values of NAPS NP190GKg PV module are of concern, since it is used in this thesis. Electrical characteristics of the module given in the manufacturer datasheet in STC are given in Table 2.1. These values are used in the calculations later on.
Table 2.1: Electrical characteristics of NAPS NP190GKg PV Module in STC Parameter Value
UOC,STC 33.1 V ISC,STC 8.02 A PMPP,STC 190 W UMPP,STC 25.9 V IMPP,STC 7.33 A
The internal connection of the PV cells inside NAPS NP190GKg PV module is presented in Fig. 2.5. The module consist of 54 series connected cells, which are divided into three groups, each of which has antiparallel bypass diode. These diodes are located in a module junction box.
upv
ipv
Figure 2.5: Internal connection of the PV cells inside NAPS NP190GKg PV module
The MPP voltage reaches its minimum value when two out of three of the bypass-diode groups are shaded. In this situation, current flows through two bypass bypass-diodes and 18 cells of non shaded group. Open-circuit voltage of a Si solar cell decreases by
rate of 2.3mV/K when temperature increases[13]. If the effect of irradiance on OC voltage is neglected, minimum MPP voltage UMPP,MIN can be approximated by (2.6).
UMPP,MIN = UMPP,STC−(TMAX−TSTC)NsKV
3 −2Ud, (2.6)
whereUMPP,STC is the MPP voltage in STC,TMAXis maximum cell temperature, TSTC
is the cell temperature in STC, which is 25◦C, KV is the temperature coefficient of the cell voltage and Ud is the forward voltage drop of the bypass diode. Minimum MPP voltage is 5.8 V, when the following values are used: TMAX = 60◦C, Ns = 54, Ud = 0.7V. This value is used in Chapter 3 as the minimum input voltage of the converter.
Maximum output voltage of the PV module was found by substituting the measure-ment data of the irradiance and the backplate temperature of NAPS NP190GKg PV module to the simplified equation (2.5). The same measurement setup has been used here as it was used to produce Fig. 2.4. Temperature was measured by using PT100 temperature sensor located on the back of the PV module. It was predicted, that the maximum peak of the OC voltage would take place in the spring when temperature is low and a peak in diffuse radiation would be formed due to cloud enhancement.
By studying the measurement data from June 2011 to May 2012, it was observed that the highest peak in OC voltage and in glogal irradiance took place on 5th of April 2012. Measured irradiance during the course of that day is shown in Fig. 2.6. As it is shown, the irradiance level fluctuates heavily and the peak takes place around noon, caused most likely by the cloud enhancement.
0 6 12 18 24
0 200 400 600 800 1000 1200 1400
Time (h) Solar Irradiance (W/m2 )
Figure 2.6: Measured irradiance on the surface of PV module on 5th of April 2012.
An extended view of the global irradiance peak value is shown in Fig. 2.7. The irradiance level stays above 1000W/m2 for several minutes, around 1300W/m2 for a minute and around 1400W/m2 for several seconds. Backplate temperature of NAPS NP190GKg PV module is presented in Fig. 2.8 measured at the same time as global
2. Properties of a Photovoltaic Module 10 irradiance. Backplate temperature is quite low and increases slowly. Actually the temperature stays almost constant compared to irradiance during the measurement period.
0 100 200 300 400 500 600 700 800
0 200 400 600 800 1000 1200 1400
Time (s) Solar Irradiance (W/m2 )
Figure 2.7: Measured peak in the irradiance on the surface of PV module during the cloud passing conditions.
0 100 200 300 400 500 600 700 800
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Time (s) Temperature (° C)
Figure 2.8: Measured temperature on the back of PV module during the cloud passing conditions.
Open-circuit voltage in Fig. 2.9 was calculated by substituting the data of Fig. 2.7 and Fig. 2.8 to equation (2.5). As it is shown, OC voltage stays around 35 V during the whole measurement period. Because this graph was achieved by the simplified model that neglects the effect of shunt and series resistance, the maximum OC voltage was also simulated by MatlabR Simulink model based on (2.1). This model is already verified to be valid for the same PV module in the prior research [12]. Temperature of 1.9◦C and irradiance of 1400W/m2 were used in the simulation. As it is visible in the simulated I-U curve of Fig. 2.10, maximum open-circuit voltage is 36.5V, which is one volt higher than the value achieved by the simplified model. Simulated value is
selected since it is obtained by more accurate model. SC current in the I-U curve of Fig. 2.10 is about 11A. This is in accordance with (2.2), which predicts that it is 1.4 times the SC current in STC when temperature depency is ignored.
Measured maximum value of 1400W/m2 for the global irradiance is consistent with the prior research [14], in which it was mentioned that the global irradiance might be around 1000W/m2 - 1500W/m2 with the duration of 20s to 140s. This obviously depends on the location of the PVG. Similarly, the maximum OC voltage of the PV module is heavily dependent on the location. For example if the ambient temperature is always high, OC voltage will not achieve high value even if the irradiance is high.
0 100 200 300 400 500 600 700 800
0 5 10 15 20 25 30 35 40
Time (s)
Voltage (V)
Figure 2.9: Calculated open-circuit voltage of NAPS NP190GKg PV module based on mea-sured irradiance and temperature data during the cloud passing conditions.
0 5 10 15 20 25 30 35 40
0 2 4 6 8 10 12
Voltage (V)
Current (A)
Figure 2.10: Simulated I-U curve of NAPS NP190GKg PV module in maximum open-circuit voltage condition.
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