Dec 1, 2001 - Department of Physics, University of Port Elizabeth, South Africa. ABSTRACT. The aim of the study was to build a system to monitor.
MONITORING CURRENT-VOLTAGE CHARACTERISTICS OF PHOTOVOLTAIC MODULES EE van Dyk, AR Gxasheka and EL Meyer Department of Physics, University of Port Elizabeth, South Africa.
ABSTRACT The aim of the study was to build a system to monitor the current-voltage (I-V) characteristics of photovoltaic (PV) modules subjected to outdoor conditions at the University of Port Elizabeth, South Africa. The monitoring of I-V characteristics, module temperature, ambient temperature, and irradiance will enable an analysis of module performance, degradation and or failure. In this paper the design of a low-cost system built to sequentially measure I-V characteristics of seven modules at regular intervals is discussed and an analysis of data obtained is presented. INTRODUCTION The performance parameters of photovoltaic (PV) modules are optimised at some reference condition, 2 usually at Standard Test Conditions (STC: 1000W/m of irradiance, 25°C cell temperature and air mass 1.5 global spectrum). However, PV modules are deployed outdoors where operating conditions are far from the reference conditions. Different module technologies respond differently to changes in irradiance, temperature and air mass. In order to characterise the response of performance parameters to various conditions, a system capable of continuously monitoring the current-voltage (I-V) characteristics of seven modules, was designed. The low-cost data acquisition system was designed, built and installed at the University of Port Elizabeth, South Africa. Results obtained from monitoring I-V characteristics can be used to investigate and compare the actual power produced by modules under realistic operating conditions. The results may also be used for degradation and failure analysis. In this paper the design of the system used to sequentially measure I-V characteristics is presented together with some initial results. Results presented here show the effect of irradiance and temperature on module performance parameters as well as a comparison between module energy production. SYSTEM DESCRIPTION The I-V sequencer system measures the I-V characteristics of seven modules. The system employs an array of resistors as load for the module, which enables the I-V characteristics to be swept from short
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circuit current (Isc) to open circuit voltage (Voc). Fig. 1 shows a schematic of the system. The system measures I-V characteristics of the different modules sequentially by selecting a module through mechanical relays. The data acquisition system (DAS) collects data every 15 minutes and employs A/D, relay and temperature cards. The current is measured using a Hall effect transducer, the voltage by a voltage transducer, and the plane of array (POA) irradiance is measured using a Licor pyranometer. Temperatures are measured using Each PV cromel-alumel (type K) thermocouples. module’s I-V data are stored as separate text files. A file of a particular PV module consists of 97 fields that go into the database as one record. These fields include each IV point on the curve, irradiance measurements at each IV point, temperature and the performance parameters such as fill-factor, Isc, Voc, Imax, Vmax, Pmax and aperture area efficiency.
Fig. 1: Schematic of the I-V sequencer system. Key to Fig. 1: PC DAS A V Rn PVn
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Computer Data acquisition System Hall effect transducer Voltage transducer Resistor array Photovoltaic module array
EXPERIMENTAL PROCEDURE The performance of seven silicon-based PV modules is being monitored (since October 2001) using the I-V sequencer described above. The modules’ I-V curves are measured every 15 minutes and stored in a database together with POA irradiation, ambient temperature and module temperature. Prior to their outdoor deployment in October 2001, the modules were first subjected to a testing procedure [1,2] to serve as a baseline for future reference. The testing procedure assesses the performance parameters. Since outdoor deployment in October 2001, the modules were periodically taken down for indoor assessment of their reliability.
3.5 2
Current (A)
2.5
Table 1: Modules used in this study.
Module
Type
Rated Power (W)
Measured Power (W)
Eff (%)
1
Mono-Si
48
46.1
11.3
2
Multi-Si
50
47.0
10.7
3
Mono-Si
70
63.7
9.3
4
Mono-Si
80
72.1
10.3
5
Multi-Si
80
73.6
10.5
6
Multi-Si
79
71.6
10.2
7
EFG-Si
50
48.6
11.9
These modules were made by different manufacturers and have different cell material technology. The lower measured power of all the modules is due to the fact that the modules have all been previously used for various applications for different lengths of time prior to this study.
1.5
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2
Curve 3: 375.0 W /m , 27.6°C
1.0 0.5 0.0
0
5
10
15
20
Voltage (V)
Fig. 2: I-V curves of module 1 at different irradiance levels and module temperatures. Table 2 lists the I-V parameters associated with curves 1, 2 and 3 in Fig. 2. Clearly, Isc is more irradiance dependent than Voc. Table 2: I-V parameters of module 1 at various irradiance levels. Irradiance (W/m²) 1042
Isc (A) 3.08
Voc (V) 20.0
FF (%) 0.69
Pmax (W) 42.5
628
1.90
20.1
0.73
28.0
375
1.19
19.8
0.75
17.7
Fig. 3 shows the influence of temperature on the I-V curve of module 1. With the use of queries in the database of module 1, two I-V characteristics meeting the desired criteria were selected. The I-V characteristics at st 30°C was traced on the 1 December 2001 and the other on the 15 December 2001. The respective irradiance values of each curve were close to 1000W/m² and were normalised to an irradiance of 1000 W/m² for uniformity.
3.0
RESULTS AND DISCUSSION
∆I
30°C
2.5
45°C
Current (A)
The effects of irradiance on the I-V curve of a PV module are illustrated in Fig 2. The three curves, curve 1, curve 2 and curve 3, of module 1 were taken on 23 January 2002 at different times. Curve 1, 2 and 3 were recorded at 12:00 PM, 9:15 AM and 8:15 AM, respectively. Also shown is the back-surface temperature of the module during each measurement. It is evident in Fig. 2 that there is a marked increase in Isc as irradiance rises. This is expected to occur since Isc is proportional to the amount of sunlight available [3]. In addition to this, the temperature is also causing an increase in Isc but that is not as large compared to that due to irradiance. Owing to the strong temperature dependency of Voc [4], the effects of irradiance on Voc are not visible on the graph since Voc is only logarithmically dependent on irradiance.
2
Curve 2: 628.3 W /m , 29.7°C
2.0
Modules under monitoring The modules currently being monitored are listed in table 1. Also shown in table 1 are the rated powers and the measured power at STC and aperture area efficiency.
Curve1: 1041.6 W /m , 33.0°C
3.0
2.0 1.5 1.0 0.5 0.0
∆V 0
5
10
15
20
Voltage (V) Fig. 3: I-V curves of Module 1 at different temperatures.
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9
1000 Module 6
Module 1
POA Irradiation
900
8
Energy Output (Wh/day)
800
7
700
6
600 5 500 4 400 3
300
2
200
1
100
Table 3: I-V parameters of module 1 at 30°C and 45°C. Temp (°C)
Isc (A)
Voc (V)
FF (%)
Pmax (W)
30
2.96
20.4
0.71
42.9
45
3.01
19.0
0.68
39.2
∆ (%)
1.7
-6.9
-4.2
-8.6
4
Isc Voc
3
3-Feb-02
15-Mar-02
Data such as daily energy production is very useful to proper system sizing because it can yield information about the total potential energy generation capabilities of PV modules. 50
15
V oc (V)
9
25-Dec-01
The daily energy production of module 1 and 6 together with the daily incident energy from the sun, POA irradiation, between October 2001 and March 2002 are shown in Fig 5. The breaks on energy curves correspond to the days when the modules were either taken down for indoor testing or some other experiments. It is clear from the graph that the energy produced by each module is different from the other, mainly due to the difference in size and power ratings. The operational efficiencies of module 1 and module 6 were 10.4 % and 9.3 %, respectively. This figure illustrates the type of useful data that can be obtained by monitoring I-V characteristics of PV modules. The long-term monitoring will show module degradation and seasonal variation of energy output.
18
2
0 15-Nov-01
Fig. 5: Energy production of modules 1 and 2 between October 2001 and March 2002.
21
12
I sc (A)
0 6-Oct-01
M a x im u m p o w e r ( W )
Figure 4 below shows Isc and Voc as a function of time of day for module 1 on the 21st February 2002. One can note the symmetry in the two curves of Isc and Voc, clearly demonstrating the symmetrical distribution of sunlight before and after solar noon. As expected in the dark hours of day Isc and Voc are zero as the device is inactive and only function as a p-n junction diode. Isc increases with time and reaches its maximum at solar noon. With the sun almost directly overhead at about 12:22 solar noon, the incident irradiance is nearly at its peak and so is Isc (as was shown in Fig. 2). A decrease in Isc was observed from the afternoon towards the evening, due to the fall in irradiance level. The drop in Voc around midday is due to the increase in temperature as discussed in Fig. 3 and table 3.
POA Irradiation (kWh/m²/day)
The effect of temperature on other I-V parameters is shown in table 3. From Fig. 3 and table 3 it is clear that Voc is more temperature dependent than Isc. A drop of 6.9 percent in Voc due to 15°C rise in temperature was observed in the module. The effect of temperature on Voc is therefore more profound than that of irradiance. It is noticeable in table 3 that there is an appreciable decrease in the maximum power (Pmax) of the module with temperature. Elevated temperatures and variation in solar irradiation, can lead to under-designing, which may in turn lead to system failure [4]. Therefore, it is important to take the negative effect of elevated temperature into account when using silicon PV devices.
M o d u le 1 M o d u le 2 M o d u le 7
40
30
20
10
0
6 :5 9
9 :2 9
1 1 :5 9
1 4 :2 9
1 6 :5 9
1 9 :2 9
--
T im e o f D a y
6
1
3 0
0 6:59
9:30
12:00 14:30 17:00 19:29
Time of day
Fig. 4: Isc and Voc as function of time of day on the 21st February 2002.
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Fig. 6: Maximum power of module 1, 2 and 7 versus time of day on 9 February 2002. Fig. 6 demonstrates the maximum power of modules 1,2 and 7 versus time of day on the 9th February 2002. A notable difference in the three curves is the top, close to the solar noon. Module 7 was the best performer of the three modules. The poor performance of module 2 compared to the other two modules could be due to delamination that has started around the edges of the frame. This delamination is caused by thermal cycling and moisture ingress. Fig. 6 also illustrates the
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distribution of peak-power, throughout the day, that can be utilized by loads connected directly to the modules as in the case of water pumping for livestock. Direct loads being run around solar noon tend to operate at their maximum efficiencies because of the high power from the modules. SUMMARY AND CONCLUSIONS A low cost I-V sequencer system capable of monitoring seven PV modules has been designed, built and successfully implemented at the University of Port Elizabeth, South Africa. Results obtained are very useful since the actual powers of the modules are obtained under realistic outdoor conditions. This type of information is crucial to system designers and for degradation and failure analyses. Monitoring the I-V characteristics under realistic outdoor conditions also allows an investigation of the effect of both temperature and irradiance on module performance. In addition, monitoring yields Isc and Voc, which can be used to obtain parameters, like ideality factor and dark saturation current [5]. ACKNOWLEDGEMENTS The authors to wish to express their gratitude to the South African National Research Foundation, ESKOM, and UPE for their financial assistance. The assistance of Mr D A O’Connor with the construction of the system is also acknowledged. REFERENCES [1]
E.E.van Dyk and E.L. Meyer (2001), “Long -term Monitoring of Photovoltaic Modules in South th Africa”, Proceedings of the 28 IEEE PV specialist conference, Anchorage, Alaska, pg 1525-1528.
[2]
E.L. Meyer and E.E. van Dyk (2001), Assessing the Performance Parameter of PV modules”, Submitted for publication in the IEEE Trans. on Rel.
[3]
E. Lorenzo (1994), Solar Electricity Engineering of Photovoltaic systems, pg 81-85.
[4]
E.E van Dyk, B.J. Scott, E.L. Meyer and A.W.R Leitch (2000), “Temperature dependence of performance of crystalline silicon photovoltaic modules", South African Journal of Science 96, pg 198-200.
[5]
E.L. Meyer and E.E. van Dyk (2002), “Extraction of I0 and n from Isc and Voc measurements”, Submitted for publication in journal of Applied Physics.
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