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Monitoring current–voltage characteristics and energy output of silicon photovoltaic modules
Renewable Energy 30 (2005) 399–411 www.elsevier.com/locate/renene
Technical note
Monitoring current–voltage characteristics and energy output of silicon photovoltaic modules E.E. van Dyk a,, A.R. Gxasheka a, E.L. Meyer b a
Department of Physics, University of Port Elizabeth, Port Elizabeth 6031, South Africa b Department of Physics, University of Fort Hare, Alice 5700, South Africa Received 19 January 2004; accepted 20 April 2004
Abstract Photovoltaic (PV) system designers use performance data of PV modules to improve system design and make systems more cost effective. The collection of this valuable data is often not done due to the high costs associated with data acquisition systems. In this paper, we report on the design of a low-cost current–voltage (I–V) measuring system used to monitor the I–V characteristics of PV modules. Results obtained from monitoring seven crystalline silicon modules between October 2001 and November 2002 are presented and discussed. Results obtained also show the value of being able to continuously monitor the current– voltage characteristics of PV modules. # 2004 Elsevier Ltd. All rights reserved. Keywords: Photovoltaic modules; Current–voltage characteristics; Performance monitoring; I–V monitoring system; Operational efficiency
1. Introduction The performance parameters of photovoltaic (PV) modules are optimized at some reference condition, usually at standard test conditions (STC) of 1000 W/m2 v 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 [1–3]. In order to characterize the response of performance parameters to various outdoor conditions, a low-cost data
Corresponding author. Tel.: +27-41-5042579; fax: +27-41-5042573. E-mail address: [email protected] (E.E. van Dyk).
0960-1481/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2004.04.016
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acquisition system (DAS) capable of continuously monitoring the current–voltage (I–V) characteristics of seven modules, was designed, built and implemented 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/or failure analysis. Studies have shown that in addition to the technology and material of the cells, the performance of PV modules is dependent on the environmental conditions at the site [4–6]. The temperature and irradiance are some of the factors influencing the performance of PV modules outdoors. This paper reports on the design of a low-cost I–V sequencer system [7,8] that was used to monitor I–V characteristics of seven modules. Important results obtained from the monitoring of I–V characteristics between October 2001 and November 2002 are presented and discussed. The influence of parameters such as temperature and irradiance on PV module performance is discussed and the energy production of the modules used in this study was also investigated.
2. Current–voltage sequencer system A system capable of sequentially measuring the I–V characteristics of up to seven modules was built for this study [7,8]. This system, which we call the I–V sequencer system, employs an array of resistors as load for the module. The resistors are switched in and out of the circuit in such a way to enable the module I–V characteristics to be swept from short-circuit current (Isc) to open-circuit voltage (Voc). Fig. 1 shows a schematic of the system. The system measures I–V character-
Fig. 1. Schematic of I–V sequencer system. Key: PC, computer; DAS, data acquisition system; A, Hall effect transducer; V, voltage transducer; Rn, resistor array; PVn, photovoltaic module array; S0 n , load relay array; Sn, module relay array.
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istics of the different modules sequentially by selecting a module through mechanical relays S1 through S7. The relays S0 1 through S0 16 are used to select a parallel combination of resistors used as a resistive load for a particular I–V pair. We used a combination of resistors that allowed 48 desired resistances, and hence I–V pairs to be measured. For an I–V curve to be traced, a particular module is selected by closing one of the relays S1 through S7, while the rest of S1 through S7 remain open. With all other relays (S0 1 through S0 16 ) open, Voc of the selected module is measured with the voltage transducer, V. When S0 1 is closed, the module is in a short-circuit condition and Isc is measured with the current transducer, A. Switch S0 1 is then opened and the desired resistances obtained by sequentially selecting various combinations of relays S0 1 through S0 16 . At each resistance value, I, V together with the plane of array (POA) irradiance are measured to obtain the full I–V characteristic. An example of an I–V characteristic of a single-crystalline silicon module, measured with the I–V sequencer, is shown in Fig. 2. Note that the resistance values are chosen to obtain a well-defined ‘‘knee’’ indicated by K in the figure. To obtain a good spread of points around the ‘‘knee’’, simulations under different current ratings and various meteorological conditions were performed. This enabled us to select a set of resistance values that yielded as many I–V points as possible around the ‘‘knee’’ for each PV module used in the study. The DAS comprises two plug-in computer cards; an ISA PC-73C temperature board and a PCI-30G A/D input/output board [9]. The ISA PC-73C is a
Fig. 2. A typical I–V curve of a single-crystalline silicon module showing the distribution of points along the curve.
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thermocouple board consisting of an auxiliary board, which has cold junction compensation and thermocouple input terminals. This auxiliary card is connected to the PC back plane via an auxiliary DB25 male connector. The PCI-30G card is used as an input/output (I/O) board for analog signals from sensors and for digital output from the software program to control relays. The board is interfaced to the analog signal lines and to the digital docking module (PC-43A2) by a 25-way A & D splitter cable. The interconnectivity of the cards is shown in Fig. 3. The DAS collects data every 15 min, employing the A/D, relay and temperature cards in a computer. The current is measured using a Hall effect transducer, the voltage by a voltage transducer, the POA irradiance is measured using a CM6B Kipp & Zonen pyranometer, and temperatures are measured using cromel–alumel (type K) thermocouples. Irradiance is measured at each I–V point in order to offset any changes in irradiance that may be caused by sudden cloud cover during measurement time. Each PV 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 point on the I–V characteristic, irradiance measurements at each I–V point, temperature and the performance parameters; fill-factor (FF), Isc, Voc, and current, voltage and power at maximum power (Imax, Vmax, Pmax), and aperture area efficiency. A program written in Hewlett
Fig. 3. Interconnectivity of I–V sequencer components and computer boards [9].
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Packard’s Visual Engineering Environment (HPVEE) software is used to control all instruments and measure all signals.
3. PV modules monitored The I–V curves of seven crystalline silicon modules were monitored for the period 1 October 2001–30 November 2002. The modules were mounted on a north v facing rack with a latitude tilt (34 ) and the I–V curves monitored using the I–V sequencer system. Table 1 lists the modules used in this study. The modules were all manufactured using crystalline silicon cells; either single-crystalline (c-Si), multicrystalline (mc-Si) or edge-defined film-fed growth (EFG-Si). Also shown in the table is the rated and measured power at STC, and the aperture efficiency of each module measured prior to commencement of monitoring. It is worth noting that all the modules had been deployed outdoors for various lengths prior to this study. This may account for the differences between rated and measured power as discussed elsewhere [8]. The purpose of this paper is not to compare rated power to measured power, but rather to illustrate the value of the information obtained by monitoring the I–V characteristics of the seven modules using the low-cost I–V sequencer.
4. Results and discussion The value of monitoring the I–V characteristic of modules while deployed outdoors is that the performance under real operating conditions may be investigated and the effects of environmental conditions are taken into consideration by the monitoring. The effects of solar irradiance and temperature on performance parameters are discussed in detail elsewhere [10,11]. These effects are shown in Figs. 4–6. Fig. 4 shows three I–V characteristics of mod1_c-Si measured at different times of day. Also shown in the figure are irradiance and module temperature values, and the maximum power produced by the module. There is a linear increase in photo-generated current with increased photon flux as irradiance levels Table 1 Modules monitored outdoors. The rated and measured powers are at STC, also shown are the module aperture area efficiencies Module
Rated Pmax (W)
Measured Pmax (W)
Efficiency (%)
Mod1_c-Si Mod2_mc-Si Mod3_c-Si Mod4_c-Si Mod5_mc-Si Mod6_mc-Si Mod7_EFG-Si
48 50 70 80 80 79 50
46.1 47.0 63.7 72.1 73.6 71.6 48.6
11.3 10.8 9.4 10.6 10.7 10.4 12.5
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Fig. 4. I–V characteristics of mod1_c-Si at different irradiances and temperatures, on 21 November 2002. Also shown are the times when the I–V characteristics were measured and the maximum power produced by the modules.
increase [12], resulting in the observed increase in current and power produced by the module with increased irradiance. The decrease in Voc with increase temperature is also clearly illustrated in the figure. The logarithmic dependence of voltage on irradiance level [13] is shown in Fig. 5 which depicts Isc and Voc plotted as a function of time of day for mod1_c-Si on 21 January 2002. For clarity, irradiance is omitted as it follows the same symmetrical distribution before and after solar noon as Isc. At solar noon, which is 12:22 p.m. for the data shown, the amount of irradiance received by the module is at its maximum; hence Isc is also at its maximum. There is, however, a drop in Voc around solar noon, which may be attributed to the elevated temperatures at this time of the day. The effect of temperature on the I–V parameters is illustrated by Fig. 6, which shows the I–V characteristics of mod1_c-Si, measured around noon on different days when the v v module temperature was 37 and 46 C. The I–V characteristics measured at 37 C v was traced on 22 November 2002 and that at 46 C on 11 January 2002. Both I–V characteristics in the figure are normalized to 1000 W/m2. Also shown in the figure v are the respective I–V parameters at the 37 and 46 C. With reference to the table in Fig. 6, it is clear that Voc is more temperature dependent than Isc. The large v drop in voltage (hV) due to 9 C increase in temperature as illustrated by the 6.3% drop in Voc which is not compensated for by the relatively small increase in current, hI (1.6% in Isc), resulting in a reduction in Pmax of 5.1%. If not taken into
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Fig. 5. Isc and Voc of mod1_c-Si as a function of time of day on 21 January 2002.
Fig. 6. I–V characteristics of mod1_c-Si at different temperatures, normalized to 1000 W/m2. I–V charv acteristics were measured on 22 November 2002 and 11 January 2002 for 37 and 46 C, respectively.
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account, excessive temperatures coupled with variation in irradiation, can lead to under-designing of PV systems, which in turn may lead to system failure [6,7]. In order to compare the module performance over the monitoring period, the module energy produced per unit area, operational efficiency and average monthly energy per unit area per day were calculated [8]. The operational efficiency is defined as the energy produced per unit aperture area expressed as a percentage of incident irradiation. The aperture area is taken to be the area of the exposed glass surface of a module. Fig. 7 shows a plot of average daily energy produced during the monitoring period October 2001–November 2002. In the figure, for clarity, only the energy produced by three of the modules, representing the three different technologies (mod1_c-Si, mod5_mc-Si, and mod7_EFG-Si), is shown together with the incident irradiation. It is clear in the figure that there is a great variation in energy produced from day to day due to the fluctuations in daily irradiation and other meteorological parameters such as temperature. These fluctuations were also observed for all the other modules. The gaps in the dataset correspond to the days when PV modules were taken down for indoor testing or other experiments. The seasonal variation caused mainly by fewer sunshine hours in winter than in summer is also evident. Plots such as those in Fig. 7 are useful for identifying how different modules perform under different meteorological conditions [14] and since all modules are subjected to the same conditions, a direct comparison of their
Fig. 7. Average daily energy of all modules used in this study.
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performance is therefore possible. An example of this type of comparison is that it is clear that mod7_EFG-Si was the best performer from the beginning of the monitoring period due to its superior efficiency, but careful examination of the data reveals that there are signs of performance degradation for this module towards the end of the period. Due to high fluctuations in daily irradiation, it is better to look at energy output versus irradiation, thereby obtaining a better comparison between the different modules [1]. Fig. 8 shows the average daily energy per unit area as a function of average daily irradiation for the same modules as in Fig. 7. Also shown in the figure (inset) are the respective slopes of the least-squares fits to the data. The linear relationship between energy produced and incident irradiance is clearly demonstrated in the figure. The scatter in the data is due to changes in meteorological conditions resulting in spectral variation and temperature changes at the specific measured irradiances. Module degradation would also contribute to increased scatter in the data due to lower energy output. The slopes of the least-squares fits which are representative of the efficiency are therefore good indicators of the overall module performance and enable one to directly evaluate how modules perform under varying irradiance while deployed outdoors. From the figure, one is able to determine how the modules will perform on average over a long time and rank their performance. It is again clear that mod7_EFG-Si is the best performer
Fig. 8. Average daily energy of mod1_c-Si, mod5_mc-Si and Mod7_EFG-Si as a function of average daily irradiation.
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(highest slope), followed by mod1_c-Si, and then mod5_mc-Si. It must, however, be noted that although this analysis gives an overall idea of operational efficiency, it does not explicitly show degradation in performance. There is a degradation of about 8% in the performance of mod7_EFG-Si, which may contribute to some scatter in the data shown in Fig. 8, particularly due to data collected towards the end of the monitoring period. This degradation, however, does not affect the analysis in this study. For the other modules, there was no meaningful degradation as discussed below. In addition to the above analysis, photovoltaic system designers and installers can use energy data and operational efficiencies as presented in Figs. 9 and 10, and Table 2 to their advantage. Fig. 9 shows the average monthly energy per unit area per day of the three modules used in the discussion above. Also shown in the figure, for comparison, is the average monthly irradiation per day. The figure gives a good visual indication of the normalized energy production of the modules. The data of Fig. 9 may be used to determine operational efficiencies. The monthly operational efficiencies for the three modules are shown in Fig. 10, enabling the monitoring of any degradation. From the figure, it is clear that mod1_c-Si and mod5_mc-Si do not degrade during the monitoring period, while mod7_EFG-Si degrades by more than 8% during this time. The degradation may be ascribed to the moisture ingress observed during visual inspection of the module [8]. An overall comparison of the modules’ performances is possible by comparing the average operational efficiencies of the modules during the monitoring period.
Fig. 9. Average monthly energy per day of mod1_c-Si, mod5_mc-Si and Mod7_EFG-Si.
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Fig. 10. Monthly operational efficiencies of all mod1_c-Si, mod5_mc-Si and Mod7_EFG-Si.
These operational efficiencies are listed in Table 2. As expected, mod7_EFG-Si and mod1_c-Si performed well compared to other modules, with operational efficiencies of 10.2% and 9.9%, respectively. The operational efficiencies are, however, well below than those measured indoors as listed in Table 1. This is due to environmental factors such as elevated temperatures, changing irradiation level, and varying spectral conditions. This shows that STC at which modules are usually specified are hardly, if ever realized outdoors [14]. As discussed above, operational efficiencies calculated for the entire monitoring period give an overall indication of the performance during monitoring and do not give any indication of degradation as Table 2 Average operational efficiencies of the seven modules for the period between October 2001 and November 2002 Module
Operational efficiency (%)
Mod1_c-Si Mod2_mc-Si Mod3_c-Si Mod4_c-Si Mod5_mc-Si Mod6_mc-Si Mod7_EFG-Si
9.9 9.5 7.6 9.2 7.5 8.7 10.2
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observed for mod7_EFG-Si. In this case, the module was, on average, the best performer with the highest overall operational efficiency, but showed signs of degradation. It is, therefore, essential that performance be monitored and that performance data are thoroughly analyzed as monitoring performance parameters is the best way to determine overall module output performance and to detect if there is any degradation. 5. Summary and conclusion The main objective of this study was to demonstrate the value of the low-cost I–V sequencer. The design and implementation of the low-cost I–V sequencer system capable of monitoring up to seven PV modules was presented and discussed. The system was used to monitor the I–V characteristics of PV modules under realistic outdoor conditions between October 2001 and November 2002. The data obtained showed the value of monitoring PV modules as the performance of a particular cell technology type could be tracked over time. The monitoring also yielded useful results on how each PV module performed over an extended period of time with regard to energy generating capability and operational efficiencies. Although secondary to this study, degradation of PV modules can also be determined using this system. With regard the modules monitored, we were able to use the data obtained to rank the modules in terms of how they performed during the monitoring period. The energy produced by the modules showed great variation from day to day due to highly variable meteorological conditions at the site. Even though mod7_EFGSi showed signs of degradation, it was the best performer with overall outdoor operational efficiency of 10.2%. It was followed by mod1_c-Si with 9.9% operational efficiency. This paper successfully emphasizes the importance of outdoor performance monitoring as there was a significant difference between indoor and outdoor measurements. Data obtained in this study can be used by system designers and consumers to correctly design and use photovoltaic systems. Acknowledgements The authors wish to thank the South African National Research Foundation, the South African Department of Trade and Industry, and Eskom for their financial support. References [1] del Cueto JA. Comparison of energy production and performance from flat plate photovoltaic module technologies deployed at fixed tilt. Proceedings of the 29th IEEE Photovoltaic Specialists Conference. 2002, p. 1523–6. [2] Meyer EL, van Dyk EE. Degradation analysis of silicon photovoltaic modules. Proceedings of the 16th European Photovoltaic Solar Energy Conference. 2000, p. 2272–5.
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[3] van Dyk EE, Meyer EL, Scott BJ, O’Connor DA, Wessels JB. Analysis of photovoltaic module energy output under operating conditions in South Africa. Proceedings of the 26th IEEE Photovoltaic Specialists Conference. 1997, p. 1197–200. [4] King DL, Boyson WE, Kratochvil JA. Analysis of factors influencing the annual energy production of photovoltaic systems. Proceedings of the 28th IEEE Photovoltaic Specialists Conference. 2002, p. 1525–8. [5] Meyer EL, van Dyk EE. The behaviour of photovoltaic modules under reduced light levels. Proceedings of the 17th European Photovoltaic Solar Energy Conference. 2001, p. 528–31. [6] van Dyk EE, Meyer EL. Long-term monitoring of photovoltaic modules in South Africa. Proceedings of the 28th IEEE Photovoltaic Specialists Conference. 2000, p. 1525–8. [7] van Dyk EE, Gxasheka AR, Meyer EL. Monitoring current–voltage characteristics of photovoltaic modules. Proceedings of the 29th IEEE Photovoltaic Specialists Conference. 2002, p. 1516–9. [8] Gxasheka AR. On the monitoring of current–voltage characteristics of photovoltaic modules. MSc Dissertation, Department of Physics, University of Port Elizabeth, 2003. [9] Eagle Technology, 2003. Available from: http://www.eagle.co.za/. [10] van Dyk EE, Meyer EL, Vorster FJ, Leitch AWR. Renew Energy 2002;25/2:183–97. [11] van Dyk EE, Scott BJ, Meyer EL, Leitch AWR. S Afr J Sci 2000;96:198. [12] Lorenzo E. Solar electricity: engineering of photovoltaic systems [Davies P, Trans.]. PROGENSA; 1994. [13] Anderson CJ. Photovoltaic translation equations: a new approach. NREL Final Subcontract Report No. DE-AC36-83CH10093, 1996. [14] Meyer EL, van Dyk EE. Renew Energy 2000;21:37–47.
Technical note
Monitoring current–voltage characteristics and energy output of silicon photovoltaic modules E.E. van Dyk a,, A.R. Gxasheka a, E.L. Meyer b a
Department of Physics, University of Port Elizabeth, Port Elizabeth 6031, South Africa b Department of Physics, University of Fort Hare, Alice 5700, South Africa Received 19 January 2004; accepted 20 April 2004
Abstract Photovoltaic (PV) system designers use performance data of PV modules to improve system design and make systems more cost effective. The collection of this valuable data is often not done due to the high costs associated with data acquisition systems. In this paper, we report on the design of a low-cost current–voltage (I–V) measuring system used to monitor the I–V characteristics of PV modules. Results obtained from monitoring seven crystalline silicon modules between October 2001 and November 2002 are presented and discussed. Results obtained also show the value of being able to continuously monitor the current– voltage characteristics of PV modules. # 2004 Elsevier Ltd. All rights reserved. Keywords: Photovoltaic modules; Current–voltage characteristics; Performance monitoring; I–V monitoring system; Operational efficiency
1. Introduction The performance parameters of photovoltaic (PV) modules are optimized at some reference condition, usually at standard test conditions (STC) of 1000 W/m2 v 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 [1–3]. In order to characterize the response of performance parameters to various outdoor conditions, a low-cost data
Corresponding author. Tel.: +27-41-5042579; fax: +27-41-5042573. E-mail address: [email protected] (E.E. van Dyk).
0960-1481/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2004.04.016
400
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acquisition system (DAS) capable of continuously monitoring the current–voltage (I–V) characteristics of seven modules, was designed, built and implemented 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/or failure analysis. Studies have shown that in addition to the technology and material of the cells, the performance of PV modules is dependent on the environmental conditions at the site [4–6]. The temperature and irradiance are some of the factors influencing the performance of PV modules outdoors. This paper reports on the design of a low-cost I–V sequencer system [7,8] that was used to monitor I–V characteristics of seven modules. Important results obtained from the monitoring of I–V characteristics between October 2001 and November 2002 are presented and discussed. The influence of parameters such as temperature and irradiance on PV module performance is discussed and the energy production of the modules used in this study was also investigated.
2. Current–voltage sequencer system A system capable of sequentially measuring the I–V characteristics of up to seven modules was built for this study [7,8]. This system, which we call the I–V sequencer system, employs an array of resistors as load for the module. The resistors are switched in and out of the circuit in such a way to enable the module I–V characteristics to be swept from short-circuit current (Isc) to open-circuit voltage (Voc). Fig. 1 shows a schematic of the system. The system measures I–V character-
Fig. 1. Schematic of I–V sequencer system. Key: PC, computer; DAS, data acquisition system; A, Hall effect transducer; V, voltage transducer; Rn, resistor array; PVn, photovoltaic module array; S0 n , load relay array; Sn, module relay array.
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istics of the different modules sequentially by selecting a module through mechanical relays S1 through S7. The relays S0 1 through S0 16 are used to select a parallel combination of resistors used as a resistive load for a particular I–V pair. We used a combination of resistors that allowed 48 desired resistances, and hence I–V pairs to be measured. For an I–V curve to be traced, a particular module is selected by closing one of the relays S1 through S7, while the rest of S1 through S7 remain open. With all other relays (S0 1 through S0 16 ) open, Voc of the selected module is measured with the voltage transducer, V. When S0 1 is closed, the module is in a short-circuit condition and Isc is measured with the current transducer, A. Switch S0 1 is then opened and the desired resistances obtained by sequentially selecting various combinations of relays S0 1 through S0 16 . At each resistance value, I, V together with the plane of array (POA) irradiance are measured to obtain the full I–V characteristic. An example of an I–V characteristic of a single-crystalline silicon module, measured with the I–V sequencer, is shown in Fig. 2. Note that the resistance values are chosen to obtain a well-defined ‘‘knee’’ indicated by K in the figure. To obtain a good spread of points around the ‘‘knee’’, simulations under different current ratings and various meteorological conditions were performed. This enabled us to select a set of resistance values that yielded as many I–V points as possible around the ‘‘knee’’ for each PV module used in the study. The DAS comprises two plug-in computer cards; an ISA PC-73C temperature board and a PCI-30G A/D input/output board [9]. The ISA PC-73C is a
Fig. 2. A typical I–V curve of a single-crystalline silicon module showing the distribution of points along the curve.
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thermocouple board consisting of an auxiliary board, which has cold junction compensation and thermocouple input terminals. This auxiliary card is connected to the PC back plane via an auxiliary DB25 male connector. The PCI-30G card is used as an input/output (I/O) board for analog signals from sensors and for digital output from the software program to control relays. The board is interfaced to the analog signal lines and to the digital docking module (PC-43A2) by a 25-way A & D splitter cable. The interconnectivity of the cards is shown in Fig. 3. The DAS collects data every 15 min, employing the A/D, relay and temperature cards in a computer. The current is measured using a Hall effect transducer, the voltage by a voltage transducer, the POA irradiance is measured using a CM6B Kipp & Zonen pyranometer, and temperatures are measured using cromel–alumel (type K) thermocouples. Irradiance is measured at each I–V point in order to offset any changes in irradiance that may be caused by sudden cloud cover during measurement time. Each PV 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 point on the I–V characteristic, irradiance measurements at each I–V point, temperature and the performance parameters; fill-factor (FF), Isc, Voc, and current, voltage and power at maximum power (Imax, Vmax, Pmax), and aperture area efficiency. A program written in Hewlett
Fig. 3. Interconnectivity of I–V sequencer components and computer boards [9].
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Packard’s Visual Engineering Environment (HPVEE) software is used to control all instruments and measure all signals.
3. PV modules monitored The I–V curves of seven crystalline silicon modules were monitored for the period 1 October 2001–30 November 2002. The modules were mounted on a north v facing rack with a latitude tilt (34 ) and the I–V curves monitored using the I–V sequencer system. Table 1 lists the modules used in this study. The modules were all manufactured using crystalline silicon cells; either single-crystalline (c-Si), multicrystalline (mc-Si) or edge-defined film-fed growth (EFG-Si). Also shown in the table is the rated and measured power at STC, and the aperture efficiency of each module measured prior to commencement of monitoring. It is worth noting that all the modules had been deployed outdoors for various lengths prior to this study. This may account for the differences between rated and measured power as discussed elsewhere [8]. The purpose of this paper is not to compare rated power to measured power, but rather to illustrate the value of the information obtained by monitoring the I–V characteristics of the seven modules using the low-cost I–V sequencer.
4. Results and discussion The value of monitoring the I–V characteristic of modules while deployed outdoors is that the performance under real operating conditions may be investigated and the effects of environmental conditions are taken into consideration by the monitoring. The effects of solar irradiance and temperature on performance parameters are discussed in detail elsewhere [10,11]. These effects are shown in Figs. 4–6. Fig. 4 shows three I–V characteristics of mod1_c-Si measured at different times of day. Also shown in the figure are irradiance and module temperature values, and the maximum power produced by the module. There is a linear increase in photo-generated current with increased photon flux as irradiance levels Table 1 Modules monitored outdoors. The rated and measured powers are at STC, also shown are the module aperture area efficiencies Module
Rated Pmax (W)
Measured Pmax (W)
Efficiency (%)
Mod1_c-Si Mod2_mc-Si Mod3_c-Si Mod4_c-Si Mod5_mc-Si Mod6_mc-Si Mod7_EFG-Si
48 50 70 80 80 79 50
46.1 47.0 63.7 72.1 73.6 71.6 48.6
11.3 10.8 9.4 10.6 10.7 10.4 12.5
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Fig. 4. I–V characteristics of mod1_c-Si at different irradiances and temperatures, on 21 November 2002. Also shown are the times when the I–V characteristics were measured and the maximum power produced by the modules.
increase [12], resulting in the observed increase in current and power produced by the module with increased irradiance. The decrease in Voc with increase temperature is also clearly illustrated in the figure. The logarithmic dependence of voltage on irradiance level [13] is shown in Fig. 5 which depicts Isc and Voc plotted as a function of time of day for mod1_c-Si on 21 January 2002. For clarity, irradiance is omitted as it follows the same symmetrical distribution before and after solar noon as Isc. At solar noon, which is 12:22 p.m. for the data shown, the amount of irradiance received by the module is at its maximum; hence Isc is also at its maximum. There is, however, a drop in Voc around solar noon, which may be attributed to the elevated temperatures at this time of the day. The effect of temperature on the I–V parameters is illustrated by Fig. 6, which shows the I–V characteristics of mod1_c-Si, measured around noon on different days when the v v module temperature was 37 and 46 C. The I–V characteristics measured at 37 C v was traced on 22 November 2002 and that at 46 C on 11 January 2002. Both I–V characteristics in the figure are normalized to 1000 W/m2. Also shown in the figure v are the respective I–V parameters at the 37 and 46 C. With reference to the table in Fig. 6, it is clear that Voc is more temperature dependent than Isc. The large v drop in voltage (hV) due to 9 C increase in temperature as illustrated by the 6.3% drop in Voc which is not compensated for by the relatively small increase in current, hI (1.6% in Isc), resulting in a reduction in Pmax of 5.1%. If not taken into
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Fig. 5. Isc and Voc of mod1_c-Si as a function of time of day on 21 January 2002.
Fig. 6. I–V characteristics of mod1_c-Si at different temperatures, normalized to 1000 W/m2. I–V charv acteristics were measured on 22 November 2002 and 11 January 2002 for 37 and 46 C, respectively.
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account, excessive temperatures coupled with variation in irradiation, can lead to under-designing of PV systems, which in turn may lead to system failure [6,7]. In order to compare the module performance over the monitoring period, the module energy produced per unit area, operational efficiency and average monthly energy per unit area per day were calculated [8]. The operational efficiency is defined as the energy produced per unit aperture area expressed as a percentage of incident irradiation. The aperture area is taken to be the area of the exposed glass surface of a module. Fig. 7 shows a plot of average daily energy produced during the monitoring period October 2001–November 2002. In the figure, for clarity, only the energy produced by three of the modules, representing the three different technologies (mod1_c-Si, mod5_mc-Si, and mod7_EFG-Si), is shown together with the incident irradiation. It is clear in the figure that there is a great variation in energy produced from day to day due to the fluctuations in daily irradiation and other meteorological parameters such as temperature. These fluctuations were also observed for all the other modules. The gaps in the dataset correspond to the days when PV modules were taken down for indoor testing or other experiments. The seasonal variation caused mainly by fewer sunshine hours in winter than in summer is also evident. Plots such as those in Fig. 7 are useful for identifying how different modules perform under different meteorological conditions [14] and since all modules are subjected to the same conditions, a direct comparison of their
Fig. 7. Average daily energy of all modules used in this study.
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performance is therefore possible. An example of this type of comparison is that it is clear that mod7_EFG-Si was the best performer from the beginning of the monitoring period due to its superior efficiency, but careful examination of the data reveals that there are signs of performance degradation for this module towards the end of the period. Due to high fluctuations in daily irradiation, it is better to look at energy output versus irradiation, thereby obtaining a better comparison between the different modules [1]. Fig. 8 shows the average daily energy per unit area as a function of average daily irradiation for the same modules as in Fig. 7. Also shown in the figure (inset) are the respective slopes of the least-squares fits to the data. The linear relationship between energy produced and incident irradiance is clearly demonstrated in the figure. The scatter in the data is due to changes in meteorological conditions resulting in spectral variation and temperature changes at the specific measured irradiances. Module degradation would also contribute to increased scatter in the data due to lower energy output. The slopes of the least-squares fits which are representative of the efficiency are therefore good indicators of the overall module performance and enable one to directly evaluate how modules perform under varying irradiance while deployed outdoors. From the figure, one is able to determine how the modules will perform on average over a long time and rank their performance. It is again clear that mod7_EFG-Si is the best performer
Fig. 8. Average daily energy of mod1_c-Si, mod5_mc-Si and Mod7_EFG-Si as a function of average daily irradiation.
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(highest slope), followed by mod1_c-Si, and then mod5_mc-Si. It must, however, be noted that although this analysis gives an overall idea of operational efficiency, it does not explicitly show degradation in performance. There is a degradation of about 8% in the performance of mod7_EFG-Si, which may contribute to some scatter in the data shown in Fig. 8, particularly due to data collected towards the end of the monitoring period. This degradation, however, does not affect the analysis in this study. For the other modules, there was no meaningful degradation as discussed below. In addition to the above analysis, photovoltaic system designers and installers can use energy data and operational efficiencies as presented in Figs. 9 and 10, and Table 2 to their advantage. Fig. 9 shows the average monthly energy per unit area per day of the three modules used in the discussion above. Also shown in the figure, for comparison, is the average monthly irradiation per day. The figure gives a good visual indication of the normalized energy production of the modules. The data of Fig. 9 may be used to determine operational efficiencies. The monthly operational efficiencies for the three modules are shown in Fig. 10, enabling the monitoring of any degradation. From the figure, it is clear that mod1_c-Si and mod5_mc-Si do not degrade during the monitoring period, while mod7_EFG-Si degrades by more than 8% during this time. The degradation may be ascribed to the moisture ingress observed during visual inspection of the module [8]. An overall comparison of the modules’ performances is possible by comparing the average operational efficiencies of the modules during the monitoring period.
Fig. 9. Average monthly energy per day of mod1_c-Si, mod5_mc-Si and Mod7_EFG-Si.
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Fig. 10. Monthly operational efficiencies of all mod1_c-Si, mod5_mc-Si and Mod7_EFG-Si.
These operational efficiencies are listed in Table 2. As expected, mod7_EFG-Si and mod1_c-Si performed well compared to other modules, with operational efficiencies of 10.2% and 9.9%, respectively. The operational efficiencies are, however, well below than those measured indoors as listed in Table 1. This is due to environmental factors such as elevated temperatures, changing irradiation level, and varying spectral conditions. This shows that STC at which modules are usually specified are hardly, if ever realized outdoors [14]. As discussed above, operational efficiencies calculated for the entire monitoring period give an overall indication of the performance during monitoring and do not give any indication of degradation as Table 2 Average operational efficiencies of the seven modules for the period between October 2001 and November 2002 Module
Operational efficiency (%)
Mod1_c-Si Mod2_mc-Si Mod3_c-Si Mod4_c-Si Mod5_mc-Si Mod6_mc-Si Mod7_EFG-Si
9.9 9.5 7.6 9.2 7.5 8.7 10.2
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observed for mod7_EFG-Si. In this case, the module was, on average, the best performer with the highest overall operational efficiency, but showed signs of degradation. It is, therefore, essential that performance be monitored and that performance data are thoroughly analyzed as monitoring performance parameters is the best way to determine overall module output performance and to detect if there is any degradation. 5. Summary and conclusion The main objective of this study was to demonstrate the value of the low-cost I–V sequencer. The design and implementation of the low-cost I–V sequencer system capable of monitoring up to seven PV modules was presented and discussed. The system was used to monitor the I–V characteristics of PV modules under realistic outdoor conditions between October 2001 and November 2002. The data obtained showed the value of monitoring PV modules as the performance of a particular cell technology type could be tracked over time. The monitoring also yielded useful results on how each PV module performed over an extended period of time with regard to energy generating capability and operational efficiencies. Although secondary to this study, degradation of PV modules can also be determined using this system. With regard the modules monitored, we were able to use the data obtained to rank the modules in terms of how they performed during the monitoring period. The energy produced by the modules showed great variation from day to day due to highly variable meteorological conditions at the site. Even though mod7_EFGSi showed signs of degradation, it was the best performer with overall outdoor operational efficiency of 10.2%. It was followed by mod1_c-Si with 9.9% operational efficiency. This paper successfully emphasizes the importance of outdoor performance monitoring as there was a significant difference between indoor and outdoor measurements. Data obtained in this study can be used by system designers and consumers to correctly design and use photovoltaic systems. Acknowledgements The authors wish to thank the South African National Research Foundation, the South African Department of Trade and Industry, and Eskom for their financial support. References [1] del Cueto JA. Comparison of energy production and performance from flat plate photovoltaic module technologies deployed at fixed tilt. Proceedings of the 29th IEEE Photovoltaic Specialists Conference. 2002, p. 1523–6. [2] Meyer EL, van Dyk EE. Degradation analysis of silicon photovoltaic modules. Proceedings of the 16th European Photovoltaic Solar Energy Conference. 2000, p. 2272–5.
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