RATING OF ANNUAL ENERGY YIELD MORE SENSITIVE TO REFERENCE POWER THAN MODULE TECHNOLOGY

Bastian ZinBer1", George Makrides2, Markus B. Schubert1, George E. Georghiou2, and JOrgen H. Werner1

1 Institut fOr Physikalische Elektronik, Universit�t Stuttgart Pfaffenwaldring 47, 70569 Stuttgart, Germany, www.ipe.uni-stuttgart.de/pvsystem

2 Department of Electrical and Computer Engineering, University of Cyprus 75 Kallipoleos Avenue, P.O. Box 20537 Nicosia, 1678, Cyprus, www.pvtechnology.ucy.ac.cy

ABSTRACT

At present, many different photovoltaic (PV) technologies share the market. Especially investors want to know how much energy each of the PV technologies produces. This paper discusses the measured annual energy yield EAc of twelve PV technologies under different climatic conditions in Germany and Cyprus over three years of operation. In order to compare the annual yield of different PV technologies, the EAC data are normalized to the rated power PN, to the flasher power Pflash, and to the measured field power Pfield. An error analysis is done for both, the energy measurement EAC and the nominal power PSTC. It is found that the typical uncertainty for an energy yield comparison is ±5 %. This means that a difference of 10 % in the annual energy yield between PV technologies can not be traced back to the technologies themselves. The performance analysis of all PV systems shows that the differences in the energy yield are smaller than the error bars on the reference power. Therefore it is not yet possible to decide which PV technology is the best. Moreover, despite obvious trends on the data, we can not unambiguously conclude that PV modules with a better temperature or low light behavior will ensure a higher energy yield in general, since the propagation of state-of­the-art nominal power rating errors outbalances the well recognized effects of low light and temperature dependencies.

THE PHOTOVOLTAIC SYSTEMS

Since June 2006, the Institut fOr Physikalische Elektronik (ipe) of the Universit�t Stuttgart operates twelve fixed mounted photovoltaic (PV) systems of different technologies (mono- and multicrystalline silicon (sc-Si, mc­Si) and thin film technologies like amorphous silicon (a-Si),

Figure 1 PV systems on top of the university building in Stuttgart (left), and at the university campus in Nicosia (right).

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Table 1 PV technologies examined in this paper.

manufacturer module type technology monocrystalline Si Atersa A·170M 24V monocrystalline silicon

BP Saturn BP7185S monocrystalline silicon

(saturn·cell)

Sanyo HIT HIP-205NHE1 monocrystalline silicon

(HIT-cell)

SunPower STM 200 FW Sun power monocrystalline silicon

(back contact-cell)

multicrystaliine Si Schott MAIN ASE-165-GT-FT/MC 170

multi crystalline silicon

(MAIN-cell)

Schott EFG ASE-260-DG-FT 250 multicrystalline

(EFG silicon)

SolarWor1d SW165 poly multi crystalline silicon

(alkaline texture)

Solon P220/6+ multicrystalline silicon

(acid texture)

thintilm Mitsubishi a-Si(1) MA100T2

amorphous silicon

(single-cell)

Schott a-Si(2) ASIOPAK-30-SG amorphous silicon

(tandem-cell)

First Solar CdTe FS60 cadmium telluride

(CdTe)

WurthCIGS WS 11007175 cooper-indium-gallium-diselenide

(CIGS)

CdTe, and Cu(lnGa)Se2) in Stuttgart, Germany, and in Nicosia, Cyprus. Figure 1 shows photographs of the PV systems. Table 1 lists the PV technologies and manu­facturers. Each system has a power of approximately PsTc = 1 kWp at the direct current (DC) side and is equipped with the same type of grid connected inverter to exclude the effects of different maximum power point (MPP) tracking methods and inverter efficiencies on the energy yield. Furthermore, similar system voltages ensure similar inverter efficiencies and 10 % oversizing of the inverters avoids power limitations. A previous paper presented details of the PV systems and the data acquisition setup for logging all relevant PV system and weather data with high temporal resolution [1).

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UNCERTAINTY OF MEASUREMENTS AND NORMALIZATION

Energy Meter and Inverter

The annual energy yield of a PV system is measured by an alternating current (AC) meter with an error of 1 %. An additional error of 1 % must be considered, for differences caused by the inverters (efficiency and maximum power point tracking accuracy [2]).

Normalization

The biggest part of the total error, however, is propagated by the error of the nominal power PSTC which is used to normalize the measured annual energy yield Emeter to the nominal annual energy yield

E = Emeter[kWh] = [kWh / kW ] AC P. [kW] P STC P

(1)

to a PV system with a size of one kilowatt peak (1 kWp). The normalization is necessary to compare different sys­tem sizes and PV technologies with different efficiencies.

Rated Power PN

The rated power PN given on the data sheet has a typical tolerance between -0/+2.5 % up to ±10 %. Today ±3 % is a typical value for crystalline PV modules, and ±5 % for thin film modules. Due to the spreading of power during module production, sorting is necessary to avoid power mismatch at array wiring. The manufacturer measures the power of each module after production with a flasher and sorts the modules into power classes. For example into 200,205,210,215,220, and 225 Wp with ±3 % tolerance. The error of measuring the power with the flasher adds to the tolerance. The rated power normalization is most inter­esting for investors, as they pay for the rated power.

Flasher Power Pflash

Round robin tests of several European test laboratories in the framework of the PERFORMANCE project showed that the tolerance of the power Pflash of a flasher measurement is about -1,5/+2.6 % for crystalline silicon modules [3]. For thin film modules ±6 % were a typical value in the past [4]. The PERFORMANCE project improved this to ±3 % in the last round robin test [5]. However, as the flasher data of the investigated modules is about five years old, this work uses an error of ±6 %. Furthermore, it is assumed that the flashers which manufacturers use to check their modules at the end of fabrication are not more accurate than the ones in the certification laboratories. Using the flasher power Pflash gives much more accurate values than the rated power PN, because it does not include the additional tolerance from module sorting. Therefore, the normalization to the

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manufacturer flasher data gives a more realistic view on the technological differences of the annual energy yield.

Field Measured Power Pfleld

The ipe has determined the power Pfield of the solar modules from real performance data during outdoor field operation [6]. To that end, the data of the complete year 2007 was filtered for geometrical AM1.5 spectrum and G = 700-900 W/m2 irradiance conditions. The next step was a linear extrapolation of the DC power to G = 1000 W/m2

irradiation plotted against the module temperature. A linear fit was done to the scattered data. Reading at a module temperature of 25 ·C, finally delivered the STC power Pfield. The uncertainties of the equipment to mea­sure irradiance, temperature and DC power result in an error of ±2.6 %. An additional error could occur due to spectral mismatch as geometrical AM 1.5 is not exactly equal to standardized AM1.5. The advantage of the measured field power is that it takes into account initial degradation of the PV module performance, which is especially important for the amorphous silicon thin film technologies.

Irradiation measurement

A ventilated pyranometer (type CM21) measures the solar irradiation G in W/m2 in the inclined plane of the PV array. Integration of the positive values of G over one year gives the irradiated energy H in kWh/m2. The manufacturer Kipp&zonen gives an uncertainty of ±2 % for the daily sum of energy. If comparing two locations, this results in an error of ±4 %, caused by irradiation measurements. If one compares the annual values of different PV systems at only one location within the same year, the error of the pyranometer does not matter, as the irradiation is the same for all PV systems. But if you compare one year with another year, e.g. for degradation, you have to consider the long time drift error of the pyranometer which is given with 0.5 %/year.

Total Uncertainty

If we assume an error of ±3 % in STC power measure­ment, and ±2 % for the energy determination, there could be a difference of 10 % between the annual yield of two PV systems at the same location. Hence, one can not say which of those systems is the better one. For thin film technologies, the error is even bigger due to nominal power deviations. The worst case would be comparing two thin film technologies on the basis of rated power at two different locations. In detail the tolerance (±10 %), plus flasher measurement error (±6 %), plus energy measure­ment error (±2 %), plus irradiance measurement error (±2 %) sum up to a possible total difference of 40 %.

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ANNUAL ENERGY YIELD

Figure 3a shows that the annual energy yield EAC,N of the different PV technologies normalized to the rated power PN given by the data sheet exhibits differences up to 18 % in Stuttgart and 13 % in Nicosia, respectively (referring to the average at each location). The error bars are in between ±3.5 and ±18 %. Therefore, the yield is the same for all PV technologies within the error margin of the data sheet. Figure 3b uses the flasher power Pflash for normalization and shows that the differences of the annual energy yield EAC,flash go down to 12 % in Stuttgart, and 9 % in Nicosia, respectively. As the sorting tolerance is eliminated, the error bars are much smaller. They are in between ±3.5 and ±8 %.

2000

1800

(a) normalized to rated power PH

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Figure 3 (a) The annual energy yield EAC normalized to rated power PN for all PV technologies is the same within the error bars. (b) Smaller error bars, using the flasher power Pflash for normalization, do not help to see significant differences. (c) Normalization to the field measured power Pfleld shows that amorphous silicon (a-Si) modules show higher yield at hot Nicosia, due to lower nominal power.

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Table 2 Annual energy yield and solar irradiation in the plane of array over three years of operation.

Stuttgart Nicoisa s:istem 2007 2008 2009 2007 2008 2009 monocrystalline Si Atersa E,c.",h IkWh/kWpJ 1072 1044 1036 1543 1610 1521

BP Saturn E,c.",h IkWh/kWpJ 1069 1034 1020

Sanyo HIT E,c.",h IkWh/kWpJ 1069 1018 999 1565 1610 1512

SunPower E,c.",h IkWh/kWpJ 1118 1092 1081 1596 1636 1565

average (sc-Si) EAC,fIash IkWh/kWp] 1082 1047 1034 1568 1618 1532 multicrystalline Si Schott MAIN E,c.flo,h IkWh/kWpJ 1086 1057 1046 1557 1615 1523

Schott EFG EAC,lIash IkWh/kWp] 1057 1029 1021 1543 1602 1512

SolarWorld E,c.flo,h IkWh/kWpJ 1114 1080 1055 1559 1608 1504

Solon E,c.flo,h IkWh/kWpJ 1085 1064 1045 1504 1535 1439

average (me-Si) E,c.flo,h IkWh/kWpJ 1086 1057 1042 1541 1590 1495 thin film Mitsubishi a-Si(l) E,c.flo,h IkWh/kWpJ 1001 935 926 1526 1559 1457

Schott a-Si(2) E,c.flo,h IkWh/kWpJ 1103 1029 1016 1450 1491 1403

First Solar CdT. E,c.",h IkWh/kWpJ 1073 1025 1009 1511 1521 1417

WiirthCtGS EAC,fIash IkWh/kWe:] 1131 1079 1048 1570 1596 1487 avera�e (thin film� EAC,fIash IkWh/kWe:) 1077 1017 1000 1514 1542 1441 irradiation H IkWh/m'l 1354 1309 1310 1968 2054 2000

Figure 3c shows the energy yield EAc,field normalized to the field power Pfield. There are differences of 9 % in Stuttgart, and 15 % in Nicosia, respectively. The error bars are ±4.6 %. Especially the amorphous silicon thin film modules show a significant higher energy yield, due to less power Pfield than rated power PN• The yield of the crystalline silicon modules is almost the same (3 %) in Stuttgart but in Nicosia the difference is still 7 %.

ANNUAL ENERGY YIELD OVER THREE YEARS

Table 2 shows the annual energy yield EAC, flash for all PV systems over the three years 2007 to 2009. The last row shows, that the irradiated amount of solar energy H was varying over the years. To analyze the degradation normalization to the irradiated solar energy H has to be done. For that purpose, we can use the performance ratio (PR).

PERFORMANCE RATIO

The ratio of the normalized annual energy yield EAC and the solar irradiation H on the area A' of a 1 kWp system with the module efficiency 11 results in the performance ratio

PR [%] = EAC = EAC [kWh I kWp] H A'1J H[kWhlm2] (2)

which benchmarks the losses of the whole system. Main losses are inverter losses, temperature losses, low light behavior, spectral and electrical module mismatch, and cable losses. An analysis of the electrical mismatch losses showed, that they range between 0.05 and 1.7 % with an average of 0.12 % [7]. As inverter and cable losses are similar for all systems, the difference in performance ratio mainly shows the behavior of the different PV technologies due to their different temperature, low light, and spectral

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behavior. Woth noting, that the value of the PR is independent of the irradiated energy H.

Figure 4 shows that for all PV systems the PR is between 70 and 84 %. In Nicosia the PR is lower, because of the higher temperatures. The PR for c-Si technologies is within the pyranometer degradation error triangles. The thin film technologies show a higher degradation during the first three years of operation. Since the systems were operated seven months before the year 2007, most of the initial degradation (especially for a-Si) should not show up in the data discussed here. Because the absolute error bars are much bigger than the differences in the PR, however, the PR of all systems must be considered similar.

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Figure 4 The performance ratio PRflash (normalized to the nominal power Pflash) of the PV systems goes down over the first three years in most cases. Especially the thin film technologies show a higher degradation. In Nicosia the loss in PR is higher than in Stuttgart. The orange error triangles show the error range given by the long term degradation of the irradiation measured by the pyranometer. However, the absolute error bars are much bigger than the observed differences in the PRo

LOW LIGHT BEHAVIOUR

Figure 5 shows that the efficiency of most thin film technologies measured at geometrical AM 1.5 conditions increases at low light levels. Combining the better low light behavior with the solar irradiance distribution of one year will give the advantage for these technologies. Figure 6 shows the European [8] and Californian [9] irradiance distribution which is well known for the efficiency specifica­tion of inverters. Taking into account these distributions, the annual energy yield will be about 3 % higher for the a-Si and CdTe thin film technologies compared to the c-Si technologies. More detailed investigations of the low light performance are published in [6].

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TEMPERATURE BEHAVIOR

Figure 7 shows that thin film technologies have a more favorable temperature coefficient than c-Si technologies. Typically the thin film temperature coefficient is about 0.2 %/K better. A time step simulation of the annual energy yield calculates an advantage of 1.4 % in energy yield for the thin film technologies in Stuttgart, and of 4 % in the hotter Nicosia, respectively. More detailed investigations of the temperature behavior are published in [6].

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CONCLUSION

Most PV technologies show little differences in annual energy yield EAc. Only thin film technologies or non­standard c-Si technologies like HIT show larger differences. The reasons are different temperature, low light and spectral dependences. But concerning the annual energy yield, differences caused by these effects are outbalanced by a significant uncertainty in determining the nominal STC power. Therefore it is very important to consider (and improve!) the error of STC power measurements for comparing the performance of different PV technologies.

ACKNOWLEDGEMENT

The authors wish to thank the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) for supporting this work under contract No. 0327553 and the Cyprus Research Promotion Foundation for support under grant number TEXNO 0506/16). We also gratefully acknowledge the support by the companies Atersa, First Solar GmbH, Phonix Sonnenstrom AG, Q­cells AG, Schott Solar GmbH, SMA Technologie AG, SolarWorld AG, Solon AG and WOrth Solar GmbH & Co KG.

LIVE DATA

You can find live operating data of the PV-systems at: www.ipe.uni-stuttgart.de/pvsystem or www.pvtechnology.ucy.ac.cy

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REFERENCES

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Energy Conference, Milan, Italy, 2007, pp. 3114-3117.

[2] H. Neuenstein, "Chancenloser Veteran," in Photon

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[3] W. Herrmann, S. Mau, F. Fabero, T. Betts, N. van der Borg, K. Kiefer, G. Friesen, and W. Zaaiman, "Advanced Intercomparison Testing of PV Modules in

European Test Laboratories, " in Proc. of the 22nd

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[4] w. Herrmann, S. Zamini, F. Fabero, T. Betts, N. van der Borg, K. Kiefer, G. Friesen, and W. Zaaiman, "Results of the European Performance Project on the

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Photovoltaic Solar Energy Conference, Valencia, Spain, 2008, pp. 2719-2722.

[5] W. Herrmann, "Results of the European PERFORMANCE project - Quality assurance for output power measurement on PV modules, " at Thin­

Film Industry Forum, Berlin, Germany, 2010, p. 27.

[6] B. ZinBer, G. Makrides, M. Schubert, G. E. Georghiou und J. H. Werner, "Temperature and Irradiance Effects on Outdoor Field Performance," in Proc. 24th European Photovoltaic Solar Energy Conference, Hamburg, Germany, 2009, pp. 4083-4086.

[7] M. Gratzke, "Konzeption und Realisierung eines Berechnungs- & Optimierungstools fOr das Fehlanpassungsverhalten bei Photovoltaikanlagen, " diploma thesis, ipe, Universit�t Stuttgart, Germany 2007.

[8] J. Nickel, "Auf den Spuren von »Euro-Eta«," in Photon

Oas Solar strom Magazin, Issue 6, pp. 62-95, Jun. 2004.

[9] W. Bower, C. Whitaker, W. Erdman, M. Behnke, and M. Fitzgerald, "Performance Test Protocol for Evaluating Inverters Used in Grid-Connected Photovoltaic Systems, "SANDIA, Albuquerque, NM, USA, 2004, p. 20.

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