of Cyprus, Nicosia, Cyprus b Institut für Photovoltaik, Universität Stuttgart, Stuttgart, Germany Published online: 25 Jan 2013.

This article was downloaded by: [University of Cyprus] On: 05 August 2015, At: 10:38 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: 5 Howick Place, London, SW1P 1WG International Journal of Sustainable Energy Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/gsol20 Seasonal performance comparison of different photovoltaic technologies installed in Cyprus and Germany G. Makrides a, B. Zinsser b, M. Schubert b & G. E. Georghiou a a Department of Electrical and Computer Engineering, University of Cyprus, Nicosia, Cyprus b Institut für Photovoltaik, Universität Stuttgart, Stuttgart, Germany Published online: 25 Jan 2013. To cite this article: G. Makrides, B. Zinsser, M. Schubert & G. E. Georghiou (2013) Seasonal performance comparison of different photovoltaic technologies installed in Cyprus and Germany, International Journal of Sustainable Energy, 32:5, 466-488, DOI: 10.1080/14786451.2012.759572 To link to this article: http://dx.doi.org/10.1080/14786451.2012.759572 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &

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International Journal of Sustainable Energy, 2013 Vol. 32, No. 5, 466 488, http://dx.doi.org/10.1080/14786451.2012.759572 Seasonal performance comparison of different photovoltaic technologies installed in Cyprus and Germany G. Makrides a *, B. Zinsser b, M. Schubert b and G.E. Georghiou a Downloaded by [University of Cyprus] at 10:38 05 August 2015 a Department of Electrical and Computer Engineering, University of Cyprus, Nicosia, Cyprus; b Institut für Photovoltaik, Universität Stuttgart, Stuttgart, Germany (Received 9 February 2012; final version received 12 December 2012) In this work, the seasonal performance of different grid-connected photovoltaic technologies installed both in the warm climate of Nicosia, Cyprus, and in the moderate climate of Stuttgart, Germany, was evaluated. The technologies presented include mono-crystalline-silicon, multi-crystalline-silicon and thin-film amorphous-silicon, cadmium-telluride and copper-indium gallium-diselenide technologies. The crystalline-silicon technologies exhibited maximum performance ratio (PR) peaks during the winter and minimum peaks during the warmer summer seasons in Nicosia. This signifies that the performance of these technologies installed in the warm climate is strongly dependent on temperature. The identical type crystalline-silicon technologies in Stuttgart exhibited a similar behaviour as in Nicosia with the exception of rapid decreases in performance due to snow in the winter. In contrast, the amorphous-silicon thin-film technologies exhibited a different seasonal performance compared with the remaining installed technologies, as their operating PR was higher during the summer and autumn months and lower during the winter. Keywords: crystalline silicon; energy; performance ratio; photovoltaic; seasonal performance; thin film 1. Introduction In this work, the seasonal performance of 12 different grid-connected photovoltaic (PV) technologies installed both in Nicosia, Cyprus, and in Stuttgart, Germany, was investigated by the acquisition of real PV operating and climatic data. Even though seasonal performance evaluations have already been presented in the literature (Marion and Atmaran 1990; King, Kratochvil, and Boyson 2000; Itoh et al. 2001; Cueto 2002; Carr and Pryor 2004; Addelstein and Sekulic 2005; Nakada et al. 2009; Makrides et al. 2010b; Nikolaeva-Dimitrova et al. 2010; Perraki and Georgitsas 2010), the work outlined here is one of the first to investigate the outdoor performance behaviour of different technologies, ranging from mono-crystalline-silicon (mono-c-si) and multi-crystalline-silicon (multi-c-si) to amorphous-silicon (a-si), cadmium-telluride (CdTe) and copper-indium gallium-diselenide (CIGS) technologies, installed side by side in two different locations. The test sites in Nicosia and Stuttgart have been operating since June 2006 and the locations selected represent the climatic exposure to the warm climatic conditions of Cyprus, typical of the Mediterranean region, and also the moderate climatic conditions of Germany, typical of central Europe. *Corresponding author. Email: eep5mg1@ucy.ac.cy 2013 Taylor & Francis

International Journal of Sustainable Energy 467 The seasonal fluctuations exhibited by each PV technology were obtained from the monthly average alternating current (AC) performance ratio (PR), PR AC,-constructed time series over the period June 2006 June 2009 for both locations. In addition, the AC PR peak-to-peak variations of each year were also calculated, and the results obtained demonstrated that each technology exhibited different performance variations throughout the year at each location. Finally, a comparison of the outdoor performance of each technology based on the monthly AC energy yield normalised to the nameplate manufacturer power, E AC(Normalised), was also carried out between the two locations. The results show significant performance differences amongst the various technologies as a function of location. 2. Outdoor PV test facilities Downloaded by [University of Cyprus] at 10:38 05 August 2015 The outdoor test facilities at the University of Cyprus, Nicosia, Cyprus, and the Institut für Photovoltaik of the University of Stuttgart include, amongst others, 12 grid-connected PV systems and an advanced monitoring and measurement platform. Specifically, the same PV technologies were installed in both locations to primarily facilitate the performance evaluations and comparisons under the two different climatic conditions. The fixed-plane PV systems installed range from mono-c-si and multi-c-si, heterojunction technologies with intrinsic thin-layer (HIT), edgedefined film-fed growth (EFG), multi-crystalline advanced industrial (MAIN) cells to a-si, CdTe and CIGS and other PV technologies from a range of manufacturers such as BP Solar, Atersa, Sanyo, Solon and Suntechnics (Zinsser et al. 2007). Each system has a nominal capacity of approximately 1 kw p and is equipped with the same type of inverter installed behind each respective system in close proximity (SMA SB 1100) (Makrides et al. 2011b). The same inverters are used in order to exclude the influence of different maximum power point tracking (MPPT) methods. Additionally, all the PV systems have approximately the same system voltage in order to avoid inverter efficiency differences. The inverters are also oversized to ensure that the systems are always working at their maximum power point (MPP; Makrides et al. 2009). Table 1 provides a brief description of the installed systems in both locations. The PV systems have been continuously monitored since the beginning of June 2006 and the measurement platform includes the acquisition of both climatic and PV operational measurements. The platform comprises meteorological and electrical sensors connected to a central data-logging system that stores data at a resolution of 1 s. The measured meteorological parameters include the total irradiation in the plane of array (POA) of 27.5 and 33 in Nicosia and Stuttgart, respectively, and wind direction and speed as well as ambient and module temperatures. The electrical parameters measured include direct current (DC) current and voltage and DC and Table 1. Installed PV technologies both in Nicosia, Cyprus, and in Stuttgart, Germany. Manufacturer Module type Technology Atersa A-170M 24V Mono-c-Si BP Solar BP7185S Mono-c-Si (Saturn cell) Sanyo HIP-205NHE1 Mono-c-Si (HIT cell) Suntechnics STM 200 FW Mono-c-Si (back contact cell) Schott Solar ASE-165-GT-FT/MC Multi-c-Si (MAIN cell) Schott Solar ASE-260-DG-FT Multi-c-Si (EFG) SolarWorld SW165 poly Multi-c-Si Solon P220/6+ Multi-c-Si MHI MA100T2 a-si (single cell) Schott Solar ASIOPAK-30-SG a-si (tandem cell) First Solar FS60 CdTe Würth Solar WS 11007/75 CIGS

468 G. Makrides et al. Table 2. Data acquisition equipment and sensors both in Nicosia, Cyprus, and in Stuttgart, Germany. Parameter Manufacturer Model Data acquisition Delphin Topmessage Temperature ambient Theodor Friedrich 2030 Temperature module Heraeus PT 100 Total irradiance Kipp Zonen CM 21 Direct normal irradiance Kipp Zonen CH 1 DC voltage Custom made Potential divider DC current Custom made Shunt resistor DC power Delphin Topmessage AC energy NZR AAD1D5F Wind speed Theodor Friedrich 4034 Wind direction Theodor Friedrich 4122 Downloaded by [University of Cyprus] at 10:38 05 August 2015 AC power at MPP as obtained at each PV system output. All the installed sensors and data-logging devices are listed in Table 2 (Makrides et al. 2010a). The measurement conditions and main sources of uncertainty in the outdoor PV performance evaluations due to the instrumentation were also investigated. In both test sites, the total irradiance was measured using a thermopile pyranometer installed at the same POA as the PV modules and regularly cleaned whenever the PV systems were cleaned. The installed pyranometer operates in the spectral range of 310 2800 nm and with a ±2% expected daily uncertainty. In practice, as the expected daily uncertainty of the pyranometer is based on a particular daily profile of irradiance, solar path and ambient temperature variations of a particular location, the application of the sensors in other climatic conditions renders the uncertainty of the pyranometer a function of many variables such as directional errors in zenith and azimuth directions, cosine response, temperature sensitivity and the level of irradiance. For a secondary standard instrument, the expected maximum errors are ±2% for the daily total error, described by the World Meteorological Organization, because some response variations cancel out each other if the integration period is long. To further reduce the remaining errors, the conversion of voltage to irradiance, obtained from the calibration sheet of the instrument, is specified and can be important as a bias. Specifically, with regard to the calibration of the pyranometer installed in the POA and used in this investigation in Nicosia, Cyprus, the initial calibration value was 11.84 μv/wm 2, and after 4 years of continuous outdoor operation, the new calibration sensitivity value was found to be 11.86 μv/wm 2, which yields an absolute percentage error (APE) of 0.17% over 4 years. Furthermore, the pyranometer was also ventilated and heated to avoid incorrect measurements caused by dew and snow (Makrides et al. 2011b). Ambient temperature measurements were accurately recorded with an uncertainty of ±0.15 C at 25 C, while PT100 (class B) sensors installed on the back plate of each module provided a measurement with an uncertainty of ±0.425 Cat25 C. Module temperature measurements suffer from additional variations due to mounting, heat transfer and temperature variations, which provide additional uncertainties. Previous temperature investigations performed based on infrared images taken on the installed PV modules clearly showed that the temperature distribution in a module is uniform apart from the areas around the junction box and the main bus-bar interconnection point which tend to be at a higher temperature as expected (around 2 3 C higher) (Makrides et al. 2009). Additionally, the annualac energy yield was primarily associated with a metering measurement uncertainty of ±1% and an additional ±1% uncertainty that accounts for differences caused by the inverters (efficiency and MPPT accuracy).

International Journal of Sustainable Energy 469 It is worth noting that during the evaluation period of the systems installed in Nicosia, the Schott Solar a-si system had a broken module since October 2006, while the performance of the BP Solar mono-c-si and Solon multi-c-si systems had been affected by partial shading during the second and third years. Accordingly, the Schott Solar a-si system in Stuttgart had a broken module since May 2008. 3. Methodology Downloaded by [University of Cyprus] at 10:38 05 August 2015 The seasonal performance patterns of the different PV technologies were determined by employing the PR, which is one of the most commonly used measures of PV performance evaluations. In addition, the energy yield of each system normalised to the nameplate manufacturer power was also recorded and compared over a certain period. For the analysis of this investigation, the first step was to construct the AC energy yield and AC PR time series on a monthly basis for each technology and in both locations. Specifically, the normalised AC energy yield, E AC(Normalised), is defined as the total AC energy yield produced in a given time period, E AC, further normalised to the nameplate manufacturer power, P 0, and is given by E AC(Normalised) = E AC. (1) P 0 Accordingly, the AC PR, PR AC, is calculated and used as a performance measure that enables the comparison of modules of different efficiencies, by normalising the energy produced under actual operating conditions to the maximum power at standard test conditions (STC) and the incident solar radiation, and is given by (Carr and Pryor 2004) PR AC = E AC G P 0 H, (2) where H is the total irradiation in the POA and G is the total irradiance at STC (G = 1000 W/m 2 ). For the analysis in this study, the normalising factor P 0 was the rated power as provided by the manufacturer s datasheets. Why the normalisation step was performed using the rated power provided by the manufacturer s datasheets and not using the power measured using a flasher or from field measurements was because this is the power readily known and which investors pay for. 4. Results 4.1. Environmental conditions The monthly total irradiation in the POA for the test sites in Nicosia and Stuttgart was measured over the 3-year period using the pyranometer installed in the POA at each site and is shown in Figure 1(a) and (b), respectively. It is evident from Figure 1 that the irradiation of both locations exhibits a seasonal pattern with the highest irradiation occurring during the summer and lowest during the winter. In addition, the measured annual total irradiation in the POA at the test site in Nicosia was approximately 33% higher than the one in Stuttgart. The monthly total irradiation plots in Stuttgart demonstrated more oscillations than the ones in Nicosia, and this is attributed to the overcast weather in the climatic conditions of Stuttgart. The solar irradiation in the POA of 27.5 in Nicosia averages over 5.5 peak sun hours (PSH) each day, varying from 3.9 PSH during the winter to 6.9 PSH during the summer season. Accordingly, in the POA of 33 in Stuttgart, the

470 G. Makrides et al. Figure 1. Monthly total irradiation in the POA over the period June 2006 June 2009 in (a) Nicosia, Cyprus, and (b) Stuttgart, Germany. average daily solar irradiation is over 3.7 PSH, varying from 1.7 PSH during the winter to 5.2 PSH during the summer. On an annual basis, the measured total irradiation in the POA over the period June 2006 June 2009 in Nicosia and Stuttgart is summarised in Table 3. In Nicosia, the period of highest annual solar irradiation was the second year of investigation, June 2007 June 2008, while in Stuttgart this was the first year, June 2006 June 2007. In addition, the monthly average ambient temperatures of both sites are also shown in Figure 2. From the plot in Figure 2(a), it is evident that the summer was the warmest season in Nicosia, while the lowest monthly average ambient temperatures were observed during the winter. The monthly average ambient temperatures ranged between 8 C and 30 C. Similarly, at the test site

International Journal of Sustainable Energy 471 Table 3. Annual total irradiation in the POA over the period June 2006 June 2009 measured at the test sites in Nicosia, Cyprus, and Stuttgart, Germany. Annual total irradiation Annual total irradiation Period POA in Nicosia (kwh/m 2 ) POA in Stuttgart (kwh/m 2 ) June 2006 June 2007 1988 1446 June 2007 June 2008 2054 1315 June 2008 June 2009 1997 1272 Average of 3 years 2013 1344 Figure 2. Monthly average ambient temperature over the period June 2006 June 2009 in (a) Nicosia, Cyprus, and (b) Stuttgart, Germany.

472 G. Makrides et al. Figure 3. Monthly average PR AC over the period June 2006 June 2009 for the different PV technologies in Nicosia, Cyprus. (a) Solon multi-c-si. (b) Sanyo HIT mono-c-si. (c) Atersa mono-c-si. (d) Suntechnics mono-c-si. (e) Schott Solar EFG-Si. (f) BP Solar mono-c-si. (g) SolarWorld multi-c-si. (h) Schott Solar MAIN-Si. (i) Würth Solar CIGS. (j) First Solar CdTe. (k) MHI a-si. (l) Schott Solar a-si. in Stuttgart, the highest temperatures were observed during the summer as shown in Figure 2(b), although, in general, lower temperatures were measured throughout the year compared with the test site in Cyprus as in Stuttgart the monthly average ambient temperatures were in the range of 2 C and 23 C. 4.2. Seasonal behaviour evaluation using outdoor measurements in Nicosia, Cyprus The monthly average PR AC of each PV technology installed in Nicosia was first investigated and is shown in Figure 3. Specifically, the seasonal behaviour in the performance of the c-si

International Journal of Sustainable Energy 473 Figure 3. Continued. technologies was evident from the monthly average PR AC peaks during the cold winter season compared with the warmer summer months presented in Figure 3(a) (h). The obvious PR AC improvements during the cooler months compared with the warmer months are attributed to the lower ambient temperatures and hence the lower module temperatures. This is why the monthly average PR AC plots of the c-si technologies, as depicted in Figure 3(a) (h), follow the opposite behaviour to the one exhibited by the monthly irradiation and ambient temperature time series in Nicosia shown in Figures 1(a) and 2(a), respectively. Essentially, this outcome further indicates that the seasonal PR AC behaviour of the c-si technologies exposed to the warm conditions of Nicosia is mainly affected by the thermal effects. In addition, the 3-year average PR AC of the c-si technologies was in the range of 70 84%, demonstrating wide variations in performance amongst

474 G. Makrides et al. Downloaded by [University of Cyprus] at 10:38 05 August 2015 the installed technologies. A significant reduction in the monthly average PR AC can be observed in Figure 3(f) for the BP Solar mono-c-si system particularly during the second and third years because of shading. In the course of a year, the Würth Solar CIGS system exhibited the same seasonal PR AC pattern as the c-si technologies, with peaks during the winter and performance decreases for the remaining warmer seasons as shown in Figure 3(i). Additionally, Figure 3(j) shows that the CdTe system exhibited a narrower peak-to-peak PR AC variation between the seasons, compared with the c-si and CIGS systems. This implies that the CdTe technology is not strongly affected by the prevailing climatic variations that exist in Nicosia. Moreover, the CIGS and CdTe systems exhibited high 3-year average PR AC of 82% and 78%, respectively, even though a decreasing tendency in the monthly average PR AC values of the third year was observed for both systems. Furthermore, the seasonal performance of the Mitsubishi Heavy Industries (MHI) and Schott Solar a-si technologies is presented in Figure 3(k) and (l), respectively. Both these technologies, more evidently the MHI system, demonstrated performance fluctuations with higher monthly average PR AC values during the summer and early autumn compared with the colder winter season. The higher PR AC values of the a-si technologies during warm periods up to the early autumn months are mainly attributed to thermal annealing and the more favourable spectrum particularly during the warm summer season compared with the less favourable spectrum during the cold winter months (Strand et al. 1996; Makrides et al. 2011a). During the first month of operation, June 2006, the average monthly PR AC of the MHI and Schott Solar a-si systems was 83% and 82%, respectively, and these were the highest values recorded for this month amongst all the installed technologies. The high initial monthly average PR AC values of the a-si technologies are attributed to the fact that these systems had not yet stabilised and subsequently showed the initial Staebler Wronski degradation (Staebler and Wronski 1977). Consequently, because of the initial stabilisation phase, the monthly average PR AC values of the MHI and Schott Solar a-si systems in June 2006 were 6% and 9% higher compared with the successive June 2007 values, respectively. The MHI and Schott Solar a-si systems demonstrated a 3-year average PR AC of 77% and 73%, respectively. A high decreasing tendency in the monthly average PR AC values was also observed for the MHI a-si system during the third year, while the Schott Solar a-si system had a broken module since October 2006, and this is a possible reason for the low 3-year average PR AC of this system. Table 4 presents the annualac PR peak-peak variations obtained by subtracting the minimum from the maximum monthly average PR AC values of each year and for each installed PV technology. In most of the cases, the c-si PV technologies showed the highest AC PR peak-peak variations amongst the installed technologies, with the Atersa mono-c-si and SolarWorld multi-c-si technologies exhibiting the highest obtained values of 16.0% and 15.4%, respectively. Accordingly, the highest AC PR peak-peak values were obtained for the c-si technologies during the second year, June 2007 June 2008. The thin-film technologies demonstrated lower AC PR peak-peak variations compared with the c-si technologies, with the exception of the Schott Solar a-si system during the first year. The high first-year AC PR peak-peak value of 13.4% is because of the unusually high June 2006 monthly average PR AC value of the Schott Solar a-si system due to the initial stabilisation phase. Conversely, the lowest AC PR peak-peak amongst all the installed technologies, of 4.3%, was exhibited by the First Solar CdTe system, signifying that this technology is not strongly affected by the seasonal climatic variations that exist in Nicosia. 4.3. Seasonal behaviour evaluation using outdoor measurements in Stuttgart, Germany The seasonal performance investigation was carried out in the same way as in Nicosia for the systems installed in Stuttgart and the monthly average PR AC plots of the different PV technologies

International Journal of Sustainable Energy 475 Table 4. Annual AC PR peak-peak variations of PV technologies over the period June 2006 June 2009 in Nicosia, Cyprus. 2006 2007 2007 2008 2008 2009 System AC PR peak-peak (%) AC PR peak-peak (%) AC PR peak-peak (%) Downloaded by [University of Cyprus] at 10:38 05 August 2015 Atersa (A-170M 24V) 11.4 16.0 12.1 BP Solar (BP7185S) 10.5 10.7 9.0 Sanyo (HIP-205NHE1) 9.1 11.9 9.4 Suntechnics (STM 200 FW) 12.4 15.1 11.7 Mono-c-Si average 10.9 13.4 10.6 Schott Solar (ASE-165-GT-FT/MC) 10.7 13.3 9.0 Schott Solar (ASE-260-DG-FT) 10.2 12.6 8.6 SolarWorld (SW165) 11.2 15.4 11.0 Solon (P220/6+) 8.0 9.4 9.7 Multi-c-Si average 10.0 12.7 9.6 MHI (MA100T2) 8.4 8.5 9.1 Schott Solar (ASIOPAK-30-SG) 13.4 7.1 6.5 Würth Solar (WS 11007/75) 8.5 9.4 6.7 First Solar (FS60) 4.3 7.8 4.3 Thin-film average 8.7 8.2 6.6 The average annual AC PR peak-peak is also given for all the mono-c-si, multi-c-si and thin-film technologies. are presented in Figure 4. The first significant observation extracted from the plots in Figure 4 is that the repetitive seasonal performance pattern exhibited by the installed technologies was obscured by the overcast weather and snow coverage in Stuttgart. As a result, performance oscillations and decreases in the monthly average PR AC plots of the different PV technologies were apparent. Both mono-c-si and multi-c-si technologies exhibited PR AC peaks during the cold winter season and performance decrease during the warmer summer months as depicted in Figure 4(a) (h). However, in all plots, a rapid decrease in performance was observed during winter, particularly during January 2007 and December 2008, for all technologies because of snow cover on the systems. Accordingly, during the evaluation period, the mono-c-si and multi-c-si systems exhibited a 3-year average PR AC in the range of 80 87% and 79 83%, respectively. Furthermore, Figure 4(i) and (j) demonstrates that both the CIGS and CdTe systems exhibited seasonal performance similar to that of the c-si system although the CdTe system exhibited narrower variations in performance. It is also important to note that the First Solar CdTe system showed the lowest performance drop during January 2007 (a month of snow coverage) due to the fact that the modules have no frame and thus snow is allowed to slide more easily and not accumulated on the surface of the modules. Snow accumulation on the pyranometer in Stuttgart was prevented by heating the sensor. The CIGS system demonstrated a 3-year average PR AC of 87%, and alongside with the Suntechnics mono-c-si system provided the highest 3-year average PR AC compared with the other installed technologies. For the CdTe system, the 3-year average PR AC was 81%. As in the case of Nicosia, both the MHI and Schott Solar a-si technologies installed in Stuttgart provided higher monthly average PR AC values during the summer and early autumn compared with the colder winter months, depicted in Figure 4(k) and (l), respectively. Furthermore, both a-si technologies provided high average monthly PR AC during the first month of operation, June 2006, because of their initial stabilisation phase. Specifically, the average monthly PR AC during June 2006 was 85% and 95% for the MHI and Schott Solar a-si systems, respectively. In addition, the Schott Solar a-si system demonstrated a 3-year average PR AC of 80%, while the MHI a-si system exhibited the lowest 3-year average PR AC of 71% amongst all the installed technologies. Both systems showed a high decreasing tendency in the monthly average PR AC values during the third year.

476 G. Makrides et al. Figure 4. Monthly average PR AC over the period June 2006 June 2009 for the different PV technologies in Stuttgart, Germany. (a) Solon multi-c-si. (b) Sanyo HIT mono-c-si. (c) Atersa mono-c-si. (d) Suntechnics mono-c-si. (e) Schott Solar EFG-Si. (f) BP Solar mono-c-si. (g) SolarWorld multi-c-si. (h) Schott Solar MAIN-Si. (i) Würth Solar CIGS. (j) First Solar CdTe. (k) MHI a-si. (l) Schott Solar a-si. Table 5 lists the annual AC PR peak-peak variations over the evaluation period for the technologies installed in Stuttgart. During the first year of evaluation, all systems showed high AC PR peak-peak variations mainly because of the snow coverage, which yielded a low PR AC particularly during January 2007. The highest AC PR peak-peak variations during the first year were exhibited by the a-si technologies because of the low PR AC due to the snow coverage and the high monthly average PR AC, which occurred during June 2006 before these technologies had fully stabilised from initial degradation. During the subsequent years, the multi-c-si and a-si technologies showed the highest annual AC PR peak-peak variations. The lowest AC PR peak-peak variation was exhibited by the First Solar CdTe system.

International Journal of Sustainable Energy 477 Figure 4. Continued. 4.4. Performance comparison of PV technologies in Nicosia, Cyprus, and Stuttgart, Germany Figure 5 presents the annual AC energy yield normalised to the nameplate manufacturer power, E AC(Normalised), of the different PV technologies installed in both locations. In Nicosia, the Suntechnics mono-c-si system produced the highest annual E AC(Normalised) over the entire 3-year evaluation period. The 3-year average E AC(Normalised) of the systems installed in Nicosia was 1572 kwh/kw p. In Stuttgart, the technology which produced the highest energy yield during the first year was the Würth Solar CIGS, while during the second and third years this technology was outperformed by the Suntechnics mono-c-si system as shown in Figure 5(b) and (c), respectively. The 3-year average E AC(Normalised) of the systems installed in Stuttgart was 1097 kwh/kw p.

478 G. Makrides et al. Table 5. Annual AC PR peak-peak variations of PV technologies over the period June 2006 June 2009 in Stuttgart, Germany. 2006 2007 2007 2008 2008 2009 System AC PR peak-peak (%) AC PR peak-peak (%) AC PR peak-peak (%) Downloaded by [University of Cyprus] at 10:38 05 August 2015 Atersa (A-170M 24V) 22.5 6.6 13.3 BP Solar (BP7185S) 20.8 8.7 12.0 Sanyo (HIP-205NHE1) 21.3 4.9 8.5 Suntechnics (STM 200 FW) 23.2 6.7 12.3 Mono-c-Si average 21.9 6.7 11.5 Schott Solar (ASE-165-GT-FT/MC) 26.1 9.4 16.3 Schott Solar (ASE-260-DG-FT) 16.4 10.9 18.4 SolarWorld (SW165) 20.8 9.5 18.8 Solon (P220/6+) 22.4 7.0 13.7 Multi-c-Si average 21.4 9.2 16.8 MHI (MA100T2) 26.6 11.0 14.1 Schott Solar (ASIOPAK-30-SG) 27.1 11.3 17.0 Würth Solar (WS 11007/75) 23.6 7.3 11.2 First Solar (FS60) 10.3 4.5 5.0 Thin-film average 21.9 8.5 11.8 The average annual AC PR peak-peak is also given for all the mono-c-si, multi-c-si and thin-film technologies. For comparison, the monthly average PR AC of the mono-c-si technologies in both locations over the period June 2006 June 2009 are plotted and depicted in Figure 6. The PV field monthly average PR AC which was calculated from the monthly average PR AC values of all installed systems is also plotted and used as a general indicator of how well each technology performed compared with the field average. In general, as the climatic conditions in Stuttgart were not as stable as in Nicosia, mainly due to the overcast weather and snow coverage, significant variations in the performance were observed. In both locations, the best-performing technology based on the average PR AC of the 3-year evaluation period and amongst the mono-c-si technologies was the Suntechnics mono-c-si system. In addition, Figure 6 shows that most mono-c-si systems in Nicosia exhibited higher monthly average PR AC compared with the field average, whereas in Stuttgart only the Suntechnics monoc-si system clearly outperformed the field average. In addition, the mono-c-si systems in Nicosia exhibited higher and more pronounced seasonal variations, with lower performance during the summer seasons, compared with the systems in Stuttgart (without considering the periods of snow coverage in Stuttgart). The high performance variations of the mono-c-si technologies in Nicosia are due to the warm climatic conditions. The multi-c-si technologies also showed higher variations in Nicosia compared with Stuttgart. In particular, the monthly average PR AC was lower during the warmer summer periods in Nicosia compared with Stuttgart as depicted in Figure 7. During the summer months, the multi-c-si technologies in Stuttgart demonstrated higher monthly average PR AC values compared with the respective ones in Nicosia. In addition, the multi-c-si technologies demonstrated good performance agreement with the field average in both locations. Furthermore, the thin-film technologies of CIGS and CdTe showed, in general, higher monthly average PR AC values in Stuttgart compared with the respective systems installed in Nicosia as shown in Figure 8. It was more evident in Nicosia that because of the warm conditions these technologies exhibited seasonal behaviour very similar to the c-si system as in Stuttgart variations during the seasons were less obvious. In both locations, the CIGS technology exhibited higher performance than the CdTe technology and the PV field average.

International Journal of Sustainable Energy 479 Figure 5. Annual E AC(Normalised) over the period June 2006 June 2009 for the different PV technologies in Nicosia, Cyprus, and Stuttgart, Germany. Over the periods (a) June 2006 June 2007; (b) June 2007 June 2008; (c) June 2008 June 2009.

480 G. Makrides et al. Figure 6. Monthly average PR AC over the period June 2006 June 2009 for the mono-c-si PV technologies in (a) Nicosia, Cyprus, and (b) Stuttgart, Germany. a The BP Solar mono-c-si system in Nicosia was affected by partial shading during the second and third years. Finally, from the plots of the monthly average PR AC of the a-si technologies presented in Figure 9, it was obvious that systems installed in Stuttgart exhibited a higher seasonal peak-to-peak performance compared with the a-si technologies installed in Nicosia. The systems in Stuttgart also showed a higher progressive performance loss and longer initial stabilisation duration compared with the systems installed in Nicosia. The MHI a-si system, which outperformed the Schott Solar a-si system in Nicosia, showed lower performance in Stuttgart due to the climatic conditions. Figure 9 therefore clearly shows that the best-performing system in Nicosia was not the best-performing one in Stuttgart. This may be due to the fact that the Schott Solar system had a

International Journal of Sustainable Energy 481 Figure 7. Monthly average PR AC over the period June 2006 June 2009 for the multi-c-si PV technologies in (a) Nicosia, Cyprus, and (b) Stuttgart, Germany. a The Solon multi-c-si system in Nicosia was affected by partial shading during the second and third years. broken module since October 2006 in Nicosia. Furthermore, another reason for the Schott Solar a-si system outperforming the MHI a-si system in Stuttgart could be that the spectral conditions were more favourable for the tandem-junction technology rather than for the single-junction technology. In Nicosia, the Schott Solar a-si system performed worse compared with the average PV field monthly average PR AC, while in Stuttgart the field performance was higher than that of the MHI a-si system. Additionally, the MHI a-si system in Nicosia and the Schott Solar a-si system in Stuttgart occasionally exhibited higher monthly average PR AC values during the summer seasons compared with the field average.

482 G. Makrides et al. Figure 8. Monthly average PR AC over the period June 2006 June 2009 for the thin-film CIGS and CdTe PV technologies in (a) Nicosia, Cyprus, and (b) Stuttgart, Germany. The seasonal performance losses caused by high module temperatures were further investigated for each technology at both test facilities, by using the module temperature measurements and the manufacturer-provided MPP power temperature coefficients, γ PMPP. The seasonal AC energy yield loss over the period of 1 year, June 2007 June 2008, due to module temperatures over 25 C was evaluated for all technologies and calculated as the APE of the seasonal AC energy yield thermal losses compared with the seasonal AC energy yield at STC, which is the maximum AC energy produced by each system without considering performance losses. The seasonal AC energy yield thermal losses calculated for the PV systems in Nicosia, Cyprus, are summarised in Table 6. In this case, all PV technologies, especially the mono-c-si

International Journal of Sustainable Energy 483 Figure 9. Monthly average PR AC over the period June 2006 June 2009 for the thin-film a-si PV technologies in (a) Nicosia, Cyprus, and (b) Stuttgart, Germany. a The Schott Solar a-si system had a broken module since October 2006 in Nicosia, Cyprus. The Schott Solar a-si system had a broken module since 9th May 2008 in Stuttgart, Germany. and multi-c-si technologies, exhibited high thermal losses during the summer and low thermal losses during the winter, demonstrating the strong correlation between the outdoor performance and module temperature. The highest loss was exhibited during the summer by the BP Solar mono-c-si system, with an APE of 14.31%. Amongst the installed technologies, the BP Solar mono-c-si modules have the highest MPP power temperature coefficient, as provided by the manufacturer s datasheet. The seasonal thermal losses of the a-si technologies were the lowest amongst the installed PV technologies because of the low temperature coefficients of thin-film technologies.

484 G. Makrides et al. Table 6. Seasonal AC energy yield thermal losses APE, using the manufacturer s MPP power temperature coefficients over the period June 2007 June 2008 in Nicosia, Cyprus. Manufacturer Summer Autumn Winter Spring System γ PMPP (%/ C) APE (%) APE (%) APE (%) APE (%) Downloaded by [University of Cyprus] at 10:38 05 August 2015 Atersa (A-170M 24V) 0.37 9.44 6.99 2.44 5.76 BP Solar (BP7185S) 0.50 14.31 10.44 4.18 9.07 Sanyo (HIP-205NHE1) 0.30 7.98 5.89 2.13 4.99 Suntechnics (STM 200 FW) 0.38 9.92 7.19 2.41 6.1 Schott Solar (ASE-165-GT-FT/MC) 0.47 12.18 8.65 2.93 7.41 Schott Solar (ASE-260-DG-FT) 0.47 13.14 9.42 3.48 8.01 SolarWorld (SW165) 0.47 13.45 9.81 3.93 8.52 Solon (P220/6+) 0.43 12.08 8.87 3.22 7.77 MHI (MA100T2) 0.20 5.08 3.62 1.18 3.06 Schott Solar (ASIOPAK-30-SG) 0.20 5.55 4.01 1.47 3.42 Würth Solar (WS 11007/75) 0.36 13.42 10.16 4.71 9.61 First Solar (FS60) 0.25 7.38 5.56 2.22 4.94 Table 7. Seasonal AC energy yield thermal losses APE, using the manufacturer s MPP power temperature coefficients over the period June 2007 June 2008 in Stuttgart, Germany. Manufacturer Summer Autumn Winter Spring System γ PMPP (%/ C) APE (%) APE (%) APE (%) APE (%) Atersa (A-170M 24V) 0.37 4.03 2.27 0.44 2.17 BP Solar (BP7185S) 0.50 5.91 3.04 0.59 2.85 Sanyo (HIP-205NHE1) 0.30 3.22 1.81 0.34 1.69 Suntechnics (STM 200 FW) 0.38 4.1 2.25 0.44 2.25 Schott Solar (ASE-165-GT-FT/MC) 0.47 4.97 2.51 0.39 2.55 Schott Solar (ASE-260-DG-FT) 0.47 4.98 2.53 0.36 2.43 SolarWorld (SW165) 0.47 5.29 2.72 0.48 2.77 Solon (P220/6+) 0.43 5.48 2.91 0.62 2.81 MHI (MA100T2) 0.20 2.09 1.07 0.19 1.17 Schott Solar (ASIOPAK-30-SG) 0.20 2.21 1.19 0.26 1.23 Würth Solar (WS 11007/75) 0.36 4.74 2.69 0.7 2.59 First Solar (FS60) 0.25 3.05 1.74 0.44 1.69 Table 7 lists the seasonal AC energy yield thermal losses calculated for the PV systems in Stuttgart, Germany. In Stuttgart, the thermal losses were lower compared with the same systems in Nicosia due to the cooler weather. For this reason, the seasonal variations in performance in Stuttgart were smaller compared with the performance variations exhibited by the same technologies in Nicosia. During the warmer summer months, the highest thermal losses were again exhibited by the BP Solar system with APE 5.91%, whereas the thin-film technologies of a-si demonstrated the lowest thermal losses for the same season. The performance dependence of each PV technology on the irradiance level was also examined by filtering DC MPP power datasets at geometric air mass (AM) conditions between AM 1.4 and AM 1.6, over the period of 1 year. The extracted datasets were first sorted into different solar irradiance classes of 50, 100, 200, up to 1200 W/m 2 (the solar irradiance measurements obtained from the pyranometer installed at each test facility). The DC power datasets were then linearly corrected to the exact irradiance class, normalised to the manufacturers rated power and plot against the module temperature. Finally, by selecting only the corrected and normalised power datasets at a module temperature of 25 C for each of the different irradiance classes, the irradiance-level performance of each technology under outdoor field conditions was obtained (Zinsser et al. 2009).

International Journal of Sustainable Energy 485 Figure 10. et al. 2009). Irradiance-level performance of PV technologies in (a) Nicosia, Cyprus, and (b) Stuttgart, Germany (Zinsser The irradiance-level performance of the installed PV technologies at both test facilities is shown in Figure 10. Most of the thin-film technologies, with the exception of the CIGS system, demonstrated better low-light performance under outdoor field conditions compared with the mono-c-si and multi-c-si technologies. Amongst the c-si technologies, the technology which exhibited the best low-irradiance-level performance was the Sanyo HIT mono-c-si technology due to its high parallel resistance. The differences in low-irradiance-level performance of the various PV technologies correspond to differences in their external quantum efficiencies and hence their response to the solar irradiance blue shift at low-light conditions. Specifically, the a-si and CdTe technologies benefit in performance from the blue shift at low irradiation, while the CIGS technology cannot benefit in performance from the blue shift because of parasitic absorption

486 G. Makrides et al. Table 8. Annual inverter efficiency over the period June 2006 June 2009 in Nicosia, Cyprus. 2006 2007 2007 2008 2008 2009 System Inverter efficiency (%) Inverter efficiency (%) Inverter efficiency (%) Downloaded by [University of Cyprus] at 10:38 05 August 2015 Atersa (A-170M 24V) 90.86 90.91 90.75 BP Solar (BP7185S) 90.74 90.69 90.58 Sanyo (HIP-205NHE1) 91.11 91.49 91.33 Suntechnics (STM 200 FW) 90.80 90.84 91.17 Mono-c-Si average 90.88 90.98 90.96 Schott Solar (ASE-165-GT-FT/MC) 90.64 90.68 90.70 Schott Solar (ASE-260-DG-FT) 90.79 90.86 90.67 SolarWorld (SW165) 90.86 91.02 90.87 Solon (P220/6+) 91.34 91.34 91.18 Multi-c-Si average 90.91 90.98 90.86 MHI (MA100T2) 90.75 90.82 90.90 Schott Solar (ASIOPAK-30-SG) 91.43 91.24 91.21 Würth Solar (WS 11007/75) 90.49 90.75 90.39 First Solar (FS60) 91.08 91.37 91.14 Thin-film average 90.94 91.05 90.91 The average annual inverter efficiency is also given for all the mono-c-si, multi-c-si and thin-film technologies. in its window and buffer layers. The relatively good blue spectral performance of c-si technologies results in an almost relatively constant performance at both low and high irradiance levels (Zinsser et al. 2009). The performance of a PV system under outdoor operating conditions is affected not only by temperature and irradiance, but also by the spectral content of the irradiance. To consider the spectral effects on the annual energy yield of the different PV technologies, a spectroradiometer was installed at the test site in Stuttgart and the spectral content of the irradiance was measured over a period of 1 year at a resolution of 1 min. The analysis of the spectral datasets in Stuttgart showed that the measured and averaged spectrum contains more blue and less infrared contributions as compared with the standard solar spectrum AM 1.5 G. For each installed PV technology, the convolution integral of the measured spectral datasets and quantum efficiency was calculated, yielding the short-circuit current. To evaluate the influence of the spectrum on the annual energy yield, the current was subsequently calculated for each 15-min interval, summed up over the period of 1 year and compared with the yield produced assuming an AM 1.5 G spectrum over the complete year. The results showed that the PV technologies in Stuttgart exhibited annual energy yield differences which were no more than 1% due to the spectrum (Zinsser, Schubert, and Werner 2011). 4.5. Inverter efficiency of PV technologies The generated DC power of each PV system is converted to AC, using an inverter in a gridconnected arrangement. The inverters installed at both the Nicosia and Stuttgart testing facilities were typically oversized by 10% in order to utilise all the energy from the PV modules and to ensure that no clipping occurred. The annual inverter conversion efficiencies of all PV technologies in Nicosia, Cyprus, over the 3-year evaluation period are summarised in Table 8. In particular, the results show that the annual inverter conversion efficiencies for all PV technologies installed outdoors were lower than the European inverter efficiency of 91.6%, provided by the manufacturer s datasheet. The lowest annual inverter conversion efficiency of 90.39% was observed during the third year for the Würth Solar CIGS system. On the other hand, the highest conversion efficiency of 91.49% was observed for the Sanyo HIT mono-c-si system during the second year. The different inverter efficiencies arise mainly due to the fact that the inverter efficiency is affected by the

International Journal of Sustainable Energy 487 DC voltage of the PV array, the higher the PV array operating voltage, the lower the conversion efficiency of the inverter. 5. Conclusions Downloaded by [University of Cyprus] at 10:38 05 August 2015 The investigations performed in this study on the monthly average PR AC over a 3-year period clearly showed the existence of seasonality in the performance of all PV technologies installed both in Nicosia, Cyprus, and Stuttgart, Germany. It was demonstrated from the monthly average PR AC that all the c-si technologies exhibited maximum PR AC peaks during the winter and minimum PR AC peaks during the summer with higher and more pronounced seasonal fluctuations occurring in Nicosia compared with the systems in Stuttgart (without considering the periods of snow coverage in Stuttgart). The c-si systems exhibited a 3-year average PR AC in the range of 70 84% and 79 87% in Nicosia and Stuttgart, respectively. In addition, it was more evident in Nicosia that the CIGS and CdTe technologies exhibited monthly average PR AC patterns very similar to the c-si technology. Moreover, the CdTe system exhibited narrower peak-to-peak PR AC variations between the seasons compared with the c-si and CIGS technologies, demonstrating that this technology is not strongly affected by the seasonal climatic variations that exist both in Nicosia and in Stuttgart. The CIGS and CdTe systems exhibited a 3-year average PR AC of 82% and 78% in Nicosia and 87% and 81% in Stuttgart, respectively. Additionally, the thin-film technologies of a-si showed evident monthly average PR AC peaks during the summer and autumn compared with the winter seasons in both locations. The MHI and Schott Solar a-si systems demonstrated a 3-year average PR AC of 77% and 73% in Nicosia, respectively. Accordingly, the Schott Solar a-si system demonstrated a 3-year average PR AC of 80%, while the MHI a-si system exhibited the lowest 3-year average PR AC of 71% amongst all the installed technologies in Stuttgart. Finally, the 3-year average E AC(Normalised) of the systems installed in Nicosia was 1572 kwh/kw p, while in Stuttgart, this was 1097 kwh/kw p. The highest E AC(Normalised) over the entire 3-year period was produced by the Suntechnics mono-c-si system in Nicosia, while the Würth Solar CIGS produced the highest in Stuttgart. References Addelstein, J., and B. Sekulic. 2005. Performance and Reliability of a 1-kW Amorphous Silicon Photovoltaic Roofing System. 31st IEEE Photovoltaic Specialists Conference, January 3 7, Lake Buena Vista, 1627 1630. Carr, A. J., and T. L. Pryor. 2004. A Comparison of the Performance of Different PV Module Types in Temperate Climates. Solar Energy 76 (1 3): 285 294. Cueto, J. A. 2002. Comparison of Energy Production and Performance from Flat-Plate Photovoltaic Module Technologies Deployed at Fixed Tilt. 29th IEEE Photovoltaic Specialists Conference, May 19 24, New Orleans, 1523 1526. Itoh, M., H. Takahashi, T. Fujii, H. Takakura, Y. Hamakawa, and Y. Matsumoto. 2001. Evaluation of Electric Energy Performance by Democratic Module PV System Field Test. Solar Energy Materials & Solar Cells, 67 (1 4): 435 440. King, D., J. Kratochvil, and W. Boyson. 2000. Stabilization and Performance Characteristics of CommercialAmorphous- Silicon PV Modules. 28th IEEE Photovoltaic Specialists Conference, September 15 22, Anchorage, 1446 1449. Makrides, G., B. Zinsser, G. E. Georghiou, M. Schubert, and J. H. Werner. 2009. Temperature Behavior of Different Photovoltaic Systems Installed in Cyprus and Germany. Solar Energy Materials and Solar Cells 93 (6 7): 1095 1099. Makrides, G., B. Zinsser, M. Norton, G. E. Georghiou, M. Schubert, and J. H. Werner. 2010a. Outdoor Performance Evaluation of Grid-Connected PV Technologies in Cyprus. Energy and Power Engineering 4 (2): 52 57. Makrides, G., B. Zinsser, M. Norton, G. E. Georghiou, M. Schubert, and J. H. Werner. 2010b. Potential of Photovoltaic Systems in Countries with High Solar Irradiation. Renewable and Sustainable Energy Reviews 14 (2): 754 762.

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