Performance loss rates of grid-connected photovoltaic technologies in warm climates

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1 GCREEDER 2013, Amman, Jordan, Sept. 10 th 12 th, Performance loss rates of grid-connected photovoltaic technologies in warm climates G. Makrides 1, A. Phinikarides 2 and G.E. Georghiou 3 1 Department of Electrical and Computer Engineering, University of Cyprus, 75 Kallipoleos Avenue, P.O. Box 20537, Nicosia, 1678, Cyprus, makrides.georgios@ucy.ac.cy 2 Department of Electrical and Computer Engineering, University of Cyprus, 75 Kallipoleos Avenue, P.O. Box 20537, Nicosia, 1678, Cyprus, phinikarides.alexander@ucy.ac.cy 3 Department of Electrical and Computer Engineering, University of Cyprus, 75 Kallipoleos Avenue, P.O. Box 20537, Nicosia, 1678, Cyprus, geg@ucy.ac.cy ABSTRACT With the current uptake and market penetration of different technology photovoltaics (PV), important questions arise regarding their lifetime power decline and degradation. Another important consideration is the influence of different climatic conditions on the exhibited degradation rates, with warmer operating conditions theoretically affecting module components to a higher extent compared to lower temperatures. The purpose of this paper is to present a comparison of the annual performance loss rates of different photovoltaic (PV) systems, ranging from mono-crystalline silicon (mono-c-si) to multi-crystalline silicon (multi-c-si) and thin-film technologies, installed in Cyprus over a period of five years. The results of the performance loss rate investigation showed that the mono-c-si and multi-c-si systems exhibited average annual performance loss rates that were lower than the 1 %/year stated by most PV module manufactures. Conversely, the average annual performance loss rate of the thin-film systems was %/year. 1. INTRODUCTION It is well known that the maximum power rating of photovoltaic (PV) systems gradually deteriorates through the years and the degradation depends on the PV module technology and the climatic conditions where the systems are installed [1]. As evidenced by the results of PV module accelerated testing, warm climates are expected to have a significant influence on the degradation rate of PV modules [2-3], as high temperature environments affect module components to a higher extent compared to lower temperature environments. At the moment, PV module manufacturers usually provide performance warranties of 20 years, with maximum power loss of no more than 20 % at the end of the 20 years. Because of the effects that climatic conditions have on the performance of PV systems, continuous monitoring under outdoor conditions is thus very important in assessing the actual field performance loss. The focus of this paper is to determine the annual performance loss rates of different PV systems, ranging from mono-crystalline silicon (mono-c-si) to multicrystalline silicon (multi-c-si) and thin-film technologies such as amorphous silicon (a-si), cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS), installed in Nicosia, Cyprus. More specifically, the systems were exposed to the warm Mediterranean climate over a period of five years and the annual performance loss rates were calculated using linear regression applied on the monthly DC performance ratio (PR) time series. As the investigation was performed on grid-connected systems the prediction of the degradation rate is difficult and complex because other factors apart from material degradation might result in reduced power output over the years; such as seasonal performance variations, soiling, shading, mismatch and faults of modules. For this reason it is more appropriate to collectively talk about performance loss which implies the gradual power loss from the maximum rated power rather than degradation. Even though investigations regarding degradation of PV technologies have been performed in the past especially for crystalline silicon (c-si) technologies, this is the first study that presents a comparison of the annual performance loss rates of different technologies installed side-by-side and exposed to the warm conditions encountered in the Mediterranean. Additionally, the fact that the technologies were installed side-by-side at the same time provides meaningful comparisons between the technologies (even though the performance loss rates might be affected by seasonal performance variations, soiling, shading, mismatch and other factors) because the evaluation is performed under the same operating conditions. 2. EXPERIMENTAL SETUP 2.1. Outdoor PV Testing Facility The outdoor PV testing facility in Cyprus was commissioned in May 2006 and includes, amongst others, twelve grid-connected PV systems of different technologies. The installed fixed-plane PV systems range from mono-c-si and multi-c-si, Heterojunction with Intrinsic Thin layer (HIT), Enhanced Film Growth (EFG), Multi-crystalline Advanced INdustrial (MAIN) cells to a-

2 GCREEDER 2013, Amman, Jordan, Sept. 10 th 12 th, Si, CdTe, CIGS and other PV technologies [4-5]. Each system has a nominal capacity of approximately 1 kw p and is equipped with the same type of inverter installed in close proximity, behind each respective system (SMA SB 1100, SMA Technology AG, Niestetal, Germany). The same inverters are used in order to exclude the influence of different maximum power point tracking (MPPT) methods to the DC yield. The inverters are also properly sized to ensure that the systems are always working at their maximum power point (MPP) [5]. The systems are laid out in two arrays, as shown in Figure Data Acquisition Infrastructure Both meteorological and PV system measurements are being acquired and stored through an advanced data acquisition platform. The platform comprises of meteorological and PV performance measurement sensors, connected to a central data-logging system. The monitored meteorological parameters include the global irradiance in the plane-of-array (POA), wind direction and speed, as well as ambient and module temperatures. The electrical parameters measured include DC current and voltage and AC power at MPP, as obtained at each PV system output. All the installed sensors and data-logging devices are listed in Table 2. Table 2: Data acquisition equipment and sensors. Parameter Manufacturer Model Data logger Delphin Topmessage Ambient temperature Theodor Friedrich 2030 Module temperature Heraeus PT 100 Global irradiance Kipp & Zonen CM 21-CV 2 DC voltage Custom made Potential divider DC current Custom made Shunt resistor AC energy NZR AAD1D5F Wind speed Theodor Friedrich 4034 Wind direction Theodor Friedrich 4122 Figure 1: Outdoor PV testing facility at the University of Cyprus. The fixed plane PV arrays are installed at the optimum annual energy yield angle for Cyprus, 27.5, and are ground-mounted on frames in an array configuration, with most modules placed in series. A brief description of the systems is given in Table 1 [6]. Table 1: PV systems installed at the University of Cyprus. Manufacturer Technology Power (kw p ) Mono-c-Si systems Atersa mono-c-si BP Solar mono-c-si (Saturn) Sanyo mono-c-si (HIT) Suntechnics mono-c-si Multi-c-Si systems Schott Solar multi-c-si (MAIN) Schott Solar multi-c-si (EFG) SolarWorld multi-c-si Solon multi-c-si Thin-film systems Mitsubishi Heavy a-si (single-junction) Industries (MHI) Schott Solar a-si (tandem) First Solar CdTe Würth Solar CIGS METHODOLOGY The different technology PV systems were gridconnected and monitored from the beginning of June Both meteorological and PV system operational measurements were acquired and stored using the data acquisition platform at a resolution of one second. The stored measurements were subsequently used to construct fifteen-minute average data-sets that were employed in the analysis of this investigation. In particular, the fifteen-minute average data-sets of the MPP Power, P MPP, and POA Global Irradiance, G POA, were used to construct a time series for the different PV systems based on a reference condition parameter, such as the DC performance ratio (PR), PR DC. The DC PR is defined as the ratio between the actual DC energy production, E real, and the DC energy the PV array would produce under standard test conditions (STC) of global irradiance 1000 W/m 2, air mass (AM) of 1.5 and cell temperature of 25 C, E STC [5] and is given by: PR E E real real DC (1) ESTC H A STC where E real is the total energy produced by the PV system at the DC side, H is the irradiation in the POA, A is the area of the PV array and η STC is the STC efficiency of the array. Subsequently, a five-year time series of the monthly DC PR was constructed and analysed for each system, by

3 employing the statistical technique of linear regression, Finally, using the gradient of the fit and multiplying it by twelve (number of months in a year), the annual performance loss rates were calculated for each system. 4. RESULTS 4.1. Environmental Conditions The monthly global irradiation in the POA of 27.5 and the monthly average ambient temperature, measured over the five-year period (June June 2011) at the outdoor PV testing facility at the University of Cyprus, are plotted in Figure 2. GCREEDER 2013, Amman, Jordan, Sept. 10 th 12 th, Figure 2: Monthly global irradiation and average ambient temperature measured over the five-year period at the outdoor PV testing facility. It is clear from Figure 2 that the monthly global irradiation exhibits seasonality, with the highest amount of irradiation occurring during summer and the lowest during winter. It is also evident that the summer seasons were the warmest, while the lowest monthly average ambient temperatures occurred during winter. Figure 3: Monthly DC PR and linear least square fit over the period June June (a) Atersa mono-c-si. (b) Suntechnics mono-c-si. (c) Sanyo HIT mono-c-si. (d) BP Solar mono-c-si. Additionally, Figure 4 demonstrates the monthly DC PR and the applied linear fit for the multi-c-si PV systems over the five-year period Annual Performance Loss Rates of c-si Technologies The monthly DC PR and the applied linear fit for the mono-c-si PV systems over the five-year period, June June 2011 are depicted in Figure 3. Figure 4: Monthly DC PR and linear least square fit over the period June June (a) Schott Solar MAIN

4 GCREEDER 2013, Amman, Jordan, Sept. 10 th 12 th, multi-c-si. (b) Schott Solar EFG multi-c-si. (c) SolarWorld multi-c-si. (d) Solon multi-c-si. The monthly PR plots in Figure 3 and 4 show that both mono-c-si and multi-c-si technologies performance is seasonally influenced, exhibiting the lowest PR during the hot summer period and peaks in performance during winter. For c-si technologies, the increased module temperature is a very important performance loss factor and all previous studies agree that the power output of c-si PV is reduced with temperature increase [7-9]. It must also be noted that partial shading had been affecting the performance of the BP Solar mono-c-si and Solon multi-c- Si systems, specifically during the second, third and fourth year Annual Performance Loss Rates of Thin-film Technologies The monthly DC PR and the applied linear fit for the thin-film PV systems over the five-year period are shown in Figure 5. respectively, it was obvious that during the summer and early autumn, the performance was higher than in the winter. The higher performance of the a-si technologies during warm periods is partly due to thermal annealing and the more favorable spectrum during the warm summer season, compared to the less favorable spectrum during the cold winter months [10-11] Performance Loss Rates Comparison The annual DC performance loss rates of the PV technologies, obtained by applying linear regression over the five-year period, are summarised in Table 3. The annual performance loss rates were in the range of to %/year for the mono-c-si systems, while the multi-c- Si systems exhibited annual performance loss rates in the range of to %/year. In contrast, most thin-film technologies showed higher annual performance loss rates compared to the c-si systems, in the range of to %/year. In particular, the CIGS, CdTe and the single-junction a-si systems exhibited annual performance loss rates which were higher than the 1 %/year stated on most manufacturer warranties. Table 3: Annual DC performance loss rates of PV technologies evaluated by applying linear regression on the monthly DC PR time series over the five-year period (with the exception of the Schott Solar a-si system which was monitored over a four-year period). System Annual performance loss rate (%/year) Atersa mono-c-si BP Solar mono-c-si Sanyo mono-c-si HIT Suntechnics mono-c-si Schott Solar multi-c-si MAIN Schott Solar multi-c-si EFG SolarWorld multi-c-si Solon multi-c-si Würth Solar CIGS First Solar CdTe MHI a-si Schott Solar a-si (four-year period) Figure 5: Monthly DC PR and linear least square fit over the period June June (a) Würth Solar CIGS. (b) First Solar CdTe. (c) MHI a-si. (d) Schott Solar a-si (the Schott Solar a-si system was monitored only for four years). The same seasonal performance pattern exhibited for c- Si technologies, was observed for the Würth Solar CIGS and First Solar CdTe technologies and shown in Figure 5a and Figure 5b, respectively. These technologies showed the highest performance loss rate gradient amongst all the installed systems. Conversely, from the PR plots of Figure 5c and 5d for the MHI a-si and Schott Solar a-si systems 5. CONCLUSIONS The annual performance loss rates of different PV technology grid-connected systems installed in the warm climate of Cyprus were evaluated by applying the statistical technique of linear regression on the constructed five-year period monthly DC PR time series. The average annual performance loss rates of the mono-c-si and multic-si systems were and %/year respectively. The results within the manufacturer datasheet warranty information. Finally, the thin-film technologies of CIGS, CdTe and the single-junction a-si exhibited annual performance loss rates which were higher than 1 %/year.

5 GCREEDER 2013, Amman, Jordan, Sept. 10 th 12 th, Acknowledgments The authors wish to thank J. H. Werner for his vision, continuous support and encouragement. The authors also gratefully acknowledge the contributions of the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU), which supported this work under contract No We also gratefully acknowledge the support by the companies Atersa, First Solar GmbH, Phönix Sonnenstrom AG, Q-cells AG, Schott Solar GmbH, SMA Technologies AG, SolarWorld AG, Solon AG and Würth Solar GmbH & Co.KG. Finally the authors would like to acknowledge the financial support by the Cyprus Research Promotion Foundation. References [1] D.C. Jordan, R.M. Smith, C.R. Osterwald, E. Gelak, S.R. Kurtz, Outdoor PV degradation comparison, 35 th IEEE Photovoltaic Specialists Conference, 2010, pp [2] D.C. Jordan, S.R. Kurtz, Photovoltaic degradation rates - An analytical review, Progress in Photovoltaics: Research and Applications, 2013; 21: [3] M. Koehl, M. Heck, S. Wiesmeier, Evaluation of the accelerated life testing conditions for PVmodules based on measured and simulated weathering stress, 37 th IEEE Photovoltaic Specialists Conference, 2011, pp [4] G. Makrides, B. Zinsser, G.E. Georghiou, M. Schubert, J.H. Werner, Temperature behaviour of different photovoltaic systems installed in Cyprus and Germany, Solar Energy Materials and Solar Cells, 2009; 93: [5] B. Zinsser, G. Makrides, W. Schmitt, G.E. Georghiou, J.H. Werner, Annual energy yield of 13 photovoltaic technologies in Germany and Cyprus, 22 nd European Photovoltaic Solar Energy Conference, 2007, pp [6] G. Makrides, B. Zinsser, M. Norton, G.E. Georghiou, M. Schubert, J.H. Werner, Potential of photovoltaic systems in countries with high solar irradiation, Renewable and Sustainable Energy Reviews, 2010; 14: [7] J.D. Sandstorm, A method for predicting solar cell current-voltage curve characteristics as a function of incident solar intensity and cell temperature, National Aeronautics and Space Administration, Technical Report , [8] C.R. Osterwald, T. Glatfelter, J. Burdick, Comparison of the temperature coefficients of the basic I-V parameters for various types of solar cells, 19 th IEEE Photovoltaic Specialists Conference, 1987, pp [9] G. Makrides, B. Zinsser, G.E. Georghiou, M. Schubert, J.H. Werner, Outdoor efficiency of different photovoltaic systems installed in Cyprus and Germany, 33 rd IEEE Photovoltaic Specialists Conference, 2008, pp [10] T. Strand, L. Mrig, R. Hansen, K. Emery, Technical evaluation of a dual junction sameband-gap amorphous silicon photovoltaic system, Solar Energy Materials and Solar Cells, 1996; 41-42: [11] G. Makrides, B. Zinsser, A. Phinikarides, M. Schubert, G.E. Georghiou, Temperature and thermal annealing effects on amorphous silicon PV, 26 th European Photovoltaic Solar Energy Conference, 2011, pp