Degradation rate and mechanisms of PV modules in the Gobi Desert of Mongolia after 14 years operated in the field

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1 Degradation rate and mechanisms of PV modules in the Gobi Desert of Mongolia after 14 years operated in the field Bat-Erdene Bayandelger School of Engineering and Applied Science National University of Mongolia Ulaanbaatar, Mongolia Amarbayar Adiyabat School of Engineering and Applied Science National University of Mongolia Ulaanbaatar, Mongolia Ueda Yuzuru Department of Electrical Engineering Tokyo University of Science Tokyo, Japan Abstract The long term performance of the PV module is strong dependent on environment and meteorological characteristics. Even though, the Gobi Desert of Mongolia is selected as most suitable area to construct 100 MW class VLS- PV, there is still needs to study more local features for solar energy application. Thus, poly-si and mono-si PV modules are installed to discover troubles, features, performance, degradation and other natural influences. In this study, we studied the degradation rate of these 2 PV modules operated in the Gobi Desert area by using outdoor measured data during 14 years and indoor measurement results. Then, visual inspection is carried out in order to investigate the reason of the power declination. The study results showed almost closer 1 %/year degradation rate for mono-si and 1.3 %/year for poly-si. These values are also presented similar results from hot-dry desert area of USA. In addition to, we observed module degradation on Pmp which is mainly corresponded to declination of Isc through indoor measurement and analysis of its parameters and then, we finally discovered by visual inspection that value of the Isc is varied cause of sand particles stricken glass cover of module. Түлхүүр үг Degradation rate; Gobi Desert of Mongolia; linear interpolation method; performance ratio. I. INTRODUCTION The Gobi Desert, Mongolia, is one of the most promising candidate sites for introduction of the 100MW class Very Large Scale Photovoltaic Systems (VLS-PV) specified by Task 8 Very Large Scale Photovoltaic Power Generation Systems conducted as part of the IEA Photovoltaic Power Systems Program (IEA PVPS) [1]. The polycrystalline silicon(poly-si) and monocrystalline silicon(mono-si) PV modules are installed in the Gobi Desert of Mongolia to verify the simulated power generation of PV system in VLS-PV of IEA PVPS program, and even, environmental condition indices, solar energy resource indices and performance indices of the PV modules, have been determined as the seasonal air temperature range ranged between from -20 to С, maximum wind speed 19.0 m/s, monthly average humidity was 40 % and 60 % during warm and cold season, respectively and the module PR showed high values of >1.0 in winter due to the effect of module temperature [2] [3]. Well accurately estimated degradation rate(r d) of PV module which is exposed above specified condition, is still necessary reason of meteorological and environmental feature strong affects to the performance of PV module and it s important not only for long term output power prediction influencing return rate of investment, but helps to make module design improvement for producers as understanding reason of power declination as well as examines the module warranties guaranteed by manufacturers. This study mainly presents long term performance of the poly and mono- Si module and module degradation analyzed by major parameters which are power (P mp), Fill Factor (FF), shortcircuit current (I sc), open-circuit voltage (V oc) and value of shunt (R sh) and series (R s) after 14 years field operation. Estimated R d using outdoor data are validated with indoor measurement results at standard test condition(stc) and causes of power declination explained through physical defects

2 on exposed module observed by visual inspection and electroluminescent(el) devices. Arizona, Nevada, Colorado, Oregon and Utah area of Great Basin Desert in North America are closer weather condition with the Gobi Desert of Mongolia and even those are all classified in arid desert cold (BWk) type of climate as described in Koppen-Gieger climate classification system [4], but they can be warmer than the Gobi area. While 1751 publications of degradation rate in the worldwide classified in term of history overview as before and after 2000, are analytically compiled and the analytical result showed 0.7 %/year average and 0.5 %/year median of R d for c-si modules in the [5], Arizona State University-Photovoltaic Reliability Laboratory(ASU-PRL) has been published study results higher than 1 %/year from respectively 4, 12, 18 and 26 years aged PV plants in hot-dry climate of Phoenix and Temp, Arizona. Major cause of the power loss is R s increment due to ribbon ribbon solder bond failure which becomes origin of delamination and burning of hotspot as well as front encapsulant browning also declines module I sc by the way affecting to transmittance [6] [7] [8]. In addition to, CR Osterwald at al. and B.Marion of NREL in Colorado also showed roughly 1 % results while Frank Vignola et al. of Oregon, presented % for c-si modules [9] [10] [11]. II. METHODOLOGY This study is generally consists of 4 Environmental condition Solar resource evaluation Long term performance analysis Degradation rate evaluation A. PV field test Two type of crystalline silicon PV module which technical specification is showed in Table 1, are installed at Sainshand PV test field ( N and E) and MP-160 I-V curve tracer measures I-V curve of those PV modules every 10 minutes. MS 802 Pyranometer inclined at 45 0 same as module angle and temperature sensor is pasted on back sheet of each PV modules. See the technical specification of PV modules exposed at the field, on Table I. TABLE I. PV MODULE DATA SHEET Type Module 1 Module 2 poly-si Mono-Si P max (W) I sc (A) V oc (V) I mp (A) V mp (V) Efficiency (%) NOCT ( 0 C) ~2 (W/ 0 C) B. Solar Simulator After 14 years, the modules exposed in field condition are measured at STC using pulsed type of solar simulator which is classified in class A and consists of Xenon-arc lamp, electronic unit measuring I-V curve and terminal computer. The measurement at STC is carried out 5 times each modules and then, measurement results are presented in subsection B of III. C. Data analysis technique In order to estimate the R d with good approximation by using outdoor measured I-V data between 2003 and 2016, 2 different type of methodologies are used in this study. First method is normalized rating or Performance ratio (PR) and second one is Linear Interpolation method (LIM). PR and LIM are both submitted respectively, for assessing the performance of PV module and system in IEC and for correcting measured I-V characteristics to other conditions of the temperature and irradiance (such as STC) in IEC While PR is most popular and its benefit is can estimate annual, monthly, weekly and daily performances of PV module and system in the literature of degradation study, LIM very well corrects the measured I-V data to parameter with good approximation in specific condition. D. Performance Ratio The performance ratio indicates the overall effect of losses including module temperature, incomplete utilization of irradiation and failures on the module s rated output as shown in equation 1. PR=(P/P n)*(g STC /G) Here, P n- rated power of PV module, G STC- solar irradiance at STC or 1000 Вт/м 2. For obtaining accuracy result before implement main calculation, following several filtrations are performed. The filtration removes spurious data. The collected data was filtered out for irradiance level less than 800 W/m 2. P n of each temperature point is corrected into STC and then, PR found by using inclined and reference or 1000 W/m 2 irradiation and power at maximum power point and STC. After initial two steps, PR value of less than 0.5 and greater 1.0 were filtered out from the estimated results as they are unrealistic for these sites. Finally, only points in the range PR AVR±σ are used, where: PR AVR is the average PR of all the points and technology σ is the standard deviation. ( )

3 E. Linear Interpolation method Before start implementation the linear interpolation calculation, distribution map of the in-plane irradiance and module temperature yearly data of each module, should initially scattered. Linear interpolation is mainly used to translate the I-V curves and predict energy output of the PV modules for the irradiance G or I sc and temperature T. The accuracy of the translation has examined by the experiment in [12]. Root mean square error(rmse) between the measured and calculated maximum power for four kinds of PV cells showed <0.5 %. These results well agree with measured and estimated power of PV modules. The present method is very useful for the power rating of the PV devices. The measured I-V characteristics is corrected into target G and T values by equations 1 and 3. V 5=V 1+a*(V 2-V 1) (2) I 5=I 1+a*(I 2-I 1) (3) Here, I 1 and V 1 are the current and voltage of the reference I-V curve measured at an irradiance that is G 1 and temperature that is T 1. I 2 and V 2 are the current and voltage of the reference I-V curve measured at G 2 and T 2. I 3 and V 3 are current and voltage of the I-V curve at G 3 and T 3 which is the target point. The pair of (I 1, V 1) and (I 2, V 2) should be chosen as I 2=I 1+ (I sc2- I sc1). Here, I sc1 and I sc2 are the short circuit current of the reference I-V curves. a is a constant for the interpolation, which has the relation with the irradiance and temperature as shown in equations (4) and (5). G 5=G 1+a*(G 2-G 1) (4) T 5=T 1+a*(T 2-T 1) (5) Estimation procedure to calculate P mp at STC by using LIM is explained in following section. Measured current, voltage and module temperature relevant to in-plane irradiance lower than 300 W/m 2 of in-plane irradiance, are subtracted for reducing influence of shading and influence of morning and evening, unexpected error at low value. Reference points 1, 2, 3 and 4 are chosen at contained I-V data as much as possible area. Interval of in-plane irradiance at reference point is ±5 W/m 2, and` temperature vicinity is ±1 0 С. The current, voltage and maximum power at 5 point are calculated from reference points 1 and 2. The point 6 is also obtained from reference points 3 and 4. Finally, estimated values at point 5 and 6 are used to calculate voltage and current at 1000 W/m 2 and 25 o С at target 7. These noted procedures are same implemented for all PV modules data of each year of 2003 and III. RESULTS AND DISCUSSION A. The result of outdoor measurement The result of two different PV modules analyzed by LIM and PR are showed in Table II. long term degradations of poly and mono-si PV modules operated under harsh and hot-dry field conditions of the Mongolian Gobi Desert, 1.5 % and % for LIM and 1.03 % and 1.28 % for PR, are presented closer results with reported results from hot-dry area of Arizona State University, US [6]- [11]. In the leading countries such Japan, Germany and Singapore, at PV field, degradation rate in range of 0.5 % and 0.7 % shows that kindly climate condition less affects to module specification. In another word, it is confirmed environmental and meteorological characteristics have strong influence. Differences of LIM and PR, respectively, ±0.16 % in poly-si and 0.25 % in mono-si, indicates accuracy within 0.25 %. It means that Poly-Si PV module is faster degraded than Mono-Si module. TABLE II. ESTIMATED ANNUAL RD BY LIM AND PR Type Description LIM PR Poly-Si Annual % of difference, R d Mono-Si Annual % of difference, R d B. Indoor test result In this section, modules exposed outdoor field condition are tested at STC using solar simulator to validate the estimation results of the R d by using outdoor measured data for long time. Main parameters including I mp, V mp, P mp, I sc, V oc, R sh and R s resistances, before and after field operation for 14 years, and variation which presents degradation on each parameters, are showed in Table III. The peak power reductions are 18.1 % and 15.2 % and it corresponds to 11.3 % and 10.4 % decrease of short circuit current, respectively, for poly and mono silicon technologies. The open-circuit voltage and fill factor are small changed during field exposure. Thus, indoor measurement results at STC show poly-si 1.29 %/year and mono-si 1.08 %/year of average R d. These results present similar values showed in 2.1. Additionally, Alexander Phinikarides et al. [13] concluded as power decrease of crystalline silicon PV modules related to short-circuit current, commonly occurs on most of the studies. The reason of the I sc declination is the UV induced discoloration of semi-conductors, oxidation of front metal contact and anti-reflecting layer, front glass breakage and soiling, delamination of EVA and R s increment due to contact soldering influences as reported in [14, 15, 9, 16, 17]. Thus, in the next section, actual causes to decrease I sc is discovered by visual inspection TABLE III. RESULTS OF INDOOR MEASUREMENTS AT STC BEFORE AND AFTER 14 YEARS FIELD OPERATION AND VARIATION Type Poly silicon Mono silicon Initial Final R d (%) Initial Final R d (%) P mp (W) I sc (A)

4 V oc (V) I mp (A) V mp (V) FF (%) C. Visual Inspection The peak power decline due to I sc decrease have been founded in previous sections defined by electrical parameters based on indoor and outdoor measurement. The I sc declination can be origin of the several factors as noted in section III-B and this section presents major impacts to down I sc, observed by the visual inspection and explains corresponds of them. All failure and degradation modes noted in other literatures published from every corner of the world, such as front grid oxidation, ribbon and encapsulant delamination, semiconductors discoloration, diode/junction box problem, frame deformation, cell crack, ethylene-vinyl acetate browning, milky pattern, busbar corrosion etc., are also checked on these modules. While the color of the absorbing layer illustrated in a) of Figure 1, changed towards black for poly-si and cell color of the mono- Si is varied center to edge in Figure 1 b). The discoloration is also one of the reasons to decrease I sc. The EL image is showed in the Figure 2. Figure 2. EL image: a) poly-si and b) mono-si As shown in Figure 3, bigger micro-spot cracks of front glass is marked by red circle and but there are lot of small microspot cracks not available to show by photo camera. Otherwise, it is confirmed by what wind speed usually higher than the 4 m/s and yearly average wind speed 2.85 m/s during the day time as reported study in [3]. Figure 1. Discoloration of the solar cell for a) poly-si and b) mono-si But, major impact showing meteorological and environmental feature of the Gobi Desert area, is small cracked spots on module glass which is created by striking sand particles.

5 Figure 3. Micro spot cracks of the module front glass: a) poly- Si and b) mono-si module IV. CONCLUSIONS Degradation rates of poly-si and mono-si PV modules operated in the Mongolia Gobi Desert during 14 years are examined by long term outdoor I-V measurement and indoor measurement of solar simulator in this paper. The study results showed almost closer 1 %/year degradation rate for mono-si and 1.3 %/year for poly-si. These values are also presented similar results from hot-dry desert area of USA. We observed module degradation on P mp which is mainly corresponded to declination of I sc through indoor measurement and analysis of its parameters and we discovered that value of the I sc is varied cause of sand particles stricken glass cover of module by visual inspection. ACKNOWLEDGEMENT We would like to acknowledge National Institute of Advanced Industrial Science and Technology of Japan and Ueda Laboratory, Department of Electrical Engineering, Tokyo University of Science for providing necessary facilities and research relevant to measurement in long term. Authors want to specially thank for financing Mongolia- Japan Engineering Education Development Program for making Join Research between Japan and Mongolia. V. BIBLIOGRAPHY [1] K. Kosuke, Energy from the Desert, Feasibility of Very Large Scale Photovoltaic Power Generation (VLS-PV) Systems, United Kingdom: James & James Ltd, [2] A. Adiyabat, "Longterm Performance Analysis of PV module in the Gobi Desert of Mongolia," in 35th IEEE Photovoltaic Specialists Conference, Honolulu, [3] A. Adiyabat, "Evaluation of Solar Energy Potential and PV module Performance in the Gobi Desert of Mongolia," Progress in Photovoltaics, p. 10, [4] M. C. Peel, "Updated world map of the Koppen-Geiger climate classification," Hydrology and Earth System Sciences, vol. 11, no. 5, p. 12, [5] D. C.Jordan, "Photovoltaic Degradation Rates - An Analytical Review," Progress In Photovoltaic, vol. 21, no. 1, p. 29, [6] J. Mallineni, "Evaluation of 12-year-old PV power plant in hot-dry desert climate: Potential use of field failure metrics for financial risk calculation," in Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th, Denver, Colorada, USA, [7] K. Olakonu, "Degradation and failure modes of 26-yearold 200 kw power plant in a hot-dry desert climate," in Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th, Denver, Colorado, USA, [8] K. Yedidi, "Failure and degradation modes and rates of PV modules in a hot-dry climate: Results after 16 years of field exposure," in Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th, Denver, Colorado, USA, [9] C. Osterwald, "Degradation analysis of weathered crystalline-silicon PV modules," in 29th IEEE photovoltaic specialists conference, New Orleans, Louisiana, [10] B. Marion, "Long-term Performance of the SERF PV Systems," in Conference paper, Denver, Colorado, [11] F. Vignola, "MEASURING DEGRADATION OF PHOTOVOLTAIC MODULE PERFORMANCE IN THE FIELD," Ashland, Oregon, USA, [12] T. Yuki, "Translation equations for temperature and irradiance of the IV curves of various PV cells and modules," in Renewable energy 2006, Tokyo, [13] A. Phinikarides, "Review of photovoltaic degradation rate methodologies," Renewable and Sustainable Energy Reviews, vol. 40, no. 12, p. 9, [14] Y. Hishikawa, "Field test results on on the stability 0f 2400 photovoltaic modules manufacured in1990s," in 29th IEEE photovoltaic specialists conference, New Orleans, Louisiana, [15] C. Chamberlin, "Comparision of PV module performance before and after 11 and 20 years of field exposure," in 37th IEEE photovoltaic specialists conference, Washington, [16] M. Quintana, "Commonly observed degradation in field aged photovoltaic modules," in 29th IEEE photovoltaic specialists conference, New Orleans, Louisiana, [17] P. Sanchez-Friera, "Analysis of degradation mechanisms of crystalline silicon PV modules after 12 years of operation in Southern Europe," Progress In Photovoltaic, vol. 19, no. 6, p. 9, [18] D. Berman, "EVA laminate browning after 5 years in a grid-connected, mirror-assisted, photovoltaic system in the Negev deser: effect on module efficiency," Solar Energy Materials and Solar Cells, vol. 36, no. 4, p. 11, [19] Ying Tang, Bindhu Raghuraman and Joseph Kuitche, "An Evaluation of 27+ Years Old Photovoltaic Modules Operated in a Hot-Desert Climatic Condition," in 4th World Conference on Photovoltaic Energy, Hawaii, [20] S. V. Janakeeraman, "A statistical analysis on the cell parameters responsible for power degradation of fielded pv modules in a hot-dry climate," in Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th, Denver, Coloroda, USA, 2014.