PERFORMANCE OF CHAMPION MODULE'S OF PV LIGHTING SYSTEMS IN INDIA

27th European Photovoltaic Solar Energy Conference and Exhibition PERFORMANCE OF CHAMPION MODULE’S OF PV LIGHTING SYSTEMS IN INDIA A. Sinha*A, O.S. SastryB, Y.K. SinghB, M. KumarB, R. GuptaA Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Mumbai-400076, India B Solar Energy Centre, Ministry of New and Renewable Energy, New Delhi-110003, India *Email: [email protected]; Tel: +91-22-25767837; Fax: +91-22-25764890 A ABSTRACT: Use of Solar Photovoltaic (SPV) for Street Lighting Systems (SLS) is an ideal application of SPV for illumination of streets and crossroads located in areas that are not connected to the power grid. This paper presents the performance and degradation analysis of two pairs of the Champion PV modules which were used for SLS installed at Solar Energy Centre, India during the year 1990. These PV modules have experienced minimal degradation in their power output even after operating for 21 years. The power loss observed in all modules is less than 10%, which is less than the specified level in IEC 61215 standard. These modules showed unique behavior and the most surprising aspect is that only one of the modules in each pair is affected by browning inspite of installed at same location and under similar environmental conditions. A detailed investigation has been performed by using the I-V measurement and characterization techniques like Lock-in thermography and Electroluminescence imaging. Keywords: Degradation, PV module, Browning, Lock-in thermography, Electroluminescence 1 INTRODUCTION in this paper. Solar Energy Centre (SEC), an R&D institute under Ministry of New and Renewable Energy (MNRE), Government of India, installed few street lighting systems (SLS) at its new campus in Gurgaon as shown in Fig. 1, when SEC started functioning during the year 1990. For meeting dusk to dawn duty cycle requirement of 11W Compact Fluorescent Lamp (CFL) of the SLS, 60Wp module(s) and 70Ah battery with charge controller were used. The 60Wp requirement was met through two 30Wp modules, connected in parallel. It should be worth mentioning here that these modules were not IEC 61215 certified. In the recent past, Solar Photovoltaic in India has experienced a significant growth among all the renewable energy resources mainly due to large scale deployment of SPV systems under the Jawaharlal Nehru National Solar Mission (JNNSM). This is also due to significant reduction in the product costs. It has opened new dimensions in the various applications like building integrated PV systems, street lighting and many more. Among them lighting systems are very popular and implemented in the urban as well as rural areas. One of the main targets of the JNNSM is 35 years of operational life for PV modules including mono/ multicrystalline silicon (c-Si) [1]. Degradation in performance during prolonged exposure under harsh environmental conditions prevailed in dry/ arid zones such as New Delhi, has been observed in all types of modules [2, 3, 4]. Outdoor conditions such as humidity, high levels of UV radiation and temperature have detrimental effects and thus a PV module undergoes different degradation mechanisms like light induced degradation (LID), degradation in packaging materials like browning, delamination, contact terminal corrosion, potential induced degradation (PID) etc. These mechanisms cause a gradual loss in module performance and eventually leading to the catastrophic failures. As a result, the lifetime cost benefit analysis of PV products changes significantly. Brown module Non-brown module Figure 1: PV modules installed for street lighting at SEC, India 2 CHAMPION PV MODULES Another interesting observation has been found out in two pair of modules used in such street lighting. This observation came into notice while removing these street lights for some construction work. They show an interesting pattern that one of the modules in a pair is affected by browning whereas the other module in a pair does not show any signs of browning even though both the modules are of same manufacturer and exposed under similar environmental conditions. It was not a coincidence because there were two such pairs as shown in Fig. 2. Modules of one pair have been marked as number M-01 and M-02 and modules of another pair have been M-03 and M-04 respectively, for detailed analysis of each module. This paper reports quite interesting observation in India on some PV modules used for street lighting purpose. These modules have shown least or no degradation even after the completion of their expected life term under outdoor conditions. The power loss observed in the degraded PV modules is less than 10% for the duration of 21 years which is less than that specified in IEC standards. IEC 61215 standard allows the degradation up to 10% in wattage within 10 years and 15% within 20 years of module’s operational life [5]. Jordan et.al. [6] also reported that the median value of degradation rate is 0.9%/ year in c-Si modules. That is why; these modules are referred as “Champion modules” 3507 27th European Photovoltaic Solar Energy Conference and Exhibition     M-01 (brown) M-03 (brown) M-02 (non-brown) clear EVA around the cell edges. The EVA regions between the neighbouring solar cells were also showing no discoloration. No major cracks were found. The busbars and finger grids were oxidised and corroded at the centre of the cells in the brown modules. Junction boxes were loosely connected to PV modules and thus taken out for making contacts and to carry out the measurements. Few bubbles were seen at the outer terminal contacts which were due to moisture ingression from backside. 4.2 I-V measurement After visual inspection, I-V characteristics of the modules were taken under STC conditions (1000 W/m2, 25oC, AM 1.5) by using Quick SUN 700 class AAA sun simulator, module tester. These characteristics are important for the quantitative analysis of the electrical parameters and also to detect any degradation in the performance parameters of a module. The electrical parameters of each module are listed in Table I. The initial rated parameters of all the modules were Isc: 2.24A, Voc: 21V, Pm: 30W and FF: 0.6. a comparison of the maximum power yield and corresponding % degradation based on the initial rated parameters (year 1990) and measured data (year 2012) are presented in Fig. 3. M-04 (non-brown) Figure 2: Two pair of PV modules, each having one brown module Table I: IV data of the modules Browning of encapsulant (EVA) is caused by the photo-thermal degradation of polymer. UV radiation (280nm-320nm) is primarily responsible for the discoloration in the presence of high temperature [7]. Several additives like UV absorbers and stabilizers are incorporated in the polymer to enhance its durability and stability from the action of UV light. But once their concentration goes below the critical value then the degradation rate increases rapidly. M-01 M-02 M-03 M-04 Isc (A) Voc (V) Pm (W) FF Rs (Ω) Rsh (Ω) 2.28 2.30 2.18 2.28 22.5 21.8 22.0 22.0 27.3 30.0 27.5 32.0 0.53 0.60 0.57 0.64 2.2 1.6 2.1 1.5 68.0 72.0 143.0 110.0 +6.7 % 3 ASSESSMENT PROCEDURE -9.0 % 0.0 % -8.3 % For detailed investigation, a proper assessment procedure for evaluating the performance and defects in these modules has been made by using following measurement/ characterization techniques  Visual inspection  I-V measurement  Dark Lock-in thermography  Electroluminescence imaging 4 Figure 3: Comparison of maximum power yield in the year 1990 and 2012 with corresponding % degradation in maximum power output RESULTS AND DISCUSSION 4.1 Visual inspection First of all, the modules were visually inspected to evaluate the physical damages occurred in them. Following observations were made:  One of the modules in each pair was affected from browning whereas there was hardly any discoloration seen in other module of the pair. The browning patterns were observed in the central parts of cells that were observed by a In Fig. 3, it shows that in first pair, brown module (M-01) generated 27.3W and the brown module (M-02) produced 30W. The maximum power degradation in M01 is 9% and no power loss in M-02. Similarly, in second pair, the brown module (M-03) generated 27.5W and non-brown module (M-04) gave 32W. The power output is reduced by 8.3% in M-03 and increased by 6.7% in M04. Thus the brown modules have higher degradation and non-brown modules have no degradation. The brown 3508 27th European Photovoltaic Solar Energy Conference and Exhibition modules have lower fill factor which indicating more mismatch in these modules. The series resistance of brown modules is also higher, which may be due to the finger corrosion. These results were re-confirmed by repeating the measurements through SPI-240A sun simulator. The data obtained were in the close proximity of the data obtained through Quick Sun 700 simulator. 4.3 Dark Lock-in Thermography Lock-in thermography (LIT) is a relatively recent technique used to image lateral-electrical nonhomogeneities of solar cells and modules by temperature difference [8, 9]. LIT has several advantages over the conventional steady state IR thermography. Due to its dynamic character, the lateral diffusion is suppressed and leading to an improved effective spatial resolution. Also the integration time is large in this technique which increases the signal to noise ratio in order to achieve mK temperature resolutions. In this technique, the solar module was periodically excited by a forward bias rectangular waveform by using a programmable power supply and the corresponding local temperature modulation was captured by the cooled IR camera, operating at a frame rate of 150Hz. The camera recorded several images and performed the lockin algorithm over them and finally produces a resultant amplitude image. The modules were investigated from the rear side (backsheet) since the front glass is not transparent in IR region of camera. The bright feature visible in LIT images corresponds to shunts whereas the dark regions are inactive or delaminated regions. The LIT images of all four modules are shown in Fig. 4. In LIT images of M-01 and M-03 (brown modules), more non-uniformity have been seen over the module compared to M-02 and M-04 (non-brown modules), which shows more electrical more electrical mismatch in brown module. This mismatch will increase the operating temperature of brown module. M-04 is the best performing module according to I-V results, shows best university and has only one shunt. Broadly there is not much difference in number of shunts among modules. Strong shunts have been marked in Fig. 4. M-03 M-04 Figure 4: LIT images of M-01 & M-03 (brown modules) and M-02 & M-04 (non-brown modules) 4.4 Electroluminescence imaging Electroluminescence imaging (ELI) is another technique which was used to investigate the defects in the solar cells and modules. In this technique, current equivalent to short-circuit current was fed into the solar module in the forward bias condition and the luminescence emission from the solar cell was captured by Si-CCD near IR camera. The camera was placed at a certain distance over the module such taht it is facing the top glass cover. The emission takes place in the near IR region. The whole experiment was performed in the dark enclosure to avoid any interference from the external light. The emission intensity of EL signal is related to the minority carrier lifetime. The nominal bright region in EL images corresponds to the sound area of the solar cell whereas dark region in the cell show low concentration of minority charge carriers, where optical transmission issues are not involved during the measurement. The EL images of all four modules are shown in Fig. 5. b a M-01 M-01 M-02 3509 M-02 27th European Photovoltaic Solar Energy Conference and Exhibition these modules are referred as “Champion modules”. Electrical and optical mismatch with in a module seems to be dominant reason of browning of module due to temperature increase. Browning of encapsulants primarily occurred due to the action of UV light but the study indicates, mismatch which increases operating temperature of PV module play a significant role in its growth. Since browning was observed only in those modules of pair which have higher mismatch support this fact. Electrical mismatch might be present in the modules from the start or it happened due to initial partial shading which started the cyclic process of temperature rise and optical mismatch due to browning. Characterization results indicated that Lock-in thermography can be used for qualitative investigation of electrical mismatch in modules and EL imaging can be used for spatial characterization of EVA discoloration. b M-01 M-02 6 ACKNOWLEDGEMENTS Figure 4: EL images of M-01 & M-03 (brown modules) and M-02 & M-04 (non-brown modules) This work has been supported by a joint India-UK initiative in solar energy through a joint project ‘Stability and Performance of Photovoltaics (STAPP)’ funded by Department of Science and Technology (DST) in India and Research Councils UK (RCUK) Energy Programme in UK (contract no: EP/H040331/1). The authors would also like to thank Mr. Jatin Roy and his team, Solar Semiconductor Pvt. Ltd., India for EL measurements. Fig. 5 shows EL images of brown modules (M-01 and M-03) are darker in compared to non-brown modules (M-02 and M-04). This is because the discolored EVA reduces the EL transmission in brown modules. In addition to this, EL intensity is not consistent over the entire solar cell which is ascribed to the non-uniform nature of discoloration or degradation. The central region of cells are darker compared to outer regions of cells in brown modules because of more browning at the centre, as shown by marked region ‘a’ in Fig. 5. In this context, ELI proved to be a diagnostic tool for the spatial characterization of the browning degradtion. Also, this information gives a vital clue to detect early signs of browning where it is difficult to detect and quantify by naked eyes. The areas around the cell edges and between the neighbouring cells are not browned which is attributed to the photo-bleaching of discolored EVA [10]. The air (or oxygen) permeates into the module from the corners and backsheet and diffuses towards the solar cells that bleach out the discoloration in EVA. The diffusion of air to the centre of the cells is very slow and remained brown. The dark lines at the centre of the solar cells are the finger defects which arised due to the corrosion and resulted in high series resistance which is also appearing in I-V results. In case of M-02 and M-04, there are no signs of browning. Among all the modules, EL is highest in M04. This module also gave the best results in I-V measurement and thermography. EL also provides the information of shunts, but it is difficult to distinguish them in EL images because of the presence of other defects related to materials and biasing situation of individual cells. Strong shunts which are present in M-02 and M-03 (depicted in LIT images) are also visible in EL images with extra high brightness as marked by region ‘b’ in Fig. 5. 5 7 REFERENCES [1] Mission document of “Jawaharlal Nehru National Solar Mission”, New Delhi, October 2010 [2] O.S. Sastry, S. Saurabh, S.K. Shil, P.C. Pant, R. Kumar, A. Kumar, B. Bandhopadhyay, Solar Materials and Solar Cells, Vol.94, pp. 1463-1468, 2010 [3] O.S. Sastry, R. Chandel, R.K. Singh, R.B. Stephan, P.K. Dash, R. Kumar, B. Bandhopadhyay, Proceedings 26th European Photovoltaic Solar Energy Conference, 2011 [4] O.S. Sastry, A. Anand, R.K. Singh, A. Kumar, R. Kumar, B. Bandhopadhyay, Proceedings 26th European Photovoltaic Solar Energy Conference, 2011 [5] “Crystalline silicon terrestrial Photovoltaic (PV) Modules-Design qualification and type removal”, IEC 61215, second edition, 2008-09 [6] D.C. Jordan, S.R. Kurtz, Progress in Photovolataics: Research and Applications, 2011 [7] F.J. Pern, NREL, 1997 [8] M. Kasemann, D. Grote, B. Walter, W. Kwapil, T. Trupke, Y. Augarten, R.A. Bardos, E. Pink, M.D, Abbott, W. Warta, Progress in Photovolataics: Research and Applications, Vol.16, pp. 297-305, 2008 [9] J. Bauer, O. Breitenstein, J.M. Wagner, ASM International, Vol.3, pp. 6-12, 2009 [10] A.W. Czanderna, F.J. Pern, Solar Energy Materials and Solar Cells, Vol.43, pp. 101-181, 1996 CONCLUSIONS The maximum observed power degradation is less than 10% in all the modules for the duration of 21 years, which is much less than IEC 61215 standard that is why 3510