Abstract: Select a batch of single crystal modules that have been used in my country for 20 years and have reliable quality, and analyze the power attenuation at the end of their service life. It is found that the average power attenuation is 26.7%, and the annual average attenuation rate is 1.54%; using a simple linear fitting function method, the power attenuation, the current attenuation and the fill factor attenuation are fitted, and it is found that the power attenuation The decrease is mainly caused by the decrease of the fill factor; the distribution law of cracks and debris in the module cells is studied and analyzed, and it is found that cracks and debris are not the main factors causing the power attenuation of the module; combined with the environmental conditions of the module before service, the power attenuation of the module is analyzed and inferred Mainly due to the increase in series resistance.
       In recent years, with the strong support of the national policy to the photovoltaic industry, photovoltaic power generation has gradually become a stable part of the hybrid power system.The warranty life of photovoltaic modules is generally 20 to 30 years, and the reliability and durability of the modules directly determine the overall performance of the photovoltaic system.What is the actual service life of photovoltaic modules? What is the power attenuation during the warranty life of the modules? This requires a lot of empirical data as support.
       Conducted outdoor aging tests on 32 mainstream photovoltaic modules, analyzed the attenuation of modules within two natural years, and simulated the power attenuation of these modules in 25 years according to the model of the linear relationship between module power attenuation and time proposed by NERL laboratory. The rate is 8% to 14%.Reference [4] conducted an accelerated aging simulation experiment on photovoltaic modules based on the environmental conditions in which the modules operate in the northwest desert region, and found that the attenuation trend of the modules in the aging box was consistent with the long-term monitoring attenuation law of NERL outdoor modules.Reference [5] adopts the wet-freeze accelerated test to analyze its failure mode, and proposes a Weibull lifetime distribution model of photovoltaic modules under the wet-freeze accelerated test condition.The software simulation calculation and accelerated aging test are of great significance for predicting the stability and reliability of photovoltaic modules, but they cannot completely and accurately reflect the real attenuation of photovoltaic modules during service.
       In this paper, a batch of single crystal modules with the earliest running time and reliable product quality in my country are selected to study the power attenuation at the end of their life, and explore and analyze the main reasons for the power attenuation of the modules.These single crystal modules have been operating in outdoor power generation for 20 years, and the empirical data at the end of their service life will have certain reference value for the research and modeling of module attenuation mechanism.
       Siemens Solar monocrystalline module SM55 was produced in 1996 and installed in the coastal area of Shenzhen (less than 500m from the ocean) in 1997, with a total of 2051 pieces.In 2014, it was all collected by the Solar Energy Research Institute of Sun Yat-sen University, Shunde, for research on module attenuation.Among them, 132 modules were re-installed in Xinyang Normal University in January 2016. As a warm-temperate module outdoor attenuation demonstration base, it is mainly used to study the re-life of old modules and the end-stage attenuation law under warm-temperature monsoon climate conditions.By 2018, this batch of monocrystalline modules has been operating normally outdoors for 20 years, and the service life is close to 25 years of commercial warranty. At present, the modules can still continue to work normally without any problems affecting safe use.
       The appearance of this batch of single crystal modules (Figure 1a) is good, the surface is not damaged, the EVA is not discolored, the junction box is intact, some module lead wires have been replaced due to damage to the MC4 connector, and the module backplane is not damaged or bubbling.However, the components have some appearance defects: (1) A layer of stains that cannot be washed off on the glass surface (Fig. 1b), (2) There are white spots near the solder ribbon, which may be EVA bubbles or flux falling off (Fig. 1c), (3) The backplane is powdered (Fig. 1d), and (4) the aluminum frame has obvious salt spray corrosion (Fig. 1e).Defects (1) and (2) may cause optical loss to affect the output performance of the module, and defects (3) and (4) may affect the safe use of the module.
       Due to the influence of artificial factors such as handling, installation, construction, or sudden changes in ambient temperature, components operating outdoors usually cause two kinds of internal defects: cracks and fragments, but these two defects are generally invisible to the naked eye and can only be passed through Only the solar module EL defect detector can detect it.The EL test in 2018 found that among the 132 modules, 38 were cracked and 21 were fragmented, accounting for 28.8% and 15.9% of the total number of modules respectively.Further statistics of the crack and fragment area and the corresponding number of components, the crack and fragment area is divided into two areas (as shown in Figure 2), the number of crack and fragment components in the two areas is shown in Table 1.
       The monocrystalline module SM55 has a total of 36 cells, the number of cells in area 1 is 26, and the number of cells in area 2 is 10. If the probability of cracks or fragments appearing in each cell is the same, then there are hidden cracks in area 1 and area 2. The ratio of the number of cracked or fragmented components should be close to 26:10, but it can be seen from Table 1 that the ratio of the number of cracked components in Zone 1 and Zone 2 is 26:12, and the ratio of the number of fragmented components is 14:7, both less than 26:10. It shows that the location of cracks or fragments is likely to occur in area 2, that is to say, the cells in the middle of the module are more prone to cracks or fragments under the influence of external force or ambient temperature.
       Further analysis of the cracks and fragments of the modules found that the failure area caused by cracks did not exceed 1/8 of the area of each cell, the failure area caused by fragments did not exceed 1/4, and the number of cells with cracks and fragments in the module did not exceed 1/8. More than 2 pieces, Figure 3 shows several cracks and fragments that cause a larger failure area.
       To test the electrical properties of these modules under STC conditions, the Swiss Pasan photovoltaic module power tester (3A+ solar simulator) was used.Due to the lack of the measured original data of the module power, this paper takes the module nameplate parameters as a reference.Due to the selection of standard components in the power test process, the accuracy of the test instrument itself and the influence of the test environment, relative to the nameplate parameters, it will bring certain systematic errors and human errors to the results of the power test. Therefore, this paper takes the average value through multiple measurements. method to ensure the accuracy of the test results.
       The test results in 2018 show that the average attenuation of module power Pm is 26.7%, the maximum attenuation is 35.9%, and the minimum attenuation is 19.6%.
       From Table 2, it is known that the current Isc and Im, the maximum power point voltage Vm and the fill factor FF of the single crystal module SM55 are seriously attenuated, while the open circuit voltage Voc remains basically unchanged.Since the module has been operating outdoors for 20 years, the annual average attenuation rate of each electrical performance parameter can be calculated. 0.02% and 0.89%.
       In order to further analyze the relationship between Pm attenuation and Isc and FF attenuation, a simple linear fitting function was used to fit the effects of Isc attenuation and FF attenuation on Pm attenuation respectively, and the obtained correlation coefficient R2 was used as the criterion for judging the quality of the relationship. ,As shown in Figure 4.
       It can be seen from Figure 4 that the Pm attenuation is significantly positively correlated with the FF attenuation, and the correlation coefficient is 0.63, indicating that the decrease in Pm is mainly caused by the decrease in FF, and the most likely reason is the increase in series resistance [6].It can also be seen from Figure 2 that there are many "bright spots" in the EL imaging of the component, and they are all distributed on the ribbon. It is likely that the delamination of the ribbon and the corrosion of the Ag electrode lead to an increase in the series resistance of these points. Therefore, in the EL test When a medium and large current is passed, the point where the series resistance increases will instantly overheat and present a "bright spot".
       According to whether cracks and fragments cause module power attenuation, the power attenuation of modules with cracks or fragments, modules without cracks and fragments, and all modules is analyzed, as shown in Table 3.
       It can be seen from Table 3 that the power attenuation of these three components is basically the same, indicating that during the power test process, cracks and fragments are not the main reasons for the power attenuation of the single crystal module SM55, but may be caused by cracks and fragments. The failure area is not enough to cause the attenuation of the component power.However, from the perspective of the long-term operation of the modules, the cracks in the cells will become fragments after continuous thermal expansion and contraction under the influence of the outdoor environment, which will eventually lead to hot spots of the modules, which directly affect the output performance of the modules and reduce the photovoltaic power station. system efficiency.
       References [6, 7] describe the factors that cause the optical loss and electrical loss of the component and how to optimize the process to reduce these two losses from the perspective of component packaging. There are many types of optical losses, including the reflection of the interface, the absorption of the packaging material, The shielding and non-cell areas of grid lines and welding ribbons; electrical losses include the power loss caused by the resistance of the battery itself, the resistance of the welding strips and the resistance of the junction box.For modules that operate outdoors for a long time, the literature [8] pointed out that the optical loss of the module mainly comes from the discoloration of the EVA packaging material and the contamination of the glass surface, which is ultimately manifested as the reduction of the module currents Isc and Im; the electrical loss mainly comes from the battery itself or the battery connection. Some internal resistance changes.Since the electrical loss in the module power test results is complex, the relationship between the electrical loss and the photovoltaic cell power will be described below.
       Under light conditions, the output of photovoltaic cells can be approximated as diode characteristics [9, 10]. Figure 5 shows the single-diode output model of photovoltaic cells.
       Where: Ip is the photo-generated current, ID is the diode forward current, ID0 is the diode reverse saturation current, q is the electron charge, A is the diode constant factor, K is the Boltzmann constant, T is the battery junction temperature, and Rs is the series connection resistance, Rsh is a parallel resistance.
       The diode constant factor A is a constant, and the general value range is [1, 1.5], so Fig. 6 only shows the influence of the model parameters Rs and Rsh on the IV curve.
       The series resistance Rs has a great influence on the shape of the IV curve near the maximum power point (Fig. 7a), and the efficiency of the photovoltaic cell decays approximately exponentially with the increase of the series resistance Rs. It affects the fill factor FF, but has no effect on the open-circuit voltage and short-circuit current [11]; the parallel resistance Rsh is the linear resistance introduced by the leakage degree of the photovoltaic cell and the contamination of conductive ions, which affects the slope of the IV curve at the short-circuit current (Fig. 7b). ), the larger the parallel resistance is, the more parallel the curve is to the voltage axis near the short-circuit point. Under normal circumstances, the larger the parallel resistance, only affects the fill factor FF, but has no effect on the open-circuit voltage and short-circuit current [12].
       The power attenuation of the single crystal module SM55 is serious, the current Isc and Im, the maximum power point voltage Vm and the fill factor FF are seriously attenuated, while the open circuit voltage Voc remains basically unchanged, which indicates that the power attenuation of the module mainly comes from electrical losses, which are mainly caused by the power loss of the module. The increase in the series resistance Rs is caused by the increase of the series resistance, and at the same time, the attenuation of the current is also partly caused by the optical loss, because during the test, it is found that there is a layer of stains on the glass surface of the module that cannot be wiped off.Since the first 18 years of the single crystal module SM55 were operated outdoors in the coastal area of Shenzhen (less than 500 m from the ocean), it was only operated outdoors in the Xinyang area for 2 years (as of 2018), so this batch of modules was mainly affected by salt in coastal areas. Fog erosion is serious, resulting in an increase in series resistance.
       The appearance defects of the SM55 module do not affect its safe use temporarily; the cells in the middle of the module are more prone to cracks or fragments under the influence of external forces or ambient temperature, but the cracks and fragments have not affected the power of the SM55 module for the time being.The power attenuation of SM55 modules mainly comes from electrical losses, and a small part comes from optical losses. Since SM55 modules have been operating in the coastal area of Shenzhen for a long time, they are seriously eroded by salt spray in coastal areas, resulting in an increase in series resistance.
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