Danny Song

The continuous development of the photovoltaic industry and the evolution of materials and technology have brought considerable advances to products. At the beginning of the 21st century, the power output of a single module was only 180 Wp, but now it can reach 700 Wp (see Figure 1). The increase in the power output of photovoltaic modules has contributed to the reduction of the LCOE (levelized cost of energy) of solar power plants and has raised the importance of photovoltaics among current energy sources. Figure 1 - Increase in the power of photovoltaic modules in recent years Since 2019, the size of photovoltaic cells has changed, first from 156,75 mm to 158,75 mm, then to 166 mm and now they reach 210 mm (Figure 2). With this increase in cells, the size of the modules also had a significant gain (see Table 1). Figure 2 - Increase in the size of silicon wafers (and cells) over the years The constructive structure of photovoltaic modules is made in layers (see Figure 3). Monofacial modules receive a polymer backsheet, while bifacial modules can have a clear polymer or glass backsheet. Figure 3 - Layered structure of the photovoltaic module. In the monofacial module, the glass represents approximately 72,3% of the total panel weight, while in the bifacial module, the glass (2,0 mm thick) represents approximately 82,96% of the total weight. Due to the weight of the glass itself, the center of the photovoltaic module may sag, as shown in the simulation result illustrated in Figure 4. The extent of this subsidence depends on many factors, the most significant of which are the panel area, type (mono or bifacial), angle and installation method. Figure 4 - Result of computer simulation of sagging of the photovoltaic module A study was carried out to evaluate the sagging of the module. Firstly, we sought to evaluate the influence of the module size. The boundary conditions are shown in Table 2 and Figure 5. Table 2: Simulation boundary conditions Type Height x Width (mm) Area（m2） Mono/Bifacial Installation Angle Method A 1754 x 1096 1.92 Monofacial 30° Fig. 5(a) Screw Fig. 5(b) B 2172 x 1096 2.38 Bifacial C 2172 x 1303 2.83 Bifacial Figure 5 - Angle and installation method In the simulation, the finite element analysis method was used to simulate the phenomenon of sinking of the center of the module, with the general algorithm Lagrange method for calculating the convergence iteration. The results are shown in Table 3 and Figure 6. Table 3: Deformation under different module sizes Module type Maximum deformation (mm) Type A 1,88 Type B 3,49 Type C 5,58 Figure 6 - Deformation with different module sizes Through simulation, it is clear that the deformation of the module center due to its own weight is a real problem – and the larger the area, the greater the deformation suffered by the module. Similarly, the amplitude of the center's sinking also varies with the installation angle. The module with the largest area (type C) is used to simulate subsidence at different installation angles (0°, 15°, 30° and 40°) – as shown in Figure 7. Figure 7 - Different installation angles used with the type C module sample Through the simulation, it can be concluded that with a decrease in the installation angle, the phenomenon of sinking in the center of the module becomes more intense. The module installed horizontally (0° angle) suffered the greatest deformation, as shown in Table 4 and Figure 8. Table 4. Deformations observed with different installation angles Sample Installation angle Maximum deformation (mm) Type C 0° 6,378 15° 6,177 30° 5,581 40° 4,936 Figure 8 - Maximum module deformations (type C sample) at different installation angles To evaluate If the central subsidence can affect the module's energy production, the boundary conditions that suffered the greatest subsidence are adopted, that is, the type C bifacial module with the largest area, installed at 0°, is selected and the load test Static mechanics is performed in accordance with IEC 61215, as shown in Figure 8. Figure 9 - Sample installation for mechanical load test In accordance with IEC 61215, the static mechanical load test sequence was conducted with 3 cycles, each carried out with a positive pressure of 5400 Pa and a negative pressure of 2400 Pa, as shown in Fig. 10. Figure 10 - Static mechanical load test Two samples were tested in accordance with the IEC 61215 standard and then power degradation was evaluated (with flash test) and EL (electroluminescence) images were obtained to check whether there was an occurrence of microcracks in photovoltaic cells after testing. It is worth mentioning that the dips measured in the test were even greater than those initially assessed through finite element analysis. From the comparison of the EL images, no microcracks were observed and no other visible flaws were found, as illustrated in Figures 11 and 12. Figure 11 - EL (electroluminescence) images of sample 1 – before (left) and after (right) the static mechanical load test Figure 12 - EL (electroluminescence) images of sample 2 – before (left) and after (right) of the static mechanical load test Power tests with flash light were carried out to compare the power degradation rate and the results were also satisfactory, as shown in Table 5. Table 5: Power degradation after static mechanical load test Through the results of the analysis performed, the following conclusions could be obtained: It is a fact that modules suffer sag, mainly in the central region, due to their own weight; The central sag of the module is mainly related to the type of module (monofacial - polymer backsheet or bifacial - glass backsheet), the area of the module and its installation angle; The sag of the bifacial module with two glass sheets (front and rear) is more evident; The larger the area of the module, the more pronounced the sag phenomenon; The smaller the installation angle, the greater the sag phenomenon; A module with the largest surface area was subjected to the static mechanical load test according to the IEC 61215 standard, which provides greater sag than that which can be obtained with the module's own weight. After this test, no reliability issues were found in the two samples tested; the deformation caused by self-weight does not affect the reliability of the photovoltaic modules, as it is less than that observed in the static load tests of the two samples analyzed.

Danny Song

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Deformation of photovoltaic modules due to their own weight