Backplanes are a key part of crystalline silicon solar modules and play a vital role in component safety, service life and reduced power attenuation. To achieve the purpose of protecting the battery, the backboard needs to have various balance properties such as good mechanical strength and toughness, weather resistance, insulation, moisture barrier, corrosion resistance and sand abrasion resistance. In recent years, under the pressure of cost reduction, some backplane materials that have not been verified by outdoor performance have been used on components, resulting in large-scale failure problems that have occurred in the outdoor work after only a few years of working outdoors. Component manufacturers need to face huge claims, and power plant developers suffer losses. It can be seen that it is important to reduce the risk of outdoor failure of the component from the source.
The backsheet structure can be roughly divided into an outer layer (also called an air layer), an intermediate layer and an inner layer. The choice and matching of each layer of material affects the overall performance of the backsheet. Currently, the outer layer of the backsheet mainly uses fluorine-containing film, especially the DuPontTM Tedlar® film which has been verified by outdoor performance to ensure that the outer layer of the backboard is outdoors. It can be used for more than 25 years under the effect of comprehensive aging stress. The inner layer of the backsheet is often overlooked because it does not directly contact the outdoor environment stress. Moreover, there are many types of inner layer materials on the back sheet of the market, and the performance is also uneven. How can I choose the right inner layer material correctly? Before answering this question, it is necessary to understand what performance is needed for the inner material of the lower back panel.
Backsheet inner layer material properties
First of all, the inner layer material of the back sheet needs to have excellent weather resistance and mechanical strength. If the inner layer material itself has undergone aging cracking, the protection effect cannot be discussed.
Secondly, the inner layer of the back sheet is one of the protective layers of the intermediate layer PET polyester material, and it needs to have a good ultraviolet blocking effect to avoid the UV damage of the PET. Figure 1 is a 250-micron PET UV test data commonly used in the market. It can be seen from the figure that the PET polyester material can reduce the elongation at break by more than 50% after receiving less than 5 kWh of UV irradiation. At a dose of 6 kWh, the mechanical properties of PET were almost completely lost (mild climate, with an annual UV dose of 57 kWh/m2 on the front of the module). This indicates that PET is prone to photoaging and requires protection of the inner and outer layers of the backsheet. The PET polyester material in the middle layer of the back sheet mainly functions as electrical insulation, chemical barrier and mechanical support. If the PET breaks, these functions will be lost, so the inner layer of the backsheet needs to block the UV of the front side of the component to protect the intermediate layer PET.
In addition, the inner layer of the back sheet is used as a bonding layer between PET and EVA, and also needs to have good bonding properties to avoid the risk of failure such as delamination.
Knowing the performance of the inner layer of the backplane, we can't help but ask, can the inner materials on the market meet these properties? Let's reclassify these inner materials on the market and discuss their strengths and weaknesses.
Backsheet inner layer material classification and its characteristics
The first category is a fluorine-containing film, mainly Tedlar® PVF film and PVDF film. PVF film, also known as polyvinyl fluoride film, is widely used in solar energy, aerospace and transportation due to its excellent UV resistance and high temperature corrosion resistance. DuPontTM Tedlar® PVF film is the only backsheet material in the photovoltaic field with more than 30 years of extensive outdoor performance verification. It has rich experience in outdoor applications and has been tested for many long-term climatic conditions. PVF film is prepared by biaxial stretching process. It is strengthened in both the transverse direction and the longitudinal direction. It has balanced mechanical properties and good aging resistance. Therefore, it is excellent after being subjected to various environmental factors such as damp heat, ultraviolet and temperature cycles. PVDF film, also known as polyvinylidene fluoride film, PVDF film is weak in the transverse direction or even stretched, which is easy to cause poor transverse mechanical properties. Adding a large amount of acrylic during the film formation process will also result in strong inherent brittleness. . These factors lead to the risk of failure of cracks such as cracking of PVDF films under various outdoor stresses. Due to its relatively low technical threshold, there are many manufacturers at present. Although each membrane product contains PVDF, the aging resistance of membranes of different manufacturers varies greatly due to different formulation systems and production processes. For heat resistance, the softening temperature point of the PVF film is 190oC, while the PVDF is only about 150oC. For PV module applications where hot spots often occur, the heat resistance of PVF films is obviously more advantageous. With the mass production of high-efficiency batteries such as PERC, the hot spot temperature will be higher, and the heat resistance of the film will be more High requirements.
The second category is non-fluorinated film, mainly including PE, EVA, PA, PO, and the like. This type of material has been tested for outdoor use in the mild climate for a certain period of time. The higher thickness has certain advantages in terms of UV resistance, mechanical properties and adhesion. The aging properties of the inner layer material are closely related to the type and content of the main resin and the inorganic filler. Different backsheet manufacturers choose their inner layer materials to combine their own positioning and characteristics, which also leads to a large difference in the performance of the selected materials. At the same time, the combination of different weather-resistant backsheet materials also determines the performance of the inner layer material. Figure 2 shows the inner layer yellowing of the inner layer of a PVDF backsheet in less than 5 years outdoors. Figure 3 shows the front side of the module with a UV dose of 540 kWh/m2 (equivalent to 9.5 years of sunlight on the front of a mild environmental component), and the inner layer of the hydrolysis-resistant HPET 1 polyester backsheet material is cracked, corresponding to Tedlar®. There is no change in the TPE backsheet material of the PVF film. From the above outdoor actual cases and indoor aging test results, it can be seen that for such backing plate inner layer materials, the outdoor-proven PVF film is used as the back plate outer layer as much as possible.
The third type is coated FEVE coating. FEVE is a copolymer resin of fluoroolefin and vinyl ether. As a backing inner layer material, it has the advantage that weather resistance and high temperature resistance are better than E layer, and it is not necessary. The glue layer is applied directly to the PET surface, eliminating the cost of glue. Due to the special structure of FEVE, it is soluble in solvents such as esters and ketones. However, the weaker ester bond in FEVE is relatively easy to crack, and the performance of the coating is greatly affected by the monomer, solvent and curing agent. Compared with the first two types of inner layer materials, the weather resistance and compactness of FEVE coatings are inferior to those of fluorine membranes. The adhesion and mechanical properties are inferior to those of E layers, and the outdoor verification time is relatively short. It is not recommended to have large temperature difference and cold and thermal stress. Used in larger weather conditions. In order to protect the intermediate layer PET from UV damage, the thickness of the coating is critical, the coating is too thin, and UV and adhesion are problematic. The relationship between the thickness of the barrier layer and the UV transmittance is generally in accordance with Beer's law. The data in Figure 4 shows that if the thickness of the UV barrier layer in the inner layer of the backsheet is less than 10 microns, the UV light begins to penetrate the barrier layer to reach PET, and the penetration ratio varies with thickness. Thinning and an increase in index, if the coating thickness is 1 micron, the ultraviolet transmittance at 365 nm will be as high as 11%, which will cause devastating damage to the intermediate layer PET. The mechanical property test data of Figure 5 further demonstrates that after 1000-1380 hours of UV irradiation, if the inner layer thickness is <10um, the elongation at break of the backsheet will decrease significantly. In order to reduce the cost, some backplane manufacturers have reduced the thickness of the inner coating of the backsheet to a very low level (as shown in Figure 6, the inner coating thickness is only 1.3 microns), which easily leads to UV aging of the backsheet PET. . This causes delamination of the inner layer of the backsheet and the intermediate PET layer. In addition, the addition amount of titanium dioxide in the layer is also important, and some coatings add too much titanium white powder, resulting in a decrease in the adhesion of the coating, which is prone to delamination.
Recommendations for the selection of the inner layer of the backsheet
Many outdoor failure cases in photovoltaic power plants show that the true weather resistance of different types of backplanes is quite different, especially in harsh environments. With the consumption of UV absorbers in EVA, the UV-penetrating components reach the inner layer of the backsheet, which is easy to cause back. The destruction of the board. In order to provide long-term reliable protection for PV modules to ensure a return on investment, it is recommended that the inner layer also use Tedlar® PVF film, a backsheet material with more than 30 years of extensive outdoor performance verification.
Double Pole Miniature Micro Switch
Features
â—† Small Compact Size,Global safety approvals
â—† Variety of lerers
â—† Multi circuit function
â—† Long life & high reliability.
â—† Wide Range of wires Terminals.
â—†Variety of actuator and terminals
â—†Customized Designs
â—†Widely used in household appliances, electric force, telecommunications, machine tool, vessel, textile, printing, Mining machine, Mustic Instrument, etc.
Double Pole Micro Switch,Double Pole Miniature Micro Switch,Double Pole Subminiature Micro Switch,Double Pole Double Throw Micro Switch
Ningbo Jialin Electronics Co.,Ltd , https://www.donghai-switch.com