The general preparation process flow of EVA film is shown in Figure 1.
1.The main problems of EVA for solar cells in China
The main problems are heat resistance, ultraviolet aging, and poor light transmittance; the uniformity, stability, and consistency of the material are poor, and different process parameters must be used to match.
China began to import 7 monocrystalline silicon solar cell production lines from the United States in the mid-1980s, and imported EVA film from the United States. The National Science and Technology Commission included the Sinicization of EVA film in the “Eighth Five-Year Plan” research plan. Zhejiang Research Institute of Chemical Industry started research in the late 1980s and successfully developed the JM-E type EVA film in 1998. Its performance indicators such as light transmittance, bonding strength, and crosslinking degree basically reached the level of imported products, and subsequently built China Article 1 An EVA film production line with a high degree of self-control, an annual output of 300,000 m², and a width of more than lm. Later, the second-generation fast-curing adhesive film was introduced to the market, and the current annual production capacity is more than 1 million m². Hangzhou Foster Hot Melt Film Co., Ltd., Zhuji Fenghua Plastic Technology Co., Ltd., and Shenzhen Swick Technology Co., Ltd. also have a certain solar cell adhesive film production capacity.
Regarding the EVA film used for PV modules, in addition to the adhesion effect, the film also has the antireflection effect. Compared with glass/air/glass and glass/EVAV glass, the adhesive layer is conducive to the improvement of the photoelectric conversion efficiency of solar cells; Under 350mm ultraviolet light radiation, the light transmittance of glass is more than 80%, while the light transmittance in the glass/EVA glass adhesive layer is only 22%, which shows that the EVA adhesive layer can absorb a lot of ultraviolet light. Xu Xueqing of the Guangzhou Institute of Energy, Chinese Academy of Sciences and others evaluated the aging resistance of EVA for PV modules in terms of mechanical properties, infrared light harmonic analysis, causes of yellowing of EVA film, thermal oxidative aging and life estimation. The results show that the current performance of Chinese EVA is equivalent to that of imported EVA.
Despite this, the EVA encapsulating film currently used in China is still unsatisfactory. China has installed and worked in Tibet for 6 years and 10 years. PV power plants have yellowing of EVA. In particular, low-power concentration will accelerate the yellowing of EVA, even cracking and degumming, causing short circuits and scrapping the modules. Therefore, improving the aging resistance and thermal conductivity of the PV module sealant, and improving the packaging technology are important issues to be solved in the solar photovoltaic power generation industry. The reason for the yellowing is: the addition of antioxidants is beneficial to improve the thermal oxidative aging resistance of EVA. The addition of ultraviolet light absorber (UV531) and light stabilizer greatly improves the anti-ultraviolet performance. Ultraviolet light absorbers have the function of cutting off ultraviolet light, but many ultraviolet light absorbers react with free radicals to generate chromophore groups. These free radicals come from the degradation products of excess crosslinking agent.
2.Research on thermal conductivity of EVA film
The operating temperature of EVA film on the market is generally around 80°C, which is stable below 120°C, but when the temperature rises to 150°C or higher, the thermal performance of the EVA film drops rapidly, and it is wet Thermal degradation may occur under the conditions of, and the film will be severely yellowed, which greatly affects the service life of the battery. Therefore, how to improve the thermal conductivity of the EVA film has become one of the key factors for the normal operation of the CPV system. At present, there are two main ways to improve the thermal conductivity of polymers: ①Synthesis of structural plastics with high thermal conductivity, mainly through the electronic heat conduction mechanism to achieve heat conduction, but the manufacturing process using this method is more complicated and the cost is higher; ②Use The high thermal conductivity filler fills the plastic. This method is cheap and easy to process and shape. For filled thermally conductive plastics, the thermal conductivity of the material depends on the synergistic effect of the plastic and the filler. The shape of the filler dispersed in the matrix is granular, flake, spherical, fibrous, etc. When the amount of filler is small, although the filler is uniformly dispersed in the matrix, they cannot contact each other, and the thermal conductivity is not improved significantly; when the amount of filler is increased When reaching a certain critical value, the fillers contact each other to form a thermally conductive network chain. When the orientation of the thermally conductive network chain is consistent with the direction of heat flow, the thermal conductivity of the material is improved quickly. When there is no thermal network chain formed in the direction of heat flow inside the system, the thermal resistance will be large and the thermal conductivity will be poor. Therefore, in order to obtain a highly thermally conductive polymer, it is critical to form a thermally conductive network chain in the heat flow direction to the greatest extent within the material. Adding certain thermally conductive fillers (such as alumina, aluminum chloride, ammonium oxide, etc.) to the matrix resin with certain bonding properties and heat resistance can effectively improve the thermal conductivity of the polymer.