The cover material is laid on the top layer of the photovoltaic module, and has the properties of high light transmission, waterproof and moisture-proof and UV resistance. The cover material of some modules also has a certain self-cleaning performance. Two points need to be considered when selecting the cover material: one is the matching of the refractive index of the cover material and the bonding material to ensure that more light is irradiated on the surface of the solar cell and improve the efficiency of the module; the other is the strength and stability, which can Long-term protection of solar cells. The most common cover material is ultra-white embossed tempered glass, and some special occasions also use plexiglass or other flexible transparent materials.
1. Ultra-white embossed tempered glass
Glass is one of the most stable inorganic materials, can be used outdoors for decades without changing its properties, and has high mechanical strength, so it has become the first choice for photovoltaic module cover materials. Ultra-white embossed tempered glass, also known as low-iron embossed tempered glass, is named for its low iron content and high light transmittance. Transmittance. The iron content of ultra-white glass is less than or equal to 120×10-6. Figure 1 shows the comparison of spectral transmittance between 3.2mm ultra-white tempered glass and ordinary glass. In the wavelength range of 380~1100nm, the average transmittance of ultra-white glass is 91.7%, but the average of non-ultra-white glass is only 87%. . In order to further improve the light transmittance of glass, anti-reflection coated glass is now widely used in the industry.
By further reducing the reflection of light by the glass, the transmittance can be increased by more than 1.5%, which can improve the output power of the module. Tempered glass is obtained by first cutting the original glass into the size required by photovoltaic modules, then heating it to a temperature near the softening point of the glass, and then rapidly and uniformly cooling it. After tempering, uniform compressive stress will be formed on the surface of the glass, while tensile stress will be formed inside, so that the mechanical properties of the glass can be greatly improved.
Ultra-white tempered glass generally uses the embossing process to produce the original sheet, which is called cloth patterned glass. Patterned glass is made by melting the glass and rolling it with upper and lower rollers. The pattern on the front and back of the glass is controlled by the pattern of the upper and lower rollers. Usually, the side in contact with the air is the cloth surface, and the surface in contact with the EVA is the suede surface. The power output of the module can be increased by optimizing the shape of the suede. Usually, part of the light that hits the surface of the solar cell is absorbed, and the other part is reflected back. Due to the internal reflection between the EVA and the glass suede, the light reflected by the cell will be reflected to the surface of the solar cell again, which can increase the amount of light reaching the cell. The effective amount of light, thereby increasing the output current and output power of the components. The shape of suede can be generally divided into two types: quadrangular and hexagonal.
The thickness of glass commonly used is 3.2mm or 4mm. As the requirements for lighter components become higher and higher, 2.5mm or even thinner glass is available in the market. The main technical indicators of ultra-white low-iron embossed tempered glass are shown in Figure 2.
2. Coated glass
The glass material and structure directly determine how much light can be incident on the surface of the solar cell, thus affecting the power generation of the photovoltaic module. Therefore, how to improve the light transmittance of the glass and reduce the shading of the glass by dust has become the focus of the industry. If the light reflection on the glass surface can be reduced, the light transmittance can be effectively increased, thereby improving the power generation efficiency of photovoltaic modules.
The industry usually increases the light transmittance by etching specific structures on the glass surface or coating the glass surface with a low-refractive-index SiO: film. The latter has become a widely used technical means in the photovoltaic industry due to its simple process control, strong refractive index adjustability, and very suitable for industrial production. Common coating processes include magnetron sputtering, chemical vapor deposition and sol-gel method, among which the sol-gel method is widely used in the coating industry due to its simple production process and low equipment cost. After adding a layer of SiO2 film, the light transmittance of the glass can be increased by 1.8%~3.0%, as shown in Figure 3, thereby improving the power output of the photovoltaic module.
In addition to improving the light transmittance of the glass, the glass surface coating can also achieve the function of self-cleaning. In the actual use environment of the module, the glass surface is easy to accumulate dust, which will affect the output power of the module and the power generation of the system. Some data show that it is very common for ash deposition to affect power generation by more than 8%. Therefore, various types of coated glass with a certain self-cleaning function have appeared on the market. The main principle is to use nanomaterials to change the surface structure and surface tension of the glass. In addition to It has a certain light trapping effect, and can also make the glass surface superhydrophobic, superhydrophilic or have photodegradation function, so that dust and other pollutants are not easy to adhere to the glass surface, or even if they adhere, they will be washed by rainwater. It is very easy to detach from the glass surface, so as to achieve the purpose of increasing power generation. As long as the performance is stable and the price is moderate, photovoltaic glass with self-cleaning and anti-reflection functions can be widely used by photovoltaic module companies.
Due to the complex and diverse installation environments of photovoltaic modules, including deserts, fields, roofs, seaside, saline-alkali land, high-altitude areas, areas with heavy snow cover, etc., this puts forward high requirements for the reliability of coated glass. The early coating structures were all open-pore structures. Now, after optimization and improvement, closed-cell structures have been adopted, and the reliability has been greatly improved, which can basically meet the requirements of long-term outdoor use.
At present, the thickness of glass used in photovoltaic modules is no longer limited to 3.2mm, 2.8mm, 2.5mm, 2mm, and even 0.85mm glass has begun to be used, and thinner and thinner glass has brought great challenges to the coating process. The current coating process can already be applied to physical tempered glass above 2mm, and can also be applied to embossed and float glass.
3. Chemical tempered glass
The demand for lightweight components has put forward higher and higher requirements for ultra-thin glass. At present, glass with a thickness of less than 1mm has appeared. However, ultra-thin glass has brought about a reduction in mechanical strength, and has At the same time as the thickness, impurities, defects, and any negative factors that reduce the strength of the glass are amplified. It is very difficult for ultra-thin glass to adopt the traditional physical tempering process. At present, most of the ultra-thin glass adopts the chemical tempering process. Some glass manufacturers represented by Japan’s Asahi Glass have launched chemically tempered glass suitable for photovoltaic modules and building materials. At present, the thickness can reach 0.85mm, and the size can also meet the needs of mainstream module sizes, and can achieve mass production, but the cost is still relatively high. high.
Chemically tempered glass mainly adopts a low-temperature ion exchange process. In an alkali salt solution at about 400 ° C, the ions with a smaller radius in the glass surface are exchanged with ions with a larger radius in the solution, and the difference in the volume of alkali ions is used to form on the glass surface. Embedded extrusion stress. Chemically tempered glass is not subjected to a high temperature process above the transition temperature during the manufacturing process, so it will not warp like physical tempered glass, and the surface flatness is consistent with the original glass, while improving glass strength and temperature resistance. Appropriate cuts can be made. The disadvantage of chemically tempered glass is that the strength of the glass will decrease to a certain extent over time, so it needs to be fully evaluated when using it.
Plexiglass is a polymer compound polymerized from methacrylate. Because of its light weight and not easy to damage, it is also used as a component cover material in some occasions. Plexiglass is the best polymer transparent material at present, and the light transmittance can reach 92%. Its tensile and impact resistance is 7~18 times higher than that of ordinary glass, and the weight of Plexiglass of the same size is only that of ordinary glass. half. The elongation at break of plexiglass is only 2% to 3%, so the mechanical properties are basically a hard and brittle plastic, and it has notch sensitivity and is easy to crack under stress. When the temperature exceeds 40°C, the toughness and ductility of the material will be improved.
Plexiglass has good dielectric and electrical insulation properties. Under the action of the arc, the surface will not produce carbonized conductive paths and arc tracks. However, due to its high cost, low surface hardness, easy to scratch, and poor weather resistance, so Only for some special occasions. Plexiglass can be divided into four categories: colorless and transparent Plexiglas, colored and transparent Plexiglas, pearl luster Plexiglass, and embossed Plexiglass. Usually, colorless and transparent Plexiglas is used in the production of photovoltaic modules, but Plexiglass is currently expensive due to its high cost. Not yet available in bulk.
5. polyvinyl fluoride
Polyvinyl fluoride film is also suitable for the front surface encapsulation of photovoltaic modules, such as transparent PVF, ETFE and a series of improved materials, of which ETFE is the most common and reliable material for thin film module encapsulation.
ETFE, namely ethylene-tetrafluoroethylene copolymer, is a material with anti-aging, self-cleaning, corrosion resistance, flexibility, tear resistance and flame retardancy. It is usually used as a surface encapsulation material for flexible components. ETFE not only has good heat resistance, aging resistance and corrosion resistance of PTFE, but at the same time, due to the addition of ethylene, its melting point is lowered, so it is easy to process, and the mechanical properties are also improved. Most importantly, the bonding performance is also greatly improved. At present, it is generally used as a front plate material in flexible photovoltaic modules.
At present, ETFE materials mainly come from DuPont in the United States and Asahi Glass in Japan. Due to its high cost, the usage is small.