A review on the performance of photovoltaic modules against extreme weather environments

Structural analysis of photovoltaic modules

The basic service life requirement of photovoltaic modules is that "after 25 years of outdoor work, it can still maintain 80% of the maximum output power of the initial value, and it is also required that it can effectively resist the impact of external forces. Photovoltaic modules are mainly composed of solar cells, backplanes, photovoltaic glass, packaging materials, junction boxes, frames, etc.

The service life and power generation performance of photovoltaic modules are largely affected by environmental factors, such as oxygen, temperature, light, relative humidity, and external shocks. These are the main reasons for the failure of modules. Among them, backsheets, photovoltaic glass, packaging materials, etc. are the short boards to ensure the service life of photovoltaic modules. However, the backplane and packaging materials are highly dependent on the environment, and are easily affected by temperature and photo-oxidative aging phenomena, resulting in performance degradation. Therefore, the photovoltaic glass, packaging materials, and backplanes are analyzed and studied separately below.

1 Photovoltaic glass

The main function of photovoltaic glass is to protect solar cells from damage by various harsh factors, and to make use of the high light transmittance of the glass itself to make the absorption of light energy of solar cells unaffected as much as possible. Photovoltaic glass is tempered glass, which is an inorganic material. It is less affected by the environment, but is greatly affected by external force impact, and is easily broken due to wind pressure, hail and other impacts. If photovoltaic modules are used in the Antarctic region, the impact of strong winds and blizzards all year round can easily cause the photovoltaic glass to break, resulting in failure of its protective performance and affecting the safety and service life of photovoltaic modules. The density of glass is proportional to its probability of impact breakage, and the impact resistance can be improved by increasing the density of the glass itself. Therefore, appropriately increasing the proportion of silica in the glass raw material formulation and reducing the content of sodium oxide and calcium oxide can effectively improve the impact resistance of tempered glass, thereby effectively reducing the impact of strong winds, blizzards and other external forces in extreme environments. Risk of glass breakage.

Studies have shown that for every 1% increase in the conversion efficiency of solar cells, the power generation cost will be reduced by 7%, and the light transmittance of photovoltaic glass will affect the conversion efficiency of solar cells, which is also an important factor affecting the conversion efficiency of photovoltaic modules. Photovoltaic glass is a kind of soda-lime glass. If it is exposed to extreme humidity for a long time, it will hydrolyze to generate sodium hydroxide and silicic acid gel; while sodium hydroxide will corrode and damage the coating layer, and the silicic acid gel will stick. Attached to glass, both of which lead to a significant decrease in the transmittance of photovoltaic glass. At the same time, the strong ultraviolet radiation in the extreme climate environment will promote the oxidation and decomposition of organic matter on the surface of the photovoltaic glass film, causing the film to wrinkle, crack and fall off, and cause rainbow spots on the glass surface, which will reduce the transmittance of photovoltaic glass. . In addition, the water molecules entering the glass substrate through the film layer are more likely to freeze at extremely low temperatures, which will cause damage to the film layer; the impact of snow seeds and hail in extreme climate environments will also cause damage to the glass film layer, and eventually lead to a decrease in light transmittance. The failure effects of these environmental factors on photovoltaic glass will seriously affect the conversion efficiency and service life of photovoltaic modules.

According to the data, iron element can color the glass and reduce the light transmittance of the glass, while the rare earth metal cerium oxide (CeO2) has the functions of clarifying agent, decolorizing agent and anti-ultraviolet absorption. Therefore, in the manufacturing process of photovoltaic glass, adjusting the content of iron in the glass and adding an appropriate amount of CeO2 can not only improve the transmittance of photovoltaic glass, reduce its reflection and absorption of sunlight, but also reduce the transmittance of ultraviolet rays and protect the solar panels. Not being damaged by strong ultraviolet rays, it can effectively improve the UV radiation resistance of photovoltaic modules, and at the same time, it can also improve the service life and conversion efficiency of photovoltaic modules.

2 Packaging Materials

The role of the encapsulation material is to bond solar cells, copper tin ribbons, backplanes and photovoltaic glass together, and is a key component of photovoltaic modules. The main packaging materials are two-component silica gel, polyvinyl butyral resin (PVB), ethylene-vinyl acetate polymer (EVA) film, etc. At present, the most widely used EVA film in the photovoltaic industry is the EVA film containing 33% vinyl acetate, which has been used in the industry for more than 20 years.

As a polymer material, EVA is prone to deethylene reaction under strong ultraviolet irradiation, and produces acetic acid and olefin. Not only the decomposition rate of EVA is proportional to the UV intensity, but also the increase in the amount of acetic acid will accelerate the aging rate of EVA. The welding tape, backplane and electrodes of photovoltaic modules are also corroded by acetic acid. The deethylene reaction causes the color change of the EVA film, which gradually changes the photovoltaic modules from colorless and transparent to yellow or even dark brown, thus affecting the light transmission of the modules. efficiency and output power, resulting in a significant decrease in the conversion efficiency and service life of solar panels.

The glass transition temperature Tg and brittleness temperature Tb are the corresponding temperatures when the mechanical properties of polymers undergo morphological changes at low temperature. Among them, the glass transition temperature is directly related to the low temperature performance of the EVA film. Below the glass transition temperature, the EVA film is in a glass state, showing a certain degree of brittleness. Some experimental data show that the glass transition temperature of EVA film is 0-10 °C. When the temperature is below 0 °C, the EVA film gradually loses its elasticity and enters a rigid state. The brittleness temperature of the EVA film is -30 to -50 °C. When the temperature drops below the brittleness temperature, the EVA film shows brittleness, and a little external force and small deformation will damage it .

At this time, the EVA film only has mechanical impact resistance. Once it is impacted by external forces such as strong wind pressure, hail or transportation, it is easy to break, and the solar cells encapsulated inside it will crack or even break. At the same time, the low temperature environment will also seriously reduce the bonding performance of the EVA film, causing delamination of the photovoltaic modules. The polar structure of the EVA film for photovoltaics is weak, and it is prone to degradation and aging under strong ultraviolet radiation. The stability of EVA film is affected by its composition, as well as additives such as anti-aging agents, stabilizers, coupling agents, and cross-linking agents. Anti-aging agent can reduce the degradation and discoloration of EVA film by ultraviolet rays, stabilizer can increase the chemical stability and environmental adaptability of EVA film, coupling agent can increase the bonding strength of EVA film, and cross-linking agent can effectively improve the The volume resistivity and mechanical strength of EVA film, etc. Therefore, the low temperature resistance can be improved by adding an appropriate proportion of additives in the production process of EVA film.

3 Backplane

The photovoltaic backsheet is located on the back of the photovoltaic module and mainly plays the role of protecting and supporting the solar cell. As a polymer material used for the outermost large-area encapsulation of photovoltaic modules, photovoltaic backsheets are the most critical material affecting the service life of photovoltaic modules. At present, a commonly used backsheet in the photovoltaic industry is a TPT backsheet, which has a 3-layer structure, namely PVF (polyvinyl fluoride film)-PET (polyester film)-PVF structure. The outer layer of PVF has good resistance to environmental erosion, the middle layer of PET has good insulating properties, and the inner layer of PVF has good adhesion to EVA film after surface treatment . According to the data, the brittleness temperature of PVF and PET are both at -70°C. Since the fluorine-containing material PVF is thin, its low-temperature performance can generally meet extreme climate environments, while PET is thicker in the backplane structure, and its elasticity is low at extreme low temperatures. will be greatly reduced, resulting in a decrease in its ability to withstand external impact, resulting in cracks or wear, and the protection performance will also be affected. At the same time, the TPT backsheet is a polymer material. Under strong ultraviolet radiation, cracks in the outer protective layer will cause the middle layer to directly contact the outdoor environment, resulting in hydrolysis and photo-oxidative aging of PET, which will eventually lead to a decline in its protective performance.

Therefore, in addition to various balanced properties such as weather resistance, insulation, water vapor barrier, corrosion resistance and sand abrasion resistance, the TPT backsheet used in extreme climate environments also needs to strengthen low-temperature mechanical strength, Toughness and anti-aging properties, so that the photovoltaic modules can effectively withstand extreme weather environments for a longer time, and ensure that the service life and power generation performance of the modules are not affected.

4 Overall performance of PV modules

To sum up, by reviewing the performance of photovoltaic glass, encapsulation materials and backsheets of photovoltaic modules, various factors that may lead to failure of photovoltaic modules in extreme climate environments are explored. The results show that:

1) By adjusting the proportion of silicon dioxide, sodium oxide and tempered calcium in the formula of photovoltaic glass, the impact resistance of photovoltaic glass can be improved, thereby reducing the probability of damage to photovoltaic modules caused by external force; at the same time, controlling the content of iron and CeO2 in the glass, It can enhance the light transmission performance of photovoltaic glass, and ultimately improve the conversion efficiency of photovoltaic modules.

2) By adopting the modification technology for the EVA film of the packaging material, the occurrence of failure phenomena such as EVA ultraviolet aging and low temperature cold brittleness can be reduced.

3) Strengthening the low-temperature mechanical strength and toughness of the TPT backsheet can improve the protection performance of the backsheet for photovoltaic modules. By researching and analyzing the reasons for the failure of each component of the photovoltaic module, and proposing technical improvement measures, the weather resistance of each component can be greatly improved, thereby further improving the overall performance of the photovoltaic module against extreme weather environments, effectively reducing the photovoltaic power consumption. The probability of aging, damage, and failure of components after experiencing extreme low temperature, strong wind, snowstorm, strong ultraviolet radiation and other harsh environments, and making it possible to maintain high conversion efficiency.

Conclusion

Through a comprehensive analysis of the performance of each component of photovoltaic modules, this paper introduces the material properties of photovoltaic glass, packaging materials, and backsheets, and how to improve the extreme weather resistance of photovoltaic modules from each component, especially in alpine regions. The further application and promotion of photovoltaic power generation systems in polar regions provides certain guidance and reference.