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A review of research on photovoltaic module materials' resistance to extreme weather conditions

A review of research on photovoltaic module materials' resistance to extreme weather conditions

2020-11-01

In today's era, human activities are spread all over the world, and energy supply is one of the difficulties that must be overcome if activities are to be carried out in extreme and complex climate environments. With the increasing depletion of traditional energy sources, new energy sources have become a new development trend, among which wind energy and solar power generation have become the first choice. For the construction of photovoltaic power generation projects in most areas, there is no need to consider the impact of extreme weather on photovoltaic modules. However, with the rapid development of global new energy and the increasing saturation of photovoltaic power generation projects in ordinary environmental areas, coupled with the extreme environment's own rich new energy resources and its demand for new energy development, photovoltaic power generation applications will inevitably become widespread.

photovoltaic module

The complexity of the working environment requires photovoltaic modules to have a very high ability to adapt to harsh environments. Research on the extreme weather resistance performance of the modules can not only improve the power generation performance and service life of photovoltaic modules in specific areas, but also provide photovoltaic power generation applications. Provide more possibilities. This article summarizes the factors and improvement measures that cause the failure of each component of the photovoltaic module under extreme climatic environments, in order to provide a reference for photovoltaic application research in extreme climate areas. Since the high temperature of the current environment is within the working temperature range that photovoltaic modules can withstand, this article only analyzes the extreme climate environment with extreme low temperature and strong radiation.


1. Structural analysis of photovoltaic modules


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

structural analysis of photovoltaic modules

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 impact. These are the main reasons for the failure of modules. Among them, the backplane, photovoltaic glass, packaging materials, etc. are the shortcomings to ensure the service life of photovoltaic modules. The backplane, packaging materials, etc. are highly dependent on the environment, and are susceptible to temperature and photo-oxidation aging, causing performance degradation. Therefore, the following analysis and research on photovoltaic glass, packaging materials, and backsheets are carried out separately.


1.1 Photovoltaic glass

photovoltaic glass

The main function of photovoltaic glass is to protect the solar cells from various harsh factors. The high light transmittance of the glass itself is used to make the solar cells' absorption of light energy unaffected as much as possible. Photovoltaic glass is tempered glass, which is an inorganic material, which is less affected by the environment, but is more affected by external impact, and is easily broken due to impacts such as wind pressure and hail. If photovoltaic modules are applied in the Antarctic region, the impact of year-round strong winds and blizzards can easily cause photovoltaic glass to break, leading to failure of its protective performance and affecting the safety and service life of photovoltaic modules. The density of glass is proportional to the probability of its impact resistance, and its 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 photovoltaic impact caused by strong winds and snowstorms in extreme environments. The risk of broken glass .

Studies have shown that for every 1% increase in the conversion efficiency of solar cells, the cost of power generation 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 be hydrolyzed to produce sodium hydroxide and silica gel; while sodium hydroxide will corrode and damage the coating layer, and the silica gel will stick. Attached to the glass, both will cause the light transmittance of photovoltaic glass to drop significantly . At the same time, strong ultraviolet radiation in extreme climates 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, and attenuate the light transmittance of photovoltaic glass. . In addition, the water molecules that enter the glass substrate through the film are more likely to freeze at extreme low temperatures, which will damage the film; the impact of snow seeds and hail in extreme climates will also cause damage to the glass film, and ultimately Cause the light transmittance to drop . The impact of these environmental factors on the failure of photovoltaic glass will seriously affect the conversion efficiency and service life of photovoltaic modules.

Data show that iron can color glass and reduce the light transmittance of glass , while rare earth metal cerium oxide (CeO2) has the functions of clarifying agent, decoloring agent and anti-ultraviolet absorption. Therefore, during the manufacturing process of photovoltaic glass, adjusting the iron content in the glass and adding an appropriate amount of CeO2 can not only increase the light transmittance of photovoltaic glass, reduce its reflection and absorption of sunlight, but also reduce the transmittance of ultraviolet rays and protect the battery. It is not damaged by strong ultraviolet rays, and while effectively improving the ultraviolet radiation resistance of photovoltaic modules, it can also improve the service life and conversion efficiency of photovoltaic modules .


1.2 Packaging materials

packaging materials

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

As a polymer material, EVA is prone to deethylene reaction under strong ultraviolet radiation, and produces acetic acid and olefin. Not only the decomposition rate of EVA is directly proportional to the intensity of ultraviolet light, but the increase in the amount of acetic acid will also accelerate the aging rate of EVA . The ribbons, backplanes and electrodes of photovoltaic modules are also corroded by acetic acid. The deethylene reaction causes the color change of the EVA film, which makes the photovoltaic module gradually change from colorless and transparent to yellow or even dark brown, which affects the light transmission of the module. The conversion efficiency and service life of the components are significantly reduced .

The glass transition temperature Tg and the brittleness temperature Tb are the corresponding temperatures when the mechanical properties of the polymer undergo a morphological mutation at low temperatures . 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 glassy and exhibits a certain brittleness . Experimental data shows that the glass transition temperature of EVA film is 0-10 ℃ . When the temperature is below 0 ℃, the EVA film gradually loses its elasticity and enters a rigid state. The brittleness temperature of EVA film is -30 to -50 ℃. When the temperature drops below the brittle temperature, the EVA film exhibits brittleness, and a small amount of external force and small deformation will cause it to be damaged .

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 cell encapsulated in 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 module. The polar structure of EVA film for photovoltaics is weak, and it is prone to degradation and aging under strong ultraviolet radiation, and it is prone to low temperature cold brittleness, cracking, and delamination under extreme climatic environments . The stability of EVA film is affected by its composition and additives such as anti-aging agents, stabilizers, coupling agents, and crosslinking 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 crosslinking agent can effectively improve The volume resistivity and mechanical strength of EVA film . Therefore, the low temperature resistance of EVA film can be improved by adding appropriate proportion of additives in the production process of EVA film.


1.3 Backplane

backplane

The photovoltaic backplane is located on the back of the photovoltaic module and mainly plays a role in protecting and supporting solar cells [20]. As a polymer material used for large-area encapsulation of the outermost layer of photovoltaic modules, photovoltaic backsheets are the most critical material that affects the service life of photovoltaic modules. At present, the most commonly used back sheet in the photovoltaic industry is the TPT back sheet, which has a three-layer structure, namely PVF (polyvinyl fluoride film)-PET (polyester film)-PVF structure. The outer layer of PVF has good resistance to environmental corrosion, the middle layer of PET has good insulation properties, and the inner layer of PVF has good adhesion to the EVA film after surface treatment [21]. According to the data, the brittle temperature of PVF and PET are both at -70℃. Because the fluorine-containing material PVF is thin, its low temperature performance can generally meet the extreme climate environment, while PET is thicker in the backsheet structure and has flexibility at extreme low temperatures. It will be greatly reduced, resulting in a decline in its ability to withstand external impacts, which will cause cracks or wear, and its 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, causing the PET to produce hydrolysis and photooxidative aging, and ultimately lead to a decline in its protective performance. [twenty two].

Therefore, TPT backplanes used in extreme weather environments need to have various balanced properties such as weather resistance, insulation, water vapor barrier, corrosion resistance and wind sand abrasion resistance [23], but also need to strengthen low temperature mechanical strength, Toughness and anti-aging performance, so that photovoltaic modules can effectively withstand extreme weather conditions for a longer time, and ensure that the service life and power generation performance of the modules are not affected.


1.4 Overall performance of photovoltaic modules


In summary, by reviewing the performance of photovoltaic glass, packaging materials and backsheets of photovoltaic modules, various factors that can cause photovoltaic modules to fail under extreme weather conditions are explored, and the results show that:

1) By adjusting the ratio of silicon dioxide, sodium oxide and calcium carbide in the photovoltaic glass formula, the impact resistance of photovoltaic glass can be improved, thereby reducing the probability of damage to photovoltaic modules caused by external forces; 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 modification technology for the packaging material EVA film, it can reduce the occurrence of failure phenomena such as EVA ultraviolet aging and low temperature cold brittleness.

3) Enhancing the low-temperature mechanical strength and toughness of the TPT backsheet can improve the protection performance of the backsheet for photovoltaic modules. Through the research and analysis of the reasons for the failure of each component of the photovoltaic module, and technical improvement measures are proposed, the weather resistance of each component can be greatly improved, so that the overall performance of the photovoltaic module to withstand extreme weather environments is further improved, and the photovoltaic module is effectively reduced. The probability of aging, damage, and failure of components after experiencing harsh environments such as extremely low temperature, strong wind, blizzard, and strong ultraviolet radiation, and allows them to maintain high conversion efficiency.


2. Conclusion

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


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