In today’s era, human activities are all over the world. If we want to carry out activities in an extreme and complex climate environment, energy supply is one of the difficulties that must be overcome. With the increasing depletion of traditional energy, new energy has become a new development trend, in which the use of wind and solar power generation to obtain energy has also become the first choice. For the construction of photovoltaic power generation projects in most regions, it is unnecessary to consider the impact of extreme weather on photovoltaic modules. However, with the rapid development of new energy in the world, and the increasing saturation of photovoltaic power generation projects in general environment regions, as well as the abundant new energy resources in extreme environments and their demand for new energy development, photovoltaic power generation applications are bound to be widespread. For example, with the deepening of Antarctic scientific research activities, scientific research teams of various countries began to pay attention to the application of new energy in Antarctica and actively explore ways to use new energy efficiently. The Antarctic contains a lot of new energy. The rapid development of new energy technology has made the development of new energy in Antarctica a reality. Some scientific research stations have made full use of the wind and solar energy in the Antarctic, and have established new energy power stations in the Antarctic, mainly based on solar energy or wind energy.
However, Antarctica has a very representative bad climate: the wind speed in coastal areas can reach 45m/s, and 95% of the land area is covered with snow with a thickness of about 2km. The average annual temperature is – 25 ℃, and the minimum temperature can reach – 89.6 ℃, which is far lower than the extremely low temperature of – 40 ℃ in general areas. The data shows that the annual total solar radiation of Zhongshan Station in 1994 was 3788MJ/m2, and the annual ultraviolet radiation was 214MJ/m2. Such an extreme environment of extremely low temperature, strong wind and high radiation has put forward high requirements for the construction capacity of photovoltaic power plants and the performance of photovoltaic modules, the core components of their photovoltaic power generation systems.
The complexity of the working environment requires that photovoltaic modules have a very high ability to adapt to harsh environments. Research on the performance of modules against extreme weather environments can not only improve the power generation performance and service life of photovoltaic modules in specific regions, but also provide more possibilities for photovoltaic power generation applications. In this paper, the factors that cause the failure of PV modules in extreme climatic environment and the improvement measures are summarized, so as to provide a reference for PV application research in extreme climatic regions. Because the high temperature of the current environment is within the operating temperature range that PV modules can withstand, this paper only analyzes the extreme climate environment with extreme low temperature and strong radiation.
1. Current situation of climate and solar energy resources in China
The use of new energy is particularly important for polar scientific research. In particular, the official delivery of the “Xuelong 2” polar research vessel on July 11, 2019 has further improved China’s polar scientific research capacity, indicating that the demand for new energy in polar regions will further increase. Therefore, it is very necessary to study the adaptability of photovoltaic modules in extreme climatic environments.
China has a vast territory, and the climate and solar energy resources in different regions are quite different.
Mohe is located in the northernmost part of China, and is the county with the lowest temperature in China. Its annual average temperature is – 4.4 ℃, and the annual extreme minimum temperature is below – 38 ℃, which sets the extreme minimum temperature in China’s meteorological history – 52.3 ℃; The annual average solar radiation in this area is 4200~5400 MJ/m2, and the sunshine duration is 2377~2625 h. The extreme minimum temperature in winter in the plateau region of China is very low. For example, the extreme minimum temperature in the southern part of Qinghai and the northern Tibetan plain is below – 17.5 ℃ on average, and the extreme minimum temperature in the Tuotuo River and Qingshui River is below – 22.5 ℃, and the ultraviolet radiation in the plateau region is about 1.3 times that in the Mohe region. In terms of the coldest period in winter, the severity of the environment is similar to that of the polar climate. Therefore, how to solve the low temperature resistance performance of photovoltaic modules is the key problem to be solved in the construction of photovoltaic power plants and the application of photovoltaic power generation in these cold and power shortage areas.
2. Structure analysis of photovoltaic modules
The basic service life of photovoltaic modules is required to be “after 25 years of outdoor work, 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”. PV 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 force impact. These are the main causes of module failure, among which, backplane, photovoltaic glass, packaging materials, etc. are the short boards to ensure the service life of photovoltaic modules. The backplane and packaging materials are highly dependent on the environment, and are vulnerable to the effects of temperature and photooxidation aging, resulting in performance degradation. Therefore, photovoltaic glass, packaging materials and backplane are analyzed and studied in the following.
2.1 Photovoltaic glass
The main function of photovoltaic glass is to protect the solar cell from being damaged by various adverse factors, and make use of the high light transmittance of the glass itself to make the solar cell absorb light energy as unaffected as possible. Photovoltaic glass is toughened glass, which belongs to inorganic material and is less affected by the environment, but is more affected by external force impact, and is easy to break due to impact of wind pressure, hail, etc. If photovoltaic modules are applied in the Antarctic region, the impact of perennial strong winds and snowstorms will easily cause the photovoltaic glass to break, which will lead to the failure of its protection performance and affect the safety and service life of photovoltaic modules. The density of glass is in direct proportion to the probability of impact breaking resistance. The impact resistance can be improved by increasing the density of glass itself. Therefore, properly increasing the proportion of silicon dioxide in the glass raw material formula and reducing the content of sodium oxide and calcium oxide can effectively improve the impact resistance of tempered glass, thus effectively reducing the risk of PV glass breakage caused by strong wind, snow and other external shocks in extreme environments.
Studies have shown that every 1% increase in the conversion efficiency of solar cells will reduce the power generation cost by 7%, while 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 soda lime glass. If it is exposed to extreme humidity for a long time, it will be hydrolyzed to produce sodium hydroxide and silicic acid gel; While sodium hydroxide will corrode and damage the coating layer, and silicic acid gel will adhere to the glass, both of which will lead to a significant reduction in the transmittance of photovoltaic glass. At the same time, the strong ultraviolet radiation in extreme climatic environment will promote the oxidation and decomposition of organic matters on the surface of photovoltaic glass film, which will wrinkle, crack and fall off the film, and cause rainbow spots on the glass surface, which will weaken the transmittance of photovoltaic glass. In addition, water molecules entering 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 the extreme climate environment will also cause damage to the glass film, which will eventually lead to a decrease in the light transmittance. The failure of photovoltaic glass caused by these environmental factors will seriously affect the conversion efficiency and service life of photovoltaic modules.
The data shows that iron can color the glass and reduce the light transmittance of the glass, while 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 iron content 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 light, protect the battery from strong ultraviolet light, and improve the service life and conversion efficiency of photovoltaic modules while effectively improving the ultraviolet radiation resistance of photovoltaic modules.
2.2 Packaging materials
The function of packaging materials is to bond solar cells, copper tin strips, backplanes and photovoltaic glass together, which is a key component 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 EVA adhesive film in the photovoltaic industry is 33% vinyl acetate, which has been used in the industry for more than 20 years.
EVA, as a polymer material, is prone to deethylation under strong ultraviolet radiation, and produces acetic acid and olefins. Not only is the decomposition rate of EVA in direct proportion to the UV intensity, but also the increase of acetic acid will accelerate the aging rate of EVA. The welding strip, backplane and electrode of the PV module will also be corroded by acetic acid. The deethylation reaction will cause the color change of the EVA adhesive film, which will gradually change the PV module from colorless and transparent to yellow or even dark brown, thus affecting the light transmittance and output power of the module, resulting in a significant decline in the conversion efficiency and service life of the module.
The glass transition temperature Tg and brittleness temperature Tb are the corresponding temperatures when the morphology of the mechanical properties of the polymer changes at low temperatures. Among them, the glass transition temperature is directly related to the low temperature performance of EVA adhesive film. Below the glass transition temperature, EVA adhesive film is in a glassy state, showing a certain brittleness. Some experimental data show that the glass transition temperature of EVA adhesive film is 0~10 ℃. When the temperature is below 0 ℃, EVA adhesive film begins to lose elasticity gradually and enter into rigid state. The brittleness temperature of EVA adhesive film is – 30~- 50 ℃. When the temperature drops below the brittleness temperature, EVA adhesive film shows brittleness. A little external force and small deformation will damage it.
At this time, EVA adhesive film only has mechanical impact resistance. Once it is impacted by strong wind pressure, hail or transportation and other external forces, it is very easy to break, and the solar cells encapsulated in it will then have hidden cracks or even break. At the same time, the low temperature environment will seriously reduce the adhesion performance of EVA adhesive film, which will lead to the delamination of photovoltaic modules. The polar structure of EVA adhesive film for photovoltaic is weak, which is prone to degradation and aging under strong ultraviolet radiation, and low temperature cold brittleness, crack and delamination under extreme climatic environment. The stability of EVA adhesive film is affected by its composition, as well as anti-aging agent, stabilizer, coupling agent, cross-linking agent and other additives. The anti-aging agent can reduce the degradation and discoloration of EVA adhesive film by ultraviolet light, the stabilizer can increase the chemical stability and environmental adaptability of EVA adhesive film, the coupling agent can increase the bonding strength of EVA adhesive film, and the crosslinking agent can effectively improve the volume resistivity and mechanical strength of EVA adhesive film. Therefore, the low temperature resistance of EVA adhesive film can be improved by adding appropriate additives in the production process.