Fire Hazard Study Of Photovoltaic Modules Considering Structural Properties Of Multilayer Non-homogeneous Polymers

With the proposal of the national dual-carbon goal,the application scope of PV in China has gradually increased,and the proportion of PV power generation in the national energy structure has also gradually increased.The national level has repeatedly proposed to accelerate the construction of large-scale wind power photovoltaic bases in areas with sufficient sunshine and the construction of distributed photovoltaic systems throughout cities and towns.Compared with natural disasters such as wind,snow and hail,fire is also one of the potential safety hazards of photovoltaic power stations.The application of distributed photovoltaic modules in urban areas has gradually deepened,including portable photovoltaic power supply and mobile photovoltaic power supply,and fire accidents occur from time to time.Once a photovoltaic fire occurs,the thermal hazard and toxic gas hazard will bring great public safety hazards.In this paper,the fire risk of crystalline silicon photovoltaic modules,which are widely used at present,has been studied by means of experiment and theoretical analysis.According to the multilayer properties of photovoltaic modules made of multiple polymers,the pyrolysis characteristics of different packaging materials in each layer of photovoltaic modules,the pyrolysis and ignition characteristics of the whole module of multi-layer polymer structure,the toxic gas hazards associated with combustion,and the pyrolysis and ignition characteristics of photovoltaic modules under multi-factor conditions.The main contents of this paper are as follows:First,TG-FTIR was used to investigate the pyrolysis mechanism of packaging materials for photovoltaic components,and the pyrolysis gas composition was analyzed and discussed.The pyrolysis kinetics analysis method was used to reveal the relationship among heating rate,pyrolysis atmosphere and pyrolysis characteristics of the related materials.The pyrolysis kinetics parameters and reaction mechanism functions of the packaging materials were determined by the mode-free function method and the mode function fitting method,respectively.Among the three packaging materials,EVA had the lowest initial pyrolysis temperature(234℃),followed by PCB(256℃)and TPE backplane membrane(316℃)under the same conditions.Increasing the heating rate will cause the thermogravimetric curves of different materials to slip to the high temperature end.The shorter the reaction time required for polymer materials to reach the same temperature,the higher the starting temperature of pyrolysis will be with increasing heating rate.The pyrolysis atmosphere also has a great influence on the pyrolysis of materials and the gas composition.The starting temperature of pyrolysis of materials in nitrogen atmosphere is significantly higher than that in air atmosphere.The composition of pyrolysis gas varies greatly under different pyrolysis atmospheres.The main intermediate products are alkanes and olefins under nitrogen atmosphere,and the most final products are oxidized under aerobic atmosphere.Analysis by infrared absorption spectroscopy shows that the heating rate has little influence on the composition of pyrolysis gas.The determination of three kinetic factors provides basic data for subsequent pyrolysis model.Secondly,under stable air conditions,the pyrolysis ignition characteristics parameters of photovoltaic components with multilayer heterogeneous polymer structure were tested and analyzed experimentally.The traditional heat transfer model and pyrolysis model based on homogeneous materials were improved,and applied to the prediction of pyrolysis process of multilayer photovoltaic components.The effects of different external thermal radiation conditions on the photovoltaic components including mass loss rate,ignition time,critical ignition temperature and heat release rate are revealed.The results show that increasing the external heat flow intensity decreases the surface ignition temperature,significantly shortens the ignition time,and promotes the heat release rate.Based on the multilayer structure characteristics of photovoltaic modules,the minimum thermal resistance rule and the equal thermal conductivity rule are used to solve the uneven thermal performance of multilayer heterogeneous polymer materials,and the thermolysis model based on Neumann is applied.The problem of different pyrolysis parameters for multi-layered heterogeneous materials under the same time node was solved by the multiple accumulation calculation of spatial thickness,and the applicability of the modified pyrolysis model to the pyrolysis of photovoltaic multi-layered polymer materials was verified.Comparing the experimental data with the simulation results,it is shown that the ignition time of the photovoltaic module is in good agreement with the one-dimensional heat transfer model,and the pyrolysis rate of the photovoltaic multilayer structure is in good agreement with the one-dimensional pyrolysis model.The instantaneous concentration of toxic gases was quantitatively analyzed by synchronous combustion gas collection system with high precision gas composition sensor,and the human hazard was evaluated by toxic gases evaluation model.The results show that the mechanisms of thermal interpretation of toxic gases by burning various polymer materials in photovoltaic modules are different,and the rate of heat release has a great effect on the instantaneous concentration of toxic gases released.Finally,the pyrolysis ignition characteristics of photovoltaic components with multilayer heterogeneous polymer structure were studied under multivariate conditions,and the intrinsic influence mechanism of ventilation and heat radiation intensity on the ignition-related characteristics of materials was obtained.Under the influence of multiple factors,not only the external heat radiation intensity,but also the increase of air supply will reduce the risk of increasing the critical ignition temperature on the material surface.However,under low heat radiation,the change of air flow has a significant effect on the ignition time.Under the conditions of high heat radiation,the influence of air flow on ignition time is greatly reduced.For the rate of heat release,the influence of heat radiation intensity remains dominant under different air supply conditions.The multilayer material pyrolysis model under multivariate conditions was further improved to solve the heterogeneous pyrolysis problem of photovoltaic modules.When the overflow velocity is low(laminar),the change of convection coefficient on the heated surface has little influence on the pyrolysis of materials,which is mainly controlled by the thermal radiation intensity.Based on the assumption of surface absorption,one-dimensional pyrolysis model can better simulate the change of mass loss rate and heat release rate of materials under high heat radiation.The model has lower accuracy under low external heat radiation.The reason is that the advection heat transfer coefficient chosen for the air flow on the heated surface of the material is introduced as an empirical formula,and the effect of lower incident absorption rate on the pyrolysis time is prolonged.In this paper,the fire hazard of PV modules is quantitatively analyzed based on the structural and material characteristics of PV modules,and the simulation prediction is carried out through theoretical methods.The research results are helpful to strengthen the understanding of the hazard of PV fire,and have important guiding value for PV fire prevention and control and safety design.