Study On Fire Characteristics Of Building-integrated Flexible And Rigid Crystalline Silicon Photovoltaic Modules

In order to alleviate the energy crisis caused by over-exploitation of fossil fuels,many countries in the world have launched new energy development strategies.Among them,solar energy has been widely concerned and developed due to its properties of universal illumination,permanent energy,non-pollution,safety and reliability.As an important form of solar energy application,building-integrated photovoltaic(PV)systems formed by integrating photovoltaic arrays onto building surfaces(building facades and roofs)have achieved rapid development in recent years.However,while these PV systems provide energy for buildings,they also bring new challenges to building fire prevention.In the past few decades,fires in building photovoltaic systems around the world have been frequent,causing serious personal injury and property damage.Statistics of these fire cases show that photovoltaic systems installed on the surface of the building generate a large amount of heat and toxic fumes under fire conditions,hindering fire fighting and rescue operations.At the same time,these photovoltaic modules change the way the fire spreads outside or through the building,and the heat feedback mechanism between them and the building surface further exacerbates the building's own combustion.Therefore,it is of great theoretical and practical value to study the heat and smoke release characteristics of building PV modules and their impact on building fire safety.This paper focuses on the pyrolysis behaviors,combustion characteristics and system fire safety of typical crystalline silicon PV modules.Combining experimental observation with theoretical analysis,the fire risk of building-integrated PV systems is studied,including the following three aspects:First,the pyrolysis characteristics of the combustible materials inside the crystalline silicon PV module,ethylene-vinyl acetate(EVA)and Tedlar film?Polyethylene terephthalate-Tedlar film(TPT),were analyzed by Thermogravimetric-Fourier Transform Infrared Spectrometry(TG-FTIR).It is found that both EVA and TPT have two pyrolysis stages.The model-free methods including Kissinger-Akahira-Sunose and Flynn-Wall-Ozawa methods were applied to estimate the activation energy values at different conversion degrees.The reaction mechanisms were determined employing the Coats-Redfern and Criado methods,both of which are model-fitting methods.The three-dimensional diffusion and first-order reaction models are responsible for the two EVA pyrolysis stages,respectively,while the nucleation and growth and three-dimensional diffusion models play important roles during the two pyrolysis stages of TPT.The FTIR analysis of the volatiles presents that the six main components produced have the following order:CH2 deformation(i.e.—CH2—and?CH2)>acetic acid>CH4>CO2>CO for EVA.During TPT degradation process,the amount of the gas components is acetaldehyde>ester>CO2>phenyl>ether>hydroxyl compounds or carboxylic acids>-CH2->HF>=C-H stretching.In addition,the similarity of infrared spectra under various heating rates indicates that the influence of heating rate on gas components is weak.Secondly,the ignition and combustion characteristics of flexible and rigid crystalline silicon photovoltaic panels were studied by cone calorimeter.The incident heat flux of the radiation source is 18,20,26,30,40,50,60 and 70 kW/m2.In the experiment,the combustion characteristics parameters such as ignition temperature and time,mass loss rate,heat release rate and toxic gas concentration were emphasized,and mathematical models for predicting these parameters under different incident radiant heat flux were established.At the same time,using the above parameters,the thermal characteristic parameters,such as ignition critical heat flux,thermal response parameters,thermal inertia and latent heat of vaporization,are derived.The comparison of fire properties of photovoltaic and polyethylene terephthalate(PET)+TPT reveals that polyethylene terephthalate is the main component responsible for decomposition and burning of flexible photovoltaic panel.In addition,the experimental results show that glass covering instead of PET covering could effectively improve the flame-retardant and smoke-suppression properties of PV modules.This conclusion stems from the fact that the rigid PV panels have higher thermal response parameters and thermal inertia as well as lower peak heat release rates and fire growth indices,less CO and CO2 production,and lower FED values.Thirdly,based on the study of the impact of building-integrated PV modules on building fire safety,the extension behavior of flames under the PV panels,including their extension length and vertical thickness,are obtained under different tilt angles of the PV modules,panel installation height and fire heat release rate(HRR).The results show that the flame extension length and vertical thickness are reduced with the increase of PV tilt angle and panel-roof distance,but are increased with increases in the fire HRR.A unified non-dimensional HRR coupled with all these factors is proposed to quantify the flame extension geometry.Furthermore,a general equation based on the physical relationship between flame radiation and flame geometry is developed to characterize the distribution of reradiation heat flux on the roof surface with the non-dimensional local flame thickness.Finally,suggestions regarding PV installations on flat roofs and the selection of roofing materials are given to decrease the possibility of flame propagation underneath the PV arrays.The results of this study can help to predict the fire characteristics of building PV modules,provide guidance for evaluating their fire hazards and have certain guiding significance for the fire safety design of building PV systems.