Thermal Characteristics Of Green Roofs And Analysis Of Building Energy Efficiency

With the acceleration of global industrialization,building energy consumption had been continuously increasing in the past few decades.The roof was the main enclosure structure of a building,therefore,the thermal performance of the roof directly affected the indoor and outdoor heat transfer.There was currently little research on the thermal delay,thermal attenuation,and plant layer thermal resistance of green roofs.In addition,most of the research on the energy-saving performance of green roofs in China was focused on a single climate zone,lacking comparison.At the same time,the energy-saving benefits of green roofs were only considered,and there was a lack of research on their carbon reduction.This study focused on the thermal characteristics of green roofs and the analysis of building energy efficiency,and conducted the following research in the following aspects.The main content is as follows:Firstly,the energy balance equations of the plant and soil layers were introduced.Based on the heat transfer model of green roofs,Energyplus was used to output the internal and external surface temperatures of green and bare roofs,and the thermal resistance,thermal attenuation factor,and thermal delay time of green roofs and bare roofs were calculated respectively;From the perspectives of thermal resistance,thermal attenuation,thermal delay,and heat flux density,the thermal performance of green roofs was analyzed.It was concluded that green roofs increased the thermal resistance of the enclosure structure,reduced the heat transfer from outdoor to indoor environment,and increased the amplitude attenuation and phase delay of outdoor periodic thermal effects;Finally,we analyzed the cooling effects of green roofs on various surfaces compared to bare roofs,as well as the energy efficiency of green roofs in different seasons.A green roof can significantly reduce the surface temperature and heat transfer to the interior.Secondly,Energyplus was used to explore the influence of different variables on the interior surface temperature of the green roof.The results showed that leaf area index and soil layer thickness had the greatest impact on the performance of green roofs.In addition,the relationship between plant layer thermal resistance,plant layer sensible heat flux,and leaf surface temperature,solar radiation,and plant leaf area index was studied,and the expression for the relationship was obtained.Then,Shanghai,Tianjin,and Guangzhou were selected as representative cities.By comparing the annual building air conditioning energy consumption under different leaf area indices and soil layer thicknesses,recommended values for soil layer thicknesses and leaf area indices were given for three different climate zones.The results indicated that the leaf area index and soil layer thickness of Shanghai and Guangzhou are recommended to be 5 and0.2 meters,respectively;The recommended values for leaf area index and soil layer thickness in Tianjin area are 5 and 0.3 meters.After installing green roofs in three cities,the energysaving rates of green roofs in different cities were compared.The results showed that the energy-saving rates of green roofs reached 16.93%,17.28%,and 8.37%,respectively in Shanghai,Guangzhou,and Tianjin.Finally,a comparison was made between the emission reduction effects of building carbon reduction technologies such as using green roofs,changing building envelope structures,improving air conditioning energy efficiency,adopting shading measures,and using natural lighting during the building operation phase.The results showed that the use of green roof technology and natural lighting had the most significant effect on building emission reduction.After obtaining the carbon emissions of green roofs in three different climate zones,this study analyzed the economic benefits that green roofs can bring from two aspects: energy conservation and emission reduction.