Heat And Mass Transfer Mechanism,Numerical Model And Application Of Soil-Covered Green Roof

Based on the sponge city and carbon emission aims,research on the hydraulic and thermal performance of soil-covered green roof（referred as“green roof”）can improve the urban ecological environment.As one of the important measures of the sponge city,green roofs have the benefits of reducing runoff,energy saving,extending building life,mitigating urban heat island and increasing urban greenery.However,the structural configurations of green roofs are mainly based on soil water holding capacity and the discharge of drainage layer.Runoff reduction and stormwater management of green roofs are weak with higher roof loads and construction costs.Further,there is a lack of hydrological models that consider the dynamics of long-term hydrological processes under rainfall and evapotranspiration conditions.As well as the lack of coupled heat and mass transfer models considering the water dynamics of soil layer and water storage layer.The regional runoff reduction performance,energy and carbon reduction benefits of green roofs are not clear.In this study,the experimental models of green roofs with different structural configurations and the experimental models of long-term heat and water transfer were developed.Based on HYDRUS-1D software,numerical simulations of the water transfer process of layered soil and green roof with water storage layer were carried out to obtain the heat and water transfer mechanism and the optimisation method of structural configuration of green roofs.A simplified water balance model was developed to simulate the optimisation of green roof configurations and irrigation management.Coupled heat and mass transfer model is developed to simulate the thermal performance of integrated green roofs and green roofs with water storage layers.Subsequently,a field experiment on green roofs was established in the South West to analyse the regional runoff reduction performance and carbon emission benefits of green roofs.The main conclusions of the study are as follows.（1）Green roofs with water storage layers and layered soils can significantly improve runoff reduction and delay water stress.The results show that during the rainy season in Southern China,the runoff reduction rate of green roofs with different structural configurations ranged from 34%to 59%.The runoff reduction rate of layered soil green roofs was 1%-4%higher than that of single soil layers.Green roofs with 25 mm water storage layer increase the annual runoff reduction rate by 13%compared to green roofs without a water storage layer and delay irrigation by approximately 9 days during dry periods.Moreover,the roof load increases linearly with increasing soil depth of green roof,while the increasing rate of rainwater retention decreases with soil depth.The runoff reduction rate of the green roof with a 25 mm water storage layer（100 mm soil layer）is equivalent to that of a green roof with 180 mm soil layer,with a corresponding increase in load of approximately 0.9 kN/m².（2）The water holding capacity of a layered soil green roof is improved by increasing the maximum soil water content and reducing water infiltration after rainfall stops.Layered soils have a higher peak runoff reduction and delayed peak drainage time than single soil layers.Further,a green roof with a water storage layer and allowing surface water infiltration can significantly increase runoff reduction rates,reduce peak drainage and delay drainage.Under surface infiltration conditions,a green roof with a 25 mm water storage layer increased the runoff reduction rate by 31%during a 20-year return period in Southern China.The drainage time was delayed by 50 min and the peak runoff was reduced by 89%as well.（3）A simplified water balance model for green roofs considering rainfall and evapotranspiration conditions is proposed.Weather data and structural configuration parameters of the green roof are used as inputs to the model.It simulates the dynamics of long-term rainwater retention capacity and soil water content of green roofs well,with an average Nash coefficient of 0.65 and an average error of 6%in rainwater retention.In Southern China,annual runoff reduction rates of up to 78%（an improvement of 41%）were obtained for green roofs with 150 mm water storage layer（100 mm soil layer）.The annual water stress time in the vegetation layer was reduced by 49%.As the depth of the water storage layer increased from 25 mm to 100 mm,the annual irrigation times for green roofs were reduced by 4 times and the cumulative irrigation volume was reduced by 50 mm.（4）The heat flux through a green roof is reduced by increasing the surface albedo and evaporating latent heat.The range of temperature differences between the soil surface temperature and concrete upper surface is lower than that of a bare roof.Moreover,the integrated green roof can replace the bare roof insulation and concrete protection layer.In Southern China,the maximum temperature on the concrete surface of a bare roof can reach 58℃,with a daily temperature difference of 32℃.However,the maximum daily temperature difference between the concrete upper surface of green roof is only 11℃.Compared to a conventional green roof,the average room temperature of an integrated green roof is lower and higher by approximately 1℃ in summer and winter,respectively.（5）A coupled heat and mass transfer model considering water dynamics in the soil and water storage layers of a green roof is proposed.The model simulates the temperature variations of different structural layers of the green roof well,with a Nash coefficient of 0.72-0.97.The green roof with a water storage layer reduces the green roof heat flux of by increasing the latent heat of surface evapotranspiration.Simulated results show that an increase in soil depth of 100 mm can reduce the average room temperature of a green roof by approximately 1℃ in summer.The average soil surface temperature is reduced by 2℃ for green roof with a 25 mm water storage layer,and the average room temperature by 1℃ in summer.Thermal performance of green with a 25 mm water storage layer（100 mm soil layer）is equivalent to a green roof with a 200mm soil layer in summer.（6）Green roofs significantly reduce regional runoff,building energy consumption,and CO₂ emissions.In a study area in the southwest,the annual runoff reduction rates were 42%and 54%for a conventional green roof and a green roof with a 50 mm water storage layer,respectively.The installation of a green roof with a 50 mm water storage layer significantly increased the regional annual runoff reduction rate by 5%compared to a bare roof.Compared to a bare roof,the average room temperature of a green roof in summer is reduced by4℃,the annual energy consumption is reduced by 10.1 kWh/m² and the regional CO₂ emissions are reduced by 60.51 tonnes/year.The annual energy consumption and CO₂ emissions of the green roof were reduced by 12%.