The Mechanism And Simulation Of The Coupled Hydro-Thermal Transfer In Roofs Covered With Soil And Plants

The concept of “Sponge City” was formally proposed by President Xi Jinping at the Central Urbanization Work Conference in 2013.The practice of covering the roofs with soil and plants,can take advantage of the rain water retention and permeability of the soil,as well as the evapotranspiration of green leafy plants,which is one of the important issues to realize the construction of “Sponge City” in China.At the same time,the heat insulation effect formed by the vegetation shading,thermal inertia of the soil and the heat dissipation by evaporation,can be one of the effective ways to improve the thermal environment inside the building,reduce the energy consumption and limit the temperature cracks in roof structures.As the carrier of evaporation,water affects soil thermal properties and the heat transfer process.Measures such as adding storage and drainage boards(S&D boards)to form a retention layer under the soil layer can improve water storage capacity and utilization,which become one of the development trends of this technology,but the overall water and heat transfer mechanism in the whole system was still unclear.Furthermore,the overall thermal insulation ability formed by the soil and plants as well as the insulation layer in the floor,are underestimated in the current building design,which may lead to the redundant insulation ability.As the development of soil-covered planting roofs,it is necessary to deeply analyze the coupled hydro-thermal transfer mechanism of the whole system,and provide reference for the optimal design.To this end,based on the field test data and the theory such as heat and moisture transfer in soil,this paper discussed the coupled hydro-thermal transfer mechanism in the roof system.A coupled heat and moisture transfer model is developed to predict the thermal performance of the whole system with “Plant-Soil-Retention Layer-Floor”.The program is developed and the calculation results are verified.On this basis,the role of moisture in the heat transfer process of green roofs,and the influence of key parameters on the heat insulation effect are further studied.The main contents of this paper are as follows:(1)A green roof test site was established on the existing building roof,and a 456-day monitoring was carried out.The meteorological data,the moisture and temperature variation,as well as the insulation effect in the green roof are recorded.The statistical analysis of green roof insulation effect is given based on the monitoring data.The parameters of the soil-water characteristic curve and the soil thermal parameters were estimated by the inverse solution method.The results show that the green roof can reduce the indoor temperature by 1.2℃during summer.The amplitude of temperature fluctuation of floor surface can be reduced by85% under the soil and plants covering.The relationship between the soil heat conductivity coefficient and the soil water content was estimated by numerical inverse solution.(2)The coupled hydro-thermal transfer mechanism in the “Plant-Soil-Retention LayerFloor” system was analyzed and a mathematical model was established.The self-developed program GRHTC for the green roof simulation was developed in this paper.The main contents included: A formula for calculating the evaporation in retention layer is proposed based on the Penman-Monteith equation.By analyzing the heat transfer process of the concave-convex type S&D boards,the calculation model of the coupled hydro-thermal transfer in retention layer is developed,and thus toward the transfer model in the whole system.The Finite volume method were used to solve the transfer model and the calculation process was programmed by using MATLAB.The calculation results were compared to the field test data to verify the accuracy of the program.Results showed that the Nash-Sutcliffe efficiency coefficient(NSE)of the short-term simulation results was between 0.4～0.9,and the long-term simulation results was close to 0.9,which justified the accuracy of this program.(3)The effect of water on the heat transfer process in green roofs is analyzed,and discussed the optimization of key parameters on hydrologic and thermal efficiency.Simulation results proved that the green roof can achieve the thermal insulation effect of the common roof even without the insulation layer in the floor.The main results included: Heat conduction is the main heat transfer mode in soil layer,but the decrease of soil moisture content will increase the proportion of heat flow transferred by water vapor flow.More water content in the green roof system can increase the attenuation effect on the temperature fluctuation.Plants with higher leaf area index could improve the water resource utilization and heat insulation ability,as well as increasing the thickness of soil layer or retention layer.The increase in retention layer thickness is more effective in reducing the indoor heat gain.The influence of floor insulation layer on the roof thermal performance is greatly reduced under the soil and plants covering.The simulation results showed that the summer indoor heat gain of green roofs without floor insulation was equivalent to the common roof,which means that the floor insulation can be removed in the design of green roofs.The above results were to provide reference for the structural optimization design of green roofs.