Thermal Analysis Of Green Roofs With Night Ventilation

Green roofs and night ventilation are well-known passive techniques for energy saving during cooling period.This study refers to the analysis of thermal properties and energy performance of the combination of green roof and night ventilation.Although there are lots of research studies have been conducted to evaluate the performance and benefits of green roofs and ventilation separately,combing them together is rare.Most researches have been carried out in no ventilation performance,which is often occurs when using HVAC system.Because the cool outdoor air temperature and sky long wave radiation at night make outer surface of the common roof be cooled quicker and easier than green roof.However,it is often non-occupied at night in the office buildings and schools,or for some residential buildings,when the air temperature is cool at night,the people would like to open the window to get fresh air instead of using air-conditioners,so it is very suitable and energy-saving to adopt night ventilation.Therefore,combining green roof and night ventilation may be a good solution for energy conservation and indoor air condition improvement.Firstly,a field experiment was conducted to compare the thermal performance of green roof and bare roof when combining with night ventilation.Three series experiments were carried out to analyze the green roof cooling effect: no night ventilation,natural night ventilation by opening vents and mechanical night ventilation by fans.Through the analysis of the experimental data,it is concluded that the green roof is equivalent to a cold source in the daytime,blocking and absorbing most of the solar radiation.When the plant is in the flourishing period,only 15% of the solar radiation heat can enter the room through the roof.But because of the strong regenerative effect of the soil,the greening roof is equivalent to the insulation layer at night and prevent the room.The internal heat is emitted to the outside.At this time,the night ventilation is adopted to bring the outdoor cold air into the room.With the intensification of the indoor and outdoor air mixing,the indoor air temperature is gradually lower than the inner surface temperature of the roof panel.At this time,the cooling effect of the night ventilation is the dominant effect.In addition,the meteorological parameters(solar radiation,outdoor air temperature,humidity,wind speed,soil moisture content)which affect the indoor temperature and the heat flow on the roof surface are also analyzed in this paper.It is concluded that the influence of solar radiation on the surface temperature difference,indoor temperature difference and heat flow difference of green roof and bare roof is the most significant,followed by outdoor air temperature and outdoor wind speed.The correlation between the moisture content of soil and the surface temperature and heat flow of the green roof is very strong.It shows that the increase of soil moisture content can greatly reduce the surface temperature of the roof inside and outside the roof,thus reducing the indoor temperature and making the heat from indoor to outdoor.The transmittance of two species of plants from ground root and Bryophyllum in Gynrasegetum was analyzed by regression analysis.The transmittance of green roofing plants was a dynamic value that changed with time.As the plant leaves grew thickly in the growth process,the sun shading performance was greatly improved.Therefore,a dynamic shading coefficient LSC(Leaf shading coeffient)of greening roof blades is proposed,which is a correlation function with days.Besides,through experimental data analysis,the heat transfer formula of the experimental box under the combined action of green roof and night ventilation is obtained.In order to quantify the potential of building energy conservation,the evaluation index of roof greening effect under the action of night ventilation is presented in this paper,the ratio of roof temperature difference RTDR and the heat absorption ratio RHR on the surface of the roof.The roof temperature difference ratio(RTDR)can be normalized to the influence of the outdoor temperature difference on the roof surface,which can be used to compare the effect of roof greening and cooling effect under different systems and climate conditions.The heat absorption ratio of RHR inside the roof can reflect the contribution of green roof and night ventilation to cooling and heat insulation respectively.According to the experimental data of 2016,this paper uses the energy consumption simulation software Energy Plus to establish the numerical model,and compares the data with the experimental data to verify the accuracy of the model.Based on this model,a series of simulations are carried out to change various parameters that affect indoor thermal environment.After single factor analysis on the thermal environment of green roof rooms under the action of night ventilation,it is found that the effect of leaf area index,soil layer thickness,irrigation amount,air change times,structure layer storage performance and day night temperature difference on indoor temperature is outstanding,and the effect of plant height on it can be ignored.At the same time,it is found that the structural layer and soil layer of green roof interact with each other on heat transfer,so we must consider it as a whole for further research.In order to strengthen the cooling effect of the joint action of the nocturnal ventilation and the greening roof,the effect of the simultaneous action of the soil layer thickness and the structure layer's heat storage performance on the indoor thermal environment and the roof heat storage and release is analyzed.Finally,on the basis of single factor analysis,the Energy Plus software is used to simulate the function curves of different roof heat storage materials,different soil thickness,and the maximum temperature difference inside and outside the room with the outdoor temperature difference under the day and night under different air changing times.It is used to measure the cooling effect of green roofs under night ventilation.The evaluation tool is simple and effective,which enables designers,openers and users to intuitively understand the cooling effect of the energy saving technology in summer.It can be widely applied to all kinds of buildings that are suitable for night ventilation and greening roofsHighlights:(1)Through the experimental study,the temperature of each layer of green roof under the action of night ventilation and the heat absorption and release of the inner surface are systematically analyzed,and the correlation coefficient of the environmental factors(solar radiation,outdoor air temperature,humidity,wind speed,moisture content of soil)and the heat transfer characteristics of green roof are obtained.The dynamic shading coefficient equation of two species of Sedum in the growth process and the heat transfer equation of the experimental box under the joint action of green roof and night ventilation were put forward.The factors affecting the design parameters of green roof under night ventilation are analyzed,and the correlation ranking of each factor and the cooling evaluation index RTDR is obtained.(2)The evaluation index of roof greening thermal insulation and cool storage under night ventilation: the temperature difference ratio of green roof interior surface RTDR and heat release ratio RHR.The combination of these two indexes can not only predict and evaluate the cooling effect of roof greening under the condition of night ventilation,but also analyze the proportion of the two in the process of roof heat transfer,and supplement the existing thermal parameters..(3)According to the simulation of the impact of the main factors affecting the green roof and night ventilation,to find out the relationship between these factors and the roof surface temperature and heat flow.At the same time,the relevant factors that affect the temperature difference ratio of RTDR inside the greening roof and the heat absorption ratio of RHR are sorted.The function curves of indoor and outdoor temperature difference and outdoor day and night temperature difference are obtained,so as to provide reference for evaluating the cooling effect of greening roof under night ventilation.