Study on Building Energy Saving Effect and Thermal Response of Roof Greening in Extremely Hot Weather

. As the main living space and carrier of the population, cities and buildings have become more seriously affected by changes in global environment in recent years. The deteriorating living environment has brought about the quality of survival of the population, the government's administrative management, and the sustainable development of the environment. This article has found that green roofs on urban environmental management and building energy consumption have found that green roofs can have an important impact on urban environmental management. In extreme weather events, green roof has a maximum cooling effect of 26 ℃ in the daytime, and its warming effect at night has been suppressed to a certain extent; the heat preservation time of the green roof can be up to 17 hours after the superposition of precipitation weather incidents, and its heat preservation effect is positively related to the temperature in the daytime. The green roof can reduce the energy consumption of the building by 106W/ ㎡ /d in the extreme weather incident, while its energy -saving effect is reduced to 45W/m 2 /d after superimposing the precipitation meteorological incident.


Introduction
Global extreme high temperature weather events have caused extremely serious impacts on the production and life of human beings, especially urban population. Considering the intensity, scope, frequency and socioeconomic consequences of these high temperature events, more attention should be paid to them in future urban environmental management and urban building energy consumption research. By changing the underlying surface properties of traditional roofs, green roofs reduce the absorption rate of solar radiation of buildings, change the thermodynamic properties of buildings, and curb the occurrence of urban heat island phenomenon from the source [1] . At the same time, transpiration of plants and evapotranspiration of water in the substrate of green roof also explain the reason why green roof can effectively link urban heat island effect [2] . N. H. Wong [3] , Oke [4] and other scholars respectively proved the contribution degree of green roof to urban heat island effect through field measurement, model construction and data fitting. After experimental verification, Salah-Eddine Ouldboukhitine screened out that matrix storage, microstructure and thermal conductivity of green roof are the main factors affecting the thermal performance of green roof [5] . Barrio [6] , Issa Jaffal [7,8] and Wolf [9] explored the influence of physiological properties of plants on the cooling effect of green roofs. Takakura [10] ; Theodosiou [11] ; Lazzarin [12] proved that air moisture and soil moisture can have a great influence on cooling effect of roof greening. D. J. Schailor established a corresponding mathematical model for the thermal performance of roof greening, and verified the accuracy of the model results through experiments [13] . Paulo Cesar Tabares-Velasco calculated the influence degree of heat transfer performance of green roof substrate, evaporation resistance of green roof substrate and plant transpiration porosity resistance on building energy conservation by using experimental data [14] . Rabah Djedjig's verification results showed that the surface temperature difference of roof greening could be as high as 25℃ due to different substrate moisture content [15] .
However, there are few reports on the response mechanism and corresponding degree of roof greening in extreme weather events caused by global warming. In view of the above problems, long-term data collection and relevant meteorological and building energy consumption models were carried out in Shanghai and Henan, China, from June to September 2022, to form research results on the impact of roof greening on building energy consumption and urban environmental management under extreme weather events.

Green roof for urban environmental management
This experiment was conducted in Shanghai and Henan Province, China. Meteorological and temperature data were recorded and observed continuously for 4 months, and data of several experimental days with similar external factors were screened out. All data were proved to be similar by difference test(Pearson significance ≥ 95%). Comparing temperature data from these dates provides a visual representation of how green roofs respond to extreme weather events.

Experimental situation
This experiment involves two experimental sites in Shanghai and Henan Province, China. In order to ensure the accuracy of the experimental data, long-term continuous observation and recording facilities were built on the experimental site. The site conditions of the experimental site are detailed in Table 1; The construction methods and instruments for the experimental platform are detailed in Table 2.  Since the sampling interval of data is 60s, this experiment involves a large number of continuous data over a long period of time. On the premise of ensuring the accuracy of experimental results, this paper takes the average result of fixed time every 300s as experimental data to reduce the workload of subsequent research. After statistical test, standard days with approximate degree of meteorological conditions ≥ 95% were selected for descriptive comparative analysis. The dates of each standard day and the main test results were shown in Table  3.

Positioning
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Result
Under abnormally high temperature conditions, the green roof exhibits a more stable cooling effect, with the maximum cooling range only decreasing by about 1℃ compared with typical sunny days. From the cooling duration, when the abnormal high temperature weather phenomenon occurred, the 10cm and 25cm soil layers maintained the cooling effect all day long, and the insulation effect of the bottom layer was reduced to 8 hours and the maximum warming effect was only about 2℃. In contrast, in ordinary summer, the warming effect of 10cm, 25cm soil layer and soil bottom lasted for 8h, 10h and 12h, respectively, and the warming time was 2 hours longer than the occurrence time of abnormal high temperature weather events. In ordinary summer, roof greening exhibits the same thermodynamic properties as other reports, that is, different soil depths will appear different warming effects. It should be noted that under different weather conditions, the maximum temperature of each soil layer has a relatively stable plateau period, maintaining a stable temperature for a relatively long period of time. This is the same as previous research results, and it is caused by the continuous evapotranspiration of plant and soil water in the event of abnormally high temperature weather events. However, the plateau period of high temperature in all soil layers was significantly shortened when the abnormal high temperature weather events occurred, which was jointly determined by the short occurrence time of the abnormal high temperature weather events and the significant cooling effect of plant evapotranspiration.
When extreme heat and precipitation occur at the same time, which is particularly concerned in this paper, the cooling effect of green roof is severely inhibited by precipitation. The effective cooling duration is only 7 hours, and the maximum cooling range is as low as 8.65℃. This indicates that the effect of summer precipitation on the urban environmental management role of green roofs needs to be re-evaluated, and the design of green roofs needs to be optimized to adapt to the possible high frequency of such situations.

Green roof for building energy consumption
Municipal buildings are the main places for people to live and work. Scientific control of building energy consumption is of great significance to alleviate urban environmental problems and improve urban environmental management level. Especially at present, with frequent extreme weather events, people stay longer in buildings, and people rely more on air conditioning and other HVAC systems, which leads to increased burden on the power system in many cities in summer. This paper and previous studies have effectively proved that roof greening has a significant impact on the reduction of building energy consumption. In order to further explore the influence of roof greening on building energy consumption when abnormally high temperature weather occurs, this paper uses Energy Plus software on the basis of previous experimental data to construct a building model of the test site for simulation experiments.

Experimental model setting
The experimental model is set strictly in accordance with the actual conditions of the test sites. Since the basic conditions of the Shanghai and Henan test sites are the same, a model is set up this time to simulate different conditions. The model has a total of seven floors on the ground, with Windows on all sides without shading. In the simulation software, the interior and exterior walls of the building are distinguished from the interior and exterior Windows. In EnergyPlus, four building function zones are divided into bedrooms, living rooms, kitchen and bathroom, and stairs. The parameter Settings are shown in Table 4.

Surface temperature of ceiling in room
The average ceiling temperature in each typical day ranked from high to low as extreme rainy day, extreme sunny day, typical sunny day, typical rainy day. At the same time, under all weather conditions, the surface temperature in the room on the top floor of the building shows the same change trend, that is, 0:00-7:00 is the cooling period, 7:00-15:00 is the heating period, and 16:00-24:00 is the stable period.
On representative days under different meteorological conditions, the variation trend of the internal surface temperature of the room on the top floor of the building obviously reflects the influence of meteorological conditions on the internal surface temperature of the building room. In the longitudinal comparison, the surface temperature of the top room of the building under extreme weather conditions is higher than that under typical weather conditions all day long under the same sunny conditions. In the time period from 9:00 to 11:00, the temperature of the inner surface temperature of the room on the top floor of the building gradually increased slowly under typical weather conditions after 13:00, and the overall temperature remained in the range of 34℃-35℃. Meanwhile, the inner surface temperature of the top floor of the building still maintained a relatively high growth trend under extreme weather conditions, and finally remained stable at 36℃-37℃. Under the condition of rainy weather, the surface temperature of the top room of green roof building has great difference in the horizontal and vertical comparison. Under the same condition of rainy day, the internal surface temperature of the top room of the green roof building in extreme rainy day fluctuates greatly throughout the day, and the temperature increases and decreases obviously. However, under typical rainy weather conditions, the surface temperature of the room showed a relatively smooth change, and the temperature did not fluctuate significantly throughout the day. The overall temperature fluctuated between 26℃ and 30℃, and there was a later heating process and an earlier cooling process. In the horizontal comparison, under the same extreme weather conditions, the internal surface temperature of the room with different weather conditions showed the change trend mentioned above. Comparing sunny and rainy days, the internal surface temperature of the room can be roughly divided into two different time periods. The first time period is 0:00-6:00. During this time period, the internal surface temperature of the room in sunny days is higher than that in rainy days. The second time period is 6:00 to 24:00. In this time period, the internal surface temperature of the room under rainy conditions is higher than that under sunny conditions. In extreme cases, the overall surface temperature of the room exceeds the typical case by 4℃-6℃. This is about the same as the average difference in soil bottom temperature in the experiment. Under different weather conditions, there are great differences in the variation of surface temperature in the room. Under extreme weather conditions, similar temperature variation as mentioned above appears, while under typical rainy day, the temperature variation period is generally delayed, the intensity decreases, and the duration is shortened. Specifically, the temperature change period is 9:00-14:00, the stable period is 14:00-19:00, and the stable period is 1900-9:00 the next day.
Meanwhile, by comparing the previous research results, it can be seen that the surface temperature variation of the top room of the green roof building is similar to that of the bottom soil temperature variation of

Building energy consumption
Because in Energy Plus software set of air conditioning components for more than set temperature would calculate the energy consumption and energy consumption of each period and the data are the same, so the energy consumption per unit floor area through the typical weather days to assess the effect of green roofs on building energy consumption of the top floor room, the daily variation of ceiling temperature in the room under the green roof under different weather types is shown in Fig.  1. The refrigeration unit is set at 1600W. According to the energy consumption per unit building area of representative days of each meteorological condition shown in Table 5, the ranking from highest to lowest is extreme sunny day, extreme rainy day, typical sunny day, typical rainy day. There is a certain difference between the overall ranking and the ranking of daily average temperature of the surface temperature of the top floor room of the green roof building. Meanwhile, the difference of energy consumption per unit building area is more significant than that of the internal surface temperature of the room.

Conclusion
This paper focuses on the impact of roof greening on urban environmental management and building energy consumption during extreme hot weather and draws relevant conclusions by comparing with previous research reports. In terms of the effect of roof greening on urban environmental management, the cooling effect of roof greening is also effective when extreme weather events occur, which indicates that the construction of green roof facilities can effectively reduce the impact of abnormally high temperature weather on urban buildings and residents. The overall reason for the warming effect of green roof in the whole experiment is that the decrease of night air temperature causes the building and roof greening soil to release heat outward rapidly. Due to the large heat capacity of soil, the deeper the soil depth, the more stable the temperature remains, which reflects the characteristics of night warming and daily maximum soil temperature continuously delaying with the increase of soil depth. As the sun rises, the presence of solar radiation interrupts the increasing warming effect and rapidly changes the effect of roof greening from warming to cooling. This indicates that it is necessary to further deepen the research on the thermal effect of green roof, especially the warming effect. If necessary, measures such as laying cooling water pipes can be taken to reduce the influence of the warming effect of green roof on the life of residents on the top floor of buildings. In this paper, we focus on the situation where abnormally high temperature weather events are superimposed with precipitation conditions. In this case, the cooling effect of green roof is not as sufficient as when abnormally high temperature weather events occur, which shows the warming effect of longer time and higher temperature. This may be related to the fact that heat stored in the substrate of green roof is difficult to transfer to the air through evapotranspiration. In order to achieve energy balance, it enhances the heat transfer path to the building. At the same time, the thermal insulation effect of green roof will be enhanced to some extent under any weather background, which indicates that in summer, weak precipitation is difficult to improve the thermal performance of green roof, and will enhance the thermal insulation effect of green roof. This situation is more likely to occur after short-term heavy precipitation in hot summer, which highlights the necessity of studying the thermal performance of green roof during the summer precipitation process. This suggests that we should pay attention to the layout of drainage facilities in the design of future roof greening. At the same time, in extreme high temperature weather, soil cooling facilities can be arranged or the water in the soil can be replaced and the heat stored in the roof greening soil can be taken away from time to time, so as to maintain the environmental management advantages of roof greening.
Studies on building energy consumption show that green roofs have varying degrees of influence on building thermal performance under different weather conditions. Combined with previous reports, the energy-saving and emission reduction effects of green roofs have been fully demonstrated [16] . The internal surface temperature of the top room of the green roof building presents a similar three-stage change under different weather conditions, which may result from the fact that the internal surface temperature of the room is mainly affected by the building body, especially the roof temperature and the air temperature, and the factors affecting the temperature of the building body and the air temperature are very diverse, but the solar radiation plays a dominant role. This variation trend and the daily variation trend of solar radiation show a high coincidence degree, and the time is delayed for 1h. The occurrence of delay phenomenon can be explained by the heat transfer process. Under extreme weather conditions, the increase of the room surface temperature is more sensitive, while the decrease is relatively slow. The whole building saves more heat in extreme weather conditions and cannot consume the stored energy through heat exchange with the air, which can better explain the occurrence of this situation. In the case of rainy days, the room surface temperature on representative days of extreme weather changes more violently than that on representative days of typical weather, and there is a maximum difference of 10℃ in the room surface temperature throughout the day. Compared with the representative day of extreme weather, the room surface temperature fluctuates more gently on the representative day of typical weather, and the heating process occurs later and the cooling process occurs earlier.
The possible reasons for this phenomenon are not only that buildings in extreme weather as mentioned above will store more heat, which is difficult to release and there is a certain difference in temperature, but also that more heat is absorbed in extreme weather precipitation, which leads to the heating of the green roof substrate and the difficulty of heat exchange in the room. At the same time, this phenomenon indicates that there are obvious differences between the short-term precipitation process in extreme weather and the thermal process in typical precipitation process, and it is difficult to play a significant role in reducing indoor temperature. Continuous ventilation to facilitate the efficient heat exchange of the interior walls will also further reduce the temperature at the base of the green roof substrate, ensuring better ecological benefits in the course of extreme hot weather that may continue to occur in the future. In the horizontal comparison under extreme weather conditions, the surface temperature of the room under sunny conditions is higher than that under rainy conditions during 0:00-6:00, and the surface temperature under rainy conditions is higher than that under sunny conditions during 6:00-24:00. The temperature in the previous period was due to the positive correlation between soil thermal conductivity and soil moisture content. The higher the soil moisture content, the higher the thermal conductivity of the soil, the faster and more heat inside the room can be absorbed so that the room surface temperature is lower than that in sunny conditions. The temperature condition in the following period may be due to the fact that the outside air temperature is 100% in rainy days, so it is difficult for the green roof substrate to reduce its temperature through water evaporation, and precipitation can improve the efficiency of obtaining air heat from the green roof substrate. But the daily difference in surface temperature between the two extreme weather days is only about 1℃, and these two thermal processes may be carried out in a very weak form. Based on the above conclusions and analysis, we can conclude the following key points for improving the thermal performance of green roofs: In rainy days in extreme weather conditions, it is necessary to carry out adequate and continuous ventilation indoors, and pay attention to the drainage process of the green roof substrate to avoid the accumulation of water in the green roof substrate. The above process can improve the thermal performance of the bottom of the green roof substrate and ensure the full play of the ecological benefits of the green roof. Sunny days in summer require that the water content of the substrate of the green roof be kept at a normal level to improve the effectiveness of the green roof in cooling the rooms on the top floor of the building.