Thermal Comfort Assessment of Green roofs on an Office Building in Humid Continental Climate, China
Abstract
China has now become the world's largest energy consumption country, and energy consumption in buildings accounted for 30% of total energy consumption in 2009 and continues to rise (Xiao et al, 2014). Faced with this situation, reducing energy consumption in buildings can reduce the world's energy consumption. Energy consumption also brings various environmental problems to the environment, such as the urban heat island effect. As a multi-purpose, high-efficiency passive design strategy, green roof is of great help to reduce building energy consumption and alleviate urban heat island effect. This report will discuss the definition, structure, operation, etc. of green roofs, and analyse how this strategy can improve thermal comfort of the office building in a humid continental climate.
1. Green roof Strategy 1.1 Definition of Green Roof Green Roof is a kind of vegetated landscapes installed on the roof of the building (Growing Green Guide, 2019), and it is also named "Living roofs" and "eco-roofs", which is a living system to solve and improve parts of the environmental problems (Abass et al ,2020). According to the literature records on green roofs, green roofs are generally classified into two types, namely extensive green roof and intensive (Peck et al, 1999; Nps.gov., 2019). Some scholars divide it into three types (Abass et al, 2020), two of which are the same as the previous method, and the additional one is semi- intensive, which is the combination of extensive and intensive. Extensive green roof is thin and light in weight, and low ground cover or herbs are often used. Intensive green roof has thick thickness and heavy weight, and shrubs or trees are often chosen as plants, so the load of the building should be considered in the design (Xiao et al, 2014). The parameters of the semi-intensive green roof fall somewhere in between. Figure 1 shows the comparison of the two green roofs.
Fig.1 Comparison of Extensive and Intensive Green Roof Systems (Source: adapted from Peck et al, 1999. p. 14.) 1.2 Structure of Green Roof The structure of the green roof consists of 5 major parts, namely vegetation, growing media, a filter fabric, a drainage layer, a root barrier, and a waterproofing (Zhou et al, 2018; Gregoire, Clausen, 2011; Peck et al, 1999). Figure 2 shows the specific structure of green roof, which provides a reference for the design of green roof. Fig.2 Green Roof Layer
1.3 Benefits of Green Roof There are many benefits of green roof, among which the most attractive may be lowering roof temperature and cooling load of the building (Zhou et al, 2018). In addition, green roofs increase the efficiency of mechanical equipment such as air conditioners (Nps.gov.,2019), and all these benefits contribute to a reduction in building energy use. The benefits of green roofs can be summarized in three parts, namely environmental, economic, and social. Figure 3 is an overview of the benefits of green roofs in these three parts.
Fig.3 Green roof Benefit Summary (Source: Stevenson,2021, Passive Design, Cardiff University. Available at: https://learningcentral.cf.ac.uk/)
2. Case Details The buildings evaluated in this study are office buildings in Beijing. With the development of the city, Beijing, as the capital of China, has a high density of population and buildings, which leads to a high consumption of building energy. Meanwhile, this situation will also reduce the urban per capita greening rate. In such a situation, green roof may be a good solution, and the Beijing government has been promoting the development of green roof and has relevant policies to encourage people to build green roof (Zhang et al, 2010).
The climate type of Beijing is humid continental climate, and the Köppen climate classification is Dwa. As can be seen from Figure 4, the climate of Beijing is snowy and dry in winter and hot in summer.
Fig.4 Köppen climate classification Map (Source: http://koeppen-geiger.vu-wien.ac.at/present.htm)
The Beijing weather data used for this analysis was obtained from the EPW file of EnergyPlus, named CHN_BEIJING.BEIJING.545110_CSWD.epw. Import the file into the Climate Consultant software and get Figure 5. The above shows that June to August in summer is the hottest period in Beijing, with the average daily high temperature exceeding 30℃. In winter, December to February is the coldest period in Beijing, with the average daily low temperature below zero. In terms of relative humidity, the average daily relative humidity in Beijing is the highest in July and the lowest in March.
Fig.5 Monthly dry bulb temperature and relative humidity in Beijing (Source: Climate Consultant, Data: EnergyPlus) 3. Literature Review Many studies have been conducted to simulate and evaluate the thermal comfort of green roofs for buildings. The key parameters affecting the efficiency of green roofs and the methods of simulation analysis can be obtained from these studies.
(Khotbehsara et al., 2019) studied the heat transfer and thermal performance of traditional and green roofs in four different climates in Iran. The simulation analysis was carried out by the simulation software DesignBuilder. The heat transfer rate of the roof was obtained from the results for comparison, and the effect of green roof in these four climates was analyzed.
(Zinzi, Agnoil,2011) compared passive cooling techniques and mitigation urban heat island techniques of residential buildings in three locations in the Mediterranean region. This study simulated the energy consumption and temperature of a standard roof, a cooling roof, a reflective coating and a green roof in three locations by DesignBuilder. Thermal comfort was analyzed by comparing the Operative Temperature.
The characteristics of the vegetation that are most important from the standpoint of impacts on the heat transfer through the roof are height, leaf area index (LAI), fractional coverage, albedo, and stomatal resistance (Sailor, 2008).
Zhou et al (2018) proposed the equation of seasonal variable LAI and combined with constant LAI values to make simulation comparison using EnergyPlus.
(Mukherjee et al., 2013) analyzed the influences of these parameters on the performance of greening roof under three different climates by changing the thickness of insulation layer, LAI, and soil thickness.
4. Methodology By combining the methods of the above literature and the parameters of the survey, this study will simulate the green roofs of office buildings in Beijing's Humid Continental Climate by means of building model simulation.
The first step was to investigate the office building and green roof in Beijing, to get the design reference values such as the structural parameters of the office building, the metabolic rate of the activity, the type of plant used for the green roof, etc., and to obtain the weather data of Beijing through EnergyPlus, which is help with the modelling using DesignBuilder.
The building model is designed based on the obtained office building structure data and combined with the requirements of the GB50189-2005 standard. The model is an office building with a length of 11m, a width of 7.8m and a height of 3.5m. There are windows on the south side, and the window-to-wall ratio is 30%. This building is a base case (Fig.6), and multiple sets of simulation comparisons are performed by changing the roof material and parameters. Figure 7 shows a base case roof structure without green roof strategy for comparison.
U-value: Wall-0.5W/㎡/K Floor-0.25 W/㎡/K Window-2.7 W/㎡/K
Fig. 6 base case modal (Source- Author, by Designbuilder)
Bitumen Cement mortar
Concrete Slab
U-value: 0.45 W/㎡/K
Gypsum Plastering
Fig.7 base case roof structure (Source- Author, by Designbuilder)
In view of Beijing's dry and cold winter and hot and rainy summer climate, the maintenance of green roofs and the selection of plant species are particularly critical. Intensive green roof has high maintenance cost and aggravates building load, so extensive green roof was chosen for this study. In terms of plant selection, the first thing is to have good cold tolerance and drought tolerance, and then to have high survival rate, the ability to cover quickly and the greening time is long (Xiao et al, 2014). According to the Beijing Roof Greening Code DB11/T 281-2005, the thickness of the substrate is required to be 10-30cm for the herbaceous or ground cover plants with a height of 0.2-1m. For reference (Zhou et al, 2018), LAI values are 0.1, 3, and 5. Set the green roof with Height (H) =0.2, LAI=0.1, and Soil thickness (ST) =0.1 as the base case. Figure 8 shows the base case structure.
Vegetation
Growing medium Filter Drainage layer Membrane Water proofing
Concrete Slab
Gypsum Plastering
Fig.8 Green roof structure (Source- Author, by Designbuilder)
The parameters investigated were plant height (H), LAI and Soil Depth (SD). It was divided into three groups for simulation comparison, corresponding to three parameters respectively. Finally, the most suitable parameters were selected as the green roof design of Office Building under Humid Continental Climate and improved. Figure 9 shows the scenarios and parameter changes that need to be simulated.
Fig.9 Simulation scenarios and parameters (Source- Author, by Excel)
Assuming that the building is a free-running building for simulation, natural ventilation is adopted, and the air change rate=3 ac/h (Engineering ToolBox, 2005). The building type is office building, people's activity is light work, and the activity's metadata=0.9 is obtained through Designbuilder. Clo (Summer) = 0.56, Clo (Winter) = 1.3 (CIBSE A, 2015). All other designerbuilder input parameters keep the default values. Figure 10 shows the activity profile input by designerbuilder.
Fig.10 activity profile (Source-from Designbuilder)
5.result According to the previous literature, operating temperature was selected as the relevant indicator of thermal comfort to observe the effect of each parameter on the green roof. Figures 11,12 and 13 respectively show the simulation results of the three groups.
Fig.11 Group1 Simulation Results (Source- Author, by Excel)
Fig.12 Group2 Simulation Results (Source- Author, by Excel)
Fig.13 Group3 Simulation Results (Source- Author, by Excel)
6.Discussion The first group showed the effect of changing the height of the plants on the green roof. It is found that green roofs can help to cool the indoor temperature from March to October, and can reduce the temperature by 0.8-1.2℃ in hot summer days. During the winter months from November to February, indoor temperatures in office buildings with green roofs are 0.4 to 0.9 ° C higher than those with traditional roofs. Changes in plant height did not make much difference in the performance of green roofs, but it was found that as the height of plants increased, indoor heat loss became less . In summer, the higher the height of the plant, the less the degree of cooling becomes. In winter, the higher the height of the plant, the stronger the heat preservation ability.
The second group shows the effect of changing the LAI value of plants on green roofs. The comparison result found that the greater the LAI value, the greater the degree of indoor cooling. Same as the first group, changing the LAI value has little effect on the performance of the green roof. In summer, the greater the LAI value, the greater the degree of indoor cooling, and in winter, the greater the LAI value, the weaker the heat preservation ability. This is the opposite of changing the height of plants.
The third group is the comparison of the results of changing the soil thickness. The trend of the comparison of the results is the same as that of the first group. The deeper the soil depth, the lower the indoor cooling degree in summer and the stronger the heat preservation ability in winter.
Sort out the effects of changing these parameters and try to combine with Beijing’s climate to get parameters suitable for Beijing’s green roofs. Considering that the outdoor temperature in Beijing in winter is lower than 0℃, while considering the cooling capacity in summer, it is also necessary to consider the indoor thermal insulation effect. From the above results, the best value of each parameter is obtained to design the green roof. In terms of plant height, a plant height of 0.7m has the best cooling effect in summer and the best heat preservation ability in winter. Select 5 for the LAI value. The reason is that LAI=5 has the best cooling effect in summer, and compared to the other two values, the indoor temperature in winter is not much different. Choose 0.3 for soil thickness. The reason is that it has the best cooling ability in summer and the best heat preservation ability in winter. The final design was simulated, and the operating temperatures of summer and winter typical days of all scenes were compared. The results show that the final design of the green roof has the best thermal comfort (Fig.14,15).
Final Green Roof
Fig.14 Comparison of operating temperatures on summer typical days of each scene (Source- Author, by Excel)
Final Green Roof
Fig.15 Comparison of operating temperatures on winter typical days of each scene (Source- Author, by Excel)
7.Limitation The final design still has some shortcomings. It is only based on the optimal value of thermal comfort and lacks the analysis of energy use. The design can be further verified and improved through energy analysis. The second point is that Beijing's climate is dry and cold in winter, so it is necessary to design an irrigation system to help plants grow. The third point is that according to the research (Zhou et al, 2018), there are seasonal changes in LAI, and it is difficult to maintain a constant LAI value, which will also affect the simulation results and thus the design.
8. Conclusion The purpose of this study is to examine the thermal performance of a wide range of green roofs on office buildings in Beijing in the Humid Continental Climate. Use DesignBuilder to compare the performance of green roofs with that of non-green roofs. Simulation results show that the green roof makes the indoor environment warmer in winter and cooler in summer. The study also investigated the effects of plant height, LAI value and soil thickness on the thermal performance of the green roof, and the optimal parameters obtained were used for the design of the green roof. The thermal performance simulation of the final green roof was carried out and compared with the previous data. The results show that the final green roof has the best thermal performance, but there are still some shortcomings that need to be improved in the future.
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