Study on the dynamic heat transfer role of vertical greening in building microclimate based on multi-objective coupling

Global climate change and the acceleration of urbanization have increasingly aggravated the problems of building energy consumption and urban heat island effect, posing major challenges to urban sustainable development and residents' quality of life. As a green building technology, the vertical greenery system (VGS) integrates vegetation into building facades, providing multiple functions such as shading, heat insulation, and environmental enhancement, thereby offering significant ecological and energy-saving benefits.^1,2 Therefore, it is of great significance to explore the performance and mechanism of vertical greening in the heat transfer process for realizing building energy-saving and improving residents' comfort.

Existing studies have shown that vertical greening can significantly reduce the surface temperature of building exterior walls, reduce the cooling load inside buildings, and improve air quality by absorbing airborne particles and pollutants.^3–5 However, most current studies mainly focus on the evaluation of the cooling and energy-saving effects of vertical greening under static conditions, lack systematic analysis of the dynamic heat transfer process, and mostly use a single model for simulation, failing to fully consider the coupling between different models, resulting in certain limitations in the accuracy and reliability of their results. In order to solve the above problems, based on the coupling model of ENVI-met, EnergyPlus, and Fluent, this paper systematically evaluates the influence of vertical greening on microclimate factors such as building surface temperature and wind speed and discusses the feedback effect of heat emission from air conditioning operation on the local environment. The research results reveal the temperature control characteristics and energy-saving effects of vertical greening systems in the dynamic heat transfer process, which not only enrich the theoretical research on the dynamic heat transfer mechanism of vertical greening in the field of building microclimate but also provide a new exploration path for the application of multi-objective coupling model in green building technology evaluation.

This study is structured as follows: (1) Introduction, which presents the study's background, analyzes the shortcomings of existing studies, and introduces the purpose and significance of this research, (2) Literature Review, summarizing existing studies on vertical greening in improving building microclimate, reducing energy consumption, and their dynamic heat transfer mechanisms, (3) Research Design, describing in detail the construction methods of the ENVI-met, EnergyPlus, and Fluent models, (4) Results and Analysis, presenting the simulation results of the models and comparing the effects of vertical greening on the microclimate and dynamic heat distribution of the target building, (5) Discussion, providing an in-depth analysis of the study's results, contributions, and limitations, and (6) Conclusion, summarizing the main findings of the study and indicating future research directions.

Architectural microclimate refers to the local climate environment around a building, generated by the building itself and its surroundings. Its formation is influenced by factors such as the building's shape, material, orientation, and layout, as well as the surrounding vegetation, water bodies, and topography.^6,7 Building microclimate can significantly affect a building's energy consumption, comfort, and indoor and outdoor environmental quality. Vertical greening, an emerging green building technology, has shown notable effects in improving building microclimate, and its role in microclimate regulation has begun to attract the attention of experts and scholars.^8,9

Studies have shown that vertical greening can significantly reduce the surface temperature of building façades, thereby reducing the cooling load inside the building. Campiotti et al. found that in July, the surface temperature of building façades with vegetation cover was 13 °C lower than that of bare walls, saving 2.22 and 1.94 kW h/m² of cooling electrical load in the summer months of 2019 and 2020,¹⁰ respectively. Freewan et al. investigated the shading and energy-saving performance of vertical greening systems (VGSs) in tropical climates and showed that the green wall system reduced external wall surface temperatures by 6–11 °C at different times and reduced internal wall surface temperatures by an average of 5 °C.¹¹ In addition, vertical greening positively affects the building microclimate by absorbing airborne particles and pollutants and improving the surrounding air quality. Viecco et al. assessed the effectiveness of green roofs and walls in mitigating urban air pollution by simulating different building heights and green infrastructure coverage. The results show that green walls with 25% coverage in low-rise buildings can effectively capture PM2.5, significantly reducing its concentration in the surrounding air. In contrast, green roofs are more effective in improving air quality for buildings less than 10 meters in height..¹² Srbinovska et al. continuously measured the concentration of particulate matter in the open area around a small green wall in Skopje and similarly found that the green wall significantly reduced PM2.5 and PM10 concentrations, improving the surrounding air quality.¹³ 

In addition, vertical greening can improve indoor air quality and reduce indoor carbon dioxide concentration by absorbing carbon dioxide and releasing oxygen through plant photosynthesis, thus improving the occupants' health. Shao et al. explored the effect of vertical greening on carbon dioxide concentration in an office building. The results showed that vertical greening can significantly reduce indoor carbon dioxide concentration and ventilation energy consumption, thereby improving indoor air quality and comfort.¹⁴ Taemthong and Cheycharoen found that green wall systems installed in classrooms were effective in reducing CO₂ concentration, with the most effective plants under optimal light reducing the concentration of CO₂ at a rate of 1.74 ppm.¹⁵ 

In general, vertical greening can significantly improve the building microclimate by lowering the building surface temperature, absorbing particles and pollutants, and reducing indoor carbon dioxide concentration. This plays an important role in energy-saving, improving air quality, and enhancing indoor environmental comfort. However, most existing studies focus on the effects of vertical greening under static conditions. Therefore, based on existing research results, this study constructs a multi-objective coupled model to further explore the dynamic heat transfer effects of vertical greening in building microclimates and address the shortcomings of previous studies.

With the acceleration of urbanization and the increase in building density, the urban heat island effect is becoming increasingly significant, resulting in higher ambient temperatures and increased energy consumption.^16,17 Vertical greening systems not only beautify the urban environment but also significantly improve the thermal performance of buildings and reduce energy consumption by incorporating plants on building façades.¹⁸ Therefore, the dynamic heat transfer mechanism of vertical greening systems has become an important research direction in the field of the built environment in recent years.

Vertical greening systems mainly include two forms: green walls and vertical gardens. Their heat transfer mechanisms are primarily realized through shading, cooling, thermal insulation, and wind barrier effects.¹⁹ Studies have shown that vertical greening systems can reduce the cooling load by lowering the building surface temperature through the shading effect of plants. Convertino et al. found that green façades significantly reduced building façade and surface temperatures through transpiration and shading. The average daily cooling effect of the wall was 16.2 MJ/m² in the summer season, with shading contributing about twice as much as transpiration.²⁰ In addition, vertical greening systems reduce heat transfer from building surfaces by consuming solar radiation through transpiration and plant photosynthesis. A study in the Kathmandu Valley, Nepal, showed that direct greening of façades significantly reduces solar radiation absorption and lowers building surface temperatures. Green walls have a significantly lower heat capacity than bare walls during the summer months, reducing heat storage and improving thermal comfort.²¹ 

In experimental studies, the thermal performance of vertical greening systems is usually evaluated by combining experimental measurements and numerical simulations. Guo et al. combined numerical simulations and field measurements to analyze the effects of vertical greening and different building layouts on the outdoor microclimate of a traditional urban neighborhood in Shanghai. The results showed that the south-facing vertical greening scenario provided the best thermal benefits in both semi-enclosed and fully enclosed neighborhood layouts.²² Bahdad evaluated the effectiveness of vertical green walls in reducing indoor air temperatures and building energy consumption using a Building Information Modeling (BIM) simulation methodology. The results showed that the green wall system could achieve total energy savings of 13.63% during the cooling season.²³ Daemei and Jamali's experimental analysis of an office building with a green wall in Rasht during winter and summer, combined with annual simulations using DesignBuilder software, showed that the green wall can reduce heat transfer and heat loss by approximately 42% during the summer season and significantly improve energy efficiency in humid climates.²⁴ 

However, the heat transfer mechanism of vertical greening systems is affected by various factors, including plant species, green coverage, building materials, and environmental conditions. Perera et al. conducted numerical simulations in three different tropical environments using the ENVI-met 4.4.5 software and evaluated the thermal performance of nine tropical plant species in vertical greening systems. The study found that Axonopus fissifolius had higher coverage on the green wall, resulting in the greatest cooling effect (5.06 °C), indicating a differential impact of plant species on the thermal performance of vertical greening systems.²⁵ Pichlhöfer et al. found that the heat transfer coefficient of a masonry wall covered with ivy was reduced by 30% compared to an ungreened wall in winter, indicating that ivy significantly improves the wall's thermal performance. This suggests that ivy can reduce energy consumption and demonstrates that the thermal performance of vertical greening systems can be influenced by building materials and environmental conditions.²⁶ In addition, the root volume and water supply of plants also have an important effect on the thermal performance of vertical greening systems. Chung et al. found that a larger root volume can enhance early plant growth and wall coverage, effectively improving the cooling effect of greening systems.²⁷ 

It is evident that vertical greening is outstanding in improving the microclimate and thermal performance of buildings, which is of great significance in enhancing indoor air quality and the health of occupants. Based on this, this study utilizes ENVI-met, EnergyPlus, and Fluent to construct a model to bi-directionally couple the microclimate and air conditioning energy consumption around the building and further analyze the integrated role of the vertical greening system in the dynamic heat transfer process.

The ENVI-met model is used to simulate and analyze changes in the urban microclimate and its interactions with green infrastructure. The ENVI-met model can detail changes in air temperature, humidity, wind speed, wind direction, and more in the urban environment. Therefore, this study uses the ENVI-met model to construct the target building model, setting the building's window-to-wall ratio to 0.25, and constructs the target building model with and without vertical greening. Scenarios with and without vertical greening are also constructed. In this study, row and column configurations are selected as the target building group type according to urban planning and design requirements. The target building is located at 31.23°N, 121.47°E, in the subtropical monsoon climate region, where the average summer temperature ranges from 21.0 to 28.0 °C. The model size of the block is 200 × 150 × 50 m³, with a grid size of 2 × 2 × 2 m³, comprising 100 × 75 × 25 pieces.

The simulation time is from 0:00 to 24:00 on July 10. The maximum temperature on that day is 31.8 °C, the minimum temperature is 24.1 °C, the minimum relative humidity is 37%, and the maximum relative humidity is 94%. The wind is from the south, and the wind speed at a height of 15 m above the ground is 6 m/s. The target building in the ENVI-met model is divided into four parts: the wall, the roof, the street, and the window. The main materials for the wall are clay bricks and concrete; for the roof, asphalt shingles and ceramic tiles; for the street, asphalt and concrete; and for the window, tempered glass. The vertical greening system of the target building consists of ivy and hanging orchids located on the walls and roof. The relevant material parameter settings and vertical greening system parameter settings of the target building in the ENVI-met model are shown in Tables I and II, respectively. The time step is set to 2 s, and the indoor environment is set to 25 °C. Meanwhile, the Modified Turbulent Kinetic Energy (TKE) Model turbulence model and Full Forcing boundary conditions are adopted.

The EnergyPlus model integrates extensive meteorological data to simulate the energy performance of a building under various climatic conditions, allowing for the evaluation of building performance in specific climates. In this study, an EnergyPlus model is constructed based on the ENVI-met results to investigate changes in air conditioning heat transfer due to microclimate changes from vertical greening. In the EnergyPlus model, the Thermal Analysis Research Program (TARP) method is used for convective heat transfer on the inner surface, the DOE2 method for convective heat transfer on the outer surface, and the heat balance is based on the conduction transfer function with a time step of 4 h. The indoor environment is set to 25 °C, and the wall temperature results generated by the ENVI-met model are entered every hour.

This study simulated the microclimate within 1.5 m around the target building and compared the results with actual meteorological data, as shown in Fig. 1. As shown in Fig. 1, the lowest temperature around the target building appeared at 6:00, before sunrise, at 23.7 °C, which was 0.6 °C lower than the actual meteorological data (24.1 °C). The highest temperature (33.6 °C) occurred at 15:00, which was 1.8 °C higher than the actual meteorological data (31.8 °C). This phenomenon is similar to the research conclusion of Nocera et al.²⁸ The temperature fluctuation around the target building throughout the day is larger than that of the actual meteorological data, with higher extremes and lower minima. This indicates that the temperature around the target building is significantly influenced by its surface temperature. Meanwhile, the ENVI-met model results show that the average wind speed around the target building is 4.5 m/s, lower than the set wind speed of 6 m/s. This is because the building directly blocks wind flow. Additionally, the narrow tube effect created by the gaps between buildings affects wind speed distribution, lowering wind speed in some areas. The average temperature around the target building is 28 °C, and the average heat island intensity is 1.15 °C.

This study investigated the microclimate effects of vertical greening on the target building from 6:00 to 24:00. Figure 2 shows the temperature comparison around the target building before and after vertical greening. After vertical greening, the maximum temperature around the target building is 30.95 °C, which is 2.65 °C lower than before vertical greening. This is because vertical greening effectively shades solar radiation by covering the walls and roof of the target building, reducing direct sunlight on the building's surface. Additionally, photosynthesis in the plants reduces leaf transpiration and absorbs heat, thus lowering the temperature of the building surface and its surroundings. The lowest temperature is 24.65 °C, an increase in 0.95 °C. This increase is because the solar energy absorbed by the plants during the day is slowly released at night, and the plant cover reduces the dissipation of long-wave radiation from the building surface and its surroundings, maintaining higher temperatures at night. The change in average temperature before and after vertical greening is 0.9 °C, which aligns with the actual situation.

Figure 3 shows a comparison of wind speed around the target building before and after vertical greening. As seen in Fig. 3, after vertical greening, the maximum wind speed around the target building decreases from 5.2 to 4.8 m/s. This is because the leaves and branches increase the roughness of the building surface, absorbing part of the wind's kinetic energy. Vertical greening also increases turbulence and eddies near the building surface, leading to wind speed dispersion and attenuation, thus decreasing the maximum wind speed. The minimum wind speed around the target building changes from 3.8 to 4.0 m/s due to the presence of vertical greening, which increases the air space between the buildings. Vertical greening increases the roughness and air resistance of the gaps between buildings, which attenuates the slit tube effect and may increase the wind speed in these areas. The presence of vegetation alters and complicates airflow paths, reducing areas of completely static air between buildings. The average wind speed slightly decreased after vertical greening, consistent with related studies.^29,30 Based on the data, the average heat island intensity around the target building decreased from 1.15 to 0.3 °C after vertical greening, indicating that vertical greening significantly mitigated the heat island effect.

In this section, the ENVI-met results are coupled with EnergyPlus to explore the impact of vertical greening on air conditioning in the target building. Figure 4 shows a comparison of air conditioning power consumption in the target building before and after vertical greening. The figure shows that the air conditioning power consumption in the target building before vertical greening is 1770 W, and after vertical greening, it is 1239 W. This indicates that vertical greening has an energy-saving effect on air conditioning in the target building. The plant cover increases the thermal resistance of the building surface, significantly reducing the amount of heat conducted through the building envelope. Additionally, the plants regulate the humidity and temperature of the surroundings, making the air around the building cooler and more humid, thereby reducing the energy demand of the air conditioning system.

In this study, Fluent was further coupled with ENVI-met and EnergyPlus to explore the effect of air conditioning heat transfer on the microclimate of the target building after vertical greening. As seen in Fig. 6, between 6:00 and 24:00, the temperature around the target building with air conditioning heat transfer ranges from 26.0 to 32.8 °C, which is higher than the temperature of the target building without air conditioning heat transfer, with a maximum difference of 1.3 °C. This is because the operation of the air conditioning system increases heat emission around the target building and exacerbates the local heat island effect. Although the vertical greening system can slow down part of the heat conduction and radiation, it cannot completely eliminate the heat impact from the air conditioning system.

Figure 7 demonstrates the variation of wind speed around the target building between 6:00 and 24:00 with and without air conditioning heat exhaust. The wind speed around the building is significantly higher with heat emission than without, with an average wind speed of 4.3 m/s without air conditioning heat exhaust, and 5.0 m/s with it. This may be due to air conditioning exhaust heat increasing the local air temperature, which enhances the hot air rise effect, increasing the wind speed around the building. Meanwhile, data showed that the average heat island intensity around the target building after vertical greening increased from 0.3 to 1.52 °C with air conditioning heat transfer, reaching the limit set by the code for average heat island intensity (1.5 °C). This result is consistent with the conclusion of Senalankadhikara et al.,³² in the study on heat dissipation of the built environment at night in summer, indicating that vertical greening cannot completely offset artificial heat emission at night.

This study aims to evaluate the impact of vertical greening systems on the interactions between the microclimate of a building complex and its energy consumption. Initially, the study explores the interactions between air conditioning and the outdoor microclimate of the target building using ENVI-met coupled with EnergyPlus. Then, Fluent is used to analyze the influence of the vertical greening system on the bidirectional coupling effect between air conditioning energy consumption and the building's external microclimate. The results indicate that the vertical greening system significantly improves the building's thermal performance, reduces air conditioning energy consumption, and mitigates the urban heat island effect.

The contributions of this study are as follows: First, existing studies have focused on the effect of greening on building surface temperatures. Campiotti et al. showed that green walls can reduce heat flux into a building by lowering the air temperature around it, especially in summer.³³ Another study using boosted regression tree modeling showed that green coverage significantly reduces surface temperature. High-rise buildings with green spaces tend to have more significant cooling effects.³⁴ This study, however, provides a more comprehensive perspective by incorporating multiple aspects of microclimate change, wind speed variations, and air conditioning heat transfer through multi-model coupling. Second, this study delves into the heat transfer mechanism of vertical greening at different times by refining the dynamic heat transfer process. Compared to studies under static conditions, such as Convertino et al.,³⁵ which simulated temperature changes in a building using an energy balance equation with input parameters including outside air temperature, relative humidity, solar radiation, wind speed, and plant and building characteristics, this study reveals the varying performance of vertical greening during day–night and seasonal variations, providing a more valuable reference for practical applications.

The limitation of this study is that, although ENVI-met, EnergyPlus, and Fluent models were used for coupling, the interaction and influence of these models remain complex issues. The coupling between the models may introduce errors, affecting the reliability of the results. Additionally, there are discrepancies between the actual environment and the models. The Fluent model simplifies air conditioning heat transfer to the form of hot air, whereas the actual air conditioning system may be more complex in operation and heat transfer processes. Therefore, the models' simplifications and assumptions may affect the accuracy of the results.

This study systematically analyzes the interaction between microclimate and air conditioning heat transfer in the target building by coupling ENVI-met, EnergyPlus, and Fluent models, using a row-type building complex as the research object. The role of the vertical greening system in the dynamic heat transfer process is further explored, with specific conclusions as follows:

1. Vertical greening significantly reduced the temperature around the target building, decreasing the maximum temperature by 2.65 °C during the day and slightly increasing the minimum temperature by 0.95 °C at night. Additionally, vertical greening reduced the maximum wind speed from 5.2 to 4.8 m/s and slightly increased the minimum wind speed from 3.8 to 4.0 m/s. The heat island intensity decreased from 1.15 to 0.3 °C. These changes indicate that vertical greening reduces the temperature of the building surface and its surroundings through shading and plant transpiration and mitigates the localized heat island effect by increasing surface roughness and absorbing wind kinetic energy.

2. Vertical greening significantly reduced air conditioning energy consumption in the target building. By reducing heat transfer from the building surface, vertical greening lowered air conditioning power consumption from 1770 to 1239 W, saving 531 W. Additionally, vertical greening reduced the outlet temperature of the air conditioning condenser by 4.91–9.45 °C, especially during the daytime, indicating its effectiveness in lowering the load and energy demand of the air conditioning system.

3. With air-conditioned heat transfer, the temperature around the target building fluctuated between 26.0 and 32.8 °C, which was higher than without air-conditioned heat transfer, with a maximum difference of 1.3 °C. The temperature of the target building was higher with air-conditioned heat transfer, with a maximum difference of 1.3 °C. Additionally, the average wind speed with air-conditioned heat transfer was 5.0 m/s, compared to 4.3 m/s without it. However, the heat island intensity increased from 0.3 to 1.52 °C, indicating that vertical greening could not completely eliminate the local heat island effect caused by air conditioning heat emission. It exacerbated local air movement and the rise of hot air, increasing the heat island intensity.

In summary, this study systematically analyzed the dynamic heat transfer effects of vertical greening on building microclimate and air conditioning energy consumption using a multi-model coupling approach. It revealed the heat transfer mechanisms and energy-saving effects of vertical greening under various environmental conditions. Future studies can employ advanced numerical algorithms and computational techniques to optimize data transfer and interaction between ENVI-met, EnergyPlus, and Fluent models, reducing coupling errors and improving result reliability. Additionally, multidisciplinary research and long-term monitoring should be combined to optimize and validate the impact of vertical greening on building microclimates, providing more scientific and effective solutions for sustainable urban development.

This research was funded by Huangshan University research startup project, Grant No. 2023xkjq011.

The authors have no conflicts to disclose.

Liang Qiao: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.