Experiment and Simulation-Based Study of Energy Efficiency of Green Façade Retrofit of Existing Buildings in Rural Northern China ^†

Abstract

As China’s urbanisation continues, the building area is expanding, of which the occupancy of rural residential buildings is also very large. However, most rural dwellings lack insulating structures and have poor thermal performance. This paper verifies and analyses the energy-saving potential of green façades for rural houses in northern China through comparative experiments as well as software simulations. The experiments were conducted from July to August 2024 to verify the reliability of the software simulations. And the simulation was carried out on an existing house in rural northern China. The experimental results show that the reference room consumes 1.84 times more electricity than the vertical greenery room, and the vertical greenery achieves a good energy saving of 45.75%. According to the simulated data, the building energy efficiency of rural houses in northern China after green façade retrofitting is obvious, the energy-saving rate reaches 14.94%, and 713.32 KWh of electricity can be saved in the whole cooling period.

1. Introduction

With a population of 1.4 billion, China is the most populous country in the world. Therefore, the demand for construction in China is extremely large. China’s construction industry continues to develop and grow in size. Compared with cities, rural development is relatively lagging behind, and most rural houses are self-built. Based on cost considerations, houses usually have no insulation, which results in poor thermal performance and forces them to require more energy to maintain indoor thermal comfort. This shows that there is a huge potential for energy efficiency retrofitting of rural buildings in China.

As we all know, green space plays an indispensable role in our daily lives. Plants can not only cool down the surrounding air through evapotranspiration, but also absorb a large amount of solar radiation, reducing the heat gain on the building surface and helping to alleviate the heat island effect. Increasing green areas can also effectively reduce carbon emissions and play the role of carbon sequestration and oxygen release.

In recent years, nature-based solution (NbS) has attracted more and more attention, as it is the key to sustainable development. In the construction industry, green façades are becoming increasingly popular. Green façades can have several positive effects on the building envelope. Firstly, the green façade provides effective shading. As a result, the building will absorb less solar radiation. Also, the surface temperature of the building is reduced. The green façade provides a passive cooling effect, which reduces the use of indoor air-conditioning systems and reduces the building’s energy consumption [1,2]. Secondly, the relative humidity around the building is increased through the transpiration of plants, thus providing a cooling effect on the air temperature around the building [3]. Finally, the leaves of green plants are also effective in reducing the wind speed near the building façade [4]. In addition, several studies have shown that green façades have other excellent effects. Examples include mitigating the heat island effect [3,5,6], improving air quality [7,8], reducing noise pollution to provide a better acoustic environment [9,10], promoting biodiversity [11,12], contributing to the physical and mental health of occupants [13], and enhancing the aesthetic attributes and values of buildings [14].

Many researchers have conducted experimental studies on the energy efficiency of green façades. For example, Hang Tan et al. studied the energy efficiency of indirect green façades in Xiangtan, China. The winter experiment was conducted in January with a 5-day experimental period, and the summer experiment was conducted in July with a 9-day experimental period. Compared with the reference room, the vertical greenery room achieved good energy savings of 18% in winter and 25% in summer [15]. Coma et al. set up three identical rooms in Spain and conducted comparative tests of living walls, indirect green façades and bare walls during the summer months. The results showed that living walls were the most energy efficient with 58.9% energy savings compared to bare walls, and indirect green façades, with 33.8%. In addition, neither the living wall nor the indirect green façade significantly increased the building’s energy consumption in winter [16]. Perez et al. studied the energy savings of indirect green façades throughout the year in Spain, where they were able to achieve 54% energy savings in early summer and 30% savings in late summer [17]. Perini et al. conducted a field experimental study of a vertical greenery system in an office building in Genoa, Italy, where the vertical greenery system significantly lowered the building surface temperature, with an energy saving of 26% [18].

Some other researchers have conducted simulation studies of green façades through software. For example, Bagheri et al. investigated the effect of indirect green façade orientation on energy efficiency through simulation using Green Building Studio. According to their study, indirect green façades oriented in the southeast and southwest directions were the most energy-efficient, reducing energy consumption by 17% and 17.9%, respectively, while the north direction was the least effective, able to reduce energy consumption by only 0.5% [19]. The effect of adding vertical greenery to the courtyard area of an existing building was studied in Ningbo, China by Li et al. They ran simulations through Designbuilder, which showed that the vertical greenery system was able to reduce the cooling load by 8.8% and the heat load by 1.85%, with the total energy saving for the year expected to reach 28% [20]. Poddar et al. investigated the role of green façades in reducing winter heating energy use in Korean universities. They also chose to use Designbuilder for their simulations, which showed that the vertical greenery system was able to save 60%, 7% and 3% of heating energy in the dormitory, research and administration buildings, respectively [21].

Currently, many studies are being carried out on some buildings or single experimental rooms in cities [2,22,23], and very few studies are carried out on residential buildings in rural areas. In order to verify the energy-saving effect of green façade retrofitting for rural houses in northern China, comparative experiments and software simulations are carried out in this paper. The power consumption of rooms with indirect green façades and bare-wall rooms was experimentally monitored and simulated for a typical rural house in northern China to determine the energy savings of green façades.

2. Methods

2.1. Experimental Setup

We constructed two identical experimental rooms in an open field in Binzhou. One was surrounded by an indirect green façade (i.e., IGF room) while the other had no vertical greenery system (i.e., Ref room) (Figure 1). Both rooms measured 3 m × 3 m × 3 m. Each room had two windows located in the south (1.2 m × 1.2 m) and north (0.9 m × 1.2 m) walls. There is a 0.9 m × 2 m lightweight entry door on the north wall of the room. The walls of the room were lightweight walls of sandwich construction with 2 mm thick steel sheets on both sides and 50 mm asbestos as the sandwich material. Air conditioners of the same make and model were installed in both rooms. Power consumption was measured through the smart meter connected to the air conditioner. The plant chosen for the indirect green façade was the five-leaf groundnut, which is a native species of the experimental area and is very suitable for the local climate, with luxuriant branches and leaves in summer, and is well-adapted to hot weather.

2.2. Simulation Setup

The simulation software used in this study was Designbuilder, which has the advantage of being both easy to operate and also having a powerful material editing function, which allows new materials to be created through the green roof module and the corresponding parameters of the green façade to be set. The green façade was then equated to an equivalent layer of thermal resistance attached to the building façade to produce accurate simulation results.

2.2.1. Reliability Verification of Designbuilder

Firstly, the experimental room was modelled in Designbuilder, then the parameters of the façade, roof, floor, air conditioning system and vertical greenery system were set. Finally, the energy consumption of the experimental room was simulated to produce relevant data. At the end of the simulation, the reliability of the simulation is verified by comparing the mean bias error (MBE) of the experimental and simulated data.

2.2.2. Energy Consumption Simulation of Typical Building

Firstly, the selected typical residence is located in a village in Binzhou City, Shandong Province. Binzhou City is located in the north of China. The annual electricity consumption of Binzhou City is among the top in China, exceeding that of megacities such as Beijing. So, it is of great significance to select buildings in this area for energy consumption simulation and energy-saving retrofit studies.

Secondly, the authors researched 50 local houses through field visits. Among them, only 2 were two-storey houses and the others were all single-storey houses (Figure 2). Moreover, all the houses were orientated in a south direction. By asking the villagers, it was learnt that this is to satisfy the need for light, especially in winter, as it raises the temperature in the room.

Finally, the typical residential buildings in northern China were modelled in Designbuilder. The model of the typical residential building was set to be single storey, facing south, and with no green façade on the south wall in order to meet the lighting requirements. Figure 3 shows the photos of the typical residential houses in northern China and the plan of the model. Based on the data obtained from the actual research, the specific parameters of the building envelope are listed in

Table 1. Building envelope parameter settings.

Projects	Structures	U-Value
External wall	Cement mortar (40 mm) + clay bricks (200 mm) + cement mortar (40 mm) + gypsum veneer (10 mm)	1.898 W/m2·K
Internal wall	Gypsum veneer (10 mm) + cement mortar (20 mm) + clay bricks (200 mm) + clay bricks (20 mm) + gypsum veneer (10 mm)	1.674 W/m2·K
Roof	Cement mortar (40 mm) + reinforced concrete (200 mm) + gypsum veneer (10 mm)	3.108 W/m2·K
Window	Single-layer glass (6 mm) and wooden frames	5.7 W/m2·K
Door	Wooden single-panel solid doors	3 W/m2·K

.

The parameter settings of the air-conditioning system are a key factor in the accuracy of the building energy simulation results. And it should be as consistent as possible with the actual situation. According to the data from the field research, the building area of a typical residential house is about 100 m², of which only the bedroom and the living room will use air conditioning, with an area of about 70 m². On average, there were about 3 people per household who are at home all year round, so the room’s personnel density was set at 0.043 people/m². In this study, cooling was carried out by split air conditioning units in summer, and then the building energy consumption was quantified in terms of electrical energy consumption, with specific parameters shown in

Table 2. Parameter settings for air-conditioning systems.

Projects	Parameter Settings
Refrigeration setting temperature	24 °C
Cooling period	1 June to 31 August
Personnel density	0.043 people/m2
Room illumination	150 lux
Equipment heat gain	12 W/m2

.

The physiological characteristics associated with the plants used in vertical greenery and the parameters that impact the thermal performance are shown in

Table 3. Green façade parameter settings.

Projects	Parameter Settings
Height of plants (m)	0.2
Cavity thickness (m)	0.2
Leaf area index	1.94
Leaf reflectivity	0.4
Leaf emissivity	0.8
Minimum stomatal resistance (s/m)	120
Max volumetric moisture content at saturation	0.5
Min residual volumetric moisture content	0.01
Initial volumetric moisture content	0.15

. In this study, it was assumed that the data of the plants remain constant and are not affected by external influences.

Finally, the energy consumption in the bare wall case and the case with vertical greenery were simulated separately to analyse the energy savings provided by the vertical greenery system.

3. Results and Discussion

The experiment was carried out from 15 July to 19 July 2024. The total cooling time was 120 h. Figure 4 and Figure 5 show the solar radiation intensity and outdoor air temperature and humidity during the experiment, respectively. According to ASHRAE standards, it is recommended to keep the indoor temperature between 22 °C and 26 °C in summer. Therefore, in this experiment, the air conditioning set temperature was taken as an average value and set to 24 °C, and the air speed was set to automatic.

Figure 6 shows the energy consumption of each room. It can be clearly seen that the reference room consumed significantly more power than the vertical greenery room. During the whole experiment period, the reference room consumed the most power, which reached 39.373 KWh, while the vertical greenery room only consumed 21.358 KWh. The energy-saving rate reached 45.75%. This shows that green façades can play a positive role in reducing the energy consumption of buildings. The plants in the green façade successfully block a large amount of solar radiation by their own biological properties. The green façade acts as an additional layer of thermal resistance, which reduces the total amount of heat gained by the building, thus reducing the load on the air-conditioning system and, ultimately, reducing energy consumption.

Subsequently, the experimental room was simulated in Designbuilder, and the simulation dates were also from 15 July to 19 July, and the simulation results showed that the reference room consumed 39.56 KWh of electricity, while the vertical greenery room consumed 21.7 KWh. Figure 7 shows the experimental and simulated trends in energy consumption for the reference and vertical greenery room. According to the figures it can be seen that the curves of the experiment and the simulation can be in good agreement. Some fluctuations in the curves are due to the difference between the weather conditions during the simulation and the experiment. By calculating the MBE (mean bias error) for the power consumption data obtained from the field experiment and the data obtained from the simulation, it is concluded that the MBE of the reference room is 3.8% and the MBE of the vertical greenery room is 6.8%, which is in the acceptable range of MBE < 10% in accordance with the requirement of ASHRAE Guideline 14. Therefore, the model developed is reasonable.

Finally, the simulation was carried out for a typical residential house in northern China for the whole cooling period, from 1 June to 31 August. Figure 8 shows the energy consumption in each month for the bare wall and after performing the indirect green façade retrofit. It can be seen that good energy savings were achieved in each month with the indirect green façade. In June, July, and August, the bare wall case consumed 1329.21 KWh, 1714.96 KWh, and 1729.35 KWh, respectively, and the green façade case consumed 1143.01 KWh, 1427.65 KWh, and 1489.54 KWh, respectively. For the whole cooling season, the total energy consumption was 4773.52 KWh for the bare wall case and 4060.2 KWh for the green façade case. And the energy-saving rate in the cooling period reached 14.94%.

4. Conclusions

The energy-saving effect of rural buildings in northern China after green façade retrofitting was evaluated through field experiments and software simulations. After the green façade retrofit, the building energy consumption during the cooling period was significantly reduced. In the hot summer, the green façade provides additional shade to the building and increases the overall thermal resistance of the building, thus achieving good energy efficiency. In the experiment, the green façade achieved an energy-saving rate of up to 45.75%. In the subsequent simulation, the energy-saving rate of residential houses with green façade retrofitting only reached 14.94%. The main reason for this difference is that the green area becomes proportionally smaller compared to the building area, so it does not achieve the best results in the experiment.

In conclusion, green façade retrofitting can have the effect of reducing the energy consumption of a building and improves the energy-saving effect for small-sized rooms. At the same time, this study still has some shortcomings, and we will continue to conduct in-depth research in the future. For example, a comparative study on the thermal performance and energy-saving effect between a direct green façade, an indirect green façade and a living wall will be conducted. A detailed study of the costs and benefits of green façade retrofitting will be conducted to explore the practical feasibility of green façade retrofitting.