1. Introduction
The latest report from the Intergovernmental Panel on Climate Change (IPCC) [
1] shows that the global surface temperature was 1.09 °C higher in 2011–2020 than in 1850–1900. It is an unequivocal fact that the global climate is warming due to pollution emissions from human activities. It is quite urgent for humans to take measures to mitigate global climate change. The rapid process of urbanization and urban densification is responsible for global climate warming [
2]. According to statistics from the United Nations [
3], 60–80% of energy consumption and 75% of carbon emissions in the world are from cities at present, although the area of land occupied by cities is only 3% of that of the earth. Furthermore, it is predicted that the proportion of the urban population around the world population will increase from 54% in 2014 to 66% by 2050 [
4]. More fossil energy will be consumed and more pollution will be released by cities if sustainable strategies are not adopted in the future.
To accommodate for this population growth, a large amount of buildings and structures have been built in cities, resulting in large amounts of permeable land, such as bare soil and vegetation, being replaced by impervious concrete surfaces [
5,
6]. Increases in impervious land cause higher urban temperatures than in the surrounding rural areas. This phenomenon is called the urban heat island (UHI) effect [
7]. The UHI can lead to many socio-economic problems, such as increasing the incidence of heat-related mortality and the cooling energy consumption in summer [
8,
9]. Therefore, strategies for mitigating the UHI effect are needed to achieve the goal of sustainable development.
Tian et al. [
10] reviewed mitigation strategies for dealing with the UHI effect. Among these strategies, the construction of green infrastructure was considered a feasible approach to mitigate the UHI effect and global climate warming, as well as other problems. Green infrastructure can be classified into urban green spaces (UGS), green roofs (GR), and green walls (GW) [
10]. UGS include the green spaces provided by urban roads, urban parks, residences, and workplaces. However, in crowded urban areas, the land is very precious and the UGS is very limited. Green roofs, also called as eco-roofs, vegetated roofs, or living roofs, contain plants vegetation on in areas that are usually idle. However, green wall plants and green vegetation on the vertical surfaces of buildings, such as the walls, façades, and blind walls, are also called vertical greening systems (VGS) [
11]. Due to the occupation of urban land, GR and GW have wider application potential as strategies to mitigate the UHI and global warming compared with UGS. Both GW and GR can be classified as greenery systems (GSs) in buildings [
12].
The environmental benefits [
13] provided by GW and GR include improvements in indoor and outdoor thermal comfort and air quality [
14,
15,
16,
17,
18]; reductions in energy consumption via enhancing building thermal performance [
19,
20,
21,
22,
23,
24,
25,
26,
27]; the mitigation of the UHI effect and global climate change by cooling the urban area and lessening of GHG emissions [
28,
29,
30,
31,
32,
33,
34,
35]; decreased urban noise pollution [
36,
37]; relief of urban drainage pressures via storm water management, which is mainly provided by GR (the role of GW on storm water management is limited) [
38,
39]; and the promotion of biodiversity in urban environment [
40,
41]. In addition, GW and GR can also bring about many social benefits, such as improving a city’s image [
42], enhancing the well-being of citizens [
43], and increasing property values [
44].
Due to the significant environmental and social benefits of GW and GF, much research has been focused on this topic, especially regarding the impacts on microclimate conditions and building energy consumption [
45]. Experimental and simulation methods have been adopted in previous studies [
46,
47]. A systematic review on the influence of GW and GF on building environments and energy was presented by Seyam [
48]. He found that six parameters, including solar radiation, ambient/indoor dry-bulb temperatures (DBT), ambient relative humidity (RH), and internal/external surface temperatures of walls and roofs, were frequently measured, while the indoor RH received little attention.
The impacts of GS on temperature reductions (TR) of external and internal surfaces [
48], which are defined as temperature differences between external and internal surfaces of bare and vegetated walls and roofs, were most frequently investigated. The magnitude of the TRs of external and internal surfaces varied depending on the building envelope, the installation location of the GS, the outdoor climate conditions, and the design of the GS. Human occupancy also has a significant impact on the TRs of external and internal surfaces [
49,
50]. For a given GS, the TR value may be positive or negative, depending on the time of the day, season, and solar radiation intensity [
48].
The reductions in temperature differences of external and internal surfaces between envelopes with and without GS have often been the focus, because these are closely associated with heat flux reductions through walls or roofs, thereby affecting the energy savings achieved by GSs. In most studies, the energy consumption was reduced via the use of a GS, while the energy saving rates varied across a wide range, depending on the vegetation coverage, outdoor climate conditions, and GS design [
48].
Compared with TR values of external and internal surfaces and the energy savings achieved by GSs, the impacts on indoor building environments have been less investigated. For an air-conditioned space, the indoor DBT and RH are controlled by the air-conditioning system and are almost unaffected by the GS. However, the experimental results from Hao et al. [
51] showed that the GS can reduce the indoor operative temperature (OT) of air-conditioned spaces by 0.4 °C on average and 2.1 °C in summer. In addition, it can also reduce the oscillations in indoor OT by 1.1 °C. According to the four seasons (spring, summer, autumn, and winter), experiments by Mangone et al. [
52] with indoor plants improved the thermal comfort of an air-conditioned office by 12% compared to an office with an identical indoor temperature set-point but without plants. Lately, research results have revealed that small indoor green walls can help relieve the mental stress of the occupants [
53].
The impacts of GSs on indoor thermal environments in non-air-conditioned spaces are more significant compared with air-conditioned spaces. The experimental results from Olivieri et al. [
49] showed that a vegetation layer reduced the indoor DBT by 4 °C on average in a continental Mediterranean climate zone under summer conditions, while a lower OT in a chamber with vegetation was achieved compared with a chamber without vegetation due to a 5 °C lower internal surface temperature. To investigate the thermal performance of a living wall system (LWS) in a hot and humid climate, Chen et al. [
54] constructed two experimental chambers of identical size, materials, and structure, except that one contained the LWS and the other did not. The experimental results for the two chambers revealed that the indoor DBT of the chamber with the LWS was reduced by 1.1 °C in summer. An LWS was applied on a school building in a hot and arid climate zone and the indoor DBT in the unconditioned school was measured in the peak summer month of July by Haggag et al. [
55]. It was found that the indoor DBT with the LWS was always lower than that without the LWS, with reductions ranging from 2 °C at night to 6 °C during the daytime. Yang et al. [
56] investigated the impact of a vertical GS on an indoor thermal environment in summer without the operation of an air-conditioning system. A double-skin green façade (DSGF) was added to a campus building and the indoor thermal environmental parameters were recorded. The experimental data indicated that the indoor DBT values were reduced on average by 0.6–1.2 °C, while the indoor OT values were lowered on average by 0.6–1.1 °C and 1.9–2.7 °C at maximum owing to the installation of the DSGF.
The transition seasons, which are between winter and summer, show different climatic condition compared with winter and summer, during which the impact of the outdoor environment on the indoor environment has its own features. To create a more comfortable indoor environment, Long et al. [
57] performed an experimental investigation on the natural ventilation performance of a solar chimney system with an earth-to-air heat exchanger during the transition seasons. Cao et al. [
58] investigated the window opening behavior of infant families during transition seasons. Yu et al. [
59] experimentally and numerically analyzed the soil temperatures using a ground heat exchanger in transition seasons. However, there are few studies on the impacts of GSs on indoor environments during the transition seasons.
A literature review showed that less attention has been paid to the impacts of GSs on indoor environments compared with those on energy performance. However, this subject deserves particular attention because humans are spending more time indoors, especially following the coronavirus disease outbreak in 2019. Indoor environments have effects not only on indoor thermal comfort and resident health, but also on energy consumption. In addition, most of the existing studies have been conducted in summer or winter [
48], while few have been conducted in transition seasons, during either spring or autumn. However, the weather fluctuates a lot in transition seasons, which could lead to severe oscillations in indoor environments, causing human discomfort, short-cycling of air-conditioning systems, and switching between heating and cooling demands. These factors not only increase a building’s energy consumption, but also shorten the service life of the air-conditioning system. Thus, technologies that can reduce the oscillations in indoor environments and decrease the energy consumption in transition seasons are preferred. The aim of this paper is to investigate the effects of a GS on an indoor environment and energy consumption in transition seasons through experimentation.
2. Materials and Methods
In order to investigate the influence of GS on indoor thermal environments and building energy performance, an experimental test platform, which consisted of two experiment rooms, both with sensors for environmental parameter measurements and a data collection and recording system, was set up on the top of an office building on the campus of Hunan University of Science and Technology, as shown in
Figure 1. The size, structure, and materials of the walls and roof, as well as the orientations of the two experiment rooms, were identical, except that one room was equipped with GS while the other one was not. The room equipped with GS was called the VGRoom. The other room was used as a reference and was referred to as the RefRoom. The experimental setup had been used to investigate the effects of GS on indoor environments and energy savings in summer and winter [
19,
51]. In this paper, it was used to investigate the performance of GS during transition seasons.
The experimental room measured 3 m × 2.5 m with a height of 3 m. It had lightweight walls and a roof, with an overall heat transfer coefficient of 1.09 W/(m2·K). Two windows measuring 0.9 m × 1.2 m and an insulated door measuring 0.9 m × 2 m were installed in each room. The U-values of the windows and doors were, respectively, 4.1 W/(m2·K) and 2.1 W/(m2·K). A VGS with a living wall structure and GR with modular trays were installed on the VGRoom. Schefflera octophylla (Lour.) Harms and Sedum linear plants were selected for the green walls and roofs, respectively. These are evergreen plants with the characteristics of light favorability, drought tolerance, and easy maintenance.
To record the indoor and outdoor environmental parameters during the experiment, many sensors were installed. Two self-recording sensors (accuracy levels of ±0.5 °C for temperature and ±3% for RH) were used to record the indoor DBT and RH values. The temperatures of interior and exterior wall and roof surfaces of each room were measured using twenty T-type thermocouples, with an accuracy level of ±0.5 °C. The indoor black-globe temperature (BGT) values were measured using two black-globe thermometers, with a globe probe diameter of 150 mm. The accuracy of the black-globe thermometer was ±0.4 °C. The solar radiation was measured using a pyranometer, with an accuracy level of ±5%. The instrument used for measuring outdoor DBT and RH values was the same as that used for indoor measurements. The accuracy levels, measurement ranges, and sampling intervals for all abovementioned instruments are listed in
Table 1.
Figure 2 shows the locations where the sensors were installed in detail. An Agilent data collector was used to automatically collect the experimental data expect for the indoor and outdoor DBT and RH and the BGT, which were automatically recorded by the measuring instruments themselves. More details about the experimental setup can be found in the references [
19,
51].
The experiment was carried out in Xiangtan, a city in Hunan Province in China at 28° N latitude and 112° E longitude. It features typical hot summer and cold winter weather conditions with four distinct seasons. The rainfall in Xiangtan is abundant, especially in spring and summer. It belongs to the Cfa category according to the Köppen–Geiger climate classification [
60]. The transition seasons in Xiangtan are quite long and include two segments, spring (from March to May) and autumn (from October to November). The daily averaged outdoor temperatures in the transition seasons usually range from 12 °C to 26 °C. The experiment was conducted from 1 April 2019 to 31 May 2019. All data were automatically recorded at intervals of 10 min. The recorded data were hourly-averaged and were used for analysis.
During the experiment, both rooms were kept under free-running mode and no air-conditioner or heater was used. The rooms remained closed and unoccupied, except occasionally people entered the rooms to collect experimental data in order to avoid the influence of occupants.
The indoor environment is significantly affected by the outdoor climatic conditions on free-running mode. In the transition seasons, the indoor environment fluctuates a lot due to large fluctuations in the outdoor weather conditions. Measures to mitigate the oscillations of the indoor environment without running an air-conditioning system are preferred, as this leads to energy savings. To evaluate the effects of GS on reducing indoor environment oscillations, two indices, the DBT oscillation weakening rate (
TOWR) and RH oscillation weakening rate (
HOWR), were defined as:
where
and
are, respectively, the average daily standard deviations (SD) of DBT (in °C) and RH (in %). The subscripts
and
denote the RefRoom and VGRoom, respectively. The
and
for RefRoom and VGRoom during the experiment will be analyzed and compared in the Discussion.
For the indoor thermal environmental evaluation, OT, which combines the effects of both convective and radiative heat transfers, is a more reasonable indicator than the DBT. It can be calculated using Equation (3):
where
is the operative temperature in °C;
and
are, respectively, the convective and linear radiative heat transfer coefficients in W/(m
2·°C);
is the indoor DBT in °C.
is the mean radiant temperature (MRT) in °C;
can be determined using Equation (4) [
61].:
where
is the indoor BGT in °C. With the experiment data for indoor DBT and BGT, the MRT and OT can be determined using Equations (3) and (4). According to the suggestions from ASHRAE [
62], values of 3.1 and 4.5 W/(m
2·K) were adopted for
and
, respectively. The impacts of GS on indoor OT will be discussed later.
5. Conclusions
In this study, the effects of GS on the indoor thermal environment and building energy performance during transitional seasons were explored via experimentation. The experiment was conducted in Xiangtan in China, which has a long transition season between the cold winter and hot summer. The experiment lasted for two months in the spring of 2019. The experimental results for two rooms, one with the GS and the other without the GS, were compared and analyzed. During the experiment, no air-conditioning system was run in either room. The findings are summarized below.
The indoor DBT and RH values of both rooms oscillated significantly in the transition season due to great fluctuations in outdoor climatic conditions when the air-conditioning system was not operated. The experimental results indicate an obvious effect of the GS in reducing the oscillation of the indoor environment in the transition season. The maximum daily DBT and RH variations were, respectively, depressed by 3.5 °C and 12.4% due to the application of the GS, although the average values during the experiment were only reduced by 0.9 °C and –2.0%. Two indices, TOWR and HOWR, were defined for evaluating the effects of GS on reducing indoor environment oscillation. The TOWR and HOWR results showed 39.3% and 28.8% oscillation reductions in indoor DBT and RH with the GS. This verified the ability of the GS to maintain a more stable indoor environment.
For various orientations, the external and internal surfaces temperatures were reduced by 1.6~4.1 °C and 0.2~1.3 °C on average by the GS, respectively. The maximum reductions in external and internal surface temperatures were 29.5 °C and 9.4 °C, respectively. For the room with the GS, the average internal surface temperatures of all walls and the roof were higher than for the corresponding external surfaces, indicating an outgoing heat flux throughout the whole day. However, the average temperature differences of external and internal surfaces for the RefRoom were either positive or negative, depending on the orientation of the wall. The temperature difference during the daytime was much higher than that throughout the whole day.
The OT for the room with the GS was lower than that without the GS during the daytime on sunny days. However, almost identical OT values were observed at night and on cloudy or rainy days. The differences in OT between the two rooms ranged from −1.8 °C to 8.2 °C, with an average of 1.0 °C. This differences were higher than those found by Yang et al. [
56] for a non-air-conditioned room and by Hao et al. [
51] for an air-conditioned room in summer. This reveals that the regulating effect of the GS is affected by the outdoor climate conditions and the air-conditioning system.
The experimental result showed that the GS can improve the indoor thermal comfort and decrease the thermal dissatisfaction by 7.9% in the transition season. A 15% longer time that the indoor conditions can meet the thermal comfort criterion was achieved by the VGRoom. The role of the GS in improving indoor thermal comfort was more significant in the daytime. The time that the indoor conditions satisfied the thermal comfort criterion was lengthened by 28% during the daytime. This indicates a reduction in air-conditioning operating time and that energy savings can be achieved.
In this paper, we presented the results of research on the impacts of the GS on an indoor microclimate. It will be helpful for designers and building owners to learn about the role of the GS in improving indoor environments and reducing energy consumption. However, the influence of a building’s dynamic properties, such as the thermal capacity of the outer envelope and the air tightness, was not considered in this paper. For future research, an investigation on the dynamic properties of the test object and a comparison and comparative analysis of CO2 concentrations in the tested rooms and the external environment shall be included.