1. Introduction
With the aggravation of global warming and the expansion of urbanization, serious environmental problems such as air pollution, large anthropogenic heat release, and urban heat island are severer day by day [
1,
2,
3,
4]. The Urban Heat Island has caused outdoor thermal environments in residential areas to become worse [
5]. Body discomfort and heat stroke rates have increased significantly because of the deterioration of the indoor and outdoor thermal environment [
6,
7]. Since people feel hotter in the summer, air conditioning is used more frequently and for a longer time; then, the exhaust gas from air conditioning makes the outdoor thermal environment in residential areas worse [
8,
9]. Therefore, avoiding the vicious cycle of the deterioration of the thermal environment, energy consumption, and further deterioration of the thermal environment’ has become an urgent problem in urban development.
China has rapidly increasing energy needs; building energy consumption accounts for approximately one-third of the total social consumption, and this energy consumption is expected to continue to grow [
10]. Passive climate adaptability architectural designs have low energy consumptions, zero energy consumptions, and even negative energy consumptions [
11], so it has attracted more research attention. Many studies have focused on optimizing the outdoor thermal environment such as greenery, water features, and pavement materials [
12,
13,
14,
15,
16,
17,
18]; other studies have focused on optimizing buildings, such as the layout, height, and density [
19,
20,
21,
22,
23,
24,
25,
26]. However, piloti, a ubiquitous architectural form in the hot summer areas of southern China, have not been sufficiently considered in the quantitative studies of passive climate adaptability design.
Setting optimal piloti is a good way to adapt to the Urban Heat Island. Piloti can form shaded space and enhance the surrounding wind speed, but it does not raise the air humidity like trees, so that the effect of the piloti on mitigating the outdoor human thermal comfort can be highly anticipated [
27,
28]. Chen et al. studied the influence of the arrangement of piloti on the outdoor wind environment in Shenzhen, China [
28]. They found that the optimal piloti arrangement could highly improve the outdoor thermal environment in summer. Li clarified the relationship between the piloti arrangement and the velocity ratio under different wind directions, but this research did not consider the influence of the surrounding buildings and no thermal comfort index was included [
29]. Xi researched the influence of the piloti ratio on the outdoor thermal environment of residential blocks in Guangzhou, China, in the summer [
30]. It was found that the outdoor thermal environment is improved with the increase of the piloti ratio, however, Xi set piloti in all the buildings studied, but this extreme situation rarely occurs outside of the research environment.
Chen et al. simulated the effects of piloti on the outdoor thermal environment of a campus building in Guangzhou [
31]. Different heights of piloti and proportions of piloti were researched. However, during the hottest months in summer the university is under the summer vacation. Zhao et al. studied the effects of piloti and non-piloti forms on the outdoor wind environment using the teachers’ apartments of Wenzhou University as an example [
32]. They found that wind environment at piloti significantly improved; the flow field within the district tends to be smooth with no significant turbulence zone, and the piloti can beneficially improve the air circulation and reduce the air pollution retention. Zhou et al. researched the effects of piloti on the outdoor thermal environment of a residential area in Wuhan in summer [
33]. They found that the pedestrian wind environment and the outdoor thermal environment improved with the increase of the piloti ratio. These studies show that the piloti can effectively improve the regional thermal environment in summer. It could be known from the above research that piloti are a really good element to improve outdoor thermal environments. However, there are some limitations in these studies: (i) Most of the researches only focus on the summer period; (ii) Many studies ignore the influence of the surrounding buildings without piloti. (iii) Some studies only discuss the relationship between the wind velocity and piloti, but do not pay attention to mean radiant temperature (MRT) and standard effective temperature (SET*).
In this study, (i) we conducted a quantitative simulation of passive climate adaptability in both summer and water for a residential area; (ii) we used piloti as the main method to improve the outdoor thermal environment and the influence of surrounding buildings without piloti is also considered; (iii) Outdoor thermal environment including wind velocity, surface temperature, MRT and SET* was studied. Our main objects were to propose an arrangement of piloti for climate adaptation and to determine the optimal piloti ratio for residential areas in a hot-summer and cold-winter city.
4. Discussion
In this paper, case studies of a residential area with different piloti arrangements and the ratio by numerical simulation were carried out, and the piloti showed good passive climate adaptability in a hot summer and cold winter city. The relationships between piloti variables and outdoor thermal environment (T, V, MRT, SET*) of the residential area both in winter and in summer in Wuhan is studied for the first time. Data in
Table 6 can be converted to the relationship between piloti ratio and wind speed in other cities with similar climate with Wuhan, which shows a certain adaptability.
Although there are some interesting results revealed by this paper, there are also limitations. First, we only studied the most basic and common determinant that arranged the residential area, while other arrangement forms (point, enclosed) with different building shapes also accounted for a large proportion. Second, we only consider the piloti variable, however, the improvement of the thermal environment is not limited by a single piloti variable. Other passive climate adaptability elements, such as reflective materials and the green wall should also be taken into account. Third, Trees, pedestrians, and cars are omitted in the simulation. Fourth, only a typical meteorological day is considered in this study, other typical days such as the hottest day and the coldest day could also be taken into account.
In our future work, the thermal environment of residential areas under different arrangement forms (point, enclosed) will be researched.
5. Conclusions
We studied the outdoor thermal environment in the hottest month (July) and coldest month (January) in Wuhan. We found that setting piloti at the two ends of the building was the best piloti arrangement for climate adaptation. It improved the wind environment and thermal environment.
Based on the optimal piloti arrangement, we studied the relationship between the piloti ratio (0%, 20%, 40%, 60%, and 80%) and the outdoor wind environment. In the summer, the wind velocity increased with the increasing piloti ratio, as the piloti could strengthen the wind path (from south to north). In the winter the wind environment changed slightly with the increasing piloti ratio because the piloti had little effect on the wind path (from east to west). The criteria for assessing wind-induced discomfort considering the temperature effects were used to evaluate the wind environment. We found that the piloti ratio should be between 12% and 38% to avoid wind-induced discomfort.
In the summer, with the increasing piloti ratio, MRT and SET* decreased. The wind path significantly affected the SET* value. MRT and SET* changed slightly with the increasing piloti ratio because most of the building and ground were in shadow in the winter cases.