The Innovation Harbor is located in Xixian District, Xi'an, China, with 34.3° north latitude, 108.93° east longitude, and an altitude of 397 m. There are three dormitory groups with a total area of 333,000 square meters. The Innovation Harbor has a warm temperate semi-humid continental monsoon climate with four distinct seasons. Winters are cold, windy, rainy and snowy; springs are warm, dry, and windy; summers are hot and rainy, and autumns are cool. The average annual temperature is 13.0–13.7°C, the average temperature in January is −1.2–0.0°C, the hottest July month with an average temperature of 26.3–26. 6°C, the annual precipitation is 522.4–719.5 mm. Annual sunshine hours 1,646.1–2,114.9 h, the annual dominant wind to the northeast wind.

Figure 1 shows the location map, the external map, the floor plan, and the internal physics of the unit apartment. The overall orientation is north-south and can accommodate more than 20,000 students. All the building adopts unit apartments with two-unit, three-unit, and multiple-unit models. The 11-floor small high-rise design has two households with one elevator. Each unit includes five independent small rooms, a shared living room, and a bathroom, as shown in Figure 1D. The separate room is about 7 square meters, with a single bed, a set of desks and chairs, a wardrobe, and windows that can be opened independently, as shown in 4# of Figure 1D. The shared living room is about 13 square meters, with a sizeable public table and five chairs. The bathroom is equipped with two toilets, two sinks, and a shower room. For the convenience of statistics and the test marking, the number of each room in the apartment is shown in Figure 1C.

The apartment is a shear wall structure, the main wall is an aerated concrete infill wall, and the outside of the wall is wrapped with a layer of 10 cm thick thermal insulation board. The outer window adopts a single frame double glass steel window. The roof surface adopts extruded foam insulation board, SBS waterproof coiled material, and a cement mortar protective layer.

Each room in the apartment is equipped with a split-type air conditioner used for cooling and temperature adjustment in summer; students can adjust the temperature by themselves. In winter, the radiant floor heating mode is adopted. The heating source is from the non-interfering geothermal heating system of medium and deep geothermal energy, “take the heat but not water,” and the heating temperature can be controlled by adjusting the valve opening in each room.

In ASHRAE standard, the definition of human thermal comfort is the conscious state of people's satisfaction with the thermal environment (38, 39). It is determined by human physiology and psychology. Two main factors are affecting human thermal comfort: (1) environmental factors: air temperature, air humidity, average radiation temperature, and air velocity; (2) Human factors: Clothing and activity (40, 41). Some environmental factors will cause local discomfort to the human body, such as the feeling of blowing, the large temperature difference between the head and feet, the asymmetry of radiation temperature (42, 43), etc. Therefore, The applicability, spatial integration, and subjective feelings of the unit apartment were investigated through questionnaires.

Since almost all students first settled in the unit-type apartment from an 8-room or a 4-Room undergraduate dormitory, their psychological feelings, space utilization, and thermal environment feelings have a great sense of contrast and effectiveness, the questions involved in the survey are diverse, so the designed questionnaire is not a standard Richter scale questionnaire. Therefore, the reliability and validity of the questionnaire were not tested. However, the content design of the questionnaire covers as much as possible the influencing factors of indoor thermal comfort of unit-type apartments, such as the objective factors (floor, room orientation, and living life, etc.), subjective physiological feelings such as indoor temperature, humidity, and wind feeling, and other potentially influencing factors such as lighting, noise, and air quality.

In total, 368 valid samples were collected, including 200 in winter and 168 in summer. The scope of the survey involves three apartment communities of the whole school and more than 100 apartments in 20 unit buildings. Among the subjects surveyed, 54% were male, and 46% were female. Male and female subjects are evenly distributed in the apartments, and 90% are on floors 2–10, and 10% are on the first and top floors. The proportion of north-facing and south-facing rooms of the apartment is uniform and representative. All the situation of the subjects meets the requirements of this survey. The winter and summer thermal comfort surveys were conducted in December 2019 and July 2020.

The test period for the winter semester is from December 25, 2019, to January 7, 2020, and the test period for the summer semester is from July 14, 2020, to August 5, 2020.

Three male apartments and three female apartments are selected as the test objects. The apartments on the 2nd, 4th, and 11th floors were chosen by considering the environmental differences between the low, middle, and top floors. Each apartment contains a south-facing and north-facing room, whose numbers are shown in Figure 1C. The measuring instrument is placed in the room's central area with a height of 1.5 m. The test instruments and their accuracy are shown in Table 8. Outdoor weather parameters are recorded by the automatic weather station (Davis-6162).

The test results in winter and summer were shown centrally in Figure 4. As a background meteorological parameter, Figures 4A,E are the outdoor solar radiation, air temperature, and relative humidity parameters during the winter and summer test period, separately.

The winter's 15-day temperature and humidity test data are shown in Figures 4B,C. According to the statistics of the temperature and humidity test data for different floors and room orientations, the temperature distribution is in the range of 24–29°C, which inferred a heat-based room environment. Among the south-facing of male and female rooms, it is similar and relatively high to that of the north-facing room caused by solar radiation, and the temperature of the male room is low than that of the female room due to the windowing habit for ventilation that is shown in Figure 4E. Furthermore, the shared living room temperature is higher than that of other rooms as a whole because of the windowing habit, shown in Figure 4D. On the other hand, the relative humidity distribution in all the rooms is generally lower than 45% shown in Figure 4C; it also accords with the characteristics of room-drying perception survey data Figure 4F.

The 21-day temperature and humidity test data in summer are shown in Figures 4F,G. Similarly, analysis with winter measurements, the temperature distribution is in the range of 25.5–29 °C, which also means a slightly heat-based room environment. The south-facing of rooms is higher than that of the north-facing room, and the temperature of the shared living room is higher than that of other rooms because there is no air conditioning in the shared living room. More importantly, the relative humidity is generally high than 75%, which fits the characteristics of room-damping perception survey data Figure 2F.

In summary, due to the small area of the individual independent room, the physical parameters are easy to mix uniformly and fluctuate greatly; on the whole, the temperature and the relative humidity fluctuation cycle in the apartment is the same as the fluctuation cycle of the outdoor environment, and it is generally preferred to open windows for ventilation in summer.

In addition, the temperature and relative humidity in the vertical height direction, the carbon dioxide concentration, and the indoor wind speed with the air conditioning were measured, as centrally shown in Figure 5.

Figure 5A is the sensor arrangement in the 4# room of a female apartment to test the temperature and the relative humidity in vertical directions, the CO₂ concentration, and the wind speed with the air conditioning in horizontal and vertical directions.

Figure 5B shows the indoor wind speed at different horizontal distances and vertical heights. In summer, 70% of males and 90% of females chose to set the air outlet of the air conditioner to blow in the direction of 1.8–2m height to avoid the air conditioner blowing directly at the human body. For all that, the wind speed is locally greater than 0.3 m/s due to the small area of the room; the wind speed fluctuates quickly at the range of standing height. The blowing feeling and the varying wind speed in the field of human movement in the room are apparent, producing local discomfort. Therefore, the air supply mode of the air conditioner should be controlled more accurately.

Figure 5C shows the 3 days consecutive measurement results of the temperature and relative humidity in the vertical height direction of the 4# room of a female apartment in winter. Due to the radiant floor heating mode, the temperature distribution in the vertical height direction shows a gradient decreasing trend, which is consistent with the human thermal comfort; the humidity does not change much in the vertical direction. It shows that the indoor temperature of 4# room is much higher than the normal winter comfortable temperature range in winter, which is the same as that of the south-facing room shown in Figure 4B. The reasons are related to the small space of 7 m², and although there is a heat source control valve in each room, few students adjust it.

Figure 5D shows a 6-day measurement result of the CO₂ concentration in the desk and the wardrobe of 4# room in winter, the CO₂ concentration is the highest in the morning (8:00), which is caused by the window-closing during sleep at night. While at noon (14:00) and night (22:00), the CO₂ concentration decreases due to the window opening habit, which also illustrates the importance of open windows for ventilation in small spaces.

The unit apartment of the Innovation Harbor is a new type of student dormitory; compared with the traditional centralized residence model of student dormitory, there are similarities and significant differences. First of all, most of the young students in recent years are growing up in the era of the one-child period; they have grown up in the internet age, emphasizing self-worth, diversified living habits, and an intimate space environment. Moreover, the residents of the unit apartment are mainly postgraduate students, the majors of the postgraduate group are divided, the research direction is meticulous, the study and work styles are more focused, and the schedules are flexible and diverse. Meanwhile, postgraduate students have a solid willingness to gather and communicate.

Therefore, the unit apartment fits young students' physical and psychological characteristics and the features of the study and work at the postgraduate level. The division of individual independent space and public space considers both the individual and gathering characteristics of the living environment.

Nearly 4,000 postgraduate apartments and more than 20,000 postgraduate students live in the Innovation Harbor. Whether heating supply in winter or air-conditioning in summer, there is a great potential for energy consumption and energy-saving. From the perspective of building a healthy and comfortable environment and building energy efficiency, the unit apartment also needs to refine the thermal environment's energy supply and regulation mode.

In summer, firstly, five small rooms in an apartment are equipped with five split air conditioners is inappropriate, it not only increases the initial investment, but also easily causes the fluctuation of environmental parameters, and the frequent start and stop of air conditioning in the whole building is also easy to cause fluctuation to the power network. Since the total area of the apartment is about 50 m², a small central air conditioning system is one of the options for this apartment form, which can be arranged in each small room with air supply outlets and control switches, respectively. On the whole, it can reduce the initial investment and achieve the purpose of energy-saving (48, 49). Moreover, for small spaces, the arrangement and analysis of the airflow field should be carried out to achieve a uniform layout without causing a feeling of blowing at the height of the pedestrian layer, Therefore, the arrangement of the air outlet position requires the simulation calculation and the detailed measurement to achieve indoor comfort (50).

In winter, the radiant floor heating mode is a recognized energy-saving and comfort heating mode that can form a flow field mode with a temperature gradient from the floor to the roof (51). However, as measured in Section Test Results, the simple rated flow mode not only causes the room temperature to be too high but also causes a waste of energy. For winter heating in small spaces, even if the floor heating pipe network was laid, the temperature can be adjusted and controlled through the automatic regulation mode of flow.

Whether heating in winter or air conditioning in summer, both of which require refined energy supply and control modes. The indoor thermal environment can be dynamically measured and adjusted by adding online data collection and intelligent personnel recognition.