Feasibility Analysis of Nearly Zero-Energy Building Design Oriented to the Optimization of Thermal Performance Parameters

Abstract

The effective control and reduction of building energy consumption are major global focuses. The building sector is responsible for over 40% of all direct and indirect CO₂ emissions. Nearly zero-energy buildings have been the subject of aims and regulations from several developed nations. An office building located in the severe cold region of China was chosen for this case study. The building was equipped with multiple NZEB technologies. Building indoor environment parameters and energy efficiency indexes were used as performance targets, and a performance-based design approach was used to optimize building design parameters. Thermal performance of the building envelope, airtightness, energy demand, and indoor thermal environment were tested according to different evaluation criteria. The total energy demand was as low as 53.93 KWh/(m²·a), and this can be attributed to the exceptional insulation of the building. In this test, the indoor thermal environment comfort was satisfactory. This study can be used as a reference for the design and evaluation of low-carbon buildings and low-energy buildings.

1. Introduction

One of the biggest concerns facing humans today is climate change. To assure that the climate will be “carbon neutral” by 2050, the European Union adopted the 2030 Climate Target Plan in December 2020 [1]. According to the report of the International Energy Agency, the building sector is responsible for over 40% of all direct and indirect CO₂ emissions and more than a third of the total energy consumption of the world [2,3,4]. Therefore, the effective control and reduction of building energy consumption are leading global focuses. Future approaches to building energy efficiency involve quick and significant changes. Nearly zero-energy buildings (NZEBs) have been the subject of aims and regulations from several developed nations [5]. The 2010 release of the revised European Union Energy Performance Directive for Buildings set the cost-optimal and virtually zero-energy level requirements for structures and compelled the member states to amend their legal frameworks to achieve NZEBs for all newly constructed structures by 2020 [6]. According to the US Department of Energy Building Technology Program, marketable net-zero energy residences were to be available by 2020 and public NZEBs will be available at a low incremental cost by 2025 [7]. The fifth iteration of the basic energy plan of Japan was approved by the Japanese Cabinet. The government intends to reduce the amount of energy produced from fossil fuels from 65% in 2011 to 56% by 2030. It is also hoped that greenhouse gases will be reduced by 26% [8]. The German government hopes to have a building stock that is almost climate-neutral by 2050. Especially, energy efficiency and the use of renewable energy sources should cut the primary energy demand of buildings by 80% from the 2008 level [9]. According to the carbon emission targets of China, 2025, 2030, and 2035 have been designated as the enforcement years for ultra-low energy buildings, NZEBs, and zero-energy buildings (ZEBs), respectively [10]. Many countries have produced similar but different definitions, such as nearly zero-energy buildings [11], zero-carbon homes [12], zero-energy houses [13], net-zero-energy buildings [14], zero-energy buildings [15], and passive houses [16], as buildings move towards lower energy consumption.

NZEBs are special structures with extremely high energy efficiency, and they require almost zero or a very small amount of energy that can be produced from renewable energy sources on site or locally [17]. In addition to energy savings, the other desirable performance metrics of NZEBs are thermal comfort, thermal insulation, life-cycle cost, and environmental protection [18]. Three techniques make up the NZEB design: (i) high-efficiency goods, (ii) passive design solutions to drastically reduce energy consumption, and (iii) diverse renewable energy sources to completely replace the use of fossil fuels [19]. Effective thermal insulation systems, high-performance windows, strong airtightness, and fresh air heat recovery systems are examples of passive design solutions [20].

The first and most important aspect of the NZEB design is to improve the envelope performance. Lapisa et al. used passive strategies, such as enhanced thermal insulation of building envelopes and solar reflectance of roofs, to achieve the best design for practically zero-energy buildings in different climates [21]. Vanaga R et al. evaluated the storage capacity and dynamic behavior of a solar facade module [22]. This module was able to store energy to lower heating and cooling loads in NZEBs. In comparison to opaque walls, a Fresnel lens used for solar radiation concentration and insulation with low thermal conductivity can raise indoor temperatures and lower building energy usage for heating. Among all building envelopes, windows account for roughly 45% of the total amount of heat gain/loss within buildings [23]. The primary factors influencing the energy consumption of a building are the window-to-wall ratio (WWR), shading, the solar heat gain coefficient (SHGC), and exterior window system heat transfer coefficients [24]. The quantities of energy used in buildings and on exterior walls are regulated by heat transfer coefficients [25]. It is reported that effective shading design can significantly reduce the peak cooling load and the energy consumption for lighting and cooling and maintain good indoor thermal and illumination conditions [26]. SHGC, which is related to solar transmittance, is a crucial consideration when evaluating the effectiveness of shading systems for windows. In order to obtain as much solar radiation as possible in winter, the value of SHGC should be adjusted as high as possible. In contrast, the SHGC value should be as low as possible to minimize the solar radiation heat gain [27]. Effective lighting and solar heat gain must be kept in perfect proportion. All the parameters required by the NZEB standard are presented with their value ranges in

Table 1. Heat transfer coefficients of envelope structures.

Performance Parameters		Severe Cold Region	Cold Region	Hot Summer and Cold Winter Region	Hot Summer and Warm Winter Region	Mild Region
Roof U values [W/(m2·K)]		0.10–0.15	0.10–0.20	0.15–0.35	0.25–0.40	0.20–0.40
Wall U values [W/(m2·K)]		0.10–0.15	0.15–0.20	0.15–0.40	0.30–0.80	0.20–0.80
Window U values [W/(m2·K)]		≤1.0	≤1.2	≤2.0	≤2.5	≤2.0
SHGC	Winter	≥0.45	≥0.45	≥0.40	—	≥0.40
	Summer	≤0.30	≤0.30	≤0.30	≤0.15	≤0.30

, and it is observable that the changes in the values greatly depend on climatic circumstances.

Building airtightness has noticeable impacts on the insulation of building envelopes. It is essential to prevent chaotic and unforeseen air exchange between interior and outdoor spaces [28,29]. Passive houses in Germany are required to have an airtightness rating of no less than 0.6 air changes per hour at 50 Pascals of pressure (ACH50), and this is generally confirmed by on-site pressure tests (conducted in both pressurized and depressurized conditions) [30]. Fresh air heat recovery systems are used as high-efficiency products in NZEBs. Effective heat recovery ventilation is an important factor in ensuring high indoor air quality and energy savings. In a passive house, a heat exchanger transfers at least 75% of the heat generated from the exhaust air to the fresh air [31]. Precooling in summer and preheating in winter can be accomplished by lowering energy usage. The implementation of renewable energy technologies can be facilitated by a comparatively low indoor heating and cooling demand. Solar photovoltaic (PV)/thermal systems, ground-source heat pumps (GSHPs), air-source heat pumps (ASHPs), and wind power are examples of renewable energy technologies [32,33,34].

Solar energy is a desirable option among the available renewable energy sources. PV systems [35], solar hot water systems [36], solar chimney [37], and solar heating systems [38] are the main techniques to utilize solar energy effectively. It is still difficult to rely entirely on solar hot water for heating, especially in the colder regions of China [39]. Solar heating and hot water systems cannot function reliably and consistently at night or on overcast days. To maintain the stability and continuity of energy usage, it is necessary to have hot water storage tanks or other energy storage devices [40]. Solar space-heating systems need a greater collector area than solar water heaters, which may independently generate domestic hot water in very small spaces. The use of solar photovoltaic systems as external energy sources that can be connected to NZEBs has been encouraged [41]. Power grids or battery storage systems can solve the problem of unbalanced energy supplements in solar power systems [42]. The energy efficiency of solar power systems can be improved by combining them with other systems, such as GSHPs and ASHPs. Vieira et al. found that after the implementation of a solar PV system in a residential structure with an energy storage system, the annual energy usage of the building decreased by about 90% [43]. The application of GSHP systems in NZEBs is a promising step [44]. In order to achieve the zero-energy aim, heat pump systems are used in newly built apartments. In low-energy houses in the Czech Republic, energy systems combined with advanced heat pumps, solar power systems, and inexpensive seasonal ground storage systems have been designed. About 84% of the energy used for system operations is covered by renewable energies [45]. Especially in hot summer and freezing winter climates, ASHP systems manifest superior performances. The constraints of complex configurations, expensive water intake systems, soil heat exchangers, and machine rooms are eliminated by ASHPs. Frosting problems and defrosting measures should be focused on during ASHP operations to avoid the reduction of heating capacity and coefficient of performance (COP). The efficiency of ASHP systems increases when they are powered by solar energy, and can be achieved by integrating hybrid solar thermal photovoltaic elements into building façades [46].

Previous research has mainly focused on design and optimization methods to achieve low energy consumption in buildings. However, there is a lack of practical testing and validation for these optimization methods. In the present work, building indoor environment parameters and energy efficiency indexes were used as performance targets and a performance-based design approach was used to optimize building design parameters. NZEBs are generally built by minimizing the energy consumption of buildings. On this basis, a design strategy for using renewable energies and efficient equipment to meet the energy demand for NZEBs is proposed. The thermal comfort of the building was tested according to different evaluation criteria.

2. Materials and Methods

2.1. Building Design Features

An office building located in the severely cold region of China was chosen for the case study (Figure 1). The building is equipped with multiple NZEB technologies, such as passive design, renewable heating and power generation systems, and energy-efficient lighting and domestic appliances, to reduce energy demand (Figure 2).

The basic framework of the building is an H-shaped steel structure made of polystyrene foam concrete (form factor = 0.54). Expanded polystyrene has been used to insulate the building and enhance the thermal performance of the envelope. The material composition of the exterior wall structure is displayed in Figure 3.

Moreover, multiple active technologies powered by renewable resources have been employed in the building to achieve net-zero carbon emissions. In order to meet heating and cooling requirements, a solar water heating system and a water source heat pump were used. The outlet temperatures of the buried pipes were 16 °C in summers and 12 °C in winters. In the ground loop, there was a 5 °C temperature difference between the supply and the return. The heating coil of the air loop and the radiant floor heating system both used hot water from the hot water loop. The hot water loop outlet temperature was 40 °C, and the supply and return temperatures differed by 8 °C. Moreover, several groups of solar collectors were used to provide additional heating functions and produce domestic hot water. A mechanical ventilation heat recovery unit (MVHR) was installed in the building to mitigate the energy loss resulting from ventilation. This unit was capable of recovering 81% of the sensible heat and 73% of the latent heat. The fan operated from 08:00 to 21:00.

2.2. Calculation and Testing of Building Thermal Parameters

2.2.1. Thermal insulation

The heat flow meter method was used to determine the heat transfer coefficient of the envelope. The heat transfer coefficient was determined using thermocouples, and heat flow meters were used to measure the internal and external surface temperatures and heat flow density of the wall. The thermal resistance of the wall was calculated by Equation (1). The heat transfer coefficient was then calculated by Equation (2).

$$R=\frac{\Delta t}{q}=\frac{\mathsf{\Delta }t}{E\times c}$$

(1)

$$K=\frac{1}{R_i+R+R_e}$$

(2)

where R is the thermal resistance, Δt is the temperature difference between the interior and outside surfaces of the wall, q is the heat flow density of the wall, E is the heat flow meter reading, c is the probe coefficient of the heat flow meter, K is the heat transfer coefficient of the wall, R_i is the heat transfer resistance of the inner surface of the wall, and R_e is the heat transfer resistance of the outer surface of the wall.

2.2.2. Airtightness

Air pressure tests were used to quantify all building envelope leakages. The test principle is shown in Figure 4. At a 50-Pa pressure difference, air changes are described by the n₅₀-value (in h⁻¹). Two methods are most commonly used to detect the overall air tightness of buildings—the air pressure method and tracer-gas concentration decay method. In the air pressure method, also known as the “blower door” method, the interior of a building is pressurized by artificial means to create a pressure difference between the interior and exterior environments. The amount of air leakage under the corresponding pressure difference is then measured. The main equipment for this pneumatic test was a building airtightness testing system, which consisted of a high-powered, calibratable blower mounted on a sealed door. The test host was connected to the blower and the test software. The entire testing process was carried out at different pressures from 60 Pa to 10 Pa. The combined differential pressure index and the flow coefficient were calculated as the air pressure varied from high to low, and then a regression equation was derived.

2.2.3. Thermal Comfort Parameters

The mental state that communicates happiness within the thermal environment is known as thermal comfort. It is challenging to please everyone in a location because there are significant physiological and psychological differences between people. The heat production and heat loss in human bodies must be balanced to attain optimal thermal comfort. Fanger’s comfort equation establishes a connection between human activities (sleeping, running, etc.), clothing, and variables affecting the thermal environment, such as air temperature, surface temperature, air humidity, air speed, and turbulence [47].

Radiant temperature describes the surface temperature around people. Natural convection heat exchange and radiation heat exchange occur simultaneously between human bodies and the surrounding air, and the corresponding evaluation index is the operating temperature. When the indoor wind speed is lower than 0.2 m/s or the difference between the average radiation temperature and the air temperature is less than 4 °C, the operating temperature can be considered as the arithmetic average of the air temperature and the average radiation temperature [48].

$$t_o=\frac{t_a+\overline{t_r}}{2}$$

(3)

$$\overline{t_r}={[{\left(t_g+273\right)}^4+2.5\times {10}^8\times v^{0.6}\times \left(t_g-t_a\right)]}^{\frac{1}{4}}-273$$

(4)

where t_o is the operating temperature, t_a is the average air temperature, t_r is the mean radiation temperature, and t_g is the black globe temperature.

The overall thermal feeling and discomfort level of humans exposed to moderate thermal settings are forecast by the methods provided in ASHRAE Standard 55-2020 [49]; the predicted mean vote (PMV) method and the predicted percentage of dissatisfaction (PPD) method are used for the analytical assessment and interpretation of thermal comfort. Thermal sensation votes (self-reported perceptions) are measured in a range from −3~+3, which corresponds to the categories of cold, cool, slightly cool, neutral, slightly warm, warm, and hot, respectively. The PMV index predicts the mean value of the thermal sensation votes (self-reported perceptions) of a large group of people on this scale. The PPD index establishes a quantitative forecast of the proportion of persons who are thermally dissatisfied based on PMV. Both indices, namely, PMV and PPD, are respectively estimated by the following relations [50]:

$$\begin{array}{c}PMV=\left(0.03e^{-0.036M}+0.028\right)\times \\ \left\{\left(M-W\right)-3.05\times {10}^{-3}\times \left[5733-6.99\left(M-W\right)-P_a\right]-0.42\times \left[\left(M-W\right)-58.15\right]-1.7\times {10}^{-5}M\left(5867-P_a\right)\right\}\\ -0.0014M\left(34-T_a\right)-3.96\times {10}^{-8}{f}_{cl}\times \left[{\left(T_{cl}+273\right)}^4-{\left(\overline{T_r}+273\right)}^4\right]-f_{cl}{h}_c\left(T_{cl}-T_a\right)\end{array}$$

(5)

$$PPD=100-95\times \exp \left(-0.03353\times PM{V}^4-0.2178\times PM{V}^2\right)$$

(6)

in which

$$T_{cl}=35.7-0.028\left(M-W\right)-I_{cl}\left\{3.96\times {10}^{-8}{f}_{cl}\times \left[{\left(T_{cl}+273\right)}^4-{\left(\overline{T_r}+273\right)}^4\right]+f_{cl}{h}_c\left(T_{cl}-T_a\right)\right\}$$

(7)

$$h_c=\left\{\begin{array}{c}2.38{\left(T_{cl}-T_a\right)}^{0.25} When 2.38{\left|T_{cl}-T_a\right|}^{0.25}>12.1\sqrt{V_{ar}}\\ 12.1\sqrt{V_{ar}} When 2.38{\left|T_{cl}-T_a\right|}^{0.25}<12.1\sqrt{V_{ar}}\end{array}\right.$$

(8)

$$f_{cl}=\left\{\begin{array}{c}1.000+1.290\times I_{cl} When I_{cl}\ll 0.078 m^{2}K/m\\ 1.050+0.645\times I_{cl} When I_{cl}>0.078 m^{2}K/m\end{array}\right.$$

(9)

The physical meaning and units of the parameters are at the end of the article.

3. Results

3.1. Thermal Performance of the Building Envelope

The heat transfer coefficients of the exterior walls, roof, floor, and windows of the building (

Table 2. Properties of the nearly zero-energy building.

Property	Value
Gross floor area (m2)	302.4
Shape coefficient	0.54
U values (W/m2·K)
Exterior wall	0.099
Roof	0.090
Base floor	0.113
Windows	1.0

) were used to analyze building performance and energy consumption. In comparison to the parameters in Table 1, all the indicators were close to the optimal values. The airtightness index of the selected exterior windows met the requirements of the national standard that suggests the airtightness performance of exterior windows should not be lower than grade 8. The heat consumption of the envelope was minimized as much as possible under the severe cold climate.

3.2. Airtightness

The overall airtightness of the building was tested using the Minneapolis blower door test system, which is considered one of the best building airtightness testing systems in the world. The test host model was DG-700. The indoor and outdoor air temperatures of the building, the outdoor wind speed, and the atmospheric pressure were tested by a German TESTO 480 multifunctional measuring instrument.

$$q_{env}=q_m\left(\frac{{\rho }_{int}}{{\rho }_e}\right)=q_m\left(\frac{T_e}{T_{int}}\right)$$

(10)

where q_env is the air infiltration of the building envelope, q_m is the air flow rate, ρ_int is the indoor air density, ρ_e is the outdoor air density, T_int is the indoor air temperature, and T_e is the outdoor air temperature. The pressure difference at each test point and the amount of air infiltration through the envelope were determined by the least-squares method [51,52].

$$q_{env}=C_{env}{\left(\Delta p\right)}^n$$

(11)

$$C_L=C_{env}{\left(\frac{{\rho }_e}{{\rho }_0}\right)}^{1-n}\approx C_{env}{\left(\frac{T_e}{T_0}\right)}^{1-n}$$

(12)

$$q_L=C_L{\left(\Delta p\right)}^n$$

(13)

where C_env is the air flow coefficient and n is the airflow index. The pressurization method and the depressurization method are the two main test procedures for airtightness. The pressure difference between the two sides of the enclosure varied from 10 Pa to 60 Pa. Air infiltration at the current differential pressure was recorded every 5 Pa. The air infiltration at each pressure level was fitted to obtain the characteristic curve. The outdoor temperature during the test varied from 22 °C to 24 °C, and the outdoor wind speed was between 0.4 m/s and 1.2 m/s. The atmospheric pressure was 101.1 kPa. The test was conducted when the outdoor wind speed was lower than 3 m/s. The installation of blower door test system is shown in Figure 5.

Test data about air infiltration and differential pressure was recorded, as shown in

Table 3. Air flow rates at different pressures.

Air Pressure Difference (Pa)	qenv (m3 h−1)	Air Pressure Difference (Pa)	qenv (m3 h−1)
−59.6	1757.8	60.2	2080.8
−54.5	1667.7	54.6	1939.7
−48.5	1591.2	49.5	1819.0
−44.7	1499.4	44.6	1705.1
−39.6	1397.4	40.0	1609.9
−34.8	1288.6	35.2	1468.8
−29.5	1166.2	29.7	1329.4
−24.5	1035.3	25.0	1201.9
−19.8	899.3	20.1	1023.4
−15.0	759.9	15.0	839.8
−9.7	569.5	9.8	598.4

. The curve corresponding to the change in air infiltration and differential pressure was obtained by fitting Equation (11) in MATLAB (Figure 6). The fitted parameter values and their confidence intervals are presented in

Table 4. Fitting parameters and confidence intervals.

Fitting Parameters	Fitting Results	Confidence Interval 1
Cenv/m3/(h·Pan)−1	144.6	(134.5, 154.2)
CL/m3/(h·Pan)−1	144.1	(134.0, 153.7)
n	0.6506	(0.5949, 0.6315)

¹ R² = 0.9985.

.

The fitting results were reliable in the least squares curve, and the fitting correlation coefficient reached 0.9987. The relationship between the building infiltration air volume and the differential pressure can be described by Equation (14):

$$q_L=144.7{\left(\Delta p\right)}^{0.6562}$$

(14)

When the pressure difference between the interior and exterior of the building was equal to 50 Pa, the airtightness index of the building was 1.04 h⁻¹ (

Table 5. Pressure difference and airtightness index.

Pressure Difference (Pa)	Air Infiltration Volume (m3 h−1)	Airtightness Index (h−1)
50	1885.09	1.04
10	655.64	0.36
5	416.04	0.23
4	359.37	0.20
3	297.55	0.16
2	228.04	0.13
1	144.70	0.08

), which is slightly higher than the energy efficiency index of NZEBs (0.6 h⁻¹).

3.3. Indoor Thermal Environment

The average outdoor temperature and average relative humidity in Shenyang during the test period were −8.3 °C and 44%, respectively. The test results of the indoor environmental parameters of the building achieved the target values specified in the technical standards for NZEBs [53]. In addition, a field test for indoor thermal comfort in winter was conducted. A total of 129 questionnaires (78 to males and 51 to females) were distributed for this test, as shown in Figure 7. The numbers of hot-feeling votes for [−1], [0], and [1] were 7, 53, and 38, respectively, accounting for 75% of the total number of votes. Thus, the indoor thermal environment of the building was recognized at 75%. The remaining 25% of the subjects chose the scale points of [+2] or [+3] and considered the indoor environment warm or hot, respectively. More than 40% of the subjects expected the indoor environment to be cooler, indicating that the indoor thermal environment did not meet the requirements of thermal comfort.

The indoor relative humidity of NZEBs should not be less than 30% in winter. As the heating progressed, the indoor relative humidity gradually decreased and reached the lowest value of 24.50%; hence, most people felt the indoor was dry. Relative humidity expectation voting chart is shown in Figure 8.

When the air flow rate becomes greater than 0.2 m/s, people can feel a sense of the blowing wind. The airflow velocity in the building varied between 0.03 m/s and 0.07 m/s, which is much lower than 0.2 m/s and did not create the sensation of blowing wind. Most people could not perceive any wind in the room. The summer test was conducted in August, and the average outdoor temperature and the average relative humidity were 28.2 °C and 64%, respectively. The maximum and minimum values of indoor temperature in the building were 25.1 °C and 23.45 °C, respectively, creating a temperature difference of 1.65 °C. The day-by-day temperature variation was not significant. The average indoor temperature in summer was about 1 °C lower than that in winter.

Voting distribution chart for summer thermal sensation is shown in Figure 9. A total of 78 indoor thermal comfort questionnaires (42 to males and 36 to females) were distributed in the summer test. The numbers of samples hot-feeling voting scores of [−1], [0], and [+1] were 18, 49, and 9, respectively, accounting for 95% of the overall number of people who participated in the test. Moreover, 19.3% of the overall population wanted to be warmer, 8.3% wanted to be colder, and 72.4% thought the ambient temperature was moderate, indicating that the vast majority of the subjects were satisfied with the feeling of the indoor temperature.

Furthermore, as shown in Figure 10, 13.9% of the people who participated in the test wanted lower humidity and 86.1% were satisfied with the wet environment. The maximum and average values of indoor wind speed were 0.18 m/s and 0.13 m/s, respectively, which met the requirements of the code.

4. Discussion

4.1. Performance-Based Design Method

The performance-based design method was adopted to control the indoor environmental parameters and energy consumption index of the building. Design solutions were quantitatively analyzed and optimized using energy-simulation calculations. In this design approach, the aim was to reduce energy consumption and meet the requirements of indoor thermal comfort. The parameter optimization was achieved based on the calculated results. The heating and cooling demand can be easily reduced with good insulation, energy-efficient windows, a ventilation system with heat or energy recovery, and an airtight building envelope. Thus, if energy consumption reduction is prioritized first, an emphasis on the use of renewable sources to power the building sector can be sustained over time. The specific optimization steps are displayed in Figure 11.

4.2. Impact of High-Performance Insulated Structures on Building Energy Consumption

The energy balance of a building is greatly influenced by the heat loss. Any heat loss must be recompensed by an equal heat gain to prevent a decline in indoor temperature. In low-energy structures, the entire building envelope must be completely airtight and insulated to provide a suitable indoor environment. It is noticeable from Table 2 that the U values of the external walls, floor slabs, and roof of the NZEB ranged from 0.10 W/(m² K) to 0.15 W/(m² K), which could achieve the required level of insulation in passive houses. Materials that are exceptionally well insulating can produce such low U values. With typical polyurethane foam, the wall thickness of the NZEB could be lowered to 20 cm.

In addition to unbroken building elements, such as walls, roofs, and ceilings, building envelopes contain edges, corners, connections, and penetrations. The “continuous uninterrupted airtight building envelope” is the main planning tenet for structural airtightness. Air cannot haphazardly flow through the walls of the building envelope in an airtight structure. Airflow caused by wind and temperature variations is insufficient to constantly produce high-quality air. Leaks in a building envelope enable warm and humid air to pass through the walls, and this erratic airflow can cause structural damage and an uncomfortable indoor environment by supplying both too much and too little air. The heating season spanned from November to March, whereas the cooling season continued from June to August. Another objective of this study was to identify the influencing factors for each category of energy consumption, such as the heating system, the cooling system, lighting, electrical appliances, fans, the water pump, and the mechanical ventilation heat recovery unit. Figure 12 illustrates the energy demand for each system separately for the NZEB. The total energy used for heating in the NZEB was 5267.81 KWh, equivalent to 17.42 KWh if divided by area. Another dominant contributor of energy consumption is the cooling system, i.e., 9.77 KWh/(m²·a), subsequently followed by lighting and electrical appliances. The total energy demand is as low as 53.93 KWh/(m²·a), and this can be attributed to the exceptional insulation of the building.

Only the installation of high-performance exterior windows cannot guarantee good airtightness in a building. Hence, the mere connection of the window frame to the masonry wall was insufficient because air leaked through the wall. The airtight layer of the exterior wall, typically the internal plaster of the building, and the window frame must be firmly attached. A plastering strip or tape that can be covered with plaster would work well in this case (Figure 13a). Any penetrations, such as power outlets in exterior walls, electric cables, and pipelines that pass through the basement ceiling, must be planned (Figure 13b). Therefore, the airtightness evaluation of low-carbon energy-saving buildings should be changed from the improvement of only door and window performance to the overall airtightness improvement of the buildings.

4.3. Thermal Comfort Parameters

Thermal comfort is the key factor for living comfort. The ASHRAE Standard provides the definition of thermal comfort as the state of mind that conveys happiness with the thermal environment. There is a broad consensus that air temperature, radiative temperature, humidity, and air velocity are the primary external physical factors that affect thermal comfort. The ideal mix of these variables primarily depends on the activity and attire of the subject. When the heat produced by the human body equals the heat released by it, optimal thermal comfort is achieved.

In addition to the thermal environment point-in-time survey, the PMV and PPD of the building were calculated based on the test results in

Table 6. Indoor environmental parameters of the building in winter.

Test Value	Clothing Insulation (clo)	Air Temperature (°C)	Relative Humidity (%)	Mean Radiant Temperature (°C)	Air Speed (m/s)	Operating Temperature (°C)
Maximum value	1.8	26.80	33.40	27.10	0.07	26.45
Minimum value	0.74	24.33	24.50	24.40	0.03	24.40
Average value	1.16	25.37	30.21	25.64	0.05	25.42

and

Table 7. Indoor environmental parameters of the building in summer.

Test Value	Clothing Insulation (clo)	Air Temperature (°C)	Relative Humidity (%)	Mean Radiant Temperature (°C)	Air Speed (m/s)	Operating Temperature (°C)
Maximum value	0.7	25.1	67	25.1	0.18	26.45
Minimum value	0.25	23.45	58	23.45	0.08	24.40
Average value	0.34	24.24	63.6	24.24	0.13	25.42

. The Center for the Built Environment (CBE) thermal comfort tool was used in this analysis [54]. The CBE thermal comfort application is an ASHRAE-compliant online application for thermal comfort calculations and visualization. The calculated results for winter and summer generated by the thermal comfort tool are summarized in

Table 8. Calculated results for winter and summer according to the thermal comfort tool.

Season	PMV	PPD	Sensation
Winter	0.65	14%	Slightly warm
Summer	−0.52	11%	Slightly cool

. The PMV levels in summer and winter were found to be very close to the ideal interval for indoor thermal comfort; however, the values did not completely comply with the ASHRAE Standard. According to the ASHRAE Standard, the PMV value in the comfort zone ranges between −0.5 and +0.5. The PPD was used to forecast the percentage of unhappy members of a population at any given PMV level. It was found that 90% of the population (10% PPD) was pleased with a PMV of 0.5.

The calculated results are also represented in the psychrometric charts in Figure 14 and Figure 15. In these psychrometric charts, the abscissa represents the operating temperature and the mean radiant temperature is equal to the dry-bulb temperature at each location. According to the ASHRAE Standard, the comfort zone is a set of circumstances where dry-bulb temperature (DBT) and mean radiant temperature (MRT) are the same and PMV ranges between −0.5 and +0.5. In the graphs, the red circle is present very close to the blue comfort zone. The blue area represents an ideal comfortable area that meets international standards.

The percentage of people choosing “slightly warm” and “warm” indoor environments in winter increased, and almost no one felt that the indoor environment was cold. The relative humidity inside the building was 30.21%, which did not meet the standard requirement. The subjective questionnaires also revealed that most people felt the room was dry. This might have happened because the building was not permanently occupied and there was no cooking or laundry inside the building, thus reducing the indoor relative humidity. In addition, the building had good airtightness to maintain a stable relative humidity in the room, preventing more air infiltration. Therefore, it is important to provide a suitable initial relative humidity for nearly zero-energy buildings.

A vast majority of people were satisfied with the indoor temperature in summer. The indoor relative humidity varied between 58% and 67%, meeting the standard requirement. The maximum indoor air velocity was 0.18 m/s, which was within the specified air velocity range. However, 23% of people still wanted to increase the indoor air flow rate, which might be achieved by the habit of people opening windows in summer to improve the thermal environment. The evaluation of indoor thermal comfort should take full account of the behavior of users.

5. Conclusions

The building performance design method is different from the traditional design method; it takes energy consumption as the control target and optimizes building thermal parameters based on quantitative analysis to minimize the heating and cooling demand of the building. The exceptional insulation of houses is primarily responsible for meeting the low energy demand of 53.93 KWh/(m² a). The use of renewable energy systems and energy-efficient building equipment can achieve high energy efficiency with low carbon emissions. This design strategy is applicable to low-energy buildings.

The indoor environmental characteristics of practically zero-energy buildings should provide a high level of thermal comfort without compromising the energy consumption control aim. A healthy and comfortable indoor environment is a basic requirement for nearly zero-energy buildings. More than 70% of the overall heat losses in existing buildings are through external walls and roofs. The most effective way to conserve energy is to improve thermal insulation. One of the most affordable approaches to making a building energy efficient is to make it airtight. In the airtightness test, the overall air leakage of the building was measured once under positive pressure and once under negative pressure. The improvement of building insulation performance and airtightness helps in enhancing thermal comfort and preventing structural damage. In this test, the indoor thermal environment comfort was satisfactory. The indoor relative humidity of the building needs to be improved in winter. The combination of good airtightness and the radiant floor heating system significantly lowered the relative humidity in the room. The experience brought by the indoor air flow rate in summer was hardly satisfactory. Most Chinese buildings do not have fresh air ventilation systems, and residents habitually open windows to achieve indoor and outdoor air replacements. Therefore, fresh air ventilators with heat recovery are essential to improve the thermal comfort of nearly zero-energy buildings.

However, in general, the research work in this paper is preliminary. The author will carry out further research in two aspects. First, the individual and combined contributions of energy efficiency measures in reducing the building’s energy performance were quantified, and their economic viability was assessed in terms of discounted payback period. Second, we will research carbon emission reductions in the building sector from an energy-saving-technology perspective.