4.1. Passive Designs for Newly Constructed Rural House
In this study, two methods were followed to create an nZEB based on the model of a typical rural house in Xi’an. The first method involved implementing the appropriate passive designs, and the second involved establishing a suitable renewable energy system in the village house. Two scenarios were considered in this study: the first scenario was the construction of a new passive house, and the second was the refurbishing of the existing house. In this section, suitable passive design strategies for the construction of a new passive house are explored. Passive designs consist of envelope improvement (orientation, wall and roof insulation, glazing type, window-to-wall ratio and sunroom application) and internal condition optimization (ceiling height and optimizing the configuration of the house).
Table 5 summarizes the passive design steps. For each step, the possible alternatives to the original condition were evaluated, and an appropriate solution was proposed.
Figure 10 presents the specific passive design exploration simulations of the possible alternatives, presented in alphabetical order. Once the results were obtained from the current step, the appropriate solution was selected and applied as the initial condition of the next step.
Table 5 and
Figure 11 also summarize the selected appropriate solutions for each step.
Figure 10a showed the energy-saving effect of the house with different orientations. Because the different orientations can help, the house benefits more from the solar gain; however, the larger solar gain can reduce the heating energy demand and increase the cooling energy demand; considering the total energy demand, the optimal orientation of the house was decided as south to west 20°.
Figure 10 and
Figure 11 show that steps 2 and 3 reduced the energy demand of the house significantly. In step 2, the annual heating energy demand was reduced from 8366 kWh to 6615 kWh, and the annual cooling energy demand was reduced from 435 kWh to 345 kWh; in step 3, the annual heating energy demand was reduced from 6615 kWh to 4856 kWh, and the annual cooling energy demand was reduced from 345 kWh to 174 kWh. The economic condition of the villages in north-western China including Xi’an was very poor in the 20th century, but it has been significantly improved in the 21st century, and most of the villagers built their new houses. In the villagers’ conventional view, the higher house represents the greater wealth of the family. Therefore, the ceiling height of the village house in Xi’an is mainly around 3.5 to 3.8 m. However, high ceiling height causes large heating and cooling energy demand due to the large volume of the space, step 2 demonstrated that the energy demand could be significantly reduced when the house ceiling height was decreased from 3.5 m to 2.8 m, which can also save a part of the construction material and reduce the labour cost. Considering the spatial comfort of the residents, the appropriate ceiling height was referred to the Chinese Residential Design Specification, which recommends a residential building with a ceiling height of 2.8 m [
28]. According to the analysis results of step 3 as
Figure 10c, the heating and cooling energy demand reduction were relatively rapid when the width of the insulation material was below or equal to 60 mm; the energy demand reduction was relatively smooth when the width of the insulation material above 60 mm; therefore, the insulation material of the external wall was selected with 60 mm width and the residents can choose wider width to pursue more significant energy performance of the house. The annual energy demand was decreased by 312 kWh when insulation material with 40 mm was implemented in the house; the reduction is relatively smooth when the width of the roof insulation is above 40 mm.
Figure 12 shows the 3D structure of the typical village house. According to the roof system in
Figure 12, there is a 120 mm reinforced concrete layer between the first floor and the half floor. Even the half-floor has the insulation effect, the insulation effect can be strengthened by adding an insulation material below the reinforced concrete.
Figure 10e discussed the insulation effect of different glazing types, results showed that the clear glazing has better energy performance due to the clear glazing has the benefit to get solar gain; therefore,
Figure 10f also discussed the energy performance of different window to wall ratios. As a result, the energy demand was decreased 241 kWh when the glazing was changed from single glazing to double glazing; the heating energy demand was decreased by 195 kWh when the window to wall ratio was changed from 0.2 to 0.5 and the cooling energy demand has a very small increase; the change of the heating energy demand was very small when the window to wall ratio was above 0.5. The effect of the double glazing and the window to wall ratio is relatively low because the solar gain only can significantly benefit the house in the day-time; however, the main energy demand from the residential house is in the morning and evening on the weekdays; the energy demand is high in the day-time only at weekends.
Table 5.
Passive design steps are applied to a typical rural house in Xi’an, China.
Table 5.
Passive design steps are applied to a typical rural house in Xi’an, China.
Steps | Designs |
---|
Original | Original condition |
Step 1 (a) | Orientation change (North to south → South to west 20 degrees) |
Step 2 (b) | Ceiling height change (3.5 m → 2.8 m) |
Step 3 (c) | Add wall insulation (60 mm Rock wool board) |
Step 4 (d) | Add roof insulation (60 mm Rock wool board) |
Step 5 (e) | Glazing type change (Single glazing → Double glazing) |
Step 6 (f) | Window to wall ratio change (South wall (0.2 → 0.5)) |
Step 7 (g) | Configuration change (Figure 13) |
Step 8 (h) | Add sunroom (1.5 m) |
Figure 13 shows the 3D model and the configuration of the house after applying passive design strategies. The configuration changes were made because the spaces with heating and cooling systems should be set next to each other to maintain proper heating or cooling. In this research model, spaces such as kitchens, bathrooms, toilets, laundry rooms and closets did not have heating and cooling systems. As shown in
Figure 13, a 1.5 m sunroom was added in front of the house. The depth of the sunroom was selected based on the solar gain effect in winter and the functional consideration of the space; the sunroom can be completely opened or disassembled in summer to avoid solar gain.
Figure 11 shows the annual energy-saving effect of the optimization design at each step. The village house in Xi’an does not have a basement, and there are four layers for ground insulation inheriting from the traditional method as shown in
Figure 12. Therefore, the ground insulation was considered sufficient in this study.
Instead of using sophisticated insulation materials such as EPS boards, using local materials can meet the insulation demand of the house at a relatively low cost. Therefore, in step 3, insulation material called rock wool board made from local materials was applied, which reduced the heating and cooling energy demand significantly. In this research, the insulation materials with various sizes were discussed, the authors selected the insulation material with a specific size in terms of the extent of the energy-saving effect; however, the residents also can select the material with larger size according to the family’s economic condition. Although changing the glazing type from single glazing to double glazing can be relatively expensive, the economic situation in Chinese villages has improved rapidly in recent years. Most new houses in the villages of Xi’an have adopted double glazing; therefore, incorporating double glazing in the design of the nZEB in step 5 is reasonable.
Table 6 summarizes the specific characteristics of the construction changes after the application of passive design strategies.
Table 7 shows the materials and their thermal coefficients of the improved rural house. The annual energy demand of each category of the passive design rural house is shown in
Figure 11. Comparing the energy demand of the original rural house with that of the passive design rural house, the annual heating energy demand decreased from 8534 kWh to 3642 kWh. Additionally, the cooling energy demand decreased from 438 kWh to 146 kWh, and the annual per-area energy consumption of the house was 68 kWh/m
2. However, according to the nZEB standard of China, the annual per-area energy consumption of the building should be equal to or below 55 kWh/m
2. Therefore, although the energy-saving effect of the passive design was significant, it still could not achieve the nZEB standard. Thus, a renewable energy system should be incorporated to achieve the nZEB standard.