Statistical analysis of residential building energy consumption in Tianjin

Abstract

To analyze the effect of energy conservation policies on energy consumption of residential buildings, the characteristics of energy consumption and indoor thermal comfort were investigated in detail in Tianjin, China, based on official statistical yearbook and field survey data. A comprehensive survey of 305 households indicates that the mean electricity consumption per household is 3215 kWh/a, in which annual cooling electricity consumption is 344 kWh/a, and the mean natural gas consumption for cooking is 103.2 m³/a. Analysis of 3966 households data shows that space heating average intensity of residential buildings designed before 1996 is 133.7 kWh/(m²·a), that of buildings designed between 1996 and 2004 is 117.2 kWh/(m²·a), and that of buildings designed after 2004 is 105.0 kWh/(m²·a). Apparently, enhancing the performance of envelops is effective in reducing space heating intensity. Furthermore, the results of questionnaires show that 18% of the residents feel slightly warm and hot respectively, while 3% feel slightly cold in winter. Therefore, the electricity consumption in summer will rise for meeting indoor thermal comfort.

Graphical abstract

Keywords

residential building / survey / statistical analysis / energy conversation / energy intensity / indoor thermal comfort

1 Introduction

Global carbon dioxide (CO₂) emissions increased by 3% in 2011, undergoing a 1% decline in 2009 and unprecedented 5% surge in 2010, reaching the highest level of 34 billion tons [1]. In 2011, the average CO₂ emissions per capita in China increased by 9% to 7.2 tons, accounting for 29% of the global CO₂ emissions at the top of the list [1].

In China, building energy consumption, including biomass energy, shared 25.1% of total energy consumption in 2010, and 24.1% of building energy was used for space heating in Northern China [2]. In recent years, dwelling area in China has increased significantly from 1.6 to 2 billion square meters per year. Based on current building energy consumption, the building energy consumption in China is expected to reach 1.089 billion tons of standard coal equivalents in 2020 [3]. With the improvement of living standard and increase of building area per capita, residential building energy consumption has become the fastest growth branch. Therefore, residential building energy efficiency has gradually become a hot issue in the field of energy conversation. The analysis of residential building energy consumption mainly concentrates on the investigation into residential building energy consumption in a given area and analysis of energy saving potential.

Chua and Chou [4] found by survey and simulation that a unit decrease in envelope thermal transfer value (ETTV) resulted in a 4% and 3.5% reduction in annual cooling energy for point block and slab block residential buildings, respectively. Theodoridou et al. [5] investigated the effects of year of construction, typology of buildings, glazing type and income on Greece typical larger and smaller urban energy consumption. Choi et al. [6] identified the characteristics of building energy consumption and found that the residents in the mixed-used apartment showed higher behavior of active heating management and more actively adjusted their indoor stay, but they consumed more electricity, particularly in summer, than those living in general residential apartments. Song and Choi [7] investigated residential buildings in Korea by field measurement and simulation, and drew a conclusion that the heating and cooling load of households without the balcony space was 39% and 22% higher than that of the unit with the balcony space. Hiller [8] measured electricity use for space heating, household electricity and electricity use for hot water production in 57 electrically heated Swedish single-family houses, and concluded that residential behavior played a key role in energy saving. Suárez et al. [9] presented an analysis of a combined solar-cogeneration installation for providing energy services in a set of four residential buildings from the legal, economic and environmental perspectives.

In 2011, Cheng et al. [10] simulated a typical residential building in southern Jiangsu and analyzed the effect of glazing ratio on total energy consumption. Through questionnaires, Li [11] drew a conclusion that personnel behaviors have an impact on air-conditioning energy consumption of residential buildings in hot summer and cold winter areas. Chen et al. [12] compared the energy use characteristics between old and new residential buildings in Shanghai, China, to probe into the influencing factors of residential energy consumption, and to analyze the reasons resulting in the differences of energy consumption quantities between high-energy and low-energy family group. Zhong et al. [13] investigated the indoor thermal conditions and the potential of energy conservation of naturally ventilated rooms in summer. Zhu et al. [14] studied temporal and spatial trends of residential energy consumption in China, and found that fuel consumption was not directly proportional to heating degree day (HDD18) and was also affected by heating days to some extent.

To reduce the energy consumption of residential buildings effectively, in China, energy performance standards follow a gradual evolutionary path beginning in 1986. Standards were revised to reduce energy consumption of air-conditioning and heating by improving the thermal performance of building envelop in 1996 and 2004. It was estimated that space heating energy consumption would decrease 50% and 65% by the base year (1980–1981) for standard 1996 and standard 2004, respectively [15,16]. The detailed envelop information for standard 1996 and standard 2004 are listed in Table 1 [16,17].

Residential energy demand is shaped by a variety of factors, including location and climate. Space heating varies with the climate, so it is soared in Northern China. To analyze the effect of energy conservation policies on total energy consumption, especially space heating energy consumption of residential buildings, a survey of energy consumption and household information was conducted in Tianjin, China.

Tianjin is located in the center of the Bohai economic circle, extending from 38° to 40°N latitude and 116° to 118°E longitude, covering approximately an area of 11917.3 km². The heating season lasts from November 15 to March 15 the next year and the annual average temperature is approximated 12.7°C. The average temperature in July which is the hottest month reaches 28°C, with a highest temperature of 40.9°C, while, the average temperature in January which is the coldest month reaches −2°C, with a lowest temperature of −17.8°C. The value of HDD18 is 2738.3°C·d.

2 Research method

In this paper, literature consultation, questionnaire and field survey were performed to gain the building energy consumption information in Tianjin. Then, a comprehensive analysis was conducted to determine the characteristics of residential building energy consumption.

Combining questionnaire and field survey, the characteristics of energy consumption was investigated in 463 energy-efficient building families thoroughly.

The data from Tianjin statistical yearbook were analyzed to figure out the changing trend of space heating intensity in residential buildings.

The data of space heating energy consumption from 2012 to 2013, including 2543 energy-efficient households and 1885 other ones (a total of 266805 m²), were supplied by the heating company. The analysis of this data provided the characteristics of space heating intensity between different levels of energy-efficient buildings.

This paper adopts the PanTa criterion to process the data, leading to a sample of 305 valid energy-efficient samples. By using the same method to process the data of space heating energy consumption, a sample of 2081 valid energy-efficient samples and 1885 valid other ones are obtained.

3 Energy consumption by end-use

3.1 Sample survey

3.1.1 Distribution of area

The detailed survey involves electricity consumption, natural gas consumption, building area, year of construction, inhabitant, heating and air-conditioning system, indoor thermal comfort, and behavioral habits etc. Figure 1 presents the distribution of effective area of samples, which indicates that building areas mainly concentrate in the 100–140 m² and a household occupies 110.3 m² on average.

3.1.2 Distribution of inhabitants

It is a fact that economic growth, changes in lifestyle and technological progress in energy efficiency technologies are both driving forces for the development of energy consumption in the building sector. Numerous studies showed that there exist indissoluble connections between building occupancy and energy behavior of dwellings [17,18].

Figure 2 indicates that the average household size is 3.06 persons for the examined samples. The family of three people and the family of two people account for 45.9% and 29.5% of the sample, respectively. The floor space per capita is 36.0 m² for the examined sample.

3.1.3 Patterns of air-conditioning and heating

The results of questionnaires concerning the ways to cool in summer show that split air conditioner and household central air conditioner account for 68.5% and 29.2%, respectively, while only 2.3% of households use a combination of natural ventilation and fan. The district heating with municipal heat source proves to be the dominant way and the heating terminal equipment includes two categories, the radiator (58.2%) and floor radiant heating (41.8%). The main patterns of heating and air-conditioning contain split air conditioner combined with radiator and split air conditioner combined with floor radiant heating.

3.2 Energy consumption distribution

3.2.1 Computing methods for energy consumption breakdown

In this paper, the energy consumption includes electricity, heat and natural gas. To conduct further analysis, all units were unified. The detailed unit conversion formula are

1)‚Heat: 1 GJ= 277.78 kWh;

2)‚Natural gas: the low value calculation is 35378 kJ/Nm³ (9.827 kWh/Nm³) in Tianjin.

End uses were classified into space heating, air conditioning, cooking and a residual category. Space heating energy consumption can be estimated by household heat meters. A portion of electricity is consumed to operate the air conditioning system. The estimate of electrical consumption due to cooling equipment is more difficult.

Electrical consumption can be divided into climate-related electrical consumption and climate- unrelated electrical consumption. Climate-related electrical consumption consists of the electrical consumption of heating and cooling system. The annual cooling energy consumption can be calculated by Eq. (1).

(1)W_{\mathrm{c}}={\displaystyle \sum _{i=\mathrm{6}}^{\mathrm{8}}\left(W_i-{\overline{W}}_{\mathrm{a}}\right)},

where W_c is the annual cooling electricity consumption, W_i is the electricity consumption per month in the cooling season, and {\overline{W}}_{\mathrm{a}} is the mean electricity consumption per month in the transition season.

Natural gas is mostly used for cooking, so it can replace the cooking energy consumption.

3.2.2 Correlation

By using statistical software SPSS, the correlations between the consumption of total electricity, cooling electricity, natural gas of 305 energy-efficient households were analyzed for multiple factors, so were the consumption of 2081 energy-efficient ones. Among the factors, air conditioning mode is sequencing variable: 0 means split air conditioning, 1 means household central air conditioning, and 2 means natural ventilation combines fan.

Table 2 shows that inhabitants and area have a significantly positive correlation with the total electricity consumption while the air conditioning mode is negatively related. That is, residential electricity consumption increases with the increase of inhabitants and area, and with the decrease of air conditioning mode sequencing variable. Therefore, inhabitants have a more significant impact on total electricity consumption, and the cooling mode using natural ventilation combined with electric fan is more efficient. Similarly, cooling electricity consumption increases with the increase of inhabitants, year of construction and air conditioning mode sequencing variable, and inhabitants have a more significant impact. Natural gas consumption increases with the increase of inhabitants and year of construction, and inhabitants have a more significant impact. Space heating energy consumption increases with the increase of heating area and year of construction, and heating area has a more significant impact. Consequently, it is more reasonable to evaluate total electricity, cooling electricity and natural gas consumption using evaluation index of energy consumption per capita, while it is more reasonable to evaluate space heating energy consumption using evaluation index of consumption per unit area.

3.2.3 Mean electricity consumption

The investigation merely obtained the monthly total electricity consumption owing to the fact that itemized meters were not installed. Figure 3 indicates that the air-conditioning season lasts from June to August, and the transition seasons are April and September. In the transition season, there are no heating and air-conditioning electricity consumption, which results in the lowest electricity consumption in April. In the air-conditioning season, high power equipment such as the air conditioning and the refrigerator increase the electricity consumption, leading to the highest electricity consumption in July. The other months are the heating season, in which many people use more electrical equipment to complement the insufficient heating by the municipal heating supply, making electric consumption higher than the transition season. Figure 4 depicts the distribution of electricity consumption per capita. The average consumption is 1041 kWh/(p·a) and the standard deviation is 349 kWh/(p·a). In addition, the average electricity consumption per household is 3215 kWh/a, and electricity consumption intensity is 29.15 kWh/(m²·a).

Figure 5 shows the distribution of household air conditioning electricity consumption. It can be calculated that the air conditioning electricity consumption accounts for approximately 11.7% of the total electricity consumption, and the air conditioning electricity consumption per household is 344 kWh/a.

The variation of the air conditioning electricity consumption with inhabitants is shown in Fig. 6. Apparently, the in-building ratio rises with the increasing inhabitants, thus the running time of air conditioning, TV and other electrical equipment increase, and so does the total electricity consumption. It can be calculated that the air conditioning electricity consumption per capita is 118 kWh/(p·a) and the standard deviation is 129 kWh/(p·a). Furthermore, the air conditioning electricity consumption intensity is 2.53 kWh/(m²·a).

3.2.4 Mean space heating energy consumption

The data of space heating energy consumption was obtained by reading meters month by month. Figure 7 indicates its relatively smooth variation with an average of 9678 kWh/a.

3.2.5 Mean natural gas consumption

In household, natural gas is mainly used for cooking. Figure 8 indicates that domestic natural gas consumption increases gradually with the increase of household size, but natural gas consumption per capita decreases. It can be calculated that natural gas consumption per household is 103.2 m³/a, which is equivalent to 1014 kWh/a. In addition, natural gas consumption per capita, of which the standard deviation is 26 m³/(p·a), is 36 m³/(p·a).

To analyze the component proportion of energy consumption, the various energy consumption units are unified as tabulated in Table 3. It can be observed that space heating energy consumption accounts for the largest share, followed by other electricity consumption and air-conditioning electricity consumption. This suggests that there is still a larger energy-saving potential in heating area.

3.3 Thermal comfort assessment

A questionnaire survey was conducted to evaluate indoor thermal comfort in energy-efficient residential buildings. In summer, 63.4% of the residents feel moderate, but 18% of them feel slightly warm while 18% of them feel hot. In winter, residents who feel warm, slightly warm and moderate account for 39.39%, 15.15% and 15.15%, respectively, but 6.06% of them feel overheating and only 3% of them feel slightly cold. Therefore, indoor thermal environment in some household cannot meet the requirements and electricity consumption in summer rises for meeting indoor thermal comfort. It was found that when residents feel overheating in winter, they usually open their windows, making the indoor environment hard to control, which increases space heating energy consumption.

4 Space heating consumption

Table 3 shows that space heating energy consumption accounts for 69.6% of the total energy consumption. Space heating in Northern China is predominately supplied by the district heating system. Many systems have no meters or switches installed in individual houses, resulting in imbalances and inability to control heat use, forcing consumers to commonly open windows to regulate overheating. With the adoption of the heat metering system and the improvement of building envelope, space heating intensity will decline. Therefore, analysis of the different factors that affect space heating energy consumption contributes to reducing the total energy consumption.

4.1 Statistical yearbook data analysis

By analyzing the data in the statistical yearbook in Tianjin from 2001 to 2011 (Fig. 9), it can be found that space heating intensity fell sharply in 2005. Considering the effect of construction period and move-in time, it can be inferred that this decline resulted from the implementation of Tianjin Energy Efficiency Design Standard for Residential Buildings in 2004.

4.2 Multi-sample analysis of space heating energy consumption

Actually, the space heating energy consumption in the statistical yearbook contains residential buildings and some of the public buildings. The indoor heat release of public buildings is large during the day and temperature heating is low during the night. Therefore, the space heating intensity is significantly lower than that of residential buildings. Consequently, the yearbook data does not reflect the space heating energy consumption of residential buildings truly.

To analyze the space heating energy consumption in detail, field surveys were conducted in 2012 and 2013. After screening out exceptional data, there are 3966 valid dwellings: 48% of which were designed before 1996, 31% of which were designed between 1996 and 2004, and 21% of which were designed after 2004. Figure 10 shows the distribution of household area for effective samples, which indicates that living areas mainly concentrate in 60−80 m².

4.2.1 Space heating energy consumption according to building area

Correlation analysis show that the correlation coefficient between building area and household energy consumption is the largest, that is, building area has a significant influence on space heating energy consumption. Figure 11 demonstrates the space heating energy consumption. It can be found that when the heating area is lower than 70 m² and greater than 100 m², there is a great increase in space heating energy consumption, but there is a slight variation from 70 m² to 100 m². Meanwhile, there is an apparent decline in space heating intensity when the heating area is greater than 70 m².

4.2.2 Space heating energy consumption according to building envelop

Figure 12 shows space heating intensity is mainly concentrated on 90–140 kWh/(m²·a), and the standard deviation is 29 kWh/(m²·a). The space heating average intensity of residential buildings designed before 1996 is 133.7 kWh/(m²·a), that of buildings designed between 1996 and 2004 is 117.2 kWh/(m²·a), and that of buildings designed after 2004 is 105.0 kWh/(m²·a). In addition, space heating energy consumption per household in energy-efficient buildings is 8577 kWh/a.

Along with the strengthening of the building energy saving consciousness and the extensive use of new building envelop structure materials, the thermal parameters of building envelop were gradually improved. Therefore, the year of construction can reflect the characteristics of the envelop. It can be found that heat transfer characteristics of the envelope have a great influence on space heating energy consumption. It can be calculated that the space heating intensity of buildings designed after 2004 is lower than that designed between 1996 and 2004 by 10.4% and lower than the others by 21.5%. Therefore, enhancing the performance of envelop is an effective way to reduce space heating energy consumption.

With the increase of energy-efficient residential construction areas in recent years, as well as the promotion of energy saving technology, space heating energy consumption of residential buildings decreases significantly. However, there are still some high heat-consumption buildings which need further energy-saving reconstruction.

5 Conclusions

A scientific investigation of official statistical yearbook and field survey data can provide detailed information for policy makers to understand the energy conservation potential of residential buildings in China. Conclusions drawn from the study in this paper are as follows:

1) The energy consumption for space heating, space cooling and cooking accounts for 69.6%, 2.5% and 7.3%, respectively. Space heating energy consumption accounts for the largest share. Therefore, there is a great energy saving potential in space heating.

2) The effective way to reduce space heating energy consumption is to enhance the performance of envelop. Space heating average intensity of residential buildings designed before 1996 is 133.7 kWh/(m²·a), that of buildings designed between 1996 and 2004 is 117.2 kWh/(m²·a), and that of buildings designed after 2004 is 105.0 kWh/(m²·a).

3) Eighteen percent of the residents feel slightly warm and 18% of them feel hot in summer, while 3% of them feel slightly cold in winter. This indicates that some household indoor thermal environment cannot meet the requirements and electricity consumption in summer rise for meeting indoor thermal comfort.

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