Building energy consumption analysis and measures: a case study from an administration building in Chengdu, China

Abstract

With the peak of carbon dioxide emissions and carbon neutrality, China is placing greater emphasis on energy expenditure. Office buildings occupy a prominent position in building energy consumption, which is one of the main energy consumption areas. Taking an administration building in Chengdu as an example, this article simulates the building energy consumption based on Design Builder software, examines the variables influencing energy consumption, and suggests energy-saving strategies combined with fresh ideas for sustainable architectural design. The results showed that the modeling building was a high-energy-consuming building, with an energy consumption of 724,857.59 kWh, and a unit area energy consumption of 288.17 kWh/m² in Chengdu. For energy conservation and emission reduction, this article proposes the following three energy-saving measures. The first is to apply heat recovery technology for air conditioning systems. The second is photovoltaic glass, which provides partial electricity demand for buildings and reduces dependence on traditional energy sources. The third is roof greening, which utilizes the plants to purify the air and beautify the environment. The results showed that the heat recovery technology in air conditioning systems reduced the total energy consumption of buildings from 642144.04 kWh/m² to 502937.83 kWh/m², photovoltaic glass reduced 552243.87 kWh/m², and roof greening reduced to 635947.35 kWh/m². All of these have good energy-saving and emission reduction effects. The above three strategies not only help reduce building energy consumption, but also provide substantial support for China to achieve carbon neutrality.

Introduction

China consumes a large amount of energy. In 2020, China consumed 5.5 billion tons of standard coal, accounting for 23.6% of global energy consumption (Liu 2023). Buildings are one of the primary energy consumption sources in China, making up 27% of total energy consumption. During 2023, public buildings accounted for 40% of all building energy, making them the most energy intensive of all building types. Simultaneously, according to statistics from China’s National Energy Administration, the total electricity consumption of urban buildings in the country accounts for about 12% of the electricity consumption of office buildings. Its yearly power usage per square meter is 10 to 20 times that of typical residential buildings, and 1.5 to 2 times that of similar buildings in wealthy nations such as Europe and Japan. Therefore, office buildings offer a high potential for improvement in terms of energy conservation and emission reduction, and have become a major research and development goal (Jiang et al. 2022).

Currently, modeling techniques are mostly used by academics to examine the primary energy consumption variables in office buildings and suggest appropriate alternatives. For example, Rana et al. (2022) assessed the energy usage of office buildings in Bangladesh, which is also located in the subtropical monsoon climate zone. They discovered that the window-wall ratio was one of the main factors influencing energy usage in air-conditioned office buildings. If the window-wall ratio was increased to between 30% and 40%, energy savings in office buildings reached 9.40%. Salehpour et al. (2023) tested the energy usage of many buildings in three different Canadian climatic zones. After the indoor temperature stabilized, constant temperature devices such as air conditioning significantly reduced energy consumption. The walls were not directly exposed to external or internal temperatures, and the thermal stratification in the walls helped maintain indoor temperature stability and save energy usage. Zhang et al. (2012) investigated the current energy consumption of Chongqing Municipal Government office buildings. The relationship between building space, air conditioning form, management system, and office building energy consumption was analyzed. The results showed that the main energy consumption of government office buildings was electricity. Central air conditioning consumed more energy than non central air conditioning (such as independent air conditioning). Zhou (2023) also mentioned that improving the building’s heating system could drastically lower its energy usage. A novel heating system that combined hollow ventilated internal walls (HVIW) with solar air collectors (SAC) was proposed to reduce emissions, increase energy consumption efficiency, and accomplish energy conservation. The results showed that under the same wind speed and heating amount, the non ventilated heat release efficiency under external heat coupling was greater than 60%, which increased by 5–10% compared with without external heat coupling.

The zero carbon technology path to achieve local architecture usually involves energy-saving design, renewable energy utilization, building material selection, intelligent systems, and carbon emission offsetting (Akram et al. 2023). Energy efficient design can improve building energy efficiency using efficient insulation materials or optimizing building structures. The intelligent control system helps optimize the operation of internal equipment in buildings, thereby improving energy efficiency. Carbon emission offsetting refers to offsetting the carbon emissions generated during construction and use through methods such as planting trees, purchasing carbon emission quotas (Haug and Hassinggaard 2023). The above technical paths requires comprehensive consideration of architectural design, equipment procurement, operation management, and the support and efforts of governments, enterprises, individuals, and other parties. Chengdu is an important hub for the economic development of western China and a key participant in the transformation of the country’s energy consumption structure. Chengdu has suggested a target of reaching an average energy saving of 70% in public buildings by 2025 in response to the National Call for Energy Efficiency. Due to the rapid economic expansion in Chengdu recently, the number of office buildings in the city has increased sharply, which has increased energy consumption. Thus, in order to reduce carbon emissions and achieve carbon neutrality, this article extensively studied the energy consumption and influencing factors of Chengdu office buildings using Design Builder software, and formulated relevant energy-saving strategies. The research aims to analyze the variables that affect the energy consumption of an administrative office building in Chengdu, and propose energy-saving strategies based on the new concept of sustainable building design. The effectiveness of the proposed strategy is verified through building simulation, providing some reference for energy-saving and emission reduction design in buildings.

The geographical location and climate of Chengdu

Chengdu is located in the central part of Sichuan Province in southwest China, with an area of around 14,000 km². As of 2022, the permanent population is approximately 21.268 million, making it a city with great development potential.

Geographical location

Chengdu is located on the western edge of the Sichuan Basin, spanning three geographical units: the western plain, the hills in the basin, and the western edge mountains of the basin. Among them, the western region is dominated by deep hills and mountains, while the eastern region is composed of plains and terraces. Chengdu has extensive plains and river systems, which provide good basic conditions for its development (Weng et al. 2023).

Meteorological characteristics

Chengdu has a humid subtropical monsoon climate. According to Chengdu’s annual climate report statistics, the annual average temperature is around 18℃. Among them, the outdoor temperature in summer from April to October is 25℃-28℃, and the maximum temperature reaches 31℃. The outdoor temperature from November to February in winter is 5℃-11℃, with the lowest temperature reaching 2.8℃ (Fig. 1). Overall, it belongs to an area with hot summers and cold winters. The humidity in Chengdu is relatively high throughout the year, ranging from 56 to 76%. From March to June, the humidity is low, around 60%. From July to September, the humidity is relatively high, around 75% (Fig. 2).

Fig. 1

Monthly temperature change trend in Chengdu area

Fig. 2

Monthly change trend of humidity in Chengdu area

Methods

This article uses Design Builder software to simulate a conventional administrative office building and analyze energy consumption and influencing factors.

Basic settings of the model

The simulated building is an administrative office building that integrates administration, office, and conference functions. The total height is about 18 m, with a total of 4 floors and an area of about 700 m² (Fig. 3). The floor plan of the modeled building is shown in Fig. 4. Based on the actual design and construction parameters of the Chengdu office building, the specific parameters of the modeling building are determined. Table 1 indicates the basic parameters. Table 2 shows the window-wall ratio parameters. Table 3 shows the building envelope parameters. On the whole, the modeling building complies with the standards for public buildings in the hot summer and cold winter areas.

Fig. 3

3D diagram of modeling administrative building

Fig. 4

Layout plan of the building

Table 1 Basic parameters for building simulation

Table 2 Window-wall ratio

Table 3 Basic information and construction of the building envelope

Specific equipment and operating settings of the modeling building

In the basic operation settings of the modeling building, the working days and rest days are operated on a weekly cycle, and their operation schedules are different. Table 4 shows the working hours, indoor personnel density, lighting density, equipment, and air conditioning system on weekdays and rest days. In addition, the power density of lighting is 15 W/m², and the power density of office equipment (e.g., computers) is 13 W/m².

Based on the investigation of air conditioning systems in office buildings in Chengdu, the commonly used fan coil unit (FCU) air conditioning system is selected as the air conditioning equipment in the modeling building. It consists of a fan coil air conditioning system, which is composed of a water chilling unit + boiler + FCU. The water chilling machine provides cooling for summer use. The gas boiler provides heating for winter use and then sends cold (hot) water to the end of FCU in each unit room through circulating water pumps and pipes, and then adjusts the flow rate of the FCU. Depending on the water supply or wind speed, it can independently process the air in the room, adjust the indoor temperature/humidity, and achieve cooling/heating in the room. The air treatment process mode is that there is only one or more FCUs in the independent unit room for local air circulation treatment, and independent exhaust pipes extract some indoor air. There is no air mixing flow caused by mechanical power in each unit room. This air handling system uses an independent FCU, which belongs to calculated HVAC and requires precise control and adjustment based on factors such as indoor air demand, water supply, or wind speed.

According to heating standards in Chengdu (Hu and Lin 2024), the operation time of the air conditioning cooling and heating system of the modeling building is divided into the heating period (November 20th to March 10th), and the cooling period (June 15th to September 14th).

Table 4 Office and equipment operation schedule of the modeling building

Results

Building energy usage

Energy demand refers to the overall demand for energy, that is, the amount of energy used. Energy consumption refers to the actual amount of energy used and consumed, that is, the actual consumption of energy demand. Conversion rate refers to the efficiency of using one energy source to produce another. For electricity, the conversion from energy demand to energy consumption can be calculated by considering the efficiency of power plants and transmission losses. For natural gas, energy demand can be converted into energy consumption by considering combustion efficiency, transportation losses, and energy losses during processing. By calculating the conversion rate, it is possible to more accurately estimate the actual energy consumption, thereby more effectively planning energy usage and resource management. Based on the modeling results, the annual energy consumption of the modeled building is 724,857.59 kWh, and its energy consumption per unit area is 288.17 kWh/m². Through the energy consumption analysis of different equipment, the results showed that there were three main types of energy consumption in the simulated building: electricity, gas, and water. Among them, gas energy consumption was the highest (214,003.29 kWh), which was air conditioning, followed by electric energy usage (155,629.31 kWh, Table 5). As shown in Fig. 5, refrigeration systems and equipment were the most important sources of electricity consumption, accounting for 36% and 27% of the total electricity consumption, respectively. The corresponding annual energy consumption was 56,496.72 kWh and 42,402.9772 kWh.

Table 5 End uses

Fig. 5

The proportion of electricity consumed by the simulated building throughout the year

Table 6 shows the energy consumption per unit area from different sources. The energy consumption density of air conditioners was the highest, with the electricity consumption density being 33.60 kWh/m² and the gas consumption density being 85.08 kWh/m². Next were equipment and lighting, both of which consumed electricity, with energy consumption densities of 16.86 and 11.42 kWh/m², respectively (Table 6).

Table 6 Energy consumption per unit area

Time distribution characteristics of different energy consumption types

Figure 5 shows the annual distribution characteristics of natural gas and electricity usage in modeled buildings. The results showed that the total annual energy consumption was the highest in winter (November-March), with an average of 48280.04341 kWh, followed by summer (June-July), with an average of 25,243.194 kWh.

The distribution characteristics of various forms of energy use varied throughout time. Most natural gas was used between October and April of the following year. The total electricity consumption was 214,003.2851 kWh. The main energy source used in winter was up to 59,474 kWh. According to the characteristics of the building air conditioning systems, its consumption mainly came from gas boiler heating.

Compared with gas energy consumption, although electricity consumption was distributed throughout the year, the overall consumption was relatively low, with a total consumption of 155,629.3101 kWh. Among them, the electricity consumption was relatively high from June to August, reaching the highest level in July (28,576 kWh). According to Table 5; Fig. 6, the electricity consumption in summer was mainly related to air conditioning.

Fig. 6

Annual distribution of electricity and natural gas consumption for the simulated building

Discussion

Energy consumption analysis

According to the building modeling results, the main energy consumption factors of Chengdu office buildings are heating and cooling. The main energy consumption for heating comes from natural gas, and the main energy consumption for cooling is electricity. This conclusion is consistent with previous research.

Energy consumption per unit building

Shenzhen, Guangzhou, and Chongqing are chosen for the investigation and comparison of the energy consumption per unit building area of office buildings, as these cities have relatively strong economic growth and are hot in summer and cold in winter. Liang et al. (2001) argued that the minimum energy consumption per unit area of office buildings in Shenzhen was 45 kWh/m²·a, and the maximum was 150 kWh/m²·a (av. 96 kWh/m²·a) through a survey of 15 high-rise office buildings in Shenzhen. The air conditioning, lighting, and office equipment accounted for approximately 30%, respectively (Liang et al. 2001). An energy consumption survey, conducted on 37 office and complex buildings in Guangzhou, found that the average cooling consumption per unit building area of office buildings in Guangzhou was 207 W/m², and the lighting consumption was 40 W/m². Tan found that the area energy usage density of office buildings in Chongqing was 132 (kWh/m²·a), of which cooling and heating accounted for 50.7% (Tan 2009).

The comparison shows that the energy usage of the office buildings in Chengdu simulated in this article is high (288.17 kWh/m²), and there is an immediate need for emission reduction and energy-saving measures.

Influence factors of building energy consumption

According to the energy usage of various equipment in the simulated building, air conditioning is the primary source of energy consumption in office buildings, accounting for 70% of total energy consumption, which is the major objective of energy saving. In addition to the high energy consumption of air conditioning, other factors that affect energy consumption include the materials used to construct structural fences, such as roofs, windows, and walls.

The better the thermal conductivity of the wall, the lower the energy consumption of the building’s air conditioning system. Air conditioner energy consumption decreased by 10.01% when the heat transfer coefficient K of the wall declined from 2.61 W/m²·K to 0.63 W/m²·K. The component energy consumption reduction rate also displayed a gradual downward trend (Fig. 7, revised from Tan 2009). The coefficients of the floor and ground of this building were relatively high, at 2.244 W/m²·K and 1.177 W/m²·K, respectively (Table 3). Therefore, improving wall performance and thermal conductivity can effectively achieve the energy-saving effect of office buildings.

Fig. 7

Wall heat transfer coefficient and building air conditioning total energy consumption curve

In addition to affecting the insulation effect of windows, exterior wall materials also affect building energy consumption. Its main function is to reduce heat transfer between indoors and outdoors through windows. The heat exchange process indicates that windows with lower heat transfer coefficients may result in less heat loss in winter and less heat inflow in summer. Therefore, the thermal insulation performance of windows is another key control signal for building energy efficiency.

Energy conservation measures

Given the high energy consumption of administrative office buildings and the sustainable development goals of the building, the study proposes measures for air-conditioning systems and building structural fences combined with new energy-saving and emission-reduction technologies. Architectural structure fence refers to the fence structure that separates the interior and exterior of a building. The research mainly analyzes glass and roof. The energy-saving effects of the three measures are shown in Table 7.

Table 7 The energy-saving effects of the three measures

Heat recovery technology of air conditioning system

Analysis shows that the main source of building energy consumption is air conditioning. Considering urban development and cost factors, replacing more energy-efficient air conditioning systems is both difficult and expensive. Therefore, this article focuses on the existing building air conditioning systems and proposes a design that optimizes air conditioning energy efficiency, which has a wider range of applications and higher value.

The “cooling” of conventional central air conditioners is not just a simple cooling process. It transfers heat from “low-temperature heat sources” to “high-temperature sources” by consuming a certain amount of external energy (such as electric, thermal, and solar). The heat generated by the “heat source” process is greater than the cooling capacity. In most current air conditioner designs, the heat is not utilized but taken away by the cooling medium (such as water and air) through the condenser. If this part of the heat can be utilized, it can achieve unidirectional energy consumption and bidirectional output, greatly improving the energy utilization rate of the refrigeration unit and saving the energy consumption of the cooling system. Therefore, this article proposes the air-conditioning heat recovery technology on the original central air-conditioning equipment to achieve energy savings.

The cooler in the air conditioning system is the primary energy-consuming component. It generates a large amount of waste heat during the refrigeration process. If these waste heat can be effectively captured and utilized, it can greatly reduce energy consumption. The cooling water design temperature of the general cooler is 37 °C for the outlet water and 32 °C for the return water, which is a low-grade heat source. This part of the heat energy cannot be fully recovered using ordinary heat exchange. Therefore, it is simple and reliable to fully recover this heat by increasing the condensation pressure or separating the cooling water from the cooler, and combining high-temperature source heat pumps or other auxiliary heat sources. The specific plan is shown in Fig. 8.

Fig. 8

Heat recovery technology for air conditioning systems

In the central air conditioning system, fresh air treatment and exhaust air treatment are two important processes. If the energy from these two processes can be fully utilized, a significant amount of energy consumption can be saved. For all-air central air conditioning systems, the general fresh air ratio is 15% or more. Regarding exhaust gas energy recovery, exhaust gas energy recovery equipment can be added to the all air system to improve the heat recovery rate.

At the same time, optimizing the design and installation process of the heat recovery device is also a very important factor. It is necessary to ensure that it can coordinate with other parts of the air conditioning system to achieve seamless connection and avoid any problems that may affect its performance and lifespan. Generally speaking, an efficient heat recovery system should be able to achieve a thermal efficiency of 60–80%, as an efficient heat recovery system can achieve better energy-saving effects, and improve indoor air quality and comfort.

Photovoltaic glasses.

Research shows that the facade affects more than 50% of building energy consumption, such as directly affecting space heating, cooling, lighting, ventilation, and lighting. At present, double-layer insulating glass is mainly used in modeling buildings and real buildings to reduce energy consumption, but it can no longer meet the increasingly stringent energy conservation and emission reduction standards. The trend in emerging glass technologies is to dynamically adapt their properties to different climatic conditions as well as energy consumption curves. There have been many studies on photovoltaic glass technology. The results show that photovoltaic glasses are effectively applied in the field of building energy conservation. This article also compares the building energy consumption before and after installing photovoltaic glasses. According to research results, solar panels reduce energy usage in buildings by 1.4%. Considering that the window walls of office buildings are usually higher, it is recommended to use solar glass to reduce emissions and save energy inside the building.

Photovoltaic glass is a special type of glass that can be used to collect sunlight and convert it into electrical energy. It has a special solar cell inside that absorbs sunlight and converts it into electricity. The produced electricity can be used to power devices such as mobile phones and tablets. Therefore, photovoltaic glass is like a self-sufficient “small power station” that can provide clean, renewable electricity for equipment. Currently, the outdoor sports brand Patagonia announced that it will install photovoltaic windows using NEXT Energy Technology Company’s transparent photovoltaic coating on 22 south-facing windows in its corporate headquarters building. The electricity generated will be used to provide energy for employees in Ventura, California. In practical applications, it is recommended to apply a transparent photovoltaic coating on one piece of glass and then seal the coating with another piece of glass.

Green roofs

In Chinese cities, especially mega cities, due to limited existing space, it is difficult to achieve quantitative breakthroughs in green space construction in central urban areas. Therefore, the environmental and ecological development potential of urban roofs can be fully tapped, which can not only reduce building energy consumption but also increase urban greening areas. Therefore, this article suggests adopting green roof design for existing office buildings to achieve energy efficiency.

Green roof is a method covering vegetation and soil on the top of a building, typically including a waterproof layer, plant planting layer, and growth medium. A green roof is a sustainable building design that covers the roof of a building with vegetation and growing media, utilizing the natural function of plants to filter water and air in urban and suburban landscapes. Green roofs can reduce the urban heat island effect, reduce the cooling load on buildings, improve energy efficiency, protect building structures, and extend the service life of roofs. Therefore, they are widely used in building design and urban planning. In addition, the thermal insulation capabilities of green roofs can reduce air condition energy consumption for buildings. The modeling results show that after installing the green roof, the total energy consumption the modeled building decreased by approximately 6196.69 kWh/m², and the energy consumption per unit area dropped from 288.17 kWh/m² to 285.39 kWh/m², with a decrease of 0.9%. Dong et al. used a heat transfer model to analyze the heat transfer efficiency of buildings, which quickly simulated and predicted the energy-saving efficiency of roof greening. The results demonstrated that the green roof saved energy consumption up to 5,000 kWh throughout the year (Dong et al. 2021).

This technology is widely used and successfully implemented in many cities. It has demonstrated significant advantages in effectively improving the urban environment, reducing energy consumption, creating beautiful landscapes, mitigating the heat island effect, and improving the city’s response to climate change, providing a healthier and more comfortable living environment. For example, the green roof of the Berlin Reichstag is a perfect combination of technology and nature. The roof made of special materials and designs can not only absorb rainwater and reduce temperature, but also reduce noise and provide a habitat for birds, realizing the harmonious symbiosis between man and nature (Zhou and Jiang 2011). For the Hudson Yards in New York, the green grassland on top of a 10-story building in the bustling area of Manhattan is like an oasis in the city. It not only absorbs rainwater and reduces temperature, but also adds natural beauty to the urban environment. Whenever citizens or tourists stand here, they can feel the tranquility and comfort coming from nature (Hu and Wang 2020).

Conclusion

This article used Design Builder software to simulate office buildings in Chengdu. The energy consumption and influencing factors were thoroughly analyzed, and corresponding energy-saving measures were formulated.

Due to its subtropical monsoon climate, Chengdu has experienced hot and humid summers as well as cold and humid winters. The building energy simulation indicated that the structure consumed 724,857.59 kWh of energy annually, or 288.17 kWh/m². By analyzing the energy consumption of different devices, this article proposed the following three energy-saving measures. The first was the heat recovery technology of air conditioning system. This technology effectively utilized the waste heat in the air conditioning system and recovered it through heat recovery devices, thereby reducing the fresh air load and reducing the total energy consumption of the building from 642144.04 kWh/m² to 502937.83 kWh/m². The second was photovoltaic glass, which was a special glass with a photoelectric conversion function that directly converted solar energy into electrical energy to provide part of the power demand for buildings and reduce the dependence on traditional energy. After installing photovoltaic glass, the total energy consumption of the building energy consumption model reduced to 552243.87 kWh/m². The third was green roofs, which reduced the total energy consumption of the model building to 635947.35 kWh/m². Plants have the natural ability to filter water and air in urban and suburban environments, and this sustainable building concept encompasses plants and growth media on building roofs. The application of the above three measures not only helps to reduce building energy consumption, but also focuses on urban beautification, providing strong support for the development of China’s green industry and practical solutions for achieving energy-saving and emission reduction goals.