Survey and analysis of energy consumption in office buildings in Tianjin

Abstract

An energy audit of 24 office buildings was conducted in Tianjin, including basic information of buildings, building energy system and energy bills. The investigation results showed that the average intensity of energy consumption in office buildings in Tianjin accounts for 161.51 kW·h/(m²·a). By breaking the energy consumption down into detailed items, it was found that the heating system consumed the highest amount of energy (46.6%), followed by equipment (35.9%), cooling system (11%) and lighting system (6.69%). The main factors for office building energy consumption were found and some feasible measures to save energy were proposed.

Keywords

office building / energy consumption intensity / energy consumption breakdown / energy saving measures

Introduction

In recent years, climate changing has brought about more and more adverse impact on people’s daily life. The promotion of energy saving and emission reduction has became a consensus around the world. At present, building energy consumption in the world accounts for 40% of world total energy consumption, while that in China more than 25%. If building energy consumption in China continues to increase at the present speed, it will reach 1.089 billion tons of standard coal at 2020, three times more than that of 2000 [1]. Of the total building energy consumption, air conditioning energy consumption alone accounts for 50%-60% [2]. With the rapid development of office construction, the energy consumption of buildings will increase more rapidly. Of the energy consumption of all buildings, the energy consumption of office buildings takes the largest share.

As a major city in north China, Tianjin shoulders a heavy burden to save energy and reduce carbon emissions. So it is necessary to conduct a comprehensive investigation on the energy consumption of office buildings in Tianjin. This paper expounded on the average energy consumption of office buildings in Tianjin and the main factors contributing to the energy consumption of office buildings. It also proposed some feasible technology and management measures to save energy and reduce carbon emissions.

Overview of energy consumption of office buildings in Tianjin at present

Energy consumption investigation objects and content

From April to August in 2012, an investigation of the annual energy consumption of office buildings in Tianjin was conducted, including the basic information of the buildings, the energy system of the buildings, and the basic information of equipment and energy bills by field surveys, interviews, and by collecting the power consumption data from related authorities. By selecting the investigation data, removing the incomplete samples, 24 buildings were valid samples for the investigation of energy consumption of office buildings in Tianjin. These 24 buildings are all government official buildings

Investigation results

Basic information of buildings

The construction area of the 24 buildings ranges from approximately 5000 to 47000 m². The heating areas of these buildings are basically equal to the building areas. The areas which are air conditioned account for about 90% of the building areas. When calculating the energy consumption, the HVAC (heating, ventilation, and air conditioning) areas are approximately equal to the building area.

The envelopes of 50% of the buildings are of concrete frame structure and the rest are of brick structure. Only 38% of the 24 buildings have exterior insulation while 12% of the buildings have internal insulation., Fifty-four percent of the buildings have single layer ordinary glass on the outer window, seventeen percent have single layer coated glass, twenty-five percent have hollow double ordinary glass, four percent have double layer coated glass. Almost every outer window has aluminum frame and internal shading, but none of them have exterior shading.

The heating and cooling equipment of the 24 buildings includes water-cooled screw chiller (accounting for 21%); air-cooled chiller (accounting for 4%); direct fired lithium bromide absorption chiller (accounting for 13%); and fission air-conditioning or VRV (accounting for 62%). The heating terminal includes radiator (accounting for 71%), which gets hot water from the municipal pipe network directly; fan coil (accounting for 8%), which is heated by central air-conditioning system; and fan coil (accounting for 8%), which gets hot water from the municipal pipe network transfer heat by heat exchangers. Moreover, a few of the buildings are heated by geothermal heating.

Value of building energy consumption

The energy used in the 24 buildings includes electricity, gas and diesel. Electricity is mainly consumed by lighting, office equipment, building equipment and the HVAC system. Gas and diesel are consumed by the HVAC system. The energy consumption systems work from 8:00 am to 8:00 pm. In order to expediently compare and analyze the energy consumption in each building, the energy consumption of each kind is converted into the same unit.

The calculation formula for the annual total energy consumption of the buildings Q (kWh/a) is

(1)Q={\displaystyle \sum Q_i}{a}_i,

where Q_i is the annual consumption of each kind of energy, α_i is the corresponding conversion factor. The value of conversion factor of each kind of energy is listed in Table 1.

The energy consumption information of 24 office buildings is presented in Table 2.

In Table 2, the total energy consumption includes building energy consumption of the HVAC system, lighting energy consumption and energy consumption of all the equipment in the building. It can be observed from Table 2 that in 2011, the average energy consumption density of office buildings in Tianjin is 161.51 kW·h/(m²·a). But the average energy consumption density of buildings vary greatly (from 124.25 kW·h/(m²·a) to 238.57 kW·h/(m²·a)). This is mainly caused by the difference of the heating and cooling system in the buildings. The average total energy consumption intensity of the buildings that use central electric refrigeration and municipal pipe network heating is greater than that of the buildings that use fission air-conditioning and municipal pipe network heating. Because not only does the heating and cooling equipment consume electricity, but also the corresponding transmission and distribution system in the central electric refrigeration. Moreover, the lack of rational operation management in the buildings also wastes lots of energy. The average annual total energy consumption per unit area of buildings using direct fired lithium bromide absorption chiller for heating and cooling is greater than those of other buildings, because of the lower efficiency of the heating in winter. The average energy consumption density of the buildings which use the same HVAC system is also different mainly because of the different building thermal insulation performance of the maintenance structure, the different building occupant density, the different efficiency of lighting equipment, and different operation and management measures.

The survey conducted by Tianjin University shows that the average energy consumption density of hospital buildings in Tianjin is 348.5 kW·h/(m²·a) [5]. The survey conducted by Tianjin Institute of Urban Construction indicates that the average energy consumption of commercial buildings in Tianjin is 200-300 kW·h/(m²·a) [6]. It is apparently seen that the average energy consumption density of the office buildings is much lower than that of hospital buildings and commercial buildings. Because the occupant density and lighting load of official buildings is smaller than that of hospital buildings and commercial buildings, and the time when the office building energy consumption system works is much shorter. The survey conducted by Beijing University of Civil Engineering and Architecture demonstrates that the average energy consumption density of an official building in Beijing is 149.1 kW·h/(m²·a) [7], which is approximately equals to that of official buildings in Tianjin.

Breakdown energy consumption

Energy consumption of heating and cooling system

Building energy consumption can be divided into climate-related energy consumption which consists of the energy consumption of heating and cooling system, and climate-unrelated energy consumption which is regarded as invariable all the year round including lighting energy consumption and equipment energy consumption etc. It can be assumed that the energy consumption of each kind is invariable in every month except that of the heating and cooling system. The months barring heating period and cooling period are defined as transitional seasons. So in transitional seasons the monthly average energy consumption is the monthly energy consumption of other kinds of energy except heating and cooling. From that, the energy consumption of heating and cooling system can be obtained by subtracting the monthly average energy consumption in transitional seasons from monthly total energy consumption during heating and cooling period. The heating energy consumption of the buildings heated by municipal pipe network can be obtained directly by converting the energy consumption from the heat meters in the external network. By calculating, the average heating energy consumption of the 24 office buildings is 74.39 kW·h/(m²·a), accounting for 46.6% of the total building energy consumption; and the average energy consumption of the cooling system is 17.63 kW·h/(m²·a), accounting for 11.0% of the total building energy consumption.

Lighting energy consumption

The lighting energy consumption is obtained according to the investigated lighting equipment power and utilization time, since most office buildings don’t have the breakdown energy consumption meters for lighting system. By calculating, the average lighting energy consumption of the 24 office buildings is 10.66 kW·h/(m²·a), accounting for 6.69% of the total building energy consumption.

Energy consumption of other equipment

The energy consumption of other equipment refers to the energy consumption of all the electrical equipments including office equipments, lifts and water heaters etc. The energy consumption of other equipment is also obtained according to the electrical equipment power and utilization time. By calculating, the average energy consumption of other equipment of the 24 office buildings is 58.84 kW·h/(m²·a), accounting for 35.9% of the total building energy consumption.

The statistical breakdown energy consumption of the 24 office buildings is depicted in Fig. 1.

Analysis of carbon emissions

The calculation formula for the annual carbon emissions per unit area of the buildings is

(2)R={\displaystyle \frac{{\displaystyle \sum (e_i\cdot {}_i)}+({\mathrm{10}}^{\mathrm{6}}G/\mathrm{29307}){}_{}}{A}},

where R is the annual carbon emissions per unit area (kgCO₂/(m²·a)); e_i the annual consumption of electricity (kW·h/a), gas (N·m³/a), diesel (kg/a), and coal (kg/a); G is the winter heating energy consumption of the buildings (GJ); 29307 is the low heat consumption of standard coal (kJ/kgce); EF_i(i = electricity, gas, diesel and coal) is the carbon emissions factor of electricity, gas, diesel and coal; EF_CE is the carbon emissions factor of standard coal (kgCO²/kgce); and A is building area (m²). The carbon emissions factor of different energy is presented in Table 3 while the annual carbon emissions per unit area of each office building in 2011 is presented in Fig. 2.

From Fig. 2, it can be found that the average carbon emissions per unit area of different buildings vary a lot (from 60 kgCO₂/(m²·a) to 109 kgCO₂/(m²·a)), and the average carbon emissions per unit area of those buildings is 81 kgCO2/(m²·a). Therefore, it can be concluded that the carbon emissions depend both on energy consumption quantities and energy structure. When the energy structure is the same, the average carbon emissions per unit area increase as the average energy consumption density per unit area of the building increases. When the energy structure is different, the average carbon emissions per unit area are related to the energy consumption and the carbon emission factors of the energy utilized. Therefore, the reduction of the total energy consumption and the reasonable adjustment of the energy structure are effective ways to reduce carbon emissions per unit area in office buildings.

Discussion

Problems existing in office buildings

1) Heating ventilation and air conditioning system: it is found from the survey that a considerable number of fans in office buildings are dirty, which increases the resistance, resulting in low air pressure and reducing the efficiency of the fans. The radiators are also covered by dust. In terms of transmission and distribution systems, the water pumps do not have variable frequency adjustment.

2) Lighting and office equipments: Some of the old office buildings still use fluorescent lamps instead of energy-saving ones. Some office equipment, such as computers, drinking fountains etc, are always left working even though no one is using them, which would result in unnecessary waste of energy.

3) Operation and management: some of the energy-using equipment and systems are not timely maintained. The centralized HVAC systems in some buildings are not reasonably adjusted according to different weather conditions. Natural ventilation is not reasonably and adequately used to eliminate indoor heat in transition seasons.

4) Energy consumption monitoring system: there exist some problems in the energy consumption monitoring system, for example, some monitoring data cannot be found.

Optimal design

1) Some energy-saving technologies, such as frequency conversion technology, variable air volume system(VAV), temperature and humidity independent control system, ground source heat pump, should be applied to improve the energy efficiency ratio of the system. Moreover the air conditioning system should be cleaned in time.

2) The operation management in office buildings should be enhanced, the literacy and knowledge of the administrative staff should be improved, and awareness of energy conservation of the staff in the office should be strengthened. Natural ventilation should be reasonably and adequately used to eliminate indoor heat in the transition seasons. The lighting in public areas should be strictly controlled. The temperature in air-conditioned rooms should be kept in a suitable range by regulating the parameters of air conditioning equipment by remote control.

3) A perfect energy consumption monitoring system should be established to monitor the energy consumption of buildings real-time, and identify and resolve the problems on time.

Conclusions

The following conclusions can be drawn from this the study in this paper:

1) The average energy consumption in office buildings in Tianjin is 161.51 kW·h/(m²·a).

2) Based on the energy consumption breakdown, it is found that the heating system consumes a major amount of the total energy (46.6%), followed by other equipment (35.9%), the cooling system (11.0%) and the lighting system (6.69%).

3) The average carbon emissions is 81 kgCO₂/(m²·a), which is mainly affected by the energy consumption and energy structure.

References

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[2] Wang Z G. Building energy conservation and new energy application research. Dissertation for the Doctoral Degree. Xi’an: Xi’an University of Architecture and Technology, 2005

[3] State Electricity Regulatory Commission. The Power Regulatory Annual Report 2010. 2011

[4] Tsinghua University Architecture Energy-Saving Research Center. China Building Energy Saving Annual Development Research Report 2009. Beijing: China Building Industry Press, 2009

[5] Jiang C X, Xing J C, Qin X N, Ling J H, Yang J. Study on energy efficiency trading baseline of hospital buildings in Tianjin. Building Science, 2012, 28(3): 1-5

[6] Lv J, Guo S W, Jiang Y, Guo J M. Analysis on energy consumption status and energy-saving of typical commercial building inTianjin. Building Science, 2008, 24(6): 1-5

[7] Zhang F, Li D Y. The energy audit and HVAC analysis of one office building in Beijing. In: Proceedings of the 17th Academic Exchange Meeting of the Architectural Society of China Building Thermal Power Branch, Beijing, 2011, 399-400

[8] Task Force on National Greenhouse Gas Inventories (TFI) of the IPCC. 2006 IPCC Guidelines for National Greenhouse Gas Inventories Volume 2 Energy. Japan: Institute for Global Environment Strategies, 2006

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Higher Education Press and Springer-Verlag Berlin Heidelberg