Promoting Chinese energy efficiency (part one)

The way that China’s urban buildings are run means that power consumption by homes and offices is lower than in developed countries. Jiang Yi shows the difference that windows, heating and cooling practices and other factors can make.

Urban construction is advancing rapidly across China: from 2000 to 2005, the total surface area of urban buildings doubled. This has led to a rapid rise in the amount of energy consumed by buildings. How do levels of building energy consumption in China compare with those in developed countries? Answering this question provides useful information on the state of architectural energy efficiency in China, and allows us to scientifically plan for future energy-efficiency work and to decide the direction that architectural energy efficiency should take.

Building energy consumption in China can be divided into five main categories: building energy consumption in agricultural areas (“the countryside”); energy consumed through heating in the urban northern areas; domestic building energy consumption in urban areas (excluding heating); energy consumption by ordinary urban public buildings; and energy consumption by large urban public buildings.

Owing to the large development gap between China’s urban and rural areas, building energy-use in the countryside is extremely low compared to urban areas. Therefore, only the last three categories – that is, urban-building energy consumption – will be used when comparing Chinese energy consumption with developed countries. In 2004, the total surface area of urban buildings stood at 14.9 billion square metres. The urban population was 543 million. Statistics show that energy consumption by urban buildings was about 170 million tonnes of standard coal, which accounted for 9% of total coal consumption. Electricity consumption by buildings was 590 billion units — 27% of total consumption.

Figure 1 (below) shows per-capita building energy consumption in China compared to a selection of developed countries. Figure 2 shows building energy consumption per metre squared compared to a selection of developed countries. It can be seen that even with only figures for urban China taken into account, building energy consumption is much lower than the average in developed countries. This is true for both per capita and per unit of area calculations. If figures for the countryside – home to 60% of China’s population and 60% of its buildings – are taken into account, then China’s building energy consumption falls even further below that of developed countries.

1·   Energy consumed in homes for heating water is much lower in China than in developed countries. Less than 70% of urban households in China own a water heater, and many of those that do so rely on solar power. The average urban Chinese household uses only 80 to 130 kilowatt-hours (kWh) for heating water per year, compared to an average of 1,404 kWh (ten times the Chinese figure) per year in Japan. This is a powerful reflection of the difference in lifestyles and living standards between the two countries.

2·   Energy consumed by residential air-conditioning in China (including by thermodynamic heating systems in the south) is far lower than in the US. Although in China there are 81 air-conditioning units for every 100 urban households, the majority of Chinese families will turn on the air conditioning only after they come home from work, or when they are at home at the weekends. When no one is at home, the air conditioning will be switched off, and when the outside temperature is within a comfortable range, people will open windows to provide natural ventilation, so energy consumption is very low. According to a survey, the average household in Beijing only uses air conditioning for less than 200 hours per year, consuming an average of five kilowatt-hours per square metre (kWh/m2) per year. In contrast, most American households have air conditioning switched on 24 hours a day for the whole year, maintaining temperatures between 18º to 26º. In order to maintain a high level of air quality, fixed ventilation systems are used, which at any time throughout the year can replace half the air in a room every hour. Simulations have shown that these differences in the way air conditioning is operated could increase residential energy consumption by close to three times.

3·   There are also differences between China and developed countries in the use of domestic lighting and electrical appliances. There is not much difference between the density of lighting in Chinese homes and those in developed countries, but the length of time for which lights are switched on varies hugely. This variance in patterns of usage is the main reason for differences in this kind of energy consumption.

4·   Let’s take another look at office buildings. Figure 3 (above) shows two typical office buildings in Beijing. On the right is an “ordinary” office building. In the daytime, the office is lit by natural light coming in through the windows; in autumn and spring, the windows can be opened for ventilation; in the summer, freestanding air-conditioning units are used. Air-conditioning units are only switched on (and the windows closed) when the office is occupied and outside temperatures are too warm to be tolerable. On the left is one of the so-called “modern” office buildings that are now springing up everywhere. Because it is so large, most of the inside has to be lit with artificial lighting. The air quality is maintained by mechanical ventilation systems. As a central air-conditioning system is fitted, this system has to be relied upon all year round to moderate temperature and humidity. As the building is so tall, lifts (elevators) are necessary. Simulations show that the modern building uses three times more electricity per square metre than the traditional building, even though the two provide an essentially similar office environment. The building on the right consumes roughly 20 units of electricity per square metre per year. This figure comes very close to the average for ordinary office buildings in Beijing. The building on the left, however, consumes 60 units of electricity per square metre per year. This example shows clearly that different architectural concepts and the pursuit of a “modern office environment” can result in huge differences in energy consumption, even though the different building styles fulfill basically similar functions as offices.

Apart from a few economically developed cities — such as Shanghai and Shenzhen — most urban offices, shops and hotels in China are similar to the “ordinary” model. However, in the developed countries exemplified by the US, the vast majority of offices take the form of the “modern” building described here. Some offices, despite covering an area of no more than 20,000 square metres, consume far more than 60 kWh/m2 per year. This is because they use central air conditioning, the majority of their windows cannot be opened, and the lights are almost always left on. All this clearly explains why non-residential building energy consumption levels in China are far lower than in developed countries.

One could conclude, from the above four points, that the reason why China’s urban building energy consumption is relatively low is that the energy consumption of residential and ordinary office buildings – apart from heating — is lower than that of their equivalents in the developed countries. This variance is mainly caused by differences in the way buildings are used and run. The main reasons for the difference in energy consumption are:

* continuous provision of artificial light, instead of provision only when it is necessary;

* continuous use of air conditioning throughout whole buildings (the central air-conditioning method), instead of using separate air-conditioning units in separate areas, operated intermittently;

* continuous use of mechanical and fixed ventilation systems, instead of opening windows for natural ventilation at appropriate times;

* fixing set interior temperatures for the whole year, instead of keeping temperatures as warm as is tolerable in summer, and as cold as is tolerable in winter; and

* using energy for lifts.

The factors outlined can lead to differences in energy consumption of up to three times.

Next: Building a healthy urban environment

Jiang Yi, professor at Tsinghua University and an academician of the Chinese Academy of Engineering, is an expert on building and environmental engineering. He is noted for his research on power-saving buildings and ecological construction.

Homepage photo by le niners

The above figures are the results of our research into building energy consumption for various types of building in several countries. The same conclusions can be reached even if different research methods and different kinds of statistical analysis are used. Currently, the United States accounts for approximately a quarter of world energy consumption, of which approximately one third is consumed by buildings. Therefore, American building energy consumption accounts for one twelfth of total global energy consumption. Chinese energy consumption accounts for approximately one eighth of world energy consumption, of which roughly one fifth is consumed by buildings — which means that Chinese building energy consumption is one fortieth of total global energy consumption. Most of this consumption occurs in the cities, which are home to 600 million people, or twice the population of the US. Taking this into account and using per-capita figures, the ratio of Chinese to American building energy consumption is 12:80, or approximately 1:6.7. This corresponds closely with the figures above.

China’s relatively low building energy consumption is not the result of advanced energy efficiency technologies, or a widespread awareness of the need for energy efficiency in buildings. Rather, it is related to China’s current level of economic development and the standard of living of its people.

Forty percent of China’s total building energy consumption is accounted for by heating urban buildings in the north of the country. The figures for average energy consumption per square metre for heating the 6.5 billion square metres of buildings in the north are very similar to the consumption figures for countries with similar climates, such as the United Kingdom, Germany, Sweden and Norway. Currently, large apartment blocks are the predominant form of housing in China, while in Europe individual houses are the norm. This means that in China the average heat-dissipating surface area of homes is smaller, so even though our heat retention is poor, average heat losses are around the same level as in the above countries.

Heating in the developed countries mainly takes the form of separate, independent heating systems for each home. These heating systems are roughly 20% to 30% more efficient than Chinese systems. However, because these heating systems are designed to provide maximum comfort, the length of time over which heating is provided is often longer than in China, even though the climates are similar. If temperatures drop below 16° or 18° centigrade, the heating turns itself on.

In contrast, China’s heating systems, which are operated centrally by the government, only run for a given period, which is shorter than the period over which heating is used in climatically similar areas of Europe. Additionally, the north of China mainly uses combined heat and power (CHP) generation and coal-burning boilers to generate heat, whereas in Europe, electric, gas and oil heating systems are predominant. This means that heat generation is slightly less efficient in Europe, which leads to a greater consumption of energy.

The second and third categories of building energy consumption are “energy consumed by residential buildings excluding heating” and “energy consumed by ordinary public buildings excluding heating”. This covers energy consumed in cooking, heating water, lighting, air-conditioning and running domestic or office appliances. Here, “ordinary public buildings” means schools, offices, shops, lower-grade hotels, hospitals and so forth, which do not use central air-conditioning and which have a surface area of less than 20,000 square metres. This category of building makes up the majority of non-residential urban buildings in China. Per unit of surface area, the energy consumption of these two categories is far lower than the equivalent categories in developed countries, standing at roughly one quarter of US energy consumption and one half of Japan’s.

Four points are important: