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Sustainable housing and urban construction in China
Energy and Buildings 36 (2004) 1287–1297
Sustainable housing and urban construction in China Yingxin Zhu∗ , Borong Lin Department of Building Science, School of Architecture, Tsinghua University, 100084 Beijing, PR China Received 23 September 2003; accepted 20 November 2003
Abstract In order to ensure China, a populous country, to develop in a sustainable way, it is an urgent task to disseminate the concept of sustainability and put it into practice for urban construction. However, it is impossible for China to simply copy the experiences of developed countries, since China has the higher population and building density and less availability of reusable energy per square meter floor area. Therefore, it is necessary to develop the sustainable building technologies applicable to various climate regions, economic conditions, and residential customs in China, as well as sustainable to the most occupants and owners. Based on an introduction of the current situation of the development of construction industry and the energy consumption of buildings in China, this paper analyses the requirements, characteristics, standards for sustainable housing and urban construction, and recommends series of technical approaches along with different phases of sustainable design and construction, which strengthen a good cooperation among researchers, designers and constructors of different majors including architecture, planning, building physics, building services, and so on. Moreover, some issues, which need further research and especially handling, are pointed out along with the recommendations. Finally, policy issue related with the sustainable development of urban construction in China is discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Sustainable housing; Urban construction; Techniques; Simulation; Evaluation
1. Background Given the construction boom since 1990s, China has been experiencing a high growth of total output value of construction industry, and an annual average speed of 11% was achieved over the past 5 years [1,2]. The residential and commercial building floor area increased by 1.6–1.9 billion square meters and resulted in a steady growth of total building stock [1,2]. Meanwhile, the possession ratio of household air-conditioners for per 100 families in South China was increasing at a speed of 20% with the rapid development of building industry and the improvement of living standard of people [1,2]. Building energy consumption is accordingly going up year after year. Statistics on total energy consumption and building energy consumption shows that the proportion of building energy consumption to the total energy consumption in China’s terminal-use sectors is rising annually, amounting to 27.8% in 1999 [1]. And according to the experience of developed countries, it will inevitably amount to 35% or so [1,3,4]. Energy efficiency in buildings has become a key factor that has a great impact of energy security, ∗ Corresponding author. Tel.: +86-10-62782746; fax: +86-10-62773461. E-mail address: [email protected] (Y. Zhu).
0378-7788/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2003.11.007
optimization of energy structure, energy efficiency improvement and Green House Gas (GHG) emission reduction. China currently consumes about 130 million tons of standard coal equivalents (Tec) per year for space heating of urban residential and commercial buildings [1]. Meanwhile, the execution of energy efficiency regulation for new buildings remains inefficient in the recent 10 years in China. By the end of 2000, only 0.5% of the total urban and rural building floor area can be complied with the energy efficiency design standards, accounting for 9% of existing heated residential building floor area [1]. Most new buildings are still consuming huge amount of energy. Despite the situation of Beijing being relatively better in the field of building energy efficient in present China, many problems and barriers still remain to be overcome. For instance, residential buildings in Beijing are estimated to consume 50–100% more energy for space heating as compared to buildings in similar cold climates in Western Europe or North America and still provide far less comfort [1,5]. Despite significant reduction of building energy consumption after its implementation of 1995 Energy Efficiency Standard for New Residential Buildings in the Heating Zone, the heat consumption of Beijing (20.6 W/m2 ) [6] is still almost one time higher than that of Sweden, Denmark and Finland (11 W/m2 ). In addition, low efficiency of heating system in residential district leads to
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enormous energy loss. Commercial buildings in Beijing are also facing the problems of energy loss and improper utilization. For instance, a great number of shopping centers in Beijing consume 40% more energy than buildings of the same type in Japan, which has similar climatic conditions, but there are still a lot of regular complains about the discomforts and noises of the HVAC system from the customers [7,8]. Another problem is that in Northern China, energy consumption for space heating of buildings accounts for 20% of local total energy consumption [4,5]. Carbon dioxide emission in heating season is obviously higher than that in non-heating season. Although the government was exploring the possibility of implementing the urban heating reform to change the situation of welfare-based heating system, the re-construction work with a simple double-pipe system instead of the traditional single-pipe heating was deviant from the right way and produced a set of problems which affected the normal life of people. China is under enormous construction booming now, and more than one-half of China’s urban residential buildings and commercial building stock in 2015 is to be constructed after the year 2000 [9]. To spread and promote advanced building energy, efficient technologies and measurements in new buildings are the most important parts of the building energy efficient practice, and also the last opportunity. Other countries may have similar situations, but the extent of new building construction makes China’s case unique. It provides not only a major opportunity to ‘grow out’ of much of the problem, but also a danger, if not addressed, of locking in enormous energy waste and inefficiencies for future generation [9]. It is not an exaggeration that China has the most to gain of any country in the world from successful implementation of an effective building energy efficiency program. Sustainable building is the key step or solution for the above questions and now China should focus more on the sustainable development of the residential buildings and urban constructions.
2. Best mode for the development of sustainable buildings in China The sustainable building is also called ‘ecological building’ and ‘green building’, which is also well known as ‘energy efficient building’ and ‘healthy building’. Only technology aspects relative to energy and environment are concerned here for the sustainable building instead of aesthetic aspect in order to consume as less as possible energy and any other resources, be nature friendly and be healthy to the occupants. Since the energy crisis of 1970s, some developed countries contributed a lot in research, design and construction of ‘energy efficient buildings’ or ‘green buildings’. Quite a lot of demonstration buildings including residential buildings, commercial buildings and public buildings have been built in these countries. The principle
technical approach used in these demonstrations was a combination of the passive measures by optimal building design and the active measures by efficient mechanical system design to control the indoor climate. These measures include utilizing reusable energy, such as solar energy, wind energy and geothermic energy, improving thermal performance of building envelope, utilizing daylight and energy efficient illuminations, increasing HVAC system energy efficiency, and developing new sustainable building materials. Even some ‘zero energy consumption building’ and experimental buildings with closed ecological circle, named ‘ecological building’, have been built and operated successfully. During nineties, 250,000 solar houses were built in US, while 2000 OM solar houses were built in Japan in the 20th century [10]. A great deal of sustainable residential and commercial buildings employing synthetic technology has also been constructed in Europe. Although many of the developed countries have successful experience in sustainable building research, design and operation, it is not suitable for China to copy their experience simply, since most big cities of China have much higher population and building density and much lesser availability of renewable energy per square meter floor area. For instance, the building density in cities of China, i.e. the ratio of floor area over the land area, is always higher than 1.5, and useful floor area of most of the apartments is less than 70 m2 [1,5]. However, the sustainable buildings in developed countries are usually in the low building density and have more available renewable energy source per unit floor area. Most of the sustainable housings are single houses with a garden. Therefore, it is possible to supply more than enough reusable energy by means of solar energy collectors, solar cell, ground-source heat pump, underground duct, green house, roof vegetation, and so on. And these houses rarely have the problems such as restrained daylighting or natural ventilation due to the adjacent buildings, and the noise and heat reject caused by the outdoor units of the neighboring air-conditioners. The household or independent heating system, garbage treatment, water treatment, and reusing system are easier to be adopted in those houses instead of the centralized systems, which are commonly applied in China. Therefore, instead of directly applying these advanced technologies from the developed countries to China, more consideration has to be taken for the various climates, different cultures, conventions, economic development level, and social situations of China comprehensively. On the other hand, there are many good sustainable building technologies that can be found in Chinese traditional dwellings. A typical case is the Wannan traditional residence in the south part of Anhui Province. The on-site measured data show that even in hot summer, when the outdoor temperature is above 38 ◦ C, the indoor thermal environment is still kept in the comfortable range even without any mechanical ventilation (Fig. 1) [11]. The technical strategies for these buildings are [11,12]: (1) the uncovered dooryard plays the role of a vertical shaft to boost ventilation induced by ther-
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Temperature (0C)
34 32 30 28 26 24 22 20 13-Aug
Zhaolan House Zhongxiu Hotel 15-Aug
17-Aug Time (d)
Jiqing House Outdoor 19-Aug
21-Aug
Fig. 1. Dry-bulb temperatures measurements of Wannan traditional residences in the summer of 2000 (the hotel is a modern building with similar room heat gain compared traditional residences).
mal pressure at night, while the narrow laneway restrains the natural ventilation in the daytime. Since the outdoor air temperature is higher than the indoor air temperature at the daytime of summer, while it is lower at night, it could always maintain a comfortable indoor environment; (2) an effective external shading by high wall of the dooryard prevents direct solar radiation while well utilizing diffusive radiation as daylighting well; (3) the wooden wall of less thermal mass speeds up the night cooling; (4) a double-pitched raft with an overhead double-layer-tile structure consists a layer of the local tile and the other layer of ‘Wang Brick’ can provide good thermal insulation and quick night free cooling. Therefore, it is more efficient to combine the advantages from both the successful Chinese traditional buildings and the experiences of developed countries to develop the sustainable buildings in China. Not only will it be beneficial to Chinese sustainable development, but also to the global environment. As a result, a series of technical strategies are recommended, which resulted from the research achievements (including the measurements, investigation and simulation of traditional dwellings [11,12]) and experiences of sustainable buildings design applications after 1990s in School of Architecture, Tsinghua University. These technical approaches should be applied into the different design stages and need a good cooperation among researchers, designers, and constructors of different majors including architecture, landscape, building physics, building services, and so on.
3. Technical strategies for sustainable buildings in China 3.1. Difference between strategies for residential buildings and commercial buildings Both the residential buildings and commercial buildings require good indoor climate and outdoor climate. However, these two kinds of buildings should be taken under different
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technical strategies because they have different building constructions and internal heat gains. The commercial building usually has larger area of inner zone and more internal heat gain than the residential building. Contrary to the commercial building, the indoor climate of the residential building is affected more by the outdoor climate than by the occupants and other internal heat gains. It is possible to merely use the passive methods such as the elaborate design and construction of building envelope to control the indoor climate of the residential building. However, the energy efficiency of the mechanical energy systems and lighting system of the commercial building may play more important roles in building sustainability, since the HVAC systems usually are necessary there, and more illumination should be used for the large inner zones. On the other hand, the thermal properties of the envelopes of the commercial buildings are still important to the building of sustainability, because unfortunate fabric design will cause great energy consumption and decrease the comfort level. In addition, major improvements can be made in commercial building HVAC systems, through relatively minor retrofits, and improved control and management. From the investigation and on-site measurements of almost 100 commercial buildings in Beijing, Shanghai, and Shenzhen [7,8], it is found that the lack of subdivisional control and cascade management are the most important factors for more energy consumption in commercial buildings compared with that of developed countries. However, it is still difficult to really realize energy efficiency in commercial buildings. The reasons are that there is no good management mechanism of commercial building which is incentive for operators, and the owner cares more about the initial cost instead of the annual energy consumption. Fortunately, there are some small specialized firms in big cities which have undertaken activities to lower energy costs and improve comfort in complex commercial building systems [8,9]. These companies are compensated from a portion of the reduction in the commercial building energy bill—an attractive arrangement for building owners. 3.2. Approaches for different stages of constructing sustainable housings If taking the design of sustainable housing as an example, a set of technologies involved with outdoor physical environmental quality, thermal performance of the building, indoor air quality, energy system and so on, should be introduced along with the design process of residential buildings (including mater planning stage, single building detailed design stage, and energy system design stage) of residential buildings. One of the most effective approaches is to use the computer simulation as an aid to improve the planning or design scheme of a building. Computer simulation can be used through all of the process of building design to improve the quality of outdoor and indoor physical environment, thermal performance of
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the envelope and the energy system with the idea of ‘sustainable design by simulation at different stages’. However, it needs better cooperation between architects and engineers since more issues should be yielded, discussed and solved. 3.2.1. Master planning stage Issues to be considered in this stage are related with the planning scheme and corresponding outdoor environment, such as sunlight, ventilation, noise level, and heat island phenomenon. It is extremely important when planning a high-density and high-rise dwelling district. The outdoor thermal environment is not only related to the thermal comfort of pedestrians, but also to the building energy consumption. Many buildings in developed countries have already considered those issues during the design phase. For instance, the local government of San Francisco issued regulations on wind velocity limitation and enough sunlight in the public area of building [13]. Following problems should be considered in the matter planning stage: • Wind environment. It includes restraining the wind speed among dwelling district in winter and promoting natural ventilation in summer. • Contamination emitting and dilution by natural ventilation. There are many residential buildings that use household gas heater for heating in winter and exhaust waste gas directly from the kitchen window. Then it is possible that the contamination concentrates around the buildings and brings health problems. • Outdoor thermal environment of the dwelling district. It means how to quantitatively evaluate the effect of the
waterscape, vegetation design and the arrangement of the materials for paving road, wall and roof on the heat island around buildings. • Daylighting. Adequate daylighting should be provided in all living spaces. Especially in the day of winter solstice, sunlight integrated time on the external window of the main habitable rooms should be the critical index to evaluate the design. • Noise pollution and sound insulation. In recent years, more high-rise residential buildings appeared in the big cities. The induced high-velocity wind around buildings is very uncomfortable and unsafe for pedestrians and is becoming a more and more serious problem. There are many failed-design buildings in Beijing, where the induced wind speed on some winter days can be as high as grade 5 or 6 (8–14 m/s). It could also increase the heat load by increasing the heat transfer coefficient of the building external surface and also by more infiltration in the building. The poor wind environment may also bring on other problems in summer. Natural ventilation may be restrained, and the contamination could not be diluted efficiently. In order to solve the above problems during the planning stage, Computational Fluid Dynamics (CFD) is a most helpful tool to predict the velocity and pollutant concentration distribution under different planning schemes [14]. Figs. 2 and 3 show an example using CFD as a tool to evaluate the planning scheme and the wind environment around a residential district in winter in Beijing (by the authors), of which the wind environment in winter is thought satisfactory (Figs. 2–3).
Fig. 2. Pressure distribution at 2 m height for north-west leading wind conditions.
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Fig. 3. Wind velocity distribution at 2 m height for north-west leading wind condition.
Heat island is also related to the wind environment in summer as well as the waterscape and vegetation design. Computer simulation is necessary to predict the effect and to optimize the site planning design [15,16]; and it is also useful to help architects to understand well how the building arrangement, waterscape, vegetation, and paved floor effect on the air temperature, radiant temperature, humility, air velocity, and the pedestrian comfort under the natural climate. Figs. 4–5 show an example of the simulation re-
sults based on a field measurement on a south residential district in Shenzhen city: with the heat balance equations and its couple with the governing equations of airflow, the distributed air parameters are numerically solved by means of CFD method. The necessary distance between buildings for adequate sunlight on habitable rooms is an important index for building planning. However, it is still not sufficient for locating buildings [17], especially when the length of building at the
Fig. 4. Analysis model in program.
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‘Target building’ in Fig. 6 could not obtain enough sunlight on the winter solstice day because of the interactive shading and self shading. Firstly, the target building could not obtain sunlight before 11:00 o’clock because of the shading from its east and south side buildings. After 11:00 o’clock, the east side of target building is shaded by itself [18]. Sound insulation should also be considered in the planning stage. It is necessary to arrange the public buildings such as service center and public clubs in the roadside as a barrier against the traffic noise. Noise level distribution prediction by computer simulation is also necessary during the planning stage. Combined with an on-site measurements, it could be used to estimate the possible noise level inside the district accurately.
Fig. 5. Horizontal air temperature distribution of 1.5 m height at August 15.
first row is too long, and/or there are spot-type and plate-type buildings laid out together inside the same district, and/or the building has complex shape and façade due to the possible self-shading and shading by around buildings (Figs. 6 and 7). In this case, computer simulation is necessary for sunlight and daylighting analysis in order to improve the indoor environment. Figs. 6 and 7 are typical cases that could illustrate all the problems mentioned above. They are the simulation results of ‘BSAT’ developed by Tsinghua University. For the master planning design, the distances between these high-rise buildings meet the building code. However, some east rooms of
3.2.2. Single building detailed design One of the important tasks in single building design stage for sustainable housing is to realize energy conservation and to improve the indoor thermal environment level. Although some building energy conservation codes and regulations have been issued, the on-site measurements results always show that near upon 80% of ‘energy saving building’ exceeded the energy consumption limitation [1,5]. Although the walls or windows of these ‘energy-saving buildings’ meet the requirement of U value limitation in the codes or regulations, but no envelope details, such as sill and shade and natural ventilation, have been well designed due to the lack of relevant regulations or guideline about that. In this stage, the orientation of the main habitable spaces, the shape coefficient, the window to wall ratio, and the location of the kitchen and toilet should be considered related to all of the aspects such as indoor thermal environment, acoustic environment, natural ventilation, and daylighting. For example the bigger window to wall ratio will always
Fig. 6. Interactive shading (before 11:00 o’clock).
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Fig. 7. Self-Shading (after 12:00 o’clock).
bring more daylighting and solar heat gain during the winter, but at the same time it will lead to heat loss by heat conduction. These problems should be considered combining the local annual climate variations and the thermal performance of the glazing. Many published literatures showed that the human thermal sensation in a naturally ventilated environment is much better than that in mechanical controlled thermal environment [19,20]. The layout of the building space and the openings, such as window, door or shaft, affect remarkably on the natural ventilation rate and then influence the indoor thermal environment. Successful design will boost natural ventilation in case of necessary, which is induced by means of wind pressure or thermal pressure. The ventilation direction should be considered carefully as well. For instance, according to the building code, external window in kitchen is necessary for both safe and indoor air quality requirement. However, poor design may cause downdraught wind from the kitchen window, and then diffuse the cooking smoke to the living spaces during the window opening seasons. It should be predicted during design stage. Multi-zone model is one of the possible tools in predicting the ventilation rate in the reference design condition. 3.2.3. Building envelope design As mentioned before, the building envelope has significant influence to the thermal performance to the building; an unsuccessful design of the envelopes may lead to failed thermal insulation due to the thermal bridge, which is not very well known by the designers. The cold bridge in the sills may cause more than 50% heat load in winter, although the external walls are well insulated and the both high insulated and high tight windows are used. Usually, insulation
on the exterior surface of the external walls can eliminate efficiently the thermal bridge at the conjunctive part to the partition walls, hence reduce the heat load to 15–20% compared with the case in which insulation only applied on the interior surface of the external walls [21]. Moreover, balcony, cantilevered roof, windowsill, rooftop, ground, and the lap joint of slabs are also easy to produce a cold bridge, and different insulation scheme should be adopted with a corresponding fine construction of the buildings. For example as exterior insulation does not cover the windowsill, there is an obvious cold bridge around the window hole. For interior insulation, because the window is inter-flushing mounted, there is no cold bridge at the interior surface although the outside surface has a large temperature gradient. The simulated results involved in those aspects are shown in Figs. 8 and 9, respectively [21]. In recent years, wall insulation has been emphasized so much and raised to a very high status in China. However, the global thermal performance of the envelope depends not only on wall insulation, but also on the shade, thermal mass, ventilation, and transparence property of the glazing according to the different climate types. The envelope design for the residential buildings in the tropics zone should be very different from that in the temperate zone, because in the tropics zone, shading, and ventilation are the most important strategies instead of wall insulation. Under the climate of which the daily temperature significantly varied, the large thermal mass of envelope is much more helpful for improving the indoor thermal environment than wall insulation. The location of the insulation layer and the thermal mass of the envelopes should be decided according to the heating and/or cooling schedule as well. In case that continuous heating, thermal insulation is the most important factor but
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design stage for testing the effect of different combination of window wall ratio and SHGC of window.
Fig. 8. Exterior insulation with inter-flush mounted window.
thermal mass is not as important in thermal comfort and energy saving. Contrarily, if the cheaper night electricity is used for heating the building through the day, large thermal mass is very important to such kind of application. In case that the occupants need heating discontinuously, less thermal mass can speed up the process of heating up or cooling down, and save more energy during the un-occupied periods. More attention has to be paid to the thermal insulation of the window or glazing due to the fact that most of heating or cooling loads from envelope come from window or glazing. To homogenize the thermal resistance on the external envelope will maximize the benefit of the insulation. Not only U value but also the shading coefficient SHGC of glazing should be attached importance according to the climate zone where the building is located. Lower SHGC can reduce cooling load in summer efficiently but increasing heating load in winter. Detailed simulation analysis is necessary during
Fig. 9. Interior insulation with inter-flush mounted window.
3.2.4. Building energy system design Building energy system usually indicates HVAC system. Four items must be considered for a sustainable building, including: (1) energy resource and heating/cooling plant; (2) air handling process; (3) duct and piping system; and (4) terminal units. The type of the heating/cooling plant has to be well chosen according to the available energy sources. The initial energy consumption, effect on natural environment (e.g. quantity of CO2 emitting), the indoor environment quality, maintenance and manage cost (may cause deleterious effect to the environment), and marketing should all be related to the building sustainability. The possible effect of different energy sources on the environment and the best strategies are listed as follows. 3.2.4.1. Coal. More than 60% of energy used in China is provided by coal [2,22]. When coal is the available energy source for heating, electricity–heat cogeneration is the optimal way for energy utilization from the viewpoints of protection, energy saving and low-cost. 3.2.4.2. Natural gas. In case that only natural gas is the available energy source for HVAC beside electricity, household gas heater is the most efficient way to reduce energy loss in transmission process. If it is necessary to install the central gas boiler to reduce the household operation risk, electricity–heat cogeneration or electricity–heat–cool tri-generation is the highly recommended method, for its high efficiency using the energy sources in different stages. 3.2.4.3. Electricity. In recently years, direct heating by electricity is regarded as a kind of ‘environment friendly, energy saving’ heating system. However, such a comment is true only when the electricity is mainly generated by hydraulic power or nuclear power. Although electric heater is flexible in control, it consumes high quality energy for the application which in fact only required low quality energy. If considering the primary energy source consumption, the energy efficiency of electric heater is only 30%. In China, electricity mainly comes from the coal-fired power plants, thus, the electric heater consumes three times of coal and emits three times of CO2 and other contaminations as heating comparing with coal boilers. Thermal storage electricity heater is one of the better substitutes comparing with the direct electric heating in case that no other energy supplies are available. Although thermal storage electricity heater cannot decrease energy consumption and pollutant emitting, it can reduce the peak load of power demand in daytime, and is benefic to the coal-fire power plant operation. The best way to use electricity for heating is heat pump. Heat pump consumes some electricity to remove
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heat between indoor environment and outdoor environment. Its energy efficiency is 2–4 times as that of electric heater. A heat pump cannot only provide heating in winter, but also provide cooling in the summer. However, the heat pumps which are wildly used now are air-cooled heat pump, of which quite lower efficiency or even breaking-down can be found in the cold period in winter. Water source heat pump can solve the problem of malfunction in cold winter. Although ground source heat pump, one kind of water source heat pump, has been well studied and widely used in developed countries. It is not suitable for the high density residential buildings in China due to the limitation of the available land area. The ground water heat pump (GWHP) uses aquifer as the thermal energy storage body, and the ground water as the medium of transferring heat to the ground. It can provide more energy to the building from the limited land area than ground source heat pump. Heat rejected from the building by refrigeration cycle is stored in the aquifer, then to be taken out by heat pump cycle in the winter. In such an application, water will not be consumed nor polluted by pollutant, because the ground water flows only in a close circuit. Furthermore, water source heat pump unit can provide both heating in winter and cooling in summer with higher COP than air-cooled heat pump, so that both initial cost and operation cost are saved. A household GWHP has no requirement to any outdoor spaces for outdoor unit installation, hence it avoids the problems such as heat rejected pollution, acoustic pollution, and uglification to the facades, which are very popular in air-cooled heat pump applications. Therefore, the household GWHP should be considered as one of the best solution to cooling/heating in high-density residential buildings in case that sufficient ground water is available. In fact, the Beijing government has decided to encourage GWHP application in the coming 2008 Olympic Games to fulfill the cooling and heating demand of Olympic Village buildings. However, there are still some problems needed to be solved. One is the lack of guides and regulations for design and building a GWHP system. The other one is the lack of detailed study of the effect of ground river, soil and the wells density on the temperature of ground water, and on the energy efficiency of GWHP systems. Figs. 10 and 11 give the simulated 10 years later temperature distribution results of the aquifer layers for two different wells arrangements of a GWHP system providing both heating and cooling for residential buildings in Beijing. Compared with scheme 1, scheme 2 could result a higher temperature for the heating wells, which is advantageous for these buildings whose heating load is larger than cooling load [23]. Based on the above analysis, co-generation combining district heating is the most efficient way for utilizing coal. If electricity is the only available energy supply, heat pump is the most efficient way for heating by electricity. Tri-generation or combined cycle co-generation should be encouraged in any possible cases when gas is the main energy supply. Heating directly by electricity in China
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Fig. 10. Temperature distribution of aquifer layer by Scheme 1 (after 10 years operation).
should not be promoted for the sake of global environment conservation and energy efficiency [24]. The only exception for considering direct heating by electricity reasonable is when building heat load is very low. In this case, installing piping system and other heating plant may bring more deleterious effect on environment. Therefore, excellent building thermal insulation is absolutely necessarily for using electric heater. 3.2.5. Renewable sources of energy Many people have been misdirected in the use of renewable energy. They simply believe that using renewable energy is an important symbol of sustainable building and pay few attention to solar cell, solar energy collectors, wind power utilization, heat recovery from waste water, and so on. In fact, in the high-density cities in China, no matter large-scale commercial buildings or high-rise residential developing, current reusable energy is far from sufficient to
Fig. 11. Temperature distribution of aquifer layer by Scheme 2 (after 10 years operation).
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provide half of the energy for operating the building. As the important complement to the conventional fossil energy source, reusable energy must be used based on the perfect thermal performance design for building envelope and highly energy efficient design for the conventional energy systems. Otherwise, a poor piping or duct design for reusable energy may get more kicks than halfpence due to the great lost in the energy transmission process.
4. Conclusion So far, based on the analysis of the requirements, characteristics, standards for evaluation and the feasibility for sustainable housing and urban construction in China, series technical approaches and suggestions in different design stages of the construction of a building are introduced and recommended. These technical approaches are based on the research achievements of School of Architecture, Tsinghua University. Some of the technical approaches for a sustainable design need good cooperation among researchers, designers and constructors. Although some technical details of them still need further research, it shows that there are many well-developed technologies related to sustainable building available in China as well as relevant products, such as energy saving building components and equipments of high efficient building energy system. If these approaches could be effectively applied into the design and construction process of a building, the sustainability would not be a dream any more. However, in order to promote the sustainable housing and urban construction in China, what is more important than technologies is how to found a mechanism which could boost the sustainable development of building automatically. So far, there are lots of problems needed to be solved by the governmental side since historical experiences told us that the executions of building energy efficiency seem not so good. Especially, market-oriented economy has come in China, new issued codes or standards for sustainable buildings should match these economic feature. Moreover, corresponding incentive policies and penalties are also needed. Fortunately, except having worked for a long time in the field of building performance simulation, building physical environment evaluation, and eco-housing design, some universities, research institutes and social organizations have also studied and developed some non-governmental approaches for improving the housing construction in China. Elicited by Leadership in Energy & Environmental Design (LEED) [25], a rating system for assessing the building sustainability and improving the design quality of the building in US, the ‘China Green Housing Rating System’ was issued in fall of 2001 with the cooperation of China Housing Industry Association, Tsinghua University, Chinese Academy of Building Research and other institutes [26]. The corresponding evaluation software was issued in March 2002. After that, more and more developers, architects, HVAC
engineers and owners are being aware of the significance of building sustainability. Since 2000, there are many evaluation and consultation practices for sustainable housing by Tsinghua University, which include projects in construction and completed that account for about 1 million m2 in cities of Beijing, Dalian, Guangzhou, Chengdu, Xian, Tianjin, etc. [27,28]. However, it is not enough compared with the total construction floor area per year in China. The state of the arts of sustainable building applications is far from satisfaction considering the high developing speed of Chinese construction. Therefore, it is still a long way for us to realize the sustainable development of urban construction in China.
Acknowledgements Some information and data about China energy consumption in buildings come from an investigation carried out by MOC, Tsinghua University and World Bank; the authors want to give their best thanks to them. This study was supported by NFSC (59836250).
References [1] MOC, Outline of the 10th Five Plan of Building Energy Efficiency of MOC, 2002 (in Chinese). [2] State Statistic Bureau, China Statistic Yearbook 2000, China Statistics Press, Beijing, China, 2001 (in Chinese). [3] MOC, Regulations for Energy Efficiency in Residential Buildings, China Architecture & Building Press, 2000 (in Chinese). [4] Y. Wu, How to make a full use of the public administrative functions of the government to accelerate the building energy efficiency development in China, Energy Efficiency in Buildings 38 (5) (2002) 1–6 (in Chinese). [5] Y. Jiang, Current situation and important issues for Building energy efficiency in China, in: Proceedings of China Civil Construction Engineering Academy, 2002 (in Chinese). [6] MOC, Technical Specification for Energy Conservation Renovation of Existing Heating Residential Building, China Architecture & Building Press, 2001 (in Chinese). [7] W.F. Zhu, Y. Jiang, Z.F. Xue, Analysis of common problems in refrigerating stations and air conditioning systems, HVAC&R 30 (6) (1999) 4–11 (in Chinese). [8] Y. Jiang, Z.F. Xue, Energy-saving technology and market analysis for commercial buildings in China, China Energy 4 (2001) 37–39 (in Chinese). [9] World Bank report, Opportunities to Improve Energy Efficiency in Buildings, 2001. [10] http://www.omsolar.com. [11] B.R. Lin, G. Tan, P. Wang, et al., Field study on the thermal performance of the Wannan traditional residential buildings in summer, Journal of Tsinghua University 42 (8) (2002) 1071–1074 (in Chinese). [12] L. Song, B.R. Lin, Y.X. Zhu, Simulation study on the summer thermal performance of a typical Chinese traditional residential building, Anhui Residence, INDOOR AIR, California, USA, 2002, pp. 718–723. [13] E. Arens, P. Bosselmann, Wind, sun and temperature—predicting the thermal comfort of people in outdoor, Space Building and Environment 24 (4) (1989) 315–320.
Y. Zhu, B. Lin / Energy and Buildings 36 (2004) 1287–1297 [14] Q. Chen, Wind in building environment design, in: Proceedings of the Second International Workshop on Energy and Environmental of Residential Buildings in China, vol. 1, 2002, pp. 142– 151. [15] Z.Q. Zhang, X.F. Li, O.O.K.A. Ryozo, et al., Experimental Researches on the Outdoor Microclimate of a Low-Rise Residential Building Cluster, ISHVAC 2 (2003) 604–608. [16] X.F. Li, Y.X. Zhu, X.T. Li, et al., Numerical prediction of microclimate in residential building cluster, in: Proceedings of the International Conference on Energy and Environment, 2003. [17] G. Wang, X. Zhang, A new idea of sunlight computation for buildings, Journal of Architect 2 (2001) 48–50 (in Chinese). [18] D.G. Gu, Y.X. Zhu, Introduction of building sunlight analysis software and its application in the energy load calculation, in: Proceedings of the 2002 National HVAC&R Symposiums China, vol. 2, 2002, pp. 657–660 (in Chinese). [19] P.O. Fanger, et al., Thermal comfort in the future—excellence and expectation, in: Proceedings of the International Conference on Moving Thermal Standards into the 21st Century, vol. 1, 2001.
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[20] Y.Z. Xia, R.Y. Zhao, Y. Jiang, Thermal comfort in naturally ventilated houses in Beijing, HVAC&R 29 (4) (1999) 1–5 (in Chinese). [21] H.M. Fan, J.L. Zeng, Y.W. Jian, et al., Numerical simulation on the 3D heat transfer of building envelope, Architecture Technology 33 (2002) 736–738 (in Chinese). [22] Y.D. Dai, Energy issue in the sustainable development of China, Power DSM 4 (5) (2002) 3–6 (in Chinese). [23] C.Z. Xin, Study on the Coupled Heat Transport of Groundwater Heat Pump System, Master thesis, Tsinghua University, 2003 (in Chinese). [24] Y. Jiang, Heating schemes analysis for medium and large cities in North China, HVAC&R 3 (2000) 30–33 (in Chinese). [25] http://www.usgbc.org/leed/leed main.asp. [26] M.S. Nie, Y.G. Qin, Y. Jiang, et al., Handbook of China Green Housing Rating System, China Architecture & Building Press, 2002 (in Chinese). [27] http://www.chinahouse.info/index/index.asp. [28] Program of Green Buildings Experts’ Panel, Assessment System Of Green Building for Beijing Olympic, China Architecture & Building Press, Beijing, 2003.
Sustainable housing and urban construction in China Yingxin Zhu∗ , Borong Lin Department of Building Science, School of Architecture, Tsinghua University, 100084 Beijing, PR China Received 23 September 2003; accepted 20 November 2003
Abstract In order to ensure China, a populous country, to develop in a sustainable way, it is an urgent task to disseminate the concept of sustainability and put it into practice for urban construction. However, it is impossible for China to simply copy the experiences of developed countries, since China has the higher population and building density and less availability of reusable energy per square meter floor area. Therefore, it is necessary to develop the sustainable building technologies applicable to various climate regions, economic conditions, and residential customs in China, as well as sustainable to the most occupants and owners. Based on an introduction of the current situation of the development of construction industry and the energy consumption of buildings in China, this paper analyses the requirements, characteristics, standards for sustainable housing and urban construction, and recommends series of technical approaches along with different phases of sustainable design and construction, which strengthen a good cooperation among researchers, designers and constructors of different majors including architecture, planning, building physics, building services, and so on. Moreover, some issues, which need further research and especially handling, are pointed out along with the recommendations. Finally, policy issue related with the sustainable development of urban construction in China is discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Sustainable housing; Urban construction; Techniques; Simulation; Evaluation
1. Background Given the construction boom since 1990s, China has been experiencing a high growth of total output value of construction industry, and an annual average speed of 11% was achieved over the past 5 years [1,2]. The residential and commercial building floor area increased by 1.6–1.9 billion square meters and resulted in a steady growth of total building stock [1,2]. Meanwhile, the possession ratio of household air-conditioners for per 100 families in South China was increasing at a speed of 20% with the rapid development of building industry and the improvement of living standard of people [1,2]. Building energy consumption is accordingly going up year after year. Statistics on total energy consumption and building energy consumption shows that the proportion of building energy consumption to the total energy consumption in China’s terminal-use sectors is rising annually, amounting to 27.8% in 1999 [1]. And according to the experience of developed countries, it will inevitably amount to 35% or so [1,3,4]. Energy efficiency in buildings has become a key factor that has a great impact of energy security, ∗ Corresponding author. Tel.: +86-10-62782746; fax: +86-10-62773461. E-mail address: [email protected] (Y. Zhu).
0378-7788/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2003.11.007
optimization of energy structure, energy efficiency improvement and Green House Gas (GHG) emission reduction. China currently consumes about 130 million tons of standard coal equivalents (Tec) per year for space heating of urban residential and commercial buildings [1]. Meanwhile, the execution of energy efficiency regulation for new buildings remains inefficient in the recent 10 years in China. By the end of 2000, only 0.5% of the total urban and rural building floor area can be complied with the energy efficiency design standards, accounting for 9% of existing heated residential building floor area [1]. Most new buildings are still consuming huge amount of energy. Despite the situation of Beijing being relatively better in the field of building energy efficient in present China, many problems and barriers still remain to be overcome. For instance, residential buildings in Beijing are estimated to consume 50–100% more energy for space heating as compared to buildings in similar cold climates in Western Europe or North America and still provide far less comfort [1,5]. Despite significant reduction of building energy consumption after its implementation of 1995 Energy Efficiency Standard for New Residential Buildings in the Heating Zone, the heat consumption of Beijing (20.6 W/m2 ) [6] is still almost one time higher than that of Sweden, Denmark and Finland (11 W/m2 ). In addition, low efficiency of heating system in residential district leads to
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enormous energy loss. Commercial buildings in Beijing are also facing the problems of energy loss and improper utilization. For instance, a great number of shopping centers in Beijing consume 40% more energy than buildings of the same type in Japan, which has similar climatic conditions, but there are still a lot of regular complains about the discomforts and noises of the HVAC system from the customers [7,8]. Another problem is that in Northern China, energy consumption for space heating of buildings accounts for 20% of local total energy consumption [4,5]. Carbon dioxide emission in heating season is obviously higher than that in non-heating season. Although the government was exploring the possibility of implementing the urban heating reform to change the situation of welfare-based heating system, the re-construction work with a simple double-pipe system instead of the traditional single-pipe heating was deviant from the right way and produced a set of problems which affected the normal life of people. China is under enormous construction booming now, and more than one-half of China’s urban residential buildings and commercial building stock in 2015 is to be constructed after the year 2000 [9]. To spread and promote advanced building energy, efficient technologies and measurements in new buildings are the most important parts of the building energy efficient practice, and also the last opportunity. Other countries may have similar situations, but the extent of new building construction makes China’s case unique. It provides not only a major opportunity to ‘grow out’ of much of the problem, but also a danger, if not addressed, of locking in enormous energy waste and inefficiencies for future generation [9]. It is not an exaggeration that China has the most to gain of any country in the world from successful implementation of an effective building energy efficiency program. Sustainable building is the key step or solution for the above questions and now China should focus more on the sustainable development of the residential buildings and urban constructions.
2. Best mode for the development of sustainable buildings in China The sustainable building is also called ‘ecological building’ and ‘green building’, which is also well known as ‘energy efficient building’ and ‘healthy building’. Only technology aspects relative to energy and environment are concerned here for the sustainable building instead of aesthetic aspect in order to consume as less as possible energy and any other resources, be nature friendly and be healthy to the occupants. Since the energy crisis of 1970s, some developed countries contributed a lot in research, design and construction of ‘energy efficient buildings’ or ‘green buildings’. Quite a lot of demonstration buildings including residential buildings, commercial buildings and public buildings have been built in these countries. The principle
technical approach used in these demonstrations was a combination of the passive measures by optimal building design and the active measures by efficient mechanical system design to control the indoor climate. These measures include utilizing reusable energy, such as solar energy, wind energy and geothermic energy, improving thermal performance of building envelope, utilizing daylight and energy efficient illuminations, increasing HVAC system energy efficiency, and developing new sustainable building materials. Even some ‘zero energy consumption building’ and experimental buildings with closed ecological circle, named ‘ecological building’, have been built and operated successfully. During nineties, 250,000 solar houses were built in US, while 2000 OM solar houses were built in Japan in the 20th century [10]. A great deal of sustainable residential and commercial buildings employing synthetic technology has also been constructed in Europe. Although many of the developed countries have successful experience in sustainable building research, design and operation, it is not suitable for China to copy their experience simply, since most big cities of China have much higher population and building density and much lesser availability of renewable energy per square meter floor area. For instance, the building density in cities of China, i.e. the ratio of floor area over the land area, is always higher than 1.5, and useful floor area of most of the apartments is less than 70 m2 [1,5]. However, the sustainable buildings in developed countries are usually in the low building density and have more available renewable energy source per unit floor area. Most of the sustainable housings are single houses with a garden. Therefore, it is possible to supply more than enough reusable energy by means of solar energy collectors, solar cell, ground-source heat pump, underground duct, green house, roof vegetation, and so on. And these houses rarely have the problems such as restrained daylighting or natural ventilation due to the adjacent buildings, and the noise and heat reject caused by the outdoor units of the neighboring air-conditioners. The household or independent heating system, garbage treatment, water treatment, and reusing system are easier to be adopted in those houses instead of the centralized systems, which are commonly applied in China. Therefore, instead of directly applying these advanced technologies from the developed countries to China, more consideration has to be taken for the various climates, different cultures, conventions, economic development level, and social situations of China comprehensively. On the other hand, there are many good sustainable building technologies that can be found in Chinese traditional dwellings. A typical case is the Wannan traditional residence in the south part of Anhui Province. The on-site measured data show that even in hot summer, when the outdoor temperature is above 38 ◦ C, the indoor thermal environment is still kept in the comfortable range even without any mechanical ventilation (Fig. 1) [11]. The technical strategies for these buildings are [11,12]: (1) the uncovered dooryard plays the role of a vertical shaft to boost ventilation induced by ther-
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Temperature (0C)
34 32 30 28 26 24 22 20 13-Aug
Zhaolan House Zhongxiu Hotel 15-Aug
17-Aug Time (d)
Jiqing House Outdoor 19-Aug
21-Aug
Fig. 1. Dry-bulb temperatures measurements of Wannan traditional residences in the summer of 2000 (the hotel is a modern building with similar room heat gain compared traditional residences).
mal pressure at night, while the narrow laneway restrains the natural ventilation in the daytime. Since the outdoor air temperature is higher than the indoor air temperature at the daytime of summer, while it is lower at night, it could always maintain a comfortable indoor environment; (2) an effective external shading by high wall of the dooryard prevents direct solar radiation while well utilizing diffusive radiation as daylighting well; (3) the wooden wall of less thermal mass speeds up the night cooling; (4) a double-pitched raft with an overhead double-layer-tile structure consists a layer of the local tile and the other layer of ‘Wang Brick’ can provide good thermal insulation and quick night free cooling. Therefore, it is more efficient to combine the advantages from both the successful Chinese traditional buildings and the experiences of developed countries to develop the sustainable buildings in China. Not only will it be beneficial to Chinese sustainable development, but also to the global environment. As a result, a series of technical strategies are recommended, which resulted from the research achievements (including the measurements, investigation and simulation of traditional dwellings [11,12]) and experiences of sustainable buildings design applications after 1990s in School of Architecture, Tsinghua University. These technical approaches should be applied into the different design stages and need a good cooperation among researchers, designers, and constructors of different majors including architecture, landscape, building physics, building services, and so on.
3. Technical strategies for sustainable buildings in China 3.1. Difference between strategies for residential buildings and commercial buildings Both the residential buildings and commercial buildings require good indoor climate and outdoor climate. However, these two kinds of buildings should be taken under different
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technical strategies because they have different building constructions and internal heat gains. The commercial building usually has larger area of inner zone and more internal heat gain than the residential building. Contrary to the commercial building, the indoor climate of the residential building is affected more by the outdoor climate than by the occupants and other internal heat gains. It is possible to merely use the passive methods such as the elaborate design and construction of building envelope to control the indoor climate of the residential building. However, the energy efficiency of the mechanical energy systems and lighting system of the commercial building may play more important roles in building sustainability, since the HVAC systems usually are necessary there, and more illumination should be used for the large inner zones. On the other hand, the thermal properties of the envelopes of the commercial buildings are still important to the building of sustainability, because unfortunate fabric design will cause great energy consumption and decrease the comfort level. In addition, major improvements can be made in commercial building HVAC systems, through relatively minor retrofits, and improved control and management. From the investigation and on-site measurements of almost 100 commercial buildings in Beijing, Shanghai, and Shenzhen [7,8], it is found that the lack of subdivisional control and cascade management are the most important factors for more energy consumption in commercial buildings compared with that of developed countries. However, it is still difficult to really realize energy efficiency in commercial buildings. The reasons are that there is no good management mechanism of commercial building which is incentive for operators, and the owner cares more about the initial cost instead of the annual energy consumption. Fortunately, there are some small specialized firms in big cities which have undertaken activities to lower energy costs and improve comfort in complex commercial building systems [8,9]. These companies are compensated from a portion of the reduction in the commercial building energy bill—an attractive arrangement for building owners. 3.2. Approaches for different stages of constructing sustainable housings If taking the design of sustainable housing as an example, a set of technologies involved with outdoor physical environmental quality, thermal performance of the building, indoor air quality, energy system and so on, should be introduced along with the design process of residential buildings (including mater planning stage, single building detailed design stage, and energy system design stage) of residential buildings. One of the most effective approaches is to use the computer simulation as an aid to improve the planning or design scheme of a building. Computer simulation can be used through all of the process of building design to improve the quality of outdoor and indoor physical environment, thermal performance of
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the envelope and the energy system with the idea of ‘sustainable design by simulation at different stages’. However, it needs better cooperation between architects and engineers since more issues should be yielded, discussed and solved. 3.2.1. Master planning stage Issues to be considered in this stage are related with the planning scheme and corresponding outdoor environment, such as sunlight, ventilation, noise level, and heat island phenomenon. It is extremely important when planning a high-density and high-rise dwelling district. The outdoor thermal environment is not only related to the thermal comfort of pedestrians, but also to the building energy consumption. Many buildings in developed countries have already considered those issues during the design phase. For instance, the local government of San Francisco issued regulations on wind velocity limitation and enough sunlight in the public area of building [13]. Following problems should be considered in the matter planning stage: • Wind environment. It includes restraining the wind speed among dwelling district in winter and promoting natural ventilation in summer. • Contamination emitting and dilution by natural ventilation. There are many residential buildings that use household gas heater for heating in winter and exhaust waste gas directly from the kitchen window. Then it is possible that the contamination concentrates around the buildings and brings health problems. • Outdoor thermal environment of the dwelling district. It means how to quantitatively evaluate the effect of the
waterscape, vegetation design and the arrangement of the materials for paving road, wall and roof on the heat island around buildings. • Daylighting. Adequate daylighting should be provided in all living spaces. Especially in the day of winter solstice, sunlight integrated time on the external window of the main habitable rooms should be the critical index to evaluate the design. • Noise pollution and sound insulation. In recent years, more high-rise residential buildings appeared in the big cities. The induced high-velocity wind around buildings is very uncomfortable and unsafe for pedestrians and is becoming a more and more serious problem. There are many failed-design buildings in Beijing, where the induced wind speed on some winter days can be as high as grade 5 or 6 (8–14 m/s). It could also increase the heat load by increasing the heat transfer coefficient of the building external surface and also by more infiltration in the building. The poor wind environment may also bring on other problems in summer. Natural ventilation may be restrained, and the contamination could not be diluted efficiently. In order to solve the above problems during the planning stage, Computational Fluid Dynamics (CFD) is a most helpful tool to predict the velocity and pollutant concentration distribution under different planning schemes [14]. Figs. 2 and 3 show an example using CFD as a tool to evaluate the planning scheme and the wind environment around a residential district in winter in Beijing (by the authors), of which the wind environment in winter is thought satisfactory (Figs. 2–3).
Fig. 2. Pressure distribution at 2 m height for north-west leading wind conditions.
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Fig. 3. Wind velocity distribution at 2 m height for north-west leading wind condition.
Heat island is also related to the wind environment in summer as well as the waterscape and vegetation design. Computer simulation is necessary to predict the effect and to optimize the site planning design [15,16]; and it is also useful to help architects to understand well how the building arrangement, waterscape, vegetation, and paved floor effect on the air temperature, radiant temperature, humility, air velocity, and the pedestrian comfort under the natural climate. Figs. 4–5 show an example of the simulation re-
sults based on a field measurement on a south residential district in Shenzhen city: with the heat balance equations and its couple with the governing equations of airflow, the distributed air parameters are numerically solved by means of CFD method. The necessary distance between buildings for adequate sunlight on habitable rooms is an important index for building planning. However, it is still not sufficient for locating buildings [17], especially when the length of building at the
Fig. 4. Analysis model in program.
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‘Target building’ in Fig. 6 could not obtain enough sunlight on the winter solstice day because of the interactive shading and self shading. Firstly, the target building could not obtain sunlight before 11:00 o’clock because of the shading from its east and south side buildings. After 11:00 o’clock, the east side of target building is shaded by itself [18]. Sound insulation should also be considered in the planning stage. It is necessary to arrange the public buildings such as service center and public clubs in the roadside as a barrier against the traffic noise. Noise level distribution prediction by computer simulation is also necessary during the planning stage. Combined with an on-site measurements, it could be used to estimate the possible noise level inside the district accurately.
Fig. 5. Horizontal air temperature distribution of 1.5 m height at August 15.
first row is too long, and/or there are spot-type and plate-type buildings laid out together inside the same district, and/or the building has complex shape and façade due to the possible self-shading and shading by around buildings (Figs. 6 and 7). In this case, computer simulation is necessary for sunlight and daylighting analysis in order to improve the indoor environment. Figs. 6 and 7 are typical cases that could illustrate all the problems mentioned above. They are the simulation results of ‘BSAT’ developed by Tsinghua University. For the master planning design, the distances between these high-rise buildings meet the building code. However, some east rooms of
3.2.2. Single building detailed design One of the important tasks in single building design stage for sustainable housing is to realize energy conservation and to improve the indoor thermal environment level. Although some building energy conservation codes and regulations have been issued, the on-site measurements results always show that near upon 80% of ‘energy saving building’ exceeded the energy consumption limitation [1,5]. Although the walls or windows of these ‘energy-saving buildings’ meet the requirement of U value limitation in the codes or regulations, but no envelope details, such as sill and shade and natural ventilation, have been well designed due to the lack of relevant regulations or guideline about that. In this stage, the orientation of the main habitable spaces, the shape coefficient, the window to wall ratio, and the location of the kitchen and toilet should be considered related to all of the aspects such as indoor thermal environment, acoustic environment, natural ventilation, and daylighting. For example the bigger window to wall ratio will always
Fig. 6. Interactive shading (before 11:00 o’clock).
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Fig. 7. Self-Shading (after 12:00 o’clock).
bring more daylighting and solar heat gain during the winter, but at the same time it will lead to heat loss by heat conduction. These problems should be considered combining the local annual climate variations and the thermal performance of the glazing. Many published literatures showed that the human thermal sensation in a naturally ventilated environment is much better than that in mechanical controlled thermal environment [19,20]. The layout of the building space and the openings, such as window, door or shaft, affect remarkably on the natural ventilation rate and then influence the indoor thermal environment. Successful design will boost natural ventilation in case of necessary, which is induced by means of wind pressure or thermal pressure. The ventilation direction should be considered carefully as well. For instance, according to the building code, external window in kitchen is necessary for both safe and indoor air quality requirement. However, poor design may cause downdraught wind from the kitchen window, and then diffuse the cooking smoke to the living spaces during the window opening seasons. It should be predicted during design stage. Multi-zone model is one of the possible tools in predicting the ventilation rate in the reference design condition. 3.2.3. Building envelope design As mentioned before, the building envelope has significant influence to the thermal performance to the building; an unsuccessful design of the envelopes may lead to failed thermal insulation due to the thermal bridge, which is not very well known by the designers. The cold bridge in the sills may cause more than 50% heat load in winter, although the external walls are well insulated and the both high insulated and high tight windows are used. Usually, insulation
on the exterior surface of the external walls can eliminate efficiently the thermal bridge at the conjunctive part to the partition walls, hence reduce the heat load to 15–20% compared with the case in which insulation only applied on the interior surface of the external walls [21]. Moreover, balcony, cantilevered roof, windowsill, rooftop, ground, and the lap joint of slabs are also easy to produce a cold bridge, and different insulation scheme should be adopted with a corresponding fine construction of the buildings. For example as exterior insulation does not cover the windowsill, there is an obvious cold bridge around the window hole. For interior insulation, because the window is inter-flushing mounted, there is no cold bridge at the interior surface although the outside surface has a large temperature gradient. The simulated results involved in those aspects are shown in Figs. 8 and 9, respectively [21]. In recent years, wall insulation has been emphasized so much and raised to a very high status in China. However, the global thermal performance of the envelope depends not only on wall insulation, but also on the shade, thermal mass, ventilation, and transparence property of the glazing according to the different climate types. The envelope design for the residential buildings in the tropics zone should be very different from that in the temperate zone, because in the tropics zone, shading, and ventilation are the most important strategies instead of wall insulation. Under the climate of which the daily temperature significantly varied, the large thermal mass of envelope is much more helpful for improving the indoor thermal environment than wall insulation. The location of the insulation layer and the thermal mass of the envelopes should be decided according to the heating and/or cooling schedule as well. In case that continuous heating, thermal insulation is the most important factor but
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design stage for testing the effect of different combination of window wall ratio and SHGC of window.
Fig. 8. Exterior insulation with inter-flush mounted window.
thermal mass is not as important in thermal comfort and energy saving. Contrarily, if the cheaper night electricity is used for heating the building through the day, large thermal mass is very important to such kind of application. In case that the occupants need heating discontinuously, less thermal mass can speed up the process of heating up or cooling down, and save more energy during the un-occupied periods. More attention has to be paid to the thermal insulation of the window or glazing due to the fact that most of heating or cooling loads from envelope come from window or glazing. To homogenize the thermal resistance on the external envelope will maximize the benefit of the insulation. Not only U value but also the shading coefficient SHGC of glazing should be attached importance according to the climate zone where the building is located. Lower SHGC can reduce cooling load in summer efficiently but increasing heating load in winter. Detailed simulation analysis is necessary during
Fig. 9. Interior insulation with inter-flush mounted window.
3.2.4. Building energy system design Building energy system usually indicates HVAC system. Four items must be considered for a sustainable building, including: (1) energy resource and heating/cooling plant; (2) air handling process; (3) duct and piping system; and (4) terminal units. The type of the heating/cooling plant has to be well chosen according to the available energy sources. The initial energy consumption, effect on natural environment (e.g. quantity of CO2 emitting), the indoor environment quality, maintenance and manage cost (may cause deleterious effect to the environment), and marketing should all be related to the building sustainability. The possible effect of different energy sources on the environment and the best strategies are listed as follows. 3.2.4.1. Coal. More than 60% of energy used in China is provided by coal [2,22]. When coal is the available energy source for heating, electricity–heat cogeneration is the optimal way for energy utilization from the viewpoints of protection, energy saving and low-cost. 3.2.4.2. Natural gas. In case that only natural gas is the available energy source for HVAC beside electricity, household gas heater is the most efficient way to reduce energy loss in transmission process. If it is necessary to install the central gas boiler to reduce the household operation risk, electricity–heat cogeneration or electricity–heat–cool tri-generation is the highly recommended method, for its high efficiency using the energy sources in different stages. 3.2.4.3. Electricity. In recently years, direct heating by electricity is regarded as a kind of ‘environment friendly, energy saving’ heating system. However, such a comment is true only when the electricity is mainly generated by hydraulic power or nuclear power. Although electric heater is flexible in control, it consumes high quality energy for the application which in fact only required low quality energy. If considering the primary energy source consumption, the energy efficiency of electric heater is only 30%. In China, electricity mainly comes from the coal-fired power plants, thus, the electric heater consumes three times of coal and emits three times of CO2 and other contaminations as heating comparing with coal boilers. Thermal storage electricity heater is one of the better substitutes comparing with the direct electric heating in case that no other energy supplies are available. Although thermal storage electricity heater cannot decrease energy consumption and pollutant emitting, it can reduce the peak load of power demand in daytime, and is benefic to the coal-fire power plant operation. The best way to use electricity for heating is heat pump. Heat pump consumes some electricity to remove
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heat between indoor environment and outdoor environment. Its energy efficiency is 2–4 times as that of electric heater. A heat pump cannot only provide heating in winter, but also provide cooling in the summer. However, the heat pumps which are wildly used now are air-cooled heat pump, of which quite lower efficiency or even breaking-down can be found in the cold period in winter. Water source heat pump can solve the problem of malfunction in cold winter. Although ground source heat pump, one kind of water source heat pump, has been well studied and widely used in developed countries. It is not suitable for the high density residential buildings in China due to the limitation of the available land area. The ground water heat pump (GWHP) uses aquifer as the thermal energy storage body, and the ground water as the medium of transferring heat to the ground. It can provide more energy to the building from the limited land area than ground source heat pump. Heat rejected from the building by refrigeration cycle is stored in the aquifer, then to be taken out by heat pump cycle in the winter. In such an application, water will not be consumed nor polluted by pollutant, because the ground water flows only in a close circuit. Furthermore, water source heat pump unit can provide both heating in winter and cooling in summer with higher COP than air-cooled heat pump, so that both initial cost and operation cost are saved. A household GWHP has no requirement to any outdoor spaces for outdoor unit installation, hence it avoids the problems such as heat rejected pollution, acoustic pollution, and uglification to the facades, which are very popular in air-cooled heat pump applications. Therefore, the household GWHP should be considered as one of the best solution to cooling/heating in high-density residential buildings in case that sufficient ground water is available. In fact, the Beijing government has decided to encourage GWHP application in the coming 2008 Olympic Games to fulfill the cooling and heating demand of Olympic Village buildings. However, there are still some problems needed to be solved. One is the lack of guides and regulations for design and building a GWHP system. The other one is the lack of detailed study of the effect of ground river, soil and the wells density on the temperature of ground water, and on the energy efficiency of GWHP systems. Figs. 10 and 11 give the simulated 10 years later temperature distribution results of the aquifer layers for two different wells arrangements of a GWHP system providing both heating and cooling for residential buildings in Beijing. Compared with scheme 1, scheme 2 could result a higher temperature for the heating wells, which is advantageous for these buildings whose heating load is larger than cooling load [23]. Based on the above analysis, co-generation combining district heating is the most efficient way for utilizing coal. If electricity is the only available energy supply, heat pump is the most efficient way for heating by electricity. Tri-generation or combined cycle co-generation should be encouraged in any possible cases when gas is the main energy supply. Heating directly by electricity in China
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Fig. 10. Temperature distribution of aquifer layer by Scheme 1 (after 10 years operation).
should not be promoted for the sake of global environment conservation and energy efficiency [24]. The only exception for considering direct heating by electricity reasonable is when building heat load is very low. In this case, installing piping system and other heating plant may bring more deleterious effect on environment. Therefore, excellent building thermal insulation is absolutely necessarily for using electric heater. 3.2.5. Renewable sources of energy Many people have been misdirected in the use of renewable energy. They simply believe that using renewable energy is an important symbol of sustainable building and pay few attention to solar cell, solar energy collectors, wind power utilization, heat recovery from waste water, and so on. In fact, in the high-density cities in China, no matter large-scale commercial buildings or high-rise residential developing, current reusable energy is far from sufficient to
Fig. 11. Temperature distribution of aquifer layer by Scheme 2 (after 10 years operation).
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provide half of the energy for operating the building. As the important complement to the conventional fossil energy source, reusable energy must be used based on the perfect thermal performance design for building envelope and highly energy efficient design for the conventional energy systems. Otherwise, a poor piping or duct design for reusable energy may get more kicks than halfpence due to the great lost in the energy transmission process.
4. Conclusion So far, based on the analysis of the requirements, characteristics, standards for evaluation and the feasibility for sustainable housing and urban construction in China, series technical approaches and suggestions in different design stages of the construction of a building are introduced and recommended. These technical approaches are based on the research achievements of School of Architecture, Tsinghua University. Some of the technical approaches for a sustainable design need good cooperation among researchers, designers and constructors. Although some technical details of them still need further research, it shows that there are many well-developed technologies related to sustainable building available in China as well as relevant products, such as energy saving building components and equipments of high efficient building energy system. If these approaches could be effectively applied into the design and construction process of a building, the sustainability would not be a dream any more. However, in order to promote the sustainable housing and urban construction in China, what is more important than technologies is how to found a mechanism which could boost the sustainable development of building automatically. So far, there are lots of problems needed to be solved by the governmental side since historical experiences told us that the executions of building energy efficiency seem not so good. Especially, market-oriented economy has come in China, new issued codes or standards for sustainable buildings should match these economic feature. Moreover, corresponding incentive policies and penalties are also needed. Fortunately, except having worked for a long time in the field of building performance simulation, building physical environment evaluation, and eco-housing design, some universities, research institutes and social organizations have also studied and developed some non-governmental approaches for improving the housing construction in China. Elicited by Leadership in Energy & Environmental Design (LEED) [25], a rating system for assessing the building sustainability and improving the design quality of the building in US, the ‘China Green Housing Rating System’ was issued in fall of 2001 with the cooperation of China Housing Industry Association, Tsinghua University, Chinese Academy of Building Research and other institutes [26]. The corresponding evaluation software was issued in March 2002. After that, more and more developers, architects, HVAC
engineers and owners are being aware of the significance of building sustainability. Since 2000, there are many evaluation and consultation practices for sustainable housing by Tsinghua University, which include projects in construction and completed that account for about 1 million m2 in cities of Beijing, Dalian, Guangzhou, Chengdu, Xian, Tianjin, etc. [27,28]. However, it is not enough compared with the total construction floor area per year in China. The state of the arts of sustainable building applications is far from satisfaction considering the high developing speed of Chinese construction. Therefore, it is still a long way for us to realize the sustainable development of urban construction in China.
Acknowledgements Some information and data about China energy consumption in buildings come from an investigation carried out by MOC, Tsinghua University and World Bank; the authors want to give their best thanks to them. This study was supported by NFSC (59836250).
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