analysis of embodied energy use in the residential building of hong kong

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Energy 26 (2001) 323–340 www.elsevier.com/locate/energy Analysis of embodied energy use in the residential building of Hong Kong T.Y. Chen * , J. Burnett, C.K. Chau Department of Building Services Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong Received 21 February 2000 Abstract Energy use in buildings accounts for nearly half of the total primary energy use in Hong Kong. Until now, studies have primarily focused on energy conservation in building operation, even though recent research has indicated that the embodied energy used in residential buildings could account for up to 40% of the life-cycle energy used in residential buildings. Accordingly, this paper presents a study on the energy embodied in the residential building envelope of Hong Kong. A model for estimating the intensities of the embodied and demolition energy for buildings has been developed. Two typical high-rise residential build- ings, the Housing Authority Harmony 1 and the New Cruciform blocks, are analysed based on the developed model. The results of the analysis provide an insight into the embodied energy usage profile in residential buildings in Hong Kong. Energy embodied in steel and aluminium ranks as the first and second largest energy demand and may account for more than three-quarters of the total embodied energy use in a residen- tial building envelope in Hong Kong. This reveals those building components with significant potential for reduction in embodied energy demand. 2001 Elsevier Science Ltd. All rights reserved. 1. Introduction With increasing concerns about ecological preservation since the late 1980s, the energy use of buildings is more broadly viewed than before, particularly in the context of global warming, energy resource depletion, and local and regional pollution. Until now, studies have primarily been focused on energy conservation in the operation of buildings. Recent research shows, however, that the embodied energy of residential buildings can contrib- ute up to 40% of the life-cycle energy use in residential buildings [1]. Accordingly, energy use * Corresponding author. Tel.: +852-2766-4858; fax: +852-2774-6146. E-mail address: [email protected] (T.Y. Chen). 0360-5442/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII:S0360-5442(01)00006-8

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Page 1: Analysis of embodied energy use in the residential building of Hong Kong

Energy 26 (2001) 323–340www.elsevier.com/locate/energy

Analysis of embodied energy use in the residential buildingof Hong Kong

T.Y. Chen *, J. Burnett, C.K. ChauDepartment of Building Services Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong

Received 21 February 2000

Abstract

Energy use in buildings accounts for nearly half of the total primary energy use in Hong Kong. Untilnow, studies have primarily focused on energy conservation in building operation, even though recentresearch has indicated that the embodied energy used in residential buildings could account for up to 40%of the life-cycle energy used in residential buildings. Accordingly, this paper presents a study on the energyembodied in the residential building envelope of Hong Kong. A model for estimating the intensities of theembodied and demolition energy for buildings has been developed. Two typical high-rise residential build-ings, the Housing Authority Harmony 1 and the New Cruciform blocks, are analysed based on the developedmodel. The results of the analysis provide an insight into the embodied energy usage profile in residentialbuildings in Hong Kong. Energy embodied in steel and aluminium ranks as the first and second largestenergy demand and may account for more than three-quarters of the total embodied energy use in a residen-tial building envelope in Hong Kong. This reveals those building components with significant potential forreduction in embodied energy demand. 2001 Elsevier Science Ltd. All rights reserved.

1. Introduction

With increasing concerns about ecological preservation since the late 1980s, the energy use ofbuildings is more broadly viewed than before, particularly in the context of global warming,energy resource depletion, and local and regional pollution. Until now, studies have primarilybeen focused on energy conservation in the operation of buildings.

Recent research shows, however, that the embodied energy of residential buildings can contrib-ute up to 40% of the life-cycle energy use in residential buildings [1]. Accordingly, energy use

* Corresponding author. Tel.: +852-2766-4858; fax: +852-2774-6146.E-mail address: [email protected] (T.Y. Chen).

0360-5442/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved.PII: S0 360- 544 2(01 )000 06- 8

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324 T.Y. Chen et al. / Energy 26 (2001) 323–340

during the lifespan of these buildings should be considered if the design of the total buildingenergy system is to be optimised.

Energy use during the lifespan of buildings consists of embodied energy, operating energy anddemolition energy. Embodied energy may be divided into two parts: initial and recurring embodiedenergy. The initial embodied energy of a building is the energy used in producing a buildingwhereas the recurring embodied energy is the energy used in maintaining and repairing of thebuilding over its effective life. The total energy demand includes energy use in producing andtransporting the building materials and components, and energy use for various processes duringthe production and demolition of the building. Operating energy is the energy use in keeping theindoor environment within the desired range while demolition energy is the energy use for thedestruction and disposal processes at the end of the lifespan of buildings.

Hong Kong is one of the most modern and most densely populated cities in east Asia, itseconomy having developed and its population grown rapidly since the mid 1960s. During the1980s to the 1990s, a substantial number of high-rise commercial and residential buildings werebuilt. Currently, the commercial and residential premises account for more than half of the totalprimary energy requirement in Hong Kong [2].

So far, only a few studies have been undertaken on embodied energy in buildings of HongKong. Instead, attention has been mainly focused on energy conservation in operating buildingsystems. Cole and Wong [1] analysed the life-cycle energy use for a typical high-rise residentialbuilding in Hong Kong and gave the relative orders of magnitude for initial and recurringembodied energy, demolition energy and operating energy. However, their studies have short-comings in that they were based on the Canadian data for estimating the energy intensities of thevarious materials and components, and an additional 5% to the embodied energy on importedmaterials was assumed for transportation. Since most of the raw building materials and productsused in Hong Kong are imported from overseas, materials transportation may have a significantimpact on the embodied energy consumed in buildings in Hong Kong. Accordingly, the impactof importing building materials and products should be further investigated.

The objectives of this study are:

� to develop a model suitable for estimating embodied energy usage in residential buildings inHong Kong;

� to understand the embodied energy usage profiles for the residential buildings, which will helpto reveal the elements with significant embodied energy reduction potential; and

� to provide information and data on the embodied energy, which may be used as a basis onwhich building regulations on energy efficiency and regional energy policy on buildings couldbe implemented.

This paper focuses on the embodied energy as well as demolition energy use in the residentialbuildings of Hong Kong. The four major building elements that should be considered in theembodied energy analysis of buildings are the structure, envelope, interior components and fin-ishes, and building services system. The first three elements will be considered in this analysissince they have been neglected and have not been thoroughly analysed for Hong Kong. Althoughthe building services component may account for up to 26% [1] of the total embodied energyuse, their impact on the total embodied energy use will be discussed in an upcoming paper.

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Prior to analysing the intensities of embodied and demolition energy of buildings, a model forestimating the embodied energy use is presented. Two typical high-rise residential buildingdesigns for accommodating lower-income groups in Hong Kong are investigated. The results ofthe analysis will show opportunities for embodied energy conservation in the construction ofresidential buildings in Hong Kong.

2. Model for embodied energy analysis

The development of a model for embodied energy analysis is based on the principle of energyaccounting for buildings. The total embodied energy considered in this study includes the energyuses in producing, transporting, installing and finishing the building materials and componentsduring initial erection as well as renovation of the building. It may be expressed by:

Ee�Em�Et�Ep (1)

where E is energy use. The subscript letters represent the following: e, embodied energy; m,energy use for manufacturing the building materials and components; t, energy required for trans-porting the building materials and components to and from the building site during erecting,renovating and demolishing the building; p, energy use in various processes, such as crane liftingand smoothing of soil, during the production and demolition of buildings.

The energy use in producing the building materials and components may be calculated by:

Em��k

j�1

(1�lj)mj��n

i�1

qi,jei,j� (2)

where k is the number of building materials and elements, qi,j is the amount of building materialsj imported from country i (kg), ei,j is the energy required for manufacturing the building materialsj in country i (MJ/kg), n is the number of countries from which building material or element jis imported, lj is a factor for waste of the materials j produced during the erection of the building,mj is a replacement factor for building elements j during the lifespan of a building. It is obviousthat mj should be equal to or greater than 1 and (mj�1) represents a factor for the recurringembodied energy of building material j. Recurring embodied energy intensities will probably bereduced if the technologies for manufacturing materials are improved. For more accurate esti-mation, an annual decrement rate for embodied energy intensities may be incorporated in Eq. (2)to take the technology improvement into account.

Some building components, such as damaged windows and doors, may be partially replacedduring the lifespan of buildings, while others, such as walls, ceilings and floor finishes, may needto be completely replaced each time. In this study, work involving less than 100% replacementis considered to be maintenance and the replacement factor is computed by:

mj�Lb/lj (3)

For full replacement, m may be found by:

mj�Lb/lj (4)

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326 T.Y. Chen et al. / Energy 26 (2001) 323–340

where Lb is the lifespan of a building, lj is the average lifespan of building materials or componentsj, and is a mathematical operator that finds the least integer which is greater than or equal toa real number within.

The energy use in transporting the building materials and components to and from the buildingsite accounts for a large percentage of embodied energy. It may be computed by:

Et��k

j�1

(1�lj)mjQj(et,j�ed) (5)

where Et is the energy required for transportation of the building materials and elements(MJ/(kg·km)), Qj is the amount of building material j (kg), ed is the energy used in demolishingbuildings and transporting the demolished building components from building site to landfill,subscript t refers to transportation, and e refers to the average energy use for transportation ofmaterial to the building site (MJ/kg), which may be obtained by:

et,j��n

i�1

qi,j

Qj��

l

et,ldl� (6)

where qi,j is the amount of building material or component j imported from country i, et,l is theenergy use for transportation of building materials by means of conveyance l (MJ/(kg·km)), anddl is the transportation distance by the conveyance l (km).

The energy required for various processes during producing and demolishing buildings may beestimated by [6]:

Ep��k

j�1

Qp,jep,j (7)

where Qp,j is the amount of building material j dealt with in a process during producing anddemolishing the building (kg, m3 or MJ/m2), and ep,j is the energy intensity required for thisprocess and building material j (MJ/kg, MJ/m3 or MJ/m2 usable floor area).

3. Fundamental data

In order to determine the breakdown of the imported building materials, the monthly HongKong Trade Statistics report is used [3]. The amounts of materials imported from different coun-tries to Hong Kong are not available, but the amounts of money spent on each country for differenttypes of building materials are presented in the report [3]. It is assumed that the average pricesof each type of materials from different locations are similar because of the large degree ofcompetition for the sale of these commodities. The percentage of building materials importedfrom different countries is derived based upon the above premises. The breakdown of the topfour imported building materials by country of origin is shown in Figs. 1–4. Countries exportingmore than 2% of any of these four building materials are given in the figures, while those countriesexporting less than 2% are collectively included in the category ‘others’.

Ideally, the energy intensity reported from those countries should be adopted in Eq. (2) since

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327T.Y. Chen et al. / Energy 26 (2001) 323–340

Fig. 1. Breakdown of imported cement by country of origin.

Fig. 2. Breakdown of imported steel by country of origin.

Fig. 3. Breakdown of imported aluminium by country of origin.

the overall energy efficiency in different countries may vary significantly. Until now, however,these data have not been available in the public domain for many countries, especially indeveloping countries such as China, Malaysia and Thailand. Therefore, the embodied energyintensities of different building materials published recently from various resources [4–14] havebeen adopted. The data used in the analysis are listed in Table 1.

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Fig. 4. Breakdown of imported timber by country of origin.

Table 1Embodied energy intensities for manufacturing common building materials, as published in Refs. 4–6, 10 and 11

Energy intensities Energy intensitiesMaterials Materials

(MJ/kg) (MJ/kg)

Aggregate (general) 0.10 InsulationVirgin rock 0.04 Cellulose 3.30River 0.02 Fibreglass 30.30

Aluminium (virgin) 191.00 Polyester 53.70Aluminium (recycled) 8.10 Glass wool 14.00Asphalt (paving) 3.40 Paint 90.40Cement 7.80 Solvent based 98.10

Cement mortar 2.00 Water based 76.00Ceramic Plasterboard 6.10

Brick and tile 2.50 PlasticsBrick (glazed) 7.20 PVC 70.00

Clay tile 5.47 Polyethylene 87.00Concrete Polystyrene 105.00

Block 0.94 Sealants and adhesivesBrick 0.97 Phenol formaldehyde 87.00Paver 1.20 Urea formaldehyde 78.20Pre-cast 2.00 Steel (recycled) 10.10Ready mix, 17.5 MPa 1.00 Reinforcing, section 8.90

30 MPa 1.30 Steel (virgin, general) 32.00Roofing tile 0.81 Galvanised 34.80

Glass Stainless 11.00Float 15.90 Timber (softwood)Laminated 16.30 Rough saw 5.18

Gypsum 8.64 Plywood 18.90

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Table 2The value of waste factors for different types of materials used in the construction of buildings from Refs. 1 and 6

Materials Waste factor Materials Waste factor

Aluminium 0.025 Polystyrene 0.05Coatings (paints and lacquers) 0.05 Polythene 0.05Concrete (reinforced) 0.025 Polyvinyl chloride (PVC) 0.05Concrete (plain) 0.025 Steel 0.05Copper 0.025 Tiles and clinkers 0.025Glass 0 Timber (planed) 0.025Gypsum wallboard 0.05 Timber (rough saw) 0.025Mineral wool 0.05 Timber (shingles and shavings) 0.025

Building materials will be wasted during the construction of buildings and the wastage should betaken into account during the estimation of embodied energy. The value of waste factors depends onthe type of building materials [1,6] and is given in Table 2. The waste factors selected are close tothose suggested in Ref. 1, but smaller than those given in Ref. 6. The smaller waste factors are inview of that some effective measures, such as prefabricated components and dimensionally coordinatedmodular flats, have already been adopted in Hong Kong to reduce material wastage.

The lifespan of some building materials may not be equal to that of the building itself. More-over, some building components, such as furniture and paint, may be replaced simply because ofthe owner’s choice. The average lifespan or replacement factors of common building materialsand elements may be found in references published by Adalberth [6] and McCoubrie and Treloar[14] which are reproduced in Table 3.

Embodied energy for transporting building materials and components may account for a largepercentage of energy embodied in the buildings of Hong Kong due to a substantial amount ofimported raw building material. However, this part of energy use has not been examined untilnow. Accordingly, the objectives of this study are to determine the primary embodied energy datafor Hong Kong, and to evaluate the impact of heavily imported material on the embodied energyuse in residential buildings.

An essential datum for calculating this part of energy use is the average energy use in trans-porting materials and components from the manufacturers to the building sites, which may be

Table 3The replacement factors of typical building materials and elements, as given in Refs. 6 and 14

Replacement ReplacementBuilding components Building components

factora factora

Structural elements (column, beams, etc.) 1.0 Plastic carpeting 2.4External and interior walls 1.0 Ceiling finishes 2.0Flooring 1.0 Floor finishes 3.0Windows and doors 1.3 Painting and wall papering 5.0Wall and roofing tiles 1.3 Others 1.2

a Replacement factor= lifespan of a building/average lifespan of building materials or components .

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computed by Eq. (5). Generally, raw materials or semi-finished products are first transported fromoverseas manufacturers to distributors or other manufacturers who further process them into build-ing products in Hong Kong, from which the products are then delivered to building sites. Theintensities of energy involved in transporting the building material within Hong Kong are negligi-bly small compared with those involved in transporting overseas. Therefore, the transportationcomponent should be broken down into three parts: from the manufacturers in foreign countriesto their seaports by truck and train, from oversea ports to the seaport of Hong Kong by ship, andfrom there to building site by truck. However, energy use in different modes of transportationreported by various sources [15–18] vary significantly. In this analysis, the smallest values havebeen used, and are given in Table 4.

Information on manufacturers exporting raw materials to Hong Kong is not available in thepublic domain. It is therefore assumed that average transportation distance from the foreign pro-ducers to their seaports is measured from the middle of their countries to the seaport. The distancemay be computed by:

d�z�Ap

(8)

where A is the area of country or region, which may be found from Li and Wang [19] and z isan area shape factor. The shape factor is approximately equal to 1 when the area shape is closeto that of a circle and increases as the area shape gradually deviates from a circle. Other factors,such as the number and geographical distribution of seaports within the country, and the lengthof the coastline also affect the value of z. Distances from foreign seaports to Hong Kong areincluded in a paper published by Wang [20]. Distances for transportation on Hong Kong Islandmay be determined by means similar to Eq. (7). Embodied energy use for the total transportationof common raw building materials is computed according to Eq. (5) and shown in Table 5. Energyuse for processes during producing and demolishing buildings is determined from Adalberth [6]and is given in Table 6.

4. Typical residential buildings

Typical residential buildings built in the 1980s and 1990s in Hong Kong are about 30–40storeys high. In this study, two 40-storey residential buildings, the first Harmony Block (H1) and

Table 4The relatively small values among the data of energy use in different modes of transportation, as published in Refs.16–18

Method of transportation Energy use (MJ/(kg·km))

Deep-sea transport 0.216Coastal vessel 0.468Truck 2.275Class railroads 0.275

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Table 5Intensities of energy use in transporting various common building products in Hong Kong

Energy use Energy useBuilding products Building products

(MJ/(kg·km)) (MJ/(kg·km))

Acrylic paints 1.04 Homogenous floor tiles 0.77Aluminium 0.84 Plasterboard 3.36Asphalt 2.34 Plywood 0.88Ceramic tiles 0.77 Steel 0.94Concrete 1.20 Steel (galvanised) 1.77Concrete roofing tiles 0.98 Steel (zinc sprayed coated) 0.79Extruded polystyrene 3.97 Timber 0.83Glass mosaic tiles 3.67 Unglazed vitreous mosaic tiles 1.55Granite wall facing slabs 2.38 UPVC 0.77

Table 6Energy use in installing and processing the different types of building component during construction of buildings [6]

Type of construction process Energy use

Drying of standard concrete on building site 0.158a

Drying concrete element 0.900a

Crane lifting 0.0072a

Lighting of construction objects 93.6b

Heating of construction objects 93.6b

Heating of sheds 50.4b

a Measured in units of MJ/kg.b Measured in units of MJ/m2 usable floor area.

the New Cruciform Block (NCB), are investigated. Their typical floor plans are shown in Figs.5 and 6, respectively.

The first Harmony Blocks, which have 40 storeys with a usable floor area of 39,040 m2 in eachblock, were completed in Tin Yiu Estate in 1992. Each block has 16 apartment units per floor witha central service core, which creates an open environment allowing daylight to enter all flats. It hasunits with one bedroom of 34 m2, two bedrooms of 43 m2 and three bedrooms of 52 m2.

The New Cruciform Block was designed in 1984 to replace earlier designs that provided mostlytwo-bedroom flats. As shown in Fig. 6, it consists of ten apartments units per floor with twobedroom flats of 37 m2 and has a layout similar to that of H1. The building under considerationis 40 storeys high with a usable floor area of 26,600 m2.

Comparing these two buildings, the Harmony Blocks were more standardised. The configur-ations of block were designed with the same modular flats and a variety of flat mix options withinthe main building envelope. This allows the population mix to be adjusted to suit the specificproject needs in different site conditions.

Standardised factory-manufactured building components can be extensively incorporated into themodular design. The modular building components include timber doors and frames, stainless steel

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Fig. 5. Typical floor plan for H1.

Fig. 6. Typical floor plan for NCB.

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cooking benches and sinks, precast concrete facade panels, precast concrete staircases, lightweightconcrete drywall panel partitions and metal security gates. Moreover, large panel steel formworkcommon to the block configurations was also used, which led to not only higher quality constructionand greater efficiency in time and cost, but also the reduction of embodied energy used.

On the other hand, in NCB larger window areas are used than in H1 in lieu of non-structuralconcrete walls. This may confer savings in the embodied energy, especially when windows aremade of low-energy materials, such as recycled aluminium.

5. Embodied energy analysis

The lifespan of residential buildings in Hong Kong generally falls between 40 and 50 years.The impact of the lifespan on initial and recurring embodied energy use has been analysed byCole and Wong [1]. Generally, the longer lifespan the building, the more annualised recurringembodied energy use, and the less annualised initial embodied energy use. The lifespan of residen-tial buildings is assumed to be 40 years in this study.

The first step in the embodied energy analysis is to determine the amount of the buildingmaterials required for each building component. These may be determined from the bills of quan-tities used by the Hong Kong Housing Authority [21,22]. As mentioned previously, buildingelements considered in this study include the structure (columns, beams, etc.), envelope (externalwalls and finishes, windows, roof and its coverings, etc.), and interior components (staircases,internal walls, partitions and finishes, burglar grilles, louvres, shelves, doors and gates, etc.).Demolishing and disposing of the entire building at the end of its life is common practice inHong Kong, which is assumed in our analysis. Eqs. (3) and (4) are used for estimating the replace-ment cycle of building materials and components over the building life.

The amounts of the ten most important materials consumed for erection of the H1 and NCBare given in Table 7. The amounts of material use per usable floor area for the nine most important

Table 7Quantities of top ten materials used in the erection of H1 and NCB

Material Elements Quantity (t)

H1 NCB

Aluminium Window frame, burglar grilles 86 124Asphalt Roof covering, damp-proofing 95 54Concrete Columns, beams, walls, slabs, staircases, etc. 7110 4200Glass Windows 73 55Paints Internal finishes 4 2Stainless steel Finishes of louvre burglar grilles, metal doors 86 59Steel Reinforcing bars, metal doors, gates, etc. 5890 3420Timber Formwork, doors 2440 1630uPVC Window frames 81 77Various tiles Internal and external finishes 1520 777

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materials are presented in Fig. 7. Table 7 and Fig. 7 reveal that the building materials consumedin the largest quantities for these two buildings are concrete and steel followed by timber andvarious tiles, while the other five materials are used much less extensively. Close examination ofFig. 7 shows that more concrete, steel and tiles are consumed in H1 than NCB. Their consumptionper usable floor area is 182, 151 and 39 kg/m2 for H1 in contrast to 158, 129 and 29 kg/m2 forNCB, respectively. On the other hand, less aluminium and uPVC are consumed in H1 and theirconsumption per usable floor area is 2.20 and 2.07 kg/m2 for H1 while 4.70 and 2.90 kg/m2 forNCB. The reason for this is that NCB uses larger window areas in replacement of external concretewalls compared with H1.

The quantities of building materials consumed are substituted into Eqs. (2), (5) and (7) in orderto determine the embodied energy in the two buildings. Energy use in manufacturing and trans-porting the raw building materials and products and in processes during producing and demol-ishing buildings is presented in Figs. 8 and 9. The results show that the energy use in the processduring the production and demolition of buildings accounts for less than 2% of the total embodiedenergy, while the production of building materials consumes more than 90%. Energy use in trans-portation of building materials and products for Hong Kong is about 7%, which is higher thanthe average 5% assumed by other researchers [1]. This figure may even be underestimated becausethe smallest values among the published energy uses for different modes of transportation areadopted in this study. Moreover, the percentage may vary significantly when different strategiesare adopted in the construction process, and will be discussed further below.

Fig. 7. A comparison of the amounts of different building materials used in H1 and NCB.

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Fig. 8. Breakdown of energy uses in different life cycle processes for H1.

Fig. 9. Breakdown of energy uses in different life cycle processes for NCB.

The further analysis of energy demand in the manufacture and transportation of different buildingmaterials may be helpful for one to fully understand the embodied energy use in the residentialbuildings of Hong Kong since this portion of energy accounts for about 97% of the total embodiedenergy. Embodied energy use in manufacturing and transporting asphalt, glass, paints and stainlesssteel accounts for less than 3% of the total embodied energy use in manufacturing and transportingall the building materials, which is neglected in the following analysis. A breakdown of initial andrecurring embodied energy use for H1 and NCB by six types of building materials is given in Figs.10 and 11. It is found that most of the embodied energy is used by steel for both buildings eventhough the material used in the largest quantity is concrete. The energy use for steel, which isprimarily used in reinforced concrete, accounts for 68% of the embodied energy for H1 and 61%for the NCB. The energy used by aluminium is about 1.17 GJ/m2 of the usable floor area in the

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Fig. 10. Breakdown of initial and recurring energy contents of various building materials used in H1.

Fig. 11. Breakdown of initial and recurring energy contents of various building materials used in NCB.

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NCB. It is approximately twice as much as that in H1, which is about 0.56 GJ/m2. Differences inenergy use for steel and aluminium between the two buildings are due to the difference in windowareas. The larger window area of NCB leads to less energy used for steel and concrete but moreenergy used for aluminium and uPVC. Figs. 10 and 11 also show that the recurring embodiedenergy accounts for only small percentages of the energy embodied in the residential buildingenvelope, 7.5% for H1 and 5.4% for the NCB. Notice that an increase in the lifespan of a buildingwill increase the recurring embodied energy. Moreover, this portion of embodied energy will alsoincrease if building services systems are considered in the analysis. This is because the lifespan ofbuilding services systems is generally shorter than that of the structure of buildings.

Energy used by steel and aluminium together accounts for 77.4% for H1 and 77.6% for NCB,which ranks the first and second largest energy use for both buildings. This may provide a primarydirection in which the embodied energy could be conserved. The use of recycled steel and alu-minium may result in large energy savings since the energy intensity is about 10 MJ/kg forrecycled steel and 8 MJ/kg for recycled aluminium, while virgin steel is about 32 MJ/kg andvirgin aluminum is about 191 MJ/kg. This makes the recycling of steel and aluminium veryimportant. Therefore, analysis has also been conducted to reveal the impact of the use of recycledsteel and aluminium. A comparison of the intensities of embodied energy use due to the use ofvirgin and recycled steel and aluminium is presented in Fig. 12. The results indicate that the useof recycled steel and aluminium may lead to savings of more than 50% in embodied energy. Theembodied energy intensity for manufacturing and transporting building materials is 7.15 GJ/m2

Fig. 12. Breakdown of intensities of energy use during manufacturing and transporting of building materials in differ-ent building designs.

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for H1 and 6.96 GJ/m2 for NCB when using the virgin steel and aluminium, while only 2.93GJ/m2 for H1 and 2.87 GJ/m2 for NCB when using the recycled steel and aluminium.

Realising the significant potential of embodied energy savings by recycling and reusing wastebuilding materials calls for novel design concepts and innovative technologies. Waste concretehas been used as a backfilling material in building foundations and as underlay in roadbed andparking lot construction [23]. New design concepts and technologies have been developed.Formwall, for instance, developed by the National Concrete Masonry Association, allows thedisassembly and reuse of concrete units [23]. However, it remains a great challenge to fullyrecycle and reuse the reinforced concrete. Although recycling or reusing aluminium windows maybe less difficult than the former, new designs may also be needed to disassemble the windowfrom the old building easily. Moreover, with the analysis result, it can be expected that the useof materials as alternatives to the concrete elements for non-structural external and internal wallsmay result in considerable savings in embodied energy. This is because, among all buildingmaterials, energy use is highest in manufacturing and transporting the reinforced concrete blocks.

Energy use in transportation should not be neglected for those countries or regions relyingheavily on imported building materials, such as Hong Kong. A breakdown of energy uses indifferent categories for the two building types using recycled steel and aluminium is given inFigs. 13 and 14. From these figures it can be seen that the percentages of energy use in transpor-tation increase considerably to 17.5% for H1 and 15.6% for NCB when recycled steel and alumi-num are used. However, the actual percentages could become even higher than these values forthe reason mentioned earlier. Therefore, further studies in this aspect are needed to properlyevaluate and minimise the embodied energy use due to different building designs.

As discussed previously, the data of embodied energy vary with different sources. The resultswill be different if the data are differently selected. Moreover, the data of embodied energy willbecome more precise as research in this field progresses. This will lead to more accurate results.

6. Conclusions

Studies on embodied energy use in the high-rise residential building of Hong Kong have beenundertaken. A model developed in this study is especially suitable for estimating the embodied

Fig. 13. Breakdown of life cycle energy uses by using recycled steel and aluminium in H1.

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Fig. 14. Breakdown of life cycle energy uses in different categories by using recycled steel and aluminium in NCB.

energy use in the building, which uses a substantial amount of imported building materials. Theamounts of energy embodied in the two buildings, H1 and NCB, are quantified, giving an insightinto energy use in manufacturing and transporting building materials and products, installingbuilding elements as well as demolishing buildings.

Energy embodied in steel and aluminium ranks as the first and second largest energy use andmay account for more than three-quarters of the total embodied energy in a residential buildingin Hong Kong, even though the building material used in the largest quantity is concrete. There-fore, the use of recycled steel and aluminium will confer savings of more than 50% in embodiedenergy and the use of materials as alternatives to non-structural concrete walls will result insignificant savings. These opportunities pose a great challenge in developing the novel buildingdesign concepts and innovative technologies to realise these potential savings.

Heavy reliance on imported building materials has a significant impact on embodied energyuse. Currently, available data on energy use in different means of transportation are considerablydifferent from each other. Further studies are needed to obtain more accurate data, which willprovide a reliable basis for proper selection of building materials for Hong Kong.

A model for the life-cycle energy analysis of residential buildings is now under development,which will allow the estimation of the total energy demand and its impact on the environmentduring the lifespan of the building. The model, which also takes economic factors into account,can be used for the optimal design of buildings and the minimisation of life-cycle energy use.

Acknowledgements

This work was financially supported by The Hong Kong Polytechnic University throughResearch Grant No G-YY19 and the Area of Excellent Project No AoE1-8628. The authors wouldlike to thank Daphne Chan of the Housing Authority for her help in data collection, Grace Ngand Kin Ting Li for their work on some estimation of building materials consumed, You Tu forhis architectural drawings and Tammy Tse for her help in editing.

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