Analysing the life cycle greenhouse gas emission and energy consumption of a multi-storied commercial building in Singapore from an extended system boundary perspective

Analysing the life cycle greenhouse gas emission and energy consumption of a multi-storied commercial building in Singapore from an extended system boundary perspective

Energy and Buildings 51 (2012) 6–14 Contents lists available at SciVerse ScienceDirect Energy and Buildings journal homepage: www.elsevier.com/locat...

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Energy and Buildings 51 (2012) 6–14

Contents lists available at SciVerse ScienceDirect

Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild

Analysing the life cycle greenhouse gas emission and energy consumption of a multi-storied commercial building in Singapore from an extended system boundary perspective Harn Wei Kua a,∗ , Chee Long Wong b a b

Department of Building, School of Design and Environment, National University of Singapore, 4 Architecture Drive, S117566, Singapore Master of Science in Environment Management Program, School of Design and Environment, National University of Singapore, 4 Architecture Drive, S117566, Singapore

a r t i c l e

i n f o

Article history: Received 17 September 2011 Received in revised form 10 March 2012 Accepted 13 March 2012 Keywords: Life cycle assessment Integrated assessment Integrated policies Sustainable development Embodied energy Singapore

a b s t r a c t Building life cycles contribute substantially to the emission of greenhouse gases and energy consumption; this is especially true for the operation or use stage of the building. This work on a commercial building in Singapore extends the traditional system boundary drawn for a whole-building life cycle assessment to include the management of wastes produced during building operations. It was found that waste management produces much more emissions than the operation stage. This reinforced the notion that waste recycling should be further promoted in buildings, possibly through building level technological innovations and design modifications. An integrated policy framework was proposed to explore ways by which building level strategies can work with other strategies to holistically address the issues of waste reduction, sorting, collection and recycling. © 2012 Elsevier B.V. All rights reserved.

1. Introduction 1.1. Building industry and climate change The building sector is a major contributor to carbon emissions. According to Ochsendorf [1], buildings in United States of America account for nearly 39% of its nationwide carbon dioxide (CO2 ) emissions. In Singapore, the construction industry has been proclaimed as the third largest emission sector, accounting for about 16% of Singapore’s overall CO2 emissions of 40,377 kilo-tons in 2005 [2]. Most of the electricity used by buildings in Singapore is for air-conditioning (40–50%), mechanical ventilation (about 20%) and lighting (15–20%). Singapore has since set high environmental targets to improve its energy efficiency by 35% and increase its waste recycling rate to 70% by 2030. In response to these targets, Singapore has adopted the approach of greening buildings as key initiative in reducing CO2 emissions for the built environment. Introduced in January 2005, the Building and Construction Authority (BCA)’s Green Mark Scheme (GMS) serves as the national assessment criteria system to recognize both new and existing buildings locally which adopts

∗ Corresponding author. Tel.: +65 6 516 3428. E-mail address: [email protected] (H.W. Kua). 0378-7788/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2012.03.027

environmentally friendly features such as increasing green cover, use of energy and water efficient technologies to reduce consumption, among others. Since April 2008, under statutory requirements all new or existing buildings above 2000 m2 in area undergoing major retrofitting has to attain at least a Green Mark Certification. As of 1 September 2010, a total of 524 buildings (existing and new) have been at least certified under the GMS [3]. Although the GMS has attained plausible achievements since its introduction, it is not without its shortcomings. In the latest version (version 4 [4]) of the GMS for nonresidential and residential buildings, the computation of carbon emissions is awarded a maximum of only 2 points (out of 155 points) for carbon footprint reporting. The reporting requires new buildings to assess their construction and operational carbon by means of either reporting through a carbon consultant or through a simple form in declaration of the amount of key materials, electricity and renewable energy source consumed. Since then, BCA has received numerous feedbacks calling for more weightage to be given to accounting for the building’s carbon footprint as an indicator of building sustainability. More specifically, there are proposals to include a life cycle approach to the computing of energy consumption by building stocks, and then look for ways to reduce their life cycle energy requirement and greenhouse gas (GHG) emissions. Therefore, it is fair to say that conducting life cycle studies on building stocks in Singapore is only beginning to catch on.

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1.2. Applying life cycle assessment (LCA) to analyze energy consumptions of entire buildings LCA is a process whereby the material and energy flows of a system are quantified and evaluated according to their life cycle stages, which, for a typical building, normally include the material extraction, production, transportation, construction, use (also known widely as operation), renovation, deconstruction and/or demolition, and recycling stages. According to international organization for standardization (specifically, ISO14040) [5], LCA studies generally consist of four phases: goal and scope definition, life cycle inventory (LCI), impact assessment and interpretation of results. The goal and scope defines the objectives, assumptions and system boundaries of the studies. The LCI involves data collection and calculations to quantify material and energy inputs and outputs of the defined system. Impact assessment evaluates the significance of potential environmental impacts based on the LCI. Finally, these impacts, which may be expressed in the form of global warming, are calculated for environmental assessments. As a whole, LCA allows for comparative evaluations of impacts of different processes and building materials on the environment. There were LCA studies done specifically on quantifying energy consumption during the different life cycle stages; this approach is known as life cycle energy analysis (LCEA). Ramesh et al. [6] divided the energy uses in the various life cycle stages into initial embodied energy (EE), recurring embodied energy (EEr ), operating energy and demolition energy. During periodic building maintenance, energy is used to refurbish the building, whereby materials are either repaired or removed and then replaced with new ones. The EEr used for such maintenance is the sum of the embodied energy of the material and the energy used for working on these materials. The sum of these energy components is known as life cycle energy. Their definitions are concurred by most of the studies in the current literature. Many LCEA have been conducted. Studies such as [7–10] showed that as much as 85% of the total energy consumption throughout a building life cycle was used in the operation phase, whereas the manufacturing of construction materials, erection and renovation of buildings collectively account for around 15% of the total energy use. Citherlet and Defaux [11] found that by reducing operation energy by 20%, through the improvement of thermal insulation of envelope, the total life cycle energy can be reduced by about 16%. Furthermore, they found that by further increasing the insulation with technologies such as 3-paned windows with low-e coatings, the operation energy can be reduced by 54%, thereby resulting in an overall reduction of 49% in the life cycle energy. However, there were also studies showing that 40–60% of the life cycle energy is used in the production and construction stages [12]. Specifically, Thormark [13] showed for Swedish low-energy houses that material substitution could reduce the share of energy use in the material production phase by 17% for a row house and by 15% for an apartment building. Even if the EE constitutes a comparatively low fraction of the total life cycle energy than operation energy, opportunities to reduce EE should never be ignored. In fact, in certain cases, materials with high thermal insulation capability have lower EE (for example, cases considered in [14,15]). In these cases, a switch to these materials will result in reductions in both embodied and operation energies. In other words, when prescribing energy reduction solutions, one should not independently focus on the operation phase. In the majority of the whole-building LCEA and life cycle GHG analysis (LCGA) in the current literature, processes that are considered under the operation phase comprise of cooling, heating, ventilation, lighting, supply of water, heating or cooling of water, and operating of other electrical appliances. The end-of-life options in these whole-building LCA are usually confined to the demolition or

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recycling of building materials. Although wastes – including waste heat, used water and solid wastes – are produced from a building’s operations, management of these wastes are usually neither considered in the operation nor end-of-life phase; only construction and demolition (C&D) wastes are usually included. Put in another way, these LCA typically restricted the system boundary of their analyses to the building scale. However, Dodoo et al. [16] considered the role played by district heating from a combined cycle heat and power (CHP) plant in their LCA of the primary energy consumption of a wooden apartment building designed to meet the Swedish building code. This exemplifies an effort to integrate a districtlevel condition/factor into a building-scale analytical framework, and hence extending the system boundary. To a lesser degree, this observation was also made by Sartori and Hestnes [17]. If the objective of a LCA is to fully understand how the life cycle of a building creates an environmental impact on its immediate environment – through creating a demand for resources and production of wastes that have to be assimilated into its environment – then extending the conventional system boundary is necessary. The authors called this approach an extended system boundary perspective. This work contributes to the literature in two important ways: it provides the first ever LCEA and LCGA on a commercial building in Singapore, and adopts an extended system boundary perspective to fully assess the environmental impacts of the case study building. The main research objective is, through a comparison of the energy consumption and GHG emissions from the different life cycle stages, provides a guide for integrated policymaking to reduce the total life cycle energy consumption and GHG emissions of this building.

2. Methodology The case study is an industrial project that is a six-storey ramped up food-factory in Singapore. It was constructed in Year 2003 at a cost of about 47 million Singapore Dollars (SGD; this is equivalent to USD 36.2 million), and occupies a land area of 27,029 m2 and has a total gross floor area of 52,094 m2 . The net tenable space is 35,262 m2 and presently it houses a total of 204 food tenants and 1 food court. The building has a total common area of 16,832 m2 . With operations round the clock, the food factory sells food products for the mass market, which includes the selling of basic food such as salt and rice, production of foods such as soy bean products, cakes and pastries, providing catering service for functions and events. Equipped with various green features, including daylighting technologies, it is poised to be exemplary for green design for multi-storied commercial buildings in Singapore, it is chosen as the main subject of this study. The facilities managers who are in-charged of the monitoring of its building performance frequently collect and collate reports on, for example, utilities consumption and waste volume collection reports. In addition, the volume of construction materials used for routine maintenance were also measured and recorded within payment systems for audit purposes. These data sets were shared with us during this study. However, due to the lack of reliable data, GHG emissions and energy usage arising from the transportation of individual building users and delivery vehicles, and sewage discharge were excluded from the study. Carbon offset potential from existing onsite greenery was also excluded. In determining what types of emissions to consider for each of these life cycle stages, we followed the guidelines of the World Resource Institute (WRI) and World Business Council for Sustainable Development (WBCSD) [18], which categorizes emissions into 3 scopes: scope 1 refers to all direct onsite emissions; scope 2 refers to the embodied emissions in purchased energy; and scope 3 refers to all indirect emissions. Table 1 shows

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Table 1 Life cycle and scopes of emission considered. Life cycle stage

Activities/materials considered

Scope of emissiona

Research methodologies

Production of building materials

Only concrete, cement and steel are considered (refer to Table 2 for quantity comparison)

2 and 3

Process analysis of cement, sand, gravel and steel

2

Process analysis of cement, sand, gravel and steel EIO analysis of construction activities in Singapore Data was derived from energy and water bills. Most of the electricity is generated from natural gas; hence, embodied energy for natural gas is derived from process analysis Embodied energies from main materials – concrete, cement, sand screed, and mild steel – were found from process analyses Process analysis of the detailed steps involved in the waste disposal

Transportation of building materials Construction

General construction activities

2

Use or operation

Electricity and water consumptions at common areas

2 and 3b

Renovation

3

Waste disposalc

1 and 2 (Indirect emissions from incineration of waste generated from daily operations of the food estate) 2

End-of-life

Demolition of construction materials a b c

Process analysis of equipment and activities in demolition

Scope 1 refers to all direct onsite emissions; scope 2 refers to the embodied emissions in purchased energy; and scope 3 refers to all indirect emissions [18]. For “water consumption”, scope 3 emissions were considered for filtration and distribution of water from source to gate. This is the proposed extended system boundary perspective.

the system boundary considered for the LCEA and LCGA and the different types of emissions that were considered for each of the life cycle stages. Data for the LCIs was obtained using two approaches. Process analysis was applied to find out the energy use and emissions for every detailed process in each of the life cycle stages considered. In the absence of reliable process data, macroscopic industry-wide data extracted from economic input-output (EIO) tables was used. This hybrid method is widely known as the EIO-LCA methodology. The detailed methodologies and source of data and information are described in the following sub-sections. 2.1. Production and transportation of building materials Table 2 shows the quantity of different types of building materials utilized in the building. This distribution is similar to the case study on a building in Thailand [19], in which the authors found that steel and concrete together accounted for 77.8% of all EE. Only the EE and EEm (embodied emissions) contributions by concrete (Ordinary Portland Cement (OPC), gravel, sand) and steel were considered in this study. This data is derived using the process analysis approach. Singapore imports most of its OPC from Taiwan. The energy consumption data associated with the stages from secondary raw materials to OPC production were obtained from companies based in Hualian, Taiwan [20]. The fuel consumption data from transportation of manufactured OPC from Hualian to Singapore was also obtained by Teo et al. [20]. Diesel consumption for transportation within Singapore and associated emission data were based on an average distance of 20 km from port to cement company and then from cement company to the location of the building. Assuming that to produce 1 kg of OPC, 0.063 kg of gypsum and 2 kg of limestone are needed; the LCI of gypsum and limestone were obtained from [21]. Furthermore, 1 kg of concrete block is assumed to be composed of 0.5 parts (by mass) water, 1 part OPC, 2 parts sand, and 3 parts gravel. The LCIs (including uncertainties in the readings) for all these constituents were taken from the study by Kua [21]. Maghimai and Kua [22] derived the LCIs for the production of primary and secondary steel. Within Singapore, the production of secondary steel via the Electric Arc Furnace (EAF) method was considered. Singapore does not produce its own supplies of primary

steel and it obtains much of its primary supplies from China. Therefore, LCIs for the extraction of iron ore, sintering and pelletizing, production via the Basic Oxygen Furnace (BOF) method and continuous casting were derived for China. Finally, the LCIs for the transportation of the finished steel products to Singapore were also estimated. The EE and EEm data for steel was taken from their work. 2.2. Building construction To find the energy required for, and the resultant emissions from, construction activities in Singapore, the economic input–output tables of Singapore were used [23]. Table 3 shows the quantities of petroleum products (mostly industrial oils imported from Thailand and China), electricity and liquefied natural gas sales to construction industry (for construction of buildings and not for civil infrastructure). The direct GHG emissions are calculated using the following formula: GHG emissions = F ×

3  c=1



Qc  × Eg Tc 3



(1)

g=1

where the total GHG emission from construction is expressed as carbon equivalent. F is floor area ratio of the building to the total floor area of building constructed in 2005. This allows us to estimate the fraction of the total construction-related emissions and energy consumption that can be attributed to the building. Since the total floor area of the building is 52,094 m2 and the total floor area built in the whole of Singapore in 2005 is 10.419 km2 [24] the ratio is 0.005. Qc is the quantity of the commodity c consumed. Tc is the average tariff of the commodity c. Eg is the emission factor for the GHG type g for every commodity c, expressed as (kgCO2 per kWh) or (kgCO2 per kg of commodity). The three types of emissions with the highest Global Warming Potential (GWP) were considered – that is, CO2 , methane and nitrous oxide. Unlike the method employed by Kofoworola and Gheewala [19], the primary energy factor was not considered explicitly in Eq. (1); this is because in calculating the emission factors for each commodity, the EE and EEm from all major life cycle stages (that is, material acquisition, production, infrastructure and transportation) were already included. In fact, this method is expected to be more accurate. The values of the variables in Eq. (1) are shown in Table 3.

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Table 2 Quantities of main building materials in the GEK. S/N

Material/plants

Total quantity

Units

Mass (ton)

%mass

1 2 3 4 5 6 7 8 9 10

Concrete Cement and sand screed (150 mm) Galvanized mild steel Formworks UPVC pipes (100 mm) Aluminum Gypsum Board Stainless Steel pipes Steel fabric Unglazed ceramic floor tile

2570 873.2 1,805,000 38.82 2.50 7.38 42.78 62.90 198.35 54.33

m3 m2 kg m2 m m2 m2 m m2 m2

6168 125.70 1805 0.47 0.006 0.099 0.385 20.55 0.38 1.09

62.28 1.27 18.22 0.005 0.000 0.001 0.004 0.208 0.004 0.011

Table 3 Input–output GHG emissions and energy consumptions for building construction in Singapore. S 

Commodity consumed

Quantity, Qc (SGD million and [USD million])

Average tariff, Tc (SGD/kWh or SGD/m3 )

Quantity (GJ or ton)

35.2 [27.1]

SGD 0.2728/kWh [USD 0.21/kWh] SGD 105,680/m3 [USD 81,373.60/m3 ] SGD 0.0314/m3 [USD 0.024/m3 ]

464,516.1 GJ

0.5863 kg/kWh [26]

41.3 tons

0.207 kg/kg [29]

57,324.8 tons

0.436 kg/kg [29]

Combined emission factor,

Eg kg/kWh

g=1

or kg/kg (for all three types of GHGs) Electricity (80% natural gas and 20% fuel oil) Petroleum products (mostly industrial oil) LNG

4.9 [3.8] 0.5 [0.4]

2.3. Building operation

Fig. 1. Similar to the electricity case, the emission from water consumption can be computed as follows:

As shown in Table 1, the energy consumptions and emissions due to electricity and water consumptions were considered. The common areas of the building hosts a range of services and facilities such as common corridors, staircases, ramped-up driveways, bin centers, car parking lots, lifts (passenger and cargo), loading/unloading bays, smoke control systems, and fire protection systems. Energy is required in these areas for different functions, including provision of lightings to all common areas, the powering of lift motors for lift operations, pumps for distribution of water supplies, and fans for smoke control systems. The power supply to common areas are tracked by a master electricity sub-meter, which is separated from the leased and sold units in the buildings (these units have their own electricity sub-meters for individual accounting and billing). This model enables building owner to monitor and manage power consumption for areas under their operations control effectively. The monthly electricity consumptions are closely monitored by facilities executives. The total amount of purchased energy for the period between 1 January 2010 and 31 December 2010 was used for this study, and it was found to be 1,946,430 kWh. The GHG emissions from the use of electricity were then computed using the tier 2 (country-specific) approach recommended by the Inter-governmental Panel on Climate Change (IPCC) [25]. That is,

emissionCO2 −e = water consumtion × emission factorwater

(3)

Wong [27] found that the emission factor of water in Singapore is approximately 0.150 kgCO2 −e /m3 of water. The total water consumption for the assessment year was 40,188.9 m3 (obtained from the facilities managers of the building). Since the design lifespan of the building is 30 years, we estimate its lifetime emissions and energy consumption during the operation phase by multiplying the values derived above by 30 years. 2.4. Building maintenance and renovation, EEr For this life cycle stage, the EEr associated with the renovation of the different areas within the building was considered. Due to possible changes in the tenants, individual units in the building may undertake their own renovation works. These may include the changing of carpets, wall tiles and installations of new computers. However, for the past few years, it has been observed that several types of materials – namely, concrete, cement, sand screed, and

Screening and Grit Removal

Inputs

emissionCO2 −e, (80/20) = electricity consumtionCO2 −e , (80/20) × emission factorCO2 −e , (80/20)

Outputs Primary Sedimentaon

(2)

Channelling of water from Malaysia (import)

Coagulaon & Flocculaon Secondary Seling

Electricity and fuel

where (80/20) means a fuel mix of 80% natural gas and 20% fuel oil, and the emission factor of 0.5863 kgCO2 /kWh was taken from Tan et al. [26]. Singapore gets 60–70% of its water supply from imported water (from Malaysia) and the local water catchment areas. Our LCA of water considered the main stages of the water treatment process – screening and grit removal, primary sedimentation, coagulation and flocculation, secondary settling, filtration, sludge processing, disinfection and distribution. The system boundary is shown in

Filtraon Chemicals Sludge processing Channelling of water from reservoirs

Disinfecon

Distribuon Process

System Boundary

Fig. 1. System boundary for water supply to GEK.

GHG emissions

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mild steel – represent about 80% of the renovation materials; only these were considered. The formula used is

 3

EEr =

i=1

mi Ei

L

GEK

Li



− 1 + Er

(4)

where Li is the average lifespan of the material i and LGEK is the design lifespan of the building, which was taken to be 30 years. mi is the quantity of the material i and Ei is the embodied energy of i material per unit mass. Er is the energy required for the renovation to be performed on material i. In summary, Eq. (4) calculates the number of times a certain material will be replaced throughout the design lifespan of the building, and multiply the result by the embodied energy of a unit mass of that material during renovation; this is then added to the energy required for working on that material to yield the total EEr . Since these materials were replaced once per year, we take the value of Li as one. The value of Er was taken from Yap [28]. Inventories for transportation of the salvaged concrete and steel to recycling centers were computed based on an average distance of around 30 km using the same model used in Ref. [29]. 2.5. Building end-of-life As mentioned earlier, traditionally, the literature covered only the demolition or recycling of demolition wastes during this life cycle stage. Besides considering these, we also adopted the extended system boundary perspective to explore the energy consumption and emissions due to management of wastes produced during the operation phase. 2.5.1. Demolition of buildings Energy consumption in this phase was calculated from the use of demolition machinery and tools. Phua [30] conducted a series of case studies on demolition projects of similar scale, each of which requires an average of 4 months to complete. Energy consumption is estimated from the use of onsite generators and from electricity bills. Diesel consumptions are found for tractors, crushers, breakers, backhoes and electro-magnetic separator/sorter. The total amount of electricity and fuels recorded for each of the demolition projects was then divided by the total mass of wastes collected in each project to give the average energy consumption and emission intensity of demolition. This intensity was applied to estimate the building’s energy requirements and emission during demolition in the future. After demolition, the wastes are transported to the sorting center in the north-western part of Singapore. The distance between the building and this center is about 35 km. Assuming that the rates of recovery of different types of building materials in the building are similar to the cases covered in Phua’s study, and that the trucks used for transporting these wastes always travel with full loads, the total fuel demand of transportation can be estimated. The total emissions and energy consumption of this stage included the EE and EEm of the fuels (mostly diesel) used in the demolition process. 2.5.2. Extended system boundary perspective: management of wastes generated during operation General Industrial Waste (GIW) accounts for at least 80% of the total 2.613 Mtons of waste generated from the building in the year of this study. Majority of the waste are wet food wastes generated from the processing of food products. As part of performance monitoring, it is a requirement for the waste collection contractor to submit daily waste disposal records reflecting the volume of waste sent to incineration plants. Such requirement had allowed for monthly waste volume output to be recorded and these records were used for this research. Recyclable wastes accounted for only

Industrial Waste Collected Inputs

Outputs Tipped off and incinerated at Waste Incineraon plant GHG Emissions

Fuel

Electricity

Incineraon Process

Ash transported to landfill site at Semakau via marine transfer at Tuas and then processed in Semakau.

System Boundary Fig. 2. System boundary for management of industrial wastes produced by GEK.

about 1% of the wastes generated, whereas the remaining 19% was divided between sewage and sludge wastes; since consistent data for these wastes was absent, they were left out from the study. GHG emissions from the incineration plant and transportation of wastes were accounted for; this included transportation of wastes from the building to incineration plant (Senoko Power Plant, situated in the north part of Singapore), transportation of ash from incineration plant to Tuas Marine Transfer Station, subsequent shipping of the ash offshore to Pulau Semakau landfill site and the final landfilling of the ash. Fig. 2 summarizes the system boundary of this stage. CO2 emissions factor from incineration in Singapore was adopted from Tan et al. [26] – namely, 1.6 kgCO2 per kg of waste incinerated. Methane (CH4 ) emissions are generated as a result of incomplete combustion during the incineration. Hence, such emissions are more significant for incinerators which are poorly maintained or having low oxygen level. In large and well-functioning incinerators, such emissions are usually small in volume. Nevertheless, we considered this type of emissions as well, using the IPCC tier 1 method: CO2 equivalence of CH4 emissions = 21 ×



(IWi × EFCH4 )

i

× 10−6

(5)

where, the factor 21 is the GWP of CH4 , IWi is the total amount of solid waste of type i (wet weight) incinerated (measured in thousand tons per year), EFCH4 is the aggregate CH4 emission factor in kgCH4 per thousand tons of waste, and the factor 10−6 is for converting the unit kilogram to thousand tons. Since the Senoko incinerator belongs to the continuous incineration, stoker type, the typical EFCH4 is taken to be 0.2 [25]. During the combustion process, Nitrous Oxide (N2 O) is emitted at relatively low combustion temperature of between 500 ◦ C and 950 ◦ C. The amount of N2 O emissions from waste combustion depends on the type of technology, combustion conditions, technology applied for NOx removal as well as the contents of the waste stream. Similar to the case of methane emissions, N2 O emissions can be calculated using the IPCC tier 1 method: CO2 equivalence of N2 O emissions = 310 ×



(IWi × EFN2 O )

i

× 10−6

(6)

where 310 is the GWP of N2 O, IWi is the total amount of solid waste of type i (wet weight) incinerated (measured in thousand tons per year), and EFN2 O is the aggregate N2 O emission factor in kgN2 O per thousand tons of waste. The typical EFN2 O for the incinerator that Senoko Power Plant typifies is 47 [25].

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Fig. 3. Contributions of life cycle stages toward life cycle GHG emissions (when waste management was not considered).

Fig. 4. Contributions of life cycle stages toward life cycle energy consumption (when waste management was not considered).

Senoko Power Plant consumes around 0.05TWhe annually for its internal operations (including the various operations related to electricity generation) – 3% of which is derived from the combustion of about 1.3 Mtons of wastes. By direct proportion, the 2613 tons of wastes produced by the building that is combusted at Senoko accounts for about 10,916 MJ of electricity generated for its operations. The remaining 97% of Senoko’s energy supplies are derived from natural gas; the EE and EEm of natural gas were also considered in this life cycle stage. Data for the entire transportation sequence and final landfilling of the incinerator ash was taken from an earlier study by Kua [29]. The energy consumptions and emissions results derived for one year were multiplied by 30 years to represent the lifetime energy consumptions and emissions of the building.

energy consumption, which also agree well with previous studies [6,19,31–34]. Lifespan operation of the building is expected to consume 86.10% of the total energy and emit 80.27% of the GHG emissions. Demolition accounted for about 0.37% of the energy consumption and 0.40% of the GHG emissions, which agree well with the study by Kofoworola et al. [19].

3. Results The raw materials acquisition and processing of building materials of the building required a total of 27.9TJ of energy and emitted about 10.94 Mtons of CO2 equivalence. A comparison of the embodied and operation energy over its lifespan of 30 years indicated that the embodied energy was about 13.2% of the operation energy (regardless of whether the embodied energy of the water consumed is also considered). This value agrees well with a similar building operating in similar tropical climatic conditions in Thailand [19]. However, the 30-year operation energy of the building was found to be 5.96 GJ/m2 ; this implies an annual energy consumption of 55.2 kWh/m2 , which is within the 15th percentile of commercial buildings in Singapore. Comparisons of the energy consumption and GHG emissions of the various life cycle stages were made with and without considering waste management. 3.1. Waste management not considered The building is expected to consume 245.45 TJ of energy and emit 42.88 ktons of GHG throughout its 30-year lifespan. The contributions of the various life cycle stages to the life cycle GHG emissions and energy consumption were shown in Figs. 3 and 4. Materials production accounted for 13.71% of the GHG emissions and 11.37% of the energy consumption. This agrees with the results of Ramesh et al. [6] and Kofoworola et al. [19]. The construction process accounted for only 0.96% of the emissions and 0.91% of the

3.2. Waste management considered If the waste management stage is considered within the system boundary, it was found that the life cycle energy consumption and GHG emissions were increased to 246.46 TJ and 169.26 ktons respectively. This means that waste management increases the total life cycle GHG emissions by 3.95 times. As shown in Figs. 5 and 6, waste management contributed 74.67% of the total emissions, and the operation and material production phases contributing 20.33% and 3.47% of the emissions respectively. The share of emissions of all other life cycle stages also decreased. Since the energy demand of waste management is typically low, it contributed 0.47% of the energy consumption and did not significantly change the distributions of energy observed in the previous section. 3.3. Sensitivity analysis An important part of a LCA is examining how results can change in the presence of uncertainties and variability in data. The comparisons involving waste management were based on the assumption that the annual waste generation rate of the building is maintained throughout its lifespan. However, the relative emission contributions of waste management and operation will change with the amount of waste generated. As shown in Fig. 7, waste management contributes more emission than operation unless the lifetime emission from waste management is only 35 ktons. For this to happen, an average annual fractional change in emission, r, is required and it can be found by solving the following equation: 35 =

4.213(1 − r 30 ) 1−r

(7)

where 4.213 ktons is the GHG emission from the 2.613 Mtons of wastes produced during the year of study. r was found to be about 0.88; that is, for waste management and operation to contribute equal percentage of emissions, there has to be a reduction of 12% of

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Fig. 5. Contributions of life cycle stages toward life cycle GHG emissions (when waste management was considered as well).

waste production every year. This is equivalent to every one of the 204 units in the building reducing his/her daily waste production by 4.2 kg for the first year. It was assumed that reducing the waste generation will neither affect nor be affected by the use of electricity and/or water during the building’s operation stage.

4. Integrated multi-level policy and technology strategies to reduce the impact of waste management

Percentage contribuon of total GHG emissions

Fig. 6. Contributions of life cycle stages toward life cycle energy consumption (when waste management was considered as well).

90 80 74.7

70 60 50

44.5 Waste management

40

Operaon

30 20 10 35

0 0

20

40

126.4

60

80

100

120

140

160

180

Total GHG emissions from life cycle stage (ktons) Fig. 7. Comparison between the GHG emission contributions of the operation stage and waste management stage.

Conventional LCEA and LCGA for buildings point to the operation stage of buildings as the stage where most attention is needed to reduce the overall environmental impact of buildings. This has given rise to technology, design and policy strategies focusing on energy conservation and renewable energy utilization. However, if the system boundary is extended to include the impacts due to the management of the wastes, this study showed that incineration of wastes produces much more emissions than the operation of buildings. Hence, the extended system boundary perspective provided a hint that attention should be put on reducing the generation of all types of wastes (and not just on C&D wastes) during the operation phase of the building and increasing the recycling of those wastes that have to be inevitably produced. It is important to note that total life cycle emissions of a building cannot be reduced merely through building-level strategies. For example, if local power stations rely on incineration of wastes as a cheap method of generating electricity, then there may be a certain degree of resistance from municipalities to fully support waste recycling. To be successful in increasing total recycling rate and hence reducing incineration, what is needed is a confluence of policies operating coherently at different scales to increase waste sorting, collection and recycling. This can be seen as a form of integrated sustainability policy, which was defined by Kua [35] as a policy that tackle multiple objectives or indicators at the same time in a coherent manner. Fig. 8 showed a multiple timeand spatial-scale model that defined the operational scales and material flow relations among different downstream life cycle stages of buildings and the physical environments in which they are embedded. To reduce the reliance on waste incineration for

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Fig. 8. Multiple time- and spatial-scale framework, describing the operational scales and possible material flow relations among different downstream life cycle stages of buildings and the physical environments in which they are embedded.

energy production, different approaches at the building, precinct and national levels are required. A near term strategy is to shift toward a higher utilization of natural gas as fuel for power plants. A long-term solution is to increase the share of renewable energy at the national or municipal/precinct levels; a detailed discussion of renewable energy policies is beyond the scope of this study. Concurrently, more efforts should be made to increase waste sorting and collection at the building and precinct levels (readers are referred to [36,37] for more discussions on this matter). As material flows across different geographical levels (Fig. 8), energy is consumed for transportation and GHG is emitted. Therefore, strategies taken at the building and precinct levels are preferred (shaded blocks in Fig. 8). In the building, the majority of the 80% of the GIW generated are food wastes, so building and precinct level strategies such as decentralized food waste digesters and micro-generators could be considered. Energy generated can be directly fed back to the building or the national grid. Asian Square Tower One in Singapore has recently installed a biodiesel generator that runs on waste cooking oil collected from the tower’s own and neighboring food centers. This system can be improved by installing an onsite modular bio-digester and connecting it to the biofuel generator. Residential buildings can be redesigned to encourage waste sorting at the building level. For example, more than 80% of Singapore’s population live in high-rise flats where wastes are disposed in common refuse chutes. Recently, the Singapore government has initiated changes to the design of new flats in a new housing estate known as Punggol New Town. The new design provides separate chutes for different types of wastes, and it is hoped that this will encourage more residents to sort their wastes into recyclables and non-recyclables at source. Nationwide and within precincts, efforts were made to re-design the immediate physical

surrounding of a building to encourage waste sorting; an example is the town councils installing more recycling bins within close proximity of a building. However, these measures may not result in substantial increase in waste recycling; this is because local waste recyclers find it difficult to clean collected recyclables that are contaminated with food wastes and hence a considerable portion of these recyclables are incinerated instead. Therefore, in addition to providing separate chutes, the government should consider providing a special chute outlet for food wastes and encourage residents to clear food wastes from recyclables before discarding recyclables in appropriate chutes. The food wastes can then be collated at the precinct-level facility, where a bio-digester can be employed to produce biogas from the wastes; the biogas can then be used to run a biofuel generator. Possible feedstock sources include food centers, supermarkets and restaurants near the precinct. These food wastes can be collected through a system comprising manual collection by commercial vendors and automated technologies (such as waste suction system). Incentives should also be provided to encourage households sort out food waste at home. For example, if separate metering is possible, each household will pay a lower electricity tariff for electricity generated from food wastes. This system, however, requires a separate micro-grid to be constructed and keeping the electricity cost down will require substantial subsidies from the government. Incentives should also be provided for firms to undertake the collection of wastes and operating the decentralized technologies (such as the biofuel generator). Local businesses trading products and goods made from recycled paper and plastic (such as indoor panels made from recycled plastic fibers) should also be given monetary incentives (such as, lower store rental rates) to operate in the precinct.

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