1. Introduction
With the fast pace of spatial urbanisation and the expansion of urban population, most cities around the world, especially those in the Asia-Pacific region, have been continuously densified with expansion and redevelopment. As a trend, it is anticipated that globally the size of urban building stock will double by 2050 [
1] to cater for soaring demand for housing, urban services, education, health, and transport facilities due to urban population growth, which has grown by 10% over the past decade [
2,
3].
Among many sustainability implications imposed by such a development, greenhouse gas (GHG) emissions (also known as carbon emissions) have become a major challenge for urban densification. Urban built forms contribute to over one third of total carbon emissions [
4,
5] and a staggering 71% of energy-related carbon emissions. Much of these can be attributed to residential buildings [
3,
6]. Significant proportions of such carbon footprint are consumption-based, including both the direct and the life-cycle carbon emissions associated with objects (e.g., buildings, equipment, infrastructure, space) and socio-economic activities (e.g., production, services, transport, lifestyle, and entertainment) within the geographical boundary of a city. These also encompass those emissions occurring outside the city boundary but related to (or ‘‘embodied’’ in) goods and services consumed by functions and residents inside the city [
7]. As reported in the study by Wiedmann et al., commercial and public sectors are responsible for only one third of consumption-based carbon emissions, whilst the rest (close to two thirds) are contributed by households, attributed to buildings, electricity and transport as well as public and commercial services [
8].
Despite the pressure exerted by urban (re)development on carbon footprint, studies have also shown that a negative correlation between urbanisation and carbon emissions can be obtained over time through positive planning and intervention strategies toward ‘‘decarbonising’’ urban systems with innovative technologies and regenerative development approaches [
9,
10]. Literature indicates that operational carbon associated with building energy consumption, embodied carbon from the building material production and construction activities as well as the carbon associated with travelling by residents for commuting are the three major emission sources for cities [
8,
11,
12]. Decarbonisation aims to achieve a reduced, or even net zero, carbon signature by balancing those emission sources with carbon removal through offsetting measures. On the carbon reduction side, size of floor area, energy efficiency of appliances as well as climate rendition of building construction and use are key influencing factors for building energy demand and carbon performance [
3,
13]. Much of the effort for reducing energy intensity has been invested in improving building envelopes and passive design features with better insulation [
14,
15], double/triple glazing [
16], orientation, and shading to enhance natural ventilation and improve lighting performance [
17]. Scenario analysis on Xiamen City, China, indicates that annual operational energy associated carbon can be reduced by up to 3.15 million tCO
2-e by increasing energy efficient design for new buildings and energy-saving retrofit for existing buildings [
18]. However, coupled with a strong growth in total floor area, new or renovated buildings equipped with energy-efficient designs and technologies can also contribute significantly to the increase of material intensities and embodied carbon [
19]. Alongside savings in operational carbon, reducing carbon embodied in construction materials and processes is now also considered as imperative and increasingly crucial for decarbonising urban development toward the net zero carbon goal [
2,
20].
Recent studies have also highlighted that carbon emissions, especially the emissions associated with household energy use and travelling behaviour, are closely linked with and strongly influenced by demographic characteristics, particularly household size, household composition, age groups, affluence/income levels, and education backgrounds [
12,
21,
22,
23]. Despite the pressure of population growth in urban densification on the carbon performance of cities, having properly managed population density and social urbanisation with improved low-carbon awareness and consumption behaviour can lead to positive outcomes of carbon reduction [
24].
Better passive building designs and strategic planning for demographic changes are effective in mitigating carbon emissions related to energy demand and consumption behaviour, which largely make up scope 1 and scope 3 emissions of cities [
8]. As a key mechanism to decarbonise electricity production, renewable-energy harvesting units (RHUs) such as solar photovoltaic (PV), solar water heaters (SWHs), and small-scale wind turbine systems, have found wide application as an affordable way to offset the scope 2 emissions from direct electricity use by households. There is abundant literature on carbon benefits from the deployment of solar-energy harvesting units (SHUs), e.g., solar PV and SWHs. Recent studies indicate that an increased coverage of roof-top SHU deployment not only ensures fulfilling energy demand with cleaner energy supply [
25,
26] but also provides viable carbon payback [
27]. Moreover, renewable energy integrated hybrid power systems can also improve energy resilience as well as balance the energy supply and demand for a maximum of economic and carbon benefits [
28]. Although SHUs are attributable to an emission-free electricity production, the systems are not entirely carbon-free throughout their life cycles when it comes to energy consumed and carbon emissions embodied in the manufacturing, maintenance, and recycling of the modules, which can account for 49.9 g CO
2-eq/kWh [
29]. Therefore, like carbon mitigation for buildings and consumption behaviour, both the benefits and burdens of SHUs for the carbon impact of urban densification need to be properly assessed and balanced in conjunction with the effects of other factors and measures for decarbonising urban built forms [
20].
Considering physical, spatial, and functional complexities of urban built forms, carbon mitigation measures need to be targeted to the relevant urban settings. As a dominant urban form that encompasses place, people, and constructed objects at a meso-scale, ‘‘precinct’’ has been increasingly used as an ideal ‘‘spatial lens’’ for development planning and urban design purposes. A precinct can be defined as part of an urban area with definite geographical boundaries and certain functions that involve interplays of land, buildings, infrastructure (energy, transport, water, and waste), and occupants [
5]. In the context of urban redevelopment and densification, a precinct can be treated as a single entity by urban planners and decision makers to analyse its morphological configuration, operations, and interactions with surrounding urban features and objects for examining and managing carbon signatures thus incurred in accordance with planning purposes [
7]. Notwithstanding a growing body of literature on smart and sustainable city development toward decarbonising urban systems [
30,
31] extant research on the methodologies of precinct-level planning and intervention for low-carbon development is still scarce. Failing to consider buildings, occupants, and the precinct environment as a whole integrated system risks reaching biased or over-simplified solutions that have limited effects on reaching zero-carbon outcomes. Despite much research on the development of low-carbon precincts, there is limited analysis on aggregated effects of population growth, building energy efficiency, renewable energy penetration, and carbon reduction targets in relation to precinct carbon signature and carbon neutral potential for precinct redevelopment and decarbonisation planning. Therefore, to assess the carbon implications of an evolving precinct and explore potential for carbon reduction, it is vital to examine and address the following key questions for informing planning decisions:
How and to what extent will population increase with demographic changes affect the carbon signature of redeveloping a precinct?
What are operational, embodied, and travelling carbon implications for a precinct in relation to different redevelopment plans?
To what extent do energy-efficient buildings, particularly residential buildings, contribute to the total carbon reduction in precinct densification?
To what extent do renewable energies, particular SHUs, affect the total carbon reduction in precinct densification?
Can carbon neutrality be realistically achieved with an increased penetration rate of SHUs?
To this end, this paper presents an integrated carbon assessment model that captures factors of urban evolution (including demographic changes as well as redevelopment and renovation of built objects) and renewable energy penetration to support the evaluation of decarbonisation options on a precinct scale. A case example of precinct redevelopment for densification is also examined with scenario analysis to identify the impacts of building and demographic and renewable energy factors on the operational, embodied, and travelling carbon signatures as well as on the potential for reaching carbon neutrality.
2. Research Methodology and Modelling Tool
Over the years, a growing body of research has been conducted to study the impacts on energy consumption associated carbon emissions resulting from physical factors (e.g., energy efficiencies of buildings, appliances, and infrastructure) and occupant behaviour on precinct objects including buildings and infrastructure operations (e.g., comfort setting and operating schedules) as well as carbon offsetting contributed by renewable energy harvesting. These studies play a crucial role in delivering solutions for urban sustainability. However, a building is not an isolated system but an evolving subsystem closely linked to the behaviour of its occupants and the dynamics of the surrounding built environment in which it is situated and operated. Therefore, in this research, an integrated model considering renewable energy penetration and precinct evolutions (including demographic changes and redevelopment of precinct objects) is developed to support the carbon performance assessment and to examine the carbon neutral potential for planning precinct (re)development.
2.1. System Boundary Selection
As the urban precinct is ‘‘a system of many interconnected systems’’, a realistic, holistic, and accurate evaluation of its carbon neutrality can be achieved by integrating the embodied carbon of precinct objects and operational and travelling carbon associated with energy consumption by occupants’ activities (e.g., use of appliances, daily commuting for work/education, recreational travels, etc.) as well as the carbon offset contributed by RHUs. Therefore, the overall precinct carbon signature can be assessed with four distinct but inter-linked components: (1) total lifecycle carbon embodied in precinct objects, RHUs, and transport systems; (2) total carbon emissions associated with energy consumption for the operation of precinct objects; (3) total carbon emissions associated with travels of precinct occupants; and (4) total carbon offsetting contributed by renewable energy harvesting.
Based on these inputs, the precinct-level carbon assessment can be formulated and evaluated with an extended consideration of integrating embodied, operational and travelling carbon with the carbon offsetting. The boundary established for modelling and assessment of precinct carbon neutrality can then be illustrated as
Figure 1 (where the carbon flow is coloured “grey”, impacts are coloured “blue”, and renewable energy harvesting is coloured “green”). Compared to the building-level assessment, precinct-level emissions should be modelled and evaluated at both temporal and spatial scales. With respect to the time scale, operational carbon varies over the lifespan of precincts due to changes in residential groups as well as the energy efficiency and recurring embodied carbon of precinct objects. At the spatial scale, morphology, location, and urban density are found to be the major influences on transport and operating associated carbon as well as SHU efficiency. Therefore, these factors, together with feasible deployment of solar PV and SWHs, are of particular interest for examining the carbon impacts of precinct (re)development.
2.2. Carbon Neutral Assessment and Performance Metrics
To support the identification of precinct carbon neutral potential in an evolutionary manner, a modelling strategy and a framework have been developed for scenario analysis and decision making based on previous work in this research [
7,
32]. As shown in
Figure 2 (where the carbon flow is coloured “grey”, energy flow is coloured “orange”, cost flow is coloured “red”, renewable energy harvesting is coloured “green”, while the half coloured boxes means conversion of data types), the precinct-scale carbon assessment is underpinned by an integrated model that consists of four key phases. At Phase 1, carbon intensities for embodied, operational, and travel-related emissions are identified. The embodied intensity represented by kg CO
2-e per square meter of floor area for each precinct object type is determined by the lifecycle carbon content of the main construction materials, the replacement cycle and waste ratio of building components, and carbon embodied in construction activities. The embodied carbon intensities of SHUs are derived from the lifecycle carbon of elements as well as related assembly and maintenance services, measured as kg CO
2-e/kWp for PVs and kg CO
2-e/each for SWHs. As for the embodied carbon of the transport systems, the intensity data is based on a previous study conducted in South Australia [
33]. To determine operational carbon intensities, operational energy intensities of precinct objects of household types are firstly assessed and then converted by using local energy-to-carbon factors. Meanwhile, travel-related carbon intensities are calculated based on the annual travelling expenditure of each family type, average fuel cost, and the carbon factor of each fuel type (measured as kg CO
2-e/dollar).
Phase 2 is developed to identify the baseline carbon emissions of the assessed precinct. In this stage, demographical factors (e.g., amount of each family type, age groups, income groups, etc.), energy efficiency of each precinct object type, and occupant lifestyle preferences (e.g., total floor area of each building type, schedule of appliances, travelling frequency and distance, etc.) are examined to support the calculation of the overall baseline carbon emissions of the precinct. Moreover, the baseline carbon offset is also calculated by converting renewable energy harvesting with the energy-to-carbon factor of local power production. Phase 3 is designed to improve the accuracy of precinct carbon assessment. At this stage, effects of precinct morphological factors (e.g., compactness, density, building orientation and obstruction angle, etc.) are incorporated to moderate the baseline carbon emissions of precinct objects and occupancy and carbon offset contributed by SHUs. The master plan of the precinct is also analysed to examine the strength of influencing factors such as urban density and solar potential. Phase 4 is established to assess the carbon neutrality of the precinct or to identify the potential if carbon neutrality has not yet been achieved. Here, the carbon neutral potential is examined through exploring scenarios by varying morphological, demographical and energy and time factors, together with sensitivity analysis.
2.3. Precinct Carbon Assessment Tool
Underpinned by the methods and framework discussed, the Precinct Carbon Assessment (PCA) tool is developed in this research to support the precinct lifecycle carbon modelling and carbon neutrality assessment. The PCA tool aims to provide both highly aggregated as well as more detailed assessments of the embodied and operational carbon of precinct objects, travel-related emissions of occupants, and carbon offset contributed by SHUs. In addition, the PCA tool can also be employed for building-level lifecycle carbon assessment (with the assumption of a “single-building precinct” as a special case). As shown in
Figure 3 and
Table 1, the PCA tool provides six main function modules, each with a drop-down menu to provide detailed modelling and dedicated assessment functions.
Targeted at different end-users, the input parameters of the PCA tool are structured for three modelling levels to satisfy the requirements of different assessment scenarios. The first level is developed to suit urban planners and government agencies for early-stage planning with limited construction details. Simulations at this level use highly aggregated data presented as energy/carbon intensity per square meter for each object type. The second level is designed for building and construction practitioners, where intensity data per square meter can be built up based on material usage as well as the units of use and the operating schedule for each appliance type. The third level allows modelling with more detailed inputs about precinct objects design and travel mode selection to support the examination of carbon neutral potential at precincts. In this study, the third level of modelling is employed for precinct carbon neutral assessment and scenario analysis.
4. Conclusions
Decarbonising the urban built environment for reaching carbon neutrality is high on the agenda for many cities undergoing rapid expansion and densification. As an important urban form, precincts have been increasingly focused on as the context for urban redevelopment planning and at the forefront for trialling carbon reduction measures. For a precinct, the dynamics of evolution, including morphological variations, demographical changes, and renewable energy penetration, have been found to be of significant impact on its carbon performance and carbon neutral potential. In this paper, an integrated modelling and assessment method is applied to examine the lifecycle carbon signature and identify the carbon neutral potential of urban precincts. By using a case study on a residential precinct redevelopment, scenario analysis is conducted to explore opportunities for decarbonising densification development in consideration of demographical and morphological changes, renewable energy penetration, and planning horizon settings. The results indicate that redevelopment of precinct buildings with higher energy efficiency and increase of renewable energy penetration can have a long-term positive impact on precinct carbon performance. Furthermore, demographical factors such as population and family types also have noticeable influence on the precinct’s carbon neutral potential. The analysis also highlights the significance of embodied carbon to the total carbon signature and the carbon reduction potential of a precinct during densification, reinforcing the notion that “develop with less” is as important as carbon offsetting measures for decarbonising the precinct toward carbon neutrality.
Based on the findings of this study, more detailed and comprehensive scenarios incorporating other factors, such as types of densification, evolution over time, hybrid renewable energy system solutions, and energy–water impacts, need to be developed and explored for further analysis on carbon reduction options and effects. In addition, due mainly to limitations in data availability and modelling assumptions, the scope of the scenario design in this paper does not have renewable energy storage systems, electric vehicle (EV) use, and EV batteries to grids, which attract increasing interest in recent research, included in the analysis. Therefore, recommendations for future studies can include further exploring carbon neutral potential with solar energy storage schemes and in-depth analysis of impacts resulting from dynamics of precinct morphology and demographics to inform an optimal planning for low-carbon (re)development of urban precincts.