Systematic embodied carbon assessment and reduction of prefabricated buildings in Hong Kong

Abstract

Buildings contribute significantly to life cycle carbon emissions. As a sustainable construction approach, the prefabrication strategy has been advocated by the Hong Kong government for all public housing developments. Carbon emissions generated from buildings are classified as embodied and operational carbon, and recently embodied carbon accounts for an increasing share of the life cycle emissions of new buildings with improved energy efficiency. Despite many studies have assessed prefabricated buildings’ embodied carbon, the system boundaries have seldom been made explicit. Reported results of different studies still display large unjustifiable variations, whereas the multifarious variables have seldom been examined systematically. In addition, no unified embodied carbon assessment model or reporting format of prefabricated buildings has been established in Hong Kong. Moreover, the carbon assessment process incorporates various uncertainties that may result in misinterpretation; however, few studies have explored them. Furthermore, embodied carbon reduction strategies have been explored mainly from the “technical potential” aspect, whereas the “social potential” has seldomly been examined until now.

Therefore, the aim of this research is to contribute a better systematic understanding of the embodied carbon assessment and reduction of prefabricated high-rise public residential buildings in Hong Kong. The research was conducted through a literature review, interviews, site surveys, and case studies. Firstly, a four-fold philosophical framework of system boundaries of buildings’ embodied carbon was established based on the Dialectical System Theory. Secondly, the methodological variables affecting the carbon results were identified via a literature review under the framework of this theory and their influences were examined through a three-step normalization and a comparative analysis of 244 information-rich case buildings collected from published studies during the last two decades. Thirdly, a systematic multi-level embodied carbon assessment model was developed for calculating and reporting embodied carbon at different levels of unit of analysis, i.e. material, component, assembly, flat, and entire building. Fourthly, a Data Quality Index (DQI)-based Monte Carlo Simulation (MCS) method was applied for parameter uncertainty analysis using SimaPro 9.0 software, followed by a scenario analysis for addressing the scenario and model uncertainties. The developed multi-level assessment model and the uncertainty analytical tools were contextualized and validated using a 30-story public rental housing (PRH) block in Hong Kong. Apart from the empirical validations, comparative and peer-review external validations were conducted. Finally, a socio-technical transition framework was established to examine embodied carbon reduction strategies in a systematic manner. Recommendations were proposed after considering policy guidance, technique improvement, and stakeholder engagement followed by a range of semi-structured interviews for validation.

The results show that buildings’ embodied carbon is affected by twelve variables, namely, building service life, life cycle stage, building element, modeling approach, width of inputs, functional unit, data source, building typology, building density, building structure, building location, prefabrication rate in the temporal, spatial, procedural and physical dimensions. It provides a theoretical foundation for identifying variables that affecting the cross-case comparisons. The analysis of the 244 cases emphasizes a remarkable variation of assessment results when selecting modeling approaches, data sources, and building structures. It bridges the knowledge gaps by addressing the quantitative influences of variables compared with traditional qualitative studies. The averaged embodied carbon of the case building was calculated as 561 kg CO2e/m2, mainly generated from the material manufacturing and prefabrication. The five-level model pushes the embodied carbon assessment of prefabricated buildings in a new and innovative direction for more effective calculating and benchmarking. The uncertainty analysis shows that a combined data-quality indicator (DQI)-based Monte Carlo simulation (MCS) method can better facilitate analysis of parameter uncertainty in a building’s embodied carbon evaluation, and the different selections of scenarios result in a wide range of alternative outcomes. Finally, the developed socio-technical transition framework indicates that solely focusing on examining the technical solutions of embodied carbon reduction may result in failure if the policy instruments and the stakeholder engagements are ignored. It provides new insights into carbon reduction by bridging the knowledge gaps between the technical potential and what is achieved in practice, which should have implications for future research and achieving nearly/net zero carbon targets by 2050. The successful implementation of these strategies in Hong Kong will also act as a guidance toward low carbon development for other high-rise high-density cities.