An ecological-thermodynamic approach to urban metabolism

Cities have evolved as centers of economic growth but are also responsible for high resource consumption and greenhouse gases emission. Rapid urbanisation due to increasing human population and resource-intensive economic activities draw concerns on urban sustainability issues in future development. In the urban metabolism framework, cities are often described as open systems where the intake of resources is heavily dependent on flows imported from the external environment to sustain economic growth. However, this raises a key question: how much of the resource available in cities is effectively consumed? To investigate the effectiveness of urban resource use, an ecological-thermodynamic approach is taken to develop an open system network effectiveness analysis (OSNEA) framework as a novel tool for urban sustainability assessment. The contributions of resources imported are maximised when the usefulness of the energy available, namely exergy, is fully utilised by destroying exergy through energy conversion processes or exported to other regions in exchange for generated capital flows into the city. Meanwhile, only a minimal amount of low-quality energy is rejected out of the system as waste. In this aspect, OSNEA introduces a set of effectiveness performance metrics to assess the ability of a system to utilise high exergy (high quality energy) and convert the resources available into economic benefits. Having incorporated the techniques from material flow analysis, ecological flow analysis, input-output analysis, exergy analysis and extended-exergy analysis, the framework formulates and translates the incoming flows into a network based on the material exchanges between different urban processes and economic activities in order to account for the metabolism of the whole system. On the downside, the use of exergy as a metric for resource effectiveness is open to challenge as it is an extensive thermodynamic property derived from the study of energy systems and may be regarded as metaphorical and not literally applicable to physical processes in reality. The thesis first presents an initial case study of the cities in England and Wales in Chapter 3 to investigate intra-city metabolism of the urban economies (monetary exchanges between economic sectors) by showing the degree of consumption-control and production-dependency relationships in their hierarchical structure. The case study addresses the shortcomings of existing urban metabolism methods when using input-output monetary transactions as resource supply and use to account for the metabolic activities in cities. However, a framework that considers the differentials in energy and material intakes through urban systems is absent from the existing assessments. OSNEA has been developed to fill this gap, Chapter 4 outlines details of the development workflow and data requirements of this new method demonstrated in this work. To verify and validate the framework, Chapter 5 demonstrates the application of OSNEA in a case study of Singapore representing a single-city model to account for the effectiveness of resource use in the city-state. The trajectory of effectiveness results throughout the years suggests a trade-off relationship between the producers and consumers to balance the production and consumption of resources in the city. Chapter 6 extends the application of OSNEA to study the urban systems of Great Britain as a multi-city model and develop a clustering taxonomy of resource use behaviours of the cities across the whole systems of cities. Next, Chapter 7 provides a synthesis of the key findings and discussion from all studies. Finally, Chapter 8 gives a conclusion and recommendations for further work. The essence of this work is the introduction of effectiveness indicators as a set of novel performance metrics to evaluate the states of resource use in the network based on the differential in resource quantity and quality between the cross-boundary inflows and outflows of the system. OSNEA promotes strategic resource management and transition to circular economy by enabling greater understanding of the sectoral interdependencies within the economy based on their supplies and demands to facilitate flow redirection to where the resources can be utilised effectively. The goal is to provide pathways for retaining and circulating resources in flow cycles for as long as possible to promote higher utilisation rate and reduce the demand for new material extraction. This will help in driving national and urban development towards the United Nation's Sustainable Development Goals for Sustainable Cities and Communities (SDG 11) and Responsible Consumption and Production (SDG 12).