Towards sustainable and resilient coastal cities : opportunities and strategies for integrating seawater and solar energy in sanitation and space cooling

Abstract

Sanitation and space cooling are essential components of "decent living standards", which are closely tied to the advancement of civilization. However, access to these amenities varies across different geographical contexts, and they remain the top two living gaps. With more than half of the world's population residing in coastal regions, where over 50% of the global economy is generated, these areas stand to benefit the most from sustainable sanitation and space cooling. Seawater-involved systems, such as seawater district cooling systems (SWDCS) and seawater toilet flushing (SWTF), as well as the decentralized rooftop solar photovoltaic systems (SPVS), offer effective solutions for sustainable space cooling and sanitation by improving energy use efficiency and renewable energy harvesting. Despite their potential, the unknown long-term environmental and economic impacts of these systems on a city-wide scale across different geographies and economies have hindered their widespread adoption. The lack of an optimal approach to integrating sustainable solutions in different urban contexts further complicates this issue. Decision-makers require a holistic and unified perspective, methods, and tools to make informed decisions about these solutions to promote sustainable development. Therefore, the primary goal of this work is to quantify opportunities and develop strategies for utilizing seawater and solar energy systems to enhance sustainability and resilience in various coastal cities. The specific objectives include i) developing a holistic and generalized water–energy management framework based on sustainability principles, ii) creating quantitative water and energy models that incorporate urban heterogeneity to map the environmental and economic impacts of the three systems, iii) designing analytical tools and decision metrics to achieve the optimal integration of these systems on a city-wide scale, and iv) understanding the strategy variations of these systems in different coastal cities worldwide. Chapter 2 of this thesis presents the first objective, which is to develop a holistic water–energy management framework based on four sustainability principles: customized solutions, efficient resource allocation, comprehensive evaluation, and optimized tradeoffs. This framework includes contextualized location analysis, urban spatial analysis, integrated sustainability assessment, and nexus analysis, which can aid decision-making regarding the technical and policy aspects of seawater uses in sanitation and space cooling. By breaking barriers between sectors and encouraging inter-municipal cooperation, coastal cities can enhance their sustainability and resilience, providing a better quality of life for their citizens. This thesis explores the potential of utilizing seawater and solar energy for sustainable sanitation and space cooling in coastal cities and provides a comprehensive analysis of their outcomes and integration strategies. Chapters 3, 4, and 5 investigate different tailor-made solutions and develop spatial quantitative models and analytical tools to identify opportunities and strategies for enhancing sustainable development through the use of seawater and solar energy. The study analyzes three system integrations, including SWDCS-SWTF (Chapter 3), SWDCS-SPVS (Chapter 4), and SWDCS-SWTF-SPVS (Chapter 5), and examines their outcomes using energy and water models based on GIS data. The results demonstrate that taking into account urban heterogeneity into decisions could result in optimal outcomes supporting seawater use in urban areas (Chapter 3), and the implementation of integrated sustainable solutions generates significant life cycle economic benefits and carbon mitigations (Chapter 4 and 5). Furthermore, Chapter 5 identifies a turning point of cost cutoff in each of the 12 coastal cities worldwide, where carbon mitigation is maximized while maintaining a positive economic impact. The turning points for cost cutoff ranged from 25 to 150 USD/tCO₂e across the 12 coastal megacities, and the life cycle carbon mitigations achieved through the integration of sustainable solutions ranged from 0.6 to 12 billion tonnes of CO₂e. In conclusion, this research highlights the potential of utilizing seawater and solar energy to save water and energy, mitigate carbon emissions, and generate economic benefits in coastal regions, transforming sanitation and space cooling into sustainable development enhancers. However, successful implementation of sustainable sanitation and space cooling requires context-specific system design, optimal implementation strategies, and practical policies. The framework and quantitative analytical tools developed in this study can aid decision-makers in identifying opportunities and exploiting the optimal potential of seawater and solar energy systems for sustainable sanitation and space cooling in diverse contexts, contributing to a sustainable future for the planet.

Date of Award	2023
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Guanghao CHEN (Supervisor) & Di Wu (Supervisor)