Urban heat transitions: Renewable integration, digital system optimization, and life-cycle perspectives for fifth-generation district heating and cooling systems

As urban areas face mounting climate and energy challenges, district heating and cooling systems are emerging as critical infrastructures for sustainable thermal transitions. This paper explores the evolution of district heating and cooling technologies toward renewable integration, digital optimization, and life-cycle sustainability from a system-level perspective. Drawing on over 112 peer-reviewed studies, the review provides a focused and integrative synthesis of fourth- and fifth-generation district heating and cooling developments, including low-temperature and bidirectional networks, wastewater and surface water heat recovery, shallow geothermal, and hybrid renewable configurations, while highlighting the enabling role of digital tools such as predictive control, artificial intelligence based optimization, and smart metering in enhancing network-level performance and flexibility. Furthermore, the study emphasizes the importance of life-cycle assessment as a comparative and methodological framework for informing environmentally responsible design and policy decisions. Despite technical and institutional challenges, the transition toward low-carbon and intelligently managed district heating and cooling networks offers significant opportunities for climate mitigation, urban resilience, and local value creation, providing an integrated yet scope-defined knowledge base to support researchers, planners, and decision-makers in shaping the future of urban heating and cooling systems. • •Recent advances in fifth-generation district heating and cooling systems are reviewed. • •Renewable and ambient heat integration approaches in urban networks are discussed. • •Digital tools (AI, MPC, digital twins) for optimized DHC operation are examined. • •Life-cycle sustainability outcomes across DHC system generations are analyzed. • •Key technical, economic, and policy barriers to widespread adoption are identified.