A state-of-the-art analysis and perspectives on the 4th/5th generation district heating and cooling systems

The evolution of district heating systems is characterized by a trend towards lower supply temperatures and higher energy efficiencies. The most recent advancement in this field is the fifth-generation district heating and cooling systems (5GDHCS). Current literature identifies three key features of 5GDHCS: operating at near-ambient temperatures, the use of distributed booster heat pumps at end-users, and the capability to provide both heating and cooling services. However, there is ongoing debate on whether 5GDHCS represents a significant technological leap beyond its predecessor, the fourth-generation district heating system (4GDHS). To resolve fundamental questions surrounding the 5GDHCS concept and to highlight its distinctive features, this paper conducts a detailed comparison between 5GDHCS and 4GDHS, examining their functional, technological, and operational differences. Based on this, limitations in existing definitions of 5GDHCS are identified, and a refined definition better characterizing its unique attributes and advancement is suggested. Additionally, considering the 5GDHCSrelated studies are growing fast, with over 30 works published each year since 2021, this paper tries to complement previous reviews with an updated perspective on its latest research progress and practical deployment, through a comprehensive state-of-the-art analysis. Results show that while 5GDHCS is a promising technology to decarbonize domestic heating and to promote sector coupling between electric power and district heating systems, it is still in the early stage of development, marked by substantial research gaps in technical, policy, regulatory, and market aspects. Analysis on an exhaustive collection of real-life cases also indicates a long way ahead to achieve its widespread commercialization.