A state of art review on the district heating systems

District heating (DH) has been widely acknowledged as the future for urban heating, as concerns of sustainability and greenhouse gas emissions are pushing, to revamp the heating sector. Although not a new technology, DH has evolved over four different generations, and is now capable of incorporating low temperature distributed renewable heat sources. The ongoing advancements and typical characteristics of a complete grid are researched upon in this paper. The research is extended to link up these characteristics with some of the most interesting DH networks in the world. The technical configurations of these DH grids along with their regulations and policies are explored upon, to understand their evolution and uniqueness. A basic overview of the economics and social aspects are presented as well. The overall trends and results of this study show that, as fossil fuel prices fluctuate and are depleting, there is a unified effort to promote DH, especially for residential heating purposes. The characteristics of modern DH are evolving to cater for distributed renewable technologies with the aim of making the entire system carbon free and sustainable.