District heating systems using seasonal thermal energy storages: a comprehensive literature review

District heating systems represent a valid alternative for cities to move towards resource-efficient and low-carbon pathways. In particular, solar district heating is a promising option, where the drawback related to the mismatch between summer supply and winter demand could be tackled by using a large seasonal thermal energy storage. This kind of tank allows to store thermal energy over weeks and months, with it being particularly important for communities where solar energy availability is high during the summer in contrast to a large demand during the winter. This study reports a comprehensive review on district heating systems using seasonal thermal storages across a broad set of case studies reported in current literature, classifying them in terms of operating schemes, location, typology of seasonal thermal energy storage, characteristics of end-users, technologies for centralised and local energy generation as well as components size. An awareness and understanding of the many factors discussed in this paper are essential in order to improve the reliability of district heating or cooling installations.