Hydrothermal energy, which can be used for cooling and heating, is an underused energy source that can be widely applied in a city. Hydrothermal energy has to be produced in areas that fulfill certain geographical conditions, meaning it is mostly useful in areas that can avoid the cost of transporting hydrothermal energy. Various hydrothermal resources are distributed throughout Seoul, such as rivers, tap water lines, sewage, and ground water, and the places of demand are nearby, which is beneficial for its utilisation. The combined available energy of usable rivers, tap water, sewage, and ground water is about 2.3M TOE, which corresponds to about six times the current renewable energy production in Seoul. Specifically, sewage from apartments is the most useful hydrothermal resource. To expand the production of hydrothermal energy, fund and loan facilities are required, and policies such as reorganisation of the energy rate system are needed to save costs. To receive benefits associated with renewable energy such as subsidies, the scope of hydrothermal energy production should be expanded to include rivers, ground water, and tap water. To begin, hydrothermal energy should be applied in all buildings near hydrothermal resources, Then, in the long-term, hydrothermal energy stations should be connected and spread throughout the region to establish a large-scale energy supply system which is linked with district heating networks.