District heating and cooling (DHC) systems have attracted interest in reducing CO2 emissions. A fifth-generation DHC (5GDHC) system that supplies a heat-transport medium at a temperature close to the ground and uses various unused heat sources is proposed. Meanwhile, some DHC systems installed in Japan are first-generation DHC (1GDHC) systems that use fossil fuels as a heat source. Based on these scenarios, this study focuses on the energy-saving effects of introducing a CO2 network system in Tokyo, which is based on the same concepts as the 5GDHC systems. The purpose of this study is to estimate the utility value of unused heat sources for the CO2 network system. An analytical model is developed that calculates the energy consumption of the system under exogenous conditions, such as those for energy consumers and heat sources. Applying a model to a domestic urban area in Tokyo, where a conventional 1GDHC system was introduced, it was confirmed that for a total head of groundwater of 50 m, river water of 10 m, and sewage water of 10 m, the potential annual CO2 reduction is approximately 5,000 ton/y compared with the 1GDHC system. These values are feasible in Tokyo, and therefore, this system looks promising.
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