District energy systems can potentially decrease the CO2 emissions linked to energy services, thanks to the implementation of polygeneration energy conversion technologies, connected to buildings over a network. To transfer the energy from centralized technologies to the users, conventional district energy systems use water with often two independent supply and return piping systems for heat and cold. This type of network is subjected to exergy losses due the temperature glide of the water and because of the level of temperature at which it operates. Furthermore, water based networks need a total of four pipes to supply heating and cooling, which is space consuming in a constraint urban environment. The present CO2 network concept aimes at improving the efficiency of the heating and cooling in urban areas, by allowing waste heat recovery and a maximum of synergy between users. In addition it could be used in the future as a CO2 collecting network as well as a part of new electricity storage schemes.