District energy systems could be used together with centralized and decentralized energy conversion technologies in order to reduce the amount of CO2 emissions. One way to transfer the produced energy from these technologies to users is to implement the conventional district system with water. However, this will result in huge energy and exergy losses and high costs. One alternative is to use CO2 for heating and cooling purposes. Here, in the first chapter the advantages and disadvantages of CO2 based district energy system compared to conventional one will be illustrated. In the next Chapter, typical energy conversion technologies which can be used in the new system will be presented. Finally, a test case will be demonstrated using and in-house energy optimization tool, OSMOSE for multi period optimization.