The electricity demands of buildings for cooling and heating, as well as for office-appliances such as computers, lighting, elevators, keep increasing [Uchiyama2002]. This increase in electricity requirement is directly accompanied by an increase in CO2-emissions. Decentralized energy conversion systems are believed to bring a solution to this ecological problem. In order to assess the ecological and economical performances of such systems, we have developped a set of integrated thermo-environomic models and evaluated the performances of the system in terms of costs and CO2-emissions of a decentralized system, using a hybrid multi-objective optimisation procedure that combines a linear programming problem and a multi-objective optimizer based on evolutionary algorithm (MOO)