SUMMARYWe evaluated total energy consumption and CO 2 emissions in the phases of a city gas utilization system from obtaining raw materials to consuming the product. Assuming monthly and hourly demand figures for electricity, heat for space heating, and hot water in a typical hospital, we explore the optimal size and operation of a city gas system that minimizes the life cycle CO 2 emissions or total cost. The cost-effectiveness of conventional cogeneration, a solar heating system, and hybrid cogeneration utilizing solar heat is compared. We formulate a problem of mixed integer programming that includes integral parameters that express the state of system devices such as the on/off condition of switches. As a result of optimization, the hybrid cogeneration can reduce annual CO 2 emissions by 43% compared with the system without cogeneration. The sensitivity of CO 2 reduction and cost to the scale of the CGS is also analyzed.