When introducing fuel cell cogeneration into an apartment house, operations in a low-load region that accord with load fluctuation occur frequently. The power generation efficiency of a fuel cell drops in a low-load region. Therefore, in order to keep energy costs low, it is necessary to introduce a method of improving power generation efficiency at the time of low load. Thus, in this paper, the hybrid cogeneration system (HCGS) that uses a hydrogen gas engine (NEG) and proton exchange membrane fuel cell (PEM-FC) together is proposed. The technology of controlling hydrogen concentration and improving the efficiency fall of gas engine at the time of low load is studied. In the gas engine, when hydrogen concentration is increased at the time of low load, the improvement of an exhaust cleanup and brake thermal efficiency is confirmed. So, the method of operating PEM-FC corresponding to a base load, and following NEG at load fluctuation and operating is proposed. There is a target of this study in proposing the energy system excellent in the power generation efficiency and the exhaust characteristic from low load to hi-h load. In this paper, the capacity of fuel consumption, and carbon dioxide emissions are investigated by numerical analysis regarding the introduction of HCGS into apartment houses in Sapporo in Japan. The carbon dioxide emissions of HCGS are very advantageous. Especially HCGS is advantageous when operations in a low-load region occur frequently. For example, concerning the power-demand pattern of a house, when operations in a low-load region occur frequently, HCGS is especially advantageous. It is thought that fuel consumption conditions with little heat demand and the power generation efficiency of HCGS are more advantageous. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.