To increase the electric power density and efficiency of the thermophotovoltaic system, the studies usually focus on the methods such as photon recycling or the matching of optical properties of thermal emission. As multiple parameters affect each other in the thermophotovoltaic system, the optimization of the geometrical parameters in the system is important. In the study, the thermophotovoltaic system was geometrically optimized to obtain more electric power density and system efficiency in the temperature range determined in the combi boiler. For the optimization, the geometrical parameters such as the emitter thickness and distance between the filter-thermophotovoltaic cell were altered. In the first step of this study, a model of the thermophotovoltaic system was designed for the combustion chamber of the combi boiler. In the second step, the lowest and highest temperature for the emitter were determined as 627.15 K and 1251.15 K, respectively. Finally, the thermophotovoltaic system was optimized by using Comsol Multiphysics in this temperature range. The optimum thickness of the emitter, the optimum distance between the filter-thermophotovoltaic cell, the optimum distance between the emitter-thermophotovoltaic cell and the optimum thickness of the filter were determined as 20 mm, 1 mm, 34 mm and 3 mm, respectively. The electric power density and system efficiency vary between 68.63-501.49 W/m(2) and 1-2.52%, respectively. Also, this system has a power output varying between 27-203 W. The results of the study show that the geometrically optimized thermophotovoltaic system provides more electric power density and system efficiency. Therefore, the optimization of the geometrical parameters is important in the design of the thermophotovoltaic system and should be considered.