FuelCell Energy, Inc. (FCE) has developed products based on its Direct FuelCell® (DFC® ) technology with efficiencies near 50% based on lower heating values of natural gas. DFC is an internally reformed molten carbonate fuel cell, which operates in the range. The combination of the internal reforming of methane and atmospheric pressure and moderately high temperature of operation has resulted in very simple power plant system configurations. Recently, FCE has developed system concepts to further increase the net electric efficiency to beyond 60% efficiency in sub-MW and MW class power plants. One of these system concepts is the arrangement of the fuel cell stacks in series for very high utilization of fuel in the stacks. Although, in principle, the concept of fuel cell stacks in series is very simple, the implementation of the concept in the actual hardware poses challenges requiring innovative solutions. These challenges include concerns with thermomechanical issues, flow and utilization patterns within the fuel cell stacks, and management of the pressure balance between the anode and the cathode. To address these issues, various analytical tools, including system-level modeling and simulation and computational fluid dynamics (CFD), were utilized. FCE has developed a comprehensive fuel cell stack operation simulation model including hydrodynamics, kinetics, electrochemical, and heat transfer mechanisms to investigate and optimize the design for performance as well as endurance. Various system configurations were developed, which included methods for fueling the second tier stacks in the series. System simulation studies using first principle mass and energy conversation laws were performed. Parametric studies were completed. Subsequent to the system modeling results, the fuel cell stack operations were analyzed using the comprehensive stack simulation model. The CFD modeling of the fuel cell stacks was performed in support of the system simulation parametric studies. The results of the CFD modeling provided insight to the thermal and flow profiles of both first and second tier stacks in series. The net outcome of the investigation was the design of the system, which met the goals of ultrahigh efficiency and yet complied with the thermomechanical requirements of the fuel cell stack components. In this paper, FCE will describe various system options for the very high efficiency systems, the issues related to the design, and the practical solutions to overcome the issues.