Hybrid System of Supercritical Carbon Dioxide Brayton Cycle and Carbon Dioxide Rankine Cycle Combined Fuel Cell

The Supercritical Carbon Dioxide (S-CO₂) Brayton cycle has been receiving a lot of attention because it can achieve compact configuration and high thermal efficiency at relatively low temperature (450∼750 °C). However, to achieve high thermal efficiency of S-CO₂ Brayton cycle, it requires a highly effective recuperator. Moreover, the temperature difference in the heat receiving section is limited for the S-CO₂ Brayton cycle to achieve high thermal efficiency results in high mass flow rate and potentially high pressure drop in the cycle. Thus, to resolve these problems while providing flexibility to match with various heat sources, authors suggest a hybrid system of S-CO₂ Brayton and Rankine cycle. This hybrid system utilizes the waste heat of the S-CO₂ Brayton cycle as the heat input to the Carbon Dioxide (CO₂) Rankine cycle. Thus, the recuperator effectiveness does not always have to be high to achieve high efficiency, which results in reduction of the recuperator volume reduction. By controlling amount of the heat transfer from the cooler of the S-CO₂ Brayton cycle to the Rankine cycle, the total system can be compact and can achieve wider operating range. Thus, the hybrid system of S-CO₂ Brayton cycle and CO₂ Rankine cycle can be coupled to various heat sources with more flexibility without trading off the performance. In this paper, Molten Carbonate Fuel Cell (MCFC) system is selected to demonstrate the feasibility of the proposed hybrid cycle system while comparing the proposed system’s performance to that of other cycle layouts as well.