Seong Jun Bae

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.© 2014 ASME

Preliminary Experimental Study of Precooler in Supercritical CO2 Brayton Cycle

The supercritical carbon dioxide (S-CO2) Brayton power conversion cycle has been receiving worldwide attention because of high thermal efficiency due to relatively low compression work near the critical point (30.98°C, 7.38MPa) of CO2. The S-CO2 Brayton cycle can achieve high efficiency with simple cycle configuration at moderate turbine inlet temperature (450∼650°C) and relatively high density of S-CO2 makes possible to design compact power conversion cycle.In order to achieve compact cycle layout, a highly compact heat exchanger such as printed circuit heat exchanger (PCHE) is widely used. Since, the cycle thermal efficiency is a strong function of the compressor inlet temperature in the S-CO2 power cycle, the research team at KAIST is focusing on the thermal hydraulic performance of the PCHE as a precooler. The investigation was performed by first developing a PCHE in-house design code named KAIST-HXD. This was followed by constructing the designed PCHE and testing it in the KAIST experimental facility, S-CO2PE. The test results of the PCHE were compared to the test results of a shell and tube type heat exchanger as well.Copyright

Comparison of Gas System Analysis Code GAMMA+ to S-CO2 Compressor Test Data

The CO2 compressor control is one of the most important issues to operate a Supercritical CO2 (S-CO2) Brayton cycle with a high thermal efficiency because it is operated near the critical point to reduce the compressing work. Therefore, our research team has accumulated the CO2 compressor data from the S-CO2 compressor test facility called SCO2PE (Supercritical CO2 Pressurizing Experiment). The data can be obtained under various compressor inlet conditions, especially near the critical point of CO2.Despite the growing interest in the S-CO2 Brayton cycle, research on the cycle transient analysis, especially in case of CO2 compressor inlet condition variation, is still in its early stage. So, in this study, the validation and verification of the gas system transient analysis code GAMMA+ is carried out by utilizing the experimental data of SCO2PE. To simulate the SCO2PE by the GAMMA+ code, the code was revised to reflect the compressor performance and add an expansion valve option. Moreover, the NIST database was connected to the GAMMA+ code for more accurate CO2 properties near the critical point. Prior to the transient analysis with the whole SCO2PE loop, major components such as a compressor and a heat exchanger were separately tested with the steady state data of SCO2PE. The loss of cooling water accident was assumed as the transient situation by observing the operating condition variations of the SCO2PE while the mass flow rate of water loop was decreased. Thus, the experimental data of SCO2PE was compared with the revised GAMMA+ code under the planned transient.Copyright