Yoonhan Ahn

The Design Study of Supercritical Carbon Dioxide Integral Experiment Loop

The Supercritical Carbon Dioxide cycle (S-CO2 cycle) can achieve relatively high efficiency in the moderate temperature (450–750°C) region because the cycle takes advantage of non-ideal properties variation near the critical point. The S-CO2 cycle was originally considered as an attractive candidate for power conversion cycle of the next generation reactors. However due to many benefits of the S-CO2 cycle, it is not only limited to the nuclear application but also considered in other conventional and renewable energy system applications including fossil fuel power plant systems, ship propulsion applications, concentrated solar power systems, fuel cell bottoming power cycles and so on. The major studies settle on the S-CO2 recompressing cycle (also known as Feher cycle) which reduces the waste heat and increases the recuperated heat by recompressing some portion of the flow without heat rejection to increase the thermodynamic efficiency of the cycle. To develop and verify the characteristics of the S-CO2 recompressing cycle, Korean Atomic Energy Research Institute (KAERI) and KAIST research team designed a Supercritical Carbon Dioxide Integral Experiment Loop (SCIEL). 550°C turbine inlet temperature and 20 MPa compressor outlet pressure condition are expected for SCIEL operation and the layout is recompressing cycle but other layouts will be studied as well. The experimental loop facility is designed for studying unique phenomena in components under various conditions and developing the strategy to improve the component performance and overall cycle efficiency. The operating condition and thermodynamic efficiency for SCIEL are evaluated from an in-house code developed by KAIST research team. The effect of the split flow, component sensitivity, and optimum cycle pressure ratio will also be analyzed for the preliminary design of SCIEL. Furthermore, turbomachinery sizes and heat exchanger sizes are estimated from other in-house codes developed by KAIST research team. The overall component specification and performance of SCIEL will be compared to other S-CO2 test loop facilities in other research institutes.Copyright

Investigation of the Bottoming Cycle for High Efficiency Combined Cycle Gas Turbine System With Supercritical Carbon Dioxide Power Cycle

The supercritical CO2 (S-CO2) power cycle has been receiving attention as one of the future power cycle technology because of its compact configuration and high thermal efficiency at relatively low turbine inlet temperature ranges (450∼750°C). Thus, this low turbine inlet temperature can be suitable for the bottoming cycle of a combined cycle gas turbine because its exhaust temperature range is approximately 500∼600°C. The natural gas combined cycle power plant utilizes mainly steam Rankine cycle as a bottoming cycle to recover waste heat from a gas turbine. To improve the current situation with the S-CO2 power cycle technology, the research team collected various S-CO2 cycle layouts and compared each performance. Finally, seven cycle layouts were selected as a bottoming power system. In terms of the net work, each cycle was evaluated while the mass flow rate, the split flow rate and the minimum pressure were changed.The existing well-known S-CO2 cycle layouts are unsuitable for the purpose of a waste heat recovery system because it is specialized for a nuclear application. Therefore, the concept to combine two S-CO2 cycles was suggested in this paper. Also the complex single S-CO2 cycles are included in the study to explore its potential. As a result, the net work of the concept to combine two S-CO2 cycles was lower than that of the performance of the reference steam cycle. On the other hand, the cascade S-CO2 Brayton cycle 3 which is one of the complex single cycles was the only cycle to be superior to the reference steam cycle. This result shows the possibility of the S-CO2 bottoming cycle if component technologies become mature enough to realize the assumptions in this paper.Copyright

Studies of Supercritical Carbon Dioxide Brayton Cycle Performance Coupled to Various Heat Sources

The concern about the global climate change and the unstable supply of fossil fuels stimulate the research of the new energy source utilization and the efficient energy system design. As the interests on the future energy sources and renovating the conventional power plants grow, an efficient and widely applicable power conversion system is required to satisfy both requirements. S-CO2 cycle is considered as a promising candidate with the advantages of 1) relatively high efficiency in the modest temperature (450–750°C) region because of non-ideal properties near the critical point, 2) effectively reduced size of the total cycle with compact turbo-machines and heat exchangers, 3) potential for using in various applications with competitive efficiency and simple layout. The S-CO2 cycle was originally considered as an attractive candidate for the power conversion cycle of the next generation nuclear reactors. However, due to many benefits of the S-CO2 cycle, it is recently considered in other conventional and renewable energy system applications including fossil fuel power plant system, ship propulsion application, concentrated solar power system, fuel cell bottoming power cycle and so on. This paper will discuss about the design of S-CO2 cycle for the various energy system applications over different temperature range. Unlike a large size power plant which usually focuses more on maximizing the cycle efficiency, a small capacity energy system is seriously concerned about the total size of the cycle. In this manner, several preliminary S-CO2 cycle designs will be compared in terms of the efficiency and the physical size. Various layouts and components of S-CO2 cycle are compared to find the optimum cycle for each energy systems. The in-house codes developed by the KAIST research team are used to evaluate the various cycle performances and component preliminary designs. The obtained results will be compared to the conventional power conversion systems along with its implication to other existing designs.Copyright

Design Methodology of Supercritical CO2 Brayton Cycle Turbomachineries

The supercritical CO2(S-CO2) Brayton Cycle is gaining attention due to its high thermal efficiency at relatively low turbine inlet temperature and compactness of turbomachineries. For designing turbomachineries of the S-CO2 Cycle, however, most of existing codes based on ideal gas assumption are not proven yet to be accurate near the supercritical condition. Furthermore, many of existing design computer programs usually focuses on a specific type of turbomachinery, e.g. axial or radial, which makes hard to compare performance of both types at the same design condition. Since both axial and radial types of turbomachineries were pointed out as an equally possible candidate for the S-CO2 Brayton cycle, in order to compare and determine the best effective type of turbomachinery requires considering both types under the same design conditions. Taking into consideration of these facts, some modifications to the conventional design methodology of gas cycle turbomachinery are necessary to design a turbomachinery for the S-CO2 cycle. Especially, a modified design method should consider non-linear property variation of CO2 near the critical point to obtain an accurate result. Thus, the modified design method for the S-CO2 Brayton cycle turbomachineries is suggested in this paper and the method was implemented in the in-house code. In addition, some preliminary results will be discussed with the plan for validation and verification of the code.© 2012 ASME

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

SCO2PE Operating Experience and Validation and Verification of KAIST_TMD

Supercritical carbon dioxide (S-CO2) Brayton cycle has gaining attention due to its compactness and high efficiency at intermediate temperature range of turbine inlet temperature. Thus, many research groups have been trying to develop their own S-CO2 Brayton cycle technology or component design technology. KAIST research team has been trying to develop a S-CO2 turbomachinery design methodology. As a part of this effort, In-House code KAIST_TMD (KAIST Turbomachinery Design) was developed based on open literatures. KAIST_TMD can reflect real gas effect since it uses precise equations and property database rather than ideal gas assumptions. Most special characteristic of KAIST_TMD is that KAIST_TMD can design both of radial type and axial type turbomachineries so it can compare performance of both radial and axial turbomachineries under the same operating conditions. KAIST_TMD provides geometry of turbomachinery and off design performance map also. This research team built a S-CO2 Pump Experiment facility (SCO2PE) to experience the S-CO2 loop operation and to perform validation and verification of KAIST_TMD in near future. Canned motor pump and shell and tube type heat exchanger were installed as the main components of SCO2PE. Main objectives of this paper are to present preliminary experimental data and share the operating experience and troubleshooting of the facility. Data analysis and detailed discussions about an experimental procedure and major issues when pump operates near the critical point will be presented in the paper. As a result, preliminary data were obtained that can be used for improving the facility to increase accuracy of the data for future validation and verification of KAIST_TMD for radial compressor/pump design.© 2013 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

Numerical Investigation of a Centrifugal Compressor for Supercritical CO2 as a Working Fluid

The supercritical carbon dioxide (S-CO2) Brayton cycle is considered as a strong candidate for power conversion systems. This includes concentrated solar power, coal power, bottoming cycle to fuel cells, and the next generation nuclear systems. In the previous studies, it was identified that the compressor consumes very small compressing work as operating condition approaches to the critical point. Thus, smaller amount of input work contributes to the enhancement of overall cycle efficiency. To achieve an efficient S-CO2 cycle, one of the major technical challenges exists in the compressor design. At KAIST, a research team is conducting a S-CO2 compressor tests to obtain fundamental data for advanced compressor design and to measure the performance of the compressor near the critical point. The measurements reveal the S-CO2 fluid to have properties of gases and liquids at the same time, but in regards to compressibility and density variation, its behavior is much closer to the liquid rather than gas near the critical point. In this paper, a CFD analysis of S-CO2 centrifugal compressor with the full geometry including diffuser and volute is presented. The numerical results are compared to the experimental data from KAIST SCO2 Pressurizing Experiment facility. A 3D grid was generated starting from the model of the compressor full geometry provided by the manufacturer. Furthermore, a property table of CO2 was generated by an in-house code and implemented to the CFD code. Then the performance characteristic of S-CO2 compressor is investigated in terms of compressor efficiency and pressure ratio. Additional flow variables inside the compressor such as velocity, pressure and viscosity are also investigated to help understanding the main reason behind the relatively higher compressor efficiency near the critical point compared to other flow conditions far from this region. In general acceptable results in comparison to the experiment are obtained (order of error from 0.5 to 7% for the compressor efficiency). Hence, the current CFD results should be able to provide additional and detailed information to be used for design enhancements of the compressor for S-CO2 Brayton power cycle.Copyright