16:00
Session 3B: CO2 Cycles
Chair: Ennio Macchi
16:00
20 mins
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Systematic Evaluation of Efficiency Improvement Options for sCO2 Brayton Cycles
Mathias Penkuhn, George Tsatsaronis
Abstract: Nonconventional working fluids have the potential to improve conventional, high-temperature thermal power generation processes and to enable the efficient exploitation of low-temperature heat resources. In particular, supercritical CO2 (sCO2) based power cycles have gained increased attention because of their potential to offer increased cycle efficiency, improved process economics, and operational flexibility. Various simple and more complex cycle layouts have been suggested and analyzed with the objective of improving cycle efficiency. However, with respect to potential further development and commercialization, the right balance between cycle efficiency, design complexity, and economics is required.
The present study analyzes possible pathways to improve the design of generic sCO2 Brayton cycles for power generation based on a structured pattern known from conventional watersteam cycles. Starting from a simple cycle design, different options such as preheating by internal recuperation, intercooled compression, reheating, and splitrecompression are investigated and their impacts on cycle efficiency and complexity are evaluated. The application of an exergy analysis for each design provides the possibility to identify the location and magnitude of thermodynamic inefficiencies for each design option. With the use of a complexity measure, the offset between efficiency improvement and the increase in system size and complexity can be quantified in the absence of economic data. The combined results thus provide a possibility to identify and evaluate promising options in system design for sCO2 cycles.
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16:20
20 mins
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Test on a CO2-based Transcritical Power Cycle (CTPC) under Various ENGIEN Conditions
Ligeng Li, Hua Tian, Gequn Shu, Lingfeng Shi
Abstract: CO2-based transcritical Power Cycle (CTPC) could be used for engine waste heat recovery as the safety and environment-friendly characteristic of fluid, which also satisfies miniaturization demand of recovery systems. Through previous investigation, the results showed that waste heat of exhaust gas and engine coolant can be combined and highly recovered by the CTPC, while other ORCs showed lower utilization capacity of engine coolant. In this study, a kW-level CTPC system was constructed as the bottoming system and experimentally investigated to recover waste heat from exhaust gas and engine coolant of a heavy-duty diesel engine (DE). Test was based on constant operating condition of CTPC, while operating condition of the DE considered the variation engine speed and engine torque. The CTPC system performance changing with different waste heat conditions are main focus points of this study. Observations of key states as well as estimations and comparisons of potential output power were carried out stepwise. Results indicated that performance of CTPC showed following trend of increase with an increase in waste heat conditions, which were caused by increasing engine speed and engine torque.
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16:40
20 mins
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Optimization of the Part-load Operation Strategy of sCO2 Power Plants
Dario Alfani, Marco Astolfi, Marco Binotti, Ennio Macchi, Paolo Silva
Abstract: Supercritical CO2 cycles for power generation are gaining a large interest from industry, institutions and academia as demonstrated by the large amount investments, founded projects and research papers. This attention is motivated by the potential of sCO2 technology of replacing conventional steam plants in a number of applications and likely to play a relevant role in the future energy scenario. The H2020 sCO2-flex project is studying the possible application of sCO2 cycles in coal fired power plants in order to enhance their flexibility and ease the integration with non-dispatchable renewable energy sources such as wind and solar. Main advantages of sCO2 power plants with respect to USC technology are: (i) potential higher efficiency, (ii) compactness of the turbomachinery, (iii) fast transients and (iv) high performance at part-load. The first two figures have been numerically evaluated in different independent studies while the assessment of flexibility still lacks of deep investigation. This study focuses on the last topic with the aim of defining the best part-load operation strategy for a power plant based on sCO2 cycles with different layout. Differently from Joule-Brayton closed cycles using perfect gases (He, N2), in sCO2 power plants the main compressor is generally designed to operate very close to the fluid critical point in a region characterized by marked real gas effects. For these plants, cycle depressurization may involve a significant variation of fluid properties along compression with an efficiency penalization that may jeopardize also the overall plant performance. The optimization of the part-load operation of sCO2 power plants is scarcely studied in literature and the main unknowns regard the design and the operation of turbomachinery. Different cases are investigated in this work referring to a recuperative cycle configuration and considering the combinations of component features: (i) turbine and compressor (fixed/variable velocity, with or without variable geometry); (ii) heat rejection unit (fixed/variable fan speed), inventory (variable/fixed). For each design combination the best operation strategy, in terms of system efficiency, is proposed providing a numerical estimation of the part-load performance attainable with sCO2 power plants and highlighting suggested design criteria for the turbomachinery.
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17:00
20 mins
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Thermo-economic Comparison of Organic Rankine and CO2 Cycle Systems for Low-to-medium Temperature Applications
Xiaoya Li, Jian Song, Michael Simpson, Kai Wang, Paul Sapin, Gequn Shu, Hua Tian, Christos Markides
Abstract: Organic Rankine cycle (ORC) power systems are a well-established technology for low-to-medium temperature heat-conversion applications. Recently, CO2-cycle power systems have emerged as another promising heat-to-power conversion technology and have been receiving increased interest due to certain advantages offered by the working fluid (e.g., non-flammable, high-temperature stability) and system compactness. However, it remains a challenge to select the appropriate technology between ORC and CO2 power-cycle systems for different applications, as these can span a wide range of scales and heat-source conditions, particularly from an integrated thermodynamic and economic perspective. This paper presents a comprehensive thermo-economic comparison of ORC and CO2 power systems with various architectures (i.e., with or without recuperation) in the specific context of power generation from two representative low- and medium-temperature heat sources, namely, brine in geothermal heat applications, and exhaust gases in waste-heat recovery from internal combustion engine applications. Expansion devices suitable for the given power scales, i.e., reciprocating-piston expanders and radial-inflow turbines, are considered and compared using comprehensive component-level models. Based on thermodynamic and economic performance analyses, technology selection maps for power generation from the pre-defined low- and medium-temperature heat sources are generated using the net power output and specific investment cost as key performance indicators. These performance selection maps allow for quick and effective decision making in choosing the optimal power cycles and system designs for geothermal exploitation, engine waste-heat recovery and other relevant heat-to-power applications.
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