Optimization of the Part-load Operation Strategy of sCO2 Power Plants

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.