Abstract
Abstract In the near future, due to the growing share of variable renewable energy in the electricity mix and the lack of large-scale electricity storage, coal plants will have to shift their role from base-load operation to providing fluctuating back-up power. However, current coal power plants, based on steam Rankine cycle, are not optimized for flexible part-load operation, resulting in an intrinsic inadequacy for fast load variations. The founding idea of the H2020 sCO2-Flex project is to improve the flexibility of pulverized coal power plants by adopting supercritical CO2 Brayton power cycles. Despite the extensive literature about the design of sCO2 plants, there is still limited discussion about the strategies to be implemented to maximize system efficiency during part-load operation. This paper aims to provide deeper insight about the potential of sCO2 power plants based on recompressed cycle with high-temperature recuperator (HTR) bypass configuration for small modular coal power plants (25 MWel). Analysis focuses on both design and part-load operation providing a preliminary sizing of each component and comparing different operating strategies. Results demonstrate that sCO2 coal power plants can achieve competitive efficiency in both nominal and part-load operation thanks to the progressive increase of heat exchangers effectiveness. Moreover, they can be operated down to 20% electric load increasing power range of coal plants. Finally, the possibility to optimize the cycle minimum pressure ensures a safe operation of the compressor far from the surge line and to increase the performance at low load.
Highlights
Supercritical CO2 cycles for power generation are gaining a large interest from industry, institutions and academia as demonstrated by the large amount of investments, funded projects and research papers
ANNUAL RESULTS From historical power production data, it is evident that, except from some niche market where coal is abundant and cheap or where renewable energies contribution is low, Ultra Super Critical (USC) coal power plants are experiencing a continuous reduction of their annual capacity factor
The rapid increase of solar photovoltaic and wind energy arises issues related to frequency stabilization on the grid because of the random behavior of non-predictable renewable energy sources (NP-RES)
Summary
Supercritical CO2 cycles for power generation are gaining a large interest from industry, institutions and academia as demonstrated by the large amount of investments, funded projects and research papers. From closed Joule-Brayton cycles using for example He and N2 that operate in the ideal gas region, in sCO2 power cycles the main compressor is generally designed to operate very close to the fluid critical point in a region characterized by marked real gas effects (i.e. a region where the gas has a compressibility factor Z significantly lower than 1) [5]. For these systems, cycle depressurization at partial load may cause a significant variation of fluid properties along the compression with an efficiency penalization that may jeopardize the overall plant performance. Two different operating strategies are investigated for a recuperative recompressed cycle configuration provided by High Temperature Recuperator (HTR) bypass selected within the sCO2-Flex project [6]
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