sCO2 power plants for waste heat recovery: design optimization and part-load operation strategies

This paper focuses on the potential of supercritical carbon dioxide closed cycle for waste heat recovery applications. The valorization of waste heat released from glass, steel and cement production facilities is recognized as one of the most effective solutions for the reduction of carbon footprint of the industrial sector. Common so-lutions rely on steam Rankine cycles and organic Rankine cycles while only few sCO2 power plants have been manufactured and operated so far as this technology has not yet reached the technical and commercial maturity. In spite of the large interest on sCO2 power plant from industry, institutions and academia, the role of this so-lution in future waste heat recovery applications is still unclear, highlighting the need of research studies focused on the performance assessment of these novel systems in both design and off-design conditions. This paper aims at bridging the gap between preliminary numerical studies and the design of real power systems by focusing on different aspects scarcely investigated in literature. The first section of this study deals with cycle design and provides a full description of the numerical complexity related to sCO2 power plant optimization, with a detailed description of the assumptions and the models implemented for the design of cycle components. Five different cycle configurations for the exploitation of a heat source consisting of a 50 kg/s stream of flue gas at 550 ◦C have been analyzed and optimized: results are presented with a set of sensitivity analyses and the most promising configurations are analyzed from both a thermodynamic and a techno-economic perspective. Simple recuperative cycle, simple recuperative cycle with recuperator bypass and turbine split flow configurations are compared in detail proving that sCO2 technology can reach overall efficiencies up to 27.5%, a value higher than ORC for the same power output (around 6 MWel), and with a similar specific cost (2000 $/kW). Simple recuperative cycle with recuperator bypass is selected as the most promising configuration and it is further studied in off design conditions in the second section of the paper. Five different part-load strategies have been implemented allowing to assess the part-load performance of the selected cycle, considering both variable CO2 inventory and constant CO2 inventory systems. Results highlight that sCO2 recuperative cycles equipped with a CO2 storage vessel present a very high and almost constant efficiency down to 50% of the normalized flue gases mass flow rate, while a lower efficiency is expected for constant inventory systems. Both solutions can be operated down to 30% load with no difficulties on system components and with a minimum plant efficiency still competitive against ORC technology