The potential of transcritical cycles based on CO mixtures: An exergy-based analysis

Abstract

This paper focuses on the thermodynamic comparison between pure supercritical Carbon Dioxide and blended transcritical Carbon Dioxide power cycles by means of a thorough exergy analysis, considering exergy efficiency, exergy destruction and efficiency losses from Carnot cycle as main figures of merit. A reference power plant based on a steam Rankine cycle and representative of the state-of-the-art (SoA) of Concentrated Solar Power (CSP) plants is selected as base-case. Two different temperatures of the energy (heat) source are considered: 575 ◦C (SoA) and 725 ◦C (next generation CSP). Compared to SoA Rankine cycles, CO2 blends enable cycle exergy efficiency gains up to 2.7 percentage points at 575 ◦C. At 725 ◦C, they outperform both SoA and pure CO₂ cycles with exergy efficiencies up to 75.3%. This performance is brought by a significant reduction in the exergy destruction across the compression and heat rejection process rounding 50%. Additionally, it has been found that the internal condensation occurring inside the heat recuperator for those mixtures with a large temperature glide improves recuperator exergy efficiency, supporting the use of simpler layouts without split-compression. Finally, CO₂ blends exhibit lower cycle exergy efficiency degradation than pure sCO₂ in the event of an increase in the design ambient temperature.