The potential of supercritical cycles based on CO₂ mixtures in Concentrated Solar Power plants: an exergy-based analysis

Abstract

This paper, developed in the context of the SCARABEUS project funded by the Horizon 2020 programme of the European Commission, focuses on the thermodynamic comparison between pure supercritical Carbon Dioxide and blended transcritical Carbon Dioxide power cycles by means of a thorough exergy analysis. A reference power plant based on a steam Rankine cycle and representative of the current state of the art of Concentrated Solar Power plants is selected as base-case. Afterwards, four cycles are added to the comparison. Two of these cycles employ pure CO2, with either a Recompression or a Partial Cooling layout, whereas two cycles employ CO2-based mixtures with either Hexafluorobenzene (CO2-C6F6) or Titanium Tetrachloride (CO2-TiCl4) with a Precompression and a Recuperated Rankine. The figures of merit used to carry out the second-law analysis are exergy efficiency and exergy destruction in the main components of the cycle. Two different cases are identified, corresponding to two temperatures of the energy (heat) source: 575oC and 725oC. The first one is representative of the peak temperatures achieved by the molten salts used in modern Concentrated Solar Power plants. 725oC will expectedly be achieved by next generation systems and it is hence assessed with the aim to unfold the true potential of the concept proposed. The results show that at 575oC pure sCO2 power cycles are clearly outperformed by steam Rankine cycles whilst, at 725oC, they are able to achieve higher thermal and exergy efficiencies, in the order of 49% and 72% respectively. When compared to state-of-the-art Rankine cycles using steam, blended-sCO2 power cycles enable thermal efficiency gains of up to 1.1 and 6 percentage points at 575oC and 725oC respectively, with exergy efficiencies of up to 75.2%.