Adoption of the CO₂ + SO₂ mixture as working fluid for transcritical cycles: A thermodynamic assessment with optimized equation of state

This paper focuses on the use of the CO₂ + SO₂ binary mixture as innovative working fluid for closed transcritical power cycles with a minimum temperature above 50 °C. Starting from a literature review of the available experimental data on the mixture, the PC-SAFT EoS is identified as a suitable model to characterize the mixture behavior. Once the proper thermodynamic model is selected for this mixture, a comparison between the innovative transcritical cycle and the sCO₂ cycle is proposed for various plant layouts in order to find out the advantages of the innovative mixture. The analysis is presented fixing the cycle maximum temperature at 700 °C and the maximum pressure at 250 bar: the results depict an increment in cycle electric efficiency and cycle specific work, along with a lower temperature of heat introduction in the cycle for any considered configuration of transcritical CO₂ + SO₂ cycle, when compared to pure sCO₂. An economic analysis of the power block is then performed to support the selection of the innovative working fluid. Two of the most promising plant layouts are evidenced: the recompression layout is selected for highly efficient power blocks, while the dual recuperated layout works effectively in applications characterized by higher hot source exploitation. The recompression layout adopting the CO₂ + SO₂ mixture presents a power block electric efficiency of 48.67% (2.33% higher than the respective sCO₂ cycle) and a reduction of the power block CAPEX from 1160 $/kW_el to 1000 $/kW_el when compared to the sCO2 configuration for a 100MW_el size, while the dual recuperated layout exploiting the CO₂ + SO₂ mixture shows a power block electric efficiency of 39.58% (0.69% above the same sCO₂ cycle), a decrease of power block CAPEX from 795 $/kW_el to 718 $/kW_el and 70 °C of additional heat recovery from the hot source with respect to the analogous sCO₂ cycle.