Entropy analysis of new proposed Brayton cycle configurations for solar thermal power plants

The present work is driven by two overarching objectives. Firstly, novel and innovative s-CO₂ Brayton cycle configurations are to be evaluated, including Partial Cooling with Recompression and Main Compressor Intercooling−PCRCMCI, PCRCMCI-RH (with Reheat) and PCRCMCI-2RH (with Dual Reheat). The latter configurations are expected to increase efficiency compared to the Partial Cooling with Recompression−PCRC. Secondly, the impact of three CO₂-based mixtures (CO₂/C₂H₆, CO₂/CH₄, and CO₂/Kr) on the efficiency of the proposed novel cycles is assessed. When the mixture attains optimal efficiency, this value increases between 0.2 and 2.4 percentage points compared to use pure s-CO₂ as working fluid. The most efficient working fluid is found to be that comprising s-CO₂ and ethane in a molar fraction of 0.30/0.70, with an efficiency value of 51.3 % for the PCRCMCI-2RH configuration. The mixture comprising s-CO₂ and methane with a molar fraction of 0.70/0.30, was the second most efficient, with a thermal efficiency of 50.5 % for the PCRCMCI-2RH configuration. The study concluded with a comparative analysis of the exergy efficiency of pure s-CO₂ and s-CO₂ mixtures in two topologies (PCRCMCI and PCRCMCI-2RH). The findings of the study demonstrated that the s-CO₂ mixtures exhibited the capacity to enhance exergy efficiency by up to 5.6 percent points. Furthermore, it was concluded that optimising heat transfer properties and designing equipment accordingly has the potential to enhance efficiency in solar field infrastructure. This finding suggests that s-CO₂ mixtures could play a pivotal role in the development of next-generation energy systems.