Advanced thermodynamic power cycles utilizing carbon dioxide based mixtures as working fluids for high temperature waste heat recovery

This paper proposed CO2 mixtures as working fluids in closed Brayton power cycles using flue gases at relatively high temperature (400-450 °C) as waste heat source. Firstly, a comprehensive selection criterion is defined for choice of working fluids to be employed as additives in CO2 mixtures. Secondly, the thermodynamic properties of the mixtures are calculated at different molar compositions using an appropriate equation of state. The binary interaction parameters involved in the equation of state are obtained with the help of available experimental VLE data or by estimation method in case of non-availability of the VLE data. As a benchmark, the study also investigates the thermodynamic performance of advanced sCO2 cycle layouts to compare with the performance of cycles operating with CO2 mixtures. Sensitivity analysis reveals that the power cycles operating with CO2-Novec5110 mixture (with 0.2 mole fraction of Novec fluids) show 3 percentage points rise in cycle thermodynamic efficiency (0.219 versus 0.252) with lower cycle operating pressures as compared to recuperative with mass split sCO2 cycle. In case of CO2-R134a mixture working fluid (with 0.3 mole fraction of R134a), total efficiency of about 0.15 is obtained at cycle maximum pressure of 200 bars compared to simple recuperative sCO2 cycle with total efficiency of 0.13 at rather higher maximum pressure of 400 bars.
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