Thermo-economic analysis and comparative study of transcritical power cycles using CO2-based mixtures as working fluids

Transcritical carbon dioxide (CO2) cycle is a promising method to utilize both low and high temperature heat sources. However, its application is restricted to lower temperature cooling source conditions since the condensing temperature must be lower than the critical temperature of CO2 (about 31.1 °C). In this study, organic fluids are selected to blend with CO2 to overcome the condensation problem by elevating the critical temperature. The effects of several key parameters on thermodynamic and exergoeconomic performances of the system are investigated for different CO2-based mixtures and pure CO2 under both low and high temperature heat source conditions. Single-objective and multi-objective optimizations are carried out to achieve the better system performances. Meanwhile, the performance comparisons between different CO2-based mixtures and pure CO2 are conducted. Results show that transcritical power cycle using CO2-based mixtures could obtain better thermodynamic and exergoeconomic performances compared with the one using pure CO2 for both low and high temperature heat source conversions. Among the CO2-based mixtures and pure CO2, CO2/R32 presents the highest exergy efficiency of 52.85% and CO2/R161 presents the lowest levelized cost per unit of exergy product of 47.909$/MWh for low temperature transcritical power cycle. For high temperature transcritical power cycle, CO2/Propane presents the lower levelized cost per unit of exergy product of 29.212$/MWh.