Exergetic performance and comparative assessment of bottoming power cycles operating with carbon dioxide-based binary mixture as working fluid

This paper presents CO2-toluene (CO2-C7H8) binary mixture as working fluid to enhance the energetic and exergetic performance of CO2 bottoming power cycles in warm ambient conditions. A criterion for selection of CO2-based binary mixture is defined, and 0.9 CO2/0.1 C7H8 composition is decided based on the required minimum cycle temperature compatible with ambient conditions. Bottoming simple regenerative cycle (BSRC) and bottoming preheating cycle (BPHC) configurations are selected, and their realistic operating conditions are determined based on sensitivity analysis. The performance of bottoming cycles using CO2-C7H8 binary mixture is compared with the bottoming cycles using pure CO2 as working fluid at different ambient temperatures. It is observed that the cycles operating with pure CO2 can only perform better at lower ambient temperature conditions, whereas, at the increased ambient temperatures, bottoming cycles with CO2-C7H8 binary mixture outperform and produce significant gains in exergetic and energetic performance compared with pure CO2 bottoming cycles. A maximum gain of exergetic efficiency for BSRC and BPHC observed is 26.83% and 18.71%, respectively, at an operating ambient temperature of 313 K, whereas an overall gain in energetic efficiencies for BSRC and BPHC observed is 28.92% and 10.12%, respectively. Taking into consideration thermodynamic performance, overall UA (product of overall heat transfer coefficient and heat transfer area for the heat exchanger) and specific investment cost, BPHC configuration is suggested as reasonable choice for higher ambient temperature conditions.