Adoption of CO2-based binary mixture to operate transcritical Rankine cycle in warm regions

This paper investigates the potential of CO2-based binary mixtures to operate a Rankine cycle with low-grade heat sources in warm regions. Utilizing low-grade heat sources in warm regions is a challenging task due to lower turbine inlet temperatures and higher pump inlet temperatures. A transcritical Rankine cycle with heat recuperation is implemented to minimize the heat losses linked with the isothermal phase change process of the working fluid. A proper selection of the working fluid suitable for a given scenario is one of the crucial tasks to maximize the thermal efficiency of the cycle. Energy and exergy analyses are presented for the cycle running with pure CO2 and CO2-based binary mixtures as a working fluid. Five different organic fluids (n-pentane, cyclopentane, cyclohexane, R1336mzz, and R600) are selected as additives to make a binary mixture. The effect of additive concentration in the mixture on the thermodynamic performance of the cycle is investigated. The cycle is found to perform significantly better if operated with a binary mixture than pure CO2. Binary mixtures of CO2 with n-pentane, cyclopentane, and cyclohexane perform substantially better than the rest with an increase in the thermal efficiency by more than 30% in comparison to the thermal efficiency of the cycle operating with pure CO2. The results indicate that the adoption of the CO2-based binary mixture for a transcritical Rankine cycle not only improves the overall thermodynamic performance but also reduces the operating pressures of the cycle which may lead to less expensive materials needed for the various components of the plant. Exergy analysis reveals that the irreversibility losses in the system during the heat transfer process are much larger than in the turbomachinery.