Simultaneous optimal design of multi-stage organic Rankine cycles and working fluid mixtures for low-temperature heat sources

• An optimization formulation for multistage Rankine cycles with mixtures is proposed.
• The efficient energy recovery from low-temperature energy sources is addressed.
• The results show that the multistage approaches are useful for the two-stage case at low temperature in terms of efficiency.
• The results indicate that the single stage approach can be effective as the process temperature increases.

Energy recovery is a process strategy seeking to improve process efficiency through the capture, recycle and deployment of normally neglected low energy content sources or streams. By proper optimal process design, such low-temperature energy sources can be a feasible and economical manner of approaching the energy recovery issue. In particular, when Rankine cycles with mixtures as working fluids are used, the amount of energy recovery can be improved. The formulation and systematic solution of this problem has shown better results when all the variables of the Rankine cycle and the compositions of the working fluid are considered simultaneously. Another interesting approach is the implementation of multiple cycles coupled together. In this work we propose a nonlinear optimization formulation of two general multistage approaches for the Rankine cycle with mixtures: the cascade and series configurations. As main decision variables, we have considered the heat source conditions and the mixture components. Then, the resulting optimization problem is solved in a deterministic approach as a nonlinear program. The results shown that for some cases the multistage configurations are useful but limited in terms of cost in comparison to the single stage cycle.