Energetic optimization of regenerative Organic Rankine Cycle (ORC) configurations

The present study focuses on the energetic optimization of regenerative Organic Rankine Cycle (ORC) configurations. More specifically, three regenerative ORCs are examined. The first includes an open preheater, in which the bleed stream is mixed with the working fluid exiting the pump of the cycle (O-ORC). The other two configurations include a closed preheater. In the second configuration, the bleed stream is throttled and conveyed to the condenser (CB-ORC), while in the third one, it is repressurized via a secondary pump and recirculates into the evaporator of the cycle (CF-ORC). The systems are optimized for different working fluids, and their energetic efficiencies are estimated and compared to that of a standard ORC (S-ORC). In all cases, the inclusion of a recuperator has also been investigated. In principle, recuperative and regenerative ORCs are mostly suitable for dry fluids, while the critical temperature can also have a positive influence on the performance improvement. Furthermore, it is estimated that while the recuperative S-ORC has a higher efficiency than the non-recuperative regenerative cycles, recuperative O-ORC and CF-ORC exhibit a relative efficiency gain ranging from 4.98% to 8.05% and 6.22% to 9.29%, respectively. The highest efficiency improvement achieved by the CB-ORC, however, is minimal.