Increased power production through enhancements to the Organic Flash Cycle (OFC)

The previously proposed Organic Flash Cycle (OFC) was shown to potentially improve power generation from finite thermal energy reservoirs. The OFC's primary advantage was improved temperature matching to the energy reservoir, thereby reducing exergy destruction and losses during energy transfer to the cycle working fluid. This advantage was negated though by irreversibilities introduced by flash evaporation. The present study proposes several design enhancements to the OFC. A theoretical analysis is conducted using the BACKONE, Span–Wagner, and REFPROP equations of state and ten aromatic hydrocarbon and siloxane working fluids previously suggested are reexamined. A comparison and discussion of the different improvements are also presented. Results showed that by splitting the expansion process into two steps and combining the liquid stream from flash evaporation in a secondary expansion stage, utilization efficiency gains of 10% above the optimized basic Organic Rankine Cycle (ORC) are achievable. The greatest efficiency gains were observed when combining this modification and replacing the flash evaporation throttling valve with a two-phase expander. Results showed that proposed enhancements had the greatest effect using aromatic hydrocarbon working fluids, increasing the utilization efficiency by 5%–20% over the optimized ORC; whereas for siloxanes, the improvement was limited to 2%–4%.

► Several improvements developed for the Organic Flash Cycle (OFC) are presented.
► Efficiency increased through reduced irreversibility in flash evaporation and lower average heat rejection temperature.
► Aromatic hydrocarbon OFC can produce 5–20% more power over the optimized ORC.
► Siloxane OFCs show 2–4% increase in power generation over optimized ORC.