Investigation of thermal efficiency for subcritical ORC and TFC using super dry working fluids

Efficiency is a crucial factor for power cycles. Generally, in the same temperature range, the cycle efficiency for a basic organic Rankine cycle (ORC) is higher than a similar trilateral flash cycle (TFC) for almost all working fluids and heat source/heat sink temperature pairs. According to some recent results, by using super dry working fluids, the thermal efficiency of TFC can outperform its ORC counterpart at high temperatures. This study performed the thermodynamics analysis using three dry working fluids with subcritical basic ORC and TFC with zero to partial recuperated heat ratio. The results showed that using recuperative heat exchange effectively contributes to increasing the efficiency of ORC and TFC. However, the rate of efficiency increase in ORC is higher than the TFC, which can be clearly seen, especially at high maximal cycle temperatures. Using super dry working fluids, the recuperator‐free TFC has favorable cycle efficiency at high heat source temperatures. In contrast, by adding heat recovery for both systems, the ORC can outperform the TFC at 0.4, 0.2, and 0.1 recuperated heat effectiveness for Dodecane, Decane, and Nonane, respectively. Basic organic Rankine cycle (ORC) thermal efficiency outperforms basic Trilateral flash cycle (TFC) for all working fluid categories, except super dry working fluids at high maximal cycle temperature. There is a maximal cycle temperature (equal efficiency temperature), where ORC and TFC subcritical basic cycles have the same thermal efficiency. By utilizing proper heat recuperation, ORC thermal efficiency outperforms TFC for super dry working fluids even at high maximal cycle temperatures.