Transcritical pressure Organic Rankine Cycle (ORC) analysis based on the integrated-average temperature difference in evaporators

Integrated-average temperature difference (Delta T-ave) was proposed to connect with exergy destruction (I-eva) in heat exchangers. Theoretical expressions were developed for Delta T-ave and I-eva. Based on transcritical pressure ORCs, evaporators were theoretically studied regarding Delta T-ave. An exact linear relationship between Delta T-ave and I-eva was identified. The increased specific heats versus temperatures for organic fluid protruded its T-Q curve to decrease Delta T-ave. Meanwhile, the decreased specific heats concaved its T-Q curve to raise Delta T-ave. Organic fluid in the evaporator undergoes a protruded T-Q curve and a concaved T-Q curve, interfaced at the pseudo-critical temperature point. Elongating the specific heat increment section and shortening the specific heat decrease section improved the cycle performance. Thus, the system thermal and exergy efficiencies were increased by increasing critical temperatures for 25 organic fluids. Wet fluids had larger thermal and exergy efficiencies than dry fluids, due to the fact that wet fluids shortened the superheated vapor flow section in condensers. (C) 2014 Elsevier Ltd. All rights reserved.