Critical temperature criterion for selection of working fluids for subcritical pressure Organic Rankine cycles

A method was proposed to couple the heat source with ORCs (Organic Rankine Cycles). The integrated-average temperature difference, Delta T-ave, quantifies the thermal match in the evaporator. Delta T-ave decreases with increase in critical temperatures (T-c). The fluid with T-c approaching T-gas,T-in (flue gas inlet temperature) can have Delta T-ave -> 0.5 x (T-gas,T-out - T-5), which is called the optimal temperature difference, at which the thermal efficiency is maximum. The ORC performance with fluids of T-c > T-gas,T-in has less deviation from the optimal condition. Thermal efficiencies are well correlated with critical temperatures. The ORC thermal efficiencies are randomly distributed against other fluid physical properties except T-c. Thus, the critical temperature can be the sole criterion for the fluid selection, as far as the thermal efficiency is concerned. The organic fluids with T-c in the range of (T-gas,T-in - 20-30 K, T-gas,T-in + 100 K) are recommended. Specific fluids are recommended for heat source temperature in the range of (100-300 degrees C) by screening 57 fluids. More fluids are available for ORCs with low heat source temperatures. Limited fluids are available for high temperature heat source applications. Due to the expanded fluids in region II, some fluids such as R245fa can be used over a wide heat source temperature range. (C) 2014 Elsevier Ltd. All rights reserved.