Thermodynamic investigation of waste heat recovery with subcritical and supercritical low temperature Organic Rankine Cycle based on natural refrigerants and their binary mixtures

The Organic Rankine Cycle (ORC) is widely considered as a very promising technology in the field of low temperature waste heat-to-power conversion and has already been implemented in various industrial scale applications. This work aims to investigate the waste heat recovery (WHR) potential from low grade industrial processes using the ORC and some of its innovative variations, such as the cycle operation under supercritical pressures and the use of binary zeotropic working fluids. Moreover, the options considered in the present work focus on the use of low ozone depletion and global warming potential (ODP and GWP) natural hydrocarbon refrigerants (pentane, hexane, and others) which are a cheap alternative to fluorinated gases. A series of simulations are performed for three heat source temperature levels (150, 225 and 300 o C) and the optimal working fluids and operating conditions are identified. Meanwhile, some important technical parameters are estimated (turbine size parameter, volume flow ratio, rotational speed, UA values in heat exchangers etc.) in each case. The performance improvement that can be achieved with the supercritical ORC and the use of binary mixture working fluids depends on the temperature of the heat source. Compared to the optimized subcritical ORC with R245fa as a working fluid, Propane90/Butane10 leads to an efficiency improvement of 80% under subcritical operation at 150 o C. At 225 o C, the exergetic efficiency of the supercritical ORC of Butane40/Propane60 is 58% higher compared to that of the subcritical R245fa, while at 300 o C the respective improvement that is attained by Butane50/Cyclopentane50 is roughly 60%. Concerning the technical evaluation parameters of the cycle, in the lower temperatures the natural refrigerants and their mixtures have equivalent or even improved UA values, turbine size parameters and volume flow ratios, while R245fa has in general lower rotational speeds. At 300 o C, on the other hand, the use of R245fa is associated with substantially favorable technical characteristics despite its inferior thermodynamic performance.