Low grade waste heat recovery with subcritical and supercritical Organic Rankine Cycle based on natural refrigerants and their binary mixtures

• Optimal heat source temperature depends on critical temperature of fluids.
• Optimal mixture component ratio depends on exergy loss during condensation.
• Supercritical ORC better for low Tcrit fluids and high heat source temperatures.
• Supercritical ORC with mixtures has best performance but intensive equipment design.

The Organic Rankine Cycle (ORC) is a promising technology for low temperature waste heat-to-power conversion. This work investigates the waste heat recovery potential of the ORC and some of its innovative variations, such as the supercritical cycle and the use of binary zeotropic mixtures. Focus is given to natural hydrocarbons having low ozone depletion and global warming potential. By performing simulations for heat source temperatures ranging from 150 to 300 °C, the optimal operation mode and working fluids are identified. Meanwhile, several technical parameters (such as the turbine size parameter and rotational speed) are calculated. The system exergetic efficiency is affected by the critical temperature of the working fluids and in the case of mixtures by their temperature glide in the condenser. The maximum efficiency ranges from 15 to 40% for heat source temperatures between 150 and 300 °C, respectively. For temperatures above 170 °C, the optimal fluids are mixtures combined with supercritical operation. Compared to the subcritical ORC with R245fa, the achievable efficiency improvement ranges between 10 and 45%. Although smaller turbines can be used due to the lower size parameter values of supercritical mixtures, R245fa has lower volume flow ratio, rotational speed and UA values.