Thermodynamic analysis of a new cascade ORC and transcritical CO2 cycle to recover energy from medium temperature heat source and liquefied natural gas

In this paper, a novel cascade cycle is proposed to recover energy from a medium temperature waste stream and a liquefied natural gas vaporization unit simultaneously. To analyze the system, its performance is compared with a single organic Rankine cycle and a single transcritical carbon dioxide cycle at the same operation conditions. Both energy and exergy analyses are applied to the system. In addition, different working fluids are examined in this study. In order to find the best configuration and working fluid, optimization algorithm is utilized. Results showed that the best performance is for a cascade cycle with Pentane as working fluid for ORC section. Its exergy efficiency is evaluated 13.08%, while the highest exergy efficiency for an ORC and a transcritical CO2 cycle is 12.3% and 11.24%, respectively. This increase in efficiency is because of better thermal match between hot and cold streams in vapor generator and condenser sections. Based on the exergy analysis, the highest rate of exergy destruction in all cycles belongs to the preheater and condenser respectively. Finally, sensitivity analysis showed that in the proposed cascade cycle, ORC's condenser pressure and pinch temperature of the heat exchanger are the most important parameters which can significantly affect the system performance and their value should be selected carefully.