Evaluation of Exergy Destruction in an Organic Rankine Cycle for Industrial Applications

The cost of electrical energy in an organic Rankine cycle (ORC), using high-temperature heat, is 0.5946 USD/kWh, with an efficiency of 40.80 % (2024), these parameters change significantly when using low temperature heat, up to 12.12 % efficiency and 0.95 USD/kWh of energy cost (2019). Exergy destruction defines the minimum area of the ORC evaporator and condenser and their total cost. The temperature of the working fluid at the evaporator outlet has a significant impact on the thermal efficiency of the system. Propane, butane and isobutane were used as working fluids to evaluate the exergy destruction in an ORC. In the analysis, the fluid temperature at the evaporator outlet is varied in a range of 80 to 120 °C. An exergo-economic evaluation was carried out for each of the working fluids. The fluid enters the turbine and undergoes an isentropic expansion of 6 bar to produce 3147.3 kW of power. For the three working fluids, the ORC evaporator presents the highest exergy destruction, followed by the condenser, the turbine and the pump. Propane exhibits the highest exergy destruction in the evaporator, 79 %. The ORC operated with butane presents the lowest exergy destruction in the evaporator among the fluids used (70 %), with an energy efficiency of 7.78. At a temperature of 110 °C, using butane as the working fluid, the smallest total area of the heat transfer equipment of 1,924 m2 and the lowest levelized cost of energy (LCOEel), 0.022 USD/kWh, were obtained. The exergo-economic evaluation allows determining the working fluid with the lowest exergy destruction, the lowest total area of the heat transfer equipment and the lowest energy cost.