Exergoeconomic analysis of utilizing the transcritical CO2 cycle and the ORC for a recompression supercritical CO2 cycle waste heat recovery: A comparative study

Two combined cogeneration cycles are examined in which the waste heat from a recompression supercritical CO2 Brayton cycle (sCO2) is recovered by either a transcritical CO2 cycle (tCO2) or an Organic Rankine Cycle (ORC) for generating electricity. An exergoeconomic analysis is performed for sCO2/tCO2 cycle performance and its comparison to the sCO2/ORC cycle. The following organic fluids are considered as the working fluids in the ORC: R123, R245fa, toluene, isobutane, isopentane and cyclohexane. Thermodynamic and exergoeconomic models are developed for the cycles on the basis of mass and energy conservations, exergy balance and exergy cost equations. Parametric investigations are conducted to evaluate the influence of decision variables on the performance of sCO2/tCO2 and sCO2/ORC cycles. The performance of these cycles is optimized and then compared. The results show that the sCO2/tCO2 cycle is preferable and performs better than the sCO2/ORC cycle at lower PRc. When the sCO2 cycle operates at a cycle maximum pressure of around 20 MPa (∼2.8 of PRc), the tCO2 cycle is preferable to be integrated with the recompression sCO2 cycle considering the off-design conditions. Moreover, contrary to the sCO2/ORC system, a higher tCO2 turbine inlet temperature improves exergoeconomic performance of the sCO2/tCO2 cycle. The thermodynamic optimization study reveals that the sCO2/tCO2 cycle has comparable second law efficiency with the sCO2/ORC cycle. When the optimization is conducted based on the exergoeconomics, the total product unit cost of the sCO2/ORC is slightly lower than that of the sCO2/tCO2 cycle.