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 (sCO(2)) is recovered by either a transcritical CO2 cycle (tCO(2)) or an Organic Rankine Cycle (ORC) for generating electricity. An exergoeconomic analysis is performed for sCO(2)/tCO(2) cycle performance and its comparison to the sCO(2)/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 sCO(2)/tCO(2) and sCO(2)/ORC cycles. The performance of these cycles is optimized and then compared. The results show that the sCO(2)/tCO(2) cycle is preferable and performs better than the sCO(2)/ORC cycle at lower PRc. When the sCO(2) cycle operates at a cycle maximum pressure of around 20 MPa (similar to 2.8 of PRc), the tCO(2) cycle is preferable to be integrated with the recompression sCO(2) cycle considering the off-design conditions. Moreover, contrary to the sCO(2)/ORC system, a higher tCO(2) turbine inlet temperature improves exergoeconomic performance of the sCO(2)/tCO(2) cycle. The thermodynamic optimization study reveals that the sCO(2)/tCO(2) cycle has comparable second law efficiency with the sCO(2)/ORC cycle. When the optimization is conducted based on the exergoeconomics, the total product unit cost of the sCO(2)/ORC is slightly lower than that of the sCO(2)/tCO(2) cycle. (C) 2016 Elsevier Ltd. All rights reserved.