An innovative cross-heat-exchange concept integrating a supercritical CO2 cycle and a subcritical high-temperature organic Rankine cycle

The current research on coupling bottoming organic Rankine cycles (ORCs) with supercritical CO2 (sCO2) Brayton cycles relies on conventional working fluids to recover the low-grade sensible heat of sCO2 at the low-temperature regenerator outlets. This approach restricts the ORC evaporation temperatures to below 100 °C and enforces unidirectional heat transfer from the sCO2 cycles to the ORCs, resulting in minimal efficiency improvements of typically below 1%. This study proposes innovative cross-heat transfer coupled systems utilizing a mixture of biphenyl-diphenyl oxide (BDO) as the ORC fluid. BDO absorbs high-grade exhaust heat from sCO2 turbine outlet and evaporates at 400 °C to drive an ORC turbine. The post-expansion superheated BDO vapor preheats the sCO2 stream bypassed from a regenerator. Consequently, the ORC's condensation heat is entirely returned to the sCO2 cycle, instead of being rejected to the environment as in the existing integrated schemes. Depending on the location and frequency of sCO2 splits, three distinct coupling structures (Systems I∼Ⅲ) are designed. Thermodynamic analysis demonstrates that System I exhibits the optimal performance, achieving the maximum thermal and exergy efficiencies of 46.23% and 71.25%, respectively. These values represent improvements of 1.35% and 2.18% compared to those of the original system without ORC coupling. It takes 5.67 years to recover the additional ORC subsystem cost.