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 CO 2 (sCO 2 ) Brayton cycles relies on conventional working fluids to recover the low-grade sensible heat of sCO 2 at the low-temperature regenerator outlets. This approach restricts the ORC evaporation temperatures to below 100 °C and enforces unidirectional heat transfer from the sCO 2 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 sCO 2 turbine outlet and evaporates at 400 °C to drive an ORC turbine. The post-expansion superheated BDO vapor preheats the sCO 2 stream bypassed from a regenerator. Consequently, the ORC’s condensation heat is entirely returned to the sCO 2 cycle, instead of being rejected to the environment as in the existing integrated schemes. Depending on the location and frequency of sCO 2 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. • A novel cross-heat-transfer cascade sCO 2 -ORC cycle configuration is proposed. • The sCO 2 turbine’s exhaust evaporates the BDO mixture-based ORC at 400 °C. • All the condensation waste heat of ORC is used for preheating the sCO 2 subcycle. • The maximum thermal and exergy efficiencies are 46.23% and 71.25%. • An equivalent payback period of 5.67 years is achievable.