Thermodynamic transition from subcritical to transcritical CO2 cycle

•Thermodynamic transition between subcritical and transcritical CO2 cycle is studied.•Vapor pressure equation is extended to pseudocritical line in supercritical region.•Optimal high pressure and condensation pressure are connected and differ at 27.79 °C.•Three approaches are proposed to define optimal operating condition of CO2 cycle.

Transcritical CO2 cycle has been widely investigated fundamentally and practically. However, transition between subcritical and transcritical CO2 cycles has not yet been well studied. In this work, temperature at maximum constant-pressure specific heat (pseudocritical temperature) in supercritical region is regarded as the extension of saturation temperature in subcritical region. The existing vapor pressure equation is therefore extended to well fit the pseudocritical temperature. Moreover, the relation between the optimal high pressure of transcritical cycle and the condensation pressure of subcritical cycle is developed. It turns out that the optimal high pressure starts at subcritical region near the critical point. Accordingly, three approaches including the optimal high pressure equation, the optimal overpressure ratio equation, and the extended optimal subcooling equation are proposed to define the optimal operating condition of CO2 cycle. This work demonstrated that knowledge of subcritical cycle can be transitioned to characterize the transcritical cycle.