Effects of initial and operating conditions on the closed supercritical carbon dioxide Brayton cycle

The supercritical carbon dioxide (s-CO2) power cycle is a potential candidate for efficient thermal energy conversion from various heat sources because of its real gas properties. It is necessary to investigate the effects of initial thermodynamic conditions because they can significantly affect the s-CO2 power cycle due to the substantial variation in the real gas properties near the critical point. In the present study, a one-dimensional analytical model was developed to analyze the thermodynamic processes of the piston engine with s-CO2 as the working fluid. Transient computational simulations were performed to analyze the thermodynamic processes of a piston–cylinder model with a dynamic mesh. The non-ideal thermodynamic properties of CO2 were then estimated using a real gas equation of state and coupled with both analytical and computational calculations. The initial conditions of T1 and P1 along the locus of the minima of Z are found to be ideal for a minimum work input, which could potentially reduce the work input up to 20% compared to a gaseous CO2. It is also found that increasing P1 slightly above the critical pressure (Pc=73.773 bar) reduces compression work. In addition to the initial conditions, the increasing temperature after heat addition and introducing heat regeneration enhanced a thermal-to-mechanical energy conversion, leading to higher work output, which increased the efficiency by up to 105%. The supercritical state significantly reduces the work input requirement compared to the gaseous state.