Comparison and analysis of the expansion process in a CO 2 transcritical cycle

The isentropic and isenthalpic expansion processes in a CO2 transcritical cycle were analysed thermodynamically. It was found that the effective refrigeration capacity of the cycle with the isentropic expansion process is higher than the cycle with the isenthalpic expansion process. The increment is slightly larger than the expansion work recovered because of the transcritical nature of the cycle. Further analysis and comparison were made for two decreasing throttling loss methods: using internal heat exchangers and using expanders instead of throttling valves. The results indicate that using high efficiency expanders in the transcritical cycle is the better method, and the work recovered by the expander is mainly from the flow work in the supercritical area, and from the internal energy change in the liquid-gas two-phase region. It is recommended that the flashing crash model can be used to simplify the characteristic equations of the liquid-gas two-phase adiabatic expansion process.