Assessment of key performance indicators for a reciprocating compressor with electronically controlled vapor injection

This study develops a comprehensive and physically consistent mathematical model of a reciprocating compressor with vapor injection (VI) to be used in a CO2 transcritical cycle (Voorhees cycle), removing the constant-volume mixing assumption adopted in a previous work. The formulation accounts for coupled thermodynamic and fluid-dynamic interactions, heat transfer and leakage correlations, and flexible vapor injection enabled by solenoid valve technology. The parameters required by the model are identified by validating the reference model without vapor injection against experimental data. After validation, the model was used to assess key performance indicators of the VI compressor under a wide range of operating conditions, namely isentropic efficiency, suction volumetric efficiency, and a novel injection volumetric efficiency that specifically describes the injection phase. At the compressor level, the injection pressure and the injection valve opening time can be set independently, and increasing either reduces the specific work, which is lowered by up to 14.1 % under favorable conditions; the compressor-level optimum is thus reached at high injection pressure and long valve opening time. At the cycle level, however, these two variables are coupled by the steady-state mass balance of the flash tank, so that the optimum shifts to low injection pressure and long valve opening time, the condition that maximizes the injected mass available from the flash expansion.