Thermodynamic analysis of CO₂ transcritical cycles with vapor injection in reciprocating compressors (Voorhees cycles) for heat pumps operating at different design conditions.

This paper presents a detailed analysis of transcritical CO2 cycles modified with vapor injection into the cylinders of reciprocating compressors (also known as Voorhees cycles). Thermodynamic cycles are evaluated under stationary conditions with reference to an air-source heat pump configuration; the performance has been evaluated for a wide range of nominal operating conditions, represented by different heat-sink temperature profiles and ambient air temperatures. The thermodynamic process related to the vapor injection into the cylinder is rigorously assessed, also taking into account the effect of the clearance volume, with the assumption of a constant-volume mixing process inside the cylinder with the piston at the bottom dead center. The results show that the optimum injection pressure increases markedly with the ambient temperature; it also increases rapidly with the average heat-sink temperature for relatively low gas cooler pressures (which result in a double pinch point in the gas cooler), whereas the rate of increase is much lower for higher pressures (which produce a single pinch point in the gas cooler). The performance of Voorhees cycles with internal heat recovery is significantly better than the reference cycle, with the COP and exergy efficiency increasing by up to 13 %; Voorhees cycles have a modest impact on the system layout and may represent a cost-effective solution to improve the performance of transcritical cycles.