Effects of the cylinder volume ratio of a twin rotary compressor on the heating performance of a vapor injection CO2 cycle

• A CO2 heat pump applying vapor injection techniques is considered.
• The effects of cylinder volume ratio of a twin rotary compressor are investigated.
• The optimum injection ratio is determined for each cylinder volume ratio.
• The optimum cylinder volume ratio for the maximum COP is determined as 0.7.

The injection mass flow rate in a vapor injection CO2 cycle is an important design factor to improve the heating performance of a heat pump at low ambient temperatures. The intermediate pressure in a twin rotary compressor strongly affects the injection mass flow rate, and can be adjusted by the cylinder volume ratio of the second-stage to the first-stage. In this study, the heating performances of the vapor injection CO2 cycles including the flash tank vapor injection (FTVI) and sub-cooler vapor injection (SCVI) cycles were measured by varying the cylinder volume ratio of the twin rotary compressor at the ambient temperature of −15 °C. From the results, the optimum injection ratio yielding the maximum COP was determined for each cylinder volume ratio. In addition, in the FTVI and SCVI cycles, the optimum cylinder volume ratio was determined as 0.7 in order to achieve the maximum COP with the designed heating capacity.