Experimental investigation and comparison on heating performance characteristics of two transcritical CO2 heat pump systems

•A new correlation about optimal vapor injection pressure was developed.•A study of a transcritical CO2 system with a flash tank was performed.•The effects of operation conditions on system performance were investigated. An experimental investigation and comparison of the heating performance of the transcritical CO2 single-stage and vapor injection cycles were conducted. The effects of operating parameters on the heating performance of the two cycles were investigated. The experimental results showed that the heating capacity of the vapor injection cycle was 7.0% higher and the discharge temperature of the refrigerant was 7.7–11.8 ℃ lower than that of the single-stage cycle under the same condition. Besides, both the compressor input power and heating capacity were increased with the increasing temperature of the glycol aqueous solution. However, the growth rate of heating capacity was greater than that of the compressor input power. Therefore, the coefficient of performance of the vapor injection cycle was 4.4% greater than that of the single-stage cycle. So the vapor injection cycle has a wider range of applications in cold conditions. When the vapor injection pressure increased, the coefficient of performance of the vapor injection cycle increased then decreased, so there was a peak of the coefficient of performance. Therefore, a new correlation of the optimal vapor injection pressure was proposed. The new correlation has a significant meaning for designing a transcritical CO2 vapor injection cycle.