Effect of vapor-injection technique on the performance of a cascade heat pump water heater

• Vapor injection increases the capacity of a cascade heat pump water heater.
• But, the cascade system cannot fully utilize the advantages of vapor injection.
• Vapor injection in upper stage cycle increases the COP of lower stage cycle.
• Vapor injection in lower stage cycle increases the overall capacity of system.

In this study, we applied a vapor-injection (VI) technique in a cascade heat pump system. The VI was applied to both upper and lower stage cycles. Test results showed that heating and cooling capacities increased by using the VI technique (12% and 6%, respectively); however, the system COP decreased (6.6% at the injection ratio of 16.7%). The cascade system which has a small compression ratio and a cascade condenser, cannot fully utilize the VI's advantages to improve the system COP. However, the VI is effective for the system reliability and capacity improvement. We also found that the VI in the upper and lower stage cycles had different effect on overall cycle operating characteristic.