Thermal performance of an absorption-assisted two-stage ejector air-to-water heat pump *

A gas-fired air-to-water ejector heat pump (EHP) for domestic hot water production is proposed and analyzed thermodynamically. The COP of the EHP is improved by using binary fluids (HFE7500 and water), and the high backpressure in the primary ejector (first-stage) is actively controlled by either a second-stage ejector and/or a LiBr-H2O absorption subsystem. A thermodynamic model is established for analysis and performance evaluation. A case study shows the backpressure of the first-stage ejector is reduced by 53.5%, and the backpressure control using the second-stage ejector increases the system COP by 21.0% compared with that using LiBr-H2O absorption. Parametric studies are conducted to investigate the roles of the first-stage ejector, the second-stage ejector, and the LiBr-H2O absorption subsystem. The first-stage ejector plays a dominant role in the EHP system performance. The EHP's heating COP can reach 2.0 with an entrainment ratio of 0.12 in the first-stage ejector. EHP performance can be improved with a high entrainment ratio and a low-temperature primary working fluid in the first-stage and second-stage ejectors, a low condensation temperature in the first-stage ejector, and a high generating temperature and a low concentration of the LiBr-H2O solution.

Automated summary

A gas-fired air-to-water ejector heat pump for domestic hot water production was proposed and analyzed thermodynamically in this study. The use of binary fluids and active control of backpressure showed improvements in COP. The first-stage ejector plays a dominant role in system performance, and optimizing various parameters can enhance the EHP performance.