The feasibility of using vapor expander to recover the expansion work in two-stage heat pumps with a large temperature lift

• Vapor expander is introduced to heat pumps for expansion work recovery.
• Two different configurations of the proposed system are modeled with pure refrigerants and mixtures.
• Slopes of binary mixtures' vapor saturation curves are found to be related to composition.
• Effect of using refrigerant mixtures on system performance is discussed.
• The thermodynamic and economic performance for each heat pump system is conducted.

Vapor expanders are introduced for the first time into heat pumps to recover the expansion work. A two-stage heat pump based on the combination of vapor expander and compressor (TSHP-E) is proposed and its feasibility in cold regions is verified theoretically. Two system configurations are modeled for wet refrigerant like R152a, R134a, R161 and R290, and dry fluids including R227ea, R1234yf and R1234ze. Results indicate that wet refrigerants are more suitable for the TSHP-E system under conditions where the evaporating and condensing temperature is −12.0 °C and 65.0 °C respectively. R152a tops all the candidates with a maximum COP of 2.41 and a COP increment of 15.0%. Using mixtures R152a/R227ea (mass fraction 60/40) will reduce the superheating degree in internal heat exchanger by 4 °C compared with R152a without obvious COP decrease. Thermodynamic and economic performance comparison between the proposed system and other two heat pump systems are conducted.