Performance analysis of a novel vapor injection cycle enhanced by cascade condenser for zeotropic mixtures

Vapor injection with flash tank has proven to be effective in improving the performance of vapor compression heat pumps at low temperatures. However, the benefit of vapor injection would be reduced when zeotropic mixtures are used, due to the loss of low-boiling component, causing by the vapor-liquid separation in flash tank. Hence, a novel vapor injection cycle (HFCC) using a cascade condenser to liquefy the separated vapor stream for evaporation was proposed in this paper. The amount and state of the injected stream was controlled via a branch circuit. The performances of the HFCC system were investigated and compared with the flash tank cycle (FTC) and the hybrid cycle (HFIC), based on the developed mathematical model. Simulation results based on R290/R600a (50/50) indicated that the HFCC had 1.9% and 2.6% improvement in COP compared with that of the HFIC and FTC, respectively, under the same conditions. HFCC became more superior to FTC in terms of COP as the cycle temperature lift increases, by either decreasing evaporation temperature or increasing condensation temperature. The COP improvement of HFCC depended largely on the circulating composition. Generally, zeotropic mixtures with large temperature glides showed higher COP improvement in comparison with FTC.