Assessment of blends of CO2 with butane and isobutane as working fluids for heat pump applications

In the present study, blends R744/R600 and R744/R600a are being proposed as working fluids in heat pumps for medium and high temperature heating applications. COP based performances have been evaluated for zeotropic mixtures of both working fluid pairs for various compositions and compared against that of pure working fluids. Effect of internal heat exchanger on blend based system performance is also presented and finally heat transfer issues in evaporation and condensation are discussed. Results show that due to gliding temperature during evaporation and condensation, the zeotropic blends, instead of pure counterparts, can be employed very effectively in heat pumps for variable temperature or simultaneous cooling and heating applications (e.g., dairy plants) at conventional high side pressure. The blend R744/R600a can be the best alternative refrigerant to R114 for high temperature heating due to superior COP (more than twice) over R600 and R600a and eliminating the requirement of extremely high side pressure of R744 systems.