چکیده (انگلیسی):

The Cascade heat pump system is commonly used to overcome the high temperature lift problem of the system. In the present investigation eight refrigerant pairs were studied including R717/R134a, R410A/R134a, R407C/R134a, and R717/R600a, R744/R134a, R744/R290, R744/R600a, and R744/R717 at HT condenser of (70)°C and (75)°C. Hot water is to be produced at temperature range (60 to 65)°C with a proper flow demand. The evaporator temperature at the LT cycle side was ranged between (-10)°C and (-2)°C. The intermediate temperatures at the cascade heat exchanger were (20, 22.5, 33, and 35)°C depending on the refrigerant pairs implemented in the Cascade heat pump. Sea water at (7)°C was used as a sustainable low temperature heat source and 30% ethylene glycol-water brine as a thermal fluid carrier for heat extraction. The evaluation of the thermal performance of the refrigerant pairs was based on a fixed heat pump extraction load at the LT cycle evaporator. The R744/R134a and R744/R290 systems revealed the highest heat pump heating load production and highest compressors power consumption accompanied with the lowest COP at (20)°C intermediate temperature and HT condensation of (75)°C. R717/R600a showed the highest COP and lowest power consumption at (35)°C intermediate temperature and HT condensation of (70)°C. The results also revealed that a band of refrigerant pairs occupied the central zone of COP range with acceptable value; they are R410A/R134a, R407C/R134a and R744/R717.