Thermodynamic Performance Evaluation for Low Temperature Heat Source Cascade System Circulating Environment Friendly Refrigerants

摘要

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.