Experimental evaluation of a high temperature cascade heat pump based on refrigerant charge

Abstract

Global energy demand continues rising with technological advancement. Renewables like solar and wind are increasingly adopted, yet heating/cooling systems responsible for 40 % of building energy use require urgent efficiency improvements. Heat pumps emerge as key solutions, particularly cascade systems. Refrigerant charge in cascade Heat pumps is critical: overcharging raises costs and leakage risks, while undercharging reduces heating capacity. To investigate the effect of refrigerant charge on the performance of the High-Temperature Cascade Heat Pump (HTCHP) system, a 20 kW HTCHP test rig was constructed. Several experiments were conducted to examine the variations in pressure, power input, heating capacity, and coefficient of performance (COP) with different refrigerant charge amounts in the Low-Temperature End Refrigeration Cycle (LS). An uncertainty analysis was performed to ensure the accuracy and reliability of the experimental results. The findings reveal that the evaporating and condensing pressures in both the LS and High-Temperature End Refrigeration Cycle (HS), as well as the power input of LS, heating capacity, and COP, increased with the refrigerant charge in LS. In contrast, the power input of HS showed little change. The HTCHP COP decreased by approximately 3.25 %–5.77 % as the refrigerant charge in LS increased by 300 g.