Characteristics and performance of very-high-temperature heat pump under unconventional conditions that may occur in normal operation ─ experimental investigation

Very-high-temperature heat pump can efficiently recover waste heat, and satisfy industrial heat needs, which constitutes a significant proportion of global energy usage. However, existing studies mostly focus on system performance for stable or normal operation, but rarely discuss the characteristics of the system operating under unconventional conditions that may occur in system development and daily use, and is important to improve system design, control and efficiency. Therefore, this study establishes a very-high-temperature heat pump experimental rig and investigates its operational characteristics under unconventional working conditions to help system development and application. The results indicate that under refrigerant overcharging condition, condensing pressure and subcooling degree is remarkably higher than those of normal condition, the evaporator outlet temperature is abnormally lower than inlet temperature, with the maximum coefficient of performance 25.5 % lower than that of normal condition. Under starting dynamic operating conditions, as starting pressure reaches 600 kPa, a high condensing pressure peak value of 2029.7 kPa is reached immediately after start, which is much higher than condensing pressure of 1834 kPa. High pressure ratio operating conditions, with constant condensing pressure, as pressure ratio increased from 7.8 to 8.4, heat transfer rates of evaporator and condenser, and coefficient of performance decreased by 24.45 %, 15.07 %, and 12.38 %, respectively. With constant evaporating pressure, as pressure ratio increased from 8.5 to 9.1, heat transfer rates of evaporator and condenser, and coefficient of performance decreased by 6.43 %, 8.05 %, and 11.89 %, respectively. These results provide important references for the development of very-high-temperature heat pump.