Where and how to place the injection structure of a cascade high-temperature heat pump with large-temperature differences: A comprehensive analysis

Abstract

In the context of utilizing cascade high-temperature heat pumps (HTHPs) for heating applications with large-temperature differences, the optimal placement and implementation of the injection structure continue to present an unresolved issue. To this end, this paper develops two otherwise-identical cascade HTHP configurations: the HT-VIE-CS with the injection structure placed in the high-temperature stage and the LT-VIE-CS with the structure placed in the low-temperature stage. Subsequently, performance and stability are compared over wide ambient temperature (T_am) and heating temperature (T_h) ranges, and several refrigerant pairs are screened. Results show that placing the injection structure in the low-temperature stage generally increases heating COP, with a maximum advantage of 4.0 % over HT-VIE-CS in the investigated conditions, while HT-VIE-CS retains a larger stability margin. Across refrigerant pairs, LT-injection remains favorable, and R152a/R245ca provides the best overall trade-off. Based on the stability-performance balance, a flexible selection strategy is proposed: as T_am rises, the recommended configuration shifts from HT-VIE-CS to LT-VIE-CS and then to a simpler two-stage injection system, with transition T_am of -1/5/10°C and 48/56/64°C for T_h = 120/130/140°C, respectively. A techno-economic and carbon assessment indicates that under current conditions, when T_am > 3°C, its operating costs are lower than those of gas boilers, and when T_am > 24°C, its emissions are lower than those of coal boilers.