Modification of figure of merit concept for evaluating optimum intermediate temperature and performance of cascade heat pump

Abstract

Heat pumps are significant technologies that provide both heating and cooling, commonly used in industrial and residential buildings due to their ease of installation and high performance. This study investigates two main types of heat pump systems: the single-stage heat pump and the cascade heat pump. Single-stage heat pumps have been widely employed for space heating and cooling, hot water production, and waste heat recovery. In this study, various pure and zeotropic refrigerants were selected as working fluids to investigate their correlation with the Figure of Merit (FOM), aiming to evaluate the performance of single-stage heat pump systems more rapidly and precisely. Additionally, the correction factor (F) for the FOM method was examined to enhance the accuracy of the correlation, especially considering the gliding temperature that occurs during phase changes in systems utilizing zeotropic refrigerants. The model results derived from this correlation showed good agreement with both the enthalpy method and experimental data from other studies, as well as our own experimental data from a cascade heat pump system located in Sakhon Nakhon, Thailand. The maximum Mean Absolute Percentage Error (MAPE) was found to be less than 7%. Furthermore, the correlation performed well even in scenarios involving sub-cooling and super-heating within the system. For this study, evaporating temperatures ranged from -10°C to 15°C, while condensing temperatures varied between 35°C and 65°C. This study focuses on a dual cascade heat pump system featuring separate low-temperature side (LS) and high-temperature side (HS) cycles, chosen for its exceptional performance and the influential factors affecting the coefficient of performance (COP). The intermediate temperature (T_m) has emerged as the most critical parameter in determining the optimal COP. Consequently, the study investigates the optimal T_m for the cascade heat pump system, examining various groups of pure refrigerants for both cycles, as well as blend refrigerants. In addition, the study explores the impact of different LS evaporating temperatures (ranging from 5°C to 15°C) and HS condensing temperatures (ranging from 65°C to 92°C) and pinch temperature on correlating with the optimal T_musing the critical temperature ratio (γ_crit). The results regarding the optimal T_m and maximum COPs, derived through correlation with the enthalpy method, demonstrate excellent agreement within ±7% and ±3%, respectively.