Thermodynamic analysis of a modified cascade high temperature heat pump with zeotropic mixtures for heating production up to 200 °C

Abstract

Developing high temperature heat pump technology is one of the key strategies to accelerate the low-carbon transformation of energy consumption in industrial sectors. However, due to inadequate temperature matching between the heat transfer fluid and the working fluid, there is still potential for performance improvement in high temperature heat pumps. Therefore, a modified zeotropic mixture cascade high temperature heat pump with liquid separation condensers and ejector is proposed in this study. Implementing liquid separation condensation to adjust the composition of the zeotropic mixture and temperature glide, while utilizing an ejector to establish dual-pressure evaporation, improves temperature matching of the heat exchange process in the system. Initially, a mathematical model is developed and validated. Subsequently, a comparison of the system performance is conducted. Finally, parameter studies and potential performance explorations of the modified cascade high temperature heat pump are performed. The main results are summarized as follows: The coefficient of performance of the modified heat pump is 5.12 % to 9.55 % higher than that of the conventional cascade high temperature heat pump. The coefficient of performance and exergy efficiency initially increase and then decrease as the vapor quality and the intermediate pressure increase. However, the influence of vapor quality in high temperature cycle is relatively insignificant. The working fluid group of R152a/R1233zd(E) and R1336mzz(Z)/toluene is the preferred choice when the heat source inlet temperature is between 40–50 °C. The working fluid group of Isobutane/R1233zd(E) and R1336mzz(Z)/toluene is effective across most temperature ranges where heat source inlet temperature is between 55–100 °C. When the heat source inlet temperature is between 75–100 °C and the heat sink inlet temperature is between 170–200 °C, the working fluid group of Propane/R1233zd(E) and R1224yd(Z)/toluene is more favorable.