A critical review of auto-cascade and cascade heat pumps with zeotropic mixtures for heating applications

Abstract

The increasing demand for sustainable heating in both residential and industrial sectors has created the need for efficient heat pump technologies capable of achieving large temperature lifts. In this context, auto-cascade and cascade heat pumps employing zeotropic mixtures have emerged as promising solutions. The benefit of these systems over the conventional heat pump cycle lies in the temperature glide of zeotropic mixtures, which leads to the improvement of heat transfer and the reduction of exergy destruction in heat exchangers, along with the phase separation in auto-cascade cycles, which changes the mixture composition in the evaporator and condenser, resulting in lower pressure ratios. This paper presents a critical review of 48 studies on auto-cascade and cascade heat pumps with zeotropic mixtures for heating applications. The reviewed studies were classified into three categories: separator-based auto-cascade cycles, ejector-enhanced auto-cascade cycles, and conventional cascade cycles with zeotropic mixtures. System configuration, operating conditions, mixture selection, thermodynamic performance and economic results were collected and discussed. The review shows that most studies so far focus on heat production below 100 °C, mainly involving separator- and ejector-based auto-cascade systems, whereas higher-temperature applications were identified for cascade cycles with zeotropic mixtures. Advanced configurations incorporating vapor injection, multiple separators and ejector devices consistently outperform the basic auto-cascade cycle. Ejector-enhanced systems frequently exhibit favorable COP values, while cascade systems with mixtures are predominantly investigated at higher temperature lifts; however, the heterogeneous operating conditions prevent a universal performance ranking. The findings of this review highlight the potential of auto-cascade and cascade heat pumps for sustainable heating and encourage further research towards higher-temperature applications and industrial process heating. Future work should focus on experimental validation, dynamic modelling with adaptive mixture composition control, comprehensive techno-economic assessment, and the integration of these technologies into polygeneration systems and Carnot battery applications.