Recent advances on performance enhancement of propane heat pump for heating applications

Heat pumps, as highly efficient water heating devices, represent one of the most promising technologies for replacing coal-fired boilers and conventional electric heating systems. The global implementation of environmental protection regulations has accelerated the adoption of low global warming potential (GWP) refrigerants applications. Therefore, the use of refrigerants in heat pump water heating (HPWH) systems is initially summarized, covering conventional refrigerants, propane (R290), and R290 refrigerant blends. R290 has been demonstrated to have excellent thermodynamic properties, making it a potential low-GWP alternative refrigerant for HPWH systems. Furthermore, the use of R290 refrigerant blends with low temperature slip and non-flammable refrigerants not only enhances the system's energy efficiency but also reduces the flammability of R290, which will be the focus of future research. The performance of the current R290 HPWH systems has been unable to meet the rapid development of heat pump market demand. Therefore, the development of high-performance, safe, and reliable R290 HPWH systems offers a viable solution. In addition, a comprehensive review of potential performance enhancement methods for R290 HPWH systems are also discussed, including sub-cooler enhancements, solar-assisted enhancements, thermal energy storage enhancements, ejector enhancements, and compressor technologies. Vapor injection and two-stage compression technologies have proven to be the most effective solutions for low-temperature heating. However, hydrocarbon type compressors remain under development. Finally, given the highly flammable nature of R290, safety application methods for R290 HPWH systems are also explored. Existing research indicates that reducing the charge of flammable refrigerants is the most direct approach to improving the safety application of the system. Consequently, the development of efficient and compact heat exchangers has become critical for the future advancement of R290 HPWH systems. This state-of-the-art review is expected to provide valuable insights for the advancement of R290 HPWH technology and draw greater attention to R290 refrigerant.