A review of recent research on hydrofluoroolefin (HFO) and hydrochlorofluoroolefin (HCFO) refrigerants

Due to the damage they have caused to the ozone layer, first-generation refrigerants known as chlorofluorocarbons have been phased out, and second-generation refrigerants known as hydrochlorofluorocarbons are being phased out. Third-generation refrigerants known as hydrofluorocarbons are also being phased out due to their high global warming potential. As successors to the above three generations of refrigerants, hydrofluoroolefins and hydrochlorofluoroolefins were designed and created in laboratories. They have been attracting increasing research interest because they are safe and environmentally friendly. In this paper, various aspects of the recent research on hydrofluoroolefin and hydrochlorofluoroolefin refrigerants, including applications in refrigeration systems and heat pump systems, applications in organic Rankine cycles, flow patterns and heat transfer characteristics, thermophysical properties, persistent degradation product, toxicity, flammability and ignitability, thermal stability and decomposition mechanisms, are reviewed and discussed. The most commonly studied hydrofluoroolefins are still HFO-1234yf and HFO-1234ze(E). HFO-1336mzz(Z) is also worthy of further attention because it may be a suitable replacement for HFC-123. Attempts can be made to establish a dedicated equation of state for commonly used hydrofluoroolefins to better predict the thermodynamic properties. Further studies on the persistent degradation product, toxicity, flammability and ignitability, thermal stability and decomposition mechanism are needed.