Water (R-718) as a refrigerant for high-temperature heat pumps: opportunities, challenges, and future directions

High-temperature heat pumps (HTHPs) using water (R-718) as a refrigerant are gaining attention as a sustainable and energy-efficient solution for industrial heating processes. This review analysis recent progress in the development of R-718-based HTHPs, focusing on theoretical modeling, simulation studies, and experimental advancements. Simulation results indicate that these systems especially those employing twin-screw or centrifugal compressors can achieve coefficients of performance (COPs) between 1.3 and 6.10 for output temperatures ranging from 130 °C to 250 °C, corresponding to temperature lifts of approximately 20 K to 100 K. Cascade configurations combining R-718 with low-GWP refrigerants such as R600 or R1234ze(Z) have demonstrated effective temperature lifts of up to 180 K with COPs between 1.5 and 4.5. Experimental studies support these findings, showing that R-718-based systems outperform traditional refrigerants such as hydrocarbons, hydrofluoroolefins (HFOs), and hydrofluorocarbons (HFCs) under high-temperature conditions (above 100 °C). Overall, R-718-based HTHPs offer a clean, efficient, and reliable option for industrial heating, making them a promising technology for reducing carbon emissions in high-temperature processes.