Review paper assesses water (R718) as a refrigerant for industrial heat pumps above 100 °C

A review paper published in Applied Thermal Engineering (vol. 304, 2026, article 132756) by Elwardany, Siva Kumar Reddy and Nassif from the University of Cincinnati (USA) and Assiut University (Egypt) consolidates findings from theoretical modelling, simulations, experimental work and prototype demonstrations on water (R718) as a refrigerant for high-temperature heat pumps (HTHPs). The paper is available in open access.

Scope and target applications

The review covers HTHP architectures that deliver process heat at temperatures from above 100 °C up to approximately 250 °C. It examines single-stage, multi-stage and cascade system configurations, including cascades that pair R718 with other low-GWP refrigerants such as R600 or R1234ze(Z). The identified application sectors are food processing, chemicals, pulp and paper, and district heating networks, where recovering industrial waste heat and supplying process heat are major energy demands currently met largely by fossil fuels.

Why water is attractive

R718 is a natural refrigerant with zero Global Warming Potential, is non-toxic and non-flammable, is widely available, and has a high latent heat of vaporisation. According to the studies reviewed, R718-based systems can achieve competitive efficiency at high temperatures, particularly when combined with advanced compressor technologies and optimised cycle configurations.

Engineering barriers and emerging solutions

Despite these advantages, water's low vapour pressure and high specific volume require compressors capable of handling large volumetric flow rates together with advanced sealing systems. High-temperature steam conditions impose additional demands on material compatibility and corrosion resistance, and precise control is needed for stable, efficient operation.

The review highlights how advances in twin-screw, centrifugal and turbo-compressors, water-injection techniques, and multi-stage or cascade configurations are helping to address these limitations and extend the achievable temperature range. Experimental prototypes are increasingly corroborating numerical results, but the authors note that further development in component design, materials, control systems, and commercial-scale demonstration is still required before R718-based HTHPs can achieve broader industrial deployment.

Economic outlook

The economic assessments reviewed in the paper indicate that R718 systems could offer attractive capital costs and payback periods under suitable industrial operating conditions.

Additional reference

The use of water as a working fluid in HTHPs is also covered in the 61st IIR Technical Brief on Refrigeration Technologies, "High-Temperature Heat Pumps for Industrial Decarbonisation."

Sources:

[1] Elwardany, M., Siva Kumar Reddy, Y., Nassif, N., 2026. Water (R-718) as a refrigerant for high-temperature heat pumps: opportunities, challenges, and future directions. Applied Thermal Engineering 304 (2026) 132756. https://doi.org/10.1016/j.applthermaleng.2026.132756