Potential and challenges of water as a refrigerant for high-temperature heat pumps

Water: a promising refrigerant for high-temperature industrial heat

Industrial process heating represents a major share of global energy consumption and remains largely dependent on fossil fuels. High-temperature heat pumps (HTHPs), capable of upgrading low-grade waste heat to useful temperatures above 100°C, are therefore attracting increasing attention as a solution to electrify industrial heating and reduce associated greenhouse gas emissions.

A recent review paper ^[1] published in Applied Thermal Engineering by a team of researchers from the University of Cincinnati, USA, and Assiut University, Egypt, examines the potential of water (R718) as a refrigerant for HTHPs, summarising findings from theoretical modelling, simulations, experimental investigations and prototype demonstrations. Water offers several attractive characteristics: it is a natural refrigerant with zero Global Warming Potential (GWP), is non-toxic and non-flammable, widely available, and has favourable thermophysical properties, including a high latent heat of vaporisation. According to the studies reviewed, R718-based systems can provide competitive efficiency at high temperatures, particularly when combined with advanced compressor technologies and optimised cycle configurations.

The review covers applications delivering heat at temperatures from above 100°C to approximately 250°C and examines different architectures, including single- and multi-stage systems as well as cascade configurations combining R718 with other low-GWP refrigerants such as R600 or R1234ze(Z). Such characteristics make water-based HTHPs particularly interesting for recovering industrial waste heat and supplying process heat in sectors such as food processing, chemicals, pulp and paper, and district heating networks.

Why is water not yet widely used in heat pumps?

Despite its advantages, using water as a refrigerant presents specific engineering challenges that have historically limited its application in vapour-compression heat pumps. Its low vapour pressure and high specific volume require compressors capable of handling large volumetric flow rates and use of advanced sealing systems, while high-temperature steam conditions introduce additional requirements related to material compatibility and corrosion. Precise control is also needed to ensure stable and efficient operation.

The review highlights how advances in twin-screw, centrifugal and turbo-compressors, water injection and multi-stage or cascade configurations are helping address these limitations and extend the achievable temperature range. Experimental prototypes increasingly support the results of numerical studies, but further development in component design, materials, control and commercial-scale demonstration is still needed before R718-based HTHPs can achieve broader industrial deployment. The reviewed economic assessments are also encouraging, indicating that R718 systems could offer attractive capital costs and payback periods under suitable industrial operating conditions.

For more information, the review paper is available in open access in Applied Thermal Engineering.

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