Reactor Engineering of Salt-Hydrate Thermochemical Heat Transformers for Industrial Waste-Heat Upgrading: A Systematic Review

Abstract

Low- and medium-temperature industrial waste heat below about 200 °C remains an underutilised energy resource because its temperature is often too low for direct reuse. Salt-hydrate thermochemical heat transformers (TCHTs) have been investigated as a means of upgrading this heat through reversible gas-solid hydration and dehydration reactions. This systematic review examined reactor engineering aspects of salt-hydrate TCHTs, including reactor configurations, reaction-transport modelling, reactor performance, exergy, techno-economic considerations and technology readiness. The review was conducted in accordance with the PRISMA 2020 guideline. Relevant publications between 2012 and July 2026 were identified through searches of Scopus, Web of Science, Google Scholar, ScienceDirect, SpringerLink, and relevant MDPI journals, and 30 studies met the predefined inclusion criteria. The literature indicated that reactor engineering has become a greater challenge to practical deployment than material selection alone. Packed-bed, fluidised-bed and moving-bed reactors were compared in terms of heat and mass transfer, operating characteristics and suitability for continuous operation. Among the reactor concepts reviewed, crossflow moving-bed reactors appeared to offer the greatest potential for industrial waste-heat upgrading, although the available evidence is still based largely on modelling studies. Experimental validation under industrial operating conditions, reactor-scale exergy analysis, techno-economic assessment and consistent performance reporting remained limited. These areas require further investigation before the technology can be evaluated confidently for large-scale industrial application.