Performance Mapping of Natural Refrigerants for Low-Pressure Steam and Hot Water Generation.

Industrial process-heating systems frequently require both hot water and low-pressure steam, creating a cycle selection problem that depends on the available heat source temperature, required steam temperature, and relative proportions of the two heating demands. This study develops performance selection maps for natural refrigerant high-temperature heat pumps supplying steam at saturation temperatures between 100°C and 150°C from heat sources represented by evaporating temperatures between approximately 10°C and 80°C. Three configurations were modelled under steady-state conditions: a single stage vapour compression heat pump with internal heat recovery, an R717-hydrocarbon cascade heat pump, and an R717 heat pump coupled with staged mechanical vapour recompression. Steam fractions of 25%, 50%, 75%, and 100% of the total useful heating demand were considered. Feasible operating points were constrained by a maximum compressor pressure ratio of 8, maximum outlet pressure of 66 bar, and maximum discharge temperature of 180°C. Candidate systems were ranked using a composite metric weighted 80% towards coefficient of performance relative to the Carnot COP and 20% towards normalised volumetric heating capacity. At steam loads of 25% and 50%, hydrocarbon cycles were preferred throughout the mapped operating range. Cascade cycles dominated at low evaporating temperatures, while single stage R600a, R600, and R601 cycles became preferable as the heat-source temperature increased. R600a was generally favoured at lower steam temperatures because of its high volumetric heating capacity, whereas R600 and R601 became competitive at progressively higher steam temperatures. At steam loads of 75% and 100%, the R717 heat pump with mechanical vapour recompression dominated the higher steam-temperature region. The resulting maps provide a preliminary screening tool for selecting natural refrigerant cycle architectures for mixed industrial steam and hot water duties.