Extending the heat delivery envelope of water-based reversed Rankine cycles using a reversed Brayton cycle top stage

High temperature heat pumps are a promising option for decarbonising industrial process heat, but conventional reversed Rankine systems become difficult to extend to very high delivery temperatures because of working fluid critical temperature limits, high pressure ratios, excessive compressor discharge temperatures, and fluid stability constraints. This study evaluates a cascaded heat pump cycle in which a water-based reversed Rankine lower stage is coupled with a reversed Brayton top stage to extend the useful heat delivery envelope of a reversed Rankine cycle. The cascaded reversed Rankine–reversed Brayton cycle is compared with a simple reversed Brayton cycle under identical external boundary conditions. Both systems are designed to deliver 1 MW of heat to a sink with inlet temperatures between 250°C and 400°C and glide temperatures between 50°C and 200°C. For each operating condition, the intermediate coupling temperature, defined by the reversed Rankine condensation temperature, is optimised to maximise the overall COP. The results show that the optimum intermediate coupling temperature increases with sink inlet temperature and decreases with glide temperature. The cascaded cycle improves COP mainly at high sink inlet temperatures and low-to-moderate glide, reaching about 15% improvement at 400°C and 50°C glide under the baseline design. However, this beneficial region also coincides with increasingly severe water-vapour compression conditions in the reversed Rankine stage. The results show that a reversed Brayton top stage can extend the heat delivery range of a reversed Rankine cycle, but the advantage over a simple reversed Brayton cycle is conditional.