Comparative evaluation of COP and phase change impacts in reversed rankine and brayton cycles for high-temperature heat pump systems

• Case study shows Rankine COP 3.48 at sink 150–200 °C and source 120–110 °C with ∼72 % from phase change. • Under the same bounds Brayton achieves COP 1.84 with sensible heat only. • The reversed Rankine is superior while the sink is below about 310 °C along with higher exergy efficiency. • Above about 310 °C the latent advantage disappears, and Brayton becomes competitive with higher exergy efficiency. • Reversed Rankine cycle outperforms when latent heat contribution exceeds 50% of total COP. The growing demand for industrial heat has sparked fresh interest in high-temperature heat pumps (HTHPs) because they can offer a low-cost method to upgrade low temperature waste heat. In this study, two conventional heat pump cycles, a reversed Rankine cycle utilising water and a reversed Brayton cycle employing air as the working fluid are compared. The analysis specifically examines the impact of phase change within the Rankine cycle on overall performance of the heat pump by breaking down the coefficient of performance (COP) into sensible and latent heat contributions. It was seen that when the heat sink temperature increased from 150 °C to 200 °C and the heat source temperature decreased from 120 °C to 110 °C, the coefficient of performance (COP) of the reversed Rankine cycle reached 3.48, with 72.13 % contribution from phase-change heat and 27.87 % from sensible heat. In contrast, the reversed Brayton cycle achieved a COP of 1.84, entirely governed by sensible heat transfer. It was observed that at lower heat sink temperatures (below approximately 310 °C), the reversed Rankine cycle clearly benefits from the latent heat associated with water phase change, leading to significantly improved COP and less exergy destruction compared to the Brayton cycle. However, most of the latent heat advantage of reversed Rankine cycle disappears as the heat sink temperature approaches the critical point of water at the given pressure and the reversed Brayton cycle becomes more competitive. Collectively, the findings provide practical guidelines for selecting suitable HTHP systems based on the target operating temperature and the feasibility of employing water or air as working fluids in real world industrial applications.