Performance analysis of a high-temperature heat pump based on a cascaded reverse Brayton and vapor compression cycle

Heat pumps are used to recover waste heat in industries and transfer it to other processes at elevated temperatures. Efforts are directed towards enhancing the heat pump's ability to achieve high heat sink temperatures, often surpassing 100 degrees C, termed as high-temperature heat pumps. Additionally, concerns over the global warming potential of refrigerants used in these heat pumps have led researchers to explore new environmentally friendly alternatives aiming to maintain functionality while minimizing environmental impact. This paper investigates the design and selection of a heat pump for an industrial project requiring the processing of waste heat from 100 degrees C to 300 degrees C. It evaluates different architectures including a reverse Brayton cycle, a multi-stage expansion reverse Brayton cycle, and a cascaded reverse Brayton cycle with a vapor compression cycle increasing the coefficient of performance from 1.74 to values exceeding 2.0. It also observes the performance of the cascaded cycle under the usage of different low global warming potential refrigerants. Refrigerants such as R1233zd(E), R1336mzz(Z), R1234ze(Z), and R1224yd(Z) are analyzed for their potential to further enhance cycle performance. The cascade system with the refrigerant R1224yd(Z) in the vapor compression part gives the best result among these four. Performance comparisons are made resulting in the cascade cycle giving a 4.7 % and 13.4% improvement when compared to the multi-stage expansion and simple reverse Brayton cycle, respectively. These results demonstrate that the cascaded cycle using eco-friendly refrigerants can enhance industrial waste heat recovery, providing a sustainable and efficient solution for high-temperature applications, such as chemical processing and food manufacturing, where significant energy savings and reduced emissions are essential.