Numerical analysis of working fluids for large scale centrifugal compressor driven cascade heat pumps upgrading waste heat

Heat pump technology has the capability to upgrade unusable low-temperature waste heat and make the upgraded heat usable for different types of industrial and domestic heating applications. Heat upgrading could allow to reach significant energy savings and reduced emissions when compared to other heat production methods. In this paper, upgrading waste heat into a higher temperature with large-scale centrifugal compressor driven cascade heat pumps was studied numerically. A combined method for heat pump thermodynamic analysis and compressor design analysis was developed and implemented in order to investigate the effect of different working fluid combinations and the effect of the cascade heat exchanger temperature level on the heat pump performance. Seven potential fluid candidates were included and condenser temperature of 90 degrees C, evaporator temperature of 20 degrees C, and evaporator heat rate of 1 MW were used in the analysis. It was concluded, that the implemented method can provide an efficient tool for evaluating suitable working fluids and design operating conditions for centrifugal compressor driven high temperature cascade heat pumps. The highest coefficient of performance of 3.08 was simulated by using R601 as the fluid in the low-temperature cycle and R245fa in the high-temperature cycle. These fluids also resulted in feasible compressor geometries and rotational speeds, as well as allowed to reach high compressor efficiencies of well above 80% based on the enthalpy loss models. Coefficient of performances of over 3.0 were simulated also with the fluid combinations R601/R600, R245fa/R245fa and R600/R245fa.