Assessment of fluid mixtures in transcritical high-temperature heat pumps exploiting waste heat with large temperature glides

The work investigates innovative compact configurations of high temperature transcritical heat pumps with sensible heat sinks that upgrade sensible waste heat with significant source exploitation. In this context, CO2 mixtures with heavier dopants are suggested as working fluids to properly match the temperature profile at the evaporator and transcritical cooler. Initially, a methodology is presented to analyze the cycle behavior and optimize its performances by modifying the necessary input parameters: for a representative mixture (CO2+acetone) and assuming a heat source of 100 °C, performance maps of the different heat pump configurations are reported as a function of the temperatures in the evaporator and cooler. The results obtained highlight that the COP in these conditions largely benefits by using a single-phase expander and by setting the cycle minimum pressure at high values, close to the pressure at the mixture cricondentherm. Then, a case study based on the production of hot pressurized water (140-200 °C) is examined, comparing simulations of the CO2+acetone mixture with mixtures previously analyzed in literature. It is further underlined that mixtures under these circumstances allow for very limited pressure and volumetric ratios at the compressors, around 2-4, exploiting the heat source for a temperature range much higher than pure fluids. Expanding the analysis to subcritical heat pumps it is demonstrated that these are less favorable than transcritical ones at same boundary conditions, as subcritical solutions present higher mixture maximum temperatures, over the thermal stability limit of the fluid, sub-atmospheric pressures and way higher compressor volume ratios.