Analysis of the CO2 + C2Cl4 mixture in high temperature heat pumps: Experimental thermal stability, liquid densities and cycle simulations

Abstract

Transcritical heat pumps working with CO₂-based mixtures with a low-volatility dopant are found to achieve good performances in thermally integrated heat pumps, especially when sensible heat sources and heat sinks are considered. This paper introduces in literature tetrachloroethylene, C₂Cl₄, as CO₂-dopant for the mixture to be adopted as working fluid in high temperature heat pumps. To calibrate the thermodynamic model used in the cycle simulations, an experimental characterization on the mixture is proposed: liquid densities of the mixtures are measured, in a wide range of concentration, optimizing the binary interaction parameter of the Peng Robinson equation of state. Moreover, the thermal stability of pure C₂Cl₄ is experimentally evaluated, identifying the maximum allowable compressor outlet temperature between 200 °C and 250 °C, with a decomposition rate below 1 %/year if the fluid is kept at temperatures around 200 °C. Then, the potentialities of this very high temperature heat pump are assessed in spray dryer applications: a coefficient of performance around 3.38 is obtained for a conventional spray dryer plant, corresponding to 73 % of second law efficiency, considering an air flow heated from ambient temperature to 200 °C as the sink, while cooling the sensible heat source, available at 76 °C, below 30 °C. As term of comparison, the same system adopting propane, instead of the CO₂ + C₂Cl₄ mixture, would achieve a coefficient of performance and second law efficiency of 2.94 and 64 %, respectively.