Thermodynamic aspect of refrigerant fluids: valorisation of CO2 and CO2 mixtures

In 1987, the Montreal Protocol, required a worldwide phase out of ozone depleting chlorofluorocarbons (CFC) and after the Kyoto Protocol in 1992 it was amended to phase-out the use of hydrochlorofluorocarbons (HCFC) as well. The usage of hydrofluorocarbons (HFC) was not affected by this protocol as they pose no harm to the ozone layer (no chlorine), however they are greenhouse gases with high GWP. In October 2016 in Kigali, a new global deal was reached to cap and reduce the use of HFC. To reach this target, one solution is the use of “natural refrigerants” such as carbon dioxide (CO2), hydrocarbons, ammonia or blends with CO2 as working fluids in refrigeration and air conditioning systems.

Understanding of phase diagrams are critical in the design of refrigeration systems and heat exchangers and this will be researched through this project. It is important to know the state of the mixture (single phase, liquid, gas) and if the blend behaves like a pure component (azeotropic behaviour). Knowledge of other thermophysical properties, such as density, calorific properties, viscosity, thermal conductivity of the fluid is also important.
The project will measure the necessary experimental data using several experimental setups (vibrating tube densitometer, viscometer, speed of sound…). Models will be used in order to correlate the data. The models are required to provide accurate determination of densities and other calorific properties to be useful in the design of refrigeration systems (compressor, valve) and other heat exchangers. The densities will be determined using SAFT type, cubic or multiparameter equations of state. The parameters will be adjusted using vapour-liquid equilibrium, critical points (for transcritical cycles) density data and speed of sound.