Refrigeration with eco-friendly fluids. Development of refrigeration systems with CO₂-based refrigerant mixtures.

This thesis explores the development of sustainable and high-performance refrigeration systems using natural, non-toxic, and non-flammable refrigerants, particularly carbon dioxide (CO₂) and carbon dioxide-based mixtures. The interest towards CO₂ is dictated by the fact that many traditional refrigerants have significant environmental impacts and are nowadays banned in Europe under the F-Gas regulation. Hydro-olefins, given their low GWP, have been regarded as a promising alternative, but require careful attention due to potential environmental long-term effects. As a first step, a general, steady-state thermodynamic model of inverse cycles was developed using Python and REFPROP 10. The model can accommodate most common cycle configurations, including regeneration and expansion work recovery through an ejector. Several fluid compositions may be implemented. Optimization algorithms were used to optimize conditions at the condenser and evaporator outlets. Mixing CO₂ with refrigerants such as R1234yf, propylene and dimethyl ether (DME) significantly improved system performance, due to reduced critical pressure, increased critical temperature and matching between the mixture glide and imposed temperature lift at the cold heat exchanger. R1234yf achieved a Coefficient of Performance (COP) increase of up to 12.8% in the base configuration with an internal heat exchanger (IHX). The highest absolute COP was obtained with the double ejector configuration, although the relative improvement compared to pure CO₂ was only 4.3%. A first outcome of this thermodynamic analysis was the idea of using the internal heat exchanger to complete the evaporation. In this way, a higher percentage of secondary refrigerant could be used for a given temperature lift at evaporator. This approach resulted in superior performance, with CO₂ - DME mixture achieving a COP increase of 25% while maintaining a GWP of 1. To this aim, a mere 8% of DME is needed, so that the mixture turns out to be non-flammable. Such outstanding result suggested to concentrate our effort on CO₂ - DME mixtures. Meanwhile, a critical review and improvement of flow boiling correlations for CO₂ and CO₂-based mixtures were conducted. A novel flow boiling correlation was proposed, achieving a mean error prediction of 28% for pure CO₂ and 37% for CO₂-propane mixtures. Another relevant aspect is the fluid solubility in lubricant oil. Experimental investigations revealed that DME has a greater solubility in polyalkylene glycol (PAG) oil compared to propylene. New empirical correlations may improve modelling accuracy. Finally, an experimental validation of the most important results was conducted, using an experimental bench equipped with a CO₂ scroll compressor and high-pressure brazed plate heat exchangers. The experimental findings confirmed the effectiveness of the CO₂-DME mixture, showing a maximum COP increase of 15% for a mixture with 10% DME and a condensation temperature of 35°C. Uncertainties concerning the actual circulating composition were addressed using gas chromatography and Coriolis mass flow meters, confirming the accuracy of the Johansson method with an average absolute error of 0.64%. The measured evaporative heat exchange was compared with calculated values, identifying the Silver and Bell & Ghaly method coupled with the Amalfi and Cooper correlations as the best-performing model, with a mean absolute relative error of 1.17%. This research concludes that CO₂-based mixtures, particularly with DME, can enhance the efficiency and performance of refrigeration systems, contributing to the development of sustainable technologies with reduced environmental impact.