Experimental Investigation of R290 and R290/R600a Mixtures in Small-Scale Refrigeration Systems Coupled with Ice Storage

This PhD thesis focuses on the optimization of small-scale vapor compression refrigeration systems coupled with latent thermal energy storage (LTES) units, with particular emphasis on the use of natural refrigerants and their zeotropic mixtures. The research was developed as part of an industrial collaboration aimed at improving the performance of Sparkling Water Dispensers (SWDs), which are typically based on Ice Bank thermal storage technology. As a first part, following an initial exploratory phase based on iterative prototyping, a modular experimental setup was developed to enable systematic investigations under controlled and repeatable conditions. The second part of this work presents a detailed evaluation of the effects of refrigerant charge, system configuration, and boundary conditions on the steady-state performance of the cycle, using pure propane (R290) as working fluid. After the introduction and experimental validation of several performance indicators, such as coefficients of performance, compressor efficiencies, and deviation indices from ideal cycle behaviour, the experimental data allowed the development of interpolated performance maps. These maps offer practical guidance for system design and optimization. Then, the experimental campaign was extended to R290/R600a blends with various mass ratios (60%/40%, 50%/50%, 40%/60%). These mixtures showed significant performance improvements, with COP increases up to 23% compared to pure R290. However, the blends also demonstrated greater sensitivity to instability phenomena, such as evaporator flooding, at high charge levels during dynamic tests. To support the refrigerant charge optimization process, a steady-state component-based model for refrigerant charge estimation was developed and experimentally validated using both pure R290 and R290/R600a mixtures. The model demonstrated good accuracy and proved to be a simple but effective tool for design applications. The third part of the thesis focuses on the ice formation dynamics on the evaporator surface during the storage phase. A series of tests was conducted to evaluate the influence of refrigerant type, charge level, and system configuration on solidification behaviour, thermal performance, and operational stability. Particular attention was given to the role of evaporating temperature and its effect on ice distribution, especially in presence of zeotropic mixtures. In the fourth section, the integration of a passive suction line heat exchanger (SLHX) between the capillary tube and compressor inlet was evaluated. Results from both steady-state and transient tests showed an improvement in system efficiency and operating stability. During the ice formation phase, the SLHX allowed a greater amount of energy to be stored before the onset of evaporator flooding and helped reduce the compression ratio in post-flooding conditions, enhancing overall system robustness. In the appendix, a complementary experimental study is reported, conducted in collaboration with the Applied Thermodynamics and Heat Transfer (ATHT) group at Ghent University. This investigation focused on a heat pump operating with R32 as refrigerant and equipped with a vapor injection compressor. Even if complementary to the core focus of the thesis, the study provided valuable insights into strategies for discharge temperature mitigation and efficiency enhancement in advanced heat pump configurations. The results highlight the role of vapor quality and injection mass flow in optimizing thermodynamic performance and contribute to the understanding of low GWP refrigerant management. The results presented in this work contribute to the understanding and optimization of compact refrigeration systems integrated with latent thermal energy storage. The methodologies and tools developed are applicable to a broader range of scenarios, including commercial, domestic, and off-grid applications. Future developments are oriented toward the integration of advanced control strategies to dynamically adapt the system to varying operating conditions. Another promising research direction concerns the design of heat exchangers optimized to take advantage of the temperature glide of zeotropic mixtures. When properly combined with distributed pressure drops, the temperature glide can help make the evaporating temperature profile more uniform, consequently improving ice distribution and enhancing system stability in LTES-integrated refrigeration units. Additional developments include the extension of the refrigerant charge model to dynamic regimes, with the goal of enabling predictive control algorithms and the deployment of intelligent energy management systems.