Numerical and experimental analysis of a Stirling refrigerator for innovative Ultra Low Temperature applications

The research focuses on advancing Ultra-Low Temperature (ULT) applications by innovating traditional cascade refrigeration systems, through the reversed Stirling cycle, aiming to achieve lower energy consumption, enhanced efficiency and using environmentally friendly fluids in collaboration with an industrial partner KW Apparecchi Scientifici. The Stirling cycle's adaptability to ULT ranges presents a promising alternative to conventional cooling technologies. To accurately simulate and optimize the system, a non-ideal thermal model was developed using a Modified Simple Analysis, which accounts for the non-ideal adiabatic behaviour of the machine and introduces actual heat and work losses that occur in real operation. A Free Piston Stirling Cooler (FPSC) with beta configuration and helium as working fluid are chosen. A focus is placed on the role of the regenerator as a critical component that directly influences the performance of the Stirling cooler with a Computational Fluid Dynamic (CFD) analysis. Theoretical modelling indicates that optimizing the regenerator's design can impact overall efficiency and cooling capacity. The model’s predictions were validated against experimental data obtained from a campaign tests on a defined prototype, as well as cross-referenced with results from published literature to ensure robustness and accuracy. The results obtained proved the reliability of the model results with the experimental ones, both of the defined test bench and of the scientific studies considered, with the same COP obtained between theoretical and experimental tests. The model developed not only offers a reliable foundation for performance analysis and a confirmation of the application of an innovative Stirling cycle over traditional ones, but also serves as a practical design tool with potential applications in commercial sectors, such as medical and biological preservation where ULT cooling is essential. A future application could fit within different thermal energy production, with the advantage of an efficient, compact system using safe and environmentally friendly natural fluids.