Theoretical and Experimental Analysis of Strategies to Improve the Performance of Transcritical CO2 Condensing Units for Low Temperature Refrigeration Applications

• Autores: Miguel Ávila Gutiérrez
• Directores de la Tesis: Bernardo Peris Pérez (dir. tes.), José Manuel Salmerón Lissen (dir. tes.)
• Lectura: En la Universidad de Sevilla ( España ) en 2026
• Idioma: inglés
• Número de páginas: 215
• Enlaces
  • Tesis en acceso abierto en: IdUS
• Resumen

  • This doctoral thesis investigates advanced strategies to enhance the performance of transcritical CO?refrigeration systems for low-temperature applications, a segment that remains particularly challenged under warm and temperate climate conditions. The work focuses on small- to medium-capacity systems based on two-stage compressors with vapor injection from a flash tank, which are widely adopted in commercial practice but still exhibit significant efficiency penalties during transcritical operation.

    The thesis begins with a comprehensive experimental characterization of a reference Flash-Tank Vapor Injection (FTVI) system. Particular emphasis is placed on the influence of refrigerant charge and phase separation behavior, demonstrating that flash-tank-based architectures can conceal undercharge conditions in commonly monitored variables while still suffering substantial losses in cooling capacity and coefficient of performance. This analysis establishes a rigorous experimental and methodological baseline, ensuring that subsequent comparisons of advanced configurations are not biased by hidden charge-related effects.

    Building upon this foundation, the work explores the integration of dedicated subcooling strategies as a means to reduce throttling losses and improve system efficiency. A semi-empirical assessment is conducted for a Dedicated Ejector Subcooling (DES) concept, in which compressor waste heat is recovered to drive a thermally activated ejector refrigeration subsystem. The results confirm that this approach enables the generation of significant subcooling with negligible auxiliary energy demand, translating almost directly into performance improvements comparable to those obtained with conventional mechanical subcooling.

    The core contribution of the thesis is the development, analysis, and experimental validation of the Ejector Subcooling Vapor Injection (ESVI) cycle. This novel configuration integrates a two-phase CO?ejector and a subcooler into the FTVI architecture, allowing the ejector to act as part of the expansion process while simultaneously providing subcooling upstream of the main throttling device. A detailed thermodynamic analysis demonstrates that the ESVI cycle yields increasing benefits under severe transcritical conditions and low evaporating temperatures. These predictions are subsequently validated through experimental testing on a purpose-built CO? prototype operating at evaporating temperatures down to ?40 °C.

    Experimental results confirm that the ESVI configuration achieves substantial liquid subcooling, leading to a pronounced reduction in flash-tank vapor quality and pressure, increased enthalpy lift across the evaporator, and measurable improvements in cooling capacity and coefficient of performance. Beyond the numerical gains, the thesis highlights the importance of critically interpreting discrepancies between modeling and experimental observations, emphasizing the role of prototype-level phenomena in advanced cycle implementations. Overall, the work demonstrates that ejector-based subcooling and vapor injection architectures constitute a technically viable and industrially compatible pathway to enhance the efficiency of transcritical CO? refrigeration systems for low-temperature applications.