Control Co-Design of a CO2-based Chiller System

Natural refrigerant R744 (i.e., CO2, Carbon Dioxide) chillers are part of the current and next generation of chillers and find applications in various sectors, ranging from industrial processes to HVAC&R (Heating, Ventilation, Air Conditioning, and Refrigeration) systems, including commercial refrigeration. This technology is considered a sustainable and environmentally friendly alternative to traditional refrigerants with practical well proven performance, enabling energy efficient and safe installations. However, designing and controlling a CO2 chiller poses new challenges due to its unique thermodynamic and fluid dynamic properties compared to conventional chillers. These characteristics strongly influence decisions made during the design or retrofitting of existing systems, where a mere refrigerant replacement approach to achieve energy-efficient system configurations would fall short. Understanding the potential improvements in system design and control, as well as their impact on costs and energy demand, represents the initial step toward fostering effective technological developments, strengthening industry capabilities, and generating market interest. Within this framework, this doctoral Thesis centers on the control co-design (CCD) of a chiller that uses CO2 as a refrigerant. This involves the concurrent and integrated design of both the control system and the physical components of the cooling apparatus. By explicitly accounting for interactions between the control algorithms and the thermodynamic processes within the chiller, CCD aims to identify innovative solutions in both physical and control system design, paving the way for new levels of performance, efficiency, and enhanced functionality. The study was funded by the Italian National Research Council (CNR) and was carried out in collaboration with the University of Illinois Urbana-Champaign (USA) and the University of Minnesota – College of Science and Engineering (USA). Key aspects of CCD for the CO2 chiller include: dynamic system modelling through a graph-based approach, the definition of the CCD problem in terms of an optimization problem and its solution via black-box optimization. The outcomes reveal that the simultaneous optimization strategy offered by the CCD yields superior performance compared to the conventional sequential design process, wherein control designs are typically developed at the end, once the mechanical, thermodynamic, electrical, and other subsystems are completely defined.