Design and optimization of a cascaded rooftop heat pump with a centrifugal compressor for cold climates
With the competitive landscape of the HVAC market characterized by high-performance systems and diverse global regulations for refrigerants, achieving competitiveness requires integrating advanced systems. Conventional rooftop heat pumps (RTHPs) often face significant challenges in severe cold climates, including capacity reduction and increased compressor discharge temperatures. To address these limitations, this study explores an advanced cascaded cold-climate rooftop heat pump (CCRTHP) architecture that utilizes ultra-low GWP refrigerants. Specifically, it employs R1234ze(E) in the upper stage with a small-sized centrifugal compressor, and CO2 in the lower stage with scroll compressors. Given the inherent lower pressures and reduced volumetric capacity of R1234ze(E), meeting thermal capacity targets demands increased volumetric flow rates and larger system footprints. Additionally, while CO2 refrigerant excels in heating performance, its efficiency diminishes during cooling operations due to supercritical operations at elevated ambient temperatures. Overcoming these challenges necessitates detailed design and optimization studies. System-level optimizations were conducted to identify the optimal configurations for heat exchangers, compressor displacement volumes, rotational speeds, and necessary efficiencies across various design conditions. A high-fidelity steady-state cycle model was employed to conduct parametric analyses and component sizing, while a dynamic cycle model was used to investigate control strategies. Preliminary findings indicate a rated cooling coefficient of performance (COP) of 3.1 at a nominal capacity of 70.3 kW (20 tons), alongside a heating COP of 4.0 at the rated conditions. Additionally, the system retains 100% of rated capacity with a COP of 2.4 at -15 °C ambient temperature. These results highlight the potential of cascaded CCRTHP systems as efficient, economically viable solutions for demanding cold climates.