Integrated Transcritical CO2 Heat Pump for a Two-Stage Heat Recovery System

Carbon dioxide (CO2/R744) is a natural refrigerant that has been widely studied as a replacement for conventional hydrofluorocarbon-based refrigerants in HVAC&R systems. However, the performance of CO2 heat pumps depends strongly on the system architecture, component parameters, operating conditions, and pressure control. This study examines a CO2-based heat pump integrated into an AHU with two-stage heat recovery. Heat recovery from the exhaust air is initially performed using a regenerative sorption wheel and then in an R744 heat pump with fin-and-tube heat exchangers located directly in the airflows. The operating performance of the heat pump was evaluated through system-level simulation based on commercially available component data and iterative balancing of the refrigeration cycle and air-side processes. For the investigated configuration and selected components, the proposed R744 system is compared with a conventional R410A-based reference system under the same air-side boundary conditions. Therefore, this comparison is interpreted as a system-level benchmark rather than an isolated effect of the refrigerant itself, since performance depends on the refrigerant’s properties, compressor performance, heat exchanger configuration, system architecture, and the operating limitations of the components. Simulations are performed for both heating and cooling modes, while preliminary experimental measurements from a physical prototype are presented for qualitative comparison with the simulated heating-mode trend.