A benchmark comparative thermodynamic analysis of transcritical CO2 heat pump configurations for heating purposes

Heating systems using R744 refrigerant are attractive and sustainable choices for heat pump configurations for space heating. The unique feature of this study is that it compares multiple fundamental transcritical CO 2 heat pumps to identify the most energy-efficient and environmentally favorable option for heating purposes. The reference system includes an evaporator, compressor, throttle device, and gas cooler. It is evaluated against a system with an internal heat exchanger, an ejector, double-stage compression, and a cascade compression system. The importance of this study lies in the comprehensive optimization and parametric analysis across the ambient temperatures from -20°C to 20°C. The systems are coupled with either fan coils or radiators, producing heat at return/supply water temperatures 40/45°C or 60/75°C, respectively. A comparison with conventional heat pump units is also performed. The systems are applied to a residential building in Athens, Greece, serving its heating energy loads, and the analysis is performed with MATLAB-developed models, validated according to other studies in existing literature. Results indicate that all investigated systems outperform the basic system across all operation conditions. The cascade CO 2 /R600 compression system achieves the highest electricity savings of 29.3% but significantly increases the total equivalent CO 2 emissions up to 423.8%. The ejector-assisted system combines energy efficiency, up to 13.2% COP improvement, and a decrease in the total equivalent warming impact, up to 6.5%. The systems with the internal heat exchanger and the double-stage compression achieve maximum COP improvement of 8.0% and 8.8% and total equivalent warming impact reduction of 4.5% and 4.0%, respectively.