Carbon dioxide (CO2), also known as R744, is one of the natural substances, which is an environmentally benign, safe and economical refrigerant used for cooling and heating systems. The transcritical CO2 heat pump water heater has attracted much attention from researchers in recent years, due to that the CO2 water heater can contribute to a significant reduction in energy consumption. The CO2 heat pump system can provide hot water with higher efficiency compared to a traditional electric or gas water heater and can achieve a higher delivery temperature compared to other water heat pumps. Based on existing studies, limited research has considered using the transcritical component of CO2 to deliver a high enough temperature suitable for high-temperature water/steam production. Although an upper-temperature limit exists in current CO2 systems, combining this with an electrically driven auxiliary heater can deliver a low-cost steam/water production solution, which can be one of the promising technologies for industrial process heating demand. The present study aims to address the limitations of existing CO2 technology by operating an auxiliary heater and employing an internal heat exchanger for further increasing discharge temperature and heat capacity. Four thermodynamic cycles have been identified and investigated with the consideration of industrial applicability. The numerical analysis for a transcritical CO2 heat pump with the auxiliary heater has been carried out. Several preliminary results with different discharge pressures are described. The results show that the total efficiency of the system will slightly increase with the discharge pressure if the targeted discharge temperature is fixed, and the results of energy distribution show that the ratio of auxiliary heating power to CO2 heating power decreases with the discharge pressure. Additionally, the mathematical model for Bitzer semi-reciprocating compressors is validated against the experimental data with acceptable agreements.     «

Carbon dioxide (CO2), also known as R744, is one of the natural substances, which is an environmentally benign, safe and economical refrigerant used for cooling and heating systems. The transcritical CO2 heat pump water heater has attracted much attention from researchers in recent years, due to that the CO2 water heater can contribute to a significant reduction in energy consumption. The CO2 heat pump system can provide hot water with higher efficiency compared to a traditional electric or gas...     »