Energy and environmental performance analysis of solar-assisted heat pump drying system using natural working fluid carbon dioxide

The solar-assisted heat pump drying system is an effective method for reducing carbon emissions. In this study, a solar-assisted heat pump drying system using the natural working fluid CO2 is proposed and its thermodynamic characteristics, as well as life cycle energy consumption and environmental performance, are analyzed. The results indicate that the highest coefficient of performance (COP) and optimal discharge pressure is achieved when two pinch points appear in the gas cooler of the transcritical CO2 heat pump drying system. The solar-assisted transcritical CO2 drying system annually consumes 59.99 tons of standard coal, which is 35.48% lower than the steam-assisted drying system. Comparing the solar-assisted transcritical CO2 drying system to the transcritical CO2 drying system and the steam-assisted transcritical CO2 drying system, it is found that the former has the lowest life cycle primary energy consumption and carbon emissions, which are 2688.816 tce and 5876.02 t, respectively. Furthermore, compared to the steam-assisted transcritical CO2 drying system, the solar-assisted transcritical CO2 drying system, when using natural gas as boiler fuel, reduces emissions of gaseous pollutants (SO2 and NOx) and solid pollutants (PM2.5 and PM10) by 28.29%-36.09%. This paper provides theoretical guidance for the development and operation of high-efficiency and low-emission high-temperature heat pump drying systems.