Hybrid CO2 air source heat pump system integrating with vapor injection and mechanical subcooling technology for space heating of global application: Life cycle techno-energy-enviro-economics assessment

Abstract

As a heating method for the sustainability of residential buildings in clean space heating, carbon dioxide (CO2) air source heat pump is an environment-friendly solution to replace fuel-fired boiler. Four hybrid configurations of vapor injection integrated with dedicated mechanical subcooling (VI + DMS) technologies are developed, to deal with the problem of traditional CO2 system energy efficiency deterioration in cold regions, and compared with traditional transcritical CO2 baseline heat pump (BASE), R410A heat pump, coal-fired boiler and four CO2 heat pump systems adopting single technique of VI or DMS. Genetic algorithm is utilized to optimize the operation variables. The comprehensive techno-energy-enviro-economics performances of the nine systems among the whole lifetime are assessed. The influences of heating terminal selection and climate condition are discussed for 40 typical cities in 8 climate zones around the world. The results demonstrate a maximum coefficient of performance (COP) is obtained for transcritical CO2 vapor injection heat pump with flash tank and dedicated mechanical subcooling (VI-FT-DMS), and the COP of VI-FT-DMS system using traditional designed radiator as heating terminal is as high as 1.94 at the ambient temperature of −30 °C. The hybrid VI + DMS CO2 heat pump achieves a significant enhancement in heating season performance factor of 10.61 ∼ 30.37 % and the throttling irreversibility can be remarkably reduced with the exergy efficiency promoted by 12.76 ∼ 64.34 %. The life cycle cost is declined by 14.71 ∼ 22.38 % in contrast to BASE, and life cycle carbon emission reduces by 4.77 ∼ 22.12 %, indicating apparent advantage in environmental performance. The hybrid VI + DMS CO2 heat pump system is a promising solution to apply for clean space heating in the cold, very cold and subarctic zones.