Advancing sustainable air conditioning with CO2 heat pump and earth-air heat exchange integration

The early development of refrigeration technology relied on natural refrigerants such as carbon dioxide (CO 2 ), ammonia, and hydrocarbons. However, technical limitations and safety challenges led to their replacement by synthetic refrigerants. With increasing environmental concerns over high global warming potential (GWP) synthetic refrigerants, renewed attention has turned to natural alternatives, particularly CO 2 , valued for its non-flammability, low toxicity, and negligible GWP. Despite these advantages, CO 2 's low critical temperature (30.98 °C) restricts its efficiency in air conditioning applications, as it often requires operation in transcritical cycles that are inherently less efficient. This study introduces a novel integration of a CO 2 air conditioning heat pump (CO 2 -ACHP) with an Earth-Air Heat Exchanger (EAHE) to overcome this limitation by maintaining the condensing temperature below CO 2 's critical point, thus enabling more efficient subcritical operation. A mathematical model was developed to simulate the subsoil thermal behaviour in Sydney, Australia, revealing stable ground temperatures between 15.5 °C and 20.1 °C at a depth of 2.5 m. Using a 35-m-long PVC pipe, the EAHE provided an outlet air temperature of approximately 19.5 °C, effectively reducing the CO 2 condenser temperature and improving system performance. The proposed CO 2 -ACHP/EAHE system demonstrated a 30 % improvement in coefficient of performance (COP) compared to conventional R-410A systems and achieved a 59 % enhancement relative to a CO 2 transcritical system. Correspondingly, total equivalent warming impact (TEWI) decreased by 32 % compared to R-410A and 59 % compared to the transcritical CO 2 baseline. These results confirm the system's potential as an energy-efficient and environmentally sustainable alternative for air conditioning, introducing a novel pathway for enhancing CO 2 cycle efficiency through passive ground temperature regulation. • Stable subsoil temperatures facilitate efficient CO 2 cooling through EAHE integration. • The hybrid system achieves a 30 % reduction in energy consumption compared to R-410A. • This study underscores the viability of CO 2 in air conditioning, broadening its scope of application. • The system reduces total environmental impact by 32 % relative to the conventional R-410A cycle. • The hybrid system demonstrates 59 % lower energy usage than standalone CO 2 transcritical cycles.