Transient simulation of R744 hybrid ground coupled heat pump with modelica

This article attempts to investigate the Air-conditioning application of R744(CO₂) as a working fluid for a ground coupled heat pump (GCHP) .The system utilises a combination of ambient air and ground as a heat sink in AC mode. According to the characteristic of reverse transcritical CO₂ cycle and ground heat exchanger, CO₂ GCHP dynamic models are developed and the transient performance is simulated in the Modelica/ Dymola environment. During the simulation, the short term response borehole model is utilized to analyse the transient thermal response of borehole to the CO₂ GCHP system. Simulation result shows the outlet fluid temperature from the borehole has a significant effect on the CO₂ heat pump system, while the introduced air-cooled gas cooler well improves the concerned performance, meanwhile the optimal capacity of air-cooled gas cooler and its optimal operating pressure is also provided. The developed hybrid CO₂ GCHP system shows good competitiveness in the air conditioning application and the optimal averaged system COP is obtained with 3.45.