In the context of extremely cold or hot climates, ground coupled heat pumps offer several environmental and energy efficiency advantages over conventional methods for heating and cooling of buildings and they are increasingly used in most near-zero and net-zero energy buildings. However, the high initial cost of the ground loop portion (geothermal boreholes) has often raised the question on the economical competitiveness of the system. In the present study, a geothermal borehole is proposed in which two-phase carbon dioxide (mixture of liquid and vapor) exchanges heat with the ground to improve the thermal performance of the borehole and thus to reduce borehole length. Carbon dioxide shows several cost and environmental advantages. Moreover, it offers superior thermophysical properties and heat transfer characteristics.

A numerical model has been developed to study the complex thermal behavior of a two-phase CO2-filled vertical geothermal borehole. The model can handle both two-phase and single-phase conditions along the borehole length. An explicit solution for fully coupled conservation equations of mass, momentum and energy as well as an equation of state are applied. The model accounts for the thermal interaction among the pipes and it predicts the fluid temperature, pressure and two-phase quality profiles. It is used to assess the thermal performance of the CO2-filled secondary loop geothermal borehole operating in heating mode. Results indicate that the proposed borehole offers superior performance due to the relatively high two-phase heat transfer characteristics of CO2.

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