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Carbon dioxide evaporation process in direct expansion geothermal boreholes

Badache, Messaoud
Eslami, Parham
Bastani, Nejad Arash
Aidoun, Zine
Ouzzane, Mohamed

Ground Heat Exchangers (GHE) play an important role in the performance of Ground Source Heat Pumps (GSHP). The impact is even more significant in direct expansion GSHP (DX-GSHP) systems as the refrigerant used in the heat pump also acts as the heat transfer fluid for the GHE. In this study, several experiments were carried out to investigate the performance of GHEs in a carbon dioxide (CO2) DX-GSHP. The evaporation of CO2 in the GHE was studied under various mass flow rates and number of active boreholes. For this purpose, a transcritical CO2 DX-GSHP test facility was built and fully equipped at CanmetENERGY-Varennes research laboratory. It was found that a partial two-phase flow regime along the GHE decreases the performance compared to the full two-phase flow and it has to be avoided for more efficient DX-GSHP systems.