Theoretical and experimental analysis of a vertical direct expansion geothermal evaporator using CO2 as refrigerant

Abstract

In direct expansion ground coupled heat pumps (DX-GCHPs), the borehole heat exchanger (BHE) plays a major role, as it is the component where heat transfer between the ground coupled heat pump (GCHP) and the soil occurs. To address the current challenges of using CO2 as refrigerant in DX-GCHP systems, theoretical and experimental investigations were performed on a single U-tube vertical direct expansion BHE using CO2 as the refrigerant. First, an experimental setup of a transcritical CO2 DX-GCHP built at CanmetENERGY laboratories was used to monitor, collect, and analyze data for refrigerant flow, pressure drop, and ground temperature along the boreholes. The CO2 evaporation process in the BHE for different mass flow rates per borehole was then experimentally analyzed. Later, a previously developed theoretical model of the BHE was validated using a set of experiments collected with the current test apparatus. Finally, the model was used for the analysis of the effect of the following parameters on the performance of the BHE: (i) the refrigerant flow rate, (ii) the borehole wall temperature and average fluid temperature over the test period during the evaporation process, (iii) the pressure drop, (iv) the required power for CO2 circulation, and (v) the evaporating temperature change in the BHE.