ABSTRACT: Thermal performances of several types of vertical ground heat exchangers (GHEs) were investigated numerically in discontinuous operation mode. The heat buildup in the ground surrounding the borehole is alleviated with stabilizing the ground temperature in the off-time. The discontinuous operation modes of 6, 10, and 12 h operations in a day of the ground source heat pump (GSHP) system are feasible for residential and commercial buildings. The performances of three types of GHEs for GSHP system including U-tube, double-tube and multi-tube have been investigated in discontinuous operation modes and also in continuous operation mode to consider the different characteristic of their performances. Water flows through the heat exchangers and exchanges heat with the surrounding ground. The GHE models were simulated using the commercial computational fluid dynamics (CFD) software FLUENT. The simulation model was applied in 10 days operation with discontinuous and continuous operation modes. The thermal performance of the GHEs descends gradually due to increasing the ground temperature around the borehole. The heat exchange rates of the three types of GHEs in discontinuous operation modes increased significantly compared with that of in continuous operation mode. As an example, the minimum heat exchange rate in discontinuous of 6 and 12 h operations in a day increase of 42.6 % and 18.6 % for U-tube, 50.7 % and 19.3 % for double-tube, and 41.8 % and 18.3 % for multi-tube, respectively. Operating the GHEs in discontinuous operation mode may reduce the borehole depth such as 2.5 to 5 m for U-tube and multi-tube, and 3.5 to 8 m for double-tube. It can be constructive information to get the optimum design of GHE system in practical engineering.
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