Performance improvements of a ground sink direct cooling system under intermittent operations

Ground heat exchangers (GHEs) present declining performance during continuous running. The purpose of this paper is to study the effects of intermittent operations on improving the heat transfer of GHEs in a ground sink direct cooling system (GSDCS), and then optimize the on-time and off-time durations of the intermittent cycles. A numerical model of a GHE was developed with experimental validation. The influences of different flow rates and ground temperatures on the GHE performance were studied in direct cooling mode. Different operation modes were adopted to investigate the intermittent performance of the GHE and analyze the recovery mechanism of the surrounding ground temperature. The results reveal that lower flow rate of the GHE is preferred to decrease the outlet water temperature in direct cooling. Intermittent operations can effectively raise the heat transfer rate of the GHE and slower the increase of its outlet water temperature. The length of on-time and off-time shows significant influence on the ground temperature recovery. Shorter on-time contributes to quicker recovery for next off period. Prolonging off-time enables ground temperature to recover more sufficiently, however, overlong time leads to inefficient process. The proper length of intermittent durations is provided in the study for better utilization of the system.