Studies On The Heat Transfer Of Vertical U-tube Ground Heat Exchangers(VGHEs)

Energy shortage has become the main concern for mankind nowadays. Thus due to the extraordinary advantage of energy saving and eco-friendly, the Ground Source Heat Pump(GSHP) has gained more and more attention. Thus, the study for the operation performance of GSHP is one of the main concerns. However, the most important part of GSHP: the ground heat exchangers can affect the operation performance of GSHP greatly. Hence, the underground heat exchange performance of GSHP was studied in this paper, which was carried out in three different aspects.The heat resistance can affect the underground heat exchange performance of GSHP greatly. Based on two-dimensional heat transfer model for single U-tube in GSHP system, we analyzed the influences of main different multi-variables on the heat resistance, including heat conductivity coefficient of backfill materials, the buried pipe spacing in the borehole and the water velocity in the pipes. Genetic Algorithm(GA) was used to optimize the heat resistance in the borehole based on foregoing multi-variables. According to the optimized result, the optimal heat resistance was obtained when the water velocity in the pipes, the heat conductivity coefficients of backfill materials and the buried pipe spacing reached their maximal designed values.Based on the single and double U-tube vertical ground heat exchangers(VGHEs) quasi-three-dimensional heat transfer model, the fluid outlet temperatures were calculated via a Matlab program. Energy efficiency coefficient and exergy efficiency were used to analyze the heat transfer performances of single and double U-tube VGHEs. The influencing parameters, including the fluid inlet temperature, the fluid mass flow rate and the borehole depth, were also studied here. The study in the paper showed that the energy efficiency coefficient and exergy efficiency of double U-tube VGHEs was higher than that of single U-tube VGHEs.Based on the two dimensional Finite Volume Method(FVM), the underground temperature of single U-tube was simulated transiently by FLUENT software. The transient simulation was achieved by UDF programs in FLUENT, which was based on heat balance equations of VGHEs. The simulation revealed that the calculation of underground heat transfer would be more accurate if the ratio of descending and ascending legs were 60% and 40% respectively. The recovery of the underground temperature was greatly affected by continuous operation time, the longer GSHP operated, the longer time ground needed to recover.