Novel pipe configuration for enhanced efficiency of vertical ground heat exchanger

Date

2021-04-29

Authors

Eswiasi, Adel

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Abstract

In this research, a novel U-Tube pipe configuration, consisting of a single U-Tube pipe with two outer fins, was proposed to enhance the thermal efficiency of the vertical ground heat exchanger (VGHE). The ground thermal behavior at and around the VGHE were studied for both conventional single U-Tube and novel U-Tube pipe configurations. The effects of different grout materials and heat injection rates were also studied for the novel U-Tube pipe configuration. To demonstrate superior thermal efficiency, the obtained temperature data from thermal response tests (TRTs) for the novel U-Tube pipe configuration were compared with the result obtained from the conventional single U-Tube pipe configuration. A small-scale experimental apparatus was designed and built for this research project, including a water supply system, a sand tank, and a data acquisition system to conduct TRTs with various pipe configurations, grout materials, and heat injection rates. The line source model was used in this research to estimate the ground thermal properties. Two TRTs were conducted for the VGHE with the conventional single U-Tube pipe and with the novel U-Tube pipe configuration, respectively, to find out which pipe configuration had a superior heat transfer rate. The results show the differences between the inlet and outlet water temperatures are 0.4 °C for the conventional single U-Tube pipe configuration, and 0.7 °C for the novel U-Tube pipe configuration, after 60 hours, indicating a superior heat injection rate for the novel U-Tube pipe configuration. The results indicate that the effective ground thermal conductivity for the novel U-Tube pipe configuration is 4.85 W/ m. K, which is 23.6% higher than that of the conventional single U-Tube pipe configuration. The borehole thermal resistance for the novel U-Tube pipe configuration was 0.680 m. K/ W, which is 29.2% lower than that of the conventional single U-Tube pipe configuration. The results show that the temperature at the borehole wall had increased by 4.87 °C when the novel U-Tube pipe configuration was used. With the novel U-Tube pipe configuration, at 45 cm radial distance from the center of the borehole and at different depths, the highest temperature had increased by 0.54 °C more than the conventional single U-Tube at a depth of 35 cm from the top of the sand tank. These observations indicate higher heat injection in the ground when the novel U-Tube pipe configuration was used. Four TRTs were also conducted in the laboratory to investigate the impacts of grout materials (bentonite and silica sand) and heat injection rates on the thermal efficiency of the novel U-Tube pipe configuration. The results show that the difference between the inlet and outlet water temperatures of the VGHE with silica sand was higher than the VGHE with bentonite as grout, i.e. the borehole thermal resistance for the novel U-Tube pipe with silica sand as grout to be less than that with bentonite as grout. The heat exchange rate also increased with an increase in the inlet water temperature entering to the VGHE. Based on the experimental results, when the novel U-Tube pipe configuration is used, the number of boreholes required for the conventional single U-Tube is decreased by about 58%, which will in turn decrease the installation and material costs.

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Keywords

Vertical ground heat exchanger

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