Numerical analysis of thermal performance of various types of horizontal ground heat exchangers

Abstract

In this paper, three types of horizontal ground heat exchangers (GHEs) such as U-tube, spiral, and slinky were numerically analyzed to investigate their thermal performance in cooling mode for 7 days of continuous operation with specific boundary conditions where a rectangular trench of 5 m in length, 2 m in width, and 5 m in depth served as the basis for the modeling of each heat exchanger. The pipe material was selected to be high density polyethylene for higher durability and corrosion resistance, as well as the soil and working fluid, which were clay and water. To confirm the accuracy of simulation results and reduce the computational time, a mesh independence test was performed, and simulation models were validated. There were four types of modifications, and in all of the cases, slinky GHE has better thermal performance. For instance, the heat exchange rate per unit trench length of slinky GHE was 20.72 W/m, which is higher than U-tube and spiral tube with a heat exchange rate of 9.75 and 13.62 W/m, which is of maximum 53% and 28% than U tube GHE, respectively, for the same pipe wall thickness and different material volumes of U-tube, spiral, and slinky GHEs. The pressure drop of slinky GHE is also higher (maximum 87% higher than U tube). To balance the heat exchange rate and pressure drop, the thermal performance capability (TPC) was examined. The slinky GHE has the highest thermal performance capability for all the cases. From the energy balance point, the TPC valued a maximum of 1.72 for the same material volume and different pipe wall thickness. The effectiveness was investigated to examine the heat exchange rate with different ground temperatures, and the slinky GHE showed higher effectiveness than spiral and U-tube GHEs. Finally, the thermal performance of slinky horizontal GHE was examined with different trench lengths to investigate the excavation work reduction with reduced trench length. With the decrease in trench length, the heat exchange rate and excavation work also decreased. The heat exchange rate of slinky horizontal GHEs with trench lengths of 1.5 and 1 m remained better than that of spiral and U-tube horizontal GHEs with a trench length of 3.5 m.