| Abstract |
The use of Horizontal Directional Drilling (HDD) has been nowadays increasing for the construction of tunnels, laying of cables or pipes due to the low installation cost and the environmentally-friendliness. The use of HDD for the installation of Ground Heat Exchangers (GHE) could significantly improve the feasibility of Ground Source Heat Pump (GSHP) systems if the applicability of HDD is well demonstrated. In this research, field tests were first carried out in shallow horizontal GHEs drilled using HDD in Saga City, Japan in 2018. One of the GHE were completed with 25 mm ID U-tube of 40 m long, while a one-way 50 mm ID pipe was installed in the other GHE of 63 m long. Thermal response tests (TRTs) were performed with a fixed heat load of 40-50 W/m for each GHE to investigate their heat exchange performances. Also, additional TRTs for the one-way GHE were conducted after inserting an inner pipe (34 mm OD) to investigate the improvement of heat exchange rate by increasing the fluid velocity. The TRTs showed that both GHEs had comparable heat exchange capacity with the conventional vertical GHEs though the installation cost is much cheaper. The use of inner pipe in the one-way GHE was also found effective for the enhancement of heat exchange rates. In the next stage of the research, numerical models were developed using a finite-element software, FEFLOW coupled with a module for directional GHEs. The models were well validated through the history matching of measured and simulated outlet temperatures. Using the validated model, sensitivity studies were then performed by changing the pipe diameters, flow velocities, GHE length, GHE depth, etc., to investigate the optimum design of the horizontal GHEs. |