Abstract
The Horizontal Directional Drilling (HDD) method is applicable in the installation of Horizontal Ground Heat Exchangers (HGHEs) at everywhere while keeping the low initial cost of Ground Source Heat Pump (GSHP) system. However, the performance of HGHEs installed using the HDD method has not been researched well due to the lack of field application. In this study, we carried out field tests and numerical simulations to investigate the heat exchange capacity of HGHEs installed using HDD. Firstly, we conducted three Thermal Response Tests (TRTs) in Saga, Western Japan to demonstrate the applicability of conventional TRTs to the directional hole drilled using HDD. The TRTs showed that the estimated thermal conductivity in the field test site by graphical interpretation were almost same as the one estimated using a numerical simulation model. These results indicate that the use of conventional graphical interpretation of TRTs is applicable to the interpretation of TRTs carried out in directional hole. The thermal resistances of HGHEs in various flow rates were then calculated based on the results of TRTs. As a result, the high flow rate was found to reduce the thermal resistance and enhance the heat exchange capacity of HGHEs. Next, seven temperature constant circulation tests applying constant inlet fluid temperature were performed in the field to investigate the effects of flow direction reversing and installation of inner pipe into the HGHEs on the heat exchange rate. These TRTs showed that the heat exchange rate declined after changing the flow direction in the middle of TRTs and that the heat exchange behavior when an inner pipe is installed was almost same as the one without the pipe. Finally, sensitivity studies were performed using a numerical model of the HGHEs to evaluate the effect of flow rate on the heat exchange capacity. The simulations showed that the flow state of heat medium in HGHEs is a crucial element to enhance the HGHEs performance and that the optimum flow rate was 10L/min in the field test area. In addition, the HGHEs of larger diameter showed higher heat exchange capacity, while a large flow rate is required to maintain the high performance.
