Evaluation of ground source heat pump system's enhancement by extracting groundwater and making artificial groundwater velocity

摘要

This study used moving line source technique as a practical approach to simultaneously determine effective thermal conductivity, groundwater velocity and borehole resistance. It mainly focused on the applicability of this approach for observing the effect of groundwater extraction, in particular for ungrouted ground heat exchangers. The thermal response tests were carried out for different ground heat exchangers at two sites on the Akita University campus, northern Japan. The test conditions varied, based on different borehole filling condition (grouted/ungrouted), heat loads and presence of an extra pump for groundwater extraction. The hydro-thermal parameters of the ground were determined by a MATLAB code, especially for consideration of the effect of artificial groundwater velocity under a special test, where a water pump extracts the groundwater from the casing of the borehole during the test. This method can estimate the groundwater velocity and the effective thermal conductivity of the soil by reproducing the temperature distribution in the well and minimizing the mot mean squared error between the calculated results and the measured data. As a result, the groundwater velocity and thermal conductivity calculated by this method improved to 205.3 cm/day and 7.48 W/m K, respectively, due to presence of the external pump, whereas the undisturbed groundwater velocity in the other cases without pumping was estimated in the range of 15 to 16 cm/day. Furthermore, the minimum groundwater velocity and thermal conductivity were reported for the grouted well as being equal to 10.79 cm/day and 1.16 W/m K, respectively.