Evaluation on the Effect of Crossflow of Groundwater on Heat Exchange Rates in Vertical Ground Heat Exchangers

Abstract

For investigating the thermal advection effects of the crossflow of groundwater in ground heat exchangers (GHEs), groundwater flow velocity surveys and thermal response tests were carried out in a GHE drilled in Hirosaki City, Japan. The GHE was completed as a cased well with casing strainers installed at three aquifers, which allows the crossflow of groundwater between aquifers. The survey of groundwater velocity in the GHE revealed that a downward flow of groundwater of 1.5 m/min to 3.0 m/min exists from the uppermost aquifer at 30 m deep to the intermediate aquifer at 60 m deep and the lowermost aquifer at 80 m deep. The results of TRTs showed that the existence of crossflow keeps the temperature increase of heat medium about 15 °C even in case applying a heat load over 180 W/m continuously for 10 days. Also, the temperature survey in the GHE after the TRT demonstrated the quick recovery of ground temperatures due to the cooling effect by groundwater flow. Though the existence of crossflow can be expected in limited geological and hydrological conditions, the utilization of the effect of crossflow is considered as a cost-effective and energy-efficient way of improving the performance of GHEs.