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.
Today, it is important to expand the use of untapped energies for tackling the climate change issues. The energy in the shallow ground is one of the most promising untapped energy sources. However, a geothermal heat pump (GHP) system is not well utilized in Japan, especially in the warm areas. In order to investigate the possibility to introduce the GHP systems in the warm areas of Japan, a comprehensive research was carried out choosing the Kumamoto Area, Kyushu, Japan, as the target area. In this study, a suitability map of GHP systems was developed applying a single ground heat exchanger model at several locations of the plain. The suitability map showed that the center of Kumamoto City, Suizenji, and Kikuyo Town were the good candidates of high performance of GHP systems. Moreover, the amount of CO2 reduction with the application of GHP systems was evaluated on the basis of the performance of the system predicted by the numerical model. From this result, the effect of about 80,000-ton CO2 reduction in a year was shown by introduction of GHP systems in Kumamoto area.
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