For reducing the initial cost of geothermal heat pump (GeoHP) systems, the existence of groundwater flow is considered important since the advection effect of groundwater flow can significantly enhance the heat exchange rate at ground heat exchangers (GHEs). In most of the planes of Japan, however, the advection effect cannot be utilized due to the slowness of the groundwater flow. In this study, therefore, we carried out field tests and numerical simulations to evaluate the effects of groundwater pumping, which could generate artificial groundwater flow around the GHEs and hence enhance the heat exchange rates. In the field tests, we carried out four thermal response tests (TRTs) in two types GHEs drilled in Hirosaki City, Japan. The results of TRTs showed that the grouting with permeable materials is more preferable than the grouting with impermeable materials in terms of heat exchange rates since the use of the permeable grouting material allows a closer contact between the heat exchange pipes and the groundwater flow. The effect of groundwater pumping was not clearly observed in the TRTs due to the existence of natural groundwater flow. In the next step, a 3D numerical simulation model was developed to simulate the heat exchange performance of the GHEs with groundwater pumping and was validated using TRT results. Sensitivity studies using the simulation model showed that groundwater pumping can remarkably enhance heat exchange rates in case groundwater flow is slow, though the effect tended to decrease as groundwater flow becomes faster. Cost calculations were also carried out on the basis on the sensitivity studies to conclude that the application groundwater pumping in GHEs could remarkably improve the feasibility of GeoHP systems.
Vapor-dominated geothermal resources were first exploited for commercial power generation, because of the easiness for utilization, i.e. wells in vapor-dominated geothermal fields produce dry steam alone. Understanding of the resource was improved first by accumulation of field experience, examples and data. Then, several different conceptual models were developed based on interpretations of the data. These conceptual models were tested and validated by numerical modeling, and then evolved into the current form. These processes led to the current understanding of the vapor-dominated geothermal resources. Thus, the key factors which contributed for the improvement are the increase of field data and experiences both in quantity and quality, and the advancement of numerical techniques. Among many vapor-dominated geothermal fields, real natural state of a reservoir has only been confirmed at Matsukawa, to date.
We have conducted gravity surveys at some areas in Fukuoka Prefecture, Japan. One of the purposes of the surveys is to reveal underground structure of non-volcanic hydrothermal systems. As the result of the gravity surveys at Yokote-Ijiri area in Fukuoka City, we concluded that one of the basement differences where some hot springs concentrate is a part of Kego Fault, and there is a permeable zone, which is like a fracture zone created by the past activities of the fault and acts as a path of hot spring water from the deeper side of the granitic body. And the survey result in Oto Town indicates an underground structure of the half graben, which is concerned with Tagawa Fault, with a strike of a north-south direction. It was concluded that Tagawa Fault is a likely target for the future hot spring well by comparison between the results of this gravity survey and the previous geological research. And the directional drilling should be adopted for the drilling of the future hot spring well because the dip of Tagawa Fault seems to be nearly perpendicular.
The applicability of geothermal heat pumps in tropical region was investigated in this study based on subsurface temperature surveys. Although generally a geothermal heat-pump system may not have thermal merit for space cooling in tropics, there may be some places in tropical regions where the subsurface can be used as "cold heat-source". In order to confirm this possibility, subsurface temperature surveys were widely conducted in the Chao-Phraya Plain, Thailand. In some cities, subsurface temperatures lower than monthly mean maximum atmospheric temperature by 5 °C or more over four months were identified. Thus underground may be used as cold heat-source even in parts of tropical regions.
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