In this paper, the quantitative evaluation of performance and advantages of Geothermal Heat Pump (GeoHP) system in Yumoto District in Fukushima Prefecture, Japan, was studied comparing to other ordinary areas, where the geothermal gradient and the thermal conductivity these values are 3-4 and 1.7 times larger than those in other ordinary areas shown in literatures, respectively. In this study, the GeoHP was operated at Yumoto Nursery School and the numerical simulation has also been carried out to analyze the dynamic behavior of GeoHP system, considering the geothermal gradients and thermal conductivities. It was revealed that the outgoing fluid temperature of circulation fluid in Yumoto was about 3-4°C higher than that of an ordinary area after the operation for 30 days, and that the thermal conductivity much affect the temperature change. Under a condition of 8 hours continuous operation a day, the electric power consumption of the compressor was 3.1-6.7% lower and the COP of the system was 0.2-0.4 higher in Yumoto than those in ordinary area. The maximum amount of extracted heat for 15 years under this operation condition in Yumoto was estimated to be 1.4-1.7 times larger than that in an ordinary area. It was also revealed that the room temperature recovers in only 30 minutes after an intermission of 8 hours operation a day to the same level as the 24 hours continuous operation. The electric power consumption of the compressor in the intermittent operation exceeded the case of continuous operation only in the first 20 minutes after the heat pump system restarted. It was suggested that these demerits of the intermittent operation, such as room temperature drop and higher electricity consumption after the intermission, are smaller than the demerits of the fall of the ground temperature in the continuous operation, and that the intermittent operations are suitable for the geothermal heat pump system.
In recent years, many people have concerned with global warming and energy conservation technique. The geoheat extraction system in soil is expected one of the important techniques and begun to use in Japan. In order to design and estimate the effectiveness of the system, the data of thermal properties of earth formation are very important. In common, the capacity and efficiency of heat exchange system is evaluated by Thermal response test (TRT) using real borehole that was drilled for the heat extraction. If we can use small aperture borehole such as drilled for Standard penetration test (SPT) to estimate thermal property, the method will be instrumental. With the proposal method, we can explain the effectiveness of geoheat extraction system and advise the system as a possible option to building constructor or customer. So far, the Cone penetration test (CPT) technique using small penetration rod applied line heat source has been conducted by present authors. However, in the case of using small borehole, we have to evaluate the effects of the filling material of borehole, the position of sensor and casing material. To confirm the applicability of the method, we have conducted both computer simulation and field experiment. As a result, we are able to demonstrate the effectiveness of the proposed method.
In Japan, high-level radioactive wastes from reprocessing plants will be disposed at the depth deeper than 300m. This disposal site will be under the stable situation, but in this region a large earthquake may form a high permeability fracture zone. By the activity of the crust, the disposal site may be affected by a non-volcanic hydrothermal system. Therefore, we estimated this influence by using HYDROTHERM Ver2.2, which is a three-dimensional numerical reservoir simulator. The model field is the northwestern part of Kego Fault, which was formed by a series of earthquakes called "the 2005 Fukuoka Prefecture Western offshore Earthquakes" (the main shock of MJMA7.0 on 20 March 2005). The results of the numerical simulations show the development of a hydrothermal system as a new fracture zone is formed. The permeability of the fracture zone is influential on the fluid flow rates. At the beginning, convection occurs in the fracture zone when the fracture zone is formed. Then, the convection reaches to the steady state in the zone. At the end, the larger convection evolves widely and slowly outside of the fracture zone. Therefore, it is inferred that a non-volcanic hydrothermal system will be formed after formation of a permeable fracture zone and the distributions of underground temperature and groundwater flow will change in the long term, even if no hydrothermal feature appears just after the seismic events. We can show the locked fault by temporal changing permeability of fracture zone.
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