Thermoluminescence (TL) ages were obtained for 11 altered samples and one pyroclastic flow deposit in the Palinpinon area. The altered samples range in age from 0.005 to 0.81Ma, while the Quaternary dacitic pyroclastic flow deposit is younger than 0.036Ma. Samples collected at or near presently-active thermal manifestation areas, tend to show the youngest TL ages, ranging from 0.005-0.055 Ma. The southernmost portion of the Nasuji-Sogongon sector is the oldest, with an alteration age of 0.81Ma. North of this is an area whose alteration ages are 0.23-0.45Ma. Based on the above TL ages and subsurface temperature data, a model was constructed showing that a heat source was coming up beneath the Sogongon area around 0.8Ma ago and surface hydrothermal activity was moving from Sogongon to Puhagan. Accordingly, geothermally hopeful area is not the presently active Puhagan area, but the Sogongon area where old alteration ages have been measured.By combining the data of annealing experiments and the temperature of formation of the altered samples, the duration of the hydrothermal activity can be inferred. Thus, for the northern area in Nasuji-Sogongon, the major hydrothermal activity is estimated to have ended at least 0.23Ma ago. However, renewed activity occurred less than 1000 years ago. Its duration varies from one year to 300 years depending on the prevailing temperatures.
The performance characteristics of an evaporator system with spring steam and a direct-contact-type condenser system for energy conversion of hot springs are described in this report. The newly devised evaporator system is composed by a spring steam separator, a hot water tank, a heat exchanger remaked of the shell-and-tube U-type, and a lubricant separator tube. The spring steam, which is separated from spring hot water by the spring steam separator, is blown up in the form of bubbles in the hot water through the bubble-making plate placed under the U-type tubes. These steam bubbles enhance turbulent flows around the U-type tubes, incresing the heat transfer rate. Therefore, the pump of hot water could be omitted in the present system. The working medium heated in the U-type tubes is first led into the lubricant separator, where it is separated from the lubricant, and then goes into the flusher. In the flusher, the necessary amount of working medium for electric power generation is evaporated, and its gas is fed into the screw expander. The rest liquid of the working medium is returned to the U-type tubes by the pump and is reheated with the hot water. The lubricant separated by the lubricant separator, and then moves to the screw expander because of its own high pressure, therefore the pump of the lubricant could be omitted in this system. In the present test, evaporator system is composed of bare copper tubes (25.4φ×2t×effective length 60 m) and the overall heat transfer coefficients with the from R11 are about 1.0 kW/m2K. Secondly, the newly devised condenser system is similary built by modifying the usual mass-produced shell-and-tube U-type heat exchanger. The working medium liquid filling the tube chamber is chilled by the cooling water which flows in the tubes. The working medium gas from the screw expander is blown up into the working medium liquid through the perforated metal plate with (408×4φ bubble holes) which is placed under the working medium liquid. The blown working medium gas bubbles are condensed efficiently through the direct-contact heat exchange. From the experiments of the new condenser system with R11, we obtained the value of 2.5 kW/m2K for the overall heat transfer coefficients of this condensor system with bare copper tubes (25.4φ×2t×effective length 85 m).
For the utilization of geothermal energy, prevention of the scale problem is one of the most important factors. Usually, scale deposition is recognized at various places along geothermal fluid flow lines in geothermal power stations. The silica scaling from geothermal fluid is brought about by solution supersaturated with silica. It is known that silica deposition is prevented by controlling the pH of solutions maintaining acidic conditions (about 5.0). There are various methods to control the pH condition of solutions. One of them is to add mineral acid into the solution. However, this method has not been applied at present, considering its environmental impact. An alternative pH-control method using geothermal gases discharged from geothermal power stations was studied. From the environmental viewpoint, this method is more desirable than the former, because the gases are mainly composed of carbon dioxide. In this paper, results of the study to prevent silica deposition will be described.
ELF-MT survey using natural electromagnetic field in the Schumann resonant frequency band (8-40Hz) was carried out to discuss a geothermal system in the Hirayu geothermal area in Gifu Prefecture, Central Japan. The 28 survey sites were distributed around the hot spring area. The data quality has been evaluated by the multiple coherency function between magnetic field components and an electric field component. The eigen state formulation (Eggers, 1982) has applied to obtaining the intrinsic tensor apparent resitivity to deal with a complicated 3-dimensional (3-D) structure. We have tried to make a 3-D resistivity model of the central part of the survey area. The finite element modeling technique which uses 20-model isoparametric hexahedral element with second order shape function has developed to improve the accuracy of calculation. And the difference between the 3-D model and the 2-D model on the same section is discussed. The 2-D model tend to be complicate excessively because the 3-D effect is superimposed. The resistivity model shows the distribution of low resistivity zones which is interpreted as the fracture zone formed in the Paleozoic formation. The model was used as the basic data of hydrothermal simulation which was performed by Yuhara et al. (1989).
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