Experiments on hydrothermal dissolution of granite using the batch system autoclave were carried out in order to study water/rock interaction in subcritical water in temperature range from 200°C to 350°C. Two types of pressure conditions for experiments were examined. One is saturated vapor pressure for a given temperature, ant the other is a fixed pressure of 18 MPa at any temperatures for 200°C, 300°C, 330°C and 350°C. Dissolution behavior of major silicate minerals in the granite, such as quartz and feldspar, was estimated on the basis of fluid component of reacted solution. Pressure dependence of dissolution behavior of granite is not so stronger than temperature dependence in the subcritical temperature range from 200°C to 350°C. Dissolution rate of feldspar at 200°C, is faster than that of quartz and feldspar dissolution controls fluid chemistry. However, at higher temperatures, 300, 330 and 350°C, dissolution of quartz controls the concentration of dissolved silica. These results indicate that hydrothermal dissolution of granite was controlled by dissolution of feldspar at relatively low temperature around 200°C, and dissolution of quartz in the granite was predominant reaction under high temperature subcritical conditions above 300°C
The authors have developed the Gaia Snow-Melting System which utilizes the ground as a heat source and a heat storage body. This system consists of Downhole Coaxial Heat Exchangers (DCHEs) developed by the authors, a heat pump and heating pipes embedded in a pavement. In this system, the geothermal heat in the shallow ground and solar heat stored in the ground over the summer are used for melting snow. The authors have carried out numerical simulations of the Gaia Snow-Melting system for designing a system for a sidewalk in Aomori City. Two systems are planned for a total of 663 m2, and each system covers 331.5 m2. The heat flux to be supplied to heating pipes is 170 W/m2. The major results obtained in this study are as follows: Four DCHEs, each 151.4 m long, and a heat pump driven by three electric motors with a total capacity of 22.5 kW are required for each system. The heat pump selected for the snow-melting system has sufficient heating capacity. The average specific heat extraction rate of the DCHE and the average COP of the heat pump over a snow-melting season were estimated to be 72.5 W/m and 5.2, respectively. In this case, the critical factor in deciding the minimum number or total length of the DCHEs was the lowest allowable operation temperature or design temperature of the heat pump, specifically the lowest antifreeze temperature at the outlet of the evaporator. Also, it was suggested that the storage of summer time solar heat in the ground is useful for reducing the required number or total length of the DCHEs, hence lower construction costs. The predicted heating capacity of the heat pump changed significantly, in the range between 56.9 kW and 105.1 kW, in a snow-melting season. This indicates that predicting the operational behavior of the system by numerical simulation is essential in selecting an appropriate heat pump
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