To prevent the formation of silica scales at the geothermal power plants, a simultaneous precipitation test of silicic acid and aluminum by addition of cethyltrimethylammonium (CTA⁺) bromide (Br^–), which is an organic ammonium salt, was carried out using a sodium silicate solution (400 ppm Si) containing aluminum (5 ppm Al). The concentration of CTA⁺ added was in the range of 10^–6 ～ 10^–3 mol dm^–3. During the polymerization of silicic acid, aluminum combined to polysilicic acid particle. The negative charge density for the polysilicic acid particle with aluminum was higher than that for pure polysilicic acid particle, suggesting that Al³⁺ may be substituted with Si⁴⁺ in the pure polysilicic acid. It was clearly found that silicic acid and aluminum can be simultaneously precipitated from aqueous solution by the electrostatic interaction between CTA⁺ and polysilicic acid particle with aluminum. However, the precipitation of polysilicic acid particle significantly depended on the aging effect (time after the polymerization started) of polysilicic acid particles. In spite of the CTA⁺ concentration, more than 90 % of the aluminum was precipitated even at 10^–6 mol dm^–3.

Ground source heat pump systems have become important worldwide since their efficiency is higher than that of conventional heat pump systems, but the cost of installing borehole heat exchangers is one of the major issues in Japan. It is important to improve the performance of vertical borehole heat exchangers, which are the most commonly used type in Japan, in order to shorten the excavation length. Previously, we proposed a lining borehole ground heat exchanger (LBHE) to improve the efficiency of ground heat utilization.

This study aims to examine the heat exchange performance of an LBHE by numerical simulations using a heat transfer model. The simulations will focus on the heat exchange rate per unit length of an LBHE and the coefficient of heat extraction as the performance index for borehole heat exchangers to compare the performance of an LBHE and a double U-tube (DUT), which is a standard borehole heat exchanger.

Three-dimensional heat transfer models of the LBHE and the DUT were developed and validated by comparison with experimental results. As a result, both the LBHE and the DUT heat transfer models were able to reproduce the outlet fluid temperature at the time of heat extraction.

The simulation revealed that the heat exchange rate per unit length of the LBHE increased when a larger drill bit diameter and shorter borehole heat exchanger length were applied to it. There was less than about 20% difference in the heat exchange rate per unit length of the LBHE after the length was increased to 40 m or longer. In addition, the coefficient of heat extraction for the 50 m LBHE was compared with that of the 50 m DUT under various ground and operating conditions. When shorter operating hours per day were applied and higher effective thermal conductivity was applied to the ground conditions, the ratio of the coefficient of heat extraction for the LBHE to that of the DUT increased. The coefficient of heat extraction for the LBHE was 11–29% larger than that of DUT when used for 6–12 hours per day. These results indicate that the LBHE heat exchange performs more efficiently than that of DUT under simulated conditions.

Vapor-dominated geothermal resources were the first to be utilized for geothermal power generation. However, since there are few examples, its characteristics, actual conditions and issues are not well known to the general public. This review intends to provide an overview of the current understanding of the vapordominated geothermal resources, the phenomena that have occurred during steam production, the responses to these phenomena, and the issues that remain unresolved. This review is divided into three parts. In the second part, we will review reservoir engineering characteristics of vapor-dominated geothermal resources, various analysis methods, and related tasks for further studies.