Several types of downhole heat exchangers, for example thermosyphon type heat pipe, concentric tube thermosyphon, downhole coaxial heat exchanger (DCHE), U-tube downhole heat exchanger, as well as others, have been proposed in order to extract heat directly from shallow geothermal resources. Among these downhole heat exchangers, U-tube downhole heat exchanger is widely used not only in Japan but also around the world, because of its cost performance and easy construction. A numerical simulation has been performed to investigate the heat extraction characteristics of a single U-tube downhole heat exchanger. First, the present computer simulation program for the heat extraction by a single U-tube downhole heat exchanger has been made and the computed results by this program were checked by both laboratory experiments and previous field research. After inspecting the effectiveness of the present computer program, a numerical simulation has been conducted for the real scale model with this program. The results showed that increasing both the flow rate of the working fluid of heat extraction and the diameter of the U-tube downhole heat exchanger increases the heat extraction rate under the prescribed numerical conditions in this study. The effect of the tube-material on the heat extraction rate was also shown, and the results suggest that the use of a resin tube, such as polyethylene, is a possible alternative, even though resin is inferior to a metal in heat transfer performance.
An organized session on sustainable development of geothermal energy was held at 2003 Annual Meeting of Geothermal Research Society of Japan. Four papers were presented based on the field experiences at Matsukawa, Otake, Ogiri and Yanaizu-nishiyama Geothermal Power Stations. Two papers were also presented from theoretical and observational viewpoints. After the presentations, detailed discussions were conducted between six reporters and attendants at the session. Furthermore, a paper on assessment technique of the geothermal resource was submitted after the session. Submitted seven papers and discussions conducted at the session are summarized in this paper. As a result, the following process for sustainable development of geothermal energy was suggested; A moderate estimate (about 50% of the predicted value) of power production is recommended at the initial stage of development. After that, the installed capacity should be increased step by step based on the geothermal reservoir monitoring and the prediction from numerical modeling of the geothermal reservoir. Through such a process, the power generation for sustainable development will be attained. We will be able to continue sustainable development of geothermal energy, if we can select such an appropriate installed capacity.
This paper reports successful operation of the Matsukawa geothermal power plant since 1966. This includes a summary of the production and electrical power records of the power plant. The wells and reservoir have shown a very slow decline, which is a result of appropriate station sizing and adequate operational criteria. The power plant was sized only to meet the demand of the company's factory for its in-house electric use, instead of full utilization of the resource. The power plant has been operated so as to maximize the profit instead of insisting on operating continuously at full power. A proper understanding of the reservoir based on some chemical and physical monitoring data has helped to maintain stable operation. Station sizing (23.5 MWe since 1993) appears to be the most important factor in the successful development and operation of the Matsukawa geothermal field.
Otake Geothermal Power Station (Rated capacity : 12.5 MW) located about 5 km northwest of Kuju volcano, central Kyushu was constructed in 1967, and since then it has achieved stable production of electricity. Such a stable and long term production was realized by sustainable management comprising moderate assessment of geothermal resources, appropriate management of production and reinjection wells and timely renewal of power plant facilities.
The first geothermal power plant in the West Kirishima geothermal area was constructed at the Ogiri field, in the western part of the Ginyu area. The 30 MW Ogiri power plant was opened in early 1996. Nittetsu Kagoshima Geothermal Co., Ltd. produces geothermal steam and the Kyushu Electric Power Co., Ltd. is generating electricity from the steam. The utilization factor of the plant over 7 years and 10 months since the beginning of commercial operation is 95.73%. This paper mainly presents a review of the development history, reservoir behavior and operation strategy from the viewpoint of a steam-supply company. It also considers the subjects of the sustainable and renewable steam production in order to promote a future geothermal development.
The Yanaizu-Nishiyama geothermal field is located on 20 km west of the Aizu wakamatsu city, Fukushima prefecture. The commercial power generation with 65 MW was started in 1995. After three years later, however, the steam production started to decline seriously to the bottom electricity of 38 MW in 2000. With the drilling effort, the steam production turned to go up gradually, and it seems to be stabilized at 45 MW. Through the micro-gravity study, the large pressure drawdown in the central part of the production zone due to excess production and the importance to re-inject water into the reservoir to maintain the reservoir pressure were pointed out. In the recent years, a few efforts of artificial recharge were started.
One of the main objectives of geothermal reservoir assessment is to find an optimum electrical generating capacity which is sustainable over the entire project life (usually fifty years). However, if the average output rate is significantly in excess of the rate of natural recharge, the capacity will decline after fifty years of successful power generation. Recently Pritchett (1998) has carried out numerical simulation of the behavior of an hypothetical geothermal reservoir which has been produced for electrical power for fifty years and then abandoned. These calculations indicate that, after a long period of shut-in, the electrical generating capacity for the field will recover ; for geothermal systems which become depleted due to large-scale cooling, the resource is still renewable, but long periods of time (centuries) are likely to be required for substantial recovery. Although it is beyond the scope of the present geothermal reservoir engineering, if we seek the sustainability of geothermal power projects for much longer time scales than fifty years, it is desirable to prepare “long-term” scenarios. Several possible scenarios are discussed in this paper.
Sustainable development of geothermal energy was discussed on the basis of the results of repeat gravity measurement. The repeat gravity measurements clarified the mass changes in the geothermal reservoirs during exploitation of the Hatchobaru and the Takigami geothermal fields and that in the volcanic geothermal reservoir of Kuju volcano after the 1995 phreatic eruption in central Kyushu, Japan. Each case showed clear decrease of gravity after the commencement of power generation or phreatic eruption. The gravity recovered after the period of gravity decrease in the initial stage of power generation or phreatic eruption, which shows recharge of fluid to the reservoir. The mass balance in a geothermal reservoir was estimated based on change in gravity. Sufficient recharge of fluid to the geothermal reservoir was estimated both in the Hatchobaru and the Takigami geothermal fields. A large amount of groundwater recharge to the central part of Kuju volcano after the 1995 phreatic eruption was detected in the volcanic geothermal reservoir. The repeat gravity measurement is an effective technique to detect the recharge of fluid to reservoir and therefore it can contribute to the discussion of sustainable development of geothermal energy.
Geothermal power generates electricity using steam as coal, oil and nuclear powers, but the power is based on natural energy as hydraulic, sunlight and wind powers being meteoric and originating in solar energy. Generally, most powers based on meteoric energy fluctuate hour by hour. In contrast, hydraulic power, though it is also meteoric, is relatively constant, because a water reservoir is prepared. Geothermal power also stores in a reservoir. In this sense, geothermal power is the same as hydraulic power. A geothermal reservoir model has many uncertainties caused by estimation errors of geophysical properties such as temperature, permeability and others. A reservoir simulator has also its own uncertainty as a facility of chemical analysis. After a geothermal field is developed, many processes happening in the reservoir will be stochastic. In order to reduce risks accompanying geothermal development and to optimize the development, we should establish an assessment technique which takes account of these uncertainties and stochastic processes.