Efficient Use of Geothermal Hot Water

by the CADDET Japanses National Team

INTRODUCTION

Kokonoe Town in Oita Prefecture, Kyushu, is the largest geothermal power generation area in Japan. It hosts three commercial geothermal power stations, totalling 147.5 MW, belonging to the Kyushu Electric Power Co: the 12.5 MW Otake station, operational since 1967; the 110 MW Hatchobaru station (the biggest geothermal power station in Japan); and the 25 MW Takigami station which began operation in 1996. In addition, there are two small-to-medium-sized demonstration test plants operated by the New Energy and Industrial Technology Development Organisation (NEDO) for binary cycle power generation. Kokonoe is a highland town, surrounded on all sides by mountains, which enjoys beautiful scenery and many hot springs. The town has a total area of about 270 km 2 , with the River Kusu flowing through the central area. The town is mostly covered by forests and wasteland, and its cultivated fields lie mainly scattered step-wise on the slopes from the river to the foot of the mountains, between 350 m and 1,050 m above sea level. Kokonoes location, deep in an inland mountainous area, means that it has a rather harsh climate and very changeable weather despite being in the southern part of Japan.

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Hatchobaru geothermal power station.

The town government of Kokonoe intends to exploit as far as possible the geothermal hot water from these power stations, together with thermal energy from the many hot springs existing in the town, to develop agriculture and tourism, and to improve the living environment.

Hatchobaru Station

Japans biggest geothermal power station consists of two units, each with an output of 55 MW. Power is generated using a double flash system, which was pioneered by Kyushu Electric Power Co in cooperation with Mitsubishi Heavy Industries. In this system, steam and hot water blowing out of geothermal wells passes through two-phase fluid transfer pipes in the mixed state and flows into steam separators, where primary (high-pressure) steam is separated from the mixed fluid. The remaining hot water flows into flashers, where it expands under reduced pressure and generates secondary (low-pressure) steam. The primary and secondary steam extracted in this way drives a turbine-generator to generate electrical power. The double flash system can increase electrical output by 15-25%, although it also increases equipment costs compared with geothermal power generation by single flash systems which use only steam separators. In the Hatchobaru Station, the system increases the output by 20%.

The Hatchobaru Station is operated remotely from the Otake Station about 2 km away. The generator output, turbine rotation, and the vacuum in the condenser are all controlled remotely using tele-control equipment, data loggers and other instruments. The Takigami Station, the third geothermal power station in the town, is also remotely operated from the Otake Station.

Geothermal Hot Water Use

The use of geothermal hot water associated with geothermal power generation started in 1965 when, in response to a request from a local farmer, Kyushu Electric Power Co made hot water from exploration wells available in the Yutsubo area of the town to heat greenhouses. In the beginning, raw geothermal hot water was supplied to users but, because the geothermal hot water contains arsenic, regulations were introduced which stated that all raw geothermal hot water must be reinjected deep into the ground. As a result, since 1974, river water heated by heat exchange from the geothermal fluid is supplied to a large number of users for horticulture, domestic heating and hotel and leisure facilities. Since 1988, the Kokonoe Bio Center has been using hot water supplied at a flow rate of 20 tonnes/hour from the Otake Station about 5 km away to produce inexpensive, virus-free seeds and saplings for farmers in the town.

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Heat supply system for rose farms (assuming an outdoor temperature of 8C).

Energy Savings in Flower Farming

A floriculture partnership organised by five farmers invested JPY 153 million (where JPY is the Japanese yen) in constructing an energy-saving rose farm consisting of 10 greenhouses with a total floor area of 5,723 m 2 . The farm grows roses all year round, and started shipments of flowers in 1984. The rose farm is provided with hot water (inlet temperature at 73C), exchanging heat with geothermal hot water at a flow rate of 16 tonnes/hour (see the figure opposite). The partnership now enjoys sales of roses exceeding the planned sum of JPY 38 million/year and completed the repayment of the investment in 12 years.

In 1994, a new floriculture partnership organised by 10 farmers began to grow roses in 20 newly-built greenhouses with a total floor area of 19,278 m 2 using geothermal hot water. JPY 678 million was invested in this new farm, which is expected to to sell roses worth JPY 180 million annually. Because the supply of geothermal hot water to the new farm is limited to 6 tonnes/hour, the farm receives hot water in a storage tank and then circulates the water repeatedly through heating circuits, thereby making hot water at 50C flow through the greenhouses at a rate of 37 tonnes/hour.

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 Annual heating load = surface area of greenhouse x average heating load coefficient x energy saving ratio x degree-hours .

The changes in outdoor temperature at the site of the rose farms are similar to those in colder, more northerly regions. Thus, when the temperature inside the greenhouses is kept at 18C throughout a year, the degree-hours are 60,210C-hours. Each year, in total, geothermal energy saves 884 kl of fuel oil and JPY 41.5 million of fuel costs (fuel oil price at JPY 47/kl).

Conclusion

The local government of Kokonoe has designated the area, where geothermal power stations are already in operation, as a promotion area for the effective use of geothermal energy. In other areas, geothermal exploration and the demonstration testing of binary cycle power generation are now under way, and the town will also consider designating those areas as the promotion areas, after reviewing and studying the developments. Plans are under way in the designated areas to systematically get geothermal energy into actual use.

For more information contact the CADDET Japanese National Team

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

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