How Geothermal Power Plants
Help to Reduce CO2 Emission
HISASHI FUKUDA*1

JUNICHI ISHIGURO*1
SHOJIRO SAITO*1

Geothermal power generation is an environment-friendly power-generation method with extremely low CO2 emission. It generates
power cleanly because it requires no combustion of fuels on the ground. Mitsubishi Heavy Industries, Ltd. (MHI) has adopted
various methods to generate power from geothermal sources such as superheated steam process, and flash cycle and binary cycle
configurations. As of this writing, MHI has delivered 25% of the geothermal power plants now operating in the world. In the future,
MHI aims to further develop and spread the geothermal power generation by improving the economy of the process. This will help
curb global warming.

wells, the water is converted into superheated steam. In otherwells, the water remains in a two phase state of water andsteam. The former type of well is called a “superheated steam
production well”; the latter, a “water-dominated production
well.”
1.2 Power-generation plant cycle
Figure 2 shows a typical example of a power-generationcycle applied for geothermal power generation. In thef lash cycle, the geothermal steam is introduced directly
to the turbine. In the binar y cycle, geothermal f luid isintroduced to a heat exchanger, the heat is recovered using
a low-boiling-point medium (a secondary working f luid),
and all of the geothermal f luid is returned underground.

1. The process of geothermal power generation
1.1 Geothermal resource
Figure 1 shows a model of a geothermal resource.
Rainwater seeps down through cracks in rock around faults
to underground depths of 1,000 m or more. It sinks further
and f urther through the crust over a period of several
decades, and stays underground. This water is heated by amagma chamber in its vicinity, forming a high temperatureand high pressure zone of hot water (aquifer). This hot water
is the energy source for geothermal power generation. When
drilling a well towards an aquifer, hot water gushes out in a
combined process of depressurization and flashing. In some

VolcanoRainwater

Geothermal
power plant
Production (steam) well
Steam separator
Water level

Reinjection pipe
Reinjection well

Shallowlayer

Cap rock

Undergroundwater
Deep hot water

Geothermalaquifer

Volcanic gas, vapor
Fault

Magma

Magma chamber

Bed rock

Source : Geothermal Energy, July 1999 Ser. No. 87
Fig. 1 Underground resource model (Model showing the origin of geothermal energy)

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)*1 Power Systems Headquarters

63

Single flash cycle
Auxiliary steam

Binary cycle
Geothermal
fluid

2.1 History of development
The world’s first geothermal power generation was carriedout using superheated geothermal steam at Larderello in
Italy, in 1904. About three-quarters of 1 horsepower was
harnessed from the steam. A geothermal power-generation
technology using water-dominated production wells was firstdeveloped in New Zealand and later applied practically at theWairakei Geothermal Power Plant in 1958.
Based on this success, MHI began developing a geothermal
power-generation facility using a water-dominated production
well jointly with Kyushu Electric Power Co., Inc. As a basis formaterial selection, we chemically analyzed the geothermal
fluid, and carried out material tests on the major components
such as the turbine under a geothermal steam atmosphere. We
also analyzed the characteristics of the major components
required for geothermal power plants, such as the directcontact type condenser, two-phase flow transmission pipe,
non-condensable gas extraction system, etc. Finally, we
performed verification tests to collect the basic data forthe component design. The Otake Power Plant of Kyushu
Electric Power Co., Inc. (11 MW), the facility designed andconstructed based on these data, commenced operation as
Japan’s first water-dominated geothermal power plant in1967. Since then, MHI has developed and applied varioustechnologies to improve the performance, economy, andreliability. The company has delivered 96 sets of geothermal
power-generation facilities, with a total capacity to produce
2,780 MW of power, in 13 countries, including Japan. This
corresponds to 25% of the geothermal power-generation
capacity in the world.
2.2 Lineup
Figure 3 shows the t y pes of components generally
required for geothermal power generation. MHI has designedand manufactured all of these types. In the flash cycle, we
cover a wide power range. At the small end of the scale, wemade a 0.2 MW power generation facility run on steam fromflashing hot water of about 100ºC under a pressure of only 0.06
MPa (below atmospheric pressure). On the large end of theNon-condensable gas

Condenser

Steam
Production
well
Hot water

Reinjection well

2. Approach of MHI

Gas extraction system(ejector)

Steam separator

TurbineHeat
exchanger

Cooling water
Cooling tower
Hotwell pump

Low-boiling-point medium
Condenser

TurbineProduction Reinjection
wellwell

Cooling waterpump

Medium pump

Cooling tower

Fig. 2 Cycle schematic diagram
Schematic diagram showing a typical geothermal power-generation cycle.

W hen a superheated steam production well is used, thegeothermal steam is introduced directly to the turbine
without processing the steam (except in some cases, when
the steam is scrubbed or otherwise treated). In a waterdominated production well, the hot water and steam areseparated by a separator, and the steam is introduced to theturbine for power generation. Unlike the artificial steam
used in a thermal power plant, geothermal steam containsimpurities and non-condensable gases with extremely highconcentration. The corrosion-related damage caused bythese impurities and the deposition of scales is a problem
unique to geothermal power generation.
1.3 Features of geothermal power generation
Geothermal power generation uses the heat in the Earth’s
interior rather than combustible fuels extracted from the ground.
Therefore, the process exhausts very little atmospheric CO2(one of the main greenhouse gases responsible for global
warming). The CO2 emission level is about as low as that ofnuclear power generation. Geothermal heat is originatedfrom the transfer of heat in the interior parts of the earth,or from the decay of radioactive materials. The amount ofthermal energy is huge. In Geysers, California, the siteof the world’s largest-scale geothermal power-generation
facilities, the thermal energy consumed for more than 30years of power generation has reportedly reached only 5% of
the thermal energy of the underground resources existingin the area. Clearly, geothermal heat is an energy that’shard to deplete. And since it isn’t affected by the weather, its
availability factor is the highest among natural energies—
nearly equal to that of a thermal power plant.

Superheatedsteam type
Flash cycle
Waterdominated type

Back pressure
turbineCondensing
turbineSingle flash

Back pressure
turbineDouble flash

Condensing
turbineBinary cycle (the media are isobutane,mixed fluid of water + ammonia, etc.)

Condensing
turbineFig. 3 Classification of facility types
Types of generally assumed geothermal power-generation facilities.

Mitsubishi Heavy Industries, Ltd.
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Table 1 Chemical compositions and mechanical properties of geothermal turbine rotor materials of MHI
Material
code

Mechanical properties(N/mm2)

Chemical composition(% of mass)
CMn

PS

Si

Ni

Cr10325MGB

0.23-0.3

0.7-1.0≤ 0.015≤ 0.005

0.2-0.4≤ 0.5

1.0-1.3

1.0-1.3 0.21-0.29

≥ 635

≥ 740

GSR1

≤ 0.06≤ 1.0≤ 0.015≤ 0.005≤ 0.305.0-5.5

11.5-12.5

0.8-1.2

≥ 635

≥ 740

scale, we made a 121 MW power-generation facility with asingle-cylinder turbine run with about 700 tons of steam perhour at a pressure of 1.68 MPa and temperature of 204ºC.
In the binary cycle, we have carried out all stages ofplant development, from design and manufacture up tocommissioning, using three types of low-boiling-point media(isobutane, Freon (HCFC-123), or a mixed fluid of ammonia+ water) in proto-model power-generation facilities (in theOtake and Takigami areas of Oita Prefecture, and in MHI’s Nagasaki Research and Development Center). These
prototype operations verified the practicability of the newsystems.
2.3 Reliability
Technology is required to protect the rotating parts of theturbine from corrosion fatigue and stress corrosion cracking(SCC), the typical failure modes under corrosive geothermal
steam contaminated with impurities and corrosive noncondensable gases. To cope with SCC of turbine rotors, wepay close attention to low-stress designs while applying rotor
materials with low SCC susceptibility. MHI has developed
two types of materials for the geothermal turbine rotors, as
shown in Table 1. The material 10325 MGB has a low SCC
susceptibility because it contains extremely low levels ofsulfur, an impurity. GSR 1 is a 12% Cr-5 Ni material applied
to turbine rotors exposed to highly corrosive geothermal
steam. To cope with corrosion fatigue of the rotor blade, weuse 12 Cr steel, 17-4 PH steel, or titanium alloy in accordancewith the corrosive environment and blade length. Applying
and integral shroud structure to a long blade, we avoid theMoV≤ 0.1

0.2% yield strength Tensile strength

corrosion fatigue by suppressing the vibrating stress to 20%
or less of the stress imposed on a conventional blade. This isachieved by the high dumping effect of the mutual contactof adjacent blades at the shroud part. These are examplesof technologies we have developed to assure reliability. The
probability of damage of rotating parts has been significantly
reduced through the accumulation of these technologies,
and the availability factor nowadays is comparable to that ofa thermal power-generation facility.

3. Conclusion
Geothermal power generation is an excellent powergeneration method with the following features:• Low CO2 emission, and environment-friendly• High availability factor• Runs on an energy resource that’s hard to deplete
But because the steam condition is low and the sizes of
the components are big, the construction cost per power
tends to be higher than that for a thermal power plant. We
will make our utmost efforts to further spread geothermal
power generation by improving its economy.

Hisashi Fukuda

Junichi Ishiguro

Shojiro Saito

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)

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