GRC Transactions, Vol. 41, 2017

Development of Hybrid Geothermal Power Plants in Japan
Yoshinobu NAKAO1, Yoshihiro MUGIKURA1, Kan OGATA2 and Norito KATSUKI2
1EERL, Central Research Institute of Electric Power Industry, Yokosuka JAPAN2 Mitsubishi Heavy Industries, LTD, Nagasaki JAPANy-nakao@criepi.denken.or.jp

Keywords
Geothermal, Renewable energy, Hybrid, Thermal efficiency, Highly efficient, Repowering
ABSTRACT
Geothermal energy is the clean and sustainable heat that lies beneath the earth’s surface, and it isconsidered a renewable energy resource. GPP (Geothermal Power Plant) has negligible
environmental impact (approximately 0.015kg-CO2/kWh) and can help reduce carbon dioxide
emissions to the atmosphere. Although geothermal energy is potentially a very valuable domestic
power source for Japan, much of the geothermal resource base in Japan cannot yet beeconomically used. Therefore, many industries and research institutes in Japan have researchedand developed technologies to harness geothermal energy.
Since 2013 we have started to develop a “highly efficient hybrid GPP” combined with otherthermal energy sources such as biomass, solar heat and exhaust heat from fuel cells. It is
expected that the thermal efficiency of a GPP can be improved by super-heating the main flow ofsteam through the use of other energy sources above. Moreover, we have proposed one more
hybrid geothermal system, “repowering hybrid GPP”. This system can be applied to existing
GPPs to recover its power output although the system cannot improve the thermal efficiency of a
GPP.
We have also looked into the feasibility of these hybrid GPPs in terms of their engineering,
economic performance, environmental laws and regulations. To report the latest status of thisproject executed by CRIEPI in 2016, we discuss system concepts of these hybrid GPPs and theexperimental results performed at the Sumikawa GPP in Japan.
This project has been supported by the New Energy and Industrial Technology Development
Organization (NEDO) program (2013 - 2017). NEDO is a semi-governmental organization in
Japan set up to promote the development and introduction of new energy technologies.

Nakao et al.
1. Introduction
Although Japan has about 100 active volcanoes and the world’s third largest reserves ofgeothermal resources after United States and Indonesia, its geothermal generation capacity was
ranked only eighth and has not improved in the last decade after the last commercial GPP was
built in 1999 on Hachijo Island by Tokyo Electric Power Company.
The main obstacle was environmental protection laws in national parks in Japan. Even though
Japan has one of the largest reserves of geothermal resources in the world, more than 80% ofthem lie within national parks. Therefore, the development of GPPs and the drilling of wells, aswell, have been strictly restricted. Rules set in the 1970s suspended the construction of new
GPPs inside national parks except for at six sites in operation or under construction at the time.
Without reforming the regulations that restrict the development of GPPs within national parks,
the geothermal potential that Japan possesses has limited use and cannot be effectively used to
produce electricity.
The second obstacle is, until recently, a lack of governmental support for geothermal
development. Geothermal energy was recognized as a “new energy” until 1997. However, sincethen geothermal energy has been excluded from the definition of new energies, subsidies werereduced every year. On the other hand, Japan enacted a RPS (Renewable Portfolio Standard) law
in 2003. This law requires electric companies to produce a certain amount of electricity byrenewable energy to increase their use and expand the market for renewable energy. However,geothermal energy (flash steam power plants) was also excluded from the RPS law. Since then,geothermal energy has lost support from the government.
In July 2011, the FIT (Feed-in tariff) was adopted by the Japanese parliament. The MoE(Ministry of the Environment) in Japan has gradually relaxed regulations in national parks by
2015. Due to these government policies, in the past four years new GPPs and binary power
plants have already been built (totaling 16MWe, as of Mar. 2017). Moreover, in May 2019, JPOWER plans to begin operating the Wasabizawa GPP (42MWe) in the Tohoku region(Yuzawa-chinetsu, 2015). Thus, now it is expected that the usage of geothermal energy will
spread more rapidly in Japan in future years. Based on the government estimates, the generation
capacity of GPPs can be tripled by 2030.
In Japan, over 10 projects related to geothermal technology are currently in progress. At MoE,
three demonstration projects have been conducted to promote social acceptance and to maintaina balance between the development of a GPP and the protection of national parks. Also, at
national institutes under the jurisdiction of METI (Ministry of Economy, Trade and Industry), 11
practical studies have been conducted with the aim of reducing the cost and development risk of
GPPs. Currently, when some projects under local coordination are added, we have 21 big
projects in Japan and their total generation capacity is approximately 560 MWe (NEDO, 2016) .
We have started to develop a highly efficient hybrid GPP combined with other thermal energy
sources such as biomass, solar heat and exhaust heat from fuel cells. It is expected that thethermal efficiency of these systems can be improved by super-heating the main flow of steam
through the use of other energy sources. Although a Japanese patent was obtained for thistechnology more than 30 years ago, a detailed study of a system based on this technology has
never been carried out in Japan (Nakao et al., 2015).

Nakao et al.
2. Concepts of Our Proposed Hybrid GPPs in Japan
In this project, our aim is to develop two types of hybrid GPP by effectively combining
geothermal energy and other types of thermal energy. One of them is a highly efficient hybrid
GPP and the other is repowering hybrid GPP.
2.1 Highly Efficient Hybrid GPP
2.1.1 System Concept
Figure 1 shows the system configuration of a highly efficient hybrid GPP, and Figure 2 shows
the T-s (Temperature-Entropy) diagram of the system. In a typical GPP, the turbine inlet steam issaturated. If there is even only a small heat drop in the turbine, the resulting thermal efficiency ofa GPP will be much lower than that of a thermal power plant. Therefore, we have investigated
the idea of improving both the power output and the thermal efficiency of a GPP by superheating the turbine inlet steam. By analyzing the performance of the highly efficient hybrid GPP,
we’ve discovered that the power output increased by 30% and the thermal efficiency was morethan 20% higher in prescribed production well steam conditions: 2,721.8kJ/kg at 0.5MPa, andcooling water conditions in winter (Nakao, 2016).

Figure 1: System configuration of a highly efficient hybrid GPP

Figure 2: T-s diagram of a highly efficient hybrid GPPNakao et al.
2.1.2 Other Thermal Energy Sources Combined with GPP
We have developed a highly efficient hybrid GPP combined with other thermal energy sources,
which are biomass, solar heat, or exhaust heat from fuel cells, and so on. It is expected that thethermal efficiencies of a GPP are improved by super-heating the main steam through the use ofother thermal energy sources. We have looked into the system feasibility of highly efficienthybrid GPPs from the point of view of engineering, economic performance, environmental laws
and regulations.
As for solar heat, it would be impractical to install solar heat collectors in national parks in Japan,
and it is not feasible to gather sufficient solar heat in these environmentally sensitive regions tosuper-heat the geothermal steam up to approximately 400℃. Fuel cells also have great feasibility
in performance and operability, but is quite expensive. As a result, we found that a highly
efficient hybrid GPP combined with biomass has greater feasibility than other thermal energy
sources in Japan. Thus, we have proceeded with investigating a geothermal-biomass highly
efficient hybrid GPP, such as the following:.
Table 2: Reliability verification of other thermal energy sources combined with a GPP in Japan
Regulationin nationalparks
Performance

Cost

OthersComprehensive
evaluation

Biomass

Solar heat

Fuel cell

Needs to meetemissions regulations

Needs large site area
for solar heatcollectors

Needs to store
combustible fuel✔
Higher than that ofbiomass power plant✖
Shortage of direct
solar radiation in
Japan✔
Very high efficiency

Micro Gas Turbine
Needs to store
combustible fuel andmeet emissions
regulations
Lower than that of
Combined Cycle Gas
Turbine✖
✖
Initial cost is stillhigh
Be able to share power generation facilities such as steam turbine, condenser, cooling tower andejectorCombination with
Wide load range
GPP has less output
Zero-emission
FIT schemes apply
High CO2 emissions
fluctuation than PV
Quick response toalone
load variation
Depends on initial
✔
✖
✖
cost

2.1.3 Plant Generation Capacity
In this project, the generation capacity of the geothermal-biomass hybrid GPP was determined tobe approximately 4MWe for the following reasons.
In the FIT scheme, geothermal energy is purchased for 40yen/kWh for smaller plants (<15MWe).
The bill introducing the FIT scheme (MoE, 2013) for renewable energy was adopted by theJapanese parliament in July 2011. The purchase rate and period are decided every year anddepend on the type of energy, the form of installation and the scale of the renewable energy
source. All of the generated renewable electricity (excess electricity produced by residential

Nakao et al.
renewable energy equipment) is purchased at a fixed price. It has been decided thatgeothermal energy will be purchased for 26yen/kWh for plants with a capacity of 15MWe orlarger and 40yen/kWh for smaller plants for the next 15 years.
As an example of best practice with minimum impact on the environment, MoE is
promoting the Hachijo Island GPP in Japan.
Although a GPP should be promoted as an environmentally friendly energy system, more than
80% of Japan’s geothermal resources lie in national parks. Therefore, MoE has relaxed
regulations on a GPP in national parks. As an example of best practice with minimum impact
on the environment, MoE is focusing on the Hachijo Island GPP. On all occasions,maintaining a balance between the GPPs and the national parks has been a very big issue.
The same thing applies to hot springs (“onsen” in japanese). Japan’s rich geothermal resources
have made it famous for its numerous hot springs and the culture that developed with them.
Currently Japan has more than 25,000 hot springs, and 90% of the direct use of geothermalenergy in Japan is for hot springs. This means that geothermal development in Japan has been
focused on its use for hot springs. Many hot spring owners worry about a reduction or
exhaustion of their resources, thus many of them resist or disagree with the development of
GPPs or binary power plants. Some of them even refuse to permit research on the geothermal
resources near their hot springs.

Biomass procurement
In Japan, it is difficult to procure lots of biomass from forests at a low cost. If we cut downtimber at the forests far from the power plant, the biomass fuel procurement cost would behigh because the cost would include the transportation cost. According to our investigation,
the maximum amount of biomass from sustainable management is 12,000 ton/year or less foreach power plant site. This means that the heat exchange amount of a standard biomass-fired
boiler is approximately 3MWth.

2.1.4 Plant Performance of Geothermal-Biomass Highly Efficient Hybrid GPP
The heat-and-mass-balance diagram of a geothermal-biomass highly efficient hybrid GPP was
calculated using EnergyWinTM (Koda, 1999; Takahashi, 2007; Nakao, 2011). Table 2 showsmajor analysis conditions for a GPP, and Figure 3 shows the configuration of a biomass heating
facility, which is additionally arranged on a GPP. The main conditions of the saturated
geothermal steam were 2,721.8kJ/kg at 0.5MPa, and the flow rate was chosen for a gross power
output of 3MWe from the GPP without biomass. The flow rates of the turbine leakage, glandseal steam, and cooling water in the oil cooler and other system components were estimated fromthe specifications of existing units in Japan. As for a biomass heating facility, we determined thecapacity of each element apparatus by calculating the heat-and-mass-balance of it, and we havemade a preliminary estimation of the auxiliary input.

Nakao et al.
Table 2: Major analysis conditions for a GPP

W : 24,131
T : 136

Chip dryer

Water content : 10wt%

24.6t/h0.5MPa2,721.8kJ/kg
1wt%
82%
685mmHg
28°C
20°C
80%
0.6

W : 13,272
T : 136

Air heating
furnaceW : kg/hr
T:℃

Exhaustgas

W : 23,459
T : 220
W : 23459
T : 608

Saturated
steam
Air

W : 9,346
T : 20Superheater

Chip(Water content : 50wt%)

CycloneW : 10,859
T : 136

Value

Bug filter

Variable
Main geothermal steam conditions-Flow rate
-Pressure-Specific enthalpy-NCG
Turbine adiabatic efficiency
Vacuum condition-Cooling water temperature-Wet-bulb air temperature
Pump efficiency
Flow ratio of steam ejector

Superheatedsteam

Figure 3: Configuration of a biomass heating facility

Figure 4 shows an advantage of a geothermal-biomass highly efficient hybrid GPP. In our
analysis, the gross power output increased by 928 kWe owing to an increase in the temperatureof the turbine inlet flow and the gross thermal efficiency is approximately two points higher thanthat of a GPP without biomass. Then, the net power output increased by 863kWe, and the net
thermal efficiency (biomass) of the system was calculated to be 22.2% (HHV :Higher Heating
Value of a biomass). It is much higher than that of a biomass-fired power plant.
Here, the net thermal efficiency (biomass) was defined as follows:
Net thermal efficiency (Biomass) [%] =(Increased net power output [kWe] – Biomass auxiliary input [kWe]) / HHV of biomass fuel [kWe]Compared to a biomass-fired power plant, the geothermal-biomass highly efficient hybrid GPP
would lead to effective utilization of biomass chips. The net power output of the system has
increased by about 10% due to combining a GPP with a biomass-fired power plant. In the case ofoperating a typical GPP and standard biomass-fired power plant respectively, the net power
output was 3,399 kWe altogether. Whereas, in the case of a geothermal-biomass highly efficienthybrid GPP, the net power output was 3,716 kWe, because the auxiliary input for biomass
heating facility would be reduced by half in comparison to that of a standard biomass-fired
power plant. Moreover, the geothermal-biomass highly efficient hybrid GPP has an advantage inefficiency and economics as both systems share a large-scale turbine and generator.

Nakao et al.

Figure 4: Advantage of a geothermal-biomass hybrid GPP

2.2 Repowering Hybrid GPP
2.2.1 System Concept
Figure 5 shows a system concept of a repowering hybrid GPP combined with other thermalenergy sources. Although the repowering hybrid GPP cannot improve the thermal efficiency of a
GPP, this concept can be applicable to existing GPPs to improve the power output. On the leftpart of Figure 5, the power output is 10% lower from the design point. In this case, as by heating
two-phase geothermal fluid until the quality of the fluid rises to 0.22 from 0.2, the power output
can be increased by 10%, as shown by the system on the right. Also,although the thermal
efficiency cannot be improved, the repowering hybrid GPP contributes to reduce the initial costrequired for the construction of a biomass-fired power plant while utilizing nearly all of theconventional power facilities of the GPP, such as the moisture separator, steam turbine,condenser, circulating pump, cooling tower, ejector and so on.
Here, “Other thermal energy sources combined with GPP” and “Plant generation capacity” of arepowering hybrid GPP are the same as that we mentioned 2.1 above.

Figure 5: System concept of a repowering hybrid GPPNakao et al.
2.2.2 Plant Performance of Repowering Hybrid GPP
For situation 2.2.1 above, a power output that is 4 times that of a biomass-fired power plant using
the same amount of biomass chips can be generated by adopting the repowering hybrid GPP.
To raise the quality of two-phase geothermal fluid (24.6t/h w/ 1wt% NCG) to 0.22 from 0.2, it isnecessary to supply latent heat of approximately 1,300kWth. Here, the net thermal efficiency ofan existing GPP is assumed to be 15% (HHV) and the biomass heating facility of Figure 3 isinstalled as an evaporator (here, the system efficiency of the biomass heating facility was
assumed to be 75%), the power output of a repowering hybrid GPP can be increased over
230kWe, and the net thermal efficiency (Biomass) defined as in section 2.2.1 above can beapproximately 13% (HHV), respectively. On the other hand, generally, it is said that the net
thermal efficiency of a small-scale biomass-fired power plant is several percent (HHV). In thiscase, this means that the power output of approximately only 60kWe is generated by a biomassfired power plant.
Thus, in the case of a small-scale system, a repowering hybrid GPP can utilize biomass chips
more economically than a biomass-fired power plant. Moreover, it is expected that the thermal
efficiency of a repowering hybrid GPP is enhanced by combining with a highly efficient hybrid
GPP system as described in Section 2.1.
3. Geothermal Fluid Heating Experiments
In order to assess the feasibility of a highly efficient hybrid GPP and a repowering hybrid GPP,
saturated geothermal steam super-heating and geothermal water evaporating experiments were
performed in the Sumikawa geothermal field in the Tohoku region of Japan in August 2016. As aresult, we confirmed that both types of hybrid GPP can be realized, although demonstration
testing directed towards practical use is needed in the future.
Figure 6 shows an experimental apparatus with arrows indicating the flow in the case of thesaturated geothermal steam super-heating experiment. Two-phase geothermal fluid was provided
to the moisture separator from an actual production well directly and saturated steam separated
form the two-phase geothermal fluid. The saturated steam (saturated water in the case of theevaporating experiments) was flowed into the bottom of the heat exchanger and while flowing
through the heat exchanger it was super-heated until its temperature became equal to an arbitrary
temperature. Then, the super-heated steam was condensed at a cooler, in order to measure its
flowrate and analyze its properties. After the super-heating experiments, we have observed aquantity of silica scale adhesion on the inside wall of the heat exchanger.
Table 3 shows analysis results of geothermal water and condensed water of saturated geothermal
steam. The geothermal fluid in the Sumikawa GPP has the property of being slightly acidic(average pH is 5.9). We found that the silica concentration of the geothermal water was
remarkably high (average of 1,512mg/L) during our experiments, and a small amount of silica
was contained in saturated geothermal steam at an outlet of the moisture separator.

Nakao et al.

Figure 6: Experiment apparatus for the saturated geothermal steam super-heating experiment
Table 3: Analysis results of geothermal fluid in the Sumikawa GPP(a) Geothermal water
DATE
2016/8/26 15:22
2016/8/26 18:02
2016/8/28 10:45
2016/8/28 12:30
2016/8/30 9:25
2016/8/30 11:00
AVERAGE

pH
6.6
5.6
5.8
5.9
5.7
5.9
5.9

SiO2
mg/L
1606
1462
1488
1535
1494
1487
1512

(b) Condensed water of saturated geothermal steam
Clmg/L
1788
4805
1936
1697
1260
1423
2151

DATE
2016/8/26 15:22
2016/8/26 17:57
2016/8/26 19:50
2016/8/27 10:45
2016/8/29 12:25
AVERAGE

pH
-

SiO2
mg/L
8.5
7.8
3.8
4.5
5.1
5.9

Clmg/L
6
3
8
0
7
5

4
4
2
2
2
3

3.1 Experimental Results of Saturated Geothermal Steam Super-Heating
Figure 7 shows the observation results of a transverse cross section of the heat exchanger (superheater) for saturated geothermal steam by electron microscopic photography. The saturated
geothermal steam separated at the moisture separator flowed into the bottom of super-heater pipe
(Length 1,000mm × Inner diameter 4mm). The inlet steam condition was 100℃ at atmosphericpressure, and the flowrate of the steam was 2.8kg/h. Then, the outlet steam temperature was
315℃ for this experiment. Thus, the top of the super-heater pipe had been exposed for 25 hours
to the super-heated steam that was at least over 300℃. After the super-heating experiments, weobserved the inside wall of the super-heater pipe by electron microscopic photography, andfound that no silica scale formation observed on it.

Nakao et al.
This result suggests that saturated geothermal steam can be super-heated without worrying aboutscale adhesion; that is, the geothermal-biomass highly efficient hybrid GPP has high feasibility.
Thus, in the summer of 2017, we are planning a demonstration test using a super-heater pipe thatis the same size as that of commercial-sized facility, in order to confirm the durability of thesuper-heater and obtain more information to design a biomass heating facility.

Figure 7: Observation results of a transverse cross section of the heat exchanger (super-heater) for saturated
geothermal steam by electron microscopic photography (25 hours later)

3.2 Experimental Results of Geothermal Water Evaporating
Figure 8 shows the observation results of a transverse cross section of the heat exchanger(Evaporator) for geothermal water by electron microscopic photography. The saturated
geothermal water separated at the moisture separator flowed into the bottom of the evaporator
pipe (Length 2,000mm × Inner diameter 4mm) through a preheater. The inlet water condition
was 95℃ at atmospheric pressure, and the flowrate of the saturated geothermal water was 8kg/h.
The quality of outlet two-phase fluid was 0.18 for this experiment. The evaporator pipe had been
exposed for 15 hours to the two-phase fluid of approximately 100℃. After the evaporating
experiment, we observed the inside wall of evaporator pipe by electron microscopic photography.
We found that no silica scale formation observed on the lower part of evaporator pipe, i.e.,
regions (a), (b), (c) and (d) that were exposed to the two-phase fluid having quality 0.09 or less,but silica scales were attached to the upper part of the evaporator pipe, such as regions (e), (f)and (g).
This result suggests that geothermal water can be evaporated without worrying about scaleadhesion if the heat amount is very small, that is, the repowering hybrid GPP has high feasibility.
Therefore, in the summer of 2017, we are planning a demonstration test using the same apparatus
as that of super-heating demonstration test.

Nakao et al.

Figure 8: Observation results of a transverse cross section of the heat exchanger (Evaporator) for geothermal
water by electron microscopic photography (15 hours later)

4. Conclusion
We have started to develop a geothermal-biomass highly efficient hybrid GPP and a repowering
hybrid GPP combined with biomass heating facility. It is expected that the thermal efficiency or
the power output of a GPP can be improved using this combination. Therefore, we have looked
into the feasibility of these hybrid GPP systems in terms of their engineering, economic
performance, environmental laws and regulations.
In August 2016, saturated geothermal steam super-heating and geothermal water evaporating
experiments were performed in the Sumikawa GPP in the Tohoku region of Japan. As a result,
we confirmed that both types of hybrid GPP (highly efficient hybrid GPP and repowering hybrid
GPP) can be realized. However, in order to confirm the durability of the super-heater or
evaporator and obtain more information to design a biomass heating facility, demonstration tests
directed in practical use are needed.
ACKNOWLEDGMENT
Most of the presented work was supported by NEDO program “Research and Development ofGeothermal Power Generation Technology”, P13009. We would like to thank NEDO for theirsupport with our project and discussions on the results.
We are also grateful to Hachimantai Geothermal Corporation, Mitsubishi Materials Corporation
and Tohoku Electric Power Company for their help with the field experiments.

Nakao et al.
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