Proceedings World Geothermal Congress 2015
Melbourne, Australia, 19-25 April 2015
Conceptual Study of Overall System Design of Geothermal Energy Systems for Achieving
Universal Use in Japanese Social Condition
Nobukazu Soma, Hiroshi Asanuma and Yasuki Oikawa
Fukushima Renewable Energy Institute, AIST, 2-2-9 Machiikedai, Koriyama, Fukushima 963-0215, Japan
n.soma@aist.go.jp
Keywords: social acceptance, system design, multiobjective-optimization, universal use
ABSTRACT
This paper will describe an object and conceptual structure of overall system design (OSD) for universal use of geothermal energy
in Japan. Social acceptance is now a serious problem while Japanese geothermal potential has again obtained public attention after
the Great East Japan Earthquake. The OSD has a function to evaluate and to optimize for a different type of geothermal energy
development, and helps stakeholders’ judgment under various valuation standards. The OSD mainly consists of two parts; one is a
system design problem, and the other is a detailed design problem. Each detailed design produces values for couples of valuation
standards, and then distribution of feasible solution of system design is shown. One can choice system design located on the line of
greatest value solution in the distribution that is called Pareto optimal solutions. Final selection from greatest value solutions should
be conducted with close discussion among stakeholders. It would be a proper way for them to reflect mind of region and to realize a
practical universal use of Japanese geothermal potential for future.
1. INTRODUCTION
In Japan, geothermal energy has been recently re-regarded as a one of future practical energy sources with a possible reduced share
of nuclear power generation after the unprecedented earthquake disaster (the Great East Japan Earthquake) in 2011 accompanied by
a nuclear power plant accident. Many Japanese people expect to increase geothermal utilization rapidly as well as to support other
renewable energy system such as photovoltaic, windmill, etc.
Japanese geothermal potential are estimated to place world’s top three from a number of active volcanic mountains (Stefansson,
2005, Muraoka, 2009). But the actual growth of geothermal use, such as capacity of power plant, has not increased since late 1990s
and not obviously changed yet after 2011. Plenty of problems have not been solved and still have existed in Japan. They are not
only detailed technical and engineering issues but also relating to many other aspects in social, political, economic, and basic
conceptual frameworks. The restriction of development permission in national parks has been recently getting loosed gradually. To
accelerate development, periods have been discussed with a possible relaxation of regulations, too. National subsidies have been
also recovered gradually. However, social acceptance for Japanese geothermal development has not been really improved yet. New
plan of geothermal energy utilization often faces serious conflicts of interests between promoters of geothermal power plants and
pre-existing stakeholders such as hot spring and nature conservation. Coordination of interests among multiple stakeholders is
strongly needed. However, it is difficult to have even mutual understanding.
We suppose that one of the main reasons of the difficulty is a lack of concept and way of overall optimization for development of
geothermal energy system in terms of not only from a geothermal company’s view but also from attitudes of pre-existing local
businesses, regional society and government, and all the potentially-impacted people that are stakeholders. We call this conceptual
way “Overall System Design (OSD)” for the universal geothermal development.
Our first idea of the OSD arose in around 2003 out of discussions in a review committee of Japanese hot dry rock (HDR) type
geothermal research and development project at Hijiori, Yamagata prefecture (Oikawa, et al., 2004, Soma et al., 2004). At that time,
the concept of optimization in the OSD mainly focused on only an adjustment among different technical elements in the HDR
development because some “Trade-Off” phenomena were observed as a consequence of the HDR project; such as between creation
of reservoir and circulation and heat extraction. Because the discussion was only for the HDR type research project, the OSD was
not a big deal in all of the review although problem consciousness for social acceptance has already pointed out. After that, the
concept of OSD has been improved and expanded for the next 10 years, while the Hijiori project itself was closed in 2002 and no
major geothermal R&D was conducted up until quite recently. In 2006, we have tried to measure and model a Japanese drilling cost
(Soma et al., 2006) in order to make an objective cost data to build the OSD. Nowadays, it seems that an optimization of
geothermal development should be done very comprehensively looking at all the development stages and covering regional society,
industry, economy and all stakeholders as well as designs of each energy system from a view of engineering and business
feasibility. We have discussed an updated concept of OSD, have re-built a basic structure of the OSD, and have started to gather
various data in order to create a new OSD for activated Japanese geothermal development.
In this paper, we describe a basic concept of our OSD, which enables to make a comprehensive optimization of a developed energy
system and to enhance regional social acceptance very much. Firstly, the main object and the role of OSD are shown, and the
conceptual structure of OSD is mentioned. Then, conceptual flow-chart of an example potential system is shown. Finally, we
discuss how to optimize a geothermal energy system development by using the OSD, and describe present agendas and future study
to complete our OSD concept.
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2. OBJECT AND ROLE OF OSD
An object of OSD is to achieve comprehensive optimization of a geothermal system for all stakeholders and then to accelerate
Japanese geothermal energy utilization. For this object, it is not satisfied only by an optimization of each type of energy system,
such as flash steam power plant, because social acceptances are not only related to the business optimal criterion. A selection of
type of energy system is itself very important for local population, too. Non-attributive manner enables us to increase universal use
of Japanese geothermal resources. On the other hand, a success and failure of one proposed energy system, such as a possibility for
a flush power plant, was only discussed in the past research and development.
Often, regional community stands on stable mutual relationship among regional human behaviors such as agriculture, industry,
society, culture, etc., that we regard one kind of regional economic circulation model. The stable flow of people, goods and money
may already exist before development of a geothermal energy system is established. A sudden rise of geothermal has risks to
destabilize the pre-existing local human behavior (Figure 1). Even if the new geothermal development has only positive impacts,
hardness to understand makes fear and resistance on the development. To make geothermal energy system development much more
clearly understandable for non-specialist stakeholders is another important object of the OSD.
With Geothermal
Pre-Geothermal
Big Company
(Headquarters)
National
Government
Big Company
(Headquarters)
Factory
(M)
Regional
Government
Manufacturing
National
Government
Factory
(M)
Regional
Government
Manufacturing
(M,G)
カ ネ(賃金)
(M) Money
(P) Persons
(G) Goods
(M,G)
Material Provider
(M)
(M,G)
(G)
Financial
Institution
(M) Tax
Trade and Service
Industries
(G)
(M) Tax
Financial
Institution
(M)
Construction
Trade and Service
Industries
House
Hold
カ ネ(賃金)
Within, Beyond
the Region
Agriculture
(M) Money
(P) Persons
(G) Goods
Balanced Regional Circulations
for Economy, Society, Culture, etc.
Hotel Business
(Hot Spring)
(M,G)
Construction
House
Hold
Within, Beyond
the Region
Agriculture
Tourism
Tourism
Customer
Material Provider
Customer
Geothermal
Increase? or Decrease?
Multifaceted Influence
Imbalance in Regional Circulations is concerned.
Hotel Business
(Hot Spring)
Extent of Impact of Geothermal
(Depends on Size of Project & etc.)
Figure 1: The conceptual diagram of regional economic circulation model before (left) and after (right) introduction of
geothermal energy system. (A balanced circulation exists for flows of money, persons, goods, etc, within some extent
of a region. Introduction of geothermal system will influence various elements in the region. The extent of impact of
geothermal depends on characteristics of developed energy system.)
In our concept, OSD has mainly two roles: one is detailed technological optimization for each proposed feasible geothermal energy
system, and the other is overall optimization for subject regional society by selecting type of geothermal utilization system and
defining its specification. The former was regarded as one of important problems around 10 years ago when Japanese geothermal
R&D was stopped. However, the later has become very serious problems in order to re-start practical Japanese geothermal
development in this 10 years.
Recently, both regional understanding and support are absolute requirements in Japanese geothermal research and development.
Therefore, the regional optimization in the OSD relates closely to the technological optimization of a specific energy system. For
example, maximum specification of a proposed energy system might be limited by an acceptance value of regional stakeholders,
although the value should not be defined without persistent discussions. A one-sided suggestion of proposed specification of
geothermal energy development often causes seriously strong opposition from local community. Regardless of implementation of
any geothermal project, outline of specifications should be determined with careful communication among all the stakeholders. The
OSD will have a function to help this.
Currently, we don’t identified “user” of the OSD. It may be better for it to be a common platform among all stakeholders to
improve a planned geothermal energy system. One possible reason why population dislike to accept geothermal is unbalance
between risk from incoming geothermal development and benefit from it on each individual. Perhaps, many opponents sense the
possibility of it. Once they touch the OSD as one of a potential user, we hope they notice what will happen and be profitable,
although everybody should understand public benefit of progress in geothermal developments for future generations after possible
future drastic change of energy resources.
3. STRUCTURE OF OSD
Designing of geothermal energy system is naturally much different from that of standard manufacturing. The designing in OSD is
totally affected by non-determinable underground condition. The parameters from underground observation are itself often used as
a “design variable” and also “constrained condition” on design variables. In addition, the observed underground parameters are not
fixed. They are generally change and improved as a geothermal development progresses. In particular, reliability of the parameters
increases as a number of drilled boreholes and conducted geophysical explorations. Furthermore, the range of influence, which is
defined by the considered area of geothermal development, varies due to the size of the energy system. The larger geothermal
system normally activates the larger size of stakeholders. The size of stakeholders affects how to optimize the geothermal system.
Therefore, the OSD is regarded not to be fixed rigidly but it is better to vary flexibly considering to various updated information
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and design variables for both underground and surface. We treat the OSD variables for “time” and “space”, in our study. The OSD
has two sides; (i) evaluation and optimization along with a development flow for each type of energy system, and (ii) evaluation,
optimization and/or selection of types of energy system at one snapshot in the development flow. The conceptual schematics of
OSD are shown in Figure 2.
Detailed Design Problem
Constrained
Condition
Stocked
Information
Geothermal
Energy
System
Size of
power generation
Feedback
- Literature
Exploration
Menu
for each
Method
- Underground
- Geology
- Region
- Geophysics
- Technology
- Geochemistry
- etc.
- Exp. drilling
< Method 1 >
ex.) Steam Flush PP
- etc.
< Method 2 >
ex.) Binary PP
Spec. of
prod. and inj. wells
Flow rate of
production
Location of
plant and wells
[Outputs from OSD]
technical possibility,
development risk,
cost,
business feasibility,
social acceptance,
etc.
Spec. of power plant
and building
Spec. of pipeline
・
・
・
Size of
power generation
Spec. of
prod. and inj. wells
・
・
・
< Method 3 >
ex.) EGS type PP
< Method 4 >
ex.) Binary PP
+ Heat-Use
Contents of stocked information
and exploration menu change
as development progresses
< Method 5 >
System Design Problem
Design Variables
・
・
・
Figure 2: Conceptual structure of OSD for geothermal energy system development. (Function of OSD is divided in two
parts; one is a system design problem, and the other is a detailed design problem. Outputs of OSD, which are results
of evaluation and optimization of each detailed design, are obtained at each stage of development, and they are used
for decision-making process in the system design problem. Exploration menu is determined for type of method and
stage of development. Obtained information from exploration and development is accumulated in order to improve
constrained condition.)
In the early part of geothermal development, the OSD is built under very few reliable information. However, it is important for all
stakeholders to be able to recognize various possibilities from the use of regional geothermal resources even at the beginning of the
geothermal project. The OSD is improved step-by-step as a function of time. The OSD may show more reliable results with the
progress of the project. The selection of optimal type and specification of geothermal energy system may be essential for some
stakeholders. So, actually, it is better not to decide a primary contractor at the beginning of geothermal project because there are
some possibilities that stakeholders’ final selections have huge mismatches from their initial expectation.
Optimization and evaluation geothermal system are conducted for various types of energy system. Here, the “type” means method
of utilization of geothermal energy including a possibility accompanying many-sided cascade heat energy use. For example, there
are steam flush power plant, binary power plant, and so on. For each type of energy system, there are many design variables such as
depth of wells, maximum extracted flow rate, extent of surface construction, specification of electric transmission line, and way of
retail electricity. We may be able to evaluate performance of each potential energy system in terms of technical possibility,
development risk, cost, business feasibility and social acceptance which are various regional judgments such as from hot spring
industry, tourism, nature conservation, regional improvement policy of local community, normal residents, etc. In order to make a
numerical evaluation and optimization in a reliable OSD, it is very important to gather objective data from both past research and
developments and regional comments from various types of local population.
We have already tried to gather some data to establish a model-curve of Japanese drilling cost for geothermal development (Figure
3, after Soma et al., 2006). It is one of the typical example what and how to gather objective data to make up the OSD. The answer
of the number of wells was about 100 obtained from hot spring wells and geothermal wells drilled by local government and
governmental fund. It was likely that proper specification of drilling with the OSD can reduce the development cost because the
preliminary model-curve indicated the expected cost is relatively lower than we believed at that time. The OSD will be able to
achieve quantified evaluation and optimization by gathering such information including important development parameters.
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Figure 3: Distribution of drilling costs versus depth based on our past questionnaire survey (Soma et al., 2006). (▼: prices
of hot spring wells drilled by Japanese local government from 1990 to 2005. △: prices of geothermal exploration
wells drilled by New Energy and Industrial Technology Development Organization from 1994 to 2005.)
The way of final decision-making is one of important issues if geothermal energy is widely accepted in Japan. It is now very
difficult in Japan that a classical government-led manner for a development program is welcomed by local community. A private
company cannot set up and progress in any geothermal projects without regional consensus, too. Therefore, the way of optimization
has a strong regional dependence considering characteristic of the region. There are certainly “Trade-Off” in an optimization
process of geothermal development. For example, acceptance by hot spring industries and nature conservations often conflicts with
profitability of private company because it is normally proportional with development scale. Although there is no true solution to
resolve such a trade-off, the OSD will help to see the subject geothermal project from various views of different values.
4. OSD IN TIME STEP
Development flow in a time step closely relates to a national support policy such as a subsidy system. So far in Japan, it seems that
the policy is intended for relatively large-scale developments of geothermal power plant. Therefore, big name companies play main
actors at the beginning of geothermal project, and often they have a lot of troubles to carry it out because local community opposes
the project strongly with perplexing fears before they understand properly. In order to avoid such unhappiness, we propose that the
main actor had better to be selected during the geothermal project going as a result of the optimization of OSD.
As shown in Figure 4, OSD provides some answers even if at the very beginning of the geothermal project. They are for example
technical possibility, development risk, cost, business feasibility and social acceptance. It helps for all the stakeholders to have
rough image of practical use of their geothermal resources. In the early part of the OSD, grasps of stakeholders’ foresight on
geothermal system, which is based on wide characteristics, such as mind, history, industry, and economic circulation, should be
valued greatly. The maximum and minimum boundaries of an acceptable geothermal energy system can be known from the
stakeholders’ foresight. They are very basic values of constrained condition to determine various design variables.
Evaluation and optimization processes are re-scanned when input observed parameters and/or constrained conditions are updated.
As input observed parameters from borehole observations are increased, reliability for the output of the OSD can be improved.
When the reliability becomes high enough, stakeholders can call some companies and/or form a business team to make a main actor
for the practical business project. Or, stakeholders ask some companies to be a potential main actor when stakeholders’ foresight is
determined at the early part of the project.
There is some possibilities that the type of energy system initially planned are not most optimum or not realizable by evaluation
from OSD. In that case, constitution of stakeholders should be updated, and then they can take other possible types of geothermal
energy system by using the OSD. It is one of the advantages to introduce the idea of OSD.
5. DETAILED OSD FOR EACH TIME STEP
At each time step of OSD, evaluation and optimization for each type of energy system can be conducted. There are many types of
geothermal energy use. Flush steam power plant using natural hydrothermal reservoir is the most typical system in Japan. EGS
(engineered geothermal system) type development has not been adopted in past practical Japanese geothermal development, but it
may become one option for future development. Combination with direct and/or cascade heat use for various purpose make
increase the number of unique type of geothermal energy system. The detailed structure of each design of geothermal system might
be very complex and different from each other.
In OSD procedure, development flowchart is used to define and evaluation of each geothermal energy system. It is used for
decision tree analysis, too. For example, to construct a flush steam power plant, rough development flow consists of five parts
following past definition of Japanese R&D; (1) agreement in subject region and establishment of consociation, (2) surface
exploration including geophysical exploration, (3) drilling exploration wells, (4) evaluation of geothermal reservoir and business
feasibility, and (5) construction of electric power plant. The agreement at (1) may be obtained not easily at the beginning of the
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project considering our idea of OSD. We redraw the detailed development flow of a flush power plant as shown in Figure 5. It is
clear that there are couples of design parameters, observable values as progress of development, constraint conditions, and turning
points of judgment. Some numerical values, such as for technical possibility, cost, and social acceptance, may be calculated based
on the past data and some proper assumption.
Feed back into input
Input
Information
Constrained
conditions
Evaluation
and
optimization
[ 1st OSD ]
Stakeholders
< Method 1 >
Technical Possibility: 20
Risk lowness: 10
Social acceptance: 10
・
・
・
< Method 2 >
Technical Possibility: 60
Risk lowness: 50
Social acceptance: 50
・
・
・
New Information
Judgment
and/or
recommendation
1st choice :
“ Method 2 ”
Re-determine stakeholders
Feed back into input
Input
Information
(Updated)
Constrained
conditions
(Updated)
Evaluation
and
optimization
[ N-th OSD ]
Stakeholders
(Updated)
Input
Information
(Updated)
[ Final OSD ]
Constrained
conditions
(Updated)
Evaluation
and
optimization
Modified
valuate standards
Stakeholders
(Updated)
< Method 1 >
Technical Possibility: 60
Risk lowness: 60
Social acceptance: 60
・
・
・
< Method 2 >
Technical Possibility: 70
Risk lowness: 70
Social acceptance: 50
・
・
・
< Method 1 >
Technical Possibility: 80
Risk lowness: 70
Social acceptance: 90
・
・
・
< Method 2 >
Technical Possibility: 80
Risk lowness: 90
Social acceptance: 40
・
・
・
New Information
Judgment
and/or
recommendation
N-th choice :
“ Method 1 ”
≒ “ Method 2 ”
Re-determine stakeholders
& Modify valuation standards
and constrained condition
Stock for operation
New Information
Judgment
and/or
recommendation
Final choice :
“ Method 1 ”
Figure 4: Conceptual diagram for time-step of OSD along with a regional geothermal development. (OSD can produce any
results for each candidate method at any stages of development, theoretically. New information on each OSD, which
is obtained during development, improves input information and constrained conditions in OSD at next time step.
Stakeholders can also be re-definable because range of influence depends on method and its specification. As
development is progressed, the OSD will produce more reliable and suitable solution.)
Various valuation standards are available to be used in the OSD. The important point is to know what elements are “Trade-off”.
There are trade-off among detailed design variables such as between drilling depth and total cost. Maximize valuation will be
obtained by many trials for different combinations of trade-off design parameters. Furthermore, there are trade-offs among
valuation standards, too. Relationship between profitability of private company and regional social acceptance is generally trade-off.
Other combination of valuations, such as between technical possibility and cost, and between development risk and cost, are also
probably trade-off in some degree, because fulfilling exploration menu gives more reliable underground information but results in
very high cost. Treatment of “Trade-off” is one of the advantages of OSD for a complex geothermal development in terms of both
underground property and mind of stakeholders.
During detailed analysis of development flow for each type of energy system, some condition setting and valuation standards will
result in impractical. If so, the type of energy system is rejected from recommendation at the subject region. In case, the type of
system is still one possibility at the subject region, results of the OSD for a specified valuation standards are gathered in order to
final selection for recommended type of energy system and its specifications to be developed at certain time.
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Start
Pre-existing information,
(Geology, Society, Local
industry, Regional Needs)
Base design of energy system
Modify the design variables
Investigation (Literature)
YES
Availability?
NO
NO
Judgment
YES
Considering other method
Investigation (from Surface)
Environmental assessment
Availability?
NO
YES
NO
Acceptance
Investigation (Exp. drilling)
YES
YES
Success?
Re-start with modified design
NO
Judgment for
re-drilling
NO
Availability
NO
YES
Rough evaluation of reservoir
Environmental assessment is performed
in conjunction with development.
Success?
NO
YES
Production test
Reservoir
potential
Considering other method
NO
ENOUGH
Detailed Design of wells and power plant
Business feasibility
Quantified data for valuable standards
are accumulated at judgment nodes, such as
technical possibility, risk, potential cost, etc.
Judgment
NO
YES
Complete power plant construction
Judgment for
further possibility
NO
YES
Re-start with modified design
Figure 5: Example of flow-chart up to judgment of power plant construction for a method of steam flush geothermal power
plant. Typical flow-chart is used for decision tree type analysis in OSD with various predicted values at judgment. In
the OSD approach, re-design and/or other method, such as binary power plant, EGS type development, combination
with heat-use, and etc., is recommended even when the target method (steam flush PP in this figure) is judged
unfeasible.
6. OPTIMIZATION, RECOMMENDATION AND JUDGMENT
The way of overall optimization in OSD is based on an idea of “Multiobjective-optimization”. Considering past and recent
geothermal exploration and development, it seems very difficult to make a widely social acceptable energy system when system
evaluation and optimization are done only along with profitability as a company’s business. Therefore, optimization in OSD is
treated with reflecting “Trade-Off” in each geothermal project.
At a certain development time, “Trade-off” valuation standards can be obtained for different types of energy systems with various
specifications as a result of OSD procedure. Results of OSD for two trade-off valuation are plotted in a distribution figure as
conceptually shown in Figure 6. In this kind of figure, optimal solutions distributed on the edge of maximum limitation line for two
trade-off valuations are called “Pareto optimal solution”. This Pareto optimal solution is theoretically definable for more number of
trade-off elements.
The designed geothermal systems corresponding to on the Pareto solution line have an objectively equivalent value. However, each
design may have different meaning for each individual in all the stakeholders. Final optimization of OSD is to make a selection of
the most suitable type of energy system and its specification from the equivalent value of system design after close and in-depth
discussions among all the stakeholders including developers, opponents and ordinary people in the region. Although this final
process looks not easy and time-consuming to get a final conclusion, it can be essential for all the stakeholders to obtain a suitable
direction of the regional geothermal development considering many people’s feelings with a correct comprehensive understanding
of regional geothermal potential. The OSD will act useful tools for it.
7. ONGOING STUDY OF OSD
As mentioned above, gathering the data is really important to construct the way of numerical evaluation and optimization in the
OSD. Now, we are conducting reviews and extracting meaningful information from the past records of Japanese geothermal
research and development in more than 40 years. The information of “mistake” and “failure” is necessity to develop a model curve,
such as technical possibility, development risk, etc. However, there is no open record for it. Therefore, we will make a hearing
investigation from people who closely touched past successful developments.
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Soma et al.
Acceptable limit
for private company’s business
Regional social acceptance
(High)
Realizing the OSD as a “system” or “software” is another issue for our study. So far we are only working to establish a conceptual
framework. We review various past works of geothermal developments and explorations, and will re-evaluate them under the
concept of the OSD in order to make the framework of OSD.
Line for Pareto optimal solutions
A
Value(A) = Value(B)
C
B
Area of
feasible solution
(Low)
Acceptable limit
for regional stakeholders
(Low)
Profitability of private company
(High)
Profitability ∝ Size of development (Plant size, Flow rate, Number of wells, etc.)
Social acceptance: Function of negative concerns (Hot spring, Nature, Scenery, Trouble, etc.)
and positive expectations (Revenue, Visitor, Employment,
Spillover, Future, etc.)
Figure 6: Conceptual model graph of solutions for geothermal development among trade-off factors, for profitability of
private company and regional social acceptance. (Developable design solution of geothermal energy system is located
within gray zone. Solutions with greatest value are distributed on the blue line, which are called Pareto optimal
solution. “A” and “B” are Pareto solution, “C” is one of feasible solution without greatest value.)
8. CONCLUSION
We are studying what and how we establish the way of overall system design (OSD) of geothermal energy system development.
The origin of our OSD was only optimization for technical elements in a type of HDR geothermal development, but now the
concept has been extended more comprehensive including another important issue of regional social acceptance. These two aspects
are closely related to each other in terms of constrained conditions for determining design values in geothermal energy system
development.
Functions of the OSD proposed in this paper are 1) detailed evaluation and optimization for each type of energy system and 2)
selection of its optimal type of energy system and its specification. The OSD can treat some “Trade-off” among different valuable
standards, and helps make a final decision established based on close discussion of all the stakeholders. Because the lack of idea of
the OSD in Japanese geothermal research and development might cause many conflictions with regional stakeholders, our proposed
concept of OSD would improve relationship between main people of development who are normally workers in a private company
and regional stakeholders such as hot spring owners, pre-existing industries, nature conservation, etc.
We are continuing the study of OSD and will complete the concept and system, which improves environment surrounding Japanese
geothermal energy development.
REFERENCES
Muraoka, H.: 3. Resource Assessment, In Geothermal power generation, Thermal and Nuclear Power Engineering Society, (2009),
61-69, (in Japanese).
Oikawa, Y., Niitsuma, H., Matsunaga, I.: Outline of a review of the Hijiori HDR project, 2004 annual meeting geothermal research
society of Japan, A16, (2004), (in Japanese).
Soma, N., Tenma, N., Matsunaga, I., Niitsuma, H., Hanano, M., Kadowaki, M., Watanabe, K.: Review of the Hijiori HDR project Concept of overall system design -, 2004 annual meeting geothermal research society of Japan, A17, (2004), (in Japanese).
Soma, N., Oikawa, Y., Sato, C.: A Questionnaire Survey Report of Japanese Drilling Cost for Overall System Design of Universal
Use of Underground Heat Energy, Renewable Energy 2006 International Conference and Exhibition, (2006)
Stefansson, V.: World Geothermal Assessment, Proceedings, World Geothermal Congress 2005, Antalya, Turkey (2005).
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