Report in reference to Trends in Sustainable Energy

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Renewable Energy 77 (2015) 430e441

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Renewable Energy

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Geothermal energy utilization trends from a technological paradigm perspective

Bobo Zheng a, b, Jiuping Xu a, b, *, Ting Ni a, b, Meihui Li a, b

a Low Carbon Technology and Economy Research Center, Sichuan University, Chengdu 610064, PR China b Business School, Sichuan University, Chengdu 610064, PR China

a r t i c l e i n f o

Article history: Received 2 July 2014 Accepted 13 December 2014 Available online 3 January 2015

Keywords: Geothermal energy utilization Technological paradigm Literature mining Electricity generation

* Corresponding author. Business School, Sichuan PR China. Tel.: þ86 028 85418191; fax: þ86 028 8541

E-mail address: [email protected] (J. Xu).

http://dx.doi.org/10.1016/j.renene.2014.12.035 0960-1481/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

The use of geothermal energy and its associated technologies has been increasing worldwide. However, there has been little paradigmatic research conducted in this area. This paper proposes a systematic methodology to research the development trends for the sustainable development of geothermal energy. A novel data analysis system was created to research the geothermal energy utilization trends, and a technological paradigm theory was adopted to explain the technological changes. A diffusion velocity model was used to simulate and forecast the geothermal power generation development in the diffusion phase. Simulation results showed that the development of installed capacity for geothermal generation had a strong inertia force along with the S-curve. Power generation from geothermal power sources reached a peak in 2008 and is estimated to be saturated by 2030. Geothermal energy technologies in hybrid power systems based on other renewable energy sources look to be more promising in the future.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The fourth assessment report of the United Nations Intergov- ernmental Panel on Climate Change has revealed that increased CO2 concentrations in the atmosphere could lead to an increase in the greenhouse effects which in turn could adversely affect climate change [1]. Forced by the need to protect the environment and promote the sustainable development of energy resources, pres- sure is growing to accelerate the adoption of renewable energy resources worldwide. Geothermal resources offer energy that is constant, and available on demand, providing an important alter- native to fossil fuels [2]. The energy savings from using geothermal resources are shown in Table 1 [3]. The case study reported by the Geothermal Energy Association compared geothermal energy with a coal plant that had been updated with scrubbers and other emissions control technologies. It was found that regardless of the emissions improvements, the coal plant emitted 24 times more carbon dioxide, 10,837 times more sulfur-dioxide, and 3865 times more nitrous oxides per megawatt hour than a geothermal steam plant.

University, Chengdu 610064, 5143.

However, due to the lack of a systematic analytical framework, much of the research in geothermal energy utilization has had little integrity or universality. In addition, there have been few studies which have specifically examined low-carbon technological para- digms, or, in particular, the geothermal energy utilization techno- logical paradigm (GEUTP). As a result, progress in this area has been slow with previous achievements partially overlapping and there- fore adding little to the area. Thus, there is a need to study the paradigm in terms of the technological evolution of geothermal energy utilization. If a geothermal energy technological paradigm is established, it could be used to investigate renewable energy so- lutions to reduce future carbon emissions.

When seeking to describe technological change and innovative research, the technological paradigm provides a sound method for the investigation of past trends as well as being able to predict future possibilities. In a broad analogy with the definition of the scientific paradigm [4], the technological paradigm is defined as “model for the solution of selected technological problems, based on highly selective principles derived from the natural sciences, together with the specific rules designed to acquire new knowl- edge, and to safeguard them as far as possible against rapid dissemination to competitors” [5]. An important concept in the above model is the technological trajectory, which is defined as the technological progress restrained by economic and technological trade-offs defined by a paradigm [6]. In other words, the trajectory

Table 1 Worldwide savings in energy, carbon and greenhouse gases using geothermal energy.

Fuel oil Carbon CO2 SOx NOx

bbl TOE TOE TOE TOE TOE

As electricity 250.0 37.5 33.2 106.9 0.74 0.022 As direct heat 125.0 18.8 16.6 53.4 0.37 0.011

Including geothermal heat pump cooling (figures in millions) in terms of fuel oil (TOE ¼ tonnes of oil equivalent).

B. Zheng et al. / Renewable Energy 77 (2015) 430e441 431

is the external form of the paradigm. For example, when targeting technologies, the trajectory may develop along the technological life cycle, which is presented in S-curves. However, the existence of a paradigm does not guarantee that its associated trajectory will develop, but it does mean that there is a complex process of se- lection within firms to determine the economic viability of the paradigm [7].

The technological paradigm is mainly applied to analyze tech- nological change and innovation in engineering fields [7e9]. By demonstrating how the various factors interact, such as scientific advancement, economic development, and organizational struc- tural change, blocks that are unable to be solved using existing technologies are exposed and future technological trajectories identified. With respect to how technological change occurs, demand-pull and technology-push have been identified as the two main driving forces [6], with supply and demand, namely push and pull, affecting the development of technological paradigms [10].

Thousands of research papers have been presented which focus on the use and benefits of geothermal energy. A novel data analysis system is created to summarize the trends to support the techno- logical paradigm to elucidate the development of geothermal en- ergy utilization. The data analysis system (DAS) is made up of the Web of Science database (WoS), NoteExpress and NodeXL, which is used to determine the internal relationship between the keywords. Compared with more simple statistical tools, this method has the ability to show the mutual relationships between the keywords over time. A diffusion velocity model is used to simulate and forecast the dominance geothermal power generation develop- ment in the diffusion phase of the GEUTP. This model is derived from a classic logistics model, combined with physics-based field theory.

The remainder of this paper is structured as follows. Section 2 uses the data analysis system (DAS) to identify the main techno- logical trajectories and uncover trends of geothermal energy utili- zation. The geothermal energy utilization technological paradigm is given in three stages in Section 3. In Section 4, geothermal power generation in the diffusion phase is illustrated as a case study and discussed based on the diffusion velocity model. Section 5 discusses the proposed framework. Conclusions are provided in Section 6.

2. Literature mining

With the explosive growth in geothermal energy research, it is difficult to screen for the most useful research foci and develop- ment directions. Literature mining, therefore, is indispensable in determining the most pertinent scientific research, especially in areas of particular interest such as geothermal utilization [11]. Literature mining is a powerful method for discovering the major trends across the years in the published scientific literature so that topic maps can be built [12]. Our primary goal for literature mining is to discover the relationships between the published years and the article keywords for geothermal utilization technologies.

2.1. The data analysis system buildup

Garfield, the “father of academic literature citation indexing”, believed that citations were the formal, explicit linkages between papersthathaveacommonlyspecificfield[13].Acitationindexisbuilt on the fact that citations in science served as linkages between similar research items, and can lead to matching related scientific literature, such as journal articles, keywords, years, and abstracts. Moreover, research which has the greatest impact in a specific field, or not less than one discipline, can be easily located using a citation index. For example, a paper's influence can be determined by its links to all the papers in which it has been cited. In this way, current developments, technologies, trends and potential fields of research emerge.

Based on the importance of the scientific research prime re- sources for observation and review can be identified. Peer- reviewed scientific research is extremely important, as these re- searchers are required to have considered the latest worldwide progress in their areas and to have ascertained the future in their respective research fields [14]. With this in mind, literature mining plays a pivotal role in the determination of the extent of geothermal utilization. To discover the keywords trends, the DAS is made up of the Web of Science database (WoS), NoteExpress and NodeXL. The WoS is chosen as the primary database, then NoteExpress is applied to review the general characteristics, and NodeXL is used to analyze the bibliographies. Therefore, the DAS, as a comprehensive inte- grated approach, can guide our research into the potential GEUTP development trajectories as shown in Fig. 1.

The Web of Science is the world's most trusted citation index as it covers all the leading scholarly research. The WoS is an online subscription-based scientific citation indexing service maintained by Thomson Reuters that provides a comprehensive citation search. It gives access to multiple databases that reference cross- disciplinary research, which allows for an in-depth exploration of specialized sub-fields within an academic or scientific discipline. Therefore, the WoS provides researchers, governments, and faculty with prompt and effective access to the world's leading citation databases. The network analysis using the WoS was conducted by Ridley et al. [15].

NoteExpress, which is similar to EndNote, is the most professional literature retrieval and management system in China. NoteExpress efficiently and automatically searches, downloads and manages researchpapersin avarietyof ways.The core functionof NoteExpress covers all aspects of knowledge management including knowledge acquisition, management, application, and mining, all of which are essential tools for academic research and knowledge management. Some scholars in China have achieved important research results by making use of NoteExpress in different fields [16e18].

NodeXL, a free analysis tool, was designed to facilitate the learning of the concepts and methods of social network analysis using visualization as a key component [19]. It is a powerful and easy-to-use interactive network visualization tool that leverages the widely available Microsoft Excel application as a platform for representing generic graphical data, performing advanced network analyses and providing a visual exploration of the networks. NodeXL generally supports multiple social network data providers that import graphical data (nodes and edge lists) into an Excel sheet and has been used widely by many researchers [20,21]. NodeXL, therefore, plays a vital role in the analysis of the keyword trends in our research.

2.2. Filtering criteria

Since the WoS is rich with knowledge, it is difficult to select useful articles if the appropriate filtering criterion is not specified. Under this circumstance, and to avoid important documents being

Fig. 1. Research documentation meta-synthesis method.

B. Zheng et al. / Renewable Energy 77 (2015) 430e441432

overlooked or duplicated, the appropriate methodology in this article is in accordance with the following rules.

� Step1: Search for geothermal utilization using keywords such as utilization, use, research, development, transformation, con- versation, application, and trends in the scope of article's “topical subject”, rather than looking at the full range. This approach ensures that the search process is more accurate and efficient.

� Step2: To ensure a high relevance in the articles, some unnec- essary research is not selected from title and abstract.

� Step3: Research themes are more concentrated by combing and merging the similar keywords, e.g. a heat pump is similar to heat pumps.

Based on these steps above, the 8541 initial geothermal energy application records were refined to 1430 articles. Taking the representative extent of literature into account, articles from 2014

Table 2 The article analysis of geothermal energy R&D.

Journal names Numbers Percentage

Renewable & Sustainable Energy Reviews 57 8.65% Geothermics 49 7.44% Renewable Energy 30 4.55% Applied Thermal Engineering 26 3.95% Energy 23 3.49% Applied Energy 20 3.04% Energy Sources 20 3.04% Energy Conversion and Management 19 2.88% Energy and Buildings 18 2.73% Energ Source Part A 12 1.82% Energ Source Part B 11 1.67% Energy Policy 10 1.52% Grundwasser 9 1.37% International Journal of Energy Research 9 1.37% Acta Montanistica Slovaca 8 1.21% Desalination 7 1.06% International Journal of Hydrogen Energy 7 1.06% Z Dtsch Ges Geowiss 7 1.06% Environmental Earth Sciences 6 0.91% Energy Exploration & Exploitation 5 0.76% J Energ Resour-ASME 5 0.76% Neth J Geosci 5 0.76% Thermal Science 5 0.76% Total 368 55.84%

were not included. After thoroughly and carefully removing duplicate and irrelevant articles using NoteExpress, 659 geothermal utilization related articles were finally selected. Classification was made according to the journals from which the 659 articles had been selected. 23 representative journals were selected in Table 2, while others which had published less than five articles were filtered out. The percentages refer to the number of articles. For example, 57 related articles in the first journal accounted for 8.65% of the final 659 articles. 368 from 1998 to 2012 articles were selected for keyword analysis and visualization.

2.3. Keywords focus

The keywords and publication years were extracted from the selected articles and put into NodeXL. Different from frequency analysis, the network developed in NodeXL shows an interaction between the ‘year’ and the ‘keyword’. Each ‘year’ and ‘keyword’ acts as two vertexes forming an edge. After calculation and rearrange- ment, the primary distribution of the edges highlights the key- words along a year-axis, as shown in Fig. 2.

After filtering the primary analysis results, the new keyword foci are laid out in years on the horizontal axis, as shown in Fig. 3. Four areas were identified. Area D below the year axis includes the keywords ‘sustainability’, ‘renewable’ and ‘environment’. These keywords demonstrate that the ultimate R&D goal of geothermal energy utilization is to promote the development of renewable energy and achieve environmental sustainability. The keywords in area A, such as ‘direct heating’, ‘greenhouse heating’ and ‘heat pumps’, refer to the direct use of geothermal energy. In area B, the keywords ‘electricity’, ‘electricity generation’ and ‘geothermal po- wer plants’ are prevalent under the general term geothermal electricity. Finally, area C indicates the future direction for the development and utilization of geothermal applications, which is primarily focused on hybrid power systems based on all low- carbon energy types.

In the above network, the size of each ‘year’ point represents the frequency of research foci. The bigger the size is, the higher the frequency is. A general yearly incremental trend is clearly from 1998 to 2012 as the size of ‘year’ point increases annually. However, 2003 and 2008 are significantly greater in size than the year before and after, which shows frequency changes around 2003 and 2008. Thus, the points ‘2003’ and ‘2008’ are identified as critical years along the horizontal year axis. When considering areas A, B and C,

Fig. 2. The primary keyword distribution of geothermal energy utilization.

Fig. 3. Keywords focus of geothermal energy utilization.

B. Zheng et al. / Renewable Energy 77 (2015) 430e441 433

the frequency changes outline relatively obvious boundaries be- tween them. Each area is summarized in a technological group with its prevalent period in Table 3.

3. Technological paradigm for geothermal energy utilization

This paper proposes a novel concept for the geothermal tech- nological paradigm (GEUTP) to illustrate the technological utiliza- tion evolution, which to the best of our knowledge is a new area of research. The technological paradigm is a long term, continuous

process which goes through a complete life cycle, from birth, growth, maturity to retirement. There are two further phases which have been distinguished. A pre-paradigmatic phase is characterized by a technical solutions selection process, and a self-structuring phase features the industrial learning of new technical solutions [22]. This paradigmatic phase was added to describe the evolution of the gas turbine paradigm [7].

From the DAS review, we concluded that geothermal energy utilization met the laws of the paradigm and could be summarized in three stages, which are characterized by the different

Table 3 Prevalent period of technology group.

Technology group names

Prevalent period

Representative technology

Direct use 1998e2012 Direct heating, the heat pumps greenhouse heating, aquaculture, hot spring spa, …

Electric production

2003e2012 Geothermal power plants, electricity generation

Hybrid power system

2008e2012 Solar energy, hydropower, wind power, hydrogen, biomass

B. Zheng et al. / Renewable Energy 77 (2015) 430e441434

technologies: competition, diffusion and shift. The model shown in Fig. 4 was derived from the well-known S-shaped curve [23]. The first stage is competition-oriented, which is defined as the early development of the technological paradigm and involves numerous trajectories and barriers. The diffusion phase is identified as the second stage in which a dominant path prevails in the potential market. Technological shift occurs with technological saturation, which predicts the technological development trends. We estimate that geothermal energy combined with other renewable sources will ultimately break through the technical barriers and enter the novel competition phase of the next advanced technical paradigm in a spiraling manner.

3.1. The competition phase

The GEUTP competition phase is the beginning of the proposed novel regulatory framework. During the technological paradigm evolution with respect to economic, institutional and social factors, selection-orientation is the main feature in this phase.

The direct-use of geothermal energy is one of the oldest, most versatile and most common forms of utilization [24]. The early history of direct geothermal use was reviewed in over 25 countries looking at geothermal use over 2000 years [25]. Information pub- lished after the World Geothermal Congress (WGC) of 1995, 2000, 2005 and 2010 is widely accepted. The early history of direct geothermal use was reviewed in over 25 countries looking at geothermal use over 2000 years [25]. Combined with other research, we show the research coverage for the direct utilization of geothermal energy in Fig. 5 [26e29].

According to the statistics from the WGC in 1995, 2000, 2005 and 2010 in Table 4, geothermal (ground-source) heat pumps (GHP) now have the largest installed capacity, accounting for 67.70% of the worldwide capacity of direct utilization. The installed capacity was 33,134 MWt and the annual energy use was 200,149 TJ/yr, with a

Fig. 4. Technological paradigm for geothermal energy utilization.

capacity factor of 0.19 (in the heating mode) [3]. Various types of geothermal heat pumps are summarized in Fig. 6 [30]. The main advantage of these technologies is that they use normal ground or groundwater temperatures (between about 5 and 30 �C), which are available in all countries of the world [31].

3.2. The diffusion phase

In the paradigm evolution, the most important common tech- nical innovation features evolve during diffusion. These can be divided into three main categories [32]:

1. Multi-generational technologies mutually coexisting and competing;

2. A dominant technology which stands out from the other current technologies;

3. The emergence of innovation diffusion.

After the competition in the preceding phase, a dominant tra- jectory (or trajectories) inevitably emerges, indicating the gradual move into the diffusion phase. From an economic viewpoint, the dominant design in this phase is most often driven by an interac- tion of market energy demand and industrial competition. This diffusion process can be seen to be similar to the contagion process of an epidemic disease [33], which is depicted in Fig. 7.

Following these laws, geothermal energy utilization also evolved from the competition phase to the diffusion stage with the development of geothermal power generation.

Commercial geothermal power generation is an established industry around the world. Italy was the first country to develop geothermal power commercially in 1913 at Larderello, with an installed capacity of 250 kWe [34]. Since 1950, other countries have followed the Italian example. By 2003, electricity generation using low to moderate temperature geothermal resources was an established technology, and successful applications in the U.S. were characterized by resource temperatures of greater than 22 �F [35]. In 2010, according to a BP statistical review, the world geothermal electrical installed capacity was 11,055 MWe, with the electrical energy generated being 67,246 GWh, representing 0.31% of the world total electrical energy. The annual electrical capacity is pre- sented in Table 6 in Section 4.

There are two types of generating sets with four kinds of geothermal plants, the use of which is determined primarily by the nature of the geothermal resource at the site [36]. Back-pressure steam turbines and condensing steam turbines are applied when the geothermal resource produces steam directly from the well, which cannot be applied to low-temperature resources. However, steam resources are the rarest of all geothermal resources and exist in only a few places in the world [37]. Thus a limitation has arisen. In binary plants, a second fluid in a closed cycle is used to operate the turbine rather than geothermal steam.

Flash steam plants are employed in cases where the geothermal resource produces high-temperature hot water or a combination of steam and hot water [37]. Depending on the temperature of the resource it may be possible to use two flash tanks stages. In the single flash case as shown in Fig. 8, the water is separated in the first stage tank and is directed to a second stage flash tank where more (but at a lower pressure) steam is separated. This so-called double flash plant delivers steam at two different pressures to the turbine, as shown in Fig. 9. This type of plant cannot be applied to low-temperature resources.

From this scenario, the global geothermal electricity share reached 0.67% in 2010 and is expected to reach 1.25, 2.03, and 2.81% by 2020, 2030, and 2040, respectively [38]. The detailed

Fig. 5. Direct step utilization of geothermal energy.

B. Zheng et al. / Renewable Energy 77 (2015) 430e441 435

development of geothermal electricity is described in the diffusion velocity model of Section 4.

3.3. The shift phase

This section attempts to describe the trends in the shift phase. Paradigm change inevitably occurs when the situation described in the original theory cannot be explained further and adapts to the new technology that is evolving as a result of scientific de- velopments [39]. The shift in the technological paradigm refers to the revolutionary changes in the industrial structure brought by new technology [40]. The natural limits closely related to the product demand in the market are a critical point for a successful technological paradigm shift [41]. Another supplementary condi- tion is the emergence of disruptive technologies originating from full competition in the specific areas [42]. Therefore, natural limi- tations and disruptive technologies act as the two main driving forces for the paradigm shift.

In area C in Fig. 2, renewable sources such as solar, hydrogen, wind, biomass, and hydropower can be found. When looking at

Table 4 Installed capacity of geothermal heat pumps and direct use of geothermal energy.

Year GHP (MWt) Direct use (MWt) Percentage (%)

1995 1854 8664 21.40 2000 5275 15,145 34.83 2005 15,384 28,269 54.42 2010 33,134 48,943 67.70

freshwater resources, hybrid power systems based on renewable sources might be a softer path for geothermal energy utilization in the future [43]. Geothermalesolar hybrid systems are the most widely accepted. Wind turbine systems which are combined with solar assisted geothermal heat pumps [44] or compression heat pumps [45] were studied. When considering biomass energy, there is a potential synergism between geothermal energy derived from hot dry rocks and the conversion of biomass to ethanol [46]. An example of a hybrid geothermalesolarehydrogen system is shown in Fig. 10.

Hybrid geothermalesolar systems generally consist of: (i) a geothermal ORC power cycle configured in a binary arrangement and (ii) a solar heating system comprised of a super heater (or solar booster), a solar pump, and solar collectors. Five cases of hybrid geothermalesolar systems are summarized in Table 5.

Besides the stand-alone systems, combined-cycle solar and geothermal heat sources are being used in adsorption desalination technology to reduce the overall cost of water treatment [53]. In a solar-assisted air-conditioning system installed in southern Spain (Almeria), the shallow geothermal energy was designed as an alternative to the cooling tower and improved the efficiency [54].

4. Geothermal generation diffusion velocity model

From the above analysis, geothermal electricity generation clearly dominates the GEUTP diffusion phase, as it has technical and economic feasibility. Geothermal power stations have an advantage over other power plants and could be a fundamental supplement to

Fig. 6. The typical geothermal heat pump system.

B. Zheng et al. / Renewable Energy 77 (2015) 430e441436

the energy market [55]. However, it is uncertain how geothermal electricity will develop. Therefore, a diffusion model based on field theory is proposed for further study.

4.1. Modeling

Based on a logistics model, field theory, which is originally derived from a physics concept, is introduced. In Eq. (1), refers to the variation in the share of specific technologies in the market at timet, V is the existing share of the market, Vpri represents the preliminary occupancy in the market, and is the natural diffusion rate.

dVðtÞ dt

¼ gV 1 � V

Vpri

! (1)

If setting a ¼ g/Vpri, the geothermal generation diffusion model is derived as Eq. (2). v(t) represents the speed in the annual geothermal electricity generation installed capacity, C is a constant coefficient, a is the self-diffusion rate, which indicates the possi- bility of new technologies. This model was used to simulate the installed capacity and show the diffusion velocity changes in the emerging technologies.

vðtÞ ¼ dV dt

¼ CVpriae �at�

1 þ Ce�at �2 (2)

4.2. Data collection

The geothermal installed electrical capacity refers to the inten- ded technical full-load sustained output of a facility such as a geothermal power plant. The accumulative total annual installed geothermal generation capacity (V) is introduced into the mathe- matical model above as an independent variable, and was collected from the Statistical Review of World Energy 2012 by BP Amoco [56]. The data can be trusted, as BP Amoco is considered as an energy industry benchmark [57]. The diffusion velocity v represents the annual added installation (v). The basic data are presented in Table 6.

4.3. Result analysis

The original values for C and Vpri were assigned. The parameter for a in the nonlinear regression function was estimated using the Statistical Package for Social Science 12.0. After the simulation, a was found to be 0.518, C was 54.950, and Vpri was 299.150. The diffusion velocity model was then constructed using Eq. (3).

vðtÞ ¼ 54:950 � 2999:150 � 0:518 � e �0:518�t�

1 þ 54:950 � e�0:518�t �2 (3)

The annual added installation was first calculated as the ‘predev’ curve in Fig.11. In Fig.11, it can be clearly seen that around 2008, the geothermal installed capacity diffusion velocity had reached a peak. When t ¼ 21(in 2021), the diffusion velocity was estimated to be less than 1 and by 2030, it was expected to be close to zero. The

Fig. 7. The evolution of technological paradigms.

Table 6 The statistics of geothermal power generation capacity worldwide.

Year Period Annual added installation v (MW)

Accumulative total annual installation V (MW)

2000 0 e 8595 2001 1 136 8731 2002 2 157 8888 2003 3 268 9156 2004 4 13 9169 2005 5 118 9287 2006 6 312 9599 2007 7 465 10,064 2008 8 466 10,530 2009 9 314 10,844 2010 10 211 11,055 2011 11 101 11,156 2012 12 290 11,446

B. Zheng et al. / Renewable Energy 77 (2015) 430e441 437

actual values for the added installation (v) had a similar trend with the predev except for the values in 2004 (t ¼ 4) and 2010 (t ¼ 10), which were 13 MWe and 101 MWe respectively. Statistics from countries which generate geothermal electricity are able to offer an explanation for this.

A total of 24 countries now generate electricity from geothermal resources [36]. The top seven countries in terms of electricity production are the USA, the Philippines, Indonesia, Mexico, Italy [58], New Zealand and Japan. The level of installed capacity in each is listed in Table 7. From 2000 to 2010, these seven countries saw an increase of above 2730.6 MWe, accounting for 88.63% of total ad- ditions. Therefore, the development of geothermal generation in these seven countries is representative of what is happening in the world.

Despite the large geothermal electrical generation potential in the USA (23,000 MWe) [59], Japan (20,000 MWe) [60], Indonesia (16,000 MWe), Philippines (6000 MWe), Mexico (6000 MWe) and New Zealand (3650 MWe) [61], the sharp decreases around 2004 and 2011 were mainly caused by severe economic crises that

Table 5 Case studies of hybrid geothermalesolar systems.

Study/project location System design System function

Tuscany [47] i Low-temperature (90 ~ 95 �C) geothermal energy

Heat and electricity (50 kWe)

ii Solar collectors iii A single turbine of 50 kW

Omaha, Nebraska, USA [48]

i An earth-to-air heat exchanger

Space cooling

ii Solar collector enhanced solar chimney

Renmark, Moomb, Longreach Australia [49]

i A geothermal ORC power cycle configured in a binary arrangement

Power generation

ii A solar heating system (a super heater or solar booster, a solar pump, solar collectors)

Florence, Italy [50] i Low-temperature (80 ~ 100 �C) geothermal energy

Electricity (50 kWe)

ii Solar field composed by evacuated solar collectors

South Korea [51,52] i Geothermal heat pump Heating and cooling ii Solar collectors iii A gas-fired backup boiler iiii Incidental facilities

adversely affected power demand and growth. However, newly- developing geothermal countries like Turkey [62] since 2005 and some policy adjustments and regulations also positively affected these fluctuations. For example, a geothermal energy law was established by the Indonesian government in 2003. Therefore, the simulated result affected by external factors in Fig. 11 shows that the development of geothermal generation installed capacity has a strong force of inertia along the S-curve. In other words, the diffusion velocity model is reasonable.

The diffusion velocity increased annually from the beginning (t ¼ 0, V ¼ 8595). The annual installation values from 2001 to 2012 were then secondly calculated and are shown as the ‘pred�V’ curve in Fig. 12. The simulation results showed that the actual adoption rate for t time was stronger than in other models. The R2 (coefficient of determination) was 0.9830, and the adjusted R2 was 0.9811 in the model output, which indicated that the predicted annual installation (pred�V curve) was highly correlated with the actually annual installation. When the sample size equaled 11, F (a test for statistical significance of the regression equation as a whole) was 519.48 with the significance being almost 0.0000, which indicated that the regression equation as a whole was statistically significant. Therefore, the simulation effect model was sound.

4.4. Forecast

The specific prediction results are shown in Fig. 13. Around 2008, the geothermal installed capacity has reached a peak. The value will not remain stable until t ¼ 21 (in 2021). By 2030, the geothermal installation worldwide is nearly unchanged, which also can be seen in Table 8.

These prediction results are basically in accordance with those [63]. In his viewpoint, because of the required geographical con- ditions and the limited reserves of energy, geothermal power would account only a tiny share of the electricity supply in the global market. From 2006 to 2020, due to technical innovations, there was and is expected to be a rapid development of geothermal power generation added capacity. However, in the end, innovations in geothermal power will gradually slow when diffusion velocity is addressing a specific level of need.

5. Evaluation and discussion

Clean and renewable energy is commonly accepted as the key to the future of the world. This is primarily because renewable energy resources have some advantages compared to fossil fuels. Both the National Energy Policy Act of 1992 and the Pacific Northwest Electric Power Planning and Conservation Act of 1980 defined

Fig. 8. The generation diagram of single flash steam turbine.

Fig. 9. The generation diagram of double flash steam turbine.

Fig. 10. An example of geothermalesolarehydrogen hybrid system.

Fig. 11. Prediction of geothermal generation diffusion velocity.

B. Zheng et al. / Renewable Energy 77 (2015) 430e441438

geothermal energy as a renewable resource [64]. Geothermal en- ergy is a clean, renewable resource providing energy around the world. Heat flows from the earth's interior constantly and will continue to for billions of years to come, ensuring an inexhaustible supply of energy. Geothermal electricity has already been proven to be a clean, reliable, and comparatively inexpensive alternative to fossil fuels.

5.1. Geothermal electricity barrier

However, geothermal systems are regarded as the most disad- vantageous renewable energy technology in respect to sustain- ability because of the availability difficult [65]. There are only a few suitable locations for geothermal energy power plants. An ideal location needs to have access to the heat at a depth that allows for easy drilling. Further, geothermal supplies are often located far from cities and other places with high energy demands. Though the estimated geothermal power available in the world that could be economically extracted exceeds 13,000 TWh, only 2600 TWh en- ergy is able to be obtained [66,67]. For example, 70% of the geothermal power available in Europe is within only 6 countries, with 33% of this coming from Sweden [68]. In total, about 54 TW h/ year of electrical energy is obtained from these resources in 24 countries.

Most of the world's high-temperature geothermal resources have already been exploited for electricity generation. Of those geothermal resources which are above 90 �C, only a quarter are at 150 �C or above. For most moderate-temperature geothermal re- sources, the generation of electricity is not economical due to the low thermal efficiencies. Therefore, it is no surprise that the elec- tricity generated by a geothermal power plant is only about one thirteenth of the heat which can be harvested and sold to pro- spective users for space, water, and process heating [69]. Therefore, geothermal applications are characterized by a high initial cost and relatively low operating and maintenance costs [69]. These costs include land surveys, power plant installation, the hire of drilling equipment, electricity tower construction and the need for specialized staff.

Another disadvantage of geothermal energy is that it is not easily transported, so can only be used to generate electricity in surrounding areas. Further, geothermal energy can only generate small amounts of electricity compared to other fuel sources. However, released substances are also a concern. The odor and noise pollution in geothermal systems and their affect on seismic activities are important social effects.

Overall, all alternative energy resources, including geothermal energy, have the disadvantage of currently being considered only complementary energy resources. Most of these technologies are still considered new, and therefore installation and operating costs

Table 7 Installed capacities of the top seven countries in 2000, 2005, 2010.

Country 2000 (MWe) 2005 (MWe) 2010 (MWe) Increase 2000e2005 Increase 2005e2010

MWe Capacity (%) MWe Capacity (%)

USA 2228 2534 3098 306 13.73 564 22.26 Philippines 1909 1930 1904 21 1.10 �26 �1.35 Indonesia 589.5 797 1197 207.5 35.20 400 50.19 Japan 546.9 535 535 �11.9 �2.18 0 0 Italy 785 791 843 6 0.76 52 6.57 Mexico 755 953 958 198 26.25 5 0.52 New Zealand 437 435 762 �2 0.46 327 75.17

Fig. 12. The simulation of geothermal generation's actual capacity and forecast capacity.

Fig. 13. The prediction of geothermal generation capacity.

Table 8 The prediction of geothermal power generation capacity worldwide.

Year Period Actual capacity V (MW/a)

Predicted capacity V (MW/a)

Year Period Actual capacity V (MW/a)

Predicted capacity V (MW/a)

2001 1 8731.00 8639.71 2016 16 e 11494.80 2002 2 8888.00 8711.85 2017 17 e 11507.38 2003 3 9156.00 8825.30 2018 18 e 11514.93 2004 4 9169.00 8996.80 2019 19 e 11519.44 2005 5 9287.00 9241.03 2020 20 e 11522.13 2006 6 9599.00 9560.66 2021 21 e 11523.74 2007 7 10,064.00 9935.46 2022 22 e 11524.70 2008 8 10,530.00 10322.07 2023 23 e 11525.27 2009 9 10,844.00 10671.50 2024 24 e 11525.61 2010 10 11,055.00 10951.68 2025 25 e 11525.81 2011 11 11,156.00 11155.65 2026 26 e 11525.93 2012 12 11,446.00 11293.99 2027 27 e 11526.01 2013 13 e 11383.45 2028 28 e 11526.05 2014 14 e 11439.50 2029 29 e 11526.07 2015 15 e 11473.92 2030 30 e 11526.09

B. Zheng et al. / Renewable Energy 77 (2015) 430e441 439

are high. Research and development activities are still generally in the early stages, so economic success has not yet been sufficiently proved. Besides these general issues, it is still widely believed that alternative energy resources are only available in small scale, dispersed, unstable production facilities, which do not have the ability to provide long term sustainable energy [70].

5.2. A new approach exploration

Regardless of these issues, this article has attempted to look for innovations that are able to offer a marked improvement on today's approaches or provide an entirely new approach [71]. Two paths were discovered: a “hard path” that relies almost exclusively on a centralized infrastructure to expand supply and increase efficiency; and a “soft path” that complements the former by investing in decentralized facilities, efficient technologies and policies, and human capital [43,72]. For the “hard path”, exploration research has focused on developing more accurate and lower cost methods for finding and mapping geothermal resources. The soft path seeks to improve overall productivity rather than to find new sources of supply. The above literature analysis and modeling shows that the “soft path” may have greater potential. Many important and chal- lenging research areas have the potential to affect our future energy needs significantly. For example, energy efficiency and the inte- gration of energy sources with electricity transmission, distribution and storage are vitally important, which is our key concept in the shift stage d the hybrid power system.

This conclusion is made in the light of the difficult geological environments, the energy characteristics, and advantages and dis- advantages of geothermal energy. Relying only on geothermal en- ergy would not meet the energy demands of any countries or regions. Therefore, hybrid renewable energy power systems energy a comprehensive energy supplies solution for the global energy market. Most sustainable energies have the capacity to relieve global environmental concerns. Current investigations into renewable hybrid energy systems have highlighted the high cost of generating electricity using stand-alone renewable power plants [73] and have suggested that one of the most effective approaches to reduce the electricity generation costs and improve plant effi- ciency is the hybridization of different renewable technology platforms [74].

The geothermal power shifting hybrid power system is heavily dependent on the diffusion velocity of added geothermal power generation capacity. It is clear from our investigation, however, that this process will be gradual rather than abrupt, so as there will possibly be no obvious increase in capacity, hybrid power systems would be more suitable in the interim. Therefore, it is estimated that hybrid power systems will play a key role in substituting for geothermal electricity generation in the diffusion stage. Hybrid power systems can meet the aims of minimizing net present costs while reliably and cost effectively meeting a given demand [75]. Further, they can offer more reliable and consistent power supply

B. Zheng et al. / Renewable Energy 77 (2015) 430e441440

than a geothermal system alone. The simulation results in this paper demonstrate that utilizing renewable generators such as a hybrid (PV and/or hydro and/or geothermal systems) generator can reduce operating costs and greenhouse gas and particulate matter emissions (CO2 and NOx) improve energy efficiency and consume less fuel. In the case of geothermal energy, such hybridization can also decelerate the depletion of heat content in the geothermal reservoir over time and hence extend its lifespan. The results also demonstrate that renewable energy technologies, such as the geothermalesolar hybrid system, have the potential to supply electricity to base station sites in a cost effective manner.

6. Conclusion

The combined effect of the widespread depletion of fossil fuels and the gradually emerging consciousness about environmental degradation has given priority to the use of conventional and renewable alternative energy sources such as geothermal, wind, hydro, solar and bioenergy sources. Geothermal energy, which is the thermal energy within the earth's interior, has attracted much attention for these reasons.

In this paper, we first analyzed the keyword focus trends using a data analysis system. The analysis results showed that the trends were divided into three stages. The first stage was the direct use of geothermal energy (heat energy) such as in space heating and cooling, agricultural crop drying, industrial processes, greenhouse and covered ground heating, snow melting and space cooling and bathing and swimming. In the second stage electricity generation was from an indirect use of geothermal energy, in which heat is converted into mechanical energy and then into electrical energy. Finally, the use of hybrid power systems was the main topic in last stage, the use of which can offer some benefits to the goal of a sustainable world.

The geothermal generation diffusion velocity model showed that geothermal generation is a mainstream technology in the implementation phase. It is estimated that use of this energy source in its present incarnation will be saturated by 2030, and will be replaced by a new technology. Finally, from the macro- environment and policy point of view, geothermal generation has some limitations, and hybrid power systems may have more advantages.

Because of worldwide concerns for the environment, a several low-carbon methodologies are beginning to have some effect. As one of the options available technologies for the reduction of car- bon emissions and the cleaner use of energy, geothermal energy technologies play a critical role in GHG emissions management. Although scientists are making common efforts to clean up the earth, there is no international framework for geothermal energy technology use at present. As a result, the development of these various technologies is at different levels, with some overlapping. In this paper, we hope our suggested novel regulatory framework can provide scholars with new future research ideas. Further, spe- cific subsidy incentives for governments and sectors should be considered to allow for acceleration in GEUTP development. However, at this stage, there are still many uncertainties about which technologies could lead to real improvements and which really have no prospects for reducing anthropogenic CO2. Further work is required to evaluate the individual cases selected in this study.

Acknowledgments

This research is supported by the Major Bidding Program of National Social Science Foundation of China (Grant No.12\&ZD217). The authors would like to thank the anonymous referees for their

insightful comments and suggestions to improve this paper, as well as Uncertainty Decision-Making Laboratory and Low Carbon Technology and Economy Research Center of Sichuan University for helpful comments and discussion.

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