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A discussion of major geophysical methods used for geothermal exploration in Africa
Renewable and Sustainable Energy Reviews 58 (2016) 775–781
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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser
A discussion of major geophysical methods used for geothermal exploration in Africa Zakari Aretouyap n, Philippe Njandjock Nouck, Robert Nouayou Laboratory of Geophysics and Geo-exploration, University of Yaounde I, PO Box: 812, Yaounde, Cameroon
art ic l e i nf o
a b s t r a c t
Article history: Received 4 February 2015 Received in revised form 21 December 2015 Accepted 27 December 2015
Geophysics provides a range of methods for the exploration of geothermal sources. This range is so broad that it can sometimes embarrass the geophysicist. The present paper classifies these methods according to several criteria: best-fitted geological environment, main assets and limitations of each method, preliminary or detailed nature of each method and even the specific objective of the exploration to be carried out. This classification could therefore help to significantly reduce costs and time loss related to trial uncertainty and bad choices in selecting the appropriate method. In order to provide necessary information for potential geothermal investors in Africa, the paper addressed several aspects such as the geological setting and the geothermal potential, the population density, the energy needs or demand, the current electricity tariff and the business environment or opportunities in the continent. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Asset Direct method Geothermal exploration Indirect method Limitation
Contents 1. 2. 3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 775 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776 3.1. Presentation of results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776 3.2. Explanatory notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776 3.2.1. Seismic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776 3.2.2. electric and magnetic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 3.2.3. Thermal methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 3.2.4. Remote sensing method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 3.2.5. γ-ray spectrometry method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 3.2.6. Induction method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 3.2.7. Frequency domain electromagnetic (FDEM) method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 4. Discussions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 4.1. Influence of the geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 4.2. The geothermal potential of Africa and business opportunity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 778 4.3. Appropriateness of those methods with the African continent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 778 4.4. Geophysical methods and levelized cost of geothermal energy exploration in Africa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 778 4.5. Other advantages of geothermal energy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 779 4.6. Useful information for geothermal investors in Africa. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 780 5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 780 Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 780 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 780
n
Corresponding author. Tel.: þ 237 675086759. E-mail address: [email protected] (Z. Aretouyap).
http://dx.doi.org/10.1016/j.rser.2015.12.277 1364-0321/& 2016 Elsevier Ltd. All rights reserved.
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1. Introduction The fight against climate change is a challenge that currently enrolls the whole world. Both developed countries, responsible for the situation and developing countries, virtually victim are expected to work together in order to curb the problem. Currently, greenhouse gases produced by fossil and oil sources (hydrocarbons) are indexed as the main cause of global warming. Hence, it is necessary to explore renewable and cleaner energy sources like geothermal sources [1]. However, Geophysics provides a so broad range of methods that professionals may be embarrassed. It becomes then important to classify those geophysical methods according to some criteria such as best-fitted geological environment, main assets and limitations of each method, preliminary or detailed nature of each method and even the specific objective of the exploration to be carried out. Geothermal energy is formed deep within the earth's crust, and is exploited for electricity generation and other direct uses. The medium of this energy transfer is geothermal fluid. On the surface, these are manifested as hot grounds, fumaroles, geysers, mudpools and hot springs [2]. The main geological parameters of the geothermal reservoir to be determined are: geological formation (lithology), tectonic structures (faults), permeability (hydraulic conductivity), temperature, and stress field. The depth to which these parameters are located must also be considered. However, the needs are not exactly the same for a hydrothermal or petrothermal project. Some of these parameters can be estimated from the surface, mainly by geophysical methods while others let themselves be measured in a borehole [3]. Anyway, geophysical methods are among the best to explore these sources [4]. Domra Kana et al. [4] drew up a review of the main geophysical methods used for this purpose. Their study classifies these methods into four groups based on the physical measured parameter and into two main categories depending on whether they are said to be direct or indirect methods. The present paper aims at promoting the use of geothermal sources by reducing costs across the world including Africa. To achieve this prodigious idea, the present paper is as an additive one designed to reduce ambiguity and speculation in choosing a method. In fact, some methods are essentially preliminary and may be used only for a "pre-exploration", others are more conducive to well-defined geological settings.
The main objective of the present paper is to clarify the assets (strengths) and limitations of each method, then classify major geophysical methods into preliminary and detailed categories.
2. Methodology The study conducted by Domra Kana et al. [4] performs a review of the most geophysical methods used for geothermal exploration. That investigation of a paramount importance was discussed mainly in terms of advantages and disadvantages for each method. The present study is a thorough analysis of these geophysical methods used in geothermal exploration. It presents the assets, the limitations and the best-fitted geological environment of each of them. These methods have also been classified into preliminary and non-preliminary ones. The asset of a method is defined as its success rate, or its ability to deliver positive results while the advantage represents a positive point of a method compared to others. Similarly, the limitation of a method lets know on its inability to perform a task while the disadvantage refers to defects or deficiencies or hazards related to a method.
3. Results For an easy operation, the main results are presented in tables. 3.1. Presentation of results The results are reported in two summary tables. Table 1 summarizes the assets, the limitations and the preferred environment of each method. 3.2. Explanatory notes 3.2.1. Seismic methods Seismic reflection predicts the depth and thickness of a desired geological formation. This may be the thickness of an aquifer for a hydrothermal project or the depth of the crystalline basement roof for a petrothermal project, for example. The permeability of a geological formation, which guarantees the success of a hydrothermal
Table 1 Summary of the main strengths and limitations of different geophysical methods. Methods
Assets
Limitations
Seismic refraction
Does not directly determine the permeability of a Volcanic and sedimentary geological formation [5]. rock assemblage [6].
Magnetic
Allows us to image directly basement (geological formations, presence and geometry of faults, predictive surveys profile). Good resolution of layering from depths of 20 m to more than several hundred meters. A significant depth resolution (magnetotelluric).
Gravimetric Thermal
Simpler and less expensive. Efficient for detecting geochemical haloes.
Remote sensing γ-ray spectrometry
Autonomously and complementarily skilled. High accuracy and enormous penetrating power.
Direct current (DC)
Simple rule of thumb with existence of several electrode configurations. A comparatively efficient reconnaissance tool because the cap response is strong in both polarization modes. Very effective for the rapid reconnaissance of an area for mapping.
Seismic reflection
Induction
Frequency domain electromagnetic (FDEM)
Extracting precise interval velocities from multilayered media is sometimes difficult [5]. Vertical sounding applications (no 2D or 3D interpretation) [7]. Does not allow an unambiguous interpretation. Limited to detecting relatively shallow features [7]. Inefficient in areas covered by thick vegetation. Intended primarily to detect radionuclides contained in a [10] rock. Ambiguity in the interpretation of results (determination of the structure or its geometry). Requires large current sources (up to 100 A) and large receiver loops (40 m 340 m) [11].
Environment/geological setting
Volcanic and sedimentary rock assemblage [6]. Volcanic environments [8]. Volcanic environments [8]. Any geological context [9]. Basement with faults [9]. Basement [8]. Any geological context [9]. All geological setting away from power lines [9].
All geological setting away Topography can be a problem in interpreting FDEM data. TDEM is not widely used for shallow from power lines [9]. studies (less than 20 m).
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project, on the contrary cannot directly be determined by seismic reflection. Seismic methods can then be used to detect substantial faults that may have a high hydraulic conductivity and may represent a target for hydrothermal projects. Large regional faults can also be associated with a higher risk of seismic activity and therefore must be identified in the framework of petrothermal projects [12]. 3.2.2. electric and magnetic methods With the exception of the magnetotelluric, electrical or magnetic methods have not deep geothermal application. Indeed, the vertical thermal gradient is an increasing function of the operation depth. However, its value depends strongly on the petrophysical and especially tectonic setting of the region except in the context of (very) low geothermal energy. Industrial geothermal power plants are located in areas where the temperature is abnormally high. This is the case of the context of volcanic rifting in Iceland or back-arc basins in the Philippines or the active plutonism in Larderello, Italy [13]. Given that these potential methods do not allow an unambiguous interpretation, they should be used in addition to other investigation methods. They can also help to validate such a geological model of the basement derived from the interpretation of the seismic reflection. In some cases, the changes in the gravity can be correlated to changes in the porosity of the rocks. This method has the advantage of being much simpler and less expensive to implement than seismic reflection campaign. To curb the limitations of DC methods, one states the double assumption that the ground is horizontal with the last layer infinitely thick, and each layer is electrically homogeneous and isotropic [14]. 3.2.3. Thermal methods Thermal methods include two distinct techniques: the first one is comprises borehole or shallow probe methods for measuring thermal gradient. Thermal gradient is very useful since it permits to measure heat flow when the thermal conductivity is known. The second technique comprises airborne or satellite-based measurements, which can be used to determine the Earth's surface temperature and thermal inertia of surficial materials, of thermal infrared radiation emitted at the Earth's surface [15]. 3.2.4. Remote sensing method Remote sensing techniques can contribute to geothermal surveys by detecting topographic features related to geothermal activities and by detecting surface thermal anomalies using thermal infrared imagery [16]. 3.2.5. γ-ray spectrometry method Gamma-ray spectrometry is a surveying technique that allows the calculation of the heat produced during radioactive decay of potassium, uranium, and thorium within rock. Radiogenic heat produced by rocks is often targets for geothermal exploration and production. Hence, refinements in gamma-ray spectrometry surveying will allow better constraint of resources estimation and help to target drilling [10]. 3.2.6. Induction method The induced polarization method provides a measure of polarizable minerals (metallic-luster sulfide minerals, clays, and zeolites) within water-bearing pore spaces of rocks. Polarizable minerals, in order to be detected, must present an active surface to pore water. Induced polarization is widely used in geothermal exploration [17].
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3.2.7. Frequency domain electromagnetic (FDEM) method Several light-weight FDEM units are available for use by single operators for rapid reconnaissance. Active source (horizontal loop) methods can be specially displayed to map ground features at several hundred meters depth. TDEM appears to have wellthought-of potential for vertically probing in areas of restricted horizontal access. Whereas TDEM has been used by the mineral industry for deep exploration for many years, it has few service providers for shallow (o 100 m) investigations. However, topography can be a problem in interpreting TDEM data [18]. Regardless of the preferred environment of each method or its assets or its limitations, all those methods have also been classified according to their preliminary or detailed and direct or indirect status as shown in Table 2. During reconnaissance surveys carried out in sedimentary basins or blank areas (unexplored), the choice of a method is based on gravimetry, magnetism, aeromagnetic, radiometric and magnetotelluric. On the contrary, in detailed and semi-detailed studies, the most appropriate methods are seismic reflection or vertical electrical soundings. And for recovered structures, the best-fitted methods are seismic refraction and polarization induction.
4. Discussions 4.1. Influence of the geology A good and precise knowledge of the geological context of the area to be explored is very important. This goes beyond the choice of the suitable exploration method and even controls the selection of the geothermal system to be implemented. Indeed, there are two major geothermal systems for the moment: firstly hydrothermal systems that use hot water from aquifers. Those systems are usually installed in the sedimentary context; secondly the petrothermal systems using the heat stored in the hard and dry rocks (crystal rocks) by artificially increasing their permeability and using a heat exchanger (Fig. 1). Two basic factors determine the performance of a deep geothermal project: reservoir temperature and permeability of the rock allowing sufficient volumes of water to flow between drilling production and injection [12]. The deep geothermal projects suffer from a fundamental conflict of interest between these two factors. In order to maximize performance and energy efficiency, high temperature is desired. This involves reservoirs located at huge depths. However, the increase in depth is accompanied by increasing compaction of sedimentary rocks. Unfortunately, too deep sedimentary rocks gradually lose their porosity and permeability which are required for high flow rates. In general, the best quality reservoirs are at shallow depths. One solution to this conflict of interest is provided by what is called the "reservoir stimulation", namely the increase Table 2 Broad classification of major geophysical methods used for geothermal exploration. Methods
Nature
Observations
Seismic Magnetic Gravimetric Thermal Remote sensing γ-ray spectrometry Direct current (DC) Induction Frequency domain electromagnetic (FEM)
Preliminary Preliminary Preliminary Detailed Preliminary Detailed Detailed Preliminary Detailed
Indirect
Direct
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Sediments
Basement
Fig. 1. Illustration of the location of hydrothermal (surface) and petrothermal (deep) systems (Source GREGE).
Greenhouse heating Bathing and swimming
40
Fish farming 30
Other uses
20
Tunisia
South Africa
Morocco
Kenya
0
Ethiopia
10
Egypt
Many African countries (from east Africa, the Horn of Africa, and parts of north and southern Africa) are endowed with geothermal energy. This energy source is likely to become a major contributor to the electrical power of those countries. Other countries may benefit by means of high voltage direct current lines [20,21]. Using technology available today, Africa has the potential to provide 9000 MW of power generation capacity from hot water and steam based geothermal resources, not including the additional potential of heat and ground source heat pump applications [22]. Kenya and Ethiopia are front-runners in the domain. With a geothermal power generation potential of more than 4000 MW, both countries have already registered significant progress in exploring geothermal energy for power generation. Nowadays, both countries have an installed capacity of about 300 MW, equivalent to about 15% of the countries installed electricity generation capacity [23]. In the near future, Kenya plans to double its installed capacity [24]. For now, following the classification criteria namely power generation, direct use and ground source heat pumps, Africa is very poorly ranked on the global market status [25,26]. This is due to the fact that geothermal energy for power generation is currently being used mainly only in Kenya (about 270 MW installed capacity). Nevertheless, geothermal energy is directly used in several countries, including Algeria, Tunisia, Kenya, South Africa, and Morocco [27]. The type of direct use differs between individual countries, however, in general, the heating of greenhouses and bathing and swimming are the two applications showing the highest shares of installed capacity [28]. This is illustrated in Fig. 2. With such a geothermal potential and such energy demand, Africa offers a pretty good business opportunity in the renewable energy sector in general and geothermal energy in particular to geothermal investors.
Space/ district heating 50
Algeria
4.2. The geothermal potential of Africa and business opportunity
60
Installed capacity (MW)
in natural permeability by injecting water under pressure into the rock [19]. It is important to mention that in order to be more apprehensive about some key parameters of a geothermal reservoir like permeability, temperature and the stress field, surface measurements are very limited. These parameters are usually extrapolated or modeled from deep wells.
Fig. 2. Installed capacity for direct use in Africa in 2010, adapted from Lund et al. [27] and RE21 [29].
4.3. Appropriateness of those methods with the African continent Independently of the implantation region, geothermal development typically consists of six major key steps undergoing systematic investigation and evaluation processes of the geothermal fields from their initial exploration and development until steam production mechanisms have been implemented: project definition and reconnaissance evaluation, detailed exploration, exploratory drilling and delineation, resource analysis and assessment of development potential, field development, and steam production and resource management [21]. However, as mentioned in Section 3.1, the choice of a method may depend strongly on the geological setting of the study area. Africa continent is divided into 6 major geology areas as illustrated in Fig. 3. 0].A combination of Table 1 and Fig. 3 may help to select the most suitable method for each region. 4.4. Geophysical methods and levelized cost of geothermal energy exploration in Africa In general, Africa's power sector is facing many challenges, mainly due to insufficient generation capacity which has limited electricity supply, resulting in low access. As a result, the average electricity tariffs in Africa are much higher than in other developing regions. For instance, in 2010, the average effective tariff in Africa was US $0.14/kW h (despite the governments subsidy) while that for South Asia was US $0.04/kW h [31]. This is partly due to
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Fig. 3. Simplified geological map of Africa [30]. Table 3 Energy and investment costs for electric energy production from renewables [32,29].
Biomass Geothermal Wind Solar (photovoltaic) Solar (thermal electricity) Tidal
Current energy cost US$/kW h
Potential future energy cost US$/kW h
Turnkey investment cost US$/kW h
5–15 2–10 5–13 25–125 12–18
4–10 1–8 3–10 5–25 4–10
900–3000 800–3000 1100–1700 5000–10 000 3000–4000
8–15
8–15
1700–2500
the type of energy creation. Indeed, fossil-fuel based power generation is the single largest source of electricity generation in Africa. However, fossil fuels are the most expensive means for generating electricity, and this could be exacerbated by high fuel prices. Geophysical methods, by their effectiveness in exploring geothermal sources can contribute significantly to reducing the high cost of electricity since geothermal energy is presented as the one with the lowest current and future cost among all renewable sources [32]. Table 3 compares energy conts from various renewable energy sources. Even using geothermal energy, the electricity cost can be further reduced by including factors such as cheaper drilling technology through advances in the state of the art of drilling, increased efficiency of the energy conversion process, cheaper
corrosion resistant materials, cheaper scaling mitigation methods, more reliable resource potential prediction minimizing the number of abandoned projects and improved exploration techniques minimizing the number of abandoned projects far into development [28]. 4.5. Other advantages of geothermal energy Geothermal energy provides various benefits and advantages including competitiveness in terms of cost, ecological or green characteristics (near zero emissions, true for modern closed cycle systems that reinject water back to the earth's crust), compactness (less cumbersome: very little space requirement per unit of power generated), autonomy from the seasonal fluctuations, versatility (with many other direct uses such as space heating and heating of greenhouses for horticultural farming). All these aspects make geothermal energy an attractive option compared to fossil fuel alternatives, more regular than hydroelectric power which is affected by low rainfall and cheaper than oil fired power plants, which can be prohibitively expensive to operate when oil prices are high [33,23,34,35]. In terms of the technologies used, geothermal power projects have very unique development timelines that are substantially different from most other energy technologies. A greenfield project typically starts with several years of exploration and drilling, followed by a brief construction period, and then several decades of operation. The advantages of geothermal power in relation to other energy technologies can be summarized in two main points: – Even with high upfront capital costs, geothermal power is a competitive renewable energy source. The absence of fuel costs
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Table 4 Summary key information useful for geothermal investors in Africa. Aspects
Observations
Geological setting
The continent has a diverse geology divided into 6 large groups: younger sedimentary rocks, younger orogens, sedimentary basins, orogens, sedimentary rocks and cratons [30]. Geothermal potential Africa has the potential to provide 9000 MW of power generation capacity from hot water and steam based geothermal resources [22] Population density The total population of Africa is estimated at 1.1 billion, representing approximately 15% of the world's population [38]. Energy needs/demand About 200 kW h per capita. From an electricity-access point of view, sub-Saharan Africa's situation is the world's worst. It has 13% of the world's population, but 48% of the share of the global population without access to electricity [36]. Current electricity tariff The average effective electricity tariff in Africa is US $0.14 per kilowatt-hour (kW h) against an average of US $0.18 per kW h in production costs [36]. Business environment/opportunities Private sector activities in many African countries are facing various obstacles such as high costs of starting a business, weak property rights, burdensome profit tax rates, unstable tax regimes, and limited access to finance [39].
and other variable costs over the long project life span give geothermal power the lowest levelized cost ($89.6/MW h) of any renewable energy technology with the exception of wind power (at $86.6/MW h) [36]. – Having no reliance upon intermittent energy sources such as wind and sunlight, geothermal facilities can produce electricity 24 h a day, 7 days a week. As a result, geothermal power plants have a high capacity factor, demonstrating a level of consistency and reliability not found in other renewable technologies. Geothermal power has the highest capacity factor (92%) of all the energy sources, higher than coal (85%), natural gas (87%), and biomass (83%). Many geothermal power plants enjoy capacity factors of more than 96%. For comparison, the capacity factors of wind, solar thermal, and solar PV are listed as 34%, 20%, and 25%, respectively [36]. However, this technology has a huge risk and disadvantage compared to other technologies in the same category [37]: – Geothermal power plant construction involves high expenditures and capital costs at the beginning of the project. This upfront capital is especially necessary for the drilling and exploration phases. This stage of the project involves considerable risks. Indeed, the return on investment is essentially random or long-term programmed. – Wind, solar, and fossil fuels are less limited by location than traditional geothermal power systems. Geothermal plants must be placed near or above the resource. 4.6. Useful information for geothermal investors in Africa Any geothermal investor in Africa must question some aspects such as the geological setting and the geothermal potential of the interest area, the population density, the energy needs or demand, the current electricity tariff and the business environment or opportunities in the region. Table 4 summarizes the state of those aspects for any potential investor.
5. Conclusions Domra Kana et al. [4] reviewed concise geophysical methods used in geothermal exploration. The present paper that offer one's service as an additive will allow faster and more efficient exploitation of that article [3] by reducing wasted time and costs associated with trial and error. The choice of a method will from now on be wiser and based on the main purpose of the investigation and on the geological setting the operation area. However, to make the choice of a method rational and more efficient the choice of a method, a preliminary geological investigation of the exploration area is required. Nevertheless, one can note other choice criteria. For example, detection of a geothermal heat source is best carried out by using a combination of gravity and
magnetic measurements, while reservoir characteristics are best imaged by the use of electric or electromagnetic techniques.
Acknowledgment The authors are very grateful to anonymous reviewers who have hugely contributed to the improvement of the manuscript. They would also like to record their gratitude to Prof. Oben Julius Enyong and Prof. Kofané Timoléon Crépin for their advice and encouragement. Madam Arétouyap Mirelle Flore and Mrs. Tchimela Clotilde are thanked for their linguistic assistance.
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Contents lists available at ScienceDirect
Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser
A discussion of major geophysical methods used for geothermal exploration in Africa Zakari Aretouyap n, Philippe Njandjock Nouck, Robert Nouayou Laboratory of Geophysics and Geo-exploration, University of Yaounde I, PO Box: 812, Yaounde, Cameroon
art ic l e i nf o
a b s t r a c t
Article history: Received 4 February 2015 Received in revised form 21 December 2015 Accepted 27 December 2015
Geophysics provides a range of methods for the exploration of geothermal sources. This range is so broad that it can sometimes embarrass the geophysicist. The present paper classifies these methods according to several criteria: best-fitted geological environment, main assets and limitations of each method, preliminary or detailed nature of each method and even the specific objective of the exploration to be carried out. This classification could therefore help to significantly reduce costs and time loss related to trial uncertainty and bad choices in selecting the appropriate method. In order to provide necessary information for potential geothermal investors in Africa, the paper addressed several aspects such as the geological setting and the geothermal potential, the population density, the energy needs or demand, the current electricity tariff and the business environment or opportunities in the continent. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Asset Direct method Geothermal exploration Indirect method Limitation
Contents 1. 2. 3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 775 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776 3.1. Presentation of results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776 3.2. Explanatory notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776 3.2.1. Seismic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776 3.2.2. electric and magnetic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 3.2.3. Thermal methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 3.2.4. Remote sensing method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 3.2.5. γ-ray spectrometry method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 3.2.6. Induction method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 3.2.7. Frequency domain electromagnetic (FDEM) method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 4. Discussions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 4.1. Influence of the geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777 4.2. The geothermal potential of Africa and business opportunity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 778 4.3. Appropriateness of those methods with the African continent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 778 4.4. Geophysical methods and levelized cost of geothermal energy exploration in Africa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 778 4.5. Other advantages of geothermal energy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 779 4.6. Useful information for geothermal investors in Africa. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 780 5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 780 Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 780 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 780
n
Corresponding author. Tel.: þ 237 675086759. E-mail address: [email protected] (Z. Aretouyap).
http://dx.doi.org/10.1016/j.rser.2015.12.277 1364-0321/& 2016 Elsevier Ltd. All rights reserved.
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1. Introduction The fight against climate change is a challenge that currently enrolls the whole world. Both developed countries, responsible for the situation and developing countries, virtually victim are expected to work together in order to curb the problem. Currently, greenhouse gases produced by fossil and oil sources (hydrocarbons) are indexed as the main cause of global warming. Hence, it is necessary to explore renewable and cleaner energy sources like geothermal sources [1]. However, Geophysics provides a so broad range of methods that professionals may be embarrassed. It becomes then important to classify those geophysical methods according to some criteria such as best-fitted geological environment, main assets and limitations of each method, preliminary or detailed nature of each method and even the specific objective of the exploration to be carried out. Geothermal energy is formed deep within the earth's crust, and is exploited for electricity generation and other direct uses. The medium of this energy transfer is geothermal fluid. On the surface, these are manifested as hot grounds, fumaroles, geysers, mudpools and hot springs [2]. The main geological parameters of the geothermal reservoir to be determined are: geological formation (lithology), tectonic structures (faults), permeability (hydraulic conductivity), temperature, and stress field. The depth to which these parameters are located must also be considered. However, the needs are not exactly the same for a hydrothermal or petrothermal project. Some of these parameters can be estimated from the surface, mainly by geophysical methods while others let themselves be measured in a borehole [3]. Anyway, geophysical methods are among the best to explore these sources [4]. Domra Kana et al. [4] drew up a review of the main geophysical methods used for this purpose. Their study classifies these methods into four groups based on the physical measured parameter and into two main categories depending on whether they are said to be direct or indirect methods. The present paper aims at promoting the use of geothermal sources by reducing costs across the world including Africa. To achieve this prodigious idea, the present paper is as an additive one designed to reduce ambiguity and speculation in choosing a method. In fact, some methods are essentially preliminary and may be used only for a "pre-exploration", others are more conducive to well-defined geological settings.
The main objective of the present paper is to clarify the assets (strengths) and limitations of each method, then classify major geophysical methods into preliminary and detailed categories.
2. Methodology The study conducted by Domra Kana et al. [4] performs a review of the most geophysical methods used for geothermal exploration. That investigation of a paramount importance was discussed mainly in terms of advantages and disadvantages for each method. The present study is a thorough analysis of these geophysical methods used in geothermal exploration. It presents the assets, the limitations and the best-fitted geological environment of each of them. These methods have also been classified into preliminary and non-preliminary ones. The asset of a method is defined as its success rate, or its ability to deliver positive results while the advantage represents a positive point of a method compared to others. Similarly, the limitation of a method lets know on its inability to perform a task while the disadvantage refers to defects or deficiencies or hazards related to a method.
3. Results For an easy operation, the main results are presented in tables. 3.1. Presentation of results The results are reported in two summary tables. Table 1 summarizes the assets, the limitations and the preferred environment of each method. 3.2. Explanatory notes 3.2.1. Seismic methods Seismic reflection predicts the depth and thickness of a desired geological formation. This may be the thickness of an aquifer for a hydrothermal project or the depth of the crystalline basement roof for a petrothermal project, for example. The permeability of a geological formation, which guarantees the success of a hydrothermal
Table 1 Summary of the main strengths and limitations of different geophysical methods. Methods
Assets
Limitations
Seismic refraction
Does not directly determine the permeability of a Volcanic and sedimentary geological formation [5]. rock assemblage [6].
Magnetic
Allows us to image directly basement (geological formations, presence and geometry of faults, predictive surveys profile). Good resolution of layering from depths of 20 m to more than several hundred meters. A significant depth resolution (magnetotelluric).
Gravimetric Thermal
Simpler and less expensive. Efficient for detecting geochemical haloes.
Remote sensing γ-ray spectrometry
Autonomously and complementarily skilled. High accuracy and enormous penetrating power.
Direct current (DC)
Simple rule of thumb with existence of several electrode configurations. A comparatively efficient reconnaissance tool because the cap response is strong in both polarization modes. Very effective for the rapid reconnaissance of an area for mapping.
Seismic reflection
Induction
Frequency domain electromagnetic (FDEM)
Extracting precise interval velocities from multilayered media is sometimes difficult [5]. Vertical sounding applications (no 2D or 3D interpretation) [7]. Does not allow an unambiguous interpretation. Limited to detecting relatively shallow features [7]. Inefficient in areas covered by thick vegetation. Intended primarily to detect radionuclides contained in a [10] rock. Ambiguity in the interpretation of results (determination of the structure or its geometry). Requires large current sources (up to 100 A) and large receiver loops (40 m 340 m) [11].
Environment/geological setting
Volcanic and sedimentary rock assemblage [6]. Volcanic environments [8]. Volcanic environments [8]. Any geological context [9]. Basement with faults [9]. Basement [8]. Any geological context [9]. All geological setting away from power lines [9].
All geological setting away Topography can be a problem in interpreting FDEM data. TDEM is not widely used for shallow from power lines [9]. studies (less than 20 m).
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project, on the contrary cannot directly be determined by seismic reflection. Seismic methods can then be used to detect substantial faults that may have a high hydraulic conductivity and may represent a target for hydrothermal projects. Large regional faults can also be associated with a higher risk of seismic activity and therefore must be identified in the framework of petrothermal projects [12]. 3.2.2. electric and magnetic methods With the exception of the magnetotelluric, electrical or magnetic methods have not deep geothermal application. Indeed, the vertical thermal gradient is an increasing function of the operation depth. However, its value depends strongly on the petrophysical and especially tectonic setting of the region except in the context of (very) low geothermal energy. Industrial geothermal power plants are located in areas where the temperature is abnormally high. This is the case of the context of volcanic rifting in Iceland or back-arc basins in the Philippines or the active plutonism in Larderello, Italy [13]. Given that these potential methods do not allow an unambiguous interpretation, they should be used in addition to other investigation methods. They can also help to validate such a geological model of the basement derived from the interpretation of the seismic reflection. In some cases, the changes in the gravity can be correlated to changes in the porosity of the rocks. This method has the advantage of being much simpler and less expensive to implement than seismic reflection campaign. To curb the limitations of DC methods, one states the double assumption that the ground is horizontal with the last layer infinitely thick, and each layer is electrically homogeneous and isotropic [14]. 3.2.3. Thermal methods Thermal methods include two distinct techniques: the first one is comprises borehole or shallow probe methods for measuring thermal gradient. Thermal gradient is very useful since it permits to measure heat flow when the thermal conductivity is known. The second technique comprises airborne or satellite-based measurements, which can be used to determine the Earth's surface temperature and thermal inertia of surficial materials, of thermal infrared radiation emitted at the Earth's surface [15]. 3.2.4. Remote sensing method Remote sensing techniques can contribute to geothermal surveys by detecting topographic features related to geothermal activities and by detecting surface thermal anomalies using thermal infrared imagery [16]. 3.2.5. γ-ray spectrometry method Gamma-ray spectrometry is a surveying technique that allows the calculation of the heat produced during radioactive decay of potassium, uranium, and thorium within rock. Radiogenic heat produced by rocks is often targets for geothermal exploration and production. Hence, refinements in gamma-ray spectrometry surveying will allow better constraint of resources estimation and help to target drilling [10]. 3.2.6. Induction method The induced polarization method provides a measure of polarizable minerals (metallic-luster sulfide minerals, clays, and zeolites) within water-bearing pore spaces of rocks. Polarizable minerals, in order to be detected, must present an active surface to pore water. Induced polarization is widely used in geothermal exploration [17].
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3.2.7. Frequency domain electromagnetic (FDEM) method Several light-weight FDEM units are available for use by single operators for rapid reconnaissance. Active source (horizontal loop) methods can be specially displayed to map ground features at several hundred meters depth. TDEM appears to have wellthought-of potential for vertically probing in areas of restricted horizontal access. Whereas TDEM has been used by the mineral industry for deep exploration for many years, it has few service providers for shallow (o 100 m) investigations. However, topography can be a problem in interpreting TDEM data [18]. Regardless of the preferred environment of each method or its assets or its limitations, all those methods have also been classified according to their preliminary or detailed and direct or indirect status as shown in Table 2. During reconnaissance surveys carried out in sedimentary basins or blank areas (unexplored), the choice of a method is based on gravimetry, magnetism, aeromagnetic, radiometric and magnetotelluric. On the contrary, in detailed and semi-detailed studies, the most appropriate methods are seismic reflection or vertical electrical soundings. And for recovered structures, the best-fitted methods are seismic refraction and polarization induction.
4. Discussions 4.1. Influence of the geology A good and precise knowledge of the geological context of the area to be explored is very important. This goes beyond the choice of the suitable exploration method and even controls the selection of the geothermal system to be implemented. Indeed, there are two major geothermal systems for the moment: firstly hydrothermal systems that use hot water from aquifers. Those systems are usually installed in the sedimentary context; secondly the petrothermal systems using the heat stored in the hard and dry rocks (crystal rocks) by artificially increasing their permeability and using a heat exchanger (Fig. 1). Two basic factors determine the performance of a deep geothermal project: reservoir temperature and permeability of the rock allowing sufficient volumes of water to flow between drilling production and injection [12]. The deep geothermal projects suffer from a fundamental conflict of interest between these two factors. In order to maximize performance and energy efficiency, high temperature is desired. This involves reservoirs located at huge depths. However, the increase in depth is accompanied by increasing compaction of sedimentary rocks. Unfortunately, too deep sedimentary rocks gradually lose their porosity and permeability which are required for high flow rates. In general, the best quality reservoirs are at shallow depths. One solution to this conflict of interest is provided by what is called the "reservoir stimulation", namely the increase Table 2 Broad classification of major geophysical methods used for geothermal exploration. Methods
Nature
Observations
Seismic Magnetic Gravimetric Thermal Remote sensing γ-ray spectrometry Direct current (DC) Induction Frequency domain electromagnetic (FEM)
Preliminary Preliminary Preliminary Detailed Preliminary Detailed Detailed Preliminary Detailed
Indirect
Direct
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Sediments
Basement
Fig. 1. Illustration of the location of hydrothermal (surface) and petrothermal (deep) systems (Source GREGE).
Greenhouse heating Bathing and swimming
40
Fish farming 30
Other uses
20
Tunisia
South Africa
Morocco
Kenya
0
Ethiopia
10
Egypt
Many African countries (from east Africa, the Horn of Africa, and parts of north and southern Africa) are endowed with geothermal energy. This energy source is likely to become a major contributor to the electrical power of those countries. Other countries may benefit by means of high voltage direct current lines [20,21]. Using technology available today, Africa has the potential to provide 9000 MW of power generation capacity from hot water and steam based geothermal resources, not including the additional potential of heat and ground source heat pump applications [22]. Kenya and Ethiopia are front-runners in the domain. With a geothermal power generation potential of more than 4000 MW, both countries have already registered significant progress in exploring geothermal energy for power generation. Nowadays, both countries have an installed capacity of about 300 MW, equivalent to about 15% of the countries installed electricity generation capacity [23]. In the near future, Kenya plans to double its installed capacity [24]. For now, following the classification criteria namely power generation, direct use and ground source heat pumps, Africa is very poorly ranked on the global market status [25,26]. This is due to the fact that geothermal energy for power generation is currently being used mainly only in Kenya (about 270 MW installed capacity). Nevertheless, geothermal energy is directly used in several countries, including Algeria, Tunisia, Kenya, South Africa, and Morocco [27]. The type of direct use differs between individual countries, however, in general, the heating of greenhouses and bathing and swimming are the two applications showing the highest shares of installed capacity [28]. This is illustrated in Fig. 2. With such a geothermal potential and such energy demand, Africa offers a pretty good business opportunity in the renewable energy sector in general and geothermal energy in particular to geothermal investors.
Space/ district heating 50
Algeria
4.2. The geothermal potential of Africa and business opportunity
60
Installed capacity (MW)
in natural permeability by injecting water under pressure into the rock [19]. It is important to mention that in order to be more apprehensive about some key parameters of a geothermal reservoir like permeability, temperature and the stress field, surface measurements are very limited. These parameters are usually extrapolated or modeled from deep wells.
Fig. 2. Installed capacity for direct use in Africa in 2010, adapted from Lund et al. [27] and RE21 [29].
4.3. Appropriateness of those methods with the African continent Independently of the implantation region, geothermal development typically consists of six major key steps undergoing systematic investigation and evaluation processes of the geothermal fields from their initial exploration and development until steam production mechanisms have been implemented: project definition and reconnaissance evaluation, detailed exploration, exploratory drilling and delineation, resource analysis and assessment of development potential, field development, and steam production and resource management [21]. However, as mentioned in Section 3.1, the choice of a method may depend strongly on the geological setting of the study area. Africa continent is divided into 6 major geology areas as illustrated in Fig. 3. 0].A combination of Table 1 and Fig. 3 may help to select the most suitable method for each region. 4.4. Geophysical methods and levelized cost of geothermal energy exploration in Africa In general, Africa's power sector is facing many challenges, mainly due to insufficient generation capacity which has limited electricity supply, resulting in low access. As a result, the average electricity tariffs in Africa are much higher than in other developing regions. For instance, in 2010, the average effective tariff in Africa was US $0.14/kW h (despite the governments subsidy) while that for South Asia was US $0.04/kW h [31]. This is partly due to
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779
Fig. 3. Simplified geological map of Africa [30]. Table 3 Energy and investment costs for electric energy production from renewables [32,29].
Biomass Geothermal Wind Solar (photovoltaic) Solar (thermal electricity) Tidal
Current energy cost US$/kW h
Potential future energy cost US$/kW h
Turnkey investment cost US$/kW h
5–15 2–10 5–13 25–125 12–18
4–10 1–8 3–10 5–25 4–10
900–3000 800–3000 1100–1700 5000–10 000 3000–4000
8–15
8–15
1700–2500
the type of energy creation. Indeed, fossil-fuel based power generation is the single largest source of electricity generation in Africa. However, fossil fuels are the most expensive means for generating electricity, and this could be exacerbated by high fuel prices. Geophysical methods, by their effectiveness in exploring geothermal sources can contribute significantly to reducing the high cost of electricity since geothermal energy is presented as the one with the lowest current and future cost among all renewable sources [32]. Table 3 compares energy conts from various renewable energy sources. Even using geothermal energy, the electricity cost can be further reduced by including factors such as cheaper drilling technology through advances in the state of the art of drilling, increased efficiency of the energy conversion process, cheaper
corrosion resistant materials, cheaper scaling mitigation methods, more reliable resource potential prediction minimizing the number of abandoned projects and improved exploration techniques minimizing the number of abandoned projects far into development [28]. 4.5. Other advantages of geothermal energy Geothermal energy provides various benefits and advantages including competitiveness in terms of cost, ecological or green characteristics (near zero emissions, true for modern closed cycle systems that reinject water back to the earth's crust), compactness (less cumbersome: very little space requirement per unit of power generated), autonomy from the seasonal fluctuations, versatility (with many other direct uses such as space heating and heating of greenhouses for horticultural farming). All these aspects make geothermal energy an attractive option compared to fossil fuel alternatives, more regular than hydroelectric power which is affected by low rainfall and cheaper than oil fired power plants, which can be prohibitively expensive to operate when oil prices are high [33,23,34,35]. In terms of the technologies used, geothermal power projects have very unique development timelines that are substantially different from most other energy technologies. A greenfield project typically starts with several years of exploration and drilling, followed by a brief construction period, and then several decades of operation. The advantages of geothermal power in relation to other energy technologies can be summarized in two main points: – Even with high upfront capital costs, geothermal power is a competitive renewable energy source. The absence of fuel costs
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Table 4 Summary key information useful for geothermal investors in Africa. Aspects
Observations
Geological setting
The continent has a diverse geology divided into 6 large groups: younger sedimentary rocks, younger orogens, sedimentary basins, orogens, sedimentary rocks and cratons [30]. Geothermal potential Africa has the potential to provide 9000 MW of power generation capacity from hot water and steam based geothermal resources [22] Population density The total population of Africa is estimated at 1.1 billion, representing approximately 15% of the world's population [38]. Energy needs/demand About 200 kW h per capita. From an electricity-access point of view, sub-Saharan Africa's situation is the world's worst. It has 13% of the world's population, but 48% of the share of the global population without access to electricity [36]. Current electricity tariff The average effective electricity tariff in Africa is US $0.14 per kilowatt-hour (kW h) against an average of US $0.18 per kW h in production costs [36]. Business environment/opportunities Private sector activities in many African countries are facing various obstacles such as high costs of starting a business, weak property rights, burdensome profit tax rates, unstable tax regimes, and limited access to finance [39].
and other variable costs over the long project life span give geothermal power the lowest levelized cost ($89.6/MW h) of any renewable energy technology with the exception of wind power (at $86.6/MW h) [36]. – Having no reliance upon intermittent energy sources such as wind and sunlight, geothermal facilities can produce electricity 24 h a day, 7 days a week. As a result, geothermal power plants have a high capacity factor, demonstrating a level of consistency and reliability not found in other renewable technologies. Geothermal power has the highest capacity factor (92%) of all the energy sources, higher than coal (85%), natural gas (87%), and biomass (83%). Many geothermal power plants enjoy capacity factors of more than 96%. For comparison, the capacity factors of wind, solar thermal, and solar PV are listed as 34%, 20%, and 25%, respectively [36]. However, this technology has a huge risk and disadvantage compared to other technologies in the same category [37]: – Geothermal power plant construction involves high expenditures and capital costs at the beginning of the project. This upfront capital is especially necessary for the drilling and exploration phases. This stage of the project involves considerable risks. Indeed, the return on investment is essentially random or long-term programmed. – Wind, solar, and fossil fuels are less limited by location than traditional geothermal power systems. Geothermal plants must be placed near or above the resource. 4.6. Useful information for geothermal investors in Africa Any geothermal investor in Africa must question some aspects such as the geological setting and the geothermal potential of the interest area, the population density, the energy needs or demand, the current electricity tariff and the business environment or opportunities in the region. Table 4 summarizes the state of those aspects for any potential investor.
5. Conclusions Domra Kana et al. [4] reviewed concise geophysical methods used in geothermal exploration. The present paper that offer one's service as an additive will allow faster and more efficient exploitation of that article [3] by reducing wasted time and costs associated with trial and error. The choice of a method will from now on be wiser and based on the main purpose of the investigation and on the geological setting the operation area. However, to make the choice of a method rational and more efficient the choice of a method, a preliminary geological investigation of the exploration area is required. Nevertheless, one can note other choice criteria. For example, detection of a geothermal heat source is best carried out by using a combination of gravity and
magnetic measurements, while reservoir characteristics are best imaged by the use of electric or electromagnetic techniques.
Acknowledgment The authors are very grateful to anonymous reviewers who have hugely contributed to the improvement of the manuscript. They would also like to record their gratitude to Prof. Oben Julius Enyong and Prof. Kofané Timoléon Crépin for their advice and encouragement. Madam Arétouyap Mirelle Flore and Mrs. Tchimela Clotilde are thanked for their linguistic assistance.
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