When considering and reporting resources, the term “geothermal potential” is often used without clearly stating what kind of potential is meant. For renewable energy resources it is nowadays common to use different potentials: theoretical, technical, economic, sustainable, developable – decreasing successively in size. In such a sequence, the potentials are progressively realizable and more and more rewarding financially. The theoretical potential describes the physically present energy, the technical potential the fraction of this energy that can be used by currently available technology and the economic potential the time- and location-dependent fraction of the previous category; the sustainable potential constrains the fraction of the economic potential that can be utilized in the long term; the developable potential is the fraction of the economic resource which can be developed under realistic conditions. In converting theoretical to technical potential, the recovery factor (the ratio extractable heat/heat present at depth) is of key importance. An example (global geothermal resources) is given, with numerical values of the various potentials. The proposed classification could and should be used as a kind of general template for future geothermal energy resources reporting.
Various possible resources reporting schemes of geothermal resources are
currently under discussion, among others in UNECE (United Nations
Economic Commission for Europe). Examples include
Geothermal Reporting Code and Geothermal Lexicon for Resources and Reserves Definitions
and Reporting produced by the Australian Geothermal Reporting Code
Committee, 1st edition 2008, 2nd edition 2010; Canadian Geothermal Code for Public Reporting of Exploration Results, Geothermal
Resources and Geothermal Reserves, 2010 Edition; “A Protocol for Estimating and Mapping Global EGS Potential” by Beardsmore et
al. (2010); New Geothermal Terms and Definitions by the Geothermal Energy Association (GEA,
2010); Resource Assessment Protocol for GEOELEC (Geothermal Electricity in Europe;
When considering and reporting resources, the term “geothermal
potential” is often used without clearly stating what kind of potential is meant. An example is
presented in Table 1; here Enhanced or
Engineered Geothermal System (EGS) “potentials” are listed without specifying their exact kind, for various countries. This uncertainty
clearly demonstrates the need of more clarity about potentials.
Reported EGS potentials from various countries (from McPherson-Grant and Baria 2009).
For renewable energy resources it is customary to refer to different potential categories (see e.g., Voivontas et al., 1998): theoretical, technical, economic, sustainable, developable – decreasing successively in size. In this order the potentials are more and more realizable and more and more rewarding financially. The various potentials in assessing geothermal resources are described in more detail below.
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The hierarchy and the interrelations among the various potentials are depicted in Fig. 1. The potentials are successively smaller and more economic along the above list; each potential is a fraction of the previous (except for the theoretical potential, which is the largest and least economic). In other words, the potentials form a series of nested subsets.
Potential definitions for renewable energy; they also apply to geothermal energy. From Rybach (2010).
Potential geothermal energy resources split into categories, e.g., theoretical, technical, economic, developable and existing supplies for power generation and direct use; from Goldstein et al. (2011).
The above-described potential systematics is a skeleton; “tissue” (solid numbers) needs to be added to define resources and reserves. This can be done at the local, regional and global level. The following example from Goldstein et al. (2011) defines and describes global geothermal potentials, but the categorizing can also be applied to the regional or local level.
Figure 2 is from Goldstein et al. (2011), with additions (red ellipses, rectangles) to highlight definitions to be discussed. Goldstein et al. (2011) uses the potential systematics to categorize geothermal resources for power generation and direct use. The figure is based on Fig. 1 of Rybach (2010).
The various subdivisions and descriptions in the figure of Goldstein et al. (2011) are discussed below, following a bottom-up approach.
The blue box in Fig. 2 concerns the theoretical potential, i.e., the stored heat in the subsurface. It is by all means immense: most of it will remain there; even with “future” technologies it will not be usable, since complete extraction will not be possible.
The green box covers technically useable resources, applying foreseeable technology. Since it is fully uncertain what technologies might develop in the near or far future the numbers in this box are only of indicative character. The technical potential can only be realized by existing established technologies. The GEOELEC (2011) resource protocol (van Wees et al., 2011) even has a category “theoretical technical potential”, which is somewhere between real, theoretical and technical potentials, i.e., not really well defined.
When it comes to converting theoretical to technical potential, the
recovery factor
The next two boxes (yellow, brown) in Fig. 2 indicate the economic and developable potentials without numbers. They correspond fully to the potential classes of Rybach (2010).
Finally the topmost (red) box is assembling geothermal energy utilizations, for power generation and direct use, already developed in 2011. These numbers will certainly increase in the future.
Evaluation of the classification by potentials in terms of the UNCF-2009 categories; from Beardsmore (2013).
The classification of energy and mineral resources is a generally important subject and currently on the agenda of UNECE. This body published the UNFC-2009.
The UNFC-2009 addresses mainly the classification of fossil (coal, hydrocarbon) resources; nevertheless it also provides a possible basis for a generic classification framework for geothermal energy.
Quantities in the UNFC-2009 are classified by three fundamental criteria: economic and social viability (E), field project status and feasibility (F), and geological knowledge (G). Combinations of these criteria create a three-dimensional framework system.
On behalf of the IEA, Geothermal Implementing Agreement (Beardsmore, 2013) undertook the task of evaluating the applicability of the UNFC-2009 scheme for geothermal energy resources. In addition, a comparison and assessment of the various geothermal reporting schemes listed in the Introduction has been attempted, with the UNFC-2009 scheme serving as the basis of comparison.
Among the other geothermal reporting schemes, the approach of classifying geothermal resources and reserves by potentials (Rybach, 2010) has also been treated in Beardsmore (2013). In particular, the following assessment is given:
The five classes of Potential proposed by Rybach (2010; Fig. 16b) define
a one-dimensional scheme that covers a broad spectrum of the
three-dimensional UNFC-2009 classification space. The step from Theoretical
Potential to Technical Potential follows the “F”-axis of the UNFC-2009
scheme, while progression from Technical Potential through Economic
Potential to Developable Potential mostly tracks the “E”-axis of
UNFC-2009.
These findings are illustrated in Beardsmore (2013) by Table 2 and Fig. 3.
In addition, Beardsmore (2013) states that “Rybach's (2010) proposed classes of “potential” overlap with most categories in the other schemes. For example, technical potential in Rybach's terminology covers measured resources (Australian and Canadian reporting codes), delineated resources (GEA), and a portion of sub-marginal contingent resources (GEOELEC)”. Moreover, it concludes that “Rybach's classes, therefore, provide a possible high-level tool for understanding and interpreting the inter-relationships between some of the other schemes.”
Among the various currently discussed reporting schemes of geothermal resources, the approach by potentials is a possible, feasible option. Whenever geothermal potentials are mentioned, it should always be clearly distinguished which of the potentials is meant.
The example of Goldstein et al. (2011) demonstrates that the resources classification by various potentials is feasible and practicable. The definitions by potential of Rybach (2010) may add further value in this context.
The example mentioned above deals with global potentials; the classification by potentials can also be applied to the regional and the local level.
Mapping of Rybach (2010) classes within the UNFC-2009 3-D framework; from Beardsmore (2013).
The proposed classification of potentials provides a useful template for standardizing the future reporting of geothermal energy resources worldwide.
The remarks and suggestions of the two reviewers helped to improve the manuscript. Edited by: G. Beardsmore Reviewed by: C. Huddlestone-Holmes and C. Harvey