Geothermal resources refer to the accessible resource base or rather the thermal energy stored between the earth’s surface and a specified depth in the crust. This is the area beneath a specific area that is measured from the local mean annual temperature. It also includes the necessary accessible resource base after excluding the part of the resource of a given area that can be extracted legally at a cost competitive with other commercial energy sources. The classification of geothermal resources occurs in various phases, which include: low enthalpy, <900C; intermediate enthalpy, 900C-1500C; and high enthalpy, >1500C, based on their reservoir temperatures (Dickson, 10).
Low enthalpy resource is best suited for direct applications of geothermal energy. The resources is available at a depth range of 1,200-1,600 m, and water temperatures of <900C. The intermediate enthalpy is used in generation of electricity using the binary-cycle power-plants. The water temperature range is 900C-1500C. On the other hand, the high enthalpy resource is also used to generate electricity, but it has a high water temperature range, >1500C. a single bore of high enthalpy resource can generate about 5 MWe.
Classification of geothermal resources is vital in that it helps in the characterization, assessment and development of energy resources (Dincer, 23). This leads to the characterization of geothermal resources through geologic settings, intrinsic properties and viability for commercial utilization. Classification also requires coherent frameworks for various purposes which include assessment of resources, exploration, development and reporting (Dincer, 27). The classification of geothermal resources requires specific systems that range from: simple and logic communication that is effective for understanding by the experts and non-experts. Similarly, classification should be valid from scientific and technical perspectives and should also meet the needs that are clearly identified for categorized resource information (International Geothermal Association). Classification of geothermal resources should also be easily translated into other classification systems that avoid misconceptions that may unintentionally adversely affect commercial activities. They should also be able to eradicate gaps and overlaps between categories.
Additionally, classification of geothermal should also avoid unnecessary predictions by focusing on fundamental physical and chemical properties of resources rather than on aspects of utilization that can be altered by evolving technological, regulatory and economic factors. The various examples of classification of geothermal resources include: assessment confidence and definitions, temperature and other thermodynamic properties, geologic setting and fluid type and convective/advective versus conductive (International Geothermal Association). The geothermal resources classified in terms of reserve define the geothermal energy extracted through legal and economic means. Similarly, geothermal resource is the energy that can be recovered through technical means and added to reserves for future purposes (Dickson, 16).
However, the classification of geothermal resources does not address the development of a clear and concise definition of a geothermal system. The assessment of oil and gas defines petroleum systems that expand the natural hydrocarbon fluid system in the lithosphere mapped to include the important elements and processes necessary for oil and gas existence through their accumulation (International Geothermal Association). The classification of geothermal resources relies on temperature that has been accepted as a classification parameter due to its simple nature of being a measured quantity. However, classification of geothermal resources through its reservoir fluid temperature is regarded as irrelevant because it requires two distinctive properties for the definition of thermodynamic state of the fluid (Tiwari, 369).
The classification of geothermal resources should depict the way they are capable of doing thermodynamic work. This incorporates their classification into low, medium and high quality resources in accordance to the specific energy indices they contain. These indices are however limited by demarcations of energies of saturated water and dry saturated steam at 1bar absolute (Tiwari, 370). Classification of geothermal resources follows various criteria that utilize enthalpy of the geothermal fluids that serve as carriers that transport heat from deep rocks to the surface. Enthalpy in this case expresses the heat contained in the fluids to produce a figurative image of their value. The criteria used require standard methods that eradicate confusion and any ambiguous aspects (International Geothermal Association).
Dickson, Mary H, and Mario Fanelli. Geothermal Energy: Utilization and Technology. Hoboken: Taylor and Francis, 2013. Print.
Dincer, Ibrahim, and Marc A. Rosen. Exergy: Energy, Environment and Sustainable Development. Oxford: Elsevier Science, 2012. Print.
"International Geothermal Association: What is Geothermal Energy (en)." International Geothermal Association: Home. N.p., n.d. Web. 14 Jan. 2014.
Tiwari, G N, and R K. R. K. Mishra. Advanced Renewable Energy Sources. Cambridge: RSC Publishing, 2012. Print.