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Review; Geothermal energy; Potentials; Electricity; Resource

Geothermal energy

Geothermal energy is the heat energy generated and stored in the Earth as a result of the radioactivity and progressive heat lost from the Earth's crust. The thermal energy which is in the form of heat reaches the surface of the Earth from the stored heat in the crust as a result of the temperature difference between the planet and the surface. The temperature of the crust can be high up to 4273°C, hence heating the water and rock at different temperature ranges (300-700°C) depending on the locations [1].

The temperature (°C) in the Earth as per depth (in km) according to Tunde et al. is shown in Figure 1. The impact of this is that there is an increase in temperature range as depth increases. Hence, a necessity to go deeper to get reasonable geothermal energy.

 

Figure 1: Earth’s temperatures.

This energy can be used for different applications such as district heating, space heating, desalination, agriculture, and electricity generation. The first geothermal plant in the world was built in 1911 [2].

In Ebere et al. research was carried out to show the installed capacity and the volume of produced energy from 1950 to 2015. Part of the result obtained through their work is shown graphically in Figure 2 as a product of world geothermal electricity need within those years.

 

Figure 2: Installed capacity from 1950 up to 2015 (Left, MW) and produced electricity (Right, GWH).

Furthermore, Mahmood et al. showed the installed capacity of several countries in the world within 2010. This corresponds with the same period Ebere et al. did and published theirs. There is therefore an adequate corroboration between the two works done at almost the same period.

Currently, the United States of America leads the world in geothermal energy exploratio n with 3450 MW of installed capacity and 2542 MW generating capacity.

The use of geothermal energy breaks down into two families namely; electricity generation and heat production. This breakdown was later grouped into four categories.

High-temperature geothermal energy (q<150°C) which is used for electricity generation or heating average geothermal energy (900<150° C) which is used for the production of electricity or heating.

Low-temperature geothermal energy is used to transfer fluid below 30°C and is also used to heat ground source heat pumps.

Geothermal hot fluids are extracted from subsoils at different temperatures and flow rates and are used directly for heating or electricity generation (Figure 3).

 

Figure 3: Geothermal according to the depth of vertical drilling used to extract groundwater.

The first two types of geothermal energy are used to produce electricity. When the geothermal gradient of the soil is high enough, wells up to more than 5 km deep permit the capture of fluids at high temperatures. These fluids which are in steam form make it possible to turn the turbines which will in turn generate electricity (Figure 4).

 

Figure 4: System that uses high temperature geothermal energy.

Geothermal resource potential

Geothermal resources vary from one location to another as a result of temperature and depth differences, rock chemistry, and the abundance of underground water. Utilization of this geothermal energy resource can be broadly classified into electric power generation and non-electric use. The type of geothermal resources beneath the Earth determines its utilization. High springs or enthalpy resources such as dry steam and hot fluids found in volcanoes can be used to generate electricity using a binary cycle plant. On the other hand, low to medium springs are found in continental areas and are used for direct heating and cooling [3]. It is important to note the potential in a location before setting up a geothermal power plant [4]. Stated that it is paramount to note the theoretical, sustainable, economic, technical, and developable potentials for a particular region or area. The theoretical potential describes the physical energy that is present, the technical potential explains the fraction that will be used, the sustainability explains the economic fraction that can be exploited, and the developable fraction is the resource that can be developed for the long term [5].

Geothermal reservoirs: This is a permeable rock unit that hosts geothermal fluid. It is usually formed when water or steam that is heated by the Earth's heat is trapped in permeable and porous rocks under a layer of impermeable rock. Hot geothermal fluids can also manifest themselves on the surface as geysers or hot springs but most of it stays deep underground trapped in porous rock and cracks [6].

Hot dry rock

Hot dry rock geothermal resources do not have water or steam circulating through the rocks. These rocks need to be fractured by hydraulic pressure to accomplish a fluid flow. The enhanced geothermal system shown in Figure 5 is a way of creating an artificial reservoir of water within the hot rock and it is typically being made at a depth of more than 3 km. An extended fracture would be made when the temperature of the crust reaches 150-200°C. With the help of production or injection wells, water from the deep wells is transmitted through these surfaces in the form of steam or hot water. Furthermore, the heat in the rock is generated from radioactive decay over a million of years and finally detained by insulating sediments [7].

 

Figure 5: Enhanced geothermal system.

Types of geothermal resources: Geothermal resources simply mean the accessible resource base [8]. The accessible resource base includes the useful accessible resource base (Resource) and the identified economic resource base (Reserve) [9]. The different categories of geothermal resources and their reserve levels are shown in Figure 6.

 

Figure 6: Diagram showing the different categories of geothermal resources.

Hydrothermal

Hydrothermal systems suitable for electric power production are normally characterized by the availability of large quantities of hot water in the range of up to 130°C. This depends on the location of the geothermal water which after heat extraction would be re-injected into the underground through another borehole or probably discharged into a nearby river. The heat from hydrothermal resources can be used for space heating, cooling, or industrial purposes as long as the temperature remains high for electric power generation. A typical hydrothermal plant with multiple heat and power utilization is shown in Figure 7.

 

Figure 7: Typical hydrothermal plant with multiple heat and power utilization.

Geothermal energy conversion technologies

This outfit is mostly seen in direct-steam power plants, flash steam plants, and binary power plants. Some examples of geothermal energy technologies are given below:

Direct-steam power plants: Direct–steam power plants have been in operation for over a hundred years ago and make use of the direct flow of geothermal steam. The most common example of a dry spring power plant is the flash power plant shown in Figure 8. The flash power plant makes use of a mixture of liquid water and steam. They are used at vapor-dominated or dry reservoirs. The dry saturated or slightly super-heated steam is produced from wells which are transmitted by pipes to the powerhouse where they will be used by turbines of impulse or reaction type. Separators are located near the wellhead to remove particulates such as dust or rock bits.

 

Figure 8: Simplified flow diagram for a direct steam thermal power plant.

Flash steam plant: Another type of geothermal power plant is liquid dominated as shown in Figure 9. The liquid is produced in two phases, liquid and vapor. The quality of the mixture is a function of the reservoir fluid conditions, the good dimensions, and the wellhead pressure. The liquid from the injector may be injected, used for its thermal energy via heat exchangers for a variety of direct heating applications. Plants in which only primary high-pressure steam is used are called single flash plants while those using high and low-pressure flash steam are called double flash power plants as shown in Figures 9 and 10.

 

Figure 9: Diagram showing a simplified flow for single flash power plant.

 

Figure 10: Diagram showing a simplified flow for double flash power plant.

Most of the acronyms used in Figures 9 and 10 explained in Table 1. This is tabulated for further ease of understanding.

Table 1: Geothermal components.

Binary power plant: Binary power plant functions as a closed loop system that makes use of resource temperatures as low as 74°C, a typical example is the Rankine cycle. For binary power plants, the thermal energy of the geofluid is transferred via a heat exchanger to a secondary working fluid which will be used in a conventional Rankine cycle. The geofluid does not come in contact with the moving parts of the power plant thus minimizing the adverse effect of erosion. A typical example of a binary power plant is the double flash power plant shown in Figure 11. Also, the parametric acronyms for any geothermal equipment are shown in Table 2.

 

Figure 11: Diagram showing a simplified flow for double flash power plant.

Table 2: Geothermal well parameters.

In the public sphere, there has been much focus on utilizing sustainable energy resources, geothermal energy does present one promising option for society's dependence on oil and natural gas. The government, recognizing the promise of geothermal energy, should enacted a variety of legal systems to regulate geothermal resources. These legal systems should be able to handle geothermal resource production when it is accomplished using existing subterranean fluids, and also handle production methodologies utilizing fluid injection and existing oil and gas wells. By designing a statutory system which classifies "hot dry rock" geothermal resources as minerals, the controlling regulatory and ownership principles will become more accurate and clearer. In addition, placing oil and gas commissions in charge of the conversion/application process for producing geothermal energy from oil and gas minerals. Indeed, enacting these few simple changes to the way laws regulate geothermal energy, can boost geothermal energy production.

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