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Geothermal Energy Reservoirs: Comparison
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Please note this is a comparison between Version 2 by Conner Chen and Version 1 by MD Shouquat Hossain.

Geothermal is a renewable energy source, but this is not as often seen as other renewable energy sources, such as solar, wind, hydro, etc. Two different types of geothermal energy sources exist in the world: (1) high entropy and (2) low enthalpy geothermal energy reservoirs. Geological characteristics and local temperature are used to classify geothermal reservoirs. 

  • technological advancements
  • challenges
  • renewable energy
  • geothermal system

1. Introduction

The sun and wind are sources of renewable energy, with global capacities of 623 GW and 586 GW in 2019, respectively. This expansion is predicted to continue on the same upward trajectory [1]. However, electric-grid-connected space conditioning in a building contributes to about 40–50% of the total energy consumed in buildings and has an adverse impact on the environment and human health [2]. While thermal rooftop solar systems might generate directly heated air naturally, sustainable energy produced by onshore and offshore wind turbines and solar installations can be converted into heat using heat pumps. In addition, such intermittent energy production from solar and wind can only be solved by energy storage. Additionally, they often produce electricity rather than heat. But geothermal energy conversion, including both heating and cooling, is available as a non-intermittent and possible endless source [3].
Some countries have used geothermal energy for thousands of years for heating and cooking. People have used geothermal energy in natural hot springs for many things, like bathing and washing, since the beginning of human history [4]. It is simply energy produced by the heat of the Earth [5]. The rocks and liquids below the crust of the Earth contain this thermal energy [6]. Nearly 4000 miles below the surface of the inner crust, geothermal energy is generated. A solid iron core surrounds molten rock in the Earth’s double-layered core. The Earth’s rocks contain radioactive elements that are deteriorating over time [7]. Thus, the continual generation of a very high temperature inside the Earth leads to the production of geothermal energy [8]. Geothermal energy is one of the sustainable energy sources available because the substantial organic heat energy reserve is located within the earth’s crust [7,9][7][9]. Geothermal energy is thought by many to have a huge amount of potential to meet our growing energy needs [10]. Over the last few decades, it has successfully satisfied the energy needs of many global locations for both home and commercial purposes [11]. Two different types of geothermal energy sources exist in the world: (1) high entropy and (2) low enthalpy geothermal energy reservoirs [12]. Geological characteristics and local temperature are used to classify geothermal reservoirs [13]. Low enthalpy geothermal storage tanks are located at around 1000 m depth and often with temperatures not exceeding 150 °C. Large geothermal reservoirs are located at a depth of 1000 m, with temperatures over 200 °C. There are many countries involved in the investigation and development of geothermal energy, such as Italy, Kenya, Mexico, Iceland, New Zealand, and the Philippines [14]. However, a wide variety of temperatures, from 60 °C to 350 °C, are accessible for the geothermal energy source [15]. On the other hand, due to technological limitations, geothermal energy generation remains uneconomical at temperatures below 80 °C and with low overall system effectiveness [16,17][16][17].
As a renewable resource, geothermal energy has several benefits. It can heat or cool buildings and generate power without putting out any potentially dangerous pollutants. It is capable of continuously generating thermal energy [9]. However, occasionally, it could release harmful gases from deep inside the ground. There are no negative effects if geothermal energy is used properly. For a few reasons, a geothermal system is very effective at cooling a space. The heat pump, which is the first component of the system, moves heat and humidity from one place to another before dispersing it into the ground, where the loop is situated. A constant, low-level air distribution provides cooling, maintaining a comfortable, uniform temperature throughout the spaces. Second, compared to conventional air conditioners, geothermal systems typically dehumidify the air up to 30% more, which naturally balances the moisture levels in any space [18]. In contrast to conventional power generation plants, geothermal power plants utilize locally available renewable resources. On average, geothermal power facilities have very little influence on the environment. Since there is no need for fuel to produce electricity from geothermal heat, geothermal plants’ operating costs are reduced [11]. Additionally, it can provide base-load electricity continuously throughout the day, something that other renewable energies cannot do. For example, the only time solar energy may be generated is during the day, or it is reduced on a cloudy day. Windmills, however, are dependent on airspeed, which is always fluctuating [19]. On the other hand, the geothermal energy supply is stable, and the power stations are thought to release only 13 to 380 g of carbon dioxide per kWh of electricity. Comparatively, power stations that use natural gas emit roughly 453 g of CO2, compared to 1042 g for coal-fired power plants and 906 g for oil-fired [20]. There have been several surveys and reviews conducted around the world regarding World Geothermal Congresses in places like Italy, Japan, Turkey, Indonesia, India, and Australia, and the scholars found that the breakdown of thermal energy usage is around 58.8% for geothermal heat pumps, 18.0% for bathing and swimming, 16.0% for space heating, 3.5% for greenhouse heaters, 1.6% for industrial uses, 1.3% for farming pond and raceway heating, 0.4% for farmland drying, 0.2% for melted snow and chilling, and 0.2% for many other purposes. Total energy saves about 596 million barrels of oil (81.0 million tonnes) every year. This keeps about 252.6 million tonnes of greenhouse gases and 78.1 million tonnes of carbon from going into the atmosphere [21].
Furthermore, an innovative cascade power generation technology includes a trans-cortical carbon dioxide compression refrigeration system (CCRS) with either a LiBr–H2O after-cooler or a proton-exchange membrane electrolyzer (PEME) to make up a tri-generation arrangement. A tri-generation system may be installed anywhere on the globe and has great potential for harnessing geothermal energy to efficiently transform low-to-medium-grade clean energy sources into electricity, cooling, and hydrogen. That system’s effectiveness was evaluated by altering the efficient input data from energy, economic, and exergy standpoints [22]. Tri-generation technologies that can generate electricity through an Organic Rankine Cycle (ORC) and offer chilling, warming, or other functions will be highly sought-after in the upcoming year. Four alternative tri-generation facility layouts are examined in two serial concepts, one parallel concept, and one serial–parallel concept.

2. Geothermal Energy Reservoirs

Geothermal energy is generated by radioactive substance depletion, and the primordial heat produced during the creation of the Earth is the source of geothermal heat. The typical heat transfer across the surfaces of that same Earth is 82 mW/cm2, while the overall production of the world exceeds 4 × 1013 W [27][23]. When the temperature of the earth rises above ambient levels, thermal energy is transferred between the host rock that makes up the planet and the natural fluid that is present in its pores and fissures. This fluid, which primarily consists of water with different levels of dissolved salts, is normally present in situ liquid phases. However, it may occasionally be a saturated liquid, liquid vapor mixture, or super-heated vapor. The position and temperature of the resource play a major role in how geothermal energy is used, whether for producing electricity or for other purposes. Even though low temperatures (below 90 °C) or middle temperatures (between 90 °C and 150 °C) seem ideal for clear benefits like warming and cooling space and processes, marine, culture, and fish farming, geothermal resources seem ideal for clear benefits like power generation [28][24]. Utilizing high-temperature geothermal resources for many purposes will improve the system’s effectiveness and save costs [29,30][25][26]. Throughout geothermal areas, the temperature of the stones increases with depth. This increase has a typical slope of 30 °C/km. However, there are regions of the earth beneath that can be drilled into where the gradient is much higher than usual. This occurs when a magma mass that is still fluid or is hardening while producing heat is also being cooled just below the layer (a few kilometers below the surface). Specific crustal geological characteristics in some places where the magnetic action is absent will promote heat accumulation, resulting in unusually elevated geothermal gradient values [31,32][27][28]. The geothermal energy recovery of the reservoir, the recharge region, and the connecting pathways via which cold superficial fluid enters the reservoir and, in most instances, exits back to the surface make up a typical hydrothermal energy system, as shown in Figure 1 [31,33][27][29].
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Figure 1.
Illustration of a perfect geothermal system.
The component in this is made up of landfill gas. For instance, heat typically moves from deep to subsurface regions first by conduction and then by convection. These substances are simply rainwater that has seeped into the crust of the earth from recharge places, heated up upon exposure to the heated stones, as well as kept in aquifers, sometimes at hot temperatures and increased pressure (up to over 300 °C). The majority of geothermal areas depend heavily on these aquifers. The reservoir is often covered with impermeable rocks, which prevent heated fluids from rising to the surface and maintain pressure in these reservoirs. Geothermal fields, unlike hydrocarbon fields, usually have a constant flow of heat and fluid. The fluid enters the reservoir through the recharge zones and leaves through the discharge zones (hot springs, wells). A viable geothermal resource must satisfy a number of requirements. Accessibility is the first requirement. This is usually done by drilling to the desired depths, frequently with traditional techniques similar to those used to extract oil and gas from subsurface reserves [34][30]. Sufficient reservoir production is the second prerequisite. To assure long-term production at levels that are acceptable commercially, hydrothermal systems typically require a substantial volume of warm, organic fluids to be trapped in an aquifer that has a significant rock formation pore structure [6,35][6][31]. A reinjection plan is required to sustain output rates whenever the hydrothermal systems do not get enough natural recharge. Integrated transportation methods (sandstone portions with porous or cracked surfaces will transfer heat to different parts of the network, and also the rock itself will conduct heat) take thermal energy first from the reservoir [34][30]. The in situ hydrologic, lithological, and geologic restrictions that currently exist were taken into consideration while designing the heat extraction method. To finish the process, all byproducts must be carefully handled and disposed of. Geothermal heat production shares many characteristics with the oil, gas, coal, and mining sectors. Due to these parallels and the usage of tools, methods, and terminology that have been adopted or taken from the oil and gas industry to be used in geothermal development, the development of geothermal resources has in some ways been sped up. Overall energy output ratio withdrawal using a reservoir is governed by a variety of resource-related factors, including temperature gradient, inherent porosities of the stone characteristics, water accumulated in the rock, tensions inside the rock, and even earthquake risk. A well in a good hydrothermal reservoir typically generates 5 MW or more of output electric power using a mixture of high fluid and temperature flow rates. For example, to provide around 4.7 MW of total electrical energy for the system, a well in a deep hydrothermal reservoir generating water at 150 °C must flow at roughly 125 kg/s [6,29,36][6][25][32]. A high saturation point in the reservoir is required for high rates of flow, while resistivity, which comprises the merge in which the fluid travels past on its journey up towards the well, may be changed by well structure and permeability. In contrast to oil and gas reservoirs, where measured transmissivity is frequently about 100 mD, geothermal systems typically have very high transmissivity (more than 100 D) [36][32]. Exceptionally pressured rocks that collapse under shear and movement caused by stimulation should result in cracks that remain open and permit fluid flow. When there are many breaches with tiny fracture ports, both a high discharge rate and modest pressure drop are possible. A linked circulation system is more likely to form stimulation in rocks that have a minimum linked susceptibility through faults or pores [36][32]. Additionally, the fracture system needs to provide injected cool water for enough time to interact with hot rock so that when it emerges from producing wells, the temperature is close to that of the formation. The greater the reservoir’s life is, the simpler the economics are under the current flow circumstances [36][32].

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