Origins
Contents
Origins#
Last edited: 01 Sep 2023
Author(s): Esteban Gomez (RWTH Aachen, Germany - ETH Zurich, Switzerland)
Wen Luo (TU Delft, Netherlands - RWTH Aachen, Germany)
What is geothermal energy?#
Geothermal energy, harnessed from the Earth’s internal heat, boasts a rich history spanning millennia. Its origins can be traced to the Earth’s core, a staggering 6,500 kilometers beneath the surface, where temperatures reach approximately 5,000 degrees Celsius [1]. This relentless heat journey from the core to the Earth’s surface establishes a perpetual wellspring of natural heat energy, known as geothermal energy. It stems from the Earth’s very formation, including heat generated by the gravitational compression of mineral during its birth, as well as the ongoing release of heat from the radioactive decay of isotopes like potassium-40 (K-40), uranium-238 (U-238), uranium-235 (U-235), and thorium-232 (Th-232) within the Earth’s crust [2].
The very term ‘geothermal’ derives from the Greek words ‘gê,’ meaning Earth, and ‘Thêrm,’ signifying energy. Geothermal energy is not a new power source, and its history spans millennia. Its origins date back to the Paleo-Indians, who settled near hot springs, utilizing natural geysers for cleansing and warmth, while harnessing minerals for medicinal purposes. Eventually, ancient Greeks and Romans similarly embraced hot springs, recognizing their healing potential [3]. Although known to humanity since ancient times, its contemporary significance lies primarily in electricity generation. Here Its historical journey from ancient springs to modern power plants began with the establishment of the first commercial geothermal power plant in Tuscany, Italy, in 1904, marking a pivotal moment in the evolution of this sustainable energy source. Today, geothermal energy serves a diverse array of functions, including heating residential and commercial spaces, electricity generation, and various industrial processes.
In the present renewable energy landscape, geothermal resources make up less than 1% of the energy of world demand, meanwhile, other renewable resource like contributing around 59% of renewable energy generation [4]. Despite this modest contribution, geothermal energy stands out due to its unique advantages for future development. These advantages include abundant untapped resources, reliability, versatility across multiple applications, and accessibility in many parts of the world at specific depths [5]. It is considered a continuous source of energy, minimally affected by weather conditions, operating approximately 98% of the time [6]. Furthermore, geothermal energy demonstrates significant potential as a renewable energy source, thanks to its low environmental impact, minimal greenhouse gas emissions, and readily available technology [7]. Nevertheless, challenges exist in its utilization, often due to remote geothermal site locations, posing logistical hurdles in delivering energy to populated areas. While initial setup costs can be substantial, long-term operational expenses remain notably lower than other energy sources. The utilization of geothermal energy takes three primary forms, which will be elaborated upon later in Geothermal Energy Utilization section:
Direct Use and District Heating: Harnessing hot water from near-surface springs or reservoirs for applications such as building heating, greenhouse cultivation, crop drying, fish farm water heating, and industrial processes.
Electricity Generation: Employing high-temperature water or steam (typically ranging from approximately 150 to 370 degrees Celsius) to power turbines, which in turn activate generators to produce electricity.
Geothermal Heat Pumps: Leveraging the consistent temperatures near the Earth’s surface to control building climates, transferring heat from the ground (or water) into structures during winter and reversing the process in summer.
Where is the geothermal energy from?#
The intrusion of magma at volcanic sites and widespread distribution of hot springs show us there is some kind of heat just under our feet. This heat has its origin in the interior of the Earth, reaching the Earth’s surface via heat conduction, convection and radiatation. But, what is the interior of the Earth?
The Earth consists of four parts, the inner core, the outer core, the mantle and the crust, shown in Fig.1.
inner core: composed of solid iron at the temperature of around 5200 ° C, with a radius of 1221 km [8];
outer core: composed of liquid iron and nickel at the temperature between 4500 ° C ~ 5500 ° C, with a radius of 2200 km approimately [8].
mantle: composed of silicate minerals, oxides and other high-density minerals whose atoms are relatively small, e.g. magnesium (Mg), titanium (Ti), calcium (Ca) and aluminum (Al)[8] at the temperature ranging from 1000 ~3700 ° C [9], with a thickness of 2890 km approximately.
crust: consists of oceanic (thickness between 6 ~ 10 km) and continental (30 ~ 60 km) crusts. Oceanic crust is composed mostly of mafic rock like basalt troctolite and gabbro, while continental crust is composed mostly of granites [9].
Fig. 1 The structure of the Earth#
The origin of the geothermal energy can be attributed to two sources: the residual heat from the early formation of the Earth’s core and the heat generated from radioactive decay [8]. The prevailing consensus suggests that a proportion of the thermal energy harnessed through geothermal utilizations, approximately 40%, is derived from the residual heat associated with the early formation of the Earth’s core, whereas the remaining 60% is sourced from the heat produced by the decay of long-lived isotopes [8].
How the heat is generated?#
Since the formation of the Earth, metal was redistributed, accompanied by density stratification. While the core is composed of iron, the mantle, which is in contact with the outer core, mainly consists of silicate minerals, oxides and other high-density minerals whose atoms are relatively small, e.g. magnesium (Mg), titanium (Ti), calcium (Ca) and aluminum (Al)[8], resulting in low existence of radioactive elements. Directly formed from mantle (extruded magma), the oceanic crust thus have a low abundance of radioactive elements [8]. In contrast, the continental crust, composed of minerals that was incompatible with the high-density minerals in the mantle, holds the largest global reservoir of radioactive elements, such as potassium (K), uranium (U) and thorium (Th).
Fig. 2 The structure of the crust#
More specifically, considering the density profile of continental crust, three layers can be characterised: lower, middle and upper crust [8], shown in Fig.2. The lower layer is mainly composed of igneous and metamorphic rock with low concentrations of radioactive elements, whereas the middle and upper crusts are more enriched in radioactive elements [8]. The heterogeneous distribution of radioactive elemtns and the largely varying thickness in the crust result in substantial variation in heat flux on the surface [8]. Consequently, the geothermal gradient throughout the shallow crust is not uniform.
The geothermal heat source
Residual heat from the early formation of the Earth, is beleived to contribute 40% of the thermal energy harnessed through geothermal utilisation.
Heat from radioactive decay of long-lived isotopes, such as potassium (K), uranium (U) and thorium (Th) that exist in the continental crust, accounts for 60% around of the thermal energy harnessed through geothermal utilization.