GEOTHERMAL HEAT SOURCES AND RESERVES

In everyday life, people rarely encounter a visible manifestation of underground heat. The underground heat is commonly referred as geothermal heat and is primary generated by the natural decay of radioactive isotops in the Earth's core and mantle as well as remains there from original residual heat from Earth's formation.

Typical examples are shown in Fig. 1: (a) a volcano, spewing molten lava flows; (b) fumaroles – cracks, from which volcanic gases or steam come out; (c) geyser – a powerful jet of water and steam that periodically flow out of the ground; (d) hot water springs, usually containing dissolved minerals. Subterranean heat is associated with the history of Earth's evolution (DiPippo, 2005; Glassley, 2015).

Figure 1.  Examples of the manifestation of geothermal energy: (a) volcano; (b) fumaroles in Iceland; (c) The Old Servant geyser, Yellowstone National Park; (d) hot spring in Yellowstone National Park. Photos (b)–(d) are taken by the author.

Earth was formed 4.56 billion years ago by accretion of material from a protoplanetary disk, which contained a significant amount of short half-life radioactive elements. The release of heat due to radioactive decay and other processes led to forming a multi-layered planet with a hot liquid metal core (iron and nickel) inside which a solid inner core with a radius of 1,220 km and a temperature of 4,500–6,600°C was subsequently formed. The outer liquid core stretches for approximately 3,480 km from the center of Earth, which makes it about 2,200 km thick. The temperature at the outer boundary of the liquid core lies within 3,700–4,500°C. The internal structure of Earth is shown in Fig. 2(a). Outside, the core is surrounded by the mantle, which has a thickness of about 2,890 km and consists of molten silicate minerals and oxides. The temperature at the outer boundary of the mantle is 1,100°C. A feature of the mantle is the presence of powerful convective flows forming convective cells. It is in this way that heat is transferred from the hot core to Earth's crust, which floats on the mantle. Earth's crust is of two types. The oceanic crust has a thickness of 6 to 10 km and is formed in areas where the ascending parts of convective cells in the mantle reach the surface. The continental crust, consisting mainly of granite and basalt, has a thickness of 30 to 60 km and contains a significant amount of long-lived radioactive isotopes of K, Rb, Th, and U. Approximately 60% of the heat flux that comes out of Earth's surface of the continents is generated by the radioactive decay of these four elements. The background heat flux, formed due to heat, stored in Earth's core is 40 mW/m². The average heat flux for the whole planet is 87 mW/m². Compared with the average energy flux from solar radiation over Earth's surface (341 W/m²), this is a very small value. However, for the entire surface of Earth, this heat flux is equivalent to a total heat output of more than 4.4 × 10¹³ W. For comparison, the total average power consumed by all human activity is approximately 1.8 × 10¹³ W. It is more important to note that in geothermal energy, heat is accumulated from volumetric underground reservoirs, rather than from the surface of Earth, which is much more efficient in terms of technology. The predominant heat transfer mechanism in Earth's crust is thermal conductivity.

Figure 2.  (a) The structure of the Earth and the scheme of convective movements; (b) the distribution map of the heat flux from the Earth; the brown line represents faults between tectonic plates.
[http://www.geophysik.rwth-aachen.de/IHFC/heatflow.html].

Due to the complexity of Earth's structure, namely, the presence of convective cells in the mantle and moving tectonic plates in Earth's crust [Fig. 2(a)], the heat flux on Earth's surface varies greatly [Fig. 2(b)]. The relationship between plate tectonics and heat on Earth's surface is fundamental. The main structural elements of plate tectonics are spreading centers, subduction zones, and faults (boundaries between tectonic plates). In Fig. 2(b), the brown line shows crust faults between tectonic plates. It can be seen that it is in these zones that the maximum heat fluxes – up to 350 mW/m² are observed. Such crust faults pass through (or along) well-known geothermal zones, involving Iceland, western North America, Indonesia, the Philippines, Kamchatka, etc. From the map indicating the location of existing geothermal power stations, it follows that most of them are situated in the area of crust fractures, or regions associated with subduction zone volcanoes and spreading centers.

Important characteristics for assessing geothermal resources are temperature distributions at different depths and the magnitude of the temperature gradient over depth. In Earth's crust, the average temperature gradient is 30°C/km, and in the town of Larderello (Tuscany, Italy) it is an order of magnitude higher – 300°C/km. An example of the temperature distribution at a depth of 10 km, which is already of interest from the point of view of practical use of deep geothermal heat is shown in Fig. 3 for the USA. As follows from the Figure, the entire Western USA has huge reserves of deep geothermal heat with temperatures of dry rocks exceeding 250°C. Such data form the basis for forecasting and developing geothermal energy.

Figure 3.  Deep temperatures distributions in the USA

Geothermal reserves refer to the amount of heat energy that can be extracted from underground sources to generate electricity or heat. Those reserves are conditionally classified into three categories:

1. Low-temperature reserves with 70–150°C range.

2. Moderate-temperature reserves with 250–200°C range.

3. High-temperature reserves with over 200°C range.

Geographical distribution of geothermal reserves is found all over the world with vast concentration area around and near tectonic plates, “Ring of Fire” around Pacific Ocean, Philippines, Japan, Mexico, and United States including Hawaii. The utilization and feasibly is a challenge for each country as all depends on local economic, technological, and deliverable conditions (IEA, 2015).