HYDROTHERMAL HEAT OVERVIEW

Hydrothermal Heat

To date, only hydrothermal resources in the form of underground hot water and (or) steam are used on a large scale for energy needs. Electricity is produced at geothermal plants (GPP) at sufficiently high temperatures of the heat carrier. At a temperature of about 100°C, the geothermal heat carrier is used for direct heat supply, and at lower temperatures, it is necessary to employ heat pumps. Heat pumps are widely used for heating individual houses also by using ground heat, that is, not only underground water (Elistratov, 2021). When hydrothermal heat utilized and converted into electricity and other forms of energy the process is known as Hydrothermal Energy.

Over the past five years, the increase in geothermal capacity amounted to 733 MWe per year (Fig. 1), which is much higher than the average in previous years. However, the share of geothermal heat in the global energy sector is still insignificant. Table 1 shows data on installed capacity and electricity production by GPPs in the top ten leading countries in 2020, as well as total figures. A total of 29 countries produce 95,098 GWh/year with a total installed capacity of 15,950 MWe. By 2025, 10 more countries will join, and the total capacity will increase to 19,361 MWe, by an average of 3.9% per year, which cannot be called impressive given the huge potential and undeniable advantages of geothermal energy.

Figure 1.  Installed electric capacity of geothermal power plants in the world from 2010 to 2025

Hydrothermal Heat, Global Practices in Developing

The undoubted leader in all indicators is the United States, followed by Indonesia, the Philippines, and Turkey. However, if talking about the contribution of subterranean heat to the national economy, the indicators here are completely different. For example, in Iceland, the share of electricity production at GPPs is quite noticeable amounting to 30%. In Russia, hydrothermal resources are huge and higher than the reserves of organic fuel by about 10 times. However, this potential is practically not utilized, since the installed capacity of the GPPs amounts to a negligible value of 82 MWe (Svalova and Povarov, 2015). The global GPP arrangement diagram is shown in Fig. 2 (Huttrer, 2020). As already noted, GPPs are situated mainly in places of faults of tectonic plates and near foci of volcanic activity. Typical geothermal energy generation costs range from $1,870 to $5,050 per kilowatt. The levelized cost of electricity (LCOE) of geothermal power plants is 0.04–0.14 US dollars per kilowatt-hour (IRENA, 2017).

Figure 2.  The GPP arrangement pattern in the world

Geothermal heat is widely used in heat supply systems both directly and using heat pumps (Butuzov et al., 2018; Elistratov, 2021; NREL, 2021). It should be noted that for closed circulation systems, coolant circulation is possible even with disconnected pumps [residual flow rate effect (Cherkasov et al., 2020)], which significantly increases the efficiency of geothermal installations. In 2015, the installed capacity of geothermal heat supply in the world amounted to 70.3 GW (of which geothermal heat pumps account for 50 GW, and direct heating – 7.5 GW), while heat generation reached 163 TWh/year (heat pumps – 90.1 TWh/year, direct heating – 24.5 TWh/year). Among the leading countries are China (installed capacity of 17.9 GW, heat generation of 48.4 TWh/year), the USA (17.4 and 21.1, respectively), and Sweden (5.6 and 14.4). Geothermal heat supply makes the greatest contribution to the economy in countries, such as Iceland, Japan, Sweden, Switzerland, Tunisia, Turkey, and the USA. In Iceland, more than 90% of buildings are heated by geothermal heat, and in Sweden, 20% of buildings are heated by geothermal heat pumps. The largest number of heat pumps are installed in the USA; in 2020, their total number amounted to two million (28 million are planned by 2050).

One of the main problems of using thermal waters is their high mineralization, which reaches 200 g/l (and even 700 g/l) (Alkhasov, 2008). Natural waters contain six main ions, which include three anions, namely, chlorine Cl^-, sulfate SO₄^2-, hydro carbonate HCO^3-, and three cations – sodium Na⁺, calcium Ca²⁺, and magnesium Mg²⁺. This results in intense processes of contamination and corrosion of equipment. However, brines from different deposits may contain valuable chemicals (lithium, rubidium, cesium, bromine, potassium, and others) that can be extracted on an industrial scale.

Hydrothermal Heat History

The first generation of electricity using geothermal steam was carried out in Larderello, Italy, in 1904. This year should be considered the year of the geothermal energy origin. Larderello is located in the province of Tuscany, which is characterized by significant volcanic activity. Here, Prince Piero Ginori Conti used a small steam generator, to lighten four light bulbs as part of a demonstration project (Fig. 3). In 1916, the generating capacity of 2,500 kW was already connected to the grid for the commercial supply of electricity to consumers.

Figure 3.  Geothermal power plants (GPP): Prince Piero Ginori Conti built the first GPP in 1904 at the Larderello dry steam field in Tuscany, Italy

A general view of a modern GPP with a capacity of 100 MW is shown in Fig. 4 (Reykjanes, Iceland). Data on the installed electric capacity of geothermal plants in the world are shown in Fig. 5. Large-scale use of geothermal power plants began in the 1960s. At that, the generated capacity grew linearly with an increment of 250 MW per year.

Figure 4.  Modern GPP with a capacity of 100 MW in Iceland (photo taken by the author)

Figure 5.  Installed electric capacity of geothermal power plants in the world