**1. Introduction**

Geothermal energy is heat energy from Earth's interior which generated from radioactive breakdown and frequently recurring heat losses from Earth's formation. The Earth's heat capacity is approximately 1 × 1019 TJ or 2.8 × 1015 TWh [1]. The Earth's heat conduction is 44.2 TW [2].

The utilization of geothermal energy according to the geological conditions are categorized as:

1.High-temperature (enthalpy) geothermal systems with temperature greater than 180°C. This system depends on recent mantle hot spot anomalies and volcanos at depths over 3.5 km. Also, high-temperature geothermal systems related to rocks at depths approximately below 3.5 km.


*a) Muffler and Cataldi (1978); b) Hochstein (1990); c) Benderitter and Cormy (1990); d) Nicholson (1993); e) Axelsson and Gunnlaugsson (2000).*

#### **Table 1.**

*Classification of geothermal resources based on temperature [3].*


Geothermal systems with middle and low-temperature are formed by decaying of radioactive isotopes and they conclude aquifers which recharge by heated water circulation. **Table 1** tabulates the classification of the geothermal resources based on temperature [3].

Geothermal energy can be utilized for both purposes – direct heat and electricity generation. As of the end of 2019, geothermal energy was used in 88 countries around the world with an annual energy consumption of around 1,020,887 TJ or 283,580 GWh [4]. According to the International Energy Agency the electricity production from geothermal energy will be increased to 1400 TWh/y and the direct use to 1600 TWh/y by 2050 [5].

The main advantage of the geothermal energy is being clean as other types of renewable sources. Other advantages include: reliability, environment-friendliness, relatively low cost of generated energy, high usage factor for the geothermal power plant operation that distinguishes the geothermal energy from other renewable energy sources. However, the heat and electricity generated by the geothermal energy should be directly utilized locally and cannot be transported.

### **2. Geothermal power plants**

In work [6], modeled types of geothermal resources are given, as shown in **Table 2**. In general, geothermal resources under 150°C are more suitable for direct use such as heating and cooling, whereas the resource of above 150°C is exploited for electricity generation. However, modern power conversion technologies allow to generate an electricity from low temperature resources up to 150°C [3].

A steam or hydrocarbon vapor are used to generate electricity from geothermal energy. While the vapor-dominated resource is applied directly, the hot-water dominated resource should be flashed by reducing the pressure to convert a steam [7]. **Table 3** provides the basic technologies and the common applications under different temperatures of geothermal fluid.

As mentioned above, the geothermal source in view of steam or hydrocarbon vapor can be used to generate electricity. In this respect, geothermal power plants (GPPs) operation is similar to those of steam power plants. However, unlike the conventional steam power plants, the geothermal power ones use natural steam of earth. History of the first GPP started from 1904 at Larderello, Italy,

### *Geothermal Power Generation DOI: http://dx.doi.org/10.5772/intechopen.97423*


#### **Table 2.**

*Types of geothermal resources for energy utilization.*


#### **Table 3.**

*Frequently used technologies for geothermal energy.*

where generator was tested to produce electricity from geothermal source. Then, it was commercialized to power plant in 1911 [7].

The simplest operational conceptualization of the GPP is presented in **Figure 1**. In this case, a natural steam from the well is directly passed to a turbine, that drives a generator for the electricity production.

There are three basic types of GPPs:

1.Dry steam plants.


#### **2.1 Dry steam plants**

Dry steam plants are simple and more efficient type of GPP. This type of power plants was firstly deployed to generate electricity in Italy in 1911. However, dry steam plants are less available in the sense that the steam should be produced from vapor-dominated reservoirs which are of few numbers in the world.

To provide a high efficiency of turbine, the steam condensation is minimized during extension of fluid fraction in steam phase. In common, isentropic efficiency of modern dry steam plants is about 85%. According to a feasibility study to extract the maximum efficiency of these plants, the generation capacity should be at least 1 MWe [8].

**Figure 1.** *Schematic diagram of dry steam power plant.*

Based on [9], electricity can be generated from the dry steam in the following conditions:


The schematic diagram of dry steam power plant is illustrated in **Figure 1**. The dry steam plant operation is based on the following: the water and steam flow from geothermal production wells are transmitted through valve to turbine and spin a steam turbine by converting thermal and kinetic energy to electrical energy.
