Electricity Production from Renewables Energies

5.2. Geothermal power

5.2.1. Introduction

The word “geothermic” comes from the Greek geo, which means Earth and thermo, which means heat. This heat has two sources: the heat generated during the creation of the Earth and the heat produced by the radioactive decay of the isotopes found in the Earth’s crust. The low thermal conductivity of the Earth has enabled the storage of this heat for billions of years.

The temperature gradient within the Earth is generally equal to 25°C per kilometer of depth. However, in some specific sites, this gradient can reach 1,000°C/km. These sites are exploited in order of the priorities of not having to go too far and for an easier exploitation of the stored energy. Moreover, nowadays in order to obtain sufficient efficiency for electricity generation, only geothermal resources at high temperature (T>90°C) are considered to be exploitable. Other more abundant resources are easily recoverable in heating applications.

5.2.2. The resource

We can distinguish four main types of geothermal resources according to their thermodynamic and hydrological characteristics:

– Hydrothermal resources with a liquid dominant: the reservoir pores are filled with water. In these reservoirs, the temperature can reach 360°C and is constant in the entire reservoir, due to convection flows. This energy is brought to the surface with the help of an extraction pump. After extraction of the energy contained in water, the latter is again injected into the reservoir with an injection ...