## 3 Geothermal Energy and How Is It Used  
Geothermal energy is the energy stored as heat in Earth's interior [26]. It can be used in various fields--electrification, space and district heating, space and district cooling, greenhouse heating, in aquaculture and in industry [27]. Geothermal energy for electricity generation has been produced commercially since 1913, and its utilization increased rapidly in the last decades [10]. The earliest known commercial use of geothermal energy through district heating system originates from 14 th century in Chaudes-Aigues Cantal, France [28]. Geothermal energy supports agriculture around the world by greenhouse heating [29; 30; 31].  
Geothermal energy can be captured from different depths and layers of the Earth. Normally, one can make a distinction between deep and shallow geothermal energy [32] (p. 38). [PERSON] and [PERSON] [33] suggest a classification which comprises the following geothermal categories: geothermal power, deep geothermal resources (direct use) and shallow geothermal resources (mainly heat pumps). Shallow geothermal energy is energy acquired from sources on less than 400 m depth [34]. It is mostly captured from Earth's layers no deeper than 150 m, and definitely no deeper than 400 m. Layers deeper than 400 m belong to the category of deep geothermal energy [32] (p. 38). Shallow geothermal energy is a good alternative for fossil fuels in heating and cooling of buildings [35]. Very shallow geothermal potentials (up to 10 m deep) are gaining more attention in the last years (This layer can be further divided into three depth layers: first (0-3 m), second (3-6 m) and third (6-10 m) [36] (pp. 227-228)) [36]. In general, every soil is suitable for very shallow geothermal applications [3]. Several parameters, such as thermal conductivity of soil, water content, pore size distribution and bulk density, have crucial influence on efficiency of very shallow geothermal applications and create different local potentials related to the prevailing soil conditions [36; 37].  
Installed thermal power for direct geothermal utilization is growing every year at a compound rate (In the period 2005-2009 it grew at a rate of 11.4% annually) [38]. Most of geothermal power capacities in 2018 were located in the United States, Indonesia, Philippines, Turkey, New Zealand, Mexico, Italy and Iceland [15] (pp. 80-83). New capacities in 2018 are installed mostly in Turkey and Indonesia. United States and Iceland are countries which are following. Except Turkey, Italy and Iceland, European countries generally do not use geothermal energy for producing electric power. They use it mostly for heating systems, and shallow geothermal applications are the largest sector of geothermal energy use in Europe [39].  
Geothermal heating can be used in two ways--as a direct heating, and as indirect heating and cooling via geothermal heat pumps [40] (p. 341). The direct use of geothermal energy refers to geothermal direct heating plants, geothermal heat in agriculture, geothermal heat in balneology, as well as geothermal heat in other and individual buildings. European countries exploiting most of geothermal direct use in 2018 were Iceland, Italy, Hungary, France and Germany [39].  
Leading countries using geothermal heat pumps in 2004 were the United States, Sweden, Germany, Switzerland, Canada and Australia, respectively [40] (p. 345). Ground source heat pumps in Europe in 2018 were mostly installed in Sweden, Germany, France, Finland and Switzerland [39].