## 1 Introduction  
Geothermal energy is hot water, steam, gas or thermal energy within hot dry rocks that has accumulated under pressure at a variety of depths in the crust. In other words, geothermal energy may be defined as thermal energy of the globe [1]. It is estimated that the crust has 5.4 billion EJ of thermal energy. If just 0.1% of this energy could be used, world energy consumption needs for 10,000 years could be met. However, it is very difficult to use the Earth's energy at this scale and it requires advanced technology [2]. Geothermal energy is considered to be a clean, reliable and safe renewable energy resource [3]. Linked to the reservoir fluid temperature, geothermal resources are classified as low (<90 \({}^{\circ}\)C), moderate (90 \({}^{\circ}\)C-150 \({}^{\circ}\)C) and high (>150 \({}^{\circ}\)C) enthalpy resources [4]. Historically Conti created the first device producing electricity from a geothermal steam well in the town of Lardello in Italy in 1904 and in 1913 the first commercial geothermal energy system was linked to the grid with a 250 kW alternator in the same place [5]. The success of these experiments led to small scale electricity production from geothermal resources in the city of Beppu in Japan in 1919 and from geysers in California in 1921. New Zealand's Wairakei station began operations in 1958, which was a turning point in the technological history of electricity production and it was the first facility to use wet steam technology (flash steam). Previous power plants were limited to the use of geothermal reservoirs containing dry steam [6]. In 1977 the first dual flash power plant began operating in Japan and in 1984 geothermal energy power plants using binary cycles started to operate [7]. Thus electricity production from geothermal energy spread around the world and over one hundred years have passed since the first day electricity was produced from geothermal resources to the present. In this respect, Turkey has a long history of electricity production from this type of energy. In 1984 Turkey's first geothermal energy power plant with 20.4 MW\({}_{\text{e}}\) installed power was opened in Denizli (Kizildere) [8]. However, to date its share within the variety of energy resources used in Turkey has not reached desired levels. Instead, the basic use of Turkey's geothermal energy is direct usage [9]. Geothermal power plants use hydrothermal resources with hydro and thermal components. Hot water or steam turns a turbine and is used to generate electricity. Geothermal power plants require hydrothermal resources with high temperatures (nearly 150 \({}^{\circ}\)C-370 \({}^{\circ}\)C) coming from hot water wells or dry steam wells [10]. In this situation, for electricity production from geothermal resources to be possible, generally a minimum temperature of 150 \({}^{\circ}\)C is required. The mean temperature gradient within the earth's crust is 25 \({}^{\circ}\)C/km-30 \({}^{\circ}\)C/km. As a result, for electricity production from geothermal, either a deep temperature drilling is required or regions with high temperature gradient should be chosen [11]. Nearly all geothermal power plants can be divided into four categories: dry steam, single flash, dual flash and binary designs. Power plant design is generally a function of the temperature and pressure of the available geothermal resource. As mentioned above, it is known that for electricity production in power plants, fluid temperatures above 150 \({}^{\circ}\)C must be ensured. Additionally, electricity generation from binary geothermal power plants may use very low temperatures. In situations where the temperature of geothermal fluid coming from the production well is below 150 \({}^{\circ}\)C (typically between 110 \({}^{\circ}\)C and 180 \({}^{\circ}\)C), it is not possible to evaporate the fluid flow to form steam. Therefore, binary geothermal power plants are typically used for these applications.