Geological Evaluation of Borehole Thermal Energy Storage Potential of Turkey

Geological Evaluation of Borehole Thermal Energy Storage Potential of Turkey

Aysegul Cetin-Bank of Provinces Inc. Com., Ankara-Halime Paksoy- Cukurova University,  Adana-Yusuf Kagan KadiogluAnkara University Faculty of Eng. Geol. Dept., Ankara, Turkey

Keywords: Turkey’s General Geology, Borehole Thermal Energy Storage, Shallow Geothermal Energy 


Turkey is located on suture zone between Europe and Asia. Turkey’s geology was formed by geotectonic evolution of Tethys Ocean as a result of the movements of Eurasian plate in the north and African plate in the south. These shaping continued from Mesozoic to Cenozoic. Crust thickness was thinned related to geotectonic evolution of Turkey and led to formation of many magmatic activities within the crust. The effects of these magmatic activities and tectonic events continue up to now. Hot springs and earthquakes are signatures on the tectonic boundaries and on the new fault zone of these terrains in Turkey. Depending on igneous rocks and tectonic structure, a number of hot-water reources have emerged and there are approximately 190 of such geothermal areas. These existing areas are far from settlements and shallow geothermal resources are waiting to be exploited. Borehole Thermal Energy Storage (BTES) is one of the shallow geothermal systems used in many countries around the world. In designing BTES several factors related to geology of the underground such as geothermal gradient and undisturbed ground temperature have to be considered. The complex geology of Turkey brings together opportunities and challenges for use of underground for BTES systems. In this paper, an overview of general geological features of Turkey in relation to BTES will be given.


Interest in renewables is increasing in Turkish Energy Policy. According to the recent “Energy Strategy Document”, Turkey aims to increase the use of renewables by 23% until 2023 (Official newspaper, 2012). Most of the renewable energy sources have an intermittent characteristic that requires being stored for efficient utilization.  Natural energy (solar, ambient air, surface water, ground) in the form of heat and cold is stored in the underground seasonally. Underground soil and/or rock provide a large, invisible and isolated storage volume.  Underground Thermal Energy Storage (UTES) technologies use the heat capacity of this volume to store thermal energy from any natural or artificial source for seasonal or diurnal applications. There are basically three types of UTES techniques: Aquifer Thermal Energy Storage (ATES), Borehole Thermal Energy Storage (BTES) and Cavern/Pits Thermal Energy Storage (CTES). UTES technologies are also shown as shallow geothermal systems.

Among the UTES techniques, BTES has the most number of applications around the world. In a BTES system thermal energy is transferred to the underground by means of conductive flow from a number of closely spaced boreholes. The boreholes are equipped with different kinds of borehole heat exchangers, making the boreholes acting as large heat exchanger with the ground. Heat or cold is injected or extracted from the underground by circulating a fluid in a closed loop through the boreholes. Ground Source Heat Pumps (GSHPs) are systems combining heat pumps with BHEs.  Earth is used as a heat source during heating mode and as a heat sink during cooling mode (Paksoy, 2008).

Since 2007, the number of shallow geothermal applications has shown an increasing trend in Turkey. Total installed capacity of applications for residential and office buildings, shopping malls, and hotels has reached to 42 MWh (Cetin and Paksoy, 2013). This is only a small fraction of the potential available in Turkey.

Thermal properties of rock are the most important parameters for a BTES system design. The ambient temperature around the rock mass (undisturbed ground temperature), geothermal gradient, thermal conductivity and thermal capacity are among these properties which have to be estimated to determine the BTES potential of a region. These properties are closely correlated with geology. In this study, general geological features of Turkey are investigated to evaluate the BTES potential.


Turkey has a complex geology that shows various formations and features, which are still changing. The geologic properties that can be relevant to predict BTES potential of a region are: tectonism, magmatism, volcanism and geothermal gradients There are a number of magmatism and volcanic features due to active tectonism in Turkey as shown in Figure 1. Main magmatic areas are shown as highlighted areas and dark black circles are for volcanic eruptions.


Figure 1. Main magmatic activity (hightligted areas) and volcanic eruptions (dark black circles) map of Turkey

  Geological features of Turkey is linked to evolution of Tethys Ocean. Turkey is divided into three main tectonic units: The Pontides, the Anatolide-Taurides and the Arabian Platform. These tectonics units were once surrounded by pieces of Tethys Ocean.  Tectonic lines, that show traces of closed Tethys Ocean, now separate them.

The Pontides have folded features and reverse faults and consits of Strandja, Istanbul and Sakarya massifs. These are made up of sandstone, limestone, gneiiss, micaschist, metagranit and metaconglomerate etc., and are aged between Carboniferous and Eocene. The Anatolide – Taurides forms the bulk of southern part of Turkey. These include metamorfic, volcanic and sedimantery rocks. Menderes massif located in Western Turkey has the highest temperature in the range of 180-2000C suitable for electricity generation. The Arabian platform is located in the southeastern region and consists of Mesozoic aged rocks, nappe consisting of ophiolitic rocks and sedimantery–volcanics rocks. The youngest rocks neogen-Quaternery aged are volcanic and sedimantery.

Magmatism and Geothermal Gradient

Plutonic and volcanic rocks, which can have acidic and basic properties, cover approximately one third of Turkey. Intrusive rocks are Paleozoic, Mesozoic and Tertiary aged. The youngest magmatic intrusions developed in Neotectonic period are located in Western part of Turkey. Magnetism of rocks or any material diminishes at the Curie-point temperature. This property can be used to estimate crustal thicknesses. For this purpose, the Curie-point depth, which is defined as the depth to the bottom of magnetic sources at which temperature reaches Curie-point, is determined to evaluate the geothermal potential of a region.  Turkey’s Curie-point isotherm map was prepared using spectral analysis technique based on aeromagnetic data (Figure 2). The regions with shallow Curie-point depths are located in Western Anatolia. This can be correlated with thinned crustal thicknesses in these regions with a value of 8-9 km. High temperature geothermal fields can be seen in such regions (MTA, 2007).


Figure 2. Curie-point isotherm map of Turkey (MTA, 2007)

Thinning of the continental crust leads to increase of geothermal gradient (Figure 3). There are approximately 190 geothermal fields in areas where continental crusts are thinner or represents young volcanic eruptions as seen in Figure 1 and 4.  The extensional tectonic at western Anatolia cause to thinner crust and resulting shallow Curie-point which led to form highest geothermal gradient in Turkey (Figure 2).


Figure 3. Geothermal gradient  (0C/100 m) distrubion map of of Turkey (Uzunlar,2006)



Figure 4. Geothermal resources map of Turkey (Basel, 2010)



Geological properties, such as tectonism, magmatism, volcanism and geothermal gradients and their relevance to important thermal parameters to determine BTES potential were investigated.  In addition other opportunities and challenges of the potential is discussed.

Undisturbed Ground Temperature and Geothermal Gradient

The ambient temperature around the rock mass is not affected by the outside seasonal changes in the climate after a certain depth. Temperature at this depth is called the undisturbed ground temperature. After this depth, increase of ground temperature is only dependent on geothermal gradient.  The value of the undisturbed ground temperature and the depth where it starts depend on surface climate conditions and geology. Curie-point isotherm map of Turkey (Figure 2) can be used to determine the geothermal gradient in relation to continental crustal thicknesses. In the regions with shallow Curie-point depths, geothermal gradient increases as seen in Figure 3.  The continental crust is thinner in Western part of Turkey, where geothermal gradients are in the range of 0.11m-1 K and 0.12 m-1 K. The undisturbed ground temperature will be higher in the regions with higher geothermal gradient due to the increased heat flux from magma. Menderes massif located in this area has the highest temperature in the range of 180-2000C, which is suitable for electricity generation


The performance of heat injection into the wells and extraction from a BTES system depend on the value of undisturbed ground temperature. The effect of geothermal gradient on heat extraction performance in a BTES system may be around 1% at low geothermal gradient values such as 0.0162 m-1 K, that are seen in Northern Europe (Eskilson,1987). At higher geothermal gradient values, Kurejiva et al. (2011) showed that total number of borehole length decreases by 5.3%, when the average geothermal gradient was taken as 0.05 Km-1 for Zagreb conditions. This will decrease the drilling and borehole heat exchanger costs of BTES system (Kurejiva et al., 2011). This may make BTES a more economic option compared to alternatives.

Geothermal gradient values in Turkey as seen in Figure 3 show differences in the range of 0.02 m-1Kto 0.12 m-1 K. Therefore, it is recommended to take geothermal gradient into account in determining the undisturbed ground temperature id designing BTES systems in Turkey. However, for geothermal fields with very high temperatures that are basically used for electricity generation, the effects of increased geothermal gradients need to be further investigated.

Thermal Conductivity

 Thermal conductivity of rocks depends on mineral structure and porosity, which can be associated with their age and origin. In general, thermal conductivities of rocks can range between 2 to 7 Wm-1K-1. In Table 1 tectonic units in Turkey and corresponding age, rock types and thermal conductivity are given.

Table 1: Main Tectonic fragments in Turkey in association with dominant rock age, type and thermal conductivity

Tectonic Units Dominant Age Main Rock Types Thermal Conductivity

(Wm-1K-1)PontidesCarboniferous – EoceneClastic rocks, limestone, gneiss, micaschist, granitoid and volcanics1,3-5,1Anatolide-TauridesPaleozoic- Early TertiaryGneiss, schist, migmatites, limestone, evaporite, volcanics (andesit, basalt, tuffs), peridotite, granitoid and gabbro0,4-6,4Arabian PlatrformLate Campanien- Early MiocenClastic rocks, marl, limestone, clay, shale, peridotite, basalt, gabbro1,0- 2,5


Ideally, the thermal conductivity of the rock has to be as high as possible for fast storage/recovery of thermal energy from the BTES system. Heat transport in a borehole heat exchanger of a BTES system can be divided into two stages (Sanner, 2011):


  1. Transport in the undisturbed ground around the borehole (controlled mainly by the thermal conductivity of the ground)
  2. Transport from the borehole wall into the fluid inside the pipes, controlled by the type of grouting, the pipe material, the borehole and pipe geometry, etc.


The first stage that is controlled by thermal conductivity of the ground is dependent on geology. Table 1 can be used to determine thermal conductivity of rocks expected according to geology in Turkey. The second stage is controlled by the design of the borehole and is independent of geological parameters.


Mechanical and geotechnical properties of the rock influence the cost of the drilling. Rate of penetration (ROT) and stability are two important parameters that will influence the duration and success of drilling process.  The location of BTES system and the expected rock types as given in Table 1 must be taken into account. For ROT parameters like fractures, texture, specific gravity and density, porosity, permeability, hardness, compressive strength, abrasivity, elasticity, plasticity and for stability diameter of the borehole, auxiliary casing needs and mud use needs are important (Arrizabalaga, 2011). Choosing suitable drilling method and working with drillers and experts who have experience with BTES wells are of utmost importance.

Waste Heat Potential

The geothermal areas of Turkey shown in Figure 4 are currently used for electricity generation, district heating, greenhouse heating and balneology.  17 of these areas are used for electricity generation. There are 18 provinces that are heated directly with geothermal resources and more than 200 balneological applications are used for health and tourism services (Parlaktuna et al., 2013).  There is a significant amount of waste heat given off even after utilization of the current geothermal applications.  This waste heat can also be considered as a potential source to be stored in BTES systems.  The users and temperature levels required have to be considered for cost-effective and feasible utilization of waste heat through BTES.


Turkey has begun to take shape approximately 70 million years ago with the closure of the Neo Tethys. The scene of different magmatic activity started and continued till Quaternary period. All the magmatic events cause a relatively warm crust in the region. Although the sedimentary units cover a major part of the lithology in the geological map of Turkey, the thicknesses of the sedimentary rocks are too thin and magmatic lithology form the basement of these sedimentary units.  Overall mean age of the igneous rocks does not exceed 20 million years. Underground heat flux can exhibit a more permanent feature in most parts of the country. In addition, Turkey has a huge amount of granitoid intrusions in all of the tectonic fragments within the upper crust. These are usually Mesozoic granitic outcrops and have Cenozoic age. According to geophysical and magmatotectonic studies, there are many young granitoid intrusions within the upper and lower crust that they are not exposed. These embedded granitoid intrusions may produce significant heat flux within the crust of Turkey. The presence of these heat flux within the crust can act as important sources for the producing the renewable energy for Turkey.

The main geological properties of Turkey comes into prominence with active fault systems, magmatic intrusions covering wide spread area, thinning of continental crust and having higher geothermal gradient.

The important thermal properties for BTES system design, undisturbed ground temperature, geothermal gradient and thermal conductivity of ground are strictly dependent on geology. The analysis of complex geology of Turkey shows that there are favourable areas in terms of geothermal gradients, which can vary between 0.02 m-1Kto 0.12 m-1 K. For correct sizing of the borehole depth, it is recommended to take geothermal gradient into account in determining the undisturbed ground temperature in designing BTES systems in Turkey, especially in areas with high values. The rock types and corresponding thermal conductivities are also important parameters to consider in determining heat transport from boreholes to the ground. Drilling is an important part of a successful and cost-effective BTES system. It is important to know the geology very well for the choice of the right drilling method and works with experts who have experience in this field. The training activities that will increase the number of experts in BTES systems are needed in Turkey.



The authors would like to acknowledge the support provided by Çukurova University BAP Project No 4319 and DPT project number 2012K120440.


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