This article addresses the issue of obtaining and using a renewable energy source (RES) for heating purposes, district heating (DH), and domestic hot water (DHW). With this energy-clean technology, it is possible to minimize the production of emission substances and replace the combustion of natural gas during heating.
Data that is publicly available and at the same time processed by us in the case study have been used for the processing of this contribution. The procedures and methods are summarized as follows:
2.1. Hydrogeological and Hydrogeothermal Conditions in Slovakia
Due to its natural conditions, the Slovak Republic has significant potential geothermal energy. Based on research and surveys to date, the energy potential of GE is important in Slovakia, and its value is at 5538 MWt. Geothermal energy sources are mainly represented by geothermal waters, which are tied to hydrogeological collectors located at depths of 200–5000 m. In Slovakia, the average temperature increase is 3–3.8 °C for every 100 m of the borehole; at a depth of 3 km, the temperature is about 100 °C. Geothermal sources are divided according to temperature (°C), into:
Low temperature—from 20 °C to 100 °C: these are geothermal sources with a moderate temperature, suitable only for heating and recreational purposes;
Medium temperature—from 100 °C to 150 °C: these are suitable for heating and using binary cycles, and for electricity generation;
High temperature—above 150 °C: these are geothermal sources suitable for electricity generation (using water vapor).
In terms of well yield, geothermal sources are distinguished as follows:
With minimum yield—up to 1.0 L·s−1;
With very little yield– from 1.0 to 5.0 L·s−1;
With a small yield—from 5.0 to 10.0 L·s−1;
With medium yield– from 10.0 to 25.0 L·s−1;
With great yield—from 25.0 to 50.0 L·s−1;
With very high yield—above 50.0 L·s
−1 [
14].
The wells that have been carried out so far (at depths of 92–3616 m) have verified temperatures at the wellhead of 18–129 °C. The yield of free-range wells ranged from tenths of a liter to 100 L·s
−1, with a predominantly Na-HCO
3-Cl, Ca-Mg-HCO
3 and Na-Cl water type, with mineralization of 0.4–90 g·L
−1 [
15].
Slovakia, considering its small surface area (49,000 km2), is very rich in mineral and thermal waters. There are 1200 springs registered on its territory, the equivalent of one spring for every 40 km2. The rich physicochemical diversity of waters and their even spread throughout the territory is conditioned by the favorable geological–tectonic construction of the territory and its geothermal activity.
From north to south, mineral and thermal waters are linked to sediments of the Flysch zone and the Klippen belt of the Paleogenic and Mesozoic eras, the crystalline rocks of the Paleozoic and Mesozoic, the dolomites and limes of the Mesozoic, and Flysch sediments of Inner Carpathian Paleogene and Neogene origin. The temperature of the waters of the springs is between 15 and 70 °C. Mining works carried out in some areas obtained water with temperatures in the range of 40–130 °C. The yield of natural springs varies between 1 and 40 L·s
−1. The well yield is from 5 to 90 L·s
−1 [
16,
17]. From the point of view of geothermal water sources, we can only consider as a possibility the Paleogene subsoil in the area of interest, which consists mainly of Triassic carbonates of Krížna Nappe and Choč Nappe, with Karst-fissure permeability. Overall, the geothermal activity of the area of interest can be assessed in terms of the density of the earth’s heat flow as average geothermal output (65–70 mW·m
−2). Temperatures at the depth of storage of the collectors (1400–3000 m below the surface) are about 45–95 °C. Mineral waters reflect the hydrogeological, hydrological, geological, and structural conditions in the monitored area. The nature of the waters indicates that Triassic structures correlate well with the appearance of Triassic carbonate rocks on the surface around the basin. The nature of the waters in the Paleogene subsoil may have been obviously marine in origin, i.e., waters that have been preserved after the transgression of the Paleogene Sea but are now more or less infiltrated surface waters from the peripheral parts of the basin. The total mineralization of the waters in the Mesozoic subsoil, based on the results of the pumping tests in the surrounding wells, can be expected in the range of 3–5 g·L
−1. Information on the hydrogeological conditions of the territory comes mainly from the quaternary sediments of Poprad, where several exploration wells were created. Several deeper wells were created in the Poprad Basin, the results of which provided valuable data on groundwater properties in individual geological units. From this point of view, valuable information can be drawn from the wells in the areas of Stará Lesná (FGP-1), Poprad (PP-1) and Vrbov (VR-1, 2, 2A, 3) and the latest from the area of Veľká Lomnica. Since the PP-1 borehole (Poprad) is relatively close to the site under examination, its results may be analogous to those of the projected well. At the PP-1 well in Poprad, the geothermal waters, of a significantly calcium-magnesium-sulfate-sulphate-hydrogen carbonate type, were verified as having an average mineralization value of 2.88 g·L
−1, with a pH value of 6.21. The water is over-gassed with CO
2. The increased sulfuret content is due to the dissolution of plaster stone and anhydrite. The incidence of sodium is low because it is released due to the low pH [
18].
Groundwater resources and reserves in Slovakia vary not only depending on the location and time but also in terms of their quality. Although they are regularly renewed, they are not unlimited and only proper use can ensure their relative inexhaustibility. Groundwater, which is a source of quality drinking water, is the most important natural wealth in Slovakia. Therefore, the most fundamental task is getting to know the laws of groundwater creation and flow as well as its protection. This requires very close monitoring, documenting and registering their basic characteristics and parameters. The total usable amounts of groundwater in Slovakia, as documented in 2018 in all categories, represent 77,175.07 L·s−1.
Those usable quantities also include usable amounts of thermal water as an integral part of groundwater and, for the sake of completeness, part of the mineral waters, in particular the usable quantities of mineral waters approved by the Hydrogeological Commission [
19].
Figure 1 shows the prospective areas of geothermal waters in the Territory of the Slovak Republic.
In order to create the best possible conditions for the use of geothermal energy, regional hydrogeothermal evaluations are carried out by determining the quantity of geothermal waters and geothermal energy in the defined 27 hydrothermal areas or structures of Slovakia [
20]. Current geothermal conditions in Slovakia are mapped out and reviewed in detail. There are currently 27 prospective geothermal areas that have been defined (
Figure 1 and
Figure 2).
A large proportion of geothermal reservoirs provide water with a temperature of up to 135 °C, which is optimal for use for heating buildings or for recreational purposes. Geothermal energy (GE) is not primarily used for efficient electricity generation. Modern technologies also make it possible to generate electricity using a binary cycle [
13].
2.3. Methodology for Determining the Suitability of Geothermal Well Usage in the Monitored Area (Podtatranska Basin)
Hydrogeologically, the projected well is situated in explored territory in the Poprad River Basin. It has a left-hand tributary from the High Tatras and the right-hand tributary is mainly from the Levoča Hills. Water management is important in the area since the surface waters of the tributary of Poprad and the groundwater from its alluvia are often used for drinking water supply. Water quality in the river is influenced by industrial enterprises and local agglomerations. The area of the Sub-Tatran Basin under investigation offers the use of several types of RES in different locations of the territory, in order to reduce negative environmental impacts, especially in terms of reducing emissions or replacing the combustion of fossil fuels. Fossil fuel heating is one of the largest sources of CO2 emissions. The best solution is the use of thermal energy from geothermal sources.
Geothermal energy is an available local, strong energy source that is characterized by stability of supply, regardless of current climatic conditions. Geothermal energy is a long-term and sustainable energy source. Based on the geological construction of the surrounding area and the conditions of geothermal wells that have already been realized, it is possible to expect a well yield in the range of 20–30 L·s
−1, with a total mineralization of about 3–5 g·L
−1 and a water temperature at the surface of 60–70 °C. One definite point of uncertainty, according to the study, may be the depth of the collectors; therefore, the study recommends counting on the final mining depth being 2800 m [
21].
Based on analyses of the available data, we assume that there is potential for the practical use of GE in the monitored Podtatranska Basin. GE may replace the combustion of natural gas in the supply of housing units under the current model. This creates ideal conditions for:
Limiting the use of energy derived from fossil and conventional fuels;
Reducing CO2 emissions, (NOx, CO, SO2, TZL)
Stabilizing heat prices,
Obtaining a stable, green, and renewable energy source.
The aim of the methodological procedure (
Figure 4) is to choose the appropriate technology to cover the energy needs of the chosen location—a housing estate—based on sustainability, local availability, and affordability, and with a positive impact on the environment, as an exemplary model of energy independence for towns and villages.
The methodology begins with the search for theoretical knowledge in the field of RES energy. This draws attention to the call for a transition to renewable and sustainable energy sources within the European Union, which aims to achieve carbon neutrality by 2050.
Following a subsequent evaluation and the selection of appropriate information, the individual available RES technologies were evaluated for:
The area of energy coverage of household needs;
The types and principle of operation; and
A large-scale and stable supply for the population throughout the year.
From the information found, geothermal energy appears to be the most suitable form of energy. With the subsequent selection of the site and the examination of existing technologies covering the energy needs of the housing estate, it is possible to proceed with an evaluation of the most appropriate RES technology. Our research has shown that the technology used has the best potential and the appropriateness of the subsequent investment is confirmed by further calculation.
The processing of the geothermal contribution was based on data from the technical study of the geothermal well GTK-1, which is publicly available [
21]. A specific site (GTK-1,
Figure 5) has been designated in which to carry out the geothermal well under investigation. Residential houses (in a housing estate with a population of 5500) were selected that met the requirements for the use of geothermal energy from the borehole. To determine the yield of a geothermal source for heating the estimated capacity needed for the source was compared with the heating volume of 3 boiler rooms in previous years (average of years 2015–2017).
The calculation of the energy potential of the geothermal source was based on the following parameters: yield, mineralization, and water temperature at the surface. Given that it is not possible to determine the exact temperature of geothermal water on the basis of current knowledge, three variants of geothermal water temperature (60 °C, 65 °C and 70 °C) have been considered when calculating the energy potential.
2.4. Procedure for Calculating the Chosen Technology’s Potential Using Geothermal Energy
A methodological procedure was used to evaluate the use of the suggested possibilities regarding suitable RES technology with the best potential and lowest subsequent investment, with a recalculation, determination, and proposal as to whether the given system (investment) is suitable. The procedure for calculating the investment technology chosen, using geothermal energy, is as follows:
ROI is the most frequently used parameter for the assessment of the economic efficiency of investments. It is the total income that results from concrete investments, divided by the amount of investment funds. This indicator is completely time-independent [
22].
Calculation of the annual cash flow is carried out on the basis of the following relationship [
22]:
CF, or annual cash flow (annual income), is the product of the unit price of natural gas (EUR 0.0219/kWh, for 2021) and the amount of heat produced (DH and DHW) in kWh (MWh) with a difference in estimated annual operating costs (EUR).
The cash flow of a project is the sum of positive and negative items, incomes and costs, connected with a certain activity. The sum of all financial flows, which results from the investment into a project, is called the cash flow produced by capitalized investments.
NPV—net present value;
I—investments;
CF—cash-flow;
a—update rate;
i—current year;
n—project duration.
If the NPV of the first project is higher than the NPV of the second project, and vice versa—the ROI of the first project is smaller than the ROI of the second project (NPV1 > NPV2; ROI1 < ROI2). In this case, there is no precise mathematical formula defining which of the two projects is better. The volume of investments and the risk level of the project will probably play the most important role. The intuition and experience of the project evaluator, as well as other arguments, can influence investment decisions.
A payback period is the project’s duration, from its beginning until the point when the cumulative cash flow becomes positive. Although in the case of some projects, the assessment results based on the payback period may seem interesting, this indicator does not say anything about the project’s future course from the viewpoint of its cash-flow development. This could be either positive or negative.
The CO
2 emission factors needed to calculate CO
2 emissions from the operation of buildings (heating, hot water preparation and the operation of other appliances) are country-specific or operational (and also different for each IPCC1 category) and are derived from specific fuel characteristics. Average CO
2 emission factors are used for natural gas, hard coal, lignite by region of origin (Slovak, Ukrainian and Czech), and coke. Due to these reasons, emission factors should be revised each year [
23,
24].
The values of the weighted arithmetic mean of the qualitative parameters of natural gas, distributed in the territory of the Slovak Republic by SPP—distribúcia, a.s, were used according to the method followed in [
25]. Density, calorific value, combustion heat and Wobbe number are given for the business unit, i.e., m
3 at 15 °C, pressure 0.101325 MPa, and relative humidity φ = 0. The formula used for the conversion of units: 1 kWh = 3.6 MJ.
Annual consumption of MNG NG at a calorific value of 34,848 MJ/m
3 [
26]:
The energy potential is an elementary indicator of the possibility of using geothermal water. The energy potential of water, which is heated by the action of the earth’s core, is its heat output [
27].
where:
Three variants of geothermal water temperature (60 °C, 65 °C and 70 °C) were considered when determining the energy balance. Natural gas savings were assessed by comparing the average natural gas consumption for 2015–2017 and the energy balance of geothermal energy as a percentage; thus, it was also possible to assume an approximate reduction in CO2 production.
When processing the energy balance of the GTK-1 geothermal source, we will take into account the following assumptions:
Geothermal energy will be used for heating the housing estate of the DH and for the preparation of DHW.
Heat loss or a temperature drop of 4 K due to the use of heat exchangers is considered.
The daily operating time of boiler rooms is assumed to be 16 h.
A maximum heating water temperature of 70 °C is assumed.
A reduction factor in the use of geothermal energy for each whole 1 °C rise in the external temperature, which takes into account differences in outdoor temperatures during the day and resulting fluctuations in the temperature of the heating water. The reduction factor varies for different geothermal water temperatures.
Thermally used geothermal water will be discharged into a nearby stream.
The energy balance will be processed for individual scenarios: the pessimistic scenario (geothermal water temperature of 60 °C), the conservative scenario (geothermal water temperature of 65 °C) and the optimistic scenario (geothermal water temperature of 70 °C).
To process the energy balance, it is necessary to know the following values:
Number of days of temperature duration from 13 °C to −16 °C;
Temperature of the supply and return heating water;
Average heat output per DH;
Amount of heat produced per DH;
Amount of heat produced per DHW;
Usable thermal output of the geothermal source for DH;
Reduction factor for the use of geothermal energy for DH.