Prospects for wider energetic utilization of subgeothermal water resources-eastern Serbia case study

Extensive worldwide usage of fossil energy sources causes high pollution and contributes to global warming. Hence, achieving energy independence by stimulating efficient use of energy and environmentally friendly exploitation of renewable sources is a main orientation of European countries. Geothermal energy is generally treated as a renewable and inexhaustible energy source. Nonetheless, direct use of low enthalpy subgeothermal resources, i.e. groundwater of 30o C or lower, for heating is commonly viewed as economically unjustified. To enable its usage, large panel surfaces or a high-temperature heat pump with excellent efficiency is required. The development of a cascade type heat pump and its wide application would enable more efficient utilization of widely available and easy replenished groundwater sources with temperatures of 10–30o C. The hydrogeological conditions in eastern Serbia are particularly favourable for exploitation of subgeothermal resources due to rich aquifer systems and notable terrestrial heat flow formed into the main geo-structures of the region (Carpathian-Balkan arch and Dachian basin). More intensive exploitation of subgeothermal sources additionally justifies the existence of a number of urbanized small and medium-size cities with a heating infrastructure already developed and centralized. Sustainable use of groundwater resources should be followed by thermal reconstruction of the previously constructed buildings as well as new legislation which supports and encourages development of renewable energy sources. It is estimated that the total potential thermal power which can be generated from subgeothermal waters in the study area is around 33 MWt, which corresponds to some 16 % of the total heat requirements.


Introduction
The utilisation of low-enthalpy ground source heat for space-heating (or cooling) is widespread in the USA and in Canada, with recent growing interest throughout Europe (BANKS 2008).The heat pump technology which supports low-enthalpy sources would help to achieve the objectives of the Kyoto protocols on "greenhouse" gas emissions.The thermal input for those pumps can be provided either directly by groundwater extracted from wells or tapped springs, or indirectly by water injected into the ground and then pumped out (rock and soil heat release) afterwards.Ground source heat pumps (GSHP) can be regarded as complementary technology inasmuch as they require a "driving" mechanical or thermal power input (commonly, electrical power to a compressor) that is used to extract renewable heat from the ground.
The heat effect delivered by a GSHP is typically 3-4 times the electrical power input.This means that it delivers heat 3-4 times more cheaply, with up to 75 % less CO 2 emission, than direct use of electrical resistance elements.This, in turn, allows GSHPs to deliver more CO 2 -efficient space-heating than even the most efficient combustion of fossil fuels.Fossil energy sources are especially limited in European countries, and their dependence on import will be even more critical in the near future.Moreover, ground source heat can also compete effectively in terms of marginal costs with coal or gas (YOUNGER et al. 2007).The energy of the geothermal water could be exploited to the maximum before it is returned to the ground through a reinjection well (as already required by legislation of the European Union).With reinjection an ecological problem is also minimized, as utilized geothermal water must not be released into local water courses.
Although the initial high capital cost of GSHP schemes is a potential weakness, within the European Union the uptake of ground source heat technology has dramatically increased (ALLEN et al. 2003).In Sweden, for example, which has few native fossil fuel resources and an energy economy that is founded on nuclear/hydroelectric electricity, there has been a strong political motivation to develop technologies that utilise electrical energy as efficiently as possible.Moreover, the strong environmental and centralist traditions of the Swedish government have stimulated the uptake of technology since the early 1990s by a generous subsidy programme.In 2007, it was assumed that 185,000 GSHPs were installed in Sweden, providing some 1700 MW of power (BOUMA 2002;YOUNGER et al. 2007;BANKS 2008).
In central and SE Europe there is also growing interest in introducing, widely applying and promoting this kind of "green energy".In 2000, fewer than 1000 GSHP were functional within Czech Republic or Poland, while just five years later the number of procured and installed systems had increased tenfold.Rapidly growing interest in GSHPs is also noticed in Italy and in Slovenia, but the less powerful economies of the Balkan countries are still waiting for the encouraged legislation and the deduction of fees for their wider application.However, concerning groundwater resources and availability most of the countries that belong to the Balkan region are strong prospects.
The efficient use of thermal potential of existing fluids would thus prevail over the possible alternative i.e. exploitation of rock and soil petrothermal resources.

Subgeothermal water sources
In most relevant references the geothermal fluids with a temperature lower than 100º C are classified as "low enthalpy": only the direct generation of electrical energy from water heat at a temperature higher than 100º C is considered to be fully feasible (NICHOLSON 1993;LUND & FREESTON 2001;HUTTRER 2001).
Recently conducted studies aiming to assess the thermal potential of lower temperature ground sources in Serbia introduced the term "subgeothermal resources" (STEVANOVIĆ et al. 2008).It has been proposed that waters with a temperature of 30ºC or lower are to be classified in this group whereas those with a temperature of 30-100ºC belong to "geothermal resources".One of the explanations is very practical: although GSHP is necessary for water with lower temperatures, the panel radiator systems allow direct use of water with a temperature of 30ºC or more without GSHP.However, as temperatures are lower, heating panels with larger surfaces are required.
Subgeothermal fluids in the territory of Serbia are typical of complex geology as well as of a moderate continental climate.They are classified as follows (Таble 1):

Brief physio geography, geology and hydrogeology of the study area
There were several reasons to choose the Carpathian region in eastern Serbia (Fig. 1) as a study area: abundant groundwater reserves, considerable subgeothermal flow, a good number of small industrial towns with developed centralized heating infrstructure, and moderate continental climate.The northern boundary is the Danube River, the eastern/northeastern boundary is the state border with Bulgaria (toward which the Carpathian-Balkan mountain range continues), and its western border runs along the edges of the Velika Morava and Južna Morava valleys (Fig. 2).The main settlements are (from north to south): Donji Milanovac, Kladovo, Negotin, Majdanpek, Bor, Zaječar, Sokobanja, Knjaževac, Pirot (all with between 10000-60000 citizens).Further west in an adjacent area of central Serbia (Velika Morava valley) there are several large cities such as Niš, Paraćin, Ćuprija, Požarevac and others that are also envisaged as possible consumers of geothermal energy produced either from local sources or from eastern Serbia (MILIVOJEVIĆ & MARTINOVIĆ 2005;MARTINOVIĆ et al. 2008).
The mountains of the Carpathian-Balkan arch are mostly unpopulated and used by the local villagers for specific crop cultivation or grassland during the summer months.Most of the hills are covered by forests or pastures.The demographic trend in the region is negative: the younger population is gravitating towards nearby industrial centers such as Bor or Zaječar.
The area is a prospect for the development of tourism.It is unpolluted and accessible by many roads, is rich in clean waters, and boasts beautiful landscapes and features such as caves, waterfalls and springs, including several spa and medical centers.
Annual average rainfall ranges from 600 mm to 750 mm for most of the cities, while the annual average air temperature for the entire region is around + 9ºC.
The complex geology of eastern Serbia comprises two main geo-structures: the Carpathian-Balkan arch and the Dachian basin, both characterized by hydrogeological heterogeneity and a variety in aquifer systems and groundwater distribution.Thus, the region is characterized by the presence of formations with small groundwater reserves but also Mesozoic carbonate rocks, Tertiary or Quaternary alluvial and terrace deposits that could be very rich in groundwater (STEVANOVIĆ 1994).
The study area consists of several mountain massifs, the majority of which are built from carbonate rocks (Fig. 2).Precambrian rocks are not often exposed to the surface and can be found in anticline cores, often corresponding with mountain central parts and peaks.They are commonly overlaid by Paleozoic rocks (Ordovician, Silurian and Devonian age).Paleozoic formations, magmatic and metamorphic rocks are mostly aquitards or aquifuges.
The major part of the study area was invaded by the Middle Jurassic marine transgression, and the sedimentation cycle continued until the end of the Lower Таble 1. Energetic potential of subgeothermal sources.Cretaceous (Albian).Throughout this period, thick deposits, primarily of carbonate rocks, were formed to a total thickness of about 1300 metres.Carbonate rocks are characterized by the non-uniformity of the facies with prevalent limestone but also by "impure" varieties in lower sections, such as sandy limestones of the Dogger, or Oxfordian-Kimeridgian chert limestones.The carbonate complex was formed mostly during the Tithonian, Valanginian, Hauterivian, Barremian and Aptian and it contains predominantly pure carbonates or magnesium carbonates.
The karst aquifer formed in carbonate rocks is rich in groundwater, and is recharged mainly from rainfall and from sinking flows which gravitate from impermeable rocks at higher altitudes.Carbonate rocks are well-karstified and contain very large groundwater reserves (STEVANOVIĆ 1994(STEVANOVIĆ , 2009)).
The karstic groundwater is used mostly for drinking purposes or for small industry supply; almost all the cities in the region are consumers.Although the total dynamic reserves in karst often surpass by far the exploitation capacities, most of the tapping structures are constructed simply to tap the natural discharge of the springs and thus depend solely on the natural flow regime.Aiming to overcome this problem, during the last three decades several successful aquifer control projects have been completed (STEVANOVIĆ et al. 2007).Karstic groundwater is extracted in a very small amount for irrigation purposes.
In the Timok tectonic trough on the eastern part of Carpathian arch, a volcanogenic-sedimentary series over 2000 meters thick (andesites, pyroclastics, tuffa) was formed during the Senonian and the Paleogene (Fig. 2).Fissured aquifer of this complex contains several important thermal and thermomineral occurrences with the water temperature ranging from 30-40ºC (Brestovačka spa, Gamzigradska spa, Sokobanja, Nikoličevo, Šarbanovac).
Clastic sediments, marls, clays, and sands were deposited in a number of the intermountain depressions filled with lake waters during the Neogene (e.g.Žagubica, Bogovina, Sokobanja, Babušnica, Pirot).The sandy water-bearing layers could have an artesian pressure (confined aquifers) and are tapped in several locations (e.g.Negotin, Zaječar).
Most recent are alluvial sediments which follow major streams in the region (Nišava, Timok, Resava etc.) and are used mostly to supply local villages.The exception is Niš where a large amount of alluvial water is tapped (Mediana source) for Niš itself, the third largest city in the country.

Utilizing geothermics by cascade heat pump -Discussion
For heating purposes three basic types of heat pumps are used (GORIČANEC et al. 2008): -a single stage heat pump, -a two stage heat pump with a flash vessel, -a two stage heat pump with a heat transmitter.Using a single stage heat pump for heating buildings, the temperature of the secondary carrier can reach 55°C maximum, which is quite a bit too low for the heating of buildings with a "classical" radiator heating system.Therefore, single stage heat pumps are primarily used for low-temperature heating systems, with the temperature of the secondary carrier up to 45°C.In high temperature heating systems, the temperature of the secondary carrier must be even higher than 60°C.With an appropriate refrigerant, a two stage heat pump with a flash vessel could be used for high temperature heating systems.But it is difficult to find a refrigerant which allows the exploitation of the temperature of geothermal water by cooling it to 10° C and at the same time reach a high temperature in the condenser of the second stage in the heat pump (GORIČANEC et al. 2009).
This problem has initiated research for a two stage heat pump with a heat transmitter (heat exchanger), schematically shown in Figure 3.The heat pump in fact consists of the two single stage heat pumps which are connected by a heat transmitter.The advantage of such a cascade heat pump is that, according to their physical-chemical characteristics, different refrigerants can be used at each stage.In choosing the refrigerant considerable attention must be paid to its physical-chemical characteristics, ecological acceptability, and use of recognized brands.
The heat transmitter between the two stages represents a condenser for the first stage and an evaporator Fig. 3. Scheme of two stage heat pump with a heat transmitter (Goričanec et al. 2009) for the second (GORIČANEC et al. 2008).The use of two stage heat pumps with a heat transmitter is suggested in the existing district heating system of buildings in the city of Lendava in Slovenia (TORHAČ et al. 2005).The heat source for the heat pumps is geothermal water of 42°C.The principle of exploiting heat from geothermal water in an individual facility is shown in Figure 4.With such a system it would be possible to exploit the heating of geothermal water to a temperature of even 10°C.
The coefficient of profitability of a two stage heat pump and the period of time for the investment to be recovered show these to be good prospective solutions.In the case of the Lendava thermal source, the coefficient of profitability has been calculated on 1.19, and the investment will be returned in 3.2 years (KOZIĆ et al. 1994;TORHAČ et al. 2005).The coefficient of performance (COP) of a heat pump is between 3.5 and 4.4.Tests and calculations for water at lower temperatures, different refrigerants and equipment used resulted in a longer period for investment recovery, but still justifiable in terms of both economy and ecology.
At present, almost 50 % of total energy production in EU countries is spent in buildings.In Serbia even more, almost 2/3 of the energy is utilized for domestic heating, which is quite logical given that more than 50 % of buildings in Serbia were built before 1970.when application of thermal insulation was not obligatory.According to some estimates, the annual energy output is in the range of 150 tо 250 kWh/m 2 .This is 2-3 times more than optimal.
Moreover, the application of active and passive renewable energy for space and water heating is still not properly regulated in Serbia: it is only recommended in different state or local government strategies.However, central heating systems exist in numerous cities and can be used for conveying energy from alternative thermal sources.

Eastern Serbia -subgeothermal potential and prospect
Several conducted studies (STEVANOVIĆ 1994(STEVANOVIĆ , 2009;;MILIVOJEVIĆ & MARTINOVIĆ 2005, 2010;MAR-TINOVIĆ et al. 2008;STEVANOVIĆ et al. 2008) have concluded that eastern Serbia is one of the regions in the country with the greatest prospect for groundwater and subgeothermal energy extraction due to: richness of the aquifer, developed heating infrastructure within moderately populated cities, proximity of the sources to the end-users.
In practical terms, most important is the karstic type of aquifer in Jurassic and Cretaceous limestones, which covers about 30 % of this region.The region features a considerable number of karstic springs, of which 16 have a minimum yield of more than 0.1 m 3 /s (STEVANOVIĆ 1995).All major cities use water from karstic aquifers; however, available reserves are several times higher than the water demand of this and neighboring regions, and will remain so for a long time to come.There is also good potential for groundwater utilization from alluvial sediments and Neogene formations in Intra-Carpathian basins.Regarding the total groundwater resources available there are some contradictions in their estimates.The Water Master Plan of Serbia from the year 2000 indicated dynamic groundwater reserves in the amount of 4.27 m 3 /s, while some other studies found that even the reserves of karst aquifers are considerably higher, reaching 12.6 m 3 /s.The karst and fissured aquifers are also rich in thermal water occurrences: there are 6 registered with temperatures over 30°C and some 20 with temperatures over 20°C.In addition, average water temperatures of artesian and subartesian waters from Neogene sediments are in the range of 10-18°C which also classifies those aquifers as ones with high potential for geothermics utilization (Table 2).
According to the preliminary assessment, some 1.7 m 3 /s of groundwater in eastern Serbia can be extracted and sustainably used for heating/cooling systems (STEVANOVIĆ et al. 2008).This potential flow resulted from the calculation which took into consideration prioritized water utilization for drinking and industrial water supply, ecological flow for downstream water dependent eco-systems, and the average water temperature ranging from 10-30° C. The subthermal flow Fig. 4. Scheme of heating of building by a two stage heat pump and reinjection of utilized waters can be separated into three categories as presented in Table 1.Total potential heat capacity is calculated by the formula: where, Q -total potential heat capacity (MWt), m -mass (kg), c -specific water thermal capacity J/(kg°C) i.e. 0.004184, ∆T -differential temperature (°C).Differential temperatures are in the range of 6-18°C depending on geothermal categories (e.g.∆T = 18°C for the waters with T 22-30°C).Available heat power is based on available water resources and tolerable COP, and is assumed to be around 49.20 MWt (Table 2).
The next step in the calculations considered the thermal heat power from sources which are located in the vicinity of the urban areas.Most of the water resources are at a viable distance from the consumers.The calcu-lation is based on apartments with an average surface of 60 m 2 and required average heat of 100 kWh/m 2 .The latter figure takes into consideration the implementation of some other energy efficiency measures to be implemented at the same time (thermal reconstruction and insulation of the buildings).Complete and systematic insulation would additionaly improve the situation and reduce required heat, but at this stage just part of these activities are envisaged.Table 3 contains data concerning the total power required to heat the apartments in major towns, the potential subgeothermal flow available in their vicinity (for each of defined three categories), the potential thermal power which can be produced by heat pumps, and the percentage of heat demands that can be met by heat power generated in such a way.The total potential thermal power which can be generated from subgeothermal waters for large settlements is assumed to be around 33 MWt, which corresponds to some 16 % of their total heat demands.While it is clear that not all potential energy can be efficiently exploited, this figure indicates that the Prospects for wider energetic utilization of subgeothermal water resources: eastern Serbia case study 137 Table 2. Available subgeothermal resources and calculated heat power in the Eastern Serbia.* = Temperature of groundwater resources: 10-30°C potential of subgeothermal energy in this region is much higher than the 4 % contribution from that kind of energy source to the Serbian energy balance recently announced in the national energy plans.
138 Fig. 5.The settlements -Main potential consumers of subgeothermal energy in eastern Serbia, their heat demands and possible cover Between 11 % and 29 % of heat demands of selected towns are covered (Fig. 5).The latter figure characterizes the town of Negotin where a relatively large groundwater resource is located near a small urban settlement.
Such promising geothermal potential requires further systematic research as well as the development of a new strategy and legislation.Therefore, greater governmental support for "green" energy production is expected.Sustainable groundwater extraction should also be followed by controlled extraction (by the provision of permits for exploitation rights) as well as permanent monitoring aiming to prevent overexploitation and potential technogenic processes.
The implementation of new legislation similar to that of the European Union is also required particularly for utilized waters which have to be regularly reinjected into the ground.A sound practice applied in developed countries suggests that reinjection should take place in the layers regularly overlying the main aquifer (Fig. 4) in order not to cool the existing temperature of the tapped waters [The heat pumps produce thermal energy by significantly decreasing initial water temperature (∆T)].A new approach also requires intensive public promotion of energy efficiency demands and achievements.And last but not least, the reconstruction of old buildings and thermal insulation should systematically be undertaken.

Conclusions
Among the main advantages of subgeothermal energy extraction from groundwater (temperature 30°C or less) are: • Easy tapping (once the groundwater resource is properly explored and defined); • Renewable energy resource which is cheap to develop; • Inexpensive heating pumps and equipment (will be even less costly with wider application and further commercialization); • Conservation and guarding non-renewable energy sources for future generations (in accordance with sustainable development principles); • Energy independence of the country; • Lesser emission of CO 2 , CO, along with numerous other ecological benefits; • Prospect for alternative economic development including tourism, recreation and the development of spa centers.The two stages (cascade) heating pumps enable the exploitation of low enthalpy sources with temperatures over 10°C.However, every additional temperature degree makes this exploitation more feasible.Some regions such as eastern Serbia are a good example to demonstrate the feasibility and advantages of the development of subgeothermal resources and wider application of cascade heat pumps.One of the main advantages is the existence of a centralized heating infrastructure in most of the small or medium sized cities in the region.

Table 3 .
Potential of subgeothermal water resources and heat power in vicinity of urban areas of Eastern Serbia.