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The Pannonian Basin, partly located in Croatia, is well known for its higher-than-average geothermal gradient with good potential for geothermal energy exploitation. Most of the currently known geothermal potential locations in the Croatian part of the Pannonian Basin (CPB) were discovered during the oil and gas exploration and exploitation from the mid-twentieth century onward. Unfortunately, recent geothermal energy utilisation in Croatia, which began in the late 1980s, developed very slowly and sporadically, even though the utilisation of it has been known since the Roman times. Most of the discovered geothermal sources are used for balneology in numerous thermal spas. In the last decade, low-temperature geothermal resources have also been used in agriculture, namely in greenhouses. However, with the change of legal framework in 2018, the market showed an increase in the number of issued geothermal exploration blocks. With Croatia’s first geothermal ORC power plant Velika 1, commissioned in 2019, the interest in developing geothermal projects is seen in 13 exploration and six production licenses issued in the last three years, focusing on deep geothermal potential. The planned use of these granted licenses varies from electricity production to agricultural use. Aside from classic geothermal brine production, there is also a good potential of geothermal brine exploitation from bottom aquifers in depleted oil and gas fields. Many hydrocarbon reservoirs in Croatia consist of oil and gas in the upper part of the reservoir and aquifer in the bottom part. During initial depletion-drive exploitation, pressure in the reservoir declines, causing brine from the aquifer to slowly invade the oil zone. While the reservoir is in its final stages of production, some waterflooded peripheral wells could be turned into geothermal ones, even if oil is still produced or after the field is abandoned. So far, several locations with relatively high temperatures of the bottom aquifer have been identified as a good potential for deep geothermal energy exploration and exploitation. This work gives an overview of the current state of geothermal energy utilisation in Croatia and future prospects.

The paper analyzes the technical solutions, economic feasibility and institutional aspects of geothermal energy use in a district heating system by retrofitting wells on a mature oil field based on the case study. The objective is to describe institutional procedures for brine production from the mature oil field. The research methodology includes the well data and the condition of the existing oil infrastructure analysis, to evaluate the brine production possibility. The methodology also analyzes existing hydrocarbon contracts. Institutional steps to obtain a geothermal concession on the brownfields are proposed. The hypothesis assumes that, with the legislation change, a vertical distribution of concessions for simultaneous production of oil and brine is achievable. The conclusion brings the novelty of this paper as the provision of guidelines for the development of the projects and the start of the production of brine from a mature oil field for the investors and the legal authorities.

The wider Zagreb city area, the capital city of the Republic of Croatia, has signifi cant potential for major implementation of geothermal heat pump systems in not just the residential sector, but in the commercial sector as well. Geothermal heat pumps often cumulatively describe diff erent earth energy heat exchanger installations, from groundwater up to shallow ground horizontal and borehole heat exchanger installations. The Zagreb area is especially favourable for both hydrogeological and thermogeological parameters, which allows for the implementation of diff erent designs for residential and commercial heating and cooling. An analysis of comparative energy and economic advantages of heating and cooling with heat pump systems (air, ground or groundwater) was made for the capital city of Zagreb. Since heat pump systems in residential areas are always a higher- class investment, there is often a need for year- round heating since projects increasingly incorporate outdoor swimming pools. Up till now, many authors investigated a comparison of winter heating energy costs for diff erent heat pump systems. However, outdoor swimming pools sometimes consume as much heating energy during the spring- summer-autumn seasons, as do houses in the autumn- winter-spring seasons. Since air temperatures during the spring-summer seasons are much more favourable than sub-cooled ground or groundwater temperatures, air-source heat pumps when used during the whole year for heating, could off set an economically unfavourable coeffi cient of performance (COP) during the coldest days of winter. On a real residential home, currently in the construction phase, techno-economic analysis was performed comparing the seasonal performance factor (SPF) for different heat pump systems during year-round heating.

Kindergarten Grdelin in the city of Buzet, Istria, Croatia, was chosen to undergo a deep retrofit of the current thermotechnical system, as a part of the HORIZON 2020 HAPPEN project1. The existing shallow spiral heat exchanger field is insufficient to cover heating loads of the building. Therefore, additional BHEs were drilled and completed to determine optimal borehole heat exchanger type within the same geological environment. Four BHEs, either single U (1U) or double U (2U), with different geometrical setting and depth were tested: BHE-1 (50 m, 2U DN32 ribbed), BHE-2 (75 m, 2U DN40 ribbed), BHE-3 (100 m, 2U DN32 smooth) and BHE-4 (150 m, 1U DN45 ribbed). A thermal response test (TRT) was performed to obtain the ground thermal properties. Furthermore, synthetic TRT curves were calculated to describe temperature response in the case of different heat pulses. This was done to determine heat extraction rates and the capacity of each BHE type, according to EN14511 norm. It was established that the BHE-4 is the optimal design for heating and cooling purposes on the selected site due to positive impact of the geothermal gradient, higher initial borehole temperature and a positive effect of the ribbed inner wall., Zgrada dječjega vrtića Grdelin, smještena u gradu Buzetu, Istarska županija, odabrana je unutar HORIZON 2020 HAPPEN projekta za obnovu i dopunu termotehničkoga sustava grijanja i hlađenja. Postojeći sustav bušotinskih izmjenjivača nije dovoljan da pokrije potrebe za toplinskom energijom te je zaključeno da je potrebna izrada dodatnih bušotinskih izmjenjivača topline. Da bi se odredio optimalan tip bušotinskoga izmjenjivača u istome geološkom okruženju, izrađene su 4 bušotine različitih dubina te su opremljene bušotinskim izmjenjivačima različitih geometrijskih karakteristika: BHE-1 (50 m, 2U DN32 žljebasta), BHE-2 (75 m, 2U DN40 žljebasta), BHE-3 (100 m, 2U DN32 glatka) i BHE-4 (150 m, 1U DN45 žljebasta). Test toplinskoga odaziva (TRT) izveden je na svim četirima bušotinama te su određena toplinska svojstva tla. Nadalje, simulacijom toplinskoga step-testa dobiveni su temperaturni odazivi pomoću kojih su određeni prinosi za pojedini bušotinski izmjenjivač. Zaključeno je da je BHE-4 izmjenjivač (150 m, 1U DN45 žljebasta), s obzirom na pozitivan utjecaj geotermalnoga gradijenta, više početne, statičke, temperature tla i pozitivnoga utjecaja žljebaste cijevi, onaj koji se pokazao kao optimalan izbor na lokaciji. Prinos svih testiranih bušotinskih izmjenjivača prema normi EN14511 i ulaznoj temperaturi 0 °C iznosi 18,8 kWt te 23,5 kWt pri vršnim uvjetima.

The main scope of this research was to develop and apply new methodology for energy prediction during a long-term period, based on the logarithmic relationship between electricity per capita and GDP per capita. This relationship was proven to be valid until 1989. It contains changeable energy and financial indicators and a dynamic rate of growth K, in the form of hyperbolic curves. With a range of slopes from 0.1 to 6, it enables a new economic classification of countries. Further research of this method, called IKoMet, has made the application of this relationship for future periods possible through the deduction of new energy and financial indicators. It was first applied to Italy, by the determination of indicators for a prediction after 1990. The results show that energy growth in the first period was more than ten times faster in comparison to the following period, while financial growth was almost twice as large in the first period. This procedure was also successfully applied for Switzerland, thus it can be concluded that IKoMet could generally be used for long-term energy prediction.

One of the most common geoexchange system in practice today is a vertical two-loop heat exchanger. There is also a growing trend of inclined drilling of multiple 40–50 m boreholes with coaxial heat exchangers, which are efficient when installing in areas where there is a favourable groundwater flow. Special care should be taken in hydraulic design of coaxial system, since laminar flow often occur in annular channel, due to glycol mixture implementation. In this research, vertical and inclined coaxial borehole exchangers were compared in identical geological environment, to express differences in heat extraction and thermal resistances, as well as in flow regimes and pressure drop. Extended Steady-State Thermal Response Step Test was implemented, which incorporate series of power steps to determine borehole extraction rate at steady-state heat transfer conditions. Results show that 2U-heat exchanger has advantages over coaxial heat exchangers, in terms of greater steady state heat extraction rate and more favourable hydraulic. Defined extraction rate at 0/-3 °C borehole temperature, in dominantly clay ground, was 43,5 W/m for coaxial system, compared to 54,5 W/m for vertical 2U-loop. Borehole resistance analysis showed almost double value for coaxial system compared to 2U-loop, due to frequent problems in cementation of inclined boreholes.

The aim of our research is to use abandoned deep-hydrocarbon reservoirs and dry wells in the Croatian part of the Pannonian Basin as a geothermal energy source. Croatia has been exploring and exploiting hydrocarbon reserves in the Pannonian Basin from the mid-20th century. Therefore, many oil and gas wells are reaching the end of their production phase and many are already abandoned. These wells could be considered for geothermal energy production through the coaxial heat exchanger principle, which is usually used in shallow geothermal energy extraction. Using the abandoned deep well Pčelić located in the Drava subbasin as a case study, we have derived the available energy and fluid temperature changes during 20 years of operation for two cases: one with a constant base heat load throughout the year and the second as a variable heat load depending on outside air temperatures. We determined that the maximum potential heat extraction in a variable system is 1750 MWh per year, with 1.5 MW of peak heating power in winter, depending on the sink temperature, climate, and consumer input data. The maximum theoretical constant heat extraction for possible industrial direct heating could be 400 kW during the entire period of 20 years, with fluid temperature reaching steady state at a favorable 50°C. To define steady-state ratio between extracted heat and consumed energy at the circulating pump, we evaluated seasonal performance factor (SPF) analysis similar to heat pump systems. Lower values of SPF linked to a higher flow rate implies higher energy extraction. Our results show that when using a lower flow, steady-state SPF ratio is as high as 280, and for a higher flow, steady-state SPF ratio drops to only 25.

Classic and extended step thermal response test were conducted on three different location in Zagreb. Measurements with the classical thermal response test were used to determine thermogeological properties of the ground and thermal resistance of the borehole for each location. Different values of thermal conductivity are result of differences in geological profile and depth of the sites. In addition, experimental research of steady-state thermal response step test (SSTRST) was carried out to determine heat rejection rates for passive and active cooling in steady state regime. Results showed that heat rejection rate is only between 8-11 W/m, which indicates that coaxial system is not suitable for passive cooling demands. Furthermore, the heat pump in passive cooling mode uses additional plate heat exchanger which causes additional temperature drop of the working fluid by approximately 1, 5 °C. Therefore, steady-state rejection rate for passive cooling is even lower for a real case project. Coaxial heat exchanger should always be designed for an active cooling regime with an operation of a heat pump compressor in a classical vapour compression refrigeration cycle.

Based on the statistical analysis of the time series of data, the influence of the change of Sava River temperature and geothermal anomalies on the changes of the groundwater temperature of the Zagreb and Samobor-Zapresic aquifers is described. In the analysis, data were used from daily measurements of the Sava River temperature and from the quarterly measurements of the groundwater temperature. Statistical methods of correlation, cross-correlation and linear regression were applied and the maximum, mean and minimum groundwater temperatures were analysed. Obtained results are presented in the form of statistical parameters, diagrams and maps of isotherms. This data is indispensable for development of shallow geothermal applications related to open loop groundwater heat pump systems. Since efficiency of the heat pump is directly dependent source temperature, presented analyses is necessary for prefeasibility study of geothermal projects and comparison between different designs. Furthermore, currently operating projects in the Zagreb and Samobor-Zapresic are systematically elaborated, giving first approximation of energy consummation from this renewable energy resource.

When considering implementation of shallow geothermal energy as a renewable source for heating and cooling of the building, special care should be taken in hydraulic design of borehole heat exchanger system. Laminar flow can occur in pipes due to usage of glycol mixture at low temperature or inadequate flow rate. This can lead to lower heat extraction and rejection rates of the exchanger because of higher thermal resistances. Furthermore, by increasing flow rate to achieve turbulent flow and satisfactory heat transfer rate can lead to increase the pressure drop of the system and oversizing of circulation pump which leads to impairment of seasonal coefficient of performance at the heat pump. Most frequently used borehole heat exchanger system in Europe is double-loop pipe system with smooth inner wall. Lately, development is focused on implementation of different configuration as well as with ribbed inner wall which ensures turbulent flow in the system, even at lower flow rates. At a location in Zagreb, classical and extended thermal response test was conducted on three different heat exchanger configurations in the same geological environment. With classic TRT test, thermogeological properties of the ground and thermal resistance of the borehole were determined for each smooth or turbulator pipe configuration. Extended Steady-State Thermal Response Step Test (TRST) was implemented, which incorporate series of power steps to determine borehole extraction rate at the define steady-state heat transfer conditions of 0/-3°C. Results show that heat exchangers with ribbed inner pipe wall have advantages over classic double-loop smooth pipe design, in terms of greater steady state heat extraction rate and more favorable hydraulic conditions.

As part of EU strategy to tackle climate changes, energy transition from fossil fuels to renewable resources is necessary. During the last few years, in the Republic of Croatia there is a strong tailwind towards promoting activities of geothermal energy exploration and exploitation. As geothermal sector was often neglected in a favour of petroleum business in the past, there is still a somewhat knowledge gap in groundwork related to defining geothermal resources potential. Since initial investigation related to defining hot spots in geothermal gradient were conducted more than 40 years ago, there was a need to remodel older imprecise maps with a completely new groundwork research in this area. More than 150 deep oil&gas exploration wells scattered in the Croatian part of the Pannonian Basin were elaborated. From the historical well development records and temperature measurements, obtained with different methods, a novel geothermal gradient map was constructed. Furthermore, to define local potential sites for direct heat utilization and electricity production, temperature maps were designed for a depth of 1000 and 2000 m, respectively.

The theory of Thermal Response Testing (TRT) is a well-known part of the sizing process of the geothermal exchange system. Multiple parameters influence the accuracy of effective ground thermal conductivity measurement; like testing time, variable power, climate interferences, groundwater effect, etc. To improve the accuracy of the TRT, we introduced a procedure to additionally analyze falloff temperature decline after the power test. The method is based on a premise of analogy between TRT and petroleum well testing, since the origin of both procedures lies in the diffusivity equation with solutions for heat conduction or pressure analysis during radial flow. Applying pressure build-up test interpretation techniques to borehole heat exchanger testing, greater accuracy could be achieved since ground conductivity could be obtained from this period. Analysis was conducted on a coaxial exchanger with five different power steps, and with both direct and reverse flow regimes. Each test was set with 96 h of classical TRT, followed by 96 h of temperature decline, making for almost 2000 h of cumulative borehole testing. Results showed that the ground conductivity value could vary by as much as 25%, depending on test time, seasonal period and power fluctuations, while the thermal conductivity obtained from the falloff period provided more stable values, with only a 10% value variation.

The utilization of shallow geothermal energy is well known in several European countries. Even though large parts of the Republic of Croatia show signifi cant potential for its use, the installation of ground source heat pump systems (heat pump and heat exchangers) is slowly progressing. Therefore, a short overview of research published thus far concerning the utilization and assessments of shallow geothermal potential for Croatia was done. In Croatia, there is no agency or government department in charge of collecting and publishing data concerning installations of heat exchangers. Therefore, a study was done to collect the available data on installed closed-loop heat exchangers either from scientifi c research or from personal contacts with drilling companies. Based on the collected data, a map was produced that shows general locations of installed heat exchangers. From the obtained data of installed heat exchanger length, a fi rst assessment of utilizing shallow geothermal potential in Croatia was given.

Quality cementation of a borehole heat exchanger is often considered to be important parameter when analysing long-term heat pump efficiency. Implementing low conductivity grout leads to an increase of borehole thermal resistance and poor heat transfer. This paper analyses one real retrofit project which comprises of 16 borehole heat exchangers with thermal enhanced grout completion. Novel approach of testing the borehole heat exchanger was implemented, so called Steady-State Thermal Response Step Testing, which incorporate series of power steps to determine borehole capacity at steady-state heat transfer conditions and thermogeological properties of ground. Paper objective was to quantify what is real benefit by implementing more expensive thermal enhanced grout, compared to traditional bentonite, silica-sand grouts. Simulation of borehole temperatures and coefficient of performance (COP) changes over time was carried out with Ground Loop Design (GLD), taking into account four different grout mixes. Results shown that in thermogeological environment with mediocre thermal conductivity, distinction in COP was modest for each of the four analysed grouting material during 30 years. Comparing investment costs with savings in electricity for each grout mix case, conclusion is that there is no real benefit of implementing enhanced grout in mediocre thermal conductivity environment such as marly-clays.

Properly sized borehole heat exchanger in geothermal heat pump systems (widely known also as ground source heat pump systems) needs to minimize long-term ground and working fluid temperature changes. These changes occurs due to imbalances of heat extracted from the ground during winter and heat rejected into the ground during summer months, as well as thermal interferences of adjacent boreholes in borehole array. Simple calculations and spreadsheet-based analogical solutions of required borehole length for heat transfer run into difficulties when dealing with such large, complex ground source heat pump heating and cooling systems which use compact borehole array. A number of analytical computer programs are available to simulate how ground loop fluid temperature varies in such complex systems, using so-called ‘g-function’ – a mathematical function dependent on the geometry and shape of the borehole array. The type of calculation involves a type of ‘step-function’, where 24 h ‘step’ of peak loading is superimposed on a top of a long-term base load. In analytical simulation models, the base load is typically specified as the heating and cooling load per month of a typical year, and is simulated as the combination of sequential monthly steps. This paper will show, by simulating long-term operation of complex geothermal heat pump system with multiple boreholes in various geometric arrays, how spacing of adjacent boreholes and thermal interferences influence required borehole length for heat transfer

Undisturbed ground temperature is one of the most crucial thermogeological parameters needed for shallow geothermal resources assessment. Energy considered to be geothermal is energy stored in the ground at depths where solar radiation has no effect. At depth where undisturbed ground temperature occurs there is no influence of seasonal variations in air temperature from surface. Exact temperature value, and depth where it occurs, is functionally dependent on surface climate parameters and thermogeologic properties of ground. After abovementioned depth, increase of ground temperature is solely dependent on geothermal gradient. Accurately determined values of undisturbed ground temperature, and depth of occurrence, are beneficial for proper sizing of borehole heat exchangers and ground source heat pump system as a whole. On practical example of building being heated and cooled with shallow geothermal resource, via heat pump system, influence of undisturbed ground temperature and geothermal gradient on size of borehole heat exchanger is going to be presented. Sizing of borehole heat exchanger was calculated with commercial software Ground Loop Designer (GLD), which uses modified line source and cylinder source solutions of heat conduction in solids.

One of the most important issues when estimating CO 2 storage capacity, especially in the case of the storage into deep saline aquifers, is the mechanical integrity of rock, i.e. estimate of cap rock fracture pressure. In the case of storage into mature oil and gas reservoirs, reservoir pressure should not present an issue since it was significantly decreased due to hydrocarbon production, so it is reasonable to assume that the rock integrity would not be disturbed by injecting CO 2 to the initial reservoir pressure. Because estimates of fracture pressure are necessary, but not convenient for regional aquifers, the analysis of pore volume changes due to pressure buildup for the chosen CO 2 site in Croatia has been made. Pressure buildup depends on reservoir fluid (brine) compressibility, CO 2 compressibility at given reservoir conditions before CO 2 injection and on rock compressibility i.e. pore compressibility. Implementing the simple method for analysis of pressure buildup considering elastic properties of fluids and rock, larger CO 2 storage capacity estimate was achieved than by using storage efficiency coefficient ( E ), as defined by U.S. DOE [1] and which was used for estimating storage capacities in regional deep saline aquifers in Croatia as the part of EU GeoCapacity project [2] .

In Croatia there is an urgent need to improve energy balance with more efficient methods of energy resources exploitation. There are approximately 3, 000 wells drilled on oil, gas/condensate and geothermal reservoirs, of which only 50 were primarily drilled for purpose of geothermal exploitation. Over 10, 000 seismic measurements were conducted. The most of drilled wells are scheduled for permanent abandonment in the near future, because of economical reasons and technological limitations of production. However, oil and gas wells which are near the end of production might be reassigned into geothermal wells, if aquifer water temperature and flow were satisfactory.

In last 10 years there was significant rise in transportation fuel consumption in Europe from 180 Mt in 1985. to 270 Mt in 2004., with gasoline representing 40% and diesel 60%. To decrease dependence upon imported fossil fuels, EU aim is to introduce biodiesel in share of 5.75% in transportation sector until 2010. and finally 8% until 2020. Because of limited production potential from some EU countries, today and in near future, large quotas of import would be required. Biodiesel is often called clean, ecological and renewable alternative fuel, but with present land-intensive methods of production it could easily be named as one of the most dangerous sources of energy for Earth’s ecosystem. Main threat from large scale biofuels utilization comes from deforestation of land that is needed for cultivation of crops. Every year large areas of rainforests in South East Asia and South America are irretrievably lost due to increasing demand. Combustion of wood and oxidation of peat during drying emits enormous quantities of CO2 into the atmosphere which is contrary to biodiesel appellation as “CO2 balanced fuel”. Unlike conventional crops that are used for production of biodiesel (rapeseed, soybean, palm etc.), possible production from algae would significantly lower unit of land needed for biofuel production. Contemporary researches give some estimation of about 25 times greater yield than palm plantation and 100 times over rapeseed, which is common biodiesel production crop in EU. Regarding lately high world oil prices, greater investment in researches upon algae, as a new source of biofuel, are bringing technological solutions for economically production start-up.

Higher costs of crude oil on world market, as well as awareness about negative environmental impact related to conventional diesel fuel utilization, are creating favourable economic conditions for production expansion and greater application of alternative fuels. Natural gas could be converted through Fischer-Tropsch synthesis technology process, dating from the beginning of the last century, to alternative form of fuel named Gas to Liquid (GTL) which can be utilized in all recent diesel engines. GTL has lower emissions in comparison to fossil diesel and it is characterized with higher cetane number which is measurement of the fuel ignition quality. Furthermore, additional advantage is that the existing infrastructure for fuel transportation and distribution could be used without any significant changes, either for pure GTL or blend with diesel. In compliance with present and future EU directives, concerning greenhouse gases reduction and decrement of imported crude oil dependence, Gas to Liquid technology imposes as one of the alternative fuels for utilization in transportation sector.

Geotermalno ležište Rečica otkriveno je dvjema bušotinama: Ka-2 i Ka-3. Na temelju dubinskih mjerenja temperature u navedenim bušotinama utvrđena je ležišna temperatura od 140 °C. Predviđeno je da bušotina Ka-3 bude proizvodna, a bušotina Ka-2 utisna. Za crpljenje geotermalne vode iz bušotine Ka-3 predviđen je proizvodni interval na dubini od 1 900 do 3 523 m, dok je za utiskivanje ohlađene vode u bušotinu Ka-2 predviđen interval na dubini od 3200 do 4 145 m. Toplinska energija iskorištavala bi se stupnjevito, Clausius-Rankinovim procesom. S obzirom na ukupan stupanj djelovanja geotermalnog izvora, snagu treba izraziti kao tehničku radnu moć ili eksergiju, pri čemu se stanje radnog medija povratnim načinom dovodi u mehaničku i toplinsku ravnotežu s okolinom. Kako bi se osigurala toplinska obnovljivost geotermalne vode i održao ležišni tlak, ohlađena voda utiskivat će se natrag u ležište. Ekonomskom analizom procijenjena su ukupna investicijska ulaganja potrebna za razvoj geotermalne elektrane, te ekonomski pokazatelji isplativosti geotermalnog projekta. Proizvodnja električne energije na lokalitetu Rečica bila bi ekonomski opravdana principom kaskadnog korištenja toplinske energije i visokog stupnja godišnjeg korištenja., Geothermal reservoir Rečica was discovered with two wells: Ka-2 and Ka-3. Based on the results of investigations from abovementioned wells, the reservoir temperature of 140 Celsius degrees was determined. Ka-3 would be production well and Ka-2 injection well. Depth interval from 1 900 to 3 523 meters is designed for hot water production in Ka-3 well and depth interval from 3 200 to 4 145 meters is designed for injection of chilled water in Ka-2 well. Gradual utilization of reservoir heat energy can be planned by, applying Clausius-Rankin process. In view of total efficiency factor, energy output of geothermal source should be expressed as maximum useful work or exergy where reversible processes lead working fluid into mechanical and thermal environment conditions equilibrium. Chilled geothermal water would be returned through injection well to the same reservoir what assures heating reproducibility and pressure maintenance in the reservoir. Economic analysis estimated total investment costs needed for development of geothermal power plant, and economic indicators of profitability of geothermal project. The production of electricity from Rečica reservoir would be economically viable with cascade principle of heat energy utilization and a high annual degree of utilization.