European Geologist Journal 59

Geothermal Heating for Urban CO₂ Mitigation: Case Studies from the Croatian Part of the Pannonian Basin

by Mario Poleto¹, Morana Petrić¹, Ivan Marković¹

¹IVICOM Consulting Ltd., Damira Tomljanovića Gavrana 11, Zagreb, Croatia

Contact: mariopoleto01@gmail.com

Abstract

Krapina-Zagorje County and the City of Sveta Nedelja have submitted projects for the financing of geothermal energy applications, and completed preparation of the necessary project documentation. For both projects in question, the carbon footprint of potential technical geothermal solutions was calculated and compared to energy-equivalent natural gas heating. Using the same principle, an assessment of the reduction of greenhouse gas emissions using the geothermal potential of the Croatian part of the Pannonian basin was carried out. The exploitation of geothermal water in the Croatian part of the Pannonian Basin and its use contributes to achieving the goal of climate neutrality set by the Low Carbon Development Strategy of the Republic of Croatia until 2030, with an outlook to 2050.

Cite as: Poleto, M., Petrić, M., & Marković, I. (2025). Geothermal Heating for Urban CO₂ Mitigation: Case Studies from the Croatian Part of the Pannonian Basin. European Geologist, 59. https://doi.org/10.5281/zenodo.16443016

This work is licensed under a Creative Commons Attribution 4.0 International License.

1. Introduction

Energy production from geothermal sources does not result in CO₂ emissions, as does the process of energy production from fossil fuels—coal, gas, or oil. Therefore, geothermal energy does not contribute to an increase in global temperature and climate change, which are the main threats to the way of life on Earth as we know it today. Geothermal plants are responsible for noticeable CO₂ emissions only during their construction and represent a viable alternative to fossil fuels. CO₂, which, depending on the chemistry of the geothermal water, may be generated during production, is often returned to the reservoirs to improve the exploitation of geothermal water.

Krapina-Zagorje County, as well as the City of Sveta Nedelja, responded to the invitation of the Ministry of Regional Development and European Union Funds in the framework of the “Energy and Climate Change” Programme, under the title “Development of Technical Documentation for the Use of Geothermal Energy”. The programme is financed by the Financial Mechanism of the European Economic Area for 2014-2021 in the Republic of Croatia (Agreement between the European Union, Iceland, the Principality of Liechtenstein, and the Kingdom of Norway). The projects “HypoKraT” (Hydrothermal potential of Krapinske Toplice) and “Development of Documentation during the Phase of Geothermal Water Research in the Area of Sveta Nedelja” were submitted.

In the territory of the Municipality of Krapinske Toplice, there is an existing borehole, Krapinske Toplice-1 (KRT-1), for which a Conceptual Project, a Feasibility Study, and a Hydrodynamic Model of the reservoir for the use of geothermal energy were prepared. In the area of the town of Sveta Nedelja, based on the confirmed geothermal potential of the Nedjelja-1 (N1) well, a new exploratory geothermal well, Sveta Nedelja GT2 (SNGT-2), was planned and located within the city limits.

2. Project Holders and Partner Institutions

The holder of the HyPoKraT project is Krapina-Zagorje County, while the project partners are the Regional Energy and Climate Agency of Northwest Croatia (REGEA), the Norwegian Research Center (NORCE), AP Consulting Ltd., and the Municipality of Krapinske Toplice. As part of the HyPoKraT project, a Base Design for the use of geothermal energy for the heating needs of industrial, public, and medical-tourist-related infrastructure and facilities in Krapinske Toplice [1], a Feasibility Study [2] and Reservoir Simulation [3] were made, on the basis of which the Environmental Protection Report on the Exploitation of Geothermal Water at the KRT-1 Well [4] was prepared.

The project “Development of documentation during the geothermal water research phase in the area of Sveta Nedelja” is managed by the City of Sveta Nedelja through the city-owned company Svenkom Ltd., while the project partners are EKOPLODOVI Ltd. and EFLA Consulting Engineers. During the investigation period, a permit was issued for the exploration of geothermal water in the Exploration Area SVETA NEDELJA 2, existing geological data were collected, and the magnetotelluric data were recorded in 2024 by Geoda Consulting Ltd. The Base Design for the construction of an exploratory geothermal well Sveta Nedelja GT-2 (SNGT-2) [5] was also prepared, based on which the Environmental Protection Report was prepared [6] and submitted to the competent Ministry for the purpose of conducting the assessment procedure on the need for an environmental impact assessment for the project in question. In the relevant procedure, the competent Ministry concluded in August of 2024 that the planned project of constructing the SNGT-2 exploratory geothermal well does not require an environmental impact assessment procedure, meaning that the project is environmentally acceptable.

3. Materials and Methods

The introduction of a renewable source of energy in the heating industry replaces the need for fossil fuels for the heating of buildings; therefore, for both projects referred to above, the reduction of greenhouse gas emissions was estimated by comparing the carbon footprint of potential technical solutions using geothermal energy and energy-equivalent natural gas heating. For the area of the Croatian part of the Pannonian Basin, the potential to reduce greenhouse gases was assessed by comparing the emissions of conventional plants for the production of electricity and thermal energy.

The carbon footprint calculation methodology is based on the instructions of the European Commission publication “Technical guidelines for the preparation of infrastructure for climate change in the period 2021-2027”. All CO_2eq emission factors for calculation purposes are taken from the Database of National Factors of Emissions and Removal of Greenhouse Gases [7], Energy in Croatia – Annual Review for 2022 [8], and Project Carbon Footprint Methodologies – Methodologies for the assessment of project greenhouse gas emissions and emission variations [9] in accordance with the Technical guidance on the climate proofing of infrastructure in the period 2021-2027 of European Commission [10].

Greenhouse gas emissions are present during the exploration phase of the geothermal reservoir, the drilling of exploration and/or exploitation wells, the drilling of injection wells, and the construction of the hot water network. After the commissioning of the geothermal well for the purpose of heating public buildings, CO_2eq emissions are present only in the form of indirect emissions from the consumption of electricity for the operation of mechanical equipment. If electricity produced from renewable sources is used, these emissions can be eliminated.

4. Municipality of Krapinske Toplice

In the area of Krapinske Toplice, there is an existing geothermal well, KRT-1 (46°05’47.1″N 15°50’19.2″E), drilled in 1986. During its drilling, the well was confirmed to have a positive yield and sealed for potential future use. After the overhaul and testing of the KRT-1 well, the exploitation of geothermal water is planned with the aim of using it for energy purposes as an alternative heat source to meet the consumption of thermal energy in social facilities in the area of the Krapinske Toplice settlement:

Special hospital for medical rehabilitation Krapinske Toplice;
Kindergarten Maslačak;
Elementary school Krapinske Toplice;
Krapinske Toplice municipality building;
Church of the Holy Trinity.

Available data on the temperature and flow of geothermal water from the KRT-1 well suggest a power output of 3,547.6 kWh, accounting for losses in the distribution network. The feasibility study [2] concluded that the total annual natural gas consumption for all considered facilities is 3,127.27 MWh and that the power of the KRT-1 well is sufficient to completely replace natural gas with geothermal energy. Considering the annual reduction in natural gas consumption at the evaluated social facilities, and applying the emission factor method, the annual estimate of CO_2eq emission reduction resulting from the transition from natural gas to geothermal energy is shown in Table 1.

Table 1: Estimate of greenhouse gas emissions in the current state and after the introduction of geothermal energy as a substitute for natural gas for HyPoKraT project.

	Current state			Introducing the use of geothermal energy	Future state
	Total energy consumption for heating [MWh/year] *	of which electricity [MWh/year]	of which natural gas [MWh/year]		Electricity consumption [MWh/year]	Consumption of geothermal energy [MWh/year]
Hospital	3,795.00	1,138.50	2,656.5		379.50	3,415.50
Hospital	3,795.00	30%	70%		10%	90%
Kindergarten	121.77	0.00	121.77		0.00	121.77
Kindergarten	121.77	0%	100%		0%	100%
School	286.62	0.00	286.62		0.00	286.62
School	286.62	0%	100%		0%	100%
Municipality	21.34	0.00	21.34		0.00	21.34
Municipality	21.34	0%	100%		0%	100%
Church	41.04	0.00	41.04		0.00	41.04
Church	41.04	0%	100%		0%	100%
KRT-1	–		–		392.45	–
In total	4,265.77	1,138.50	3,127.27		771.95	3,886.27
Emission of CO₂ [t] **	–	267.33	688.62		181.26	–
Total [t]		993.17			181.26
* Real consumption data in 2022, data from project holder
** Emission factors: Natural gas consumption (stationary) – 232.1 kg CO_2eq/MWh [7]; Specific CO_2eq emission factor per total electricity consumption in Croatia in 2022 – 234.81 kg CO_2eq/MWh [8].

The calculations indicate that the implementation of the project would reduce greenhouse gas emissions by 811.91 t CO_2eq annually in the area of Krapinske Toplice.

5. City of Sveta Nedelja

The SNGT-2 exploratory well is planned to be drilled in the undeveloped area of the Mala Gorica settlement, within the administrative boundaries of the City of Sveta Nedelja, near the Sports and Recreation Center adjacent to the Kerestinec Elementary School, the existing undeveloped industrial zone planned for construction and expansion, and the Kerestinec Sports and Recreation Center [5]. The referenced buildings can potentially be supplied with thermal geothermal energy.

As previously stated, greenhouse gas emissions will occur during the exploration and drilling phase of the SNGT-2 geothermal exploration/exploitation well. During the construction of the drilling work area (hereinafter referred to as the DWA), trucks and construction machinery will be used, resulting in greenhouse gas emissions (such as carbon dioxide, nitrogen oxides, and sulfur dioxide). During the construction of the exploration/exploitation well, a diesel-powered drilling rig will be used, which also results in greenhouse gas emissions. For the purposes of this paper, these emissions will be referred to as upstream emissions.

The construction of the DWA is estimated to take 25 days, operating in two shifts with an effective working time of 10 hours per day. The construction is expected to require two backhoe excavators, two trucks with a capacity greater than 16 t, a loader, a bulldozer, and a roller. In order to estimate the emission of greenhouse gases, the worst-case scenario, assuming all aforementioned machines operate simultaneously throughout the construction of the DWA, was taken into account. The calculations and estimated greenhouse gas emissions associated with the construction of the DWA are presented in Table 2.

Table 2: Calculation of estimated upstream greenhouse gas emissions for the planned SNGT-2 geothermal well.

Machinery	Fuel consumption [l/h] *	Estimated working time [h]	Total fuel consumption during DWA construction and test drilling [l]	Emission factor of diesel consumption	Estimated CO_2eq emissions
Machinery	Fuel consumption [l/h] *	Estimated working time [h]		[kg CO_2eq/l] **	[t CO_2eq/ duration of works]
Bulldozer	33.16	250	8,290.00	2.7	22.38
Backhoe loader 2x	14.59	450	6,565.50	2.7	17.73
Loader	21.96	250	5,490.00	2.7	14.82
Roller	8.49	200	1,698.00	2.7	4.58
Trucks 2x	18.78	450	8,451.00	2.7	22.82
Drilling rig	104	720	74,880.00	2.7	202.18
Total estimated CO_2eq upstream emissions [t]					284.51
* Klanfar et al. [11], except for the drilling rig, as stated by the developer of the well Base Design.
** Table A1.1: Liquid fossil fuels – diesel oil usage, Project carbon footprint methodologies [9].

The calculated upstream emissions in this paper were increased by 100% to account for the emissions generated during exploration activities (seismic surveys, etc.) that precede the drilling of the exploration well and the subsequent construction of the hot water network. These additional emissions are estimated at around 570 t CO_2eq.

The Base Design of the SNGT-2 exploratory well [5], based on the geological investigation, estimated that the geothermal reservoir was classified as a low-to medium-temperature aquifer. In addition, the exploitation of geothermal water from well SNGT-1 with a flow rate of at a temperature of 87 °C was evaluated. After the thermal energy is utilised, the temperature of water drops to 40 °C, which makes the temperature difference ΔT = 47 °C. Based on the input parameters, the theoretical gross power () of the circulating fluid flow at the well SNGT-2 was estimated:

– the specific heat capacity of water [kJ/kgK]

(1)

Taking into account distribution losses estimated at 5%, and the use of geothermal heating from September to May (8 months per year), the annual production of heat energy () can be estimated as follows:

(2)

Based on the estimated annual thermal energy production of the SNGT-2 well, and assuming a positive outcome of well testing that allows it to replace the use of natural gas for heating purposes, the annual CO_2eq emission reduction is estimated at approximately 6,332 t CO_2eq. This estimate considers the emission factor for natural gas (232.1 kg CO_2eq/MWh) and assumes the use of “green” electricity to power the mechanical equipment of the heating circuit.

6. Geothermal Potential of the Croatian Part of the Pannonian Basin

The continental part of the Republic of Croatia (roughly from the city of Karlovac to the city of Ilok) is part of the Pannonian Basin system, bounded by the Alps, the Carpathians, and the Dinarides. Its structure is shaped by geotectonic movements throughout geological history, which controlled the depositional and erosional processes that led to its present-day geomorphological forms. The prevailing model for the origin and evolution of the Croatian part of the Pannonian Basin is based on the lithospheric . The complex geotectonic history of the region resulted in a thinned lithosphere, approximately 30 km thick across continental Croatia, creating favorable conditions for the formation of oil, gas, and geothermal deposits. The thinned lithosphere, combined with intense volcanism documented in boreholes and outcrops along the edges of the Slavonian, Moslavina, Zagreb, and Zagorje mountains, has contributed to an elevated geothermal gradient in the Croatian part of the Pannonian Basin, particularly in the Sava and Drava depressions, ranging from 2.29 to 6.23 °C/100 m.

The Croatian part of the Pannonian Basin is rich in deposits of geothermal water at lower temperatures (50-220 °C), suitable for heating and for the construction of binary geothermal power plants. In these plants, steam to drive turbines is generated indirectly by heating a working fluid with a boiling point lower than that of water. This type of geothermal power plant was built at Velika Ciglena, Croatia, with a capacity of about 15 MW_e.

In the document Analysis and basis for the development of the Energy Development Strategy of the Republic of Croatia – GREEN BOOK [12], the minimal geothermal potential of the Pannonian basin in the Republic of Croatia was estimated, depending on thermal utilisation, . At the time of writing this paper, project documentation is being prepared for at least three geothermal power plants in continental Croatia, whose projected net installed capacity already exceeds the upper estimate of 67.6 MWe. Therefore, the data in question in this paper will be regarded as the minimum geothermal potential of the Croatian part of the Pannonian Basin. Assuming an average annual operation of 8,640 working for electricity production and 5,840 working hours for heat production, the minimal potential for reducing CO_2eq emissions in the Croatian part of the Pannonian Basin by implementing geothermal energy in the production of heat and electricity is given in Table 3.

Table 3: Estimate of greenhouse gas emission mitigation potential of the Croatian part of Pannonian Basin.

	Geothermal potential		Yearly energy production		Greenhouse emissions reduction [t CO_2eq/annually]
	Electricity [kWh]	Heating [MWh]	Electricity [kWh]	Heating [MWh]	Electricity*	Heating**
Croatian Pannonian Basin	56,500	456	488,160,000	2,663,040	85,428	618,092
				TOTAL:	703,520
*Specific CO₂ emission factor for total electricity production in Croatia in 2022 – 0.175 kg CO_2eq/kWh [8]
**Specific CO₂ emission factor for natural gas consumption – 232.1 kg CO_2eq/MWh [7]

If the minimum geothermal potential of the Croatian part of the Pannonian Basin were to be used, the calculations indicate that it could reduce greenhouse gas emissions by 703,520 tons annually.

7. Conclusion

As a renewable source of energy without CO₂ emissions, the impact of geothermal plants on the environment is significantly lower than that of other energy production technologies. Although greenhouse gas emissions are unavoidable during the construction and testing phases of geothermal wells, for the SNGT-2 geothermal well, these emissions represent approximately 11% of the annual CO_2eqemissions associated with natural gas usage.

The Hrvoje Požar Energy Institute, in its annual report on energy in the Republic of Croatia for 2022 [8], states that 3,498.96 TJ of electricity and heat energy was produced in the same year. Therefore it can be concluded that the geothermal potential of the Pannonian Basin alone is not sufficient for the energy needs of the Republic of Croatia. Nevertheless, this paper demonstrates that the estimated potential of the Pannonian Basin to reduce greenhouse gas emissions in the production of electricity and thermal energy is highly significant.

This type of energy, apart from the need for a geothermal reservoir, requires only electricity, the availability of which is widely available in Europe today, and can also be produced with renewable (‘green’) sources. Geothermal energy is, in every sense, a renewable source of energy, and compared to wind and solar energy, it leads the way in terms of constant energy availability, as its production does not depend on meteorological conditions, seasonal variations, and environmental impact. Its disadvantage is the high initial investment in research, accompanied by certain risks during the exploration phase. However, in the case of a positive outcome of geological exploration, a substantial return of investment is possible over a relatively short timeframe.

To mitigate climate change by implementing a geothermal energy source, the principle of sustainable energy production is applicable, i.e., increasing the economic value while simultaneously reducing the consumption of energy and natural resources with negligible greenhouse gas emissions in the exploration phase of geothermal projects. Additionally, due to the low risk of climate change impacts on geothermal plants, it is unnecessary to implement any climate change adaptation measures. Exploitation of geothermal water in the Croatian part of the Pannonian Basin and its use for heating purposes and electricity production contributes to the achievement of the goal of climate neutrality set by the Low Carbon Development Strategy of the Republic of Croatia by 2030, with the aim of achieving neutrality by 2050 (Official Gazette 63/21), which includes a gradual reduction of emissions until 2030.

The Exploration Area is an officially approved zone, delineated by defined geographic coordinates and designated under the name SVETA NEDELJA 2 (the numeral “2” indicating differentiation from another area labeled “1”). This area is covered by all necessary permits and documentation, and the competent National Authority has granted permission for the execution of predefined exploration activities within its boundaries.

References

AP Consulting Ltd., 2023; Base Design for the use of geothermal energy for heating needs of industrial, public and medical-tourist buildings and facilities of the Municipality of Krapinske Toplice, Varaždin, May 2023
IVICOM Consulting Ltd., 2023; Feasibility study and CBA analysis – Hydrothermal potential of Krapinske Toplice (HyPoKraT), Zagreb, Croatia, December 2023.
Norwegian Research Center – NORCE, 2023; Status Report HypoKraT project, Bergen, November 2023.
IVICOM Consulting Ltd. 2023; Environmental protection report for the exploitation of geothermal water for energy needs as part of the HyPoKraT project, Krapina-Zagorje County, Zagreb, December 2023.
Geoda Consulting Ltd., 2024; Base Design for the construction of an exploration geothermal borehole Sveta Nedelja GT-2 (SNGT-2) with drill working area for accommodation of drilling rig on EA Sveta Nedelja 2, Zagreb, Croatia, March 2024
IVICOM Consulting Ltd., 2024: Environmental protection report for drilling of the exploratory geothermal well Sveta Nedelja GT-2 (SNGT-2) in the research area Sveta Nedelja 2, Zagreb County, Zagreb, Croatia, March 2023
Croatian Ministry of Economy, 2024; Database of national factors of emissions and removal of greenhouse gases, https://mingo.gov.hr/o-ministarstvu-1065/djelokrug/uprava-za-klimatske-aktivnosti-1879/ugljicni-otisak/8960 (accessed on 5 December 2024).
Energy Institute Hrvoje Požar – EIHP, 2023; Energy in Croatia – Annual review for 2022, Ministry of Economy and Sustainable development, Zagreb, Croatia
European Investment Bank – EIB, 2023; Project Carbon Footprint Methodologies – Methodologies for the assessment of project greenhouse gas emissions and emission variations, Version 11.3, Luxembourg, January 2023
European Commission. Technical guidance on the climate proofing of infrastructure in the period 2021-2027, 2021/C 373/01. Available online: https://ec.europa.eu/newsroom/cipr/items/722278/en (accessed on 5 December 2024)
Klanfar, M., Korman, T., & Kujundžić, T., 2016; Fuel consumption and engine load factors of equipment in quarrying of crushed stone. Tehnički vjesnik, 23(1), 163-169., Zagreb, Croatia, DOI: 10.17559/TV-20141027115647
Energy Institute Hrvoje Požar – EIHP, 2019; Analysis and basis for the development of the Energy Development Strategy of the Republic of Croatia – GREEN BOOK, Zagreb, UG-18-00098/1

This article has been published in European Geologist Journal 59 – UN Sustainable Development Goals – where the geology lies

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European Geologist Journal 59

Geothermal Heating for Urban CO₂ Mitigation: Case Studies from the Croatian Part of the Pannonian Basin

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