Geothermal energy 


Geothermal energy is ubiquitous renewable source of energy, which can be exploited using thermal water or geothermal heat pumps. In addition to the energy utilization, thermal water can also be used as a natural remedy in health and spa resorts. Geothermal energy is the subject of geothermal research.


The GeoZS team of experts in geothermics combines specialists in the fields of hydrogeology, regional geology, hydrogeochemistry, geothermal measurements and numerical modeling. The team performs basic geothermal research, manages a database and energy balance of geothermal energy in Slovenia, and plans sustainable use of geothermal energy in the context of numerous national and European projects. Besides, the team also contributes to highly applied projects for implementation and monitoring of geothermal wells and boreholes for geothermal heat pumps. Sizing of borehole heat exchanger fields for exploitation of shallow geothermal energy is also their expertise.

  • Supplying thermal and thermomineral water

Heat production from thermal water and its spa and recreational use have a long tradition in Slovenia. The use of thermal water with many dissolved substances, organic compounds, or free gases, such as CO2, is usually technically very demanding, yet the specific chemical composition is of importance for defining its healing effects. Planning the use of such "special" types of groundwater is a major challenge of the GeoZS team due to growing number of users and rising need to ensure groundwater with a specific physical-chemical composition.

GeoZS provides professional and technical support in the following activities:

  • Groundwater research
    • Elaboration of hydrogeological expert bases for the location and construction of wells,
    • Setting up research wells, monitoring of geological and hydrogeological changes during drilling,
    • Implementation of pumping and injection tests,
    • Logging measurements in boreholes (Robertson Geologging equipment with 1000 m long cable,,
    • Hydrogeochemical research (origin, quality and retention time of groundwater, gases in water),
    • Determination of water usability (suitability for different categories of use),
    • Report of these studies with hydrogeological and geothermal interpretation.
  • Acquisition of a water permit for using open-loop geothermal heat pumps
    • Elaboration of hydrogeological reports to obtain a water permit.
  • Obtaining a concession for using thermal water or geothermal energy source:
    • Elaboration of hydrogeological report for the concession permit,
    • Designing a monitoring plan of geothermal wells,
    • Establishment and implementation of operational monitoring of geothermal wells (abstracted quantity, groundwater level, temperature and electrical conductivity of water),
    • Elaboration of annual monitoring reports on the state of wells and aquifers.
  • Expert studies, expertise, opinions
    • Determining potential for the exploitation of thermal, thermomineral and natural mineral water
    • Comparative analysis of the efficiency of thermal water use (benchmarking analysis)
    • 2D and 3D numerical modeling of groundwater flow and heat transfer for the designing of wells and optimization of exploitation
    • Definition of water protection areas
    • Assessment of technical and hydraulic conditions of wells.

  • Geothermal research (geothermal potential)

Geothermal potential is estimated on the basis of a heat-flow density map and maps of temperature at various depths below the ground, which we produce at GeoZS. Geothermal maps can be produced for any depth, which also applies to prognostic temperature profiles at any location in Slovenia. Temperature maps to a depth of 250 m are primarily used to assess the potential for geothermal heat pumps, while temperature maps at depths between 1000 and 3000 m are most convenient for evaluation of thermal water potential. The potential of geothermal electricity can be evaluated from temperature maps at depths between 4000 and 5000 m.

These maps are produced based on the measurements of thermal properties of soil (sediments and rocks) encountered in boreholes. Thermal conductivity measurements of rocks and other poorly conductive materials (for example, building materials) is carried out with the new TCS meter, which operates on the principle of scanning the surface of a sample with a focused, flexible and continuous heat source in combination with the infrared temperature sensors. The measurement range is from 0.2 to 25 W / (m.K). Loose (unbound) and crumbly soil is measured with MTP-1 and MTP-4 meters, which operate according to unsteady hot wire method. Their measuring range is from 0.5 to 10 W / (m.K).

  • Geothermal heat pumps - GHPs (design, operation, monitoring)

Services of planning shallow geothermal energy systems (analysis and evaluation of local geothermal parameters, feasibility studies – verification of technical and legal constraints, risks and comparison of possible variants, simulation and design of systems – EED, FEFLOW as a basis for detailed planning and support at decision-making, monitoring and follow-up the effectiveness for the purpose of optimization).

When we have simultaneous requests for domestic hot water, heating and cooling of a building, the GHP is a unique technology that covers these needs. What type of GHP will be chosen depends on heating and cooling needs, geological and hydrogeological characteristics at the GHP locality, and the legal system. Geological situation is that part of the GHP plan, which the planner cannot change and has to adapt all solutions to natural conditions which need to be thoroughly familiar to him. These conditions include:

  • Type of rock and its hardness (for drilling)
  • Ground thermal characteristics (for operation of GHP)
  • Groundwater situation (for drilling and operation of GHP).

When deciding on a source of energy for heating and cooling, it is necessary to know the Local energy concept (LEC) of a municipality and to consult with sectoral energy advisor (ENSVET). A good plan must take into account the entire system by adjusting the components to be able to reach most efficient operation and maximum comfort. It is reasonable that operation of GHP is observed with the appropriate monitoring system.

  • Analysis of local geothermal conditions

By analyzing local geothermal conditions at an early stage of planning, we provide information about general geological conditions for the GHP. Existing data are obtained and used for this purpose, and providing an input for the design and evaluation of investment with optimal costs. For larger and more complex systems, a preliminary investigation is accomplished using not only the existing data but also supplementary field investigations. Based on these findings, a pre-investment analysis and feasibility study is carried out in the next step.

  • Feasibility studies / economics

A feasibility study is accomplished on the basis of: identified local geothermal conditions, assessment of monthly energy demand for heating and cooling, duration of the peak daily needs in individual months, and monthly surplus heat due to possibility of heat storage. In the case of a favorable performance, the investment is also done. The analysis includes a comparison of different possibilities of optimizing costs and benefits. Both, technical and legal constraints are included. A model by the Swiss standard SIA 480 is used for calculation of economic indicators and internal rate of return of the investment.

  • Simulation / design

Professional design and simulation in the field of shallow geothermal energy (for proper configuration of borehole heat exchangers (BHEs) and wells) provide important groundwork for construction of BHEs and decision support systems. Simulations of BHEs are carried out throughout the life of the system. We use software Earth Energy Designer (EED) for determining the behavior of the system in its lifecycle, verification of the configuration, and cost estimation.

For more complex problems where significant groundwater flow occurs, seasonal storage or water reinjection (recovery) is planned, energy piles will be used ... we use the software tools FEFLOW and MODFLOW.


At Geological Survey of Slovenia we evaluate the contribution of GHP technology with the following steps:

  • Record keeping of GHP contribution (number of devices, energy and CO2 reduction)
  • Local energy concepts - support and guidance to municipalities to achieve the overall national target of 7% geoenergy in RES for heating and cooling
  • Information system (GIS) for the exchange of information by integrating data on the use of GHPs in the cadaster of real estates
  • Establishing and updating a geothermal GIS database and a series of maps with recommendations for widespread use of geothermal resources
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