Geothermal Systems—An Overview

Abstract

This editorial aims to summarize 10 articles (7 scientific papers and 3 review papers) that contributed to the Special Issue “Geothermal Systems—An Overview”. This Special Issue contains information on both shallow and deep geothermal systems, and analyzes geothermal resources (low and high temperature) in various locations around the world.

Geothermal energy, a clean and renewable energy source, has been used for thousands of years all over the world. Depending on the existing resources and available technologies, this kind of energy can be used for different purposes and in different ways. This variety and specificity of utilization cause geological and engineered geothermal systems to be classified differently based on their geological location, method of formation, dominant heat transfer mechanism, or availability. The world needs an energy transition. The current use of fossil fuels is causing environmental damage, leading to the depletion of natural resources and, in many regions, to a significant degradation of environmental quality due to the emission of significant amounts of pollutants into the atmosphere. It is necessary to transform the energy sector, which is largely responsible for greenhouse gas emissions. Renewable energy sources, including precisely clean and environmentally friendly geothermal energy, play an important role in the transformation process. Problems associated with the effective use of geothermal energy were dedicated to an Energies Special Issue under the title Geothermal Systems.

Geothermal system is a very broad concept. A thorough understanding of geothermal systems requires the integration of expertise from various disciplines such as geology, physics, chemistry, hydrogeology, and many others. The concept of geothermal systems has been well illustrated in previous studies [1,2]. Geothermal systems are not only technological systems that extract geothermal energy, but also reservoirs of heat. The description of geothermal systems therefore covers issues such as geophysical and geochemical signatures of geothermal systems, heat transfer and regional heat flow, geothermal anomalies, mathematical modelling of geothermal systems, heat extraction from geothermal reservoirs, geothermal resource assessment, environmental aspects of geothermal energy development, as well as many other aspects of efficient use of geothermal energy.

This Special Issue was designed to cover all types of geothermal systems, from low to high temperatures. Papers analyzing the possibilities of using geothermal energy in conventional or unconventional ways were invited. As a result of the work, the Special Issue contains 10 articles [3,4,5,6,7,8,9,10,11,12], of which 7 are scientific articles [3,4,5,6,7,8,9] and 3 are review papers [10,11,12]. This editorial presents an overview of the articles published in this Special Issue. The Special Issue contains works related to shallow and deep geothermal systems as well as energy storage. The issues of various utilization of low- and high-temperature geothermal resources are widely discussed. Review papers address the use of shallow geothermal systems for heating and cooling [10]; the development of the enhanced geothermal system (EGS); carbon capture, utilization, and storage (CCUS) technologies [11]; and the prospects for using hydrocarbon deposits from the autochthonous Miocene formation (Eastern Carpathian Foredeep, Poland) for geothermal purposes [12].

The keywords in these 10 articles are: low-enthalpy geothermal sources, electromagnetic heating, flows in porous media, partial differential equations, geothermal energy, fluid geochemistry, deep geothermal resources, sedimentary basin, carbonate reservoirs, geothermal, hot dry rock (HDR), enhanced geothermal system (EGS), CO2 sequestration, cross-impact method, MICMAC, silica sinter, thermoluminescence dating, ground penetrating radar, northwestern basin-and-range province, Alvord/Pueblo valleys, confined aquifer, heat storage, ATES, groundwater modeling, heat transport modeling, thermal imbalance, urban engineering, geographical information system (GIS), ground source heat pump (GSHP), shallow geothermal energy, borehole heat exchanger (BHE), geothermal waters, chemical composition, Outer Carpathians, Carpathian Foredeep, principal component analysis, statistical analysis, shallow geothermal energy, borehole heat exchanger, collector, grout, geoenergetics, techno-economic analysis, optimization, CCUS (carbon capture, utilization, and storage), Poland, earth sciences, and petroleum field.

One of the important issues related to the use of renewable energy sources, including geothermal energy, is energy storage. This topic appears in several publications of the special issue. Almeida et al. [3] presented down-hole electromagnetic heating of deep aquifers for renewable energy storage. Electromagnetic heating is an emerging method for storing renewable energy, such as photovoltaic solar and wind electric power, into aquifers. The authors have demonstrated that underground electromagnetic heating is a highly efficient method of storing renewable energy that can help solve the problem of its intermittency.

Bulte et al. [7] present numerical interference modeling of thermally unbalanced aquifer energy storage systems in Belgium. This paper presents the results of a numerical model built using FEFLOW^® software to simulate groundwater flow and heat transport in a confined aquifer. The study was conducted in Brussels, where two thermal energy storage systems (ATESs) were installed in adjacent buildings that use the same aquifer consisting of mixed sandy and silty sublayers. The results of the simulation indicated that in the long term, the joint operation of two ATES systems is inefficient.

The Special Issue also published a study related to the use of shallow geothermal resources. Ramos-Escudero et al. [8] presented a case study in the region of Murcia, Spain. The authors assessed the potential of shallow geothermal systems used for heating and cooling, integrated with the socio-economic environment. To achieve results, they developed a GIS-based methodology to estimate the resource, energy, economic, and environmental potential of shallow geothermal on a regional scale. The method was applied to the Region of Murcia (Spain). The authors conclude that by combining resource and technical potential depending on the climatic conditions in the analyzed region, the average energy demand of a residential building for heating varies from 3200 to 6200 kWh/year, and for cooling from 3800 to 5200 kWh/year. The average energy needs of the residential building for the heating needs vary from 3200 to 6200 kWh/year and the cooling needs vary from 3800 to 5200 kWh/year. Economic analysis shows that the payback period for GSHP investments ranges from 8 to 20 years, with an average of 11 years. Considering the 10-year period, the cost of producing energy from shallow geothermal in the region ranges from EUR 0.13 to EUR 0.18/kWh.

Ahmed et al. [10] present a literature review on the use of shallow geothermal energy (SGE) for heating and cooling purposes, as well as the latest developments in materials, as well as new innovative structures and approaches associated with the techno-economic optimization of these systems. The article contains state-of-the-art and technological advances regarding borehole heat exchangers (BHTs), including information on the thermal conductivity of the rock surrounding the BHE, innovative borehole heat exchanger structures, grouting materials, bore heat exchanger collectors, and heat carrier fluids. The paper also touches on the important topic of underground thermal energy storage (UTES) systems, which store thermal energy in natural underground formations. A UTES uses aquifers (ATESs) or boreholes (BTESs) to store sensible heat by lowering or raising the temperature of water. In conclusion, the authors state that ground source heat pump (GSHP) is a promising technology with great potential toward decarbonizing the heating and cooling sector in Europe. The authors also pointed out the benefits of using BTES technology to store surplus solar, and thermal energy waste collected in the summer that can be used in the winter. Techno-economic analysis indicated that the high initial cost of shallow geothermal systems creates financial obstacles for households. Therefore, GSHPs without subsidies are more expensive heating solutions compared to heating systems based on solid fuels.

An important aspect is the topic of the development of the enhanced geothermal systems (EGSs). Pająk et al. [5] presented the criteria of selecting locations and technologies used in CO_2-EGS Systems. This is the first paper included in this special issue on the EnerGizerS project (CO₂-Enhanced Geothermal Systems for Climate Neutral Energy Supply) implemented by a Polish-Norwegian consortium. Another article on the same project is a review paper prepared by Sowiżdżał et al. [11]. Within the EnerGizerS project, the international consortium of scientists has conducted research aimed at the detailed identification of potential geological structures for the location of CO₂-EGS systems in Poland and Norway, combining the requirements for both the EGS (enhanced geothermal system) and CCS (carbon, capture, and storage) technologies. The first is a technology for harnessing the energy stored in hot dry rocks, while the second is an important technology used for improving the elimination of carbon dioxide emissions from the burning of fossil fuels. A system combining both technologies is an enhanced geothermal system using supercritical carbon dioxide as the working fluid (CO2–EGS). This kind of system is an interesting alternative due to the added benefit of the geological storage of CO2 during geothermal energy extraction. The development of CO₂-EGS technology is the goal of the Polish–Norwegian project currently underway in Poland (EnerGizerS). The authors in paper [5] presented the results of a study to determine the most important factors relevant to the selection of the location of CO2-EGS systems. These factors are connected with reservoir parameters, as well as existing wells and other infrastructure, formal restrictions, the availability of CO₂ sources, and others presented and discussed in detail in the paper [5].

Review article prepared by Sowiżdżał et al. [11] focuses on the combination of two technologies—the enhanced geothermal system (EGS) and carbon capture, utilization, and storage (CCUS). An overview of the world’s major CCUS and EGS systems provides a good basis for the development of such investments, especially in those countries that are just beginning to develop such technology. The paper presents 15 examples of CCUS hubs in North America, South America, Australia, Europe, and Asia, as well as 9 selected EGS projects located in the European Union, Japan, South Korea, Australia, and the United States.

Two of the presented papers concern the Carpathian Foredeep area located in the southern Poland [7,12]. This is primarily an area of hydrocarbon exploration, but in recent years the province has also become an object of geothermal interest, as can be seen in the papers. In this area, possibilities surrounding the effective utilization of low-temperature geothermal resources in hydrogeothermal systems are analyzed.

Jasnos [7] presented the hydrogeochemical characteristics of geothermal waters from the Mesozoic formations in the basement of the central part of the Carpathian Foredeep and the Carpathians in Poland. The research was conducted using multivariate statistical methods. The study provided results of water sampling of the Cretaceous, Jurassic, and Triassic aquifers of the Miechów Trough, submerged under the central part of the Miocene formations of the Carpathian Foredeep and the Flysch formations of the Outer Carpathians. Detailed analysis of the chemical composition of potentially geothermal waters from a statistical point of view can be diagnostic, indicating certain anomalies in relation to their background. These may indicate the existence of zones that would be of interest from the point of view of exploiting the waters for various purposes.

Chmielowska et al. [12] presented prospects for using hydrocarbon deposits from the autochthonous Miocene formation located in Eastern Carpathians (Poland) for geothermal purposes. The paper had two main goals. The first objective was to review geothermal and petroleum facts surrounding the eastern Carpathian foredeep. Another equally important objective is to find out the location of abandoned oil and gas wells (AOGWs) with a prospective geothermal potential. In many regions characterized by high geothermal potential, there are a significant number of abandoned oil wells. Their conversion to geothermal wells can bring a number of benefits in many cases. The analysis made by the authors in this paper indicates that the area of natural gas deposits in the eastern part of the Carpathian foredeep in Poland may also be of interest for geothermal investments.

The hydro-chemical characterization of groundwater was presented in [7], but was also the focus of the paper [4]. Guglielmetti et al. shows results on the hydrochemical characterization of groundwaters’ fluid flow through the upper Mesozoic carbonate geothermal reservoirs in the Geneva Basin. The study analyzed major ions, trace elements, and isotopes of oxygen, hydrogen, sulfur, strontium, and carbo, and the results indicate that the sampled waters are of meteoric origin, the carbonate aquifers act as preferential host rocks for geothermal waters, and a partial contribution of Cenozoic sediments can be observed in some samples.

Mowbray and Cummings [6] presented the results of a study of hot springs in the Al-vord/Pueblo valleys in southeastern Oregon, USA. In this area, hot springs have been active in a 4-acre area in Mickey Springs for more than 30,000 years. The present study investigated the evolution of these springs, where three different sinter morphologies were tentatively identified. Currently, the high-temperature springs, steam vents, and mud boilers are concentrated in a 50 m × 50 m area near the southern edge of the source area.

In summary, the Special Issue contains 10 interesting articles on geothermal systems. It contains information on both shallow and deep systems, and analyzes geothermal resources (low and high temperature) in various locations around the world (in countries such as Belgium, Poland, Spain, Switzerland, and the USA).

I invite you to read more in papers [3,4,5,6,7,8,9,10,11,12].