Shallow Geothermal Potential Research Articles

Impact of simplifications on numerical modelling of the shallow subsurface at city-scale and implications for shallow geothermal potential

Anthropogenic infrastructures in the shallow subsurface, such as heated basements, tunnels or shallow geothermal systems, are known to increase ground temperatures, particularly in urban areas. Numerical modelling helps inform on the extent of thermal influence of such structures, and its potential uses. Realistic modelling of the subsurface is often computationally costly and requires large amounts of data which is often not readily available, necessitating the use of modelling simplifications. This work presents a case-study on the city centre of Cardiff, UK, for which high resolution data is available, and compares modelling results when three key modelling components (namely ground elevation, hydraulic gradient distribution and basement geometry) are implemented either ‘realistically’, i.e. with high resolution data, or ‘simplified’, utilising commonly accepted modelling assumptions. Results are presented at a point (local) scale and at a domain (aggregate) scale to investigate the impacts such simplifications have on model outputs for different purposes. Comparison to measured data at individual locations shows that the accuracy of temperature outputs from numerical models is largely insensitive to simplification of the hydraulic gradient distribution implemented, while changes in basement geometry affect accuracy of the mean temperature predicted at a point by as much as 3.5 °C. At the domain scale, ground temperatures within the first 20 m show a notable increase (approximately 1 °C volume-averaged and 0.5 °C surface-averaged), while the average heat flux over the domain is about 0.06 W/m2 at 20 m depth. These increased temperatures result in beneficial conditions for shallow geothermal utilisation, producing drilling cost savings of around £1700 per typical household system or about 9% increase in thermal energy potential. Simplifications of basement geometry and (to a lesser degree) the hydraulics can result in an overestimation of these temperatures and therefore over-predict geothermal potential, while the elevation simplification showed little impact.

Upgrading a District Heating System by Means of the Integration of Modular Heat Pumps, Geothermal Waters, and PVs for Resilient and Sustainable Urban Energy

Krakow has an extensive district heating network, which is approximately 900 km long. It is the second largest city in terms of the number of inhabitants in Poland, resulting in a high demand for energy—for both heating and cooling. The district heating of the city is based on coal. The paper presents the conception of using the available renewable sources to integrate them into the city’s heating system, increasing the flexibility of the system and its decentralization. An innovative solution of the use of hybrid, modular heat pumps with power dependent on the needs of customers in a given location and combining them with geothermal waters and photovoltaics is presented. The potential of deep geothermal waters is based on two reservoirs built of carbonate rocks, namely Devonian and Upper Jurassic, which mainly consist of dolomite and limestone. The theoretical potential of water intake equal to the nominal heating capacity of a geothermal installation is estimated at 3.3 and 2.0 MW, respectively. Shallow geothermal energy potential varies within the city, reflecting the complex geological structure of the city. Apart from typical borehole heat exchangers (BHEs), the shallower water levels may represent a significant potential source for both heating and cooling by means of water heat pumps. For the heating network, it has been proposed to use modular heat pumps with hybrid sources, which will allow for the flexible development of the network in places previously unavailable or unprofitable. In the case of balancing production and demand, a photovoltaic installation can be an effective and sufficient source of electricity that will cover the annual electricity demand generated by the heat pump installation, when it is used for both heating and cooling. The alternating demand of facilities for heating and cooling energy, caused by changes in the seasons, suggests potential for using seasonal cold and heat storage.

Defining the Shallow Geothermal Heat-Exchange Potential for a Lower Fluvial Plain of the Central Apennines: The Metauro Valley (Marche Region, Italy)

In this work we assessed the shallow geothermal heat-exchange potential of a fluvial plain of the Central Apennines, the lower Metauro Valley, where about 90,000 people live. Publicly available geognostic drilling data from the Italian Seismic Microzonation studies have been exploited together with hydrogeological and thermophysical properties of the main geological formations of the area. These data have been averaged over the firsts 100 m of subsoil to define the thermal conductivity, the specific heat extraction rates of the ground and to establish the geothermal potential of the area (expressed in MWh y−1). The investigation revealed that the heat-exchange potential is mainly controlled by the bedrock lithotypes and the saturated conditions of the sedimentary infill. A general increase in thermal conductivity, specific heat extraction and geothermal potential have been mapped moving from the coast, where higher sedimentary infill thicknesses have been found, towards the inland where the carbonate bedrock approaches the surface. The geothermal potential of the investigated lower Metauro Valley is mostly between ~9.0 and ~10 MWh y−1 and the average depth to be drilled to supply a standard domestic power demand of 4.0 kW is ~96 m (ranging from 82 to 125 m all over the valley). This investigation emphasizes that the Seismic Microzonation studies represent a huge database to be exploited for the best assessment of the shallow geothermal potential throughout the Italian regions, which can be addressed by the implementation of heating and cooling through vertical closed-loop borehole heat exchanger systems coupled with geothermal heat pumps.

The effect of urban heat island on groundwater located in shallow aquifers of Kutahya city center and shallow geothermal energy potential of the region

The effect of urban heat island on groundwater located in shallow aquifers of Kutahya city center and shallow geothermal energy potential of the region

Impacts of progressive urban expansion on subsurface temperatures in the city of Amsterdam (The Netherlands)

Subsurface temperatures are substantially higher in urban areas than in surrounding rural environments; the result is a subsurface urban heat island (SUHI). SUHIs and their drivers have received attention in studies world-wide. In this study, a well-constrained data set of subsurface temperatures from Amsterdam, The Netherlands, is presented. The study demonstrates that, through modeling of centuries-long (from fourteenth to twenty-first century) urban development and climate change, along with the history of both the surface urban heat-island temperatures and ground surface temperatures, it is possible to simulate the development and present state of the Amsterdam SUHI. The results provide insight into the drivers of long-term SUHI development, which makes it possible to distinguish subterranean heat sources of more recent times that are localized drivers (such as geothermal energy systems, sewers, boiler basements, subway stations or district heating) from larger-scale drivers (mainly heat loss from buildings and raised ground-surface temperatures due to pavements). Because these findings have consequences for the assessment of the shallow geothermal potential of the SUHIs, it is proposed to distinguish between (1) a regional, long-term SUHI that has developed over centuries due to the larger-scale drivers, and (2) local anomalies caused by anthropogenic heat sources less than one century old.

Detection of Geothermal Potential Zones Using Remote Sensing Techniques

The transition towards a new sustainable energy model—replacing fossil fuels with renewable sources—presents a multidisciplinary challenge. One of the major decarbonization issues is the question of to optimize energy transport networks for renewable energy sources. Within the range of renewable energies, the location and evaluation of geothermal energy is associated with costly processes, such as drilling, which limit its use. Therefore, the present research is aimed at applying different geomatic techniques for the detection of geothermal resources. The workflow is based on free/open access geospatial data. More specifically, remote sensing information (Sentinel-2A and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)), geological information, distribution of gravimetric anomalies, and geographic information systems have been used to detect areas of shallow geothermal potential in the northwest of the province of Orense, Spain. Due to the variety of parameters involved, and the complexity of the classification, a random forest classifier was employed, since this algorithm works well with large sets of data and can be used with categorical and numerical data. The results obtained allowed identifying a susceptible area to be operated on with a geothermal potential of 80 W·m−1 or higher.

Alternative heating systems for northern remote communities: Techno-economic analysis of ground-coupled heat pumps in Kuujjuaq, Nunavik, Canada

Nunavik is a subarctic remote region covering the northern third of Québec, Canada, where low efficiency and environmentally adverse diesel furnaces are currently used to meet residential heating demand. Studies focusing on building heating alternatives in subarctic climate are limited and hence, this work can help with the development of such systems in remote off-grid communities. Shallow geothermal potential was mapped for Kuujjuaq, the largest village in Nunavik. Four ground-coupled heat pump scenarios were analysed for a simulated 5-occupant residential dwelling, with heating needs of 71 MWh/year. Resulting maps show a relatively high ground potential for such cold region, ranging between 5.8 MWh/year and 22.9 MWh/year per borehole for heat exchanger lengths of 100 m–300 m. A 50-year life-cycle cost analysis of such systems reveal that a compression heat pump with electricity derived from solar photovoltaic panels has a net present cost as low as approximately CAD$179,000, representing the most economically attractive heating option in Kuujjuaq as compared to the currently subsidized, diesel furnace heating at CAD$277,000. This work verifies that shallow geothermal energy through state-of-the-art heat pumps is a financially interesting option in Kuujjuaq. Results can be extended to similar subarctic settings in Canada and worldwide.

An improved volumetric method of geothermal resources assessment for shallow ground combining geophysical data

Assessment of available heat capacity in the shallow ground is important to the development of ground source or geothermal heat pumps. A new volumetric method-based approach is proposed for evaluating shallow geothermal potential by combining microtremor survey results. Parameters such as thickness of clay and sandy gravel layers can be achieved by processing the microtremor filed data. The study area was divided into grids based on the different exploration sites. The heat present in each small unit was calculated using the revised volumetric method. This division into small units improved the accuracy of assessment of the geothermal potential and helped plot the distribution map of the available heat amount, which indicated optimal locations for development and utilization of shallow geothermal resources. Jimo, China was used as an example to depict the calculation flow. Since the microtremor survey method can identify the thickness of clay, sandy gravel, and fresh bedrock layers in shallow ground, the amount of shallow geothermal resources present above fresh bedrock was calculated. In Jimo, the amount of exploitable geothermal resources, using the traditional volumetric method, was found to be 4.92 × 1012 kJ. The proposed calculation method not only provided a higher corresponding value of 6.96 × 1012 kJ but also showed the regions with the highest potential of shallow geothermal resources. Hence, this approach provides an alternative method for geothermal potential assessment which can be used globally.

A machine learning approach for mapping the very shallow theoretical geothermal potential

The very shallow geothermal potential (vSGP) is increasingly recognized as a viable resource for providing clean thermal energy in urban and rural areas. This is primarily due to its reliability, low-cost installation, easy maintenance, and little constraints regarding ground-related laws and policies. We propose a methodology to extract the theoretical vSGP (installed in the uppermost 10 m of the ground, and mostly at depths of 1–2 m) at the national scale for Switzerland, based on a combination of Geographic Information Systems, traditional modelling, and machine learning (ML). The theoretical vSGP is based on the estimation of three thermal characteristics of the ground that impact significantly the geothermal potential, namely the monthly temperature at various depths in the surface layer, the thermal conductivity, and the thermal diffusivity. Each of the three variables is estimated separately, to a depth of 1 m below the surface, using the following general strategy: (1) collect significant data related to the variable, (2) if not existing, extract values for the variable at available locations with the help of traditional models and part of the data as input for these models, (3) train a ML model (with the Random Forests algorithm) using the extracted variable values as examples (training output labels) and related information contained in the data as features (training input samples), (4) use the trained ML model to estimate the variable in unknown locations, (5) estimate the uncertainty attached to the estimations. The methodology estimates values for (200 × 200) (m2) pixels forming a grid over Switzerland. The strategy, however, can be generalized to any country with significant data (topographic, weather, and surface layer/soil data) available. The results indicate a very non-negligible potential for very shallow geothermal systems in Switzerland.

A framework for assessing the technical and economic potential of shallow geothermal energy in individual and district heating systems: A case study of Slovenia

The deployment of geothermal energy systems can significantly contribute to climate change mitigation and play a part in the transition to a low carbon society. This study proposes a framework for identifying the shallow geothermal energy potential of new individual and district heating (DH) systems. The model accounts for thermal interference between neighbouring wells and borehole heat exchangers, where the main criteria for analysis are ground temperature, thermal conductivity, heat flow, and heat capacity. The paper presents a cost-effective area method for the identification of potential new DH areas, while considering the cost competitiveness of the heat supplied. Economic potential is determined based on the cost-effectiveness of the competing technologies, separately for urban and rural areas. The results show that although 54% of technical DH potential in Slovenia remains untapped, the future of shallow geothermal energy systems lies predominantly in individual systems, which have proven to be the most cost efficient solution on locations with favourable geological or hydro-geological conditions. Where possible, shallow geothermal energy can contribute from 2% to 25% of energy for heat production in analysed existing fossil fuel based DH systems, thus making shallow geothermal energy suitable for supplying base load power in an economical manner.

Techno-Economic Mapping for the Improvement of Shallow Geothermal Management in Southern Switzerland

Cantone Ticino, a mountainous region located in the southern part of Switzerland, is greatly affected by the continuous growth of subsurface exploitation through the use of both closed-loop and open-loop geothermal systems. In this study, techno-economic maps for shallow geothermal potential of Cantone Ticino are produced, considering closed-loop systems. The work starts with the identification of the main parameters affecting the techno-economic potential such as GST and thermal conductivity. Maps for different indicators of techno-economic feasibility are created and compared against real data/measurements. An empirical method is tailored to derive a map of the techno-economic geothermal potential, expressed as meters required to provide 1 kW of installed power. The produced map shows an overall discrepancy from real installed length data of approximately ±23%. Moreover, compared with current regulation, the produced maps show an unoptimized management of the shallow geothermal resource, since high potential zones are commonly located where the installation of BHE is not permitted and often closed-loop systems are installed where the estimated potential is lower, mainly in alluvial fans. In light of these considerations, the authorization process in Cantone Ticino for BHE should be revised taking into account the real techno-economic potential.

SYSTEMATIC REVIEW OF RESEARCH AND UTILIZATION OF SHALLOW GEOTHERMAL ENERGY IN CROATIA

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.

Research of the geological and geothermal conditions for the assessment of the shallow geothermal potential in the area of Ljubljana, Slovenia

Research of the geological and geothermal conditions for the assessment of the shallow geothermal potential in the area of Ljubljana, Slovenia

A methodology to calculate long-term shallow geothermal energy potential for an urban neighbourhood

Ground source heat pumps typically use Borehole Heat Exchangers (BHEs), which are drilled vertically into the ground to extract heat. Space limitations in urban areas can limit the total length of BHEs. Continuous heat extraction over long periods from BHEs especially those with restricted lengths, will result in cooling of the ground. Furthermore, the presence of neighbouring BHEs adds to the cooling effect, thereby reducing the overall geothermal energy potential. Due to these restrictions, it is important to consider spatial constraints and the thermal interference from neighbouring BHEs when evaluating the geothermal energy potential of urban areas. In this paper, a methodology to calculate the long-term geothermal energy potential for an urban neighbourhood is developed. A GIS based workflow is used to design the BHEs for multiple buildings, in order to satisfy their total heating/cooling demands with consideration of spatial constraints. A model that accounts for thermal interference between neighbouring BHEs is developed in order to simulate their operation and calculate their long-term geothermal energy potential. The method is applied to an urban neighbourhood in Zurich, Switzerland with 170 buildings. Results show that the geothermal energy potential is overestimated if thermal interference between BHEs are not accounted for. A long-term declining trend is observed in the geothermal energy potential due to ground cooling.

Assessment and mapping of the closed-loop shallow geothermal potential in Cerkno (Slovenia)

The economic viability of Borehole Heat Exchangers (BHEs) depends on the ability of the ground to exchange heat, and maps of the shallow geothermal potential are therefore useful planning tools for future installations. In this work, we present the assessment of shallow geothermal potential in Cerkno, a mountain town of 5,000 inhabitants in western Slovenia. The recently developed G.POT method was applied, taking into account site-specific ground thermal parameters and usage profiles depending on climate conditions. This work is part of the EU-funded project GRETA, aiming at supporting the diffusion of GSHPs in the Alpine territory.

Development of the hybrid model for estimating the undisturbed ground temperature using the finite element method and geostatistical technique

Shallow geothermal potential needs to be evaluated when introducing the ground source heat pump (GSHP) system. Accurate evaluation of shallow geothermal potential can be done by estimating underground thermal properties with precision. The data previously established from measuring underground thermal properties, however, have limitations in that the locations where the measurements were made were not identical. Therefore, this study aimed to develop a hybrid model for estimating UGT using the finite element method (FEM) and the kriging method. This study was conducted in three steps: (i) data collection; (ii) estimation of the UGT in 216 non-measurement locations using FEM and the kriging method; and (iii) estimation of the shallow geothermal potential using the geothermal potential (G.POT) method. The average mean absolute percentage error of the hybrid model (3.67%) was lower than those of the other estimation models using FEM and the kriging method (FEM: 4.62%; Kriging method: 6.34%). Thus, the analysis showed that the developed hybrid model was the most suitable method for estimating UGT in all the length ranges (65–250m) where the ground heat exchanger was to be installed. The hybrid model can help policy makers evaluate shallow geothermal potential at the national level.

EXPERIMENTAL CALIBRATION OF UNDERGROUND HEAT TRANSFER MODELS UNDER A WINERY BUILDING IN A RURAL AREA

Ground temperature and hydrogeological conditions are key parameters for many engineering applications, such as the design of building basements and underground spaces and the assessment of shallow geothermal energy potential. Especially in urban areas, in the very shallow depths, it is diffi cult to fi nd natural undisturbed underground thermal conditions because of anthropic interventions. The assessment of underground behaviour in disturbed conditions will become more and more relevant because of increasing awareness to energy effi ciency and renewable energy topics. The purpose of this paper is to show a three-dimensional representation - based on models calibrated on experimental data - of the underground thermal behaviour aff ected by a building in a rural area in Italy. Temperature varies in space and time and it depends on ground, climate and building characteristics, and all these parameters are taken into account by the seasonal periodic modelling implemented. The results obtained in a context of low urbanization indirectly suggest the importance of these eff ects in dense urban areas; taking greater account of these aspects could lead to improvements in the design of underground spaces and geo-exchanger fi elds for geothermal energy exploitation.

Assessment and mapping of the shallow geothermal potential in the province of Cuneo (Piedmont, NW Italy)

Ground Source Heat Pump (GSHP) is a low carbon heating and cooling technology which can make an important contribution for reaching the ambitious CO2 reduction targets set by the European Union. The economic and technical suitability of this technology strongly depends on the thermal and hydrogeological properties of the ground at the installation site, which need to be assessed in detail. A common indicator adopted to define such suitability is the geothermal potential, i.e. the thermal power that can be exchanged with the ground through a GSHP with a certain setup. In this paper, we present the assessment and mapping of the shallow geothermal potential in the province of Cuneo, a 6900 km2 wide county in NW Italy. Geological, hydrogeological and climatic information are collected and processed to estimate the relevant ground properties. The shallow geothermal potential is then estimated with different methods for closed-loop installations (Borehole Heat Exchangers, BHEs) and open-loop installations (Ground Water Heat Pumps, GWHPs) systems in order to identify the most suitable areas for different technologies. The maps of the geothermal potential are an important planning tool for the installation of GSHPs and for the growth of this renewable energy source.

Quantification of exploitable shallow geothermal energy by using Borehole Heat Exchanger coupled Ground Source Heat Pump systems

In previous studies, the amount of exploitable shallow geothermal energy was estimated by assuming a uniform temperature drop of 2–6°C in the aquifer. In this work, a more comprehensive numerical model has been employed to evaluate the available amount of shallow geothermal energy by using Borehole Heat Exchanger coupled Ground Source Heat Pump systems. Numerical experiments have been performed by simulating the long-term evolution of the subsurface temperature field, which is subject to the operation of borehole heat exchangers and varying parameters like subsurface thermal conductivity and groundwater flow velocity. The concept of equivalent temperature drop is proposed as an auxiliary quantity for the subsurface. With the help of this parameter, a procedure has been established to quantify the amount of shallow geothermal potential. Following this approach, a realistic equivalent temperature reduction is found to be from −1.8 to −4.4°C in the subsurface over a period of 30years. This can be translated to an annual extractable geothermal energy value in a unit surface area, and it ranges from 3.5 to 8.6kWhm−2a−1. The exact value is site specific and heavily depends on the soil thermal conductivity, groundwater velocity, and borehole arrangement.

G.POT: A quantitative method for the assessment and mapping of the shallow geothermal potential

GSHPs (Ground source heat pumps) exchange heat with the ground to provide sustainable heating or cooling. Their technological feasibility and economic viability depend on the site-specific thermal properties of the ground and on the usage profile of the plant. These parameters influence the shallow geothermal potential, which is defined as the thermal power that can be efficiently exchanged by a BHE (Borehole Heat Exchanger) of a certain depth. We present a general method (G.POT) for the determination of shallow geothermal potentials. This method was derived using a comprehensive set of analytical heat transfer simulations, performed by varying (i) the thermal properties of the ground, which comprise its thermal conductivity and capacity, (ii) the thermal properties of the borehole, and (iii) the operational and design parameters of the plant, namely, the BHE length, the threshold temperature of the heat carrier fluid, the duration of the heating/cooling season and the simulated lifetime. Therefore, the G.POT method is a simple and flexible tool that can be implemented in a wide range of different scenarios for large-scale mapping of geothermal potentials. We also assess G.POT by discussing its application to map the geothermal yield in the Province of Cuneo (Piemonte, NW Italy).