Shallow Geothermal Applications Research Articles

Use of electrical resistivity tomography measurements for investigation of different grouting materials for very shallow geothermal applications within varying seasonal conditions; Applied on a geothermal earth-air heat exchanger system

To achieve the current climate targets, heat pump systems like very shallow geothermal applications are gaining ground. However, the dimensioning of these ground coupled systems is soil-dependent and thus subject to significant differences in efficiency. Furthermore, the thermal properties of the grouting materials are essential for heat transfer in the direct vicinity of the geothermal application. To provide a non-invasive assessment of relevant soil and grouting characteristics, electrical resistivity tomography (ERT) measurements were performed.In this case, the surrounding of an Earth Air-Heat Exchanger – system (EAHE) with different grouting materials (fine sand and fine sand with bentonite) was investigated. For investigation the electrical resistivity (ER) of initial and modified soil conditions were measured regarding soil texture and moisture content. Thereby, the different grouting materials are clearly identified. Thus, based on the ERT measurements, a characterisation of key soil and grouting material properties for applications focusing on heat transfer in soil has been carried out.Furthermore, the depth of soil disturbance due to the EAHE installation could have been traced.Due to a monitoring over different season (February and May) changes in raw ER data could have been ascribed to temperature changes.

Efficient operation of earth–air heat exchanger system for air cooling in a hot climate based on a new control strategy

The present research contributes to filling a gap not covered in the previous studies on shallow geothermal applications using earth-to-air heat exchanger (EAHE) systems. The authors think that operating the EAHE continuously for long periods, as presented in many of the previous studies, is not efficient, and this is due to the following reasons:- At specific periods, especially during the night and early morning, the difference between the ambient air temperature (Tamb) and the undisturbed ground temperature (UGT) is small and even negative, meaning the ambient air is colder than the ground.- The continuous operation of the EAHE for a long time leads to the ground saturation with heat, which means that the ground cannot absorb more heat from the EAHE inlet air.So, the present study investigates performance improvement by applying a control strategy to the EAHE system according to two different modes: mode 1 and mode 2. These two operational modes depend on the difference between the ambient air temperature and the UGT value. In mode 1, the ambient air flows inside the EAHE and supplies fresh air to the building. This mode is applied when the temperature difference (Tamb-UGT) exceeds 1.5 οC. When the condition of mode 1 is not satisfied, mode 2 is used, where the ambient air bypasses the EAHE and is supplied directly to the ventilated space. To carry out the present investigation, a numerical model of the EAHE system is developed, and its results are validated against published data from the literature.The simulation of EAHE in Madinah City, KSA, presenting a hot climate region, has been carried out. The study's results confirmed that the control strategy is an efficient solution for the continuous operation of the EAHE. The applied control strategy guarantees that the air supplied to the building is the coldest among the ambient air and the EAHE outlet air. Also, it has been shown that when the control is applied (mode 2), the temperature of the air supplied to the building is 1.5 °C lower than that of the case related to mode 1. Finally, the results showed that the ground can be recovered thermally when the air bypasses the EAHE (mode 2). The ground temperature recovers almost to its undisturbed value during specific periods, which enhances the potential for subsequent heat absorption.

New trilobular geometry using advanced materials for experimentally validated enhanced heat transfer in shallow geothermal applications

The Adapted Conductivity Trilobular (ACT) design, comprising an up-flow central pipe and three satellite downward pipes, represents a novel configuration of a borehole heat exchanger. The novelty lies not only in the geometry of the heat exchanger, but also in the use of materials perfectly adapted to the thermal use for which they have been specified. The central pipe is constructed of a composite bilayer material with very low thermal conductivity, whereas the satellite pipes are based on a novel highly conductive plastic material. The primary objective behind this design was to simplify installation, by using essentially the same drilling and grouting methods used for conventional single-U or double-U borehole heat exchangers. In response to the rising demand for more efficient and affordable ground heat exchangers to be connected to Ground Source Heat Pump (GSHP) systems, this innovation was one of the promising outcomes of the GEOCOND European project. To assess ACT performance, the Geothermal Laboratory on the campus of the Universitat Politècnica de Valencia conducted a number of thermal response tests (TRTs), the experimental results of which are collected and analyzed in this paper. Additionally, a modified version of the Composite Two Region Line Source modeling scheme is described and used to assess the experimental data. A total of four demonstration boreholes – two single-U baseline and two ACT boreholes – have been tested. The experimental investigation using the C2RLS methodology reveals that the ACT configuration has a significantly lower thermal than the typical BHE arrangement and allows a greater specific heat injection rate. Specifically, a borehole resistance drop from 0.149 (m.K)/W to 0.07 (m.K)/W allowing higher injection rates to be achieved without an unsustainable increase in ground temperature. These findings may have significant implications for the development of more efficient and cost-effective GSHP systems.

Generation of gridded temperature map of constant-temperature layer based on meteorological data for shallow geothermal applications

The energy exchange between the subsurface and the infrastructure may serve as an effective way to reduce energy consumption in space heating. In China, the concepts of climate battery and shallow geothermal have obtained growing attentions. The depth and the temperature of the constant-temperature layer (CTL), also called neutral zone, in the ground are significant factors during the design modeling. Unfortunately, the lack of knowledge in these site-dependent variables often leads to conservative design of ground heat exchangers and thus limits their implementations due to the high upfront cost. This study presents an improved analytical model based on the subsurface energy balance equation, which can be used to estimate the temperature and depth of CTL based on the meteorological and geological datasets. The model was first validated against the temperature records from 77 shallow geothermal case studies in China. With the development of remote sensing technology and agricultural dataset, the spatial mapping of CTL was then generated based on the variability of the geological and meteorological parameters. Finally, the simplifications were made to the proposed model when there is no available meteorological data and proven as sufficiently robust in results delivery. By performing this model at the initial stage of the engineering practice, the long-term heat exchange potential of the geology can be anticipated with higher fidelity, and the suitability of a certain region to be treated as a climate battery may be determined without costly drilling efforts.

A new semi-analytical solution addressing varying heat transfer rates for U-shaped vertical borehole heat exchangers in multilayered ground

Existing analytical solutions for vertical borehole heat exchangers (BHEs) in a multilayered ground normally require a prescribed heat transfer rate at the borehole wall, such as a constant heat transfer rate. However, in reality, the heat transfer rate between a BHE and its surrounding ground varies along the length of the borehole. To account for this variation, this study proposed a new semi-analytical solution named the “MVQ solution”—Multilayered ground with Varying heat transfer rates (Q). The MVQ solution was first verified with an equivalent three-dimensional (3-D) finite-element (FE) model. The results showed that the MVQ solution had a mean absolute percent error (MAPE) of less than 2.50% for the average borehole wall temperature. In comparison, the conventional constant heat transfer rate solution was found to potentially overpredict the temperature by up to 50.82%. Lastly, a parameter study revealed that the inhomogeneity of ground thermal properties strengthened the heat transfer rate variation along the borehole length. Thus, the MVQ solution is indispensable for thermal performance predictions of BHEs in the multilayered ground, and it can improve the design of BHE systems in shallow geothermal applications.

Laboratory assessment of corrosion rate of carbon steel ground heat exchangers

Abstract. The materials used in the manufacture of geothermal heat exchangers for shallow geothermal applications play an important role in the overall system performance, especially if grout is not being used to seal the boreholes in which the heat exchanger is installed. The subject of this study is the durability evaluation of a vertical coaxial ground heat exchanger made of steel that is coupled directly to the ground. This solution minimizes the thermal resistance between the heat exchanger and the ground, but presents the important drawback of removing any protection toward the surrounding environment Among the materials proposed for manufacturing such vertical geothermal heat exchanger, carbon steel is suitable and have potential, due to its low cost and high thermal conductivity. The main disadvantage of this material is that it is strongly subject to corrosive attack, according to the chemo-physical properties of the underground. This study investigated the corrosion behaviour of carbon steel used in an experimental underground heat exchanger and assessed its durability over time. Corrosion rate of steel samples were measured in the laboratory by weight loss method after exposure over a specified period in a selected ground medium. Different ground conditions were tested, resulting in different densities and moisture contents of ground samples collected on the field. Based on the results, the corrosion rate of carbon steel is evaluated as a function of water content and rate of ground compaction. This information has allowed to advance more accurate quantitative forecast of the expected operational life of installed geothermal exchangers and their safety over time.

An Analytical Solution for the Heat Conduction–Convection Equation in Non-homogeneous Soil

Heat transfer through the soil is important in shallow geothermal applications, in plant growth as well as in the surface energy balance. In this work, an analytical solution for a conduction–convection equation that models heat transfer is presented for non-homogeneous soil. The main assumption underlying the solution is that the thermal diffusivity of soil is piecewise-constant. A method to determine the depth-dependent thermal diffusivity and the water flux density through the porous medium based on temperature measurements is also developed and applied to field data from a site in Ioannina, Greece. The thermal diffusivity was found to increase in the layer below the surface and decrease for larger depths, while convection through the porous medium was found to be present in wet conditions and to account for about 10% of the heat flux in terms of the annual variability of temperature. Finally, the capability of the novel method in capturing the spatio-temporal variability of soil temperature is compared to three commonly used algorithms: the amplitude, the phase, and the conduction–convection algorithm. The novel method is able to reduce the root-mean-square error for the predicted variability of temperature at all depths by an order of magnitude compared to the other three algorithms.

Geophysical exploration for shallow geothermal applications: A case study in Artà, (Balearic Islands, Spain)

Within the installation of a shallow geothermal system, the lack of information on the subsoil frequently leads to errors in the design of the geothermal wellfield. This research presents the application of geophysics, combining 2D and 3D electrical resistivity tomography surveys and the geological information of a certain area for defining the structural distribution of the underground. Processed electrical resistivity data allow elucidating possible geological units and the thermal behavior of the in-depth materials. Two different assumptions (with different locations of the wells) are designed by using the specific geothermal software GES-CAL. Results show, that Case 1 (based on the geophysical results, so avoiding complex areas) allows the reduction of the global drilling length, and hence, the general initial investment of the system (around 20% lower). Meanwhile, Case 2 (without considering the geophysics) is less economically advantageous and could also present technical difficulties during the drilling process, as well as the possible alteration to the normal system operation. The study highlights the benefits of geophysics as an effective approach to characterize the underground and to help to understand its thermal behavior, which is, in turn, crucial for a proper geothermal design.

European and municipal scale drillability maps: A tool to identify the most suitable techniques to install borehole heat exchangers (BHE) probes

The most suitable drilling technology for shallow geothermal applications in a given geological context plays a key role in the techno-economic evaluation of shallow geothermal solutions. The installation costs are one of the main constraints to the wider application of shallow geothermal heat exchangers and are due mainly to the drilling time and fees.This paper defines drillability as a tool to select the most suitable drilling technique for borehole heat exchanger installations in a given geological and hydrogeological setting. Drillability maps at European and municipal scale are proposed as a guideline for drillers and ground source heat pump designers. The former summarizes the most suitable drilling technology based on lithology and borehole heat exchangers type, whilst the latter provides an insight in the ground thermal properties and the installation timing/costs. In the GEO4CIVHIC Project, a database correlating drilling technique to rock types, time needed to drill a borehole at 100 m depth and drilling costs was created. Local drillability features are compared to in situ real data in four test sites. Two out of four test sites, completed at the time of writing, shows a good agreement between local maps and real site conditions, validating the innovative methodology proposed.

Development of a Calculation Concept for Mapping Specific Heat Extraction for Very Shallow Geothermal Systems

Horizontal shallow geothermal applications are easy to install, and their installation process is less liable to legislation than other geothermal systems. Due to a lack of planning guidance, the opportunity to implement such systems is often overlooked, although geothermal installations are urgently needed as a sustainable energy source. To give a foundation for including very shallow geothermal systems in local heat supply planning, potential maps are crucial. To enable their utilization in energy use plans or similar elaborations for municipalities, location-specific and system-specific heat extractions are required. Since applicable standards are not available, it is nearly impossible to provide aggregate propositions, which are essential for potential maps. In this study, a concept was evolved for deriving very shallow geothermal potential maps with location-specific and system-specific heat extraction values. As a basis, VDI 4640 Part 2 information regarding heat extraction and respective climate zone references was utilized. Furthermore, climate information and a soil map were needed to apply the concept to the study area. The application of the concept in an Austrian study area resulted in appropriate potential maps. Moreover, this concept is similarly applicable in other areas of interest.

A simplified approach to predicting the Heat Extraction Rate of Borehole Heat Exchangers from parametric analysis

Predicting Borehole Heat Exchanger (BHE) performance is crucial in shallow geothermal applications, and efforts have been made to develop more efficient and accurate Heat Extraction Rate (HER) prediction approaches. In this investigation, a simple analytical approach to estimating the HER of a BHE has been proposed through the parametric analysis from the results of a validated numerical simulation framework. At first, 12 factors considering geotechnical condition, carrying fluid material, U-pipe configuration, etc. and their connection with HER were studied, the results showed that a linear function was able to express the tendency of HER with each factor, and the R2 ranges between 0.56 and 1.0. Next, the coefficients and the intercept of the linear function was obtained. Subsequently, the obtained analytical approach was compared with the conventional Infinite Line Source (ILS) model by randomly generated numerical simulations and in-situ measurement. The results showed that the proposed linear function model based on parametric analysis matches the numerical simulation results with a R2 of 0.72, while the ILS method is with a R2 of 0.45. Further, the proposed approach has a smaller error than the ILS method, compared with the in-situ measurement. The investigation provides a simple approach to estimating the heat exchange potential of BHEs, which is especially prospective for projects without the information of equivalent thermal conductivity.

Evaluating anisotropy ratio of thermal dispersivity affecting geometry of plumes generated by aquifer thermal use

Studies on shallow geothermal applications have demonstrated that mechanical thermal dispersion is an important heat transport mechanism in saturated porous media. However, most previous studies have assumed the thermal dispersivity ratio, which has not been sufficiently examined. This study investigates the validity of the general assumption that the transverse dispersivity is one-tenth of the longitudinal dispersivity. For this purpose, heat transport experiments, specifically designed at the laboratory scale for estimating dispersivity, were conducted using two different heat sources at Darcy fluxes of 4.74 to 37.47 m/d (Reynolds number Re < 0.3). The results from the analytical models showed that the dispersivity ratios differed from the thermal dispersivity assumption, indicating that the ratio can vary depending on the properties of the porous media. Additionally, heat transport modeling was performed to confirm the influence of the dispersivity ratio on the temperature plumes that develop from the thermal use of shallow aquifers, such as groundwater heat pump systems. We found that, under steady-state conditions, the decrease in the dispersivity ratio led to smaller plume dimensions in all directions, owing to the increased heat dissipation in the transverse direction. Transient simulations for flow and heat transport indicated that the effect of the thermal dispersivity ratio was time-dependent and heterogeneous and increased with the injection rate. These results suggest that the thermal dispersivity ratio is crucial for assessing the environmental impacts of aquifer thermal use, especially for long-term and large-scale applications. Therefore, the magnitude and ratio of thermal dispersivity should be evaluated at the design stage for more efficient and sustainable use of shallow geothermal resources.

Evaluation of the Effect of Anti-Corrosion Coatings on the Thermal Resistance of Ground Heat Exchangers for Shallow Geothermal Applications

The materials and the technology used to build the ground heat exchangers significantly affect the heat transfer performance of a geothermal system, in addition to the local geological and hydrogeological context. Among expense items such as the coupled heat pumps and the applied drilling technology, the heat exchangers play a key role in the shallow geothermal market. For this reason, they are usually made with plastic. Metal tubes are not widely used because of corrosion issues, which can compromise the reliability of the system over time. According to best practices, metal is an unfavorable choice if the pipes are not made of corrosion resistant alloys, such as stainless steel, but the overall performance is strongly related to the heat transfer efficiency. In this study, laser-flash technique is applied on carbon steel samples with anti-corrosion coatings and on corrosion resistant materials (stainless steel grades used for pipes), thus, allowing the comparison of their thermophysical properties. These properties are used to evaluate each solution in terms of thermal resistance. This study demonstrates that there are no particular corrosion resistant steel pipe configurations that are thermally favorable over others in a critical way.

Phase change material-sand mixtures for distributed latent heat thermal energy storage: Interaction and performance analysis

In this study two phase change materials (PCMs) mixed with sand were evaluated for distributed latent heat thermal energy storage (LHTES) coupled with a novel Flat-Panel ground heat exchanger (GHE) for shallow geothermal applications. N-Octadecane and a commercial paraffin-based PCM were mixed (30% v/v) separately with sand, which is commonly used as backfilling material for GHE. Both two mixtures underwent 16 thermal cycles and specimen’s temperatures and their variation over time were analyzed to evaluate phase change stability and supercooling. Grain size laser diffraction and pore analysis were performed together with optical microscopy, environmental scanning electron microscopy coupled with X-Ray spectrometry (ESEM-EDS) and Fourier transform infrared spectroscopy (FTIR) analysis to evaluate PCMs-sand dynamic interaction over time and temperature. Results shown that sand addition halves n-Octadecane phase change time, although leading to a limited supercooling equal to 1 °C. Sand addition to commercial PCM leaded to a similar increasing in heat transfer, however in absence of supercooling phenomena. These performances were constant through 16 thermal cycles. Therefore, PCMs mixing in sand as mixture for GHEs backfilling material can be considered a strategy to enhance thermal storage of backfilling material, by increasing the underground thermal energy storage and then the exploitation carried out by shallow geothermal applications.

The relevance of microbial processes in geo-energy applications

The subsurface is a vast reservoir which we exploit in various ways. We extract energy in the form of oil/gas or heat from it. We use it for the storage of energy, e.g., in shallow geothermal applications or for the underground storage of natural gas. A lot of recent research has studied the potential for storing hydrogen (H 2 ) in the subsurface. We also use the subsurface to dispose of energy-related waste, e.g., radioactive materials, carbon dioxide (CO 2 ), and acid gas. For a long time, the subsurface was considered sterile below a few metres, probably stemming from work carried out in the 1950s which suggested that bacteria in Pacific sediments most likely disappeared somewhere just below 8m (Morita and ZoBell, 1955). This observation seems to have been extrapolated to the subsurface in general, but over time, as methods developed and microbiologists probed harder, this view changed, and it is now recognised that microbial communities exist at depths where the subsurface is exploited for most types of geo-energy. This raises questions about what sort of microbial community exists, how active it is, what limits and drives that activity and how this might impact geo-energy operations. Cited as : Ebigbo, A., Gregory, S. P. The relevance of microbial processes in geo-energy applications. Advances in Geo-Energy Research, 2021, 5(1): 5-7, doi: 10.46690/ager.2021.01.02

Development of advanced materials guided by numerical simulations to improve performance and cost-efficiency of borehole heat exchangers (BHEs)

One promising way to improve the efficiency of borehole heat exchangers (BHEs) in shallow geothermal applications is to enhance the thermal properties of the materials involved in its construction. Early attempts, such as using metal tubes in the 1980s or the utilization of thin–foil hoses, did not succeed in being adopted by the market for diverse reasons (cost, corrosion, fragility, etc…). In parallel, the optimization of pipe size, the use of double-U-tubes, thermally enhanced grout, etc. were able to bring the measure for the BHE efficiency, the borehole thermal resistance, from 0.20 to 0.15 K/(Wm) down to 0.08–0.06 K/(Wm) in the best solutions today. A further improvement cannot be expected without development of new, dedicated materials, combining the versatility of plastic like PE with an increased thermal conductivity that matches the respective properties of the rock and soil. This goal was included in the Strategic Research and Innovation Agenda of the European Technology Platform on Renewable Heating and Cooling in 2013.Within an EU supported project, both BHE pipes and grouting materials have been produced prototypically in small amounts, suitable for the first tests in the intended environment.The present work explains the research pathways envisaged and the resulting sensitivity analysis to highlight the influence of some of the most critical parameters that affect the overall performance of a GSHP system. The results have allowed guiding the real development of more efficient new advanced materials for different scenarios representative of different European regions. Finally the developed materials and their properties are discussed, including a comparative assessment about their compliance with reference material properties as currently seen in the BHE market.

Evaluating the thermal impacts and sustainability of intensive shallow geothermal utilization on a neighborhood scale: Lessons learned from a case study

This paper presents a case study using calibrated numerical models to evaluate the thermal impacts and long-term sustainability of intensive geothermal use on a neighborhood scale. The subsurface heat transport model is configured with site-specific parameters and validated against monitoring data from a typical urban living quarter in Germany. Based on the simulated ground temperature profile, the heat pump performance is approximated. In addition, the effects of groundwater flow on the thermal interaction and economical operation of the shallow geothermal systems are examined. The results indicate limited thermal impacts as the groundwater temperature will maintain above 3.2 °C and that the area undergone severe temperature drop is less than 1% size of the neighborhood. Since the estimated seasonal coefficients of performance (SCOPs) are at least 3.8, the sustainability of the shallow geothermal applications is confirmed economically. Nevertheless, financial disadvantages up to 92 € year-1 are anticipated due to gradual efficiency losses of the heat pump, which are meant for the owners of downstream installations. In addition, uncertainties in groundwater flow rate are also analyzed. For the negligible advection case, simulation results suggest that some systems can only operate sustainably for eight years. Conclusions are drawn regarding the general feasibility of neighborhood-scale shallow geothermal utilization and the importance of hydrogeological site investigations during the planning phase of such projects.

Underground thermal heat storage and ground source heat pump activities in Turkey

This study presents an overview of Underground Heat Storage and Ground Source Heat Pumps situation and activities of Turkey. Shallow geothermal installations started in the early 2000s with Ground Source Heat Pump (GSHP) systems for small houses. In the last five years, shallow geothermal energy applications have been increasing with large shopping malls, governmental building and schools. The national expert group on shallow geothermal energy has been established as part of International Energy Agency Energy Conversation Through Energy Storage (IEA ECES) Annex27 “Quality Management in Design, Construction and Operation of Borehole Systems”. The purpose of this group is to increase awareness, to prepare guidelines, carry out training programs and determine market barriers and suggest actions needed to speed up the success of shallow geothermal applications in Turkey. This paper will give an overview of the current status of UTES and GSHP systems in Turkey and activities of national Annex27 experts group.

IDENTIFICATION OF RIVER SAVA TEMPERATURE INFLUENCE ON GROUNDWATER TEMPERATURE OF THE ZAGREB AND SAMOBOR-ZAPREŠIĆ AQUIFER AS A PART OF SHALLOW GEOTHERMAL POTENTIAL

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.

Daily soil temperatures predictions for various climates in United States using data-driven model

As an important indicator of soil characteristics, soil temperature has a great impact on agricultural production, building energy savings and for shallow geothermal applications. Data-driven models have been developed and achieved good accuracy in monthly soil temperatures or daily soil temperatures predictions of a single site taking air temperature, solar radiant and time as inputs. Models’ accuracy obviously dropped if they are applied for predicting daily soil temperatures for various climates on a continental scale. We proposed a new data-driven model based on the support vector machine (SVM). The new model considers daily soil temperature variations as superposition of annual average ground temperatures predictions (long-term climates impact) and daily ground temperature amplitude predictions (short-term climates impact). Annual average soil temperature are determined by air temperature, solar radiant, wind speed and relative humidity; daily soil temperature amplitudes by air temperature amplitudes, solar radiant and day of year. For daily soil temperature predictions at 16 sites located in arid or dry summer climates, warm climates and snow climates in United States, the new model’s mean absolute error is 1.26 °C and root mean square error is 1.66 °C. Meanwhile, traditional SVM model’s mean absolute error is 2.20 °C and root mean square error is 2.91 °C.