Shallow Geothermal Energy Systems Research Articles

The influence of geometry on the heat exchange potential of embedded planar retaining walls with seasonally-balanced thermal load

A number of operational cases exist where embedded retaining walls, used in the construction of underground spaces such as basements and shallow tunnels, have also been utilised as ground-heat exchangers in shallow geothermal energy systems. These are complex structures in terms of their geometry, the surrounding temperature field and boundary conditions, and there are currently no methods to assess their heat exchange capacity in a simple and expedient manner. This contribution uses the finite element method to validate the use of the method in predicting heat flow for this application and then, to assess the influence of wall and excavation geometry in the heat exchange process. The influence of the soil and wall thermal conductivity is shown to be quasi-linear with the latter showing the greatest influence on peak heat exchange. The work identifies a geometric parameter - the ratio of excavation depth to total wall panel depth, H/L which in combination with the wall thickness (D), provide a consistent and simple means by which the heat exchange potential can be estimated, for a given set of wall and soil thermal properties and boundary conditions.

Analysis of the operation characteristics of solar-assisted shallow geothermal energy systems in rural residential areas in southern Hebei Province

Analysis of the operation characteristics of solar-assisted shallow geothermal energy systems in rural residential areas in southern Hebei Province

Assessing the Impact of a Shallow Geothermal System Operation through Multi-Layer Temperature Monitoring in a Mediterranean Climate

Shallow Geothermal Energy (SGE) exchanges heat with the ground. In continuous, long-term operation, the initial temperature field can be disturbed, and subsurface thermal changes can be developed. In this paper, the thermal impact of a SGE system under a Mediterranean climate is handled. Temperature monitoring was conducted on 15 investigation boreholes equipped with a total of 92 thermal sensors placed at specific depths. Investigation boreholes were drilled 1–2 m from SGE system borehole heat exchangers installed in a university building. The analysis handles a one-year monitoring period of SGE system operation. Temperature depth profiles, reaching up to 140 m depth, were registered with a 10 min time step, resulting in a large amount of data. Ground thermal conductivity was estimated experimentally and semi-empirically, allowing us to obtain, using a numerical model, the initial undisturbed ground temperature profiles and compare them with the monitored values. Climate data were recorded by the university meteorological station. Globally, the measured and computed data were coherent, and a non-negligible impact of the SGE system operation in the first year was observed. The building orientation as well as the nearby departments had significant impacts on the shallow ground temperature. Maximum ground temperature changes observed at depths higher than 10–20 m, ranging from 2 to 3 °C as observed in different boreholes, indicate that the system is operating efficiently.

Temperature dependence of soil-structure interface behaviour in the context of thermally-activated piles: A review

Thermally-activated (TA) piles are cost-effective technologies with the dual role of transferring structural loads to the ground while exchanging heat with the surrounding soil as part of shallow geothermal energy systems. As TA piles are subjected to both thermal and mechanical loads, the behaviour on the soil-structure interface is particularly complex and is key for the analysis and design of these structures. The present paper aims to review the current state of knowledge regarding the thermal dependency of soil-structure interface behaviour and provide an overview of experimental results obtained from non-isothermal tests investigating soil and soil-structure interface behaviour. This overview includes comparison of the different experimental equipment and procedures, soil types, initial soil state overconsolidation ratio (degree of consolidation or relative density) and thermal loadings. It was found that it is not straightforward to reach a unique interpretation regarding possible variation of the soil-structure interface behaviour at different temperatures: the framework of the experimental evidence is very complex due to the wide variation in testing conditions. Therefore, it was not possible to compare the studies like-for-like, leading to an apparently ambiguous interpretation of the results. Overall, the consensus across this and other studies is that the potential variation of interface resistance with temperature typically appears to be limited and not very significant.

Fast calculation of the technical shallow geothermal energy potential of large areas with a steady-state solution of the finite line source

Shallow geothermal energy systems may play a significant role in the energy transition as they can strongly reduce the carbon emissions from the residential heating and cooling sector. For urban and rural planning, policy making, and the development of regulations, regional scale estimations of the heating potential of borehole heat exchangers (BHEs) are required. For such regional estimations of the technical geothermal potential the thermal interference between BHEs is a crucial parameter to take into account as it can strongly reduce the heat extraction rate in borehole fields with a high BHE density. Here, we propose an analytical solution of the steady-state finite line source solution to calculate thermal response factors, or g-functions, within large BHE fields with variable distances between, and lengths of, boreholes. We show that the methodology can be used to rapidly calculate the thermal interference of boreholes on a regional scale and apply it to estimate the technical shallow geothermal potential of the German state of Baden-Württemberg. The results highlight areas where BHEs can offer a good alternative to fossil fuel-based heating options and will be used by municipalities within the study area for the development of local carbon neutral heating plans.

Geothermal energy system application: From basic standard performance to sustainability reflection

Shallow Geothermal Energy (SGE) systems are emerging as an attractive solution to enhance building energy efficiency and contribute to a low-carbon economy. This research investigates a SGE system comprising 40 boreholes installed in a university building located in Aveiro City under Mediterranean climate conditions. Both the building and the underground soil were monitored during the SGE operation, and EnergyPlus® software was subsequently employed for the integrated dynamic simulation. The building energy model was calibrated using field data, and a good agreement between the measured and the simulated temperatures was archived, with a Coefficient of Variance of the Root Mean Square Error (CVRMSE) below 10 %. The case study model performance was analysed and compared to other alternatives. The impact of the climate on the system performance was investigated by analyzing nine Portuguese cities situated in different climate contexts. A sustainability indicator was proposed, and an initial sustainability assessment was conducted. The results emphasize that the SGE system offered advantages compared to other alternative systems, achieving a 55 % reduction in heating and cooling energy consumption, and mitigating climate change by 405.67 ton of CO2 per 25-year lifespan. This study can guide further implementation of sustainable geothermal systems in Mediterranean climates.

Fossil energy reduction for heating and cooling of buildings using shallow geothermal integrated energy systems – a comprehensive review

Ground source heat pumps (GSHP) are a very efficient system for space heating and cooling, and it was established in 1904. GSHPs can minimize the environmental effect of buildings by using the ground as a renewable energy source. The ground will act as a heat sink or heat source. The research collection aims at finding the various possible opportunities for the effec-tive integration of shallow geothermal energy (SGE) to decrease the fossil energy in the built environment and to reduce emission associated with it. The direct utilization of SGE using a ground source heat pump (GSHP) has been reviewed in detail for global north and global south countries, with a primary focus on heating application. The punctual information of results of various authors have been extensively summarized. This review discusses the GSHP installation status, SGE availability, GSHP system simulation, feasibilities, and performance. Worldwide more than one million GSHP systems have been installed, and the system is prev-alent in Europe, the Americas, and Asia. Most of the systems are installed for heating-domi-nated buildings in the global north. This paper also contains the research details pertaining to the last two decades about refrigerants and compressors for the development of GSHP. Finally, the feasibility study and the performance of the GSHP unit for different climatic conditions are reviewed and it is found that the technique is more feasible for cold and dry climatic con-ditions. This paper highlights the recent research findings and a potential gap in the above components for further research and development.

Thermo-mechanical behaviour of microbial induced carbonate precipita-tion (MICP) sand for geothermal pavements

Introduction
 Ground source heat pump (GSHP) or shallow geothermal energy systems are gaining attention for helping combat global warming and the negative effects of urbanisation caused by human activities. In recent decades, energy geo-structure techniques have developed by using subsurface infrastructures to exchange heat with the ground. These techniques can provide space heating and cooling while still preserving the primary structural function. As a result, they have become a valuable part of geothermal energy systems. Researchers have developed an understanding of incorporating ground heat exchangers into foundation piles, retaining walls, and tunnel linings with modest additional cost [1-4]. Pavements are also structures in contact with the ground that have the potential to be used as energy geo-structures (i.e., geothermal pavements), yet, their use has not been extensively studied [5-8].
 Soil thermal conductivity is an important factor influencing the efficiency of geothermal pavements since heat transfer in soils occurs primarily by conduction [9]. Microbially induced calcium carbonate (CaCO3) precipitation (MICP) is an innovative technique for strengthening sandy soils by coating and binding soil grains with calcium carbonate crystals. The distribution and arrangement of the CaCO3 within the pore spaces play a crucial role in determining the resulting strength of the treated sand. In addition, these crystals can act as thermal bridges to enhance the soil's thermal conductivity [10]. Combining geothermal pavements with MICP sand is still nascent, and the limited number of studies that exist mainly focus on the associated thermal property changes [11]. However, since the principal function of pavements is transmitting (dynamic) loads to the subbase and the underlying soil, the thermal conductivity of the MICP-treated pavement may vary as a result of the applied mechanical loads (e.g., due to the partial or total loss of thermal bridges and/or particle rearrangement). This research thus investigates the changes in the thermal conductivity of MICP-treated sands as they are subjected to quasi-static triaxial compression. The experimental results collected can deepen our understanding of the thermo-mechanical behaviour of MICP-treated sands and provide practical insights for using MICP to reinforce the subbase or underlying soil of geothermal pavements.
 Methodology
 This research performed a series of quasi-static triaxial tests on MICP-treated Houston sand, fine-grained, high-purity silica sand. Sporosarcina pasteurii (strain designation DSM 33) was used for the MICP treatment of the soil specimen. To study the effect of CaCO3 content on the thermo-mechanical performance of MICP-treated sand, three cementation solution treatment cycles were applied, yielding theoretical CaCO3 contents of 0.6%, 1.6% and 2.7% by weight, respectively. Details on the MICP treatment of the samples can be found in [12]. Samples for triaxial testing were treated in cylindrical tubes of 50mm inner diameter and 100mm height. To investigate the influence of the CaCO3 content on the soil thermal conductivity, MICP-treated specimens were air-dried prior to triaxial testing Triaxial tests were subsequently conducted in dry conditions to isolate the effect of the MICP and avoid the influence of water content on soil thermal conductivity (λ). Furthermore, a new miniaturised transient sensor was embedded in the triaxial samples to monitor the λ changes during the sharing phase [12].
 Results
 An example of the evolution of the deviatoric stress with axial strain under 50kPa confining stress (σ3) in the triaxial cell is shown in Figure 1a. Compared to the untreated sand, MICP-treated samples lead to higher peak strength and stiffness. Importantly, λ changes during triaxial testing for different CaCO3 contents are summarised in Figure 1b. Results indicate that the increase in CaCO3 content can significantly improve λ, and that λ rapidly decreases post-peak strength due to dilation and CaCO3 bond breakage. Once the samples reach its ultimate state, λ remains unchanged.

Thermomechanical behaviour of soils and soil-structure interfaces in thermo-active geo-structures

In recent years, the demand for sustainable, low carbon-emission energy solutions has risen, leading to an increased interest in shallow geothermal energy systems. These systems, such as ground source heat pump (GSHP) systems, harness the Earth's stable temperature at shallow depths as a heat sink and source of energy. Energy piles are a type of GSHP system that performs both structural and thermal energy-exchange functions [e.g. 1]. Despite the technological advantages of these systems, understanding the thermomechanical behavior of soils and soil-structure interfaces remains a challenge, limiting safe, efficient, and cost-effective designs. This research aimed to address these challenges by developing thermomechanical constitutive models for soils, implementing these models in boundary-value solvers, and analyzing the experimental results from direct shear tests performed on soils to understand their thermomechanical behavior at the soil-structure interface. By comprehensively examining the complex interactions between energy piles and surrounding soils, the research results offer a foundation for improved design standards and implementation.
 The methodology followed the following three main directions:
 Development of thermomechanical constitutive models for soils: A thermomechanical constitutive model based on the thermodynamic framework of Hyperelasticity-Hyperplasticity was developed to capture the major thermomechanical behaviors of fine-grained soils when subjected to temperature variations [2]. By utilizing a flexible yield surface within the framework, a more precise representation of soil behavior can be simulated. This framework was extended to develop a dual-surface thermomechanical model with an additional yield surface and a temperature-dependent kinematic rule to capture the shakedown behavior of soils subjected to heating-cooling cycles more accurately [3], as illustrated in Figure 1 (left) where two triaxial tests are simulated. The enhanced model addresses the need for improved representation of diverse soil behaviors and their implications for the performance of energy pile systems.
 Numerical aspects to improve simulation capability: A new flexible yield function was introduced that addresses the common issue of undesired elastic domains and erratic or divergent gradients in the numerical implementation of constitutive models with flexible yield surfaces and plastic potentials for use in implicit stress integration schemes [4]. By providing a robust and efficient framework for return mapping algorithms, this new yield function supports more precise simulations of soil behavior. An advanced numerical algorithm was devised to implement the thermomechanical constitutive model within a boundary-value solver such as the finite-element method [5]. The algorithm incorporates Gibbs energy potential, Lode angle dependency, and additional features to ensure accuracy, robustness, effectiveness, and convergence during simulations.
 Experimental analysis of the soil-structure interface including the impact of initial shear stress and thermal cycles: Laboratory-scale direct shear tests were conducted on fine- and coarse-grained soils to investigate their thermomechanical behavior at the interface with a concrete structure [6, 7]. A focus here was on the impact of thermal cycles; in particular, under realistic working stress conditions. Figure 1 (right) presents example results, where thermal cycles are applied in a temperature controlled shear box, with a clay-concrete interface which had a constant shear stress. Thermal creep is observed, which also has a hardening effect on future shear loading. By exploring the variations in soil behavior under differing temperature and loading conditions, the research deepens the understanding of the complexity inherent in soil-pile interactions for energy pile applications.
 This comprehensive investigation of the thermomechanical behavior of soils and soil-structure interfaces in the context of thermo-active geo-structures represents an essential step forward in optimizing energy pile systems for diverse applications. The developed constitutive models, numerical algorithms, and experimental analyses contribute valuable insights for improving energy pile design, ultimately leading to more efficient, safe, and cost-effective solutions.
 Future research should continue refining our understanding of the complex processes governing soil behavior in energy pile systems, with emphasis on consolidating the developed models and their implementation for a range of soil types and conditions. Additionally, further experimental studies should focus on elucidating the interactions between soils, piles, and the broader environment to identify potential opportunities for maximizing the performance of energy pile systems through innovative design and implementation.

A Numerical Study of the Behavior of Micropile Foundations under Cyclic Thermal Loading

Micropiles are small-diameter foundation elements that are widely used in building refurbishment to reinforce existing foundations or provide new foundations where access for construction is difficult. Thermally-activated (TA) micropiles could be useful as an efficient means of providing cost-effective ground-coupling when shallow geothermal energy systems are considered in building rehabilitation. It is well-established that thermal activation of pile foundations results in thermo-mechanical interactions between the pile and the surrounding soil. These thermally-induced effects need to be examined to ensure that they do not adversely impact the load transfer function of the micropile. Numerical analysis is able to produce reliable predictions of thermo-mechanical behavior of TA piles, and this study applied this technique to examine the cyclic thermal behavior of micropiles, isolated and in groups. For the situations considered in this study, it is shown that during cyclic thermal activation, irrecoverable movements are unlikely to be significant in design terms, if the initial mobilization of the shaft resistance is low. Though stable, cyclic thermal movement amplitudes are large enough that they should be considered in design. The study highlights that large changes in thermal stress can develop and be locked-in to the response of long flexible piles, and that these should be verified in design. Further, as pile spacing reduces, thermal interference results in a loss of heat exchange capacity per pile, which has to be considered in the design of large groups of TA micropiles. Therefore, TA micropiles can offer an efficient and secure means of providing ground coupling in shallow geothermal energy systems.

Estimating cooling capacities from aerial images using convolutional neural networks

In recent decades, the global cooling demand has significantly increased and is expected to grow even further in the future. However, knowledge regarding the spatial distribution of cooling demand is sparse. Most existing studies are based on statistical modelling, which lack in small-scale details and cannot accurately identify individual large cooling producers. In this study, we implement and apply a novel method to identify, map and estimate nominal cooling capacities of chillers using deep learning. Chillers typically use air-cooled condensers and cooling towers to release excess heat, and produce most of the cooling needs in the commercial and industrial sectors. In this study, these units are identified from aerial images using specifically trained object detection models. The corresponding nominal cooling capacity is then estimated based on the number of fans of air-cooled condensers and the fan diameters of the cooling towers, respectively. Both detection and capacity estimations are first evaluated on test data sets and subsequently applied to an industrial area (Brühl) and the city center in Freiburg, Germany. In Brühl, aerial images show chillers with an estimated nominal cooling capacity of 205 MW, of which the model detected 88%, while 88% of all detections are correct. In the city center, a nominal capacity of 18.6 MW is estimated, of which the model detected 87% with 77% of all detections being correct. Hence, the developed approach facilitates a reliable analysis of the installed nominal cooling capacity of individual buildings at large scales, such as districts and cities. This information could be further used to locate areas for investments and support planning of eco-friendly, centralized supply of cooling energy, for example district heating and cooling systems or shallow geothermal energy systems such as aquifer thermal energy storage (ATES).

Improvement in Operation Efficiency of Shallow Geothermal Energy System—A Case Study in Shandong Province, China

Shallow geothermal energy (SGE) is a renewable energy source that has the advantage of being low-cost, clean, and locally sourced compared to fossil fuels, and is thus significant for China to achieve its future goals of carbon peaking and carbon neutrality. However, determining how to improve the operational efficiency of SGE systems is a key factor in the sustainable development and utilization of geothermal energy. This study examined the long-term operational efficiency of SGE systems and applied numerical simulation methods of hydro-thermal coupling to a SGE utilization project in Shandong Province, China. The effect of the distribution of pumping and injection wells on the operation efficiency of the SGE system was analyzed, and the parameter of operation efficiency, defined by the ratio of the practical minable shallow geothermal energy to the theoretical shallow geothermal energy, was applied to quantify the operation efficiency of the SGE system. The simulated results show that the phenomenon of heat transfixion is significant in the current operation scheme, where one of three pumping wells is located downstream of the study area, which indicates that the local groundwater flow field mainly controls the operation efficiency of the SGE system. In the optimized operation scheme, the distribution of pumping and injection wells can be adjusted according to the feature of groundwater flow and temperature fields. The degree of heat transfixion significantly declines and the operation efficiency increases by 71.5%. In addition, further improvements in the operational efficiency of the SGE system can be considered through the running time. The findings of this paper will be useful for the construction and management of SGE systems.

Fifth-Generation District Heating and Cooling Networks Based on Shallow Geothermal Energy: A review and Possible Solutions for Mediterranean Europe

This document presents a comprehensive review of research works, regulatory frameworks, technical solutions, and commercial trends related to the integration of shallow geothermal energy (SGE) technologies in modern 5th-generation district heating and cooling (5GDHC) networks. This literature and market analysis is contextualized by the present geopolitical, environmental, and societal scenario in Europe. In this sense, decarbonization of the heating and cooling sector is a crucial piece in the energy transition puzzle to keep global warming below the critical threshold of 1.5 °C by the next century. Moreover, Ukraine war has added urgency to end up with fossil fuel dependency. The most relevant outcome of this literature review is the synergistic relationship between SGE, 5GDHC networks, and urban environments. SGE is most efficiently deployed in urban environments when it is part of a district heating and cooling network, and the modern concept of 5GDHC is the most suitable scenario for it. Since the potential contribution of SGE to the decarbonization of the heating and cooling supply is mostly untapped across Europe, this synergistic effect represents a possible boost. Hybridization with solar photovoltaics and/or storage makes it even more attractive. Outstanding cases are reviewed, challenges for the future are presented, and tools to overcome social reluctance and/or lack of awareness are described, along with a discussion of the stimuli for the deployment of SGE and 5GDHC networks. A particular focus on Mediterranean countries is presented, where SGE systems and DHC networks of any kind show a particularly low deployment compared to the rest of Europe. To this end, the second part of this work evaluates, justifies, and analyzes the possibilities and potentialities of their application in this zone.

A systematic review on shallow geothermal energy system: a light into six major barriers

Shallow geothermal energy systems (SGES) are being widely recognized throughout the world in the era of renewable energy promotion. The world is aiming to promote and implement the concept of nearly zero energy consumption in the building sector. Shallow geothermal energy systems have huge potential to meet the heating and cooling demand of a building with low carbon emissions. However, the shallow geothermal system exploration rate and its global contribution to renewable energy used in the buildings sector is yet relatively low. Therefore, this study explores specific barriers which hinder the promotion of shallow geothermal energy systems through a systematic review of the literature. The study was carried out by investigating published papers indexed in Scopus and Web of science core collection databases. The selected papers are focused on shallow geothermal energy systems and barriers to their promotion. Only review and research articles types were included in the analysis and constrained to the topic of closed-loop shallow geothermal energy systems. This system’s promotion has been influenced by the lack of legislation, little knowledge about the conductivity of soil and by high initial investment cost at its topmost. The least influencing barrier is considered to be the heating and cooling efficiency of shallow geothermal energy systems.

Application of a method for the sustainable planning and management of ground source heat pump systems in an urban environment, considering the effects of reciprocal thermal interference.

The "Most Easy, Efficient and Low Cost Geothermal Systems for Retrofitting Civil and Historical Buildings" (GEO4CIVHIC) project aims to accelerate the deployment of shallow geothermal systems for heating and cooling purposes when retrofitting existing and historical buildings. Analyzing the implementation process of borehole heat exchangers (BHEs), allows the understanding of how to promote the long-term sustainability of shallow geothermal energy systems. The thermal interference between BHE systems represents a problem, especially due to the increasing deployment of this technology and its spread in densely built-up areas. The main goal of this paper is to propose a conceptual model and to apply this to different case studies. The methodology includes phases to adopt an integrated approach for preventing long term thermal interference in neighbouring borehole heat exchangers, by providing management strategies and technical suggestions for design and operation. The method developed follows the following steps: 1) literature review to determine what are the main drivers for thermal interference between shallow geothermal systems, in the context of the GEO4CIVHIC project case study sites; 2) to create a conceptual model to limit thermal interference at both design and operational phases; 3) to apply the developed method to real and virtual case studies in countries with different regulatory frameworks and to test its main strengths and weaknesses. The application of this conceptual model to specific case studies provides evidence of critical planning and operational characteristics of GSHP systems and allows the identification of measures to mitigate impacts of thermal interference to be identified.

A data-driven approach to low-enthalpy shallow geothermal energy extraction: A case study on indoor heating for precision agriculture applications

Shallow geothermal energy systems utilize the upper layers of the ground to provide space heating and cooling, with a relatively high efficiency. The research presented aims to assess the potential harnessing of this source and applying it for indoor conditioning of a precision agriculture facility. An experimental phase was conducted to measure the temperature of the ground at 0.75 m, 1.5 m, 2.25 m, and 3.0 m. Field testing was performed employing two probes; one uncovered and a second insulated with a polyethylene sheet, to evaluate the effect soil water content variations exert over the soil temperature. It was found that, given the high permeability and good drainage of the soil material, water content is not statistically significant, ruling out the presence of microconvective phenomena within the interstices of the undisturbed earth. Based on the temperature profile obtained, achievable heat extraction capacity was computed by modelling a buried pipe system. A parametrization was conducted showing that, of all the involved factors, pipe depth, pitch and their interaction, are the ones that contribute the most to the heat output variability. The computed available heat flow was used to simulate the performance of a geothermal heat pumping system for greenhouse heating based on real operation data, provided by the owners. The results show that, shallower layers of the ground, can provide adequate solutions for immediate thermal needs; a 5-pipe, 100m-length ground loop at 0.75 m, could provide 90% of the total thermal demand, whilst an 11-pipe, 200m-length loop, at 1.5 m depth, reaches 84% of the same demand, with a variability reduction in availability of 10% to 20% depending on the severity of surface conditions. However, for deeper layers, i.e., 2.25 m and 3.0 m, respective maximum heat output amounts to 43% and 49% of the total thermal demand. Nonetheless, these configurations yield the most stable and constant heat flow of the lot, making them particularly suitable for preheating/subcooling processes or even long-term thermal storage. Despite not fulfilling the total thermal demand of the greenhouse, these setups are more than capable of providing support to on-site conditioning devices; requiring a lower upkeep and yielding a higher efficiency, which would reduce overall production costs. Furthermore, from the conducted data analysis, it was found that controlling the agronomic values, associated to indoor agriculture, facilitates the physical aspect of greenhouse energy management, paving the way to more optimized thermal solutions, tailored to different sets of operation conditions.

A Critical Review on the Use of Shallow Geothermal Energy Systems for Heating and Cooling Purposes

The reduction of CO2 emissions has become a global concern. In this regard, the EU intends to cut CO2 emissions by 55% by 2030 compared to those of 1990. The utilization of shallow geothermal energy (SGE) in EU countries is considered the most effective measure for decarbonizing heating and cooling. SGE systems utilize heat energy collected from the earth’s crust to provide secure, clean, and ubiquitous energy. This paper provides a literature review on the use of SGE for heating and cooling purposes. The latest advances in materials, new innovative structures, and techno-economic optimization approaches have been discussed in detail. Shallow geothermal energy’s potential is first introduced, and the innovative borehole structures to improve performance and reduce installation cost is outlined. This is followed by an extensive survey of different types of conventional and thermally enhanced collectors and grouts. Attention is mainly given to the techno-economic analysis and optimization approaches. In published case studies, the least economic break-even point against fossil fuel-based heating systems occurs within 2.5 to 17 years, depending on the local geological conditions, installation efficiency, energy prices, and subsidy. Ground source heat pumps’ cost-effectiveness could be improved through market maturity, increased efficiency, cheap electricity, and good subsidy programs.

A method for the sustainable planning and management of ground source heat pump systems in an urban environment, considering the effects of reciprocal thermal interference

The “Most Easy, Efficient and Low Cost Geothermal Systems for Retrofitting Civil and Historical Buildings” (GEO4CIVHIC) project aims to accelerate the deployment of shallow geothermal systems for heating and cooling purposes when retrofitting existing and historical buildings. Analyzing the implementation process of borehole heat exchangers (BHEs), allows the understanding of how to promote the long-term sustainability of shallow geothermal energy systems. The thermal interference between BHE systems represents a problem, especially due to the increasing deployment of this technology and its spread in densely built-up areas. The main goals of this paper are: a) to analyze the design phase of a BHE system in order to prevent mutual thermal interference, b) to propose a model that encloses phases to adopt an integrated approach for preventing long term thermal interferences, c) to give technical and management suggestions to minimize thermal interference between closed-loop geothermal systems. The method developed follows the following steps: 1) literature review to determine what are the main drivers for thermal interference between shallow geothermal systems, in the context of the GEO4CIVHIC project case study sites; 2) to create a conceptual model to limit thermal interference at both design and operational phases; 3) to apply the developed method to real and virtual case studies in countries with different regulatory frameworks and to test its main strengths and weaknesses. The application of this conceptual model to specific case studies provides evidence of critical planning and operational characteristics of GSHP systems and allows the identification of measures to mitigate impacts of thermal interference to be identified.

Site characterization for the design of thermoactive geostructures

This paper addresses the topic of site characterization for the design of Shallow Geothermal Energy (SGE) systems, namely of thermoactive geostructures, which are geotechnical structures, such as piles, retaining walls and tunnel linings, also used as heat exchangers as part of closed-loop SGE systems. Such solutions, being increasingly adopted for buildings’ and infrastructures’ heating and/or cooling, are considered sustainable and cost effective. For the design of the primary circuit of the SGE system, which is embedded within the superficial soil layers, a comprehensive knowledge of the ground condition at the site is mandatory. This includes the evaluation of the energy features and whether the system can provide the required energy needs during the operational period, as well as the verification of the structural and geotechnical safety and functionality requirements. The site characterization for SGE systems involves different stages, from desk studies to detailed characterization, including in-situ trials, laboratory testing of undisturbed soil samples and the study of possible interferences. The specific aspects that will be addressed are: (i) the assessment of the site undisturbed ground temperature and its hydrogeological features; (ii) the thermal and thermomechanical characterization of the different soil layers; (iii) the investigation of the ground-heat exchanger thermal resistance; (iv) the collection of information related to the environmental constraints and to potential interferences among multiple users, which are related to the service life of the structure. The overall aim is to ensure a proper design of the SGE system for guaranteeing its sustainability in the long term.

The influence of initial shaft resistance mobilisation in the response of seasonally, thermally-activated pile foundations in granular media

The thermal-activation of pile foundations for use within shallow geothermal energy systems has received much attention and the results from various studies have allowed the development of a broad understanding of the thermo-mechanical behaviour of thermally-activated (TA) pile foundations. One area that requires further attention is the clarification of the foundation response under seasonal cyclic thermal loading. This study systematically assesses the impact of balanced cyclic thermal loading in relation to the initial mechanical loading, for isolated floating piles and pile groups in a frictional soil medium. The pile thermal response was evaluated for differing levels of mobilisation of the shaft resistance with respect to the initial mechanical loading and for piles at varying spacing based on a unit cell approach. Isolated piles with high shaft mobilisation were found to develop significant irrecoverable settlement during thermal cycling however the maximum thermal stress was similar irrespective of the initial shaft mobilisation. These results are remarkably similar to those reported in small scale tests of TA piles in sand, confirming that the behaviour reported in these is likely to be due to the result of low shaft restraint, rather than thermally-induced loss of resistance on the pile–soil interface. On the other hand, piles in close proximity were found to behave quasi thermo-elastically at the same initial mechanical load, which is attributed to an alteration in mobilised pile resistance due to pile–soil–pile interference under the initial mechanical loading and, to a lesser extent, thermal activation of the ground between the piles both of which reduce the relative movement between the pile and the soil. This pile group thermal effect has major beneficial implications for the geotechnical and structural design of TA pile foundations.