Shallow Geothermal Systems Research Articles

Shallow geothermal energy systems for district heating and cooling networks: Review and technological progression through case studies

Heating and Cooling constitute a major part of society's final energy use and a significant contributor to greenhouse gas emissions. The world society ought to mitigate climate change through decarbonisation, which must include the transition to low-temperature, sustainable and renewable heating and cooling technologies. Shallow Geothermal Energy is one of the most energy efficient and least greenhouse gas emitting available alternatives to provide space heating and cooling. The decarbonisation of the heating and cooling sector may have to comprise both individual systems and shared electrified heating and cooling systems from renewable sources of energy, where economies of scale and synergies between different types of consumers can be exploited. To this end, the focus of this paper is on the integration of shallow geothermal energy technologies into district heating and cooling systems. A key contribution of this work is the illustration of a number of practical case studies, highlighting the potential of existing shallow geothermal systems for DHC networks, which, as front runners in adopting such technologies, serve as paradigms for future development. Follows a discussion providing an outlook over the next 25 years. All in all, the future of utilizing shallow geothermal energy for district heating and cooling seems to be promising to play a pivotal role in sustainable urban development and decarbonizing the heating and cooling sector.

Small-scale physical modelling of vertically loaded, cyclically thermally-activated helical piles

To investigate the use of thermally-activated helical piles in shallow geothermal energy systems, a 1-g modelling study was conducted. Helical piles with either a single- or double- helix were installed in a medium dense, dry sand, and subjected to mechanical, thermal only and thermo-mechanical loading. The results indicate that during the thermal tests (1 – 3 cycles), a small upwards residual displacement was observed and pile head movements ranged between about 90% and 100% of the free expansion of the pile shaft above the shallowest helix, suggesting that the helices fixed the shaft and little restraint was offered by the surrounding soil. In the thermo-mechanical tests (30 thermal cycles), the pile head developed irrecoverable settlement as a function of the number of helices (more helices, less settlement) and initial load (higher load, greater settlement). No significant alteration in pile axial stiffness or resistance was found for piles with zero mechanical load that underwent only a few thermal cycles; however, an increase in stiffness and resistance, beyond that due to inherent variability in the test setup, was observed for piles with an initial load and following a large number of thermal cycles. The testing of thermally-activated helical piles in sand has confirmed that the response is similar to conventional piles and that thermal ratcheting effects can be managed by the application of suitable margins of safety in design and/or the use of multi-helix piles.

The role of pit lake thermal dynamics on the thermal performance of ground heat exchangers

The addition of ground heat exchangers (GHEs) to a pit lake’s basin has the potential for abundant, clean and renewable geothermal energy extraction using shallow geothermal systems. Basin-embedded GHEs avoid direct interaction with mine water, which has been shown to impact efficiency and longevity in mine open-loop geothermal systems negatively. The now accelerated closure of open-pit coal mines presents itself as an opportunity to use this technology. However, no guidelines currently exist for designing or operating GHEs embedded in the sediment of water bodies. Furthermore, the two-way coupling between the complex annual thermal fluid dynamics that lakes are naturally subjected to and heat fluxes on the sediments and the GHE system has not been explored. In this study, we develop and validate finite element models to assess the relevance of lake thermal stratification in the performance of a geothermal system embedded in water bodies basins, e.g., on open-pit mine closures, under temperate residential thermal loads. The results show that the pit lake’s role as a thermal sink improves significantly when the lake’s thermal dynamics are accounted for, with an increase of up to 292% in the lake’s available energy budget. A minor variation in energy budget (~8%) was found whether the lake is modelled explicitly or simplified as a transient Dirichlet temperature boundary condition. This small difference vanishes if horizontal circulation along the lake is considered, highlighting the lake’s thermal energy potential. Finally, the impact on the GHE Coefficient of Performance (COP) is evaluated, with a maximum of ~15% difference among all cases.

The impact of shallow geothermal HVAC systems with heat pump units on the ground and ground water

The impact of shallow geothermal HVAC systems with heat pump units on the ground and ground water

Decommissioned open-pit mines are potential geothermal sources of heating or cooling for nearby population centres

In the shift to cleaner technologies, coal thermal power plants and mines will be decommissioned earlier than planned. These open-pit coal mines show potential as a geothermal source/sink for space heating and cooling, yet this post-closure use option remains unexplored. Here, we assess the techno-economic feasibility of providing heating and cooling using a decommissioned mine pit as a source or sink of heat for nearby population centres. Mixed integer quadratically constrained programming is employed for operational optimisation. The results show that thermal provisioning, considering centralised and decentralised arrangements, is economically competitive under specific scenarios regarding commodities prices, thermal demands and pit source temperature. Notably, a higher pit temperature reduces the overall thermal provisioning costs for residential thermal demands. Improvements in the economics due to transmission pipeline insulation are limited. While an expected decline in the system lifecycle costs occurs for decreasing electricity prices, the results highlight that the relative economic competitiveness of the thermal systems is tied to the individual alternatives’ performance at the same commodities prices. Overall, this paper identifies general conditions for techno-economic competitiveness for the implementation of shallow geothermal systems in the context of mine closure.

Integrated Underground Analyses as a Key for Seasonal Heat Storage and Smart Urban Areas

The design and performance of a shallow geothermal system is influenced by the geological and hydrogeological context, environmental conditions and thermal demand loads. In order to preserve the natural thermal resource, it is crucial to have a balance between the supply and the demand for the renewable energy. In this context, this article presents a case study where an innovative system is created for the storage of seasonal solar thermal energy underground, exploiting geotechnical micropiles technology. The new geoprobes system (energy micropile; EmP) consists of the installation of coaxial geothermal probes within existing micropiles realized for the seismic requalification of buildings. The underground geothermal system has been realized, starting from the basement of an existing holiday home Condominium, and was installed in dry subsoil, 20 m-deep below the parking floor. The building consists of 140 apartments, with a total area of 5553 m2, and is located at an altitude of about 1490 m above sea level. Within the framework of a circular economy, energy saving and the use of renewable sources, the design of the geothermal system was based on geological, hydrogeological and thermophysical analytical studies, in situ measurements (e.g., Lefranc and Lugeon test during drilling; Rock Quality Designation index; thermal response tests; acquisition of temperature data along the borehole), numerical modelling and long-term simulations. Due to the strong energy imbalance of the demand from the building (heating only), and in order to optimize the underground annual balance, both solar thermal storage and geothermal heat extraction/injection to/from a field of 380 EmPs, with a relative distance varying from 1 to 2 m, were adopted. The integrated solution, resulting from this investigation, allowed us to overcome the standard barriers of similar geological settings, such as the lack of groundwater for shallow geothermal energy exploitation, the lack of space for borehole heat exchanger drilling, the waste of solar heat during the warm season, etc., and it can pave the way for similar renewable and low carbon emission hybrid applications as well as contribute to the creation of smart buildings/urban areas.

Thermal-force response analysis and applicability evaluation of strata energy shaft in Shanghai

Energy underground structures, such as energy shafts and tunnels, are a class of energy-saving and environmentally-friendly underground structures. They can be used as shallow geothermal energy harvesting systems for heat exchanges with the ground. Theoretical research and practical applications of energy shafts are still in the initial stages, and related tests are still relatively few. This study used the Shanghai Taihe energy shaft test section as the research background. The results of the thermal force response test of the energy shaft was numerically analyzed using the COMSOL software and compared with that of Beijing Qinghuayuan energy tunnel for applicability evaluation and analysis. The results show that the Shanghai strata energy shaft has a better heat transfer capacity than the Beijing strata energy tunnel owing to the different geological environments and is more economical. Simultaneously, the temperature stress generated in the heat transfer process of the Shanghai Strata energy shaft was smaller than that of the Beijing strata energy tunnel, which is safer. Therefore, underground energy structures can be widely applied to the Shanghai strata, which can significantly improve economic applicability while ensuring safety.

Effects of varying heat transfer rates for borehole heat exchangers in layered subsurface with groundwater flow

The existence of groundwater flow adds a convective heat component to shallow geothermal heat pump systems, significantly improving heat transfer efficiency. Numerous analytical and numerical methods have been proposed to assess the impact of groundwater flow on ground heat exchangers. However, numerical simulations often require significant resources and time, while conventional analytical solutions overlook the time and depth-varying heat transfer rates, leading to inaccurate temperature predictions. This study proposes a computationally efficient semi-analytical solution by extending the line source solution and coupling it with a thermal resistance model. This approach addresses variations in heat transfer rate over time and depth, overcoming the limitations of conventional analytical solutions. Compared with a three-dimensional numerical model constructed in COMSOL Multiphysics, the average absolute percentage error (MAPE) of borehole wall temperatures for a U-shaped borehole heat exchanger (BHE) is approximately 1 %, across a wide range of water flow velocities and thermal conductivity ratios between two contacting geological layers. A significant advantage of the proposed solution is its computational efficiency. The proposed solution can complete long-term simulations with over 9,000 time steps in about 10 min, as opposed to numerical models that typically require several hours. The excellent computational efficiency and substantial accuracy make the proposed solution a simple and effective tool for the design and optimization of BHEs in engineering applications.

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.

Using thermal response factors with time dependent thermal properties

This paper presents a simple and fast methodology to consider changing thermal properties in, for example, the design of shallow geothermal systems, by using thermal response factors. The simulation period is split into multiple sections according to the changes in thermal properties. By transforming the temperature response from one section to the next and subsequent superposition, any changes in the thermal conductivity properties of the media through which heat travels can be taken into account. The method is verified by numerical simulation and its efficiency is demonstrated in an application example. Results show that even though the computational effort increases exponentially with the variation of thermal parameters in time, the computational time is significantly shorter than comparable numerical simulations.

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

The impact of soil layering and groundwater flow on energy pile thermal performance

Shallow geothermal energy pile systems have emerged as cost-effective and low-carbon alternatives for heating and cooling buildings, compared to traditional air-conditioning systems. Geothermal applications have been researched extensively in recent years under the assumption of ground homogeneity, and the effect of ground stratification remains mostly unexplored. To investigate this, a 3D finite element numerical model is developed and validated against laboratory-scale experimental data, to study the transient diffusion-convection heat transfer linked with Darcy groundwater flow around energy piles in multi-layered lithology. The model is used to undertake long-term assessments under balanced and unbalanced thermal loads to evaluate the thermal effects of soil layering and discrepancies against commonly assumed equivalent homogeneous stratum, for soil profiles with different thermal conductivity distributions. The groundwater flow effect at various depths and seepage velocities on the thermal performance of the energy pile is investigated as well. Results demonstrate the need to account for the spatial variability in thermal properties, particularly for unbalanced thermal loading scenarios. The thermal yield can be underestimated by up to 19.6 % due to an inaccuracy of 48.2 % in the accumulated temperature after 25-year of operation with respect to an equivalent homogeneous ground with depth-weighted average thermal conductivity. This discrepancy grows as the contrast between layers increases. An empirical derived formula is presented and tested, presenting a correction to the effective thermal conductivity of the layered systems in this study that considers the thermal contribution of the ground beneath the pile. Groundwater seepage is shown to have a positive impact on the heat exchanger efficiency, and in the layered geology the efficiency under-estimation becomes more critical at low to moderate Darcy velocities, if neglected or inaccurately measured. These findings contribute to a broader understanding of energy piles and can assist engineers and practitioners in optimising energy geo-structure design, boosting the technology’s viability.

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.

Shallow geothermal systems in dense urban areas: The issue of thermal interference and long-term sustainability

Shallow geothermal systems, namely Ground Source Heat Pumps (GSHP) and Ground Water Heat Pumps (GWHP), are expected to give an increasing contribution to the decarbonization of the buildings climatization sector. A fully sustainable use should guarantee fair access to the shallow geothermal sources for new systems, given the potential thermal interference among neighbouring ones in dense urban areas, and address environmental concerns related to thermal pollution of ground and groundwater. In this paper the state of the art concerning environmental concerns, regulation approaches and sustainability metrics is firstly reported. Then, focusing on closed-loop systems, a simulation case study is developed to study the long-term thermal footprint in the ground. The Energy Imbalance indicator, summarizing the annual energy balance in the ground, drives the thermal drift produced by the bore-field and is therefore proposed as the main sustainability indicator. For given ground conditions, a maximum Energy Imbalance is identified, which limits the thermal perturbation distance to the borehole spacing and minimizes thermal interference with other systems.

Assessing the potential of using geothermal energy in buildings: Parametric analysis

For the development of more energy-efficient buildings, pursuing comprehensive sustainable solutions can help decrease greenhouse gas emissions, and using natural resources sustainably and at low cost are challenging objectives for the achievement of the United Nations (UN) sustainable development goals. In this context, the main objective of this research is the evaluation of the energy-efficiency improvement of shallow geothermal systems, based on one case study building located at Aveiro University. The present research undertakes the improvement of several parameters with influence on the shallow geothermal systems efficiency, using as a demonstrator a university building in Aveiro University, the CCCI department building. The performed work was developed in three phases: (i) the acquisition of vertical temperature profiles in 15 exploration boreholes, (ii) the simulation of the buildings’ energy needs using EnergyPlus® software, and (iii) the evaluation of the energy demand of the geothermal system using HYGCHP® software. A sensitivity analysis was carried out with HYGCHP® by changing several parameters (average ground temperature, soil thermal conductivity, pipe radius, and total installation length) to assess the reduction potential of the annual energy consumption. Combining different parameters in the design phase allows a considerable reduction in the annual energy consumption. Two parameters are highlighted with a higher influence: the soil thermal conductivity and the average soil temperature (measured in the boreholes). A synthesis of the simulations carried out will lead to concrete recommendations and guidelines for future planning actions regarding geothermal systems installation to ensure sustainable use conditions, comfort, and health in the campus environment.

Mapping of very shallow geothermal potentials in rural areas of Bavaria

Climate change is a rising issue which strongly influences contemporary society. Therefore, the utilization of sustainable non-fossil energy sources is one of the most important goals in order to reduce greenhouse gas emission. Utilization of geothermal energy for heating and cooling buildings or residential units is one of the significant steps in providing sustainable and renewable energy supply. This paper presents very Shallow Geo-thermal Potentials (vSGP) of German federal state Bavaria, with special focus on rural are-as. Main goal of the study was to analyze the potentials for utilization of very shallow geo-thermal systems in terms of thermal conductivity and heat extraction. High-resolution soil maps containing information of grain size conditions served as an area-wide data basis for the research, while the analysis and visualization of the results were conducted by GIS software. Thermal conductivity as well as system-specific heat extraction were calculated depending on soil texture and climate conditions. Thermal conductivity results are intend-ed to be further used as the basic parameter for planning and installing horizontal geo-thermal heating and cooling systems.

Optimization approaches for the design and operation of open-loop shallow geothermal systems

Abstract. The optimization of open-loop shallow geothermal systems, which includes both design and operational aspects, is an important research area aimed at improving their efficiency and sustainability and the effective management of groundwater as a shallow geothermal resource. This paper investigates various approaches to address optimization problems arising from these research and implementation questions about GWHP systems. The identified optimization approaches are thoroughly analyzed based on criteria such as computational cost and applicability. Moreover, a novel classification scheme is introduced that categorizes the approaches according to the types of groundwater simulation model and the optimization algorithm used. Simulation models are divided into two types: numerical and simplified (analytical or data-driven) models, while optimization algorithms are divided into gradient-based and derivative-free algorithms. Finally, a comprehensive review of existing approaches in the literature is provided, highlighting their strengths and limitations and offering recommendations for both the use of existing approaches and the development of new, improved ones in this field.

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.

Evaluation of different methodologies for calculating the energy demand and their influence on the design of a low enthalpy geothermal system

The increasing importance of shallow geothermal resources in the decarbonization of heating and cooling systems requires the correct management of all the project stages. One of the fundamental steps in this process is determining the space energy demand, which plays a significant role in the subsequent geothermal design. In the context of Spain, different tools are available for the estimation of the mentioned parameter. For evaluating these procedures, this research applies the principal energy demand calculation tools and uses the outcomes for the later design of the shallow geothermal system. Results show how the Spanish official tools (HULC and CE3X) provide lower energy demand values adjusted to the construction conditions of the building that allow the optimization of the geothermal well field. On the contrary, simpler, and more intuitive applications (regular spreadsheets and GES-CAL) assume higher heating energy demands, which in turn implies an oversizing of the geothermal scheme. Even though all the procedures ensure to cover the energy requirements of the building, the most precise tools manage to reduce the initial investment of the system and its operating costs, in addition to reducing the global CO2 emissions because of the lower power of the associated geothermal heat pump.