Despite the advantages of using Bayesian networks for probabilistic risk assessment, adoption in practice has been limited due to the lack of realistic, facility-scale studies. Scaling up from systems to facility-level safety assessments poses challenges in (i) integrating external hazards and their cascading effects, and (ii) resolving non-homogeneity of various technical and human reliability models. The novelty of the study is in formalising risk integration using Bayesian networks, at facility scale, and demonstrating its effectiveness in addressing associated challenges. A Bayesian network-based multi-hazard risk framework is introduced and demonstrated for a nuclear power plant subject to flooding and earthquake hazards, capturing dependencies among hazards and consequences. Individual reliability models – conventionally extraneous to facility-wide risk models – are included as subnetworks by using Bayesian network-based surrogate models for technical systems and a Bayesian networks approach for human reliability modelling. Two approaches are used for subnetwork integration – object-oriented and unified Bayesian networks. The unified approach allows for prediction, diagnostics and inter-causal reasoning since Bayesian inference is bi-directional. Conversely, in the object-oriented approach, diagnostics are limited to within individual subnetworks and as a consequence the model can potentially neglect dependencies between objects. However, the object-oriented model requires only 50 % of the computational memory and consumes less than 25% of the runtime as the unified network, while improving visual clarity of the risk model. The model reveals key insights – for example, variations in operator stress or available response time during a hazard event can result in up to a 77 % change in top event probability – demonstrating its effectiveness in capturing critical relationships in complex, facility-scale risk scenarios. These findings can be used to suitably allocate resources towards risk mitigation and plant safety management.

High-temperature aquifer thermal energy storage (HT-ATES) can play a key role in the energy transition. For well completion of conventional low-temperature ATES and groundwater wells, grout and/or clay pellets are typically utilised as annular materials to ensure the long-term well integrity. It is not yet known if such materials can also be used in HT-ATES working conditions. In this work, a novel approach to evaluate the sealing performance for such completion materials is proposed and tested over multiple thermal heating and cooling cycles representative of the conditions of HT-ATES operation. The experimental framework utilises a novel experimental design to test the apparent transmissivity of the annular material, followed by micro-CT scanning. During each test, up to 11 thermal cycles are applied, with temperature variations between 22oC and 90oC. For grouts after 7 days of curing, micro-CT scans reveal debonding and the occurrence of micro-annuli with an equivalent diameter of approximately 26% of the original cross-section. After 28 days of curing, the thermal cycles had a much reduced impact on micro-annulus formation. The corresponding apparent transmissivity decreased up to 80% for samples containing a high percentage of cementitious minerals and a low water-to-grout ratio. The clay pellets, saturated with fresh water, demonstrated effective sealing capacity and an impermeable behaviour. However, clay pellets saturated with 0.25 mol/L NaCl, showed up to an 85% decrease in swelling capacity yet still exhibited impermeable behaviour. The results indicated that thermal cycles affect the integrity of grouts, while clay pellets show resilience to them. Furthermore, longer curing periods and specific chemical compositions improve sealing performance and provide resilience to thermal cycles.

Cow-dung is a widely used stabiliser applied in traditional earthen buildings with one objective to improve water resistance. However, most research has focused on explaining its mechanical strength, with only one study suggesting water resistance mechanism via formation of insoluble compounds at high pH, a phenomenon uncommon in natural cow dung and soil mixtures. This article investigates the water-resistance behaviour of cow-dung stabilised compressed earthen blocks (CD-CEBs) through an extensive experimental programme to understand the influence of cow-dung and soil related factors and to characterise the components of cow-dung responsible for its water resistance. It was found that the small-sized microbial aggregates (SSMA) present in cow-dung, which are negatively charged hydrophobic aggregates of low specific surface area, are responsible for enhanced water resistance of CD-CEBs. The insights gained from experiments are compiled to recommend the following strategies for improved performance of CD-CEBs: (i) The use of wet cow-dung is advised over dry cow-dung as it provided over 80 times better water resistance; (ii) Adopting a higher compaction liquid content (by 3%) improved the water resistance by over 40 times; (iii) The water resistance of CD-CEBs was improved over 30 times by using soils rich in low-swelling clay minerals such as kaolinite. A case study applying these findings demonstrates the successful scaleup from the lab to field showcasing potential of cow-dung and soil in low-carbon construction.

Integrating renewable energy into district heating creates a heat supply–demand mismatch that High-Temperature Aquifer Thermal Energy Storage (HT-ATES) can help address. However, the potential greenhouse gas emission reduction and financial benefits of HT-ATES have received limited attention. Additionally, the interplay between the demand, supply components, and HT-ATES has been overlooked, while the assessment of integrating HT-ATES into a district heating system is crucial to understanding the benefits of the HT-ATES implementation. This study evaluates the integration of HT-ATES into a district heating system, focusing on both economic and environmental performance indicators. It novelly accounts for the dynamic operational interactions between HT-ATES and other system components, enabling a more realistic assessment of operational choices. The model is applied to a case study of a simplified district heating system. The results show that the relative size of the heat supplier compared to heat demand is a key determinant of the cost-effectiveness of HT-ATES. In the case study, a geothermal doublet reduced the levelized cost of heat by 25–37 €/MWh compared to a gas boiler, while also reducing reliance on fossil fuels. In contrast, HT-ATES had a limited impact on total system costs, regardless of whether it operated when stored heat was available or was used for peak shaving. Nevertheless, HT-ATES increased the renewable energy share by 9%–18% across all scenarios. Furthermore, the optimal geothermal capacity differed depending on whether HT-ATES was included. Finally, while a high renewable energy share can be cost-effective, achieving 100% renewable heat was found to be highly cost-ineffective in this case. These results support informed decision-making on HT-ATES implementation under appropriate system design conditions.

Energiepfähle werden zunehmend als multifunktionale Gründungselemente eingesetzt, die tragende Funktionen mit der Nutzung geothermischer Energie kombinieren. Während das mechanische Verhalten in der Bemessung üblicherweise berücksichtigt wird, werden thermische Einwirkungen häufig in vereinfachter elastischer Form behandelt. Langzeitiges Heizen und Kühlen kann jedoch irreversible Bodenverformungen und Spannungsumlagerungen hervorrufen, insbesondere in weichen und geschichteten Böden. Dieser Beitrag untersucht die Relevanz thermoplastischen Bodenverhaltens für die Bemessung von Energiepfählen unter Verwendung eines thermo-visko-hypoplastischen Stoffmodells für feinkörnige Böden (AVISA-T), das zyklische Belastung berücksichtigt. Das Modell wird in einem Finite-Elemente-Programm implementiert und anhand großmaßstäblicher Energiepfahlversuche in Delft (NL) unter unterschiedlichen axialen Lastniveaus und langzeitiger thermischer Beanspruchung evaluiert. Die Ergebnisse zeigen, dass thermoplastische Effekte maßgeblich die Pfahlkopfverformungen sowie die Umlagerung der Normalkräfte entlang des Pfahls bestimmen können, was mit konventionellen thermo-elastischen oder thermo-elasto-plastischen Ansätzen nicht erfasst wird.

In this study, a newly developed rate-dependent thermo-plastic constitutive model was enhanced to incorporate thermally accelerated creep and implemented into the Plaxis finite-element code, enabling the simulation of the behaviour of a well-instrumented energy pile in multilayered soft soils under thermomechanical loads. First, the model was validated against non-isothermal laboratory tests on soils surrounding the pile, and then against simulations of field tests. The results revealed that the inclusion of thermally accelerated creep improves the prediction of irreversible pile settlement, which is primarily attributed to the accumulation of volumetric contraction in the surrounding soil after each thermal cycle. The analysis also distinguishes between drag down effects resulting from thermo-elastic and thermo-plastic behaviour, as well as those induced by long-term creep.

Rocks can undergo fatigue failure when subjected to cyclic mechanical, hydraulic, or thermal loadings, or a combination of these. Therefore, accounting for possible fatigue damage is important for subsurface engineering projects, such as the cyclic stimulation of geothermal reservoirs. However, existing models do not simultaneously account for degradation of both tensile strength and stiffness under varying-amplitude loading and coupled thermo-hydro-mechanical (THM) conditions. To address this, a new cohesive zone model is developed to account for the effect of fatigue on tensile strength and stiffness. The model is then used within the framework of zero-thickness interface elements to simulate the response of pre-existing or new fractures. Hydraulic and thermal processes are included in both the cohesive interface elements and the continuum elements, allowing the consideration of coupled thermo-hydro-mechanical processes. The fatigue damage variable is set to evolve with the number and magnitude of cycles according to Palmgren-Miner's rule. The proposed method is validated against three laboratory tests from the literature, including cyclic Brazilian test, cyclic hydraulic fracturing test and cyclic thermal stimulation test. All three validation results show that the fatigue damage or reduced breakdown pressure can be well reproduced. Mesh sensitivity based on the simulation of the Brazilian test, in which interface elements are inserted in-between all the continuum elements, highlights the influence of the mesh orientation and mesh density on the simulation results. In addition, stabilisation of the method is demonstrated by increasing the mechanical viscosity, which must be used with care to avoid predicting a longer fatigue life. The ability of the method to handle varying-amplitude cyclic loading is demonstrated by the simulation of a synthetic cyclic loading scheme based on the Brazilian test. The proposed method can be used to support the design of cyclic thermal stimulation campaigns for geothermal (or other) reservoirs, by being able to simulate the reduction in strength due to fatigue, and thus reducing stimulation pressures needed.

Soft stimulation technologies have been proposed as a means to reduce the breakdown pressure and mitigate the risk of induced seismicity during geothermal reservoir stimulation. Yet, the underlying mechanisms remain poorly understood due to the complexity of the coupled thermo-hydro-mechanical (THM) processes. In this work, a fully coupled THM model is developed to evaluate and compare the performance of different stimulation scenarios (monotonic, stepwise injection rate, cyclic injection rate or temperature, and stepwise combined with cyclic injection rate stimulation) on a synthetic, highly permeable reservoir with near-borehole clogging. Simulation results show that stepwise injection rate stimulation yields the most favourable outcomes, followed by the stepwise injection rate combined with cyclic injection rate stimulation. On the other hand, fatigue effects are seen to play a negligible role in the improved performance since the tensile stress at the fracture tip is relaxed with the continuous fracture growth. In addition, cyclic injection temperature stimulation is generally neither better nor worse than monotonic stimulation, but has slightly different characteristics, creating more local damage controlled by the period of the injection cycle. Cyclic injection rate stimulation can slightly reduce the peak pressure, compared with monotonic stimulation, but only when the injection rate is low. The reduction in peak pressure occurs due to the combination of thermally-induced stresses associated with cooling and incremental damage rather than any influence of fatigue. Stepwise or low-frequency cyclic injection rate stimulation are suggested rather than a high-frequency cyclic injection rate stimulation, while injection with cyclic temperatures is suggested when more local damage is wanted.

Low-enthalpy geothermal heat production is becoming increasingly common, which leads to the potentially competitive use of the available subsurface space, especially in densely populated urban areas. A specific challenge presented by the high density of different geothermal systems is understanding the details of convective and conductive heat flow processes and detailed monitoring of properties and processes in the subsurface.

On the TU Delft campus, we aim to drill a borehole of around 4.5 km depth to be used for the exploration, observation, and monitoring of subsurface processes that will be part of a larger research infrastructure under development. This so-called urban energy laboratory includes – in addition to the deep multi-use borehole – a well-instrumented geothermal doublet drilled in 2023, reaching to a depth of 2.2 km; a local seismic monitoring system (installed in 2022); an ultra-sensitive portable seismic monitoring array; and a high-temperature aquifer heat storage system (HT-ATES), for which a pilot well was drilled in 2024. With this urban energy laboratory, we want to tackle problems and better understand processes related to multiple and/or competing subsurface uses in urban environments. The deep exploration and monitoring borehole is designed specifically to monitor fluid and/or flux movement in 3D with unprecedented precision, aiming to understand the propagation of the geothermal cold front and reservoir pressures.

During the 3 d International Continental Scientific Drilling Program (ICDP)-sponsored UrbEnLab workshop, 75 scientists from 17 countries met in Delft, the Netherlands, in June 2024 to prioritize the scientific ambitions of the deep exploration and monitoring borehole and to discuss potential techniques that could be applied to tackle them. Assessing the life cycle of a geothermal system situated in a complex heterogeneous sedimentary system was defined as the broad aim, with revealing the detailed flow field established being a key priority.

Realistic numerical modeling of energy piles in soft soil requires advanced constitutive relationships capable of capturing the inherent thermo-plastic behavior of the surrounding ground. In this study, a newly developed rate-dependent, thermo-plastic constitutive model, called AVISA-T, is employed within the Plaxis Finite Element (FE) code to simulate the response of a well-instrumented energy pile embedded in multilayered soft soils subjected to thermo-mechanical loading. Following material parameter calibration and model prediction validation using non-isothermal laboratory tests on the soils surrounding the pile, the model was employed to simulate full-scale in-situ tests. In these simulations, the pile was initially subjected to either 0 % or 60 % of its bearing capacity and then exposed to continuous cooling over a period of up to three months. The AVISA-T model effectively reproduces the development of contractive and expansive strains, as well as compressive and tensile stresses that coexist along the pile shaft, including the accumulation of residual strains and stresses. In the absence of axial mechanical load, both residual contractive and expansive strains were observed, accompanied by irreversible uplift of the pile head, primarily attributed to non-uniform, unrecovered temperature changes. Moreover, under higher mechanical loading, the model captures dragdown effects resulting from thermal shrinkage of the surrounding soil, which contributes to the accumulation of permanent strains, stresses, and settlements. A comparison between simulations using the common Modified Cam Clay (MCC) model, the AVISA model without thermal effects and the AVISA-T model highlights the importance of using models including thermal plasticity for engineering practice.

This document describes the drilling, completion and testing of the pilot borehole (DEL-HTO-P01), which will later be utilized as a monitoring well for High Temperature Aquifer Thermal Energy Storage (HT-ATES). The work includes all materials, components, tools, and services that are related to the preparation, delivery, installation, measurement, and completion of the components belonging to the test drilling and monitoring well. The following analysis combines various data sources to characterize the subsurface for the Delft Demonstration site (WP1). The location of the DEL-HTO-P01 is demonstrated in Figure 1, which is located at RD-coordinates X = 85,257 m, Y = 445,757 m.

All data presented in this report have been published via TU Delft institutional data repository 4TU.ResearchData under the CC BY 4.0 license. The initial borehole dataset and the CT scan data can be found via https://doi.org/10.4121/1694ba82-db41-4017-8d1c-9de3ce1a785e and https://doi.org/10.4121/9902ebc7-dbd6-43b1-a3c3-85ff9ab645a9.

This paper presents quantitative data from a field test on a new type of energy pile, called a displacement cast in situ energy pile. The test pile was installed in a multilayered soft soils and subjected to a continuous cooling for 3 months, with no mechanical load. Afterwards, the pile was loaded to a specific target of 20 or 60 % of its calculated ultimate bearing capacity and then subjected to up to five thermal cycles. Under zero mechanical load, the results revealed that the compressive/tensile stresses coexist along the pile. Under low mechanical load (20 %), thermal cycles induced irreversible residual contractive strains and stresses as well as a limited pile head settlement. Under high mechanical load (60 %) and extreme operating conditions, i.e., negative temperatures which could have indicated a frozen interface, further irreversible settlements observed at the end of this test. Mechanical pile tests however indicated no impact of stress history (including the freezing test) on the shaft resistance and the overall pile-bearing capacity.

Underground Thermal Energy Storage (UTES) technologies are essential for advancing low-carbon heating and cooling systems, particularly in urban areas where space constraints and retrofitting challenges pose significant barriers. In this study the performance of a system of novel coaxial diagonal borehole heat exchangers (BHE) is analyzed during September–December 2024.

The Home Smart Energy (HSE) system, implemented in Medemblik, Netherlands, features a nine-borehole diagonal array arranged in a circular configuration. The boreholes are drilled at a 60° or 45° angle to depths of up to 40 meters, operating in a closed-loop coaxial setup. A brine mixture of water, operates with a flow rate of 3100 l/h, and 14% glycol lowers the freezing point below 0°C, allowing the system to supply higher capacities. The heat pump extracts the heat from the BHE’s, supported by solar thermal collectors to charge the BHE’s in summer, ensuring efficient year-round heating. An extensive monitoring framework, including Distributed Temperature Sensing (DTS), provides detailed insights into system performance during operation.

The HSE system demonstrated consistent performance under varying configurations and conditions. With all nine boreholes active, the system achieved a seasonal Coefficient of Performance (COP) ranging from 3.8 to 5.2, with daily energy outputs averaging 125 to 220 kWh/day. During December 2024, tests were conducted using three boreholes in different configurations at a reduced flow rate of 2800 l/h. These tests showed that borehole arrangement moderately influenced system performance, with the adjacent configuration achieving slightly higher energy outputs and COP, compared to the dispersed configuration.

The system also demonstrated significant energy cost savings of €954 during November and December 2024, attributed to a reduction in gas consumption by over 700 m³ compared to the previous year. These findings confirm that diagonal shallow co-axial borehole arrays are a scalable and sustainable UTES solution, offering substantial energy savings and CO₂ reductions in dense urban settings.

Long-term geothermal production is subject to considerable uncertainty due to limited data availability and inherent geological heterogeneity. While observation and data acquisition improve our understanding of the reservoir, they also contribute significantly to project costs. It is essential to identify the most informative observation strategy. In this study, we apply a previously developed scenario-based data assimilation framework that integrates rapid geological modelling, efficient numerical simulation, and Ensemble Smoother with Multiple Data Assimilation (ESMDA) to constrain uncertainties in reservoir properties and production forecasts to a synthetic but geologically realistic fluvial geothermal system and conduct a data worth analysis to evaluate the impact of different observations (production temperature and injection pressure, well temperature and pressure profiles, etc.) on uncertainty reduction. Results show that production temperature and injection pressure alone, though cost-effective, are insufficient to significantly reduce uncertainties in reservoir performance forecasts. In contrast, well temperature and pressure profiles exhibit substantially higher data worth, leading to much better-constrained predictions. Moreover, incorporating a monitoring borehole further constrains uncertainty by capturing subsurface dynamics between the injector and producer. These findings underscore the importance of monitoring pressure and temperature profiles in the wells of a geothermal doublet.

Energy Quay Walls (EQWs) are innovative energy geostructures with the unique capability to exchange heat with both soil and open water. Although previous laboratory testing demonstrated a promising energy efficiency for this type of system, its novelty necessitated thorough research to advance comprehension of its thermal behaviour and optimise energy efficiency. This paper conducts an in-depth examination of EQWs, employing numerical models validated against real data from a full scale test in Delft, The Netherlands.

Two Finite Element numerical models were developed to (i) reconstruct the undisturbed (i.e. pre-geothermal activation) temperature profile within the soil and (ii) conduct a comprehensive (3D) analysis of heat exchange processes in an EQW application (i.e. during geothermal activation), calibrating relevant parameters with field test data, providing valuable insights into its energy efficiency. Following validation, the geothermal activation model was employed to assess the impact of the flow regime within the heat exchanger pipes and the velocity of the open water on the energy efficiency of the EQW system. Additionally, the contributions of soil, water, and air to the energy gain are investigated. The results indicate that the primary source of energy gain is from open water, and the dominance of this contribution is further increased by the presence of turbulent flow within the heat exchanger pipes. However, the soil can play a key role in short term energy delivery. Furthermore, this study emphasises the importance of the open water movement, revealing a 48% reduction in energy extraction for fully stationary water scenarios.

The effect of the load level on long-term thermally induced pile displacements and the impact of cyclic thermal loads on the bearing capacity of energy piles are investigated via a full-scale in situ test in Delft, The Netherlands. The pile was loaded to a specific target of 0, 30, 40, or 60% of its calculated ultimate bearing capacity. At the end of each loading step, up to ten cooling–natural heating cycles were applied. The pile behavior during monotonic cooling and cyclic cooling–natural heating in terms of the displacement along the pile is reported, with a focus on permanent displacements. During monotonic (pile/ground) cooling, a settlement of the pile head and an uplift of the pile segment near the pile tip were observed in all four tests. In addition, under higher mechanical load, the pile head displacement was larger while the uplift was lower due to the imposed mechanical load. During cyclic thermal load, under zero mechanical load, pile head displacement was fully reversible while permanent uplift of the lowest pile segment was observed and attributed mainly to the permanent dragdown of the surrounding soil. Under moderate mechanical loads (30 and 40%), thermal cycles induced an irreversible pile head settlement, which stabilized with an increasing number of cycles. In addition, a permanent pile settlement along the pile was observed at the end of these tests. Under high mechanical load (60%), the irreversible settlement along the pile continued to increase with only a slight reduction in rate, being higher compared to moderate mechanical loads. In this test, a normalized pile head settlement of 0.124% was observed after ten thermal cycles. The permanent settlement of the pile under thermo-mechanical loads was mainly attributed to the contraction of sand beneath the pile tip and thermal creep at the soil–structure interface. The pile bearing capacity was observed to increase after thermo-mechanical tests, mainly due to the residual/plastic pile head displacement, which in turn densified sand leading to an increase in tip resistance.

Cold water injection into geothermal reservoirs is a common, sometimes necessary, technique for multiple reasons including the replenishment and stimulation of the reservoirs, and the disposal of waste water. The injection of cold water results in a thermo-hydro-mechanical (THM) impulse, which can cause near-wellbore cracking. A method is presented to simulate coupled thermo-hydro-mechanical processes, including the re-activation of existing fractures and fracturing of the rock matrix. The model is based on the finite element method, and utilises a newly developed cohesive interface element to represent discontinuities. The interface element belongs to the family of zero-thickness elements and is triple-noded. It is developed to allow the simulation of longitudinal and transversal fluid/heat flow. The cubic law is used to simulate the fracture transmissivity as a function of its aperture, while a elasto-damage law is used to characterise the mechanical response of the discontinuity. The method is successfully verified against analytical solutions for hydraulic fracturing (KGD model) and for the thermo-hydraulic response of a single fracture (Lauwerier’s problem). As numerical oscillations are observed due to the high Péclet number, an artificial diffusion is added to stabilise the numerical solution with sufficient accuracy. Qualitative validation is achieved against experimental data of cold water injection in granite samples. Fracture branching is observed in the case with large cooling shock, while a single fracture is induced in the case with smaller cooling shock, as was observed in the experiment. The validation demonstrates the capability of the proposed model to simulate fracturing processes under THM couplings.

Distributed acoustic sensing (DAS) that uses optical fibres as sensing units is attracting increasing interest for micro-seismic monitoring in geothermal projects. Standard optical fibres provide one-component measurements along the fibre and this pose challenges in determining certain characteristics of the source, such as its azimuth and its full moment tensor. Full source characteristics can be obtained via offset downhole measurements and/or measurements from horizontal well sections but these come with substantial extra costs. This paper proposes a single-well dual-cable DAS configuration to reduce the need for drilling additional wells or sections, where two DAS cables are assumed to be positioned within a single vertical well at opposite sides of the well. Synthetic DAS signals are generated by an open-source code that assumes plane-layered media and are used to study the feasibility of the dual-cable DAS for localising a seismic source and resolving its moment tensor. A localisation procedure is presented, and a sensitivity analysis of localisation accuracy is conducted with respect to source parameters and noise levels. In addition, an analysis is performed to assess the resolvability of the moment tensor components from the dual-cable DAS configuration. Results suggest the source location can be fully determined, yet low signal-to-noise ratio and azimuth close to 0∘ (North, aligned with the two cables) lead to a decrease in accuracy. The full moment tensor can be resolved only if the epicentral distance is 5 m or less, while non-double-couple components can be reliably resolved with an epicentral distance up to 20 m, showing improvement compared to installations with a single cable. Consequently, near-borehole failures, regardless of the source mechanisms, can be characterised within an epicentral distance of 5 m. With epicentral distance increasing, resolvability of the mix-mode failures is reduced first, followed by the resolvability of the pure shear or tensile failures, which depends on the azimuth. Overall, the results demonstrate that a single-well dual-cable configuration has the potential for monitoring and understanding near-borehole micro-seismic events induced during geothermal reinjection and stimulation operations.

It is clear that to address climate change, an energy transition which makes a large-scale use of the subsurface is needed. The subsurface will play a critical role in this transition, serving as a resource for new sources of energy production and storage, a foundation for energy infrastructure, and a repository for waste by-products from energy production (e.g., radioactive waste disposal, CO2 geo-sequestration). Furthermore, there are challenges in understanding material behaviour due to complex coupled phenomena, measuring material properties and upscaling the physical phenomena to engineering scale structures. Uncertainties, heterogeneities and long timescales offer additional challenges, as does bringing technology ever closer to dense populations. This is the topic of Energy Geotechnics. In the next decade and decades, society needs to complete the energy transition, and to do so the already substantial changes need to be vastly accelerated. This brings many challenges, which academics, consultants, contractors and authorities need to address together. [...]

Energy Quay Walls (EQWs) are innovative energy geostructures which exchange thermal energy with both soil and open water while providing a structural function. A full-scale EQW with thermally activated sheet piles was tested, measuring 8.4 m in length along a 1.75m deep canal, with the sheet piles embedded 13m into the underlying soil. Two different length heat exchangers were used: shallow (3 m length) loops primarily extracting thermal energy from the open water, and deep (15 m length) loops extracting energy also from the soil. The shallow loops demonstrated a high heat extraction rate per activated surface area (∼200 W/m2 at 8 °C water temperature, compared with ∼60 W/m2 for the deep loops), with their performance closely linked to the open water temperature. The shallow loops did not require time to restore surrounding temperatures, indicating stable long-term performance, yet can extract the least energy at the coldest time periods. In contrast, the deeper loops exhibit greater stability across varying open water temperatures and achieve the highest total energy extraction per quay wall length (∼900 W/m at 8 °C water temperature, compared with ∼600 W/m for the shallow loops). Realistic operation of the deep loops lowered the soil temperature by ∼2 °C.