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.

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.

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.

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.

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.

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.

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.

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.

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.