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... 3 presents the state of the art on energy piles, based on the integration of the information from 157 energy pile projects, in terms of extracted thermal power with respect to the diameter and length of the piles. On the other hand, Figure 4a and 4b represent extracted and injected heat for heating and cooling purposes for energy walls (17 projects) and energy tunnels (11 projects), respectively. ...
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... Extensive theoretical and numerical studies were conducted to investigate the thermal-mechanical responses of clay soils under temperature variations [14][15][16]. The developed mechanical models were implemented into numerical programs for carrying out THM analysis for shallow geothermal energy projects [17][18][19][20]. However, minimal emphasis was placed on investigating the influence arising from the dehydration of bound water in clay minerals, a phenomenon that should be regarded as an independent deformation mechanism [21][22][23][24][25]. ...
... Energy piles are an emerging technology, with several successful applications throughout the world (see, i.e., Brandl, 2006). They allow the exploiting of renewable and clean source of energy for air conditioning of buildings (Amatya et al., 2012;Murphy et al., 2014;Laloui et al., 2019). In urban areas, heating and cooling of buildings represents one of the main sources of energy consumption and one of the main causes of CO2 production. ...
In this paper a 2D Finite Element model is presented for the simulation of the coupled Thermo-Hydro-Mechanical (THM) processes induced by the operation of energy piles. The model, implemented in the Plaxis 2D Finite Element (FE) code, was used to simulate the behaviour of a well-known instrumented energy pile during a test performed at Lambeth College in London (Bourne-Webb et al., 2009), comparing the results obtained from different constitutive models used to describe the soil behaviour, namely: i) the simple elasto-plastic Mohr-Coulomb (MC) model; ii) the Modified Cam Clay (MCC) model; iii) the Hardening Strain (HS) model. Comparisons with the experimental results show that all the models, if correctly calibrated, are able to realistically reproduce the behaviour of the pile-soil system from a qualitative point of view, while the best prediction was found using the HS model.
... Thermal piles potentially offer higher heat transfer rates per drilled metre than conventional borehole heat exchangers 13 due to the ability to include more pipes 14 in the larger cross section of the ground heat exchanger. They have also been shown to offer good system energy efficiencies overall. ...
Ground source heat pump systems, operating in conjunction with vertical ground heat exchangers, will play a key role in decarbonising heating and cooling of buildings. Design of traditional borehole heat exchangers relies on tools which implement routine analytical relationships between heat transferred and the temperature change in the ground and circulating thermal fluid. However, for novel piled foundations used as ground heat exchangers, there are few such analytical solutions available that are practical for routine implementation. This paper examines the use of a radial approximation to simulate the dynamic thermal behaviour of pile heat-exchangers. Originally developed for small diameter and high aspect ratio borehole heat exchangers, the approach is more challenging for piles since unsteady heat transfer within the pile material is more significant over typical timescales. Nonetheless, we demonstrate that for pile diameters between 300 mm and 1200 mm, generally the error is <1oC with centrally placed heat transfer pipes or four or more pipes placed near the edge with circumferential spacing less than 550 mm. The radial model is therefore practical for most pile configurations. The strong performance of the model is demonstrated for a year of hypothetical heating and cooling cycles, and also against a field-scale thermal response test.
Energy geostructures have been identified as a cost-effective mitigating strategy for the adverse impact of climate change. Operation of energy geostructures results in temperature fluctuation and subsequent water migration, particularly at the soil–structure interface, determining the shear response of soil and soil–structure interface. This state-of-the-art paper brings together experimental data from direct shear tests carried out on the soil–structure interface from several laboratory investigations, presenting a comprehensive review to gain a thorough understanding of the interface response in different thermo-hydro-mechanical states, which is critical in the analysis and design of energy geostructures. First, the evolution of shear strength parameters, i.e., adhesion and friction angle, with matric suction and temperature, are investigated. Then, a more detailed analysis of the impact of matric suction and temperature on the shear strength of the soil–structure interface is provided. Furthermore, a comprehensive discussion is provided in this section on the role of the most recent stress history in determining the non-isothermal shear strength of an interface. Data on the effect of matric suction and temperature on shear parameters of the corresponding fundamental soil is reviewed as a reference to the interface behaviour throughout the study, revealing potential underlying mechanisms. In general, a higher matric suction results in higher shear strength of the interface, whereas non-isothermal variations in adhesion and friction angle may lead to a higher or lower shear strength of a saturated interface.
