Energy Geostructures Research Articles

The importance of boundary conditions on the modelling of energy retaining walls

Abstract Shallow geothermal technologies have proven to efficiently provide renewable energy for space heating and cooling. Recently, significant attention has been given to utilising sub-surface structures, primarily designed for stability, to also exchange heat with the ground, converting them into energy geo-structures. This research includes investigations into the feasibility of applying this technology to retaining walls, focusing on the usually neglected interaction between the energy retaining wall and the air inside the underground space it contains (e.g., a building basement, a metro station). Even though soldier pile walls are adopted for the study, the results are applicable for any retaining wall type. Two commonly adopted boundary conditions on the surfaces of the underground structure (thermal insulation and a defined temperature) are used as well as the computationally expensive approach of fully modelling the air inside the underground space. The results show that if these boundaries are not carefully considered, a significant amount of heat can flow into/out of the underground space (up to about 75% in this study). Importantly, adopting inappropriate boundary conditions for these surfaces can result in erroneous and misleading results, a potentially under-designed heating, ventilation and air-conditioning (HVAC) system and subsequently thermal discomfort within these spaces.

Thermal conductivity of sands treated with microbially induced calcite precipitation (MICP) and model prediction

Thermal resistance between underground energy geo-structures and surrounding soils limits heat exchange efficiency particularly when soils are at low saturation degrees or nearly dry conditions. Microbially induced calcite precipitation (MICP) technique has great potential to enhance soil thermal properties thus increasing the possibility to expand geothermal applications to arid environments. This study investigates the thermal conductivity (k) of MICP treated sands in drying process by using a newly developed dual probe heat pulse (DPHP) sensor. The results show that thermal conductivity of sands was significantly improved after MICP treatment, and k value increased as the number of treatment cycles increased. The improvement in k is attributed to the MICP induced CaCO3 crystals functioning as “thermal bridges” among sand grains, which provides more highly effective heat transfer path and increases surface contact area in heat exchange process. Based on the normalized thermal conductivity (kr) concept and the experimental data obtained in this study, a modified thermal conductivity predictive model was proposed for MICP treated sands. The model prediction matched the experimental results very well, and the predicted values of k agreed with the measured values with only 10% deviation in the entire saturation range.

Improving a thermal conductivity model of unsaturated soils based on multivariate distribution analysis

Soil thermal conductivity (k) is a key parameter for the design of energy geo-structures, and it depends on many soil properties such as saturation degree, porosity, mineralogical composition, soil type and others. Capturing these diversified influencing factors in a soil thermal conductivity model is a challenging task for engineers due to the nonlinear dependencies. In this study, a multivariate distribution approach was utilized to improve an existing soil thermal conductivity model, Cote and Konrad model, by quantitatively considering the impacts of dry density (ρd), porosity (n), saturation degree (Sr), quartz content (mq), sand content (ms) and clay content (mc) on thermal conductivity of unsaturated soils. A large database containing these seven soil parameters was compiled from the literature to support the multivariate analysis. Simplified bivariate and multivariate correlations for improving the Cote and Konrad model were derived analytically and numerically to consider different influencing factors. By incorporating these simplified correlations, the predicted k values were more concentrated around the measured values with the coefficient of determination (R2) increased from 0.83 to 0.95. It is concluded that the developed correlations with the information of different soil properties provide an efficient, rational and simple way to predict soil thermal conductivity more accurately. Moreover, the quartz content is a more important factor than the porosity that shall be considered in the establishment of thermal conductivity models for unsaturated soils with high quartz content.

Thermal Conductivity of Dry Sands Treated with Microbial-Induced Calcium Carbonate Precipitation

With an increasing energy demand, exploration and utilization of new energy resources become more significant recently. Geothermal energy, characterized as a clean, renewable, and sustainable energy, has various engineering applications. Microbial-induced calcium carbonate precipitation (MICP) technique has a potential to improve soil thermal properties for geothermal applications. In this study, thermal conductivity of dry sands treated using MICP technique with different treatment cycles was investigated in laboratory. The results showed that thermal conductivity of MICP-treated sands was much higher than that of the untreated sand under dry condition and it increased with increasing treatment cycles. Based on the scanning electron microscopy (SEM) analyses, it is found MICP-induced CaCO3crystals are being formed among sand particles functioned as “thermal bridge,” which provided more highly effective heat transfer path. It is concluded that the MICP technique could significantly improve the thermal conductivity of sands and the overall heat transfer efficiency. It is advantageous to use MICP-treated soils as enhanced grout materials for underground energy geostructures.

Thermal design and full-scale thermal response test on Energy Walls

Energy geostructures (EG) are an innovative technology in the sustainable energy agenda that can be used to satisfy the heating and cooling needs of the built environment. EGs include several types of geostructures such as piles, walls, tunnels, shafts, sewers. The application of this technology to infrastructure projects is particularly interesting because of the important thermal potential offered by the large surfaces that can be thermally-activated. This study deals with thermo-active walls (Energy walls, EW), which are retaining structures used to sustain the sides of excavations. Aspects related to the hydro-thermal interactions and to the thermal design are here presented. Finally, the testing setup for the execution of a thermal response test on a recently-built EW in western Switzerland is discussed.

Energy and mechanical aspects on the thermal activation of diaphragm walls for heating and cooling

Underground geotechnical structures, such as deep and shallow foundations, diaphragm walls, tunnel linings and anchors are being increasingly employed as energy geostructures to exchange heat with the ground by installing absorber pipes into the structural elements. This paper focuses on the application of this technology to reinforced concrete diaphragm walls used for construction of underground car parks, basements and metro stations, with the purpose of heating and cooling the adjacent buildings. Preliminary numerical modelling allowed optimising the geothermal plant design of the diaphragm wall. Then its energy efficiency is investigated through finite element thermo-hydro coupled analyses together with the effects of the thermal activation on the surrounding soil. Finally, finite difference thermo-mechanical analyses are used to study the mechanical effects induced by the thermal activation.

A robust prediction model approach to energy geo-structure design

Energy geo-structures, such as piles or retaining walls, provide geothermal space heating and cooling, in addition to their structural purposes. The thermal design of these structures is undertaken on a case by case basis, commonly using costly finite element simulations, especially for complex geometries. This work introduces a simple but robust prediction methodology that can be used alongside such simulations to significantly reduce computational time and resources for the analysis of any energy geo-structure. An evaluation is presented and exemplified with energy diaphragm walls, for a range of geometrical and material conditions, showing insignificant prediction errors and vast computational savings.

Thermal response prediction of a prototype Energy Micro-Pile

The paper presents the results of an ongoing research project aimed at developing an innovative technology for energy geostructures for the exploitation of low-enthalpy geothermal energy: the energy micro-piles (EMP). The EMP has the double role of structural support and heat exchanger since it is equipped with a primary circuit of a traditional ground source heat pump system. Given the particular nature of the EMP considered, in terms of pile geometries and characteristics of the primary circuit, its performance has been investigated by means of both field testing, carried out on a full-scale prototype, and 3D finite element simulations. The numerical model has been calibrated on the results of a thermal response test carried out on the full-scale prototype. Then, a parametric study has been conducted to gain useful insight on the performance of the EMP under long-term operating conditions, by varying: (i) the mass flow rate of the heat-carrier fluid; (ii) the soil thermal conductivity; (iii) the thermal properties of the circulation pipe; (iv) the material filling the bottom portion of the prototype. The results of the experimental and numerical activities showed that the value of the specific heat flux generated by the EMP falls in the same range of that of conventional energy piles and that the performance of the EMP is mainly affected by the thermal properties of the foundation soil.

Energy tunnels: concept and design aspects

Geotechnical structures are increasingly employed as energy geostructures in Europe and worldwide. Besides being constructed for their primary structural role, they are equipped to exchange heat with the ground and supply thermal energy for heating and cooling of buildings and de-icing of infrastructures. This technology can play a fundamental role in the current challenge of addressing the increasing need for clean and renewable sources of energy. This study investigates the possibility of thermal activation of tunnel linings. Particularly, attention will be paid on a new energy segment, which can be used together with tunnel boring machine tunneling to create so-called energy tunnels. Thermal and mechanical designs need to be developed by making effective use of computational methods to quantify the exploitable heat and assess the possible consequences on the surrounding ground and the structure itself. Guidance on how to proceed in this direction will be provided in this study, showing how thermo-hydro and thermo-mechanical numerical analyses can be used to achieve a proper and effective design of energy tunnels. Two examples of possible applications will also be presented.

Influence of Temperature on the Volume Change Behavior of Saturated Sand

The effect of temperature on the volume change of sand is rarely reported but may be relevant to the performance of energy geostructures. This study involves an investigation of the thermal volume change behavior of saturated, dense sand through a series of temperature-controlled, isotropic hollow cylinder triaxial compression tests. Variables measured during a heating stage include the volume of water expelled from the sand specimen, the temperatures at the top, bottom, and inside of the specimen, and the axial and volumetric strains. The volumes were used along with thermo-elastic relationships for the pore water and soil skeleton to infer the axial and volumetric strains during drained heating. It was observed that the thermally induced axial and volumetric strains were negative, reflecting expansion. The pore water was observed to flow out of the sand specimen during heating, reflecting differential thermal expansion of the pore water and sand particles. The thermal volume changes were observed to be independent of the mean effective stress, as the dense sand specimens were all in normally consolidated conditions. Three linear equations incorporating the effects of temperature on the volume change behavior of dense sand were proposed and match well with the experimental data. The experimental approach proposed in this study can be used in the future to evaluate the role of sand density and stress state on the parameters of these equations.

Characterisation of Ground Thermal and Thermo-Mechanical Behaviour for Shallow Geothermal Energy Applications

Increasing use of the ground as a thermal reservoir is expected in the near future. Shallow geothermal energy (SGE) systems have proved to be sustainable alternative solutions for buildings and infrastructure conditioning in many areas across the globe in the past decades. Recently novel solutions, including energy geostructures, where SGE systems are coupled with foundation heat exchangers, have also been developed. The performance of these systems is dependent on a series of factors, among which the thermal properties of the soil play a major role. The purpose of this paper is to present, in an integrated manner, the main methods and procedures to assess ground thermal properties for SGE systems and to carry out a critical review of the methods. In particular, laboratory testing through either steady-state or transient methods are discussed and a new synthesis comparing results for different techniques is presented. In situ testing including all variations of the thermal response test is presented in detail, including a first comparison between new and traditional approaches. The issue of different scales between laboratory and in situ measurements is then analysed in detail. Finally, the thermo-hydro-mechanical behaviour of soil is introduced and discussed. These coupled processes are important for confirming the structural integrity of energy geostructures, but routine methods for parameter determination are still lacking.

Site investigation for energy geostructures

Energy geostructures are structure or infrastructure foundations used as heat exchangers as part of a ground source heat pump system. Although piles remain the most common type of energy geostructure, increasingly infrastructure projects are considering the use of other buried structures such as retaining walls and tunnels for heat exchange. To design and plan for construction of such systems, site investigations must provide appropriate information to derive analysis input parameters. This paper presents a review of what information regarding the ground, and also the structures themselves, would be required for the ground energy system design process. Appropriate site investigation methods for energy geostructures are reviewed, from desk study stages through in situ testing to laboratory testing of samples recovered. Available methods are described and critically appraised and guidance for practical application is given.

Thermal conductivity of sand–kaolin clay mixtures

Thermal conductivity is a key parameter for the design of various geothermal structures such as geothermal energy piles, ground source heat pumps and soil borehole thermal energy storage. In this paper, the thermal conductivity of sand–kaolin clay mixtures prepared at varied moisture content, dry density and clay content was measured using a newly designed thermo-time domain reflectometry probe. The data revealed that the thermal conductivity of the sand–kaolin clay mixtures increased with an increase in moisture content and dry density. The thermal conductivity of the moist sand–kaolin mixtures decreased with an increase in clay content in two distinct regimes including a slow decrease in thermal conductivity above a critical clay content, which was found to be 10%. Five soil thermal conductivity models were selected to predict the thermal conductivity of the sand–kaolin clay mixtures, and the predictions were compared with the experimental results. Two models were modified to account for the effect of clay content on the thermal conductivity of the sand-clay mixtures. Recommendations regarding the design of energy geostructures were made based on the measured thermal conductivity data.

Thermoplastic Analysis of a Thermoactive Pile in a Normally Consolidated Clay

AbstractThe use of shallow geothermal systems for buildings and infrastructure conditioning has numerous recognized environmental and potential economic benefits and is expected to contribute to climate change mitigation. These systems are already used in many countries and are foreseen in many more. The study of their behavior is a subject of current research interest, including thermoactive or energy geostructures. In this work the mechanical effects of a thermal action, reflecting seasonal atmospheric conditions prevalent in Lisbon, Portugal, on a single 20-m-long thermoactive floating pile with different load levels, embedded in a normally consolidated saturated clay (speshwhite kaolin), are evaluated. For this purpose a critical state elastoplastic soil model with thermal hardening was formulated, implemented, and calibrated with experimental results for thermal expansion available in the literature. The numerical analyses performed revealed that the mechanical effects on the pile caused by thermal a...

Application of energy tunnels to an urban environment

Deep and shallow foundations, diaphragm walls, tunnel linings and anchors are being increasingly employed as energy geostructures in Europe and all around the world. Besides being constructed for their primary structural role, they are equipped to be able to exchange heat with the ground and supply thermal energy for heating and cooling of buildings and de-icing of infrastructures. This technology can play a fundamental role in the current challenge of addressing the increasing need for clean and renewable sources of energy. This paper investigates the possibility of thermal activation of a new section under construction of the Metro Torino line 1 (Italy) to heat and cool adjacent buildings. The design and optimization of the geothermal plant, the quantification of the exploitable heat and the assessment of the eventual consequences on the surrounding ground are here discussed. For this purpose, thermo-hydro finite element analyses, able to capture the key aspects of the problem, were conducted. A 3D model is devoted to study the efficiency of the system, reproducing one ring of the instrumented tunnel segmental lining, while a 2D large scale model of the Torino aquifer is conceived to investigate the sustainability of the technology in terms of effects on the surrounding environment. Based on the results of the computations, it can be anticipated that, thanks to the favorable underground water flow conditions in Torino, the system would allow 53 and 74W per square meter of tunnel lining to be exchanged during winter and summer respectively.

On the Exploitation of Ground Heat Using Transportation Infrastructure

The use of shallow geothermal energy systems that employ heat-exchange loops within trenches and boreholes is well established. The use of civil engineering structures that are in contact with the ground (geo-structures) to replace the more conventional heat-exchange methods is creating great interest in many countries. Bearing piles have been used for this purpose since the mid-1980s, retaining walls since the late-1990s and the use of tunnels has been explored since the early-2000s. With regards to transportation infrastructure shallow geothermal may be used to provide renewable heating and cooling to the infrastructure itself or adjacent users, or even to enhance safety by providing e.g. heat to de-ice bridges, station platforms, airport run-ways and the like. This paper presents an overview of the potential application energy geo-structures in transportation infrastructure through case studies and numerical simulations, and then goes on to discuss some of the issues associated with the potential use of geo-structures for heat exchange, in terms of construction, thermal operation and the impact of heating and cooling on the geo-structure.

Design of plane energy geostructures based on laboratory tests and numerical modelling

Energy geostructures are an up-coming technique for the thermal utilization of the ground. Due to the complexity of the system, it is necessary to apply numerical simulations for a proper design of plane energy geostructures. However, varying scales, different heat transfer mechanisms, and a missing rotational symmetry require complex numerical models and long computing times.This motivated the authors to develop a semi-analytical calculation model based on thermal resistances. This model was implemented in the general 3D coupled heat and flow transport code SHEMAT-Suite. Large-scale laboratory tests were carried out for verification and validation of the model. The results from the novel numerical approach were compared with results from the laboratory and with results from a fully discretized finite element model. All comparisons show a good agreement. As expected, computing times are significantly smaller than for fully discretized numerical models. Thus, the new approach is suitable for the design of plane energy geostructures. The new model was used for an extensive parameter study. As a result, the flow rate in the heat exchanging system, the ground temperature, the groundwater flow, the pipe arrangement in the structural element and the structure of the element were identified as the decisive parameters.

Response of soil subjected to thermal cyclic loading: Experimental and constitutive study

The response of soil subjected to thermal cyclic loading plays an important role in certain engineering applications, such as high-level nuclear waste disposal, heat storage systems, CO2 sequestration plant and energy geostructures. For instance, energy geostructures impose temperature variations that are daily and seasonally cyclic to the soil and might have consequences of engineering relevancy, mainly in terms of foundation displacements. This paper aims to experimentally investigate the response of a natural silty clay soil to thermal cyclic loading in drained conditions. The experimental programme includes: (i) oedometric tests at various constant temperatures aimed at studying the sensibility of the material to temperature and (ii) thermal cyclic tests under constant vertical effective stress in oedometric conditions, with temperature ranging between 5 and 60°C. As expected, the silty clay tested under normal consolidation conditions (NC) undergoes thermo-plasticity, and the results indicate that most of the irreversible deformation occurs during the first heating–cooling cycle, exhibiting an accommodative behaviour during the subsequent cycles. In other words, increments of irreversible deformation are observed in the thermal cycles successive to the first one, which generally become smaller and smaller cycle after cycle until stabilisation. In the end, the material tends to remain inside the elastic domain, exhibiting thermo-elastic expansion and contraction during heating and cooling. In the second part of the paper, an extension of an existing thermoelastic–thermoplastic constitutive model that aims to tackle the accommodative response is proposed. The extended model is able to reproduce the accommodation phenomenon thanks to an additional parameter called the cyclic plastic radius. The experimental results obtained are innovative in clarifying the response of soils subjected to drained thermal cyclic loading, and the constitutive model presented allows this aspect to be considered in the numerical analysis of the engineering applications where it is of concern.

Introduction to the Special Issue of Geotechnical and Geological Engineering Entitled: “Thermo-Hydro-Mechanical Behavior of Soils and Energy Geostructures”

This special issue of Geotechnical and Geological Engineering highlights state-of-the-art research on the thermo-hydro-mechanical behavior of soils, rocks and energy geostructures. Although the impetus of the special issue was to collect journal papers on subjects related to an NSF-funded international workshop on ‘‘Thermally Active Geotechnical Systems for Near Surface Geothermal Energy’’ held in March 2013 at Ecole Polytechnique Federale de Lausanne in Switzerland, the special issue expanded to collect a broader set of papers from the field of Energy Geotechnics that involve thermal soil behavior. A total of 15 journal papers were selected to be published as part of the special issue, and went through the standard peer review process for the journal. Two papers focused on the thermal properties of unsaturated soils. Likos (2014) presented a pore-scale model for predicting the thermal conductivity of unsaturated sands using basic soil properties. Dong et al. (2014) presented an evaluation of various approaches to predict the thermal conductivity of unsaturated soils, and compared them with experimental data for sand, silt and clay. One paper was presented on the thermal volume change of unsaturated soils, and the impact of these volume changes on the response of a heated shallow foundation (Houston et al. 2014). Three papers were published on different aspects of geothermal heat exchangers in conventional applications (boreholes and horizontal trenches). Wu et al. (2014) studied relationships between the thermal conductivity dryout curve for unsaturated soils and the performance of geothermal heat exchangers in horizontal trenches, Ozudogru et al. (2014a) compared the performance of different numerical simulations to evaluate the response of geothermal heat exchangers in vertical boreholes, and Walker et al. (2014) studied the impact of different stratigraphic units on the overall thermal response of a geothermal heat exchanger in a vertical borehole. Seven papers were focused on the thermal and thermo-mechanical response of energy piles. Loveridge et al. (2014) analyzed the response of different types of auger cast piles that were converted into energy piles. Yu et al. (2015) used numerical modeling to evaluate the thermal conductivity of different soil layers from the analysis of a thermal response test on an energy pile. Kramer et al. (2014) evaluated the thermal response of an energy pile in sand constructed within a laboratory container. Murphy and McCartney (2014) studied the J. S. McCartney (&) Department of Structural Engineering, University of California San Diego, 9500 Gilman Dr, La Jolla, CA 92093-0085, USA e-mail: mccartney@ucsd.edu

Energy from geo-structures: a topic of growing interest

Underground geotechnical structures (piles, diaphragm walls, tunnel linings, anchors etc.) can be instrumented to become energy geo-structures. This short paper focuses on the use of energy tunnels, which are shown to have a number of advantages. An example of a possible application to the Turin Metro Line 1 south extension is discussed. Preliminary results of numerical analyses, performed to study the hydro-thermal interaction and the influence of the energy tunnel on the surroundings, show that the energy stored in the ground can be exploited without generating relevant effects on the aquifer. This highlights the importance of additional research to improve the understanding of the geothermal process and exploit the potential of energy tunnels allowing for great economic and environmental benefit.