Dr Nikolas Makasis

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.

Shallow geothermal energy systems are known to efficiently provide renewable energy for heating and cooling purposes. Energy geo-structures constitute a relatively recent application of these systems where the use of traditional purpose-made boreholes or trenches as ground heat exchangers (GHEs) is minimised or avoided by incorporating geothermal piping in underground structural elements such as piles, retaining walls and tunnels. This study explores the application of energy tunnels in the M4 – M5 Link project in New South Wales, Australia, which includes the construction of twin motorway tunnels of around 7.5 km in length for up to 4 lanes of traffic. The presented work examines this premise in detail by utilising advanced numerical modelling approaches and high-performance computing applications to investigate the long-term applicability of energy tunnels for this case study. The results indicate that thermally activating the entire tunnel could provide up to about 38.6 GWh per year for heating and cooling. A number of pipe configurations are also investigated, suggesting that placing the pipes only on either side of lining of the tunnel (as opposite to top or bottom) can result in a better thermal performance due to thermal interference and in this case provide up to about 17.5 GWh per year.

Geothermal pavement systems are a novel type of energy geostructure. They use sub-surface structures to exchange heat with the ground and, thereby, provide thermal energy in addition to structural support. The thermo-activation of pavements has been largely overlooked in the literature. This research focuses on the development of a detailed three-dimensional (3D) finite-element (FE) model to explore the thermal performance of geothermal pavement systems. The 3D FE model developed was successfully validated with both data measured from a full-scale experiment undertaken in Adelaide, South Australia and other published data. The validated model is further employed to evaluate the long-term performance of a geothermal pavement system under both a traditional system configuration and a hybrid system. Furthermore, a life-cycle cost analysis is performed to explore the cost implication of such pavement systems. Results show that a geothermal pavement with total pipe length of 640 m, or a hybrid system (a geothermal pavement system with a pipe length of 320 m and an auxiliary system) can provide for sufficient space heating and cooling for a typical residential building in Australia. It is found that, compared with conventional heating and cooling systems, the geothermal pavement system is indeed a cost-effective solution. This research study indicates that this pavement technology can be successfully implemented in the field and accurately modelled using FE techniques.

Energy geo-structures are a promising application of shallow geothermal energy technologies utilising underground structures primarily build for stability to also convert them to ground heat exchangers and make thermal energy provision their secondary function. One type of energy geo-structures that has received little attention is energy soldier pile walls. This work adopts advanced numerical modelling approaches to investigate the thermal performance of these energy walls and important parameters affecting this performance including the soldier pile depth, spacing, pipe length and thermal load. The results indicate that both the soldier pile depth and spacing can impact the thermal performance with higher values being desirable. Non-linear/logarithmic performance trendlines have been identified. The scenario of activating less piles overall to increase their (thermal) spacing is also investigated, showing a decrease in the thermal performance but noting that in certain cases this decrease could be acceptable compared to the capital cost savings of activating less piles. The pipe configuration is found to result in relatively insignificant returns after utilising more than about 3 U-loops connected in series, suggesting the potential suitability of an easy to adopt rule-of-thumb for these structures.(C) 2021 The Author(s). Published by Elsevier Ltd.

Rapid rates of urbanisation are placing growing demands on cities for accommodation and transportation, with increasing numbers of basements and tunnel networks being built to meet these rising demands. Such subsurface structures constitute continuous heat sources and sinks, particularly if maintained at comfortable temperatures. At the city-scale, there is limited understanding of the effect of heat exchange of underground infrastructures with their environments, in part due to limited availability of long-term underground temperature data. The effects of underground temperature changes due anthropogenic heat fluxes can be significant, impacting ventilation and cooling costs of underground spaces, efficiency of geo-energy systems, quality and quantity of groundwater flow, and the health and maintenance of underground structures. In this paper we explore the impact of anthropogenic subsurface structures on the thermal climate of the shallow subsurface by developing a heat transfer model of the city of Cardiff, UK, utilising a recently developed semi-3D modelling approach.

Energy geo-structures have received rapid attention as part of the pursuit for renewable energy since they can exchange heat between the ground and under- or above-ground spaces, in addition to their primary structural functions. However, their efficiency in cooling-dominated conditions has not been adequately studied. This paper tackles a key challenge regarding transport tunnels: sustainable cooling of underground substations by introducing a cooling system that integrates heat exchangers into tunnel lining. This system takes advantage of the tunnel air and the ground potential as sustainable heat sinks to which the heat from the substations is rejected and evaluates the efficiency of the proposed cooling system for different configurations of heat exchangers. The efficiency is evaluated by numerically investigating temperature changes in the ground, tunnel air, tunnel structure and carrier fluid circulating within the heat exchangers. Moreover, the cost of the proposed systems is compared with those of the conventional direct expansion (DX) systems. Results show that the proposed cooling system can effectively improve the efficiency of cooling underground substations by yielding a higher Coefficient of Performance (COP) and lower Net Present Cost (NPC) than the conventional DX systems, without imposing unsustainable practices on the ground, tunnel structure or tunnel air.

•A novel approach for sustainable cooling of deep underground substations is shown.•Efficiency of energy tunnels in cooling dominant condition is numerically studied.•The proposed system effectively supplies total cooling demand for substations.•The substantial cost benefits of the proposed cooling system are demonstrated.•Airflow in the tunnel is the key factor for efficient operation of this system. With rapid rates of urbanisation and significant improvements in construction technologies, the number of subsurface infrastructure projects has drastically increased in recent years. In addition to their primary functions, these structures have shown great potential as energy geo-structures, exchanging heat with the ground to heat and cool spaces. Given their large contact area with the ground, energy tunnels are proving to be a sustainable source of thermal energy for effectively heating under- and above-ground spaces. However, their efficiency in cooling-dominated conditions has not yet been adequately studied. This paper tackles one of the key challenges regarding transport tunnels: sustainable cooling of underground substations, by introducing an efficient and cost-effective cooling method. The method takes advantage of airflow in the tunnels and relatively stable ground temperatures and involves heat exchangers in the form of water-filled high-density polyethylene (HDPE) pipes being integrated into the tunnel space. The efficiency of the proposed system is numerically assessed by analysing the spatio-temporal variations of temperature in the ground, substation, tunnel air, tunnel structure and heat exchangers caused by continuous heat rejection from the substations. A detailed 3D finite element heat and mass transport model is used, and alternative placements of heat exchangers are investigated. Results show that heat exchangers placed on the tunnel lining, and hence exposed to the tunnel airflow, could efficiently supply a substation’s cooling demand, without significantly increasing the temperature of the tunnel air or the ground. The substantial economic benefits of this cooling system compared to a conventional cooling system is also demonstrated.

Geothermal pavements represents a novel approach to shallow geothermal energy applications, in which horizontal ground heat exchangers are implemented within the pavement structure instead of traditional purpose-built trenches, thus decreasing capital costs. This study aims to better understand the feasibility and potential of these systems, by investigating the thermal response of the ground in a full scale geothermal pavements system, the first of its kind in Australasia. For this purpose, a fully instrumented geothermal pavements segment, measuring 20 m × 10 m, was constructed in the city of Adelaide, Australia. The geothermal pavement was subjected to Thermal Response Testing (TRT) and numerical modelling is adopted herein and validated using TRT data to further understand the ground response. In addition to providing insights on the capacity of energy provision for geothermal pavements, this work also introduces and discusses various methods of TRT data analysis for the geothermal pavement, specifically for obtaining the effective ground thermal conductivity, a key parameter in shallow geothermal design. The results indicate that from the considered methods, the conventional semi-log method can lead to overestimation of thermal conductivity, the guarded hot plate model tends to underestimate its value and detailed numerical modelling is most accurate, but computationally more expensive. The results also show that the radius of influence of the geothermal pavement in the examined case is close to 0.5 m and even though the geothermal pavement is considerably affected by the ambient temperature, it can be a viable solution for heating and cooling purposes, showing a heat exchange of 50.2 W/m2 and a rate per length of the pipe of 25 W/m in the case analysed here.

Thermal response tests (TRT) are an essential in situ test for the sizing of ground source heat pump systems. Its application on energy piles is becoming an established practice, involving key differences in relation to testing boreholes. Recently, group TRTs emerged as an alternative to testing single short energy pile elements. This work assesses the execution and interpretation of those tests through different analytical heat transfer models implemented in a unit-response calculation methodology. To assess the suitability of the method, a comprehensive parametric analysis using a validated numerical model is undertaken, considering different energy pile sizes, thermal properties for both concrete and soil and group pile arrangements. The analytical methodology is validated against experimental and numerical results, and then tested to interpret the numerically simulated TRTs using a curve fitting algorithm. The broad parametric investigation and the evaluation of the capacity of different analytical methods on modelling each scenario provide directions to execute and interpret group TRTs. The soil effective thermal conductivity and energy pile thermal resistance are obtained with less than 5% precision error for most scenarios and less than 10% considering all scenarios. These results can be further improved with specific individual G-functions for the pile elements, that can be implemented on the proposed methodology.

Energy geo-structures utilise underground structures primarily designed for structural and geo-mechanical stability to also provide renewable geothermal energy for heating and cooling purposes. Piping is incorporated in the structures to exchange heat with the ground via a carrier (water) and connected to a ground-coupled heat pump on the building side. This work focuses on energy diaphragm walls, expanding on the limited available knowledge and undertaking a comprehensive parametric analysis using experimentally validated numerical modelling. Focus is put on the wall pipe configuration and spacing, which are parameters the geo-thermal design can directly control, however, the effects of ground thermal conductivity and wall depth are also considered. The wall depth is shown as a critical factor to the thermal performance and low thermal conductivity material sites might require deep energy walls for a cost-effective design. Larger pipe spacing (>= 500 mm) appears preferable, despite less piping being placed, since small spacing leads to increased costs but insignificant thermal performance gains. Comparing the horizontal and vertical pipe configurations, relatively small temperature differences of less than 1 degrees C are found. Moreover, the former can be less expensive for multiple-section deeper walls, while the latter for shorter walls or when construction delays are non-critical. (C) 2020 Elsevier Ltd. All rights reserved.

The dataset in this article is related to shallow geothermal energy systems, which efficiently provide renewable heating and cooling to buildings, and specifically to the performance of the vertical ground heat exchangers (GHE) embedded in the ground. GHEs incorporate pipes with a circulating (carrier) fluid, exchanging heat between the ground and the building. The data show the average and inlet temperatures of the carrier fluid circulating in the pipes embedded in the GHEs (which directly relate to the performance of these systems). These temperatures were generated using detailed finite element modelling and comprise part of the daily output of various one-year simulations, accounting for numerous design parameters (including different pipe geometries) and ground conditions. An expanded explanation of the data as well as comprehensive analyses on how they were used can be found in the article titled “Ground-source heat pump systems: the effect of variable pipe separation in ground heat exchangers” (Makasis N, Narsilio GA, Bidarmaghz A, Johnston IW, 2018) [1].

Shallow geothermal technologies have proven to efficiently provide renewable energy for heating and cooling. Recently much attention has been given to utilising sub-surface structures, primarily designed for stability, to also transfer heat to and from the ground, converting them into energy geostructures. This work investigates the potential of applying this technology to the geo-structures of underground train stations in the city of Melbourne (Australia) to fulfil some of their heating and cooling demands. The diaphragm retaining walls and slabs that form part of a case study station are designed to also incorporate geothermal pipe loops. A finite element numerical model comprising the station walls and slabs is presented and used to investigate the thermal performance of these systems, for the temperate climate and geological conditions of Melbourne, adopting an expected lifespan of at least 25 years. The technical applicability of this technology is discussed for different thermal load scenarios, showing the importance of thermal storage and the balanced distribution of the thermal load.

Geothermal pavements can be used with ground-source heat pump systems to sustainably provide energy for heating and cooling by incorporating ground heat exchanger elements underneath pavement surfaces. This work investigates the suitability of geothermal pavements at scale, adopting the city of Cardiff, UK, as a case-study. A two-scale modelling framework, combining detailed small-scale with holistic large-scale approaches, is presented, incorporating the accuracy of the former with the continuity of the latter. The results show that between 184 kWh and 345 kWh of thermal energy per metre length of pavement can be supplied annually, depending on soil profile. Moreover, geothermal operation can reduce anthropogenic heat flux into the ground from heated basements, and its associated negative impacts, by about 390 MWh/year. A city-scale analysis using population-consistent geographic areas called LSOAs, estimates that geothermal pavements can supply about 23% of the entire city residential heat demand, or up to 75% with heat sharing between LSOAs. The suitability of geothermal pavements for larger LSOAs is highlighted, supplying up to 100% of the annual domestic heat demand. Investigating the carbon emissions of heating and cooling technologies shows potential reductions of up to 75% when replacing gas boilers and resistance heating with geothermal pavement systems. 

Utilising foundation systems as heat exchangers has received significant public interest worldwide, as these energy geo-structures can constitute a clean, renewable, and economical solution for space heating and cooling. Despite their potential, the thermal performance of energy retaining walls, especially soldier pile walls, has not been sufficiently studied and understood and thus further research is required. This work utilises the first ever energy soldier pile wall in the currently under-construction Melbourne CBD North metro station as a case study. A section of this wall has been instrumented and monitored by the University of Melbourne. Full scale thermal response tests (TRTs) have been conducted on a single thermo-active soldier pile at two different excavation levels. Thermal response testing field data results are presented in terms of mean fluid temperatures and further analysed to show the potential impact of the excavation level on the structure’s thermal performance. To further explore this impact of excavation depth (or pile embedment depth) and the long-term thermal performance of energy pile walls, a detailed 3D finite element numerical model is developed in COMSOL Multiphysics and validated against the field-testing results. The simulation suggests that thermally activating all the soldier piles in the station can provide enough energy to fulfil the heating and cooling demand of the station and to satisfy partial heating demand to the surrounding buildings. Furthermore, results suggest that current energy pile design approaches may be adapted for designing energy pile walls.

Geothermal pavements comprise a newfound type of thermal geo-structures, an innovation in pavement construction contributing to ground source heat pump (GSHP) systems, resulting in a better cost-efficiency compared to conventional GSHP systems. Ground heat exchangers are formed by embedding pipe into the pavement structure (e.g., base, sub-base or subgrades). Developing accurate models with appropriate boundary conditions is crucial to obtaining a deep understanding of the performance and function of geothermal pavement systems. This paper introduces an experimentally validated 3D finite element model (FEM) and compares the effect of surface boundary condition choices on the results of the simulation. The model uses two different approaches for boundary conditions: i) using ambient temperature and ii) implementing energy balance equations on the surface, as surface boundary conditions. The model is further utilised to perform a parametric study and the results are employed by a statistical tool (Minitab) to determine the optimum heat exchanger design for a geothermal pavement project in Adelaide, an Australian city subjected to temperate climate conditions and good solar irradiation throughout the year. Results show that the choice of both boundary conditions leads to similar fluid temperature trends, but with differences in values, particularly during the heating season, resulting in a reduced embedded pipe length of up to 30 % and up to a 10 % better annual average coefficient of performance (COP (when using the energy balance equations on the surface. The parametric study shows that the system annual COP increases with the length of the pipe and spacing between the pipes, whilst the pipe depth placement is inversely proportional to the annual average COP given the advantages obtained during the heating season over the cooling season. Finally, considering the interaction effect between various parameters, a pipe heat exchanger with 400 m length, 0.76 m pipe spacing and 0.45 burial depth is evaluated as the best design to maximise COP for the case study at hand.

Closed loop ground-source heat pump (GSHP) systems can efficiently provide clean and renewable energy for heating and cooling purposes using direct geothermal energy. These systems use Ground Heat Exchangers (GHE) to transfer heat to and from the ground. Vertical GHEs contain loops, pipes with circulating fluid, which transfer energy between the ground and the fluid. One very common assumption made in designing GSHP systems is that, when installed, the loops containing the circulating fluid remain straight and evenly separated along the length of the GHE. However, this is rarely true, as the high-density polyethylene (HDPE) pipes can flex within the GHE before being grouted into position. This can result in thermal interference not accounted for accurately in the design, with the worst case scenario represented by direct contact between the inlet and outlet pipes, leading to a negative impact on the performance of the system. This paper investigates the effect of this interference and the implications of ignoring it in design.

AbstractEnergy piles are a consolidated underground heat exchanger alternative to traditional boreholes in ground source heat pump (GSHP) systems. Previous works focused on assessing the differences between piles and boreholes, but few assessed small piles in operational conditions. Moreover, most of these studies centered around cylindrical concrete piles, overlooking short screw piles. Using in-situ testing, established analytical methods, and advanced three dimensional (3D) finite element model simulations, this work assesses three thermal response tests (TRT) executed in different energy pile structures, one being a unique group of eight short energy screw piles connected in series, located in the same site in Melbourne, Australia. Detailed numerical analysis provided reliable soil and structure thermal parameter predictions and detailed computations allowed the study of thermal effects for the energy screw piles steel components. The results show limited impact of the steel components on effective thermal conductivity, but a reduction in thermal resistivity that may provide a speedier thermal exchange in short term GSHP operation. In addition, the more traditional TRT rigs and analytical interpretation provided reasonable results for the pile group in series, and show a similar performance to a borehole heat exchanger of similar pipe length; however, the short piles engage only the upper soil layers, with potentially lower thermal conductivity. TRT in single short screw piles require careful consideration, because common rigs may be unable to cater for the required low fluid flow rates and heating power. Thus, for the cases assessed herein, the pile group TRT proved to be more reliable than individual pile testing, due to their short length.

Ground-source heat pump (GSHP) systems efficiently heat and cool buildings by using sustainable geothermal energy accessed by way of ground heat exchangers (GHEs). Loess covers vast parts of the world, about 10% of the landmass; therefore, the use of piles or 'micropiles' is extensive in these areas, particularly where the thickness of loessic soils is significant. These deep foundations have the potential to be used as 'energy piles' in GSHP systems, with a minimal additional cost. This paper presents a case study of a representative real building in Cordoba, Argentina, where foundations are also used as GHEs. The thermal properties of local soils were experimentally measured and used in recently developed detailed state-of-the-art finite-element models. Results from the realistic simulations show that the partial substitution of electrical heating and cooling systems with geothermal systems could significantly reduce energy consumption and the size of associated infrastructure, despite the relatively low thermal conductivity of local loess. Moreover, the effects of surface air temperature fluctuations, which are routinely ignored in GHE design, are accounted for in these simulations. This case study shows the potential of GSHP technology in loessic environments and gives incentives to engineers to start considering the technology in their designs and practices.

Incorporating ground heat exchangers (GHEs) into building foundations allows them to also provide thermal energy for space heating and cooling. However, this introduces certain constraints to ground-source heat pump (GSHP) design, such as on the geometry, and thus a different design approach is required. One such approach, introduced in this article, uses machine learning techniques to very quickly and accurately determine the maximum amount of thermal energy that can reasonably be provided. A comprehensive validation of this methodology for energy piles is presented, using different geometries and thermal load distributions, drawing conclusions about how the approach can best be utilised.

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.

Shallow geothermal energy utilises the ground at relatively shallow depths as a heat source or sink to efficiently heat and cool buildings. Geothermal pavement systems represent a novel concept where horizontal ground source heat pump systems (GSHP) are implemented in pavements instead of purpose-built trenches, thus reducing their capital costs. This paper presents a geothermal pavement system segment (20m × 10m) constructed and monitored in the city of Adelaide, Australia, as well as thermal response testing (TRT) results. Pipes have been installed in the pavement at 0.5 m depth, and several thermistors have been placed on the pipes and in the ground. A TRT has been performed with 6kW heating load to achieve an understanding of the thermal response of the system as well as to estimate the effective thermal conductivity of the ground. The results show that the conventional semi-log method may be applicable to determine the thermal conductivity for geothermal pavements. The geothermal heat exchanger at shallow depth is considerably under the influence of the ambient temperature; however, it is still acceptable for exchanging the heat within the ground. It is also concluded that the impact radius of heat exchanger in geothermal pavement during the TRT is around 0.5m in the vertical and horizontal directions for this case study.

Ground-source heat pump systems are renewable and highly efficient HVAC systems that utilise the ground to exchange heat via ground heat exchangers (GHEs). This study developed a detailed 3D finite element model for horizontal GHEs by using COMSOL Multiphysics and validated it against a fully instrumented system under the loading conditions of rural industries in NSW, Australia. First, the yearly performance evaluation of the horizontal straight GHEs showed an adequate initial design under the unique loads. This study then evaluated the effects of variable trench separations, GHE configurations, and effective thermal conductivity. Different trench separations that varied between 1.2 and 3.5 m were selected and analysed while considering three different horizontal loop configurations, i.e., the horizontal straight, slinky, and dense slinky loop configurations. These configurations had the same length of pipe in one trench, and the first two had the same trench length as well. The results revealed that when the trench separation became smaller, there was a minor increasing trend (0.5 degrees C) in the carrier fluid temperature. As for the configuration, the dense slinky loop showed an average that was 1.5 degrees C lower than those of the horizontal straight and slinky loop (which were about the same). This indicates that, when land is limited, compromises on the trench separation should be made first in lieu of changes in the loop configuration. Lastly, the results showed that although the effective thermal conductivity had an impact on the carrier fluid temperature, this impact was much lower compared to that for the GHE configurations and trench separations.

Over the last decades, converting foundation elements such as piles, retaining walls and tunnel linings into energy geo-structures has gained popularity around the world. These geo-structures form part of ground source heat pump (GSHP) systems to constitute a clean, renewable, and economical solution for space heating and cooling. This work utilises experimental investigations in combination with numerical simulations to study the thermal performance of energy piled walls, specifically regarding the effect of thermo-active pile spacing. This knowledge can aid towards better design guidelines and the determination of the number of piles to be thermally activated in the wall system. The experimental results from a section of a pilot energy soldier piled wall in Melbourne (Australia) and the numerical results from a validated finite element model suggest that the thermal performance of piled walls can be significantly affected by close pile spacing. In addition, parametric analyses were performed to understand the role of some other key design parameters. The results indicate that the depth of the wall impacts the thermal performance of the GSHP system more significantly than the thermal conductivity of the ground. Considering a real-world scenario where a fixed number of piles are constructed for a typical building at relatively close spacings, one may increase the “thermal” pile spacing by activating less piles overall. It is found that activating every other pile (1/2 of the total number of piles) or every other second pile (1/3 of them) can still provide over 70% or 50%, respectively, of the maximum thermal energy compared to activating every pile. •Thermal response testing and monitoring data on an energy piled wall are presented.•FE models are built to simulate thermal performance of energy walls.•The FE model has been successfully validated by the presented field-testing data.•The importance of pile spacing on energy piled walls’ performance are highlighted.•Activating all piles in the wall provides the highest thermal energy potential.

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.

Understanding the subsurface is crucial in building a sustainable future, particularly for urban centers. Importantly, the thermal effects that anthropogenic infrastructure, such as buildings, tunnels, and ground heat exchangers, can have on this shared resource need to be well understood to avoid issues, such as overheating the ground, and to identify opportunities, such as extracting and utilizing excess heat. However, obtaining data for the subsurface can be costly, typically requiring the drilling of boreholes. Bayesian statistical methodologies can be used towards overcoming this, by inferring information about the ground by combining field data and numerical modeling, while quantifying associated uncertainties. This work utilizes data obtained in the city of Cardiff, UK, to evaluate the applicability of a Bayesian calibration (using GP surrogates) approach to measured data and associated challenges (previously not tested) and to obtain insights on the subsurface of the area. The importance of the data set size is analyzed, showing that more data are required in realistic (field data), compared to controlled conditions (numerically-generated data), highlighting the importance of identifying data points that contain the most information. Heterogeneity of the ground (i.e., input parameters), which can be particularly prominent in large-scale subsurface domains, is also investigated, showing that the calibration methodology can still yield reasonably accurate results under heterogeneous conditions. Finally, the impact of considering uncertainty in subsurface properties is demonstrated in an existing shallow geothermal system in the area, showing a higher than utilized ground capacity, and the potential for a larger scale system given sufficient demand.

A ground source heat pump (GSHP) system provides efficient space heating and cooling and thus is regarded as a contributor to achieve net-zero emissions targets. This study focuses on an economical type of GSHP system - energy walls where earth retaining walls are equipped with pipes to act as geothermal heat exchangers in addition to being geotechnical structures. A detailed numerical investigation is performed to study the long-term heat exchange mechanism between the walls and the surrounding ground. The work highlights the significance of the existence and magnitude of groundwater flow on the temperature distribution of the ground and the thermal performance of energy walls. Compared to the case when there is no subsurface flow, the energy retaining wall can offer up to a 6 % better coefficient of performance (COP) or provide up to 1.3 times higher thermal yield for relatively slow groundwater flow velocities of 0.013 m/d. The COP can improve up to 93 % and thermal yield up to 23 times for high velocities of 2 m/d, even in extremely cooling-dominant thermal demand cases. The study also shows that the ground thermal conductivity plays a crucial role on performance when groundwater flow is minimal, however, as the convective heat transfer resulting from the groundwater flow becomes more dominant, the influence of thermal conductivity gradually diminishes. It is also found that the absorber pipe flow rate has a rather small effect on the system thermal performance, of less than a 10 % COP difference in all studied cases. This suggests that obtaining reliable hydrogeological information specifically on the groundwater flow velocity and direction are crucial for accurate predictions and optimal design for energy retaining walls and large scale GSHP installations. The insights from this study can improve the design and enhance the uptake of retaining walls for efficient heating and cooling of above/underground spaces - critical components in achieving a clean energy future.

The flipped classroom has been increasingly employed as a pedagogical strategy in the higher education classroom. This approach commonly involves pre-class learning activities that are delivered online through learning management systems that collect learning analytics data on student access patterns. This study sought to utilize learning analytics data to understand student learning behavior in a flipped classroom. The data analyzed three key parameters; the number of online study sessions for each individual student, the size of the sessions (number of topics covered), and the first time they accessed their materials relative to the relevant class date. The relationship between these parameters and academic performance was also explored. The study revealed patterns of student access changed throughout the course period, and most students did access their study materials before the relevant classroom session. Using k-means clustering as the algorithm, consistent early access to learning materials was associated with improved academic performance in this context. Insights derived from this study informed iterative improvements to the learning design of the course. Similar analyses could be applied to other higher education learning contexts as a feedback tool for educators seeking to improve the online learning experience of their students.

Anthropogenic infrastructures in the shallow subsurface, such as heated basements, tunnels or shallow geothermal systems, are known to increase ground temperatures, particularly in urban areas. Numerical modelling helps inform on the extent of thermal influence of such structures, and its potential uses. Realistic modelling of the subsurface is often computationally costly and requires large amounts of data which is often not readily available, necessitating the use of modelling simplifications. This work presents a case-study on the city centre of Cardiff, UK, for which high resolution data is available, and compares modelling results when three key modelling components (namely ground elevation, hydraulic gradient distribution and basement geometry) are implemented either ‘realistically’, i.e. with high resolution data, or ‘simplified’, utilising commonly accepted modelling assumptions. Results are presented at a point (local) scale and at a domain (aggregate) scale to investigate the impacts such simplifications have on model outputs for different purposes. Comparison to measured data at individual locations shows that the accuracy of temperature outputs from numerical models is largely insensitive to simplification of the hydraulic gradient distribution implemented, while changes in basement geometry affect accuracy of the mean temperature predicted at a point by as much as 3.5 °C. At the domain scale, ground temperatures within the first 20 m show a notable increase (approximately 1 °C volume-averaged and 0.5 °C surface-averaged), while the average heat flux over the domain is about 0.06 W/m2 at 20 m depth. These increased temperatures result in beneficial conditions for shallow geothermal utilisation, producing drilling cost savings of around £1700 per typical household system or about 9% increase in thermal energy potential. Simplifications of basement geometry and (to a lesser degree) the hydraulics can result in an overestimation of these temperatures and therefore over-predict geothermal potential, while the elevation simplification showed little impact. [Display omitted] •Heated basements shown to increase volumetric ground temperature by up to 1.1 °C•Locally, modelling simplifications are more appropriate within groundwater flow.•At global scale, modelling simplifications somewhat overestimate ground temperature.•Simplifying heat source (basement) geometries shows greatest impact on temperature.•In this case study, heated basements increase shallow geothermal potential by 9–11%.

•The effect of variable pipe separation (VPS) in ground heat exchangers is explored.•Detailed 3D finite element model accounting for VPS is introduced.•Large scale analyses identify most influential design parameters for performance.•Thermal conductivity of borehole grout found highly important to the effect of VPS.•A significant capital cost increase may be needed to compensate for VPS. Closed loop ground-source heat pump (GSHP) systems use ground heat exchangers (GHEs) to transfer heat to and from the ground and efficiently provide clean and renewable energy for heating and cooling purposes. Vertical GHEs contain pipes with circulating fluid (loops), which transfer thermal energy between the ground and the fluid. One very common assumption made in designing GSHP systems is that, when installed, these loops remain evenly separated along the length of the GHE, something that due to the nature of construction is rarely true. This can result in thermal interference not accounted for in the design, leading to a potential negative impact on the performance of the system. This paper investigates the effect of this interference, using detailed numerical simulations to compare different geometries, modelling fixed and variable pipe separations. A comprehensive parametric analysis is conducted to identify some of the most influential design parameters and the potential consequences on running and capital costs. Amongst the key findings of this study is the importance of the borehole filling material, as a highly thermally conductive material can minimise these negative effects from the thermal interference by up to 60%. Moreover, potential increases in drilling (capital) costs of up to 24% are shown, while the potential increases in running costs due to the reduced efficiency were found to be relatively minor.

Geothermal permeable pavement (GPP) systems are a nascent type of energy geo-structures. GPP combine ground-source heat pump (GSHP) systems with permeable pavements to enable the additional functionality of heating and cooling, through energy harvesting using roads alongside existing drainage controls. GPP thermal performance is relatively unknown, hence, this research focuses on the development of a detailed 3D finite element (FE) model that considers local thermal non-equilibrium (LTNE) for exploring the influence of rainfall infiltration on the thermal performance of GPPs. The 3D numerical model was successfully validated and was used to evaluate the short-term thermal response of GPPs during rainfall events in both cooling (summer) and heating (winter) demand conditions. Furthermore, the diurnal rainfall effects and the influence of raindrop temperature and rainfall duration on GPPs were also examined. Results show that rainfall can have significant impacts on the thermal performance of GPPs and the raindrop temperature is an important parameter in yield predictions. Compared to no-rainfall events, the GSHP system fluid temperature during rainfall can differ by up to 19% when in the cooling operation model (air conditioning) and 63% in heating mode. These results indicate the significantly better performance of GPPs than impervious geothermal pavements, depending on the climatic conditions and end-use.

The use of monitored data to improve the accuracy of building energy models and operation of energy systems is ubiquitous, with topics such as building monitoring and Digital Twinning attracting substantial research attention. However, little attention has been paid to quantifying the value of the data collected against its cost. This paper argues that without a principled method for determining the value of data, its collection cannot be prioritised. It demonstrates the use of Value of Information analysis (VoI), which is a Bayesian Decision Analysis framework, to provide such a methodology for quantifying the value of data collection in the context of building energy modelling and analysis. Three energy decision-making examples are presented: ventilation scheduling, heat pump maintenance scheduling, and ground source heat pump design. These examples illustrate the use of VoI to support decision-making on data collection.

The use of energy piles as heat exchangers for Ground Source Heat Pump (GSHP) systems, providing heating and cooling, is a well researched application worldwide [1]. However, a broader implementation in practice still faces resistance, mainly because of the lack of accessible, easy to implement design methods and uncertainty regarding the thermo-mechanical effects. These issues need to be addressed to close the gap between research and practice. This work presents data of a full-scale thermal response test (TRT) undertaken in a group of eight energy screw piles connected in series, that are part of an operational GSHP system of a building located in Melbourne, Australia. The temperature was measured in the inlet and outlet of the pipe circuit (circulating water temperature) and at the bottom of each pile (external pipe wall temperature). Besides providing insights regarding the thermal performance of short energy pile groups, the test was used to validate a finite element numerical model (FEM). The model was then used to expand the database of thermal performance of energy pile groups by simulating several long thermal response tests, considering different energy pile group geometries, configurations and material properties. The experimental data presented can be used for analyses and validation of thermal modelling methodologies that consider the group effect of energy piles, given the lack of TRTs performed in groups of energy piles reported in literature. Moreover, the extensive set of simulated data can be analysed to understand the thermal behaviour of energy pile groups and evaluate how alternative simpler heat transfer models, feasibly applied in industry practice, perform in a range of scenarios that could be encountered in daily practice.