A robust prediction model approach to energy geo-structure design

doi:10.1016/j.compgeo.2018.08.012

Computers and Geotechnics

Volume 104, December 2018, Pages 140-151

https://doi.org/10.1016/j.compgeo.2018.08.012 Get rights and content

Abstract

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.

Section snippets

Ground source heat pump systems and energy geo-structures

Transitioning towards cleaner and more renewable sources of energy and finding ways to reduce energy consumption are considered as essential means to ensure a sustainable future. A technology that can contribute towards these goals is shallow geothermal, which uses the ground as a heat source or sink to efficiently heat and cool buildings, by utilising the fact that the ground temperature a few tens of metres below the surface is relatively constant throughout the year [1]. Shallow geothermal,

Energy geo-structures and current approaches

As explained in Section 1, an important question needed to be answered in geothermal design as well as in research of energy geo-structures is how much energy can the system provide [21], [22], instead of how does the system need to be designed to provide the target energy demand, as in traditional GSHP design. Knowing how much energy the system can reliably provide is important, as it enables an evaluation of its applicability for a given project to be performed, including a cost-benefit

Prediction model evaluation methodology

This section presents the tools used to evaluate the accuracy of the prediction methodology presented. Firstly, an overview of the finite element modelling used to generate the numerical simulations is given, including the details of the case study adopted, such as the geometry and material parameters used. An explanation of the thermal loads adopted in the testing is also presented, showing how the percentages of cooling and heating are calculated. Lastly, a more comprehensive expanded

Results and Discussion

This section utilises the outputs of 2520 solved numerical simulations (each requiring a running time between 1 and 3 h utilising high performance computing) to present a thorough evaluation of the prediction methodology, starting from a specific set of design parameters and continuing to an expanded overall evaluation for all 360 possible parameter combinations (i.e., all possible combinations of λ_ground, λ_concrete, $L_{wall}$ , $s$ for the values in Table 3). The effect of the fluid temperature

Summary and Conclusions

This work presented a prediction methodology that can be used alongside numerical simulations to greatly hasten and ease the design process for energy geo-structures. The methodology uses a simple but robust prediction model created using results from two numerical simulations and can aid in calculating how much thermal energy the GSHP system can reliably provide. This work selected energy retaining walls as a case study to perform the evaluation of the prediction model; however, it is expected

Acknowledgements

Funding from the Australian Research Council (ARC) FT140100227, The University of Melbourne, the Cambridge Centre for Smart Infrastructure and Construction (via the Global University Alliance), and the Victorian Government is much appreciated.

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Energy diaphragm walls have attracted increasing attention due to their high heat transfer performance. Thus, the factors that affect the thermal performance of energy walls have been investigated to provide insights for further practical applications. Numerical simulation is widely used in energy wall analysis due to the complexity of conducting field tests. Therefore, the choice of boundary conditions in the simulations is crucial; it refers to whether the model reflects reality and has a significant impact on the results. In most cases, the assumed ground surface thermal boundary is not the same (e.g., thermal insulation, constant temperature), and the ground temperature is also assumed to be constant with depth, which may result in a large loss of thermal energy or inaccurate assessment of thermal performance for the energy wall. In this respect, this paper investigates the long-term differences in thermal performance under these boundaries and considers the ground temperature that varies with depth and time, which has not been comprehensively considered before in the analysis of energy walls. The results show that accounting for air temperature or ground temperature variations with depth and time can result in a great increase in thermal performance, e.g., a 34% increase in the cooling thermal load compared with the thermal insulation boundary. In the performed parametric analysis, the results highlight the significant influence of the thermal load and wall height on the thermal differences between these boundaries. These findings provide a reference for choosing ground surface thermal boundaries and better thermal design of energy diaphragm walls.
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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.
Investigating the thermal performance of energy soldier pile walls
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Citation Excerpt :
These investigations consider characteristic stresses, strains etc. and are essential to ensure there will not be any undesirable effects, such as settlement. The second area investigates the thermal performance of these structures, considering the heat transfer in the various media and answering the key question of how much energy can be provided by the system.33–35 The presented work is part of this second category, investigating the thermal design of energy soldier pile walls and considering the effect of important design parameters, such as the pipe configuration, wall depth and pile spacing on the thermal performance of the system.
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.
Early-stage thermal performance design of thermo-active walls implemented in underground infrastructures
2022, Geomechanics for Energy and the Environment
Citation Excerpt :
The transition from regular geostructures to EGs is simple, it only requires heat exchangers (HEs) to be installed and secured to the reinforcing cage whenever a new geostructure is needed. Some of the main advantages of this technology are that energy is continuously available, regardless of weather conditions, it is available almost everywhere and it is usable in a wide range of applications3–10. Possible applications of EGs and shallow geothermal systems include: (i) heating and cooling of civil engineering structures (residential and commercial buildings, industries, etc.)11–14, (ii) production of hot water for agricultural needs15–17, (iii) de-icing of deck, bridge and road pavements18,19.
Energy geostructures (EGs) represent an innovative technology in the sustainable energy agenda and are useful for satisfying the energy needs of the built environment. They are usually involving geostructures such as piles, walls, tunnels, shafts, and sewers. The application of such technology to infrastructure projects may have considerable thermal potential because of the large surfaces that can be thermally activated. This study focuses on thermo-active walls (energy walls, EWs), which are retaining structures used to sustain the sides of excavations. Key features related to their thermal design are examined, and a design methodology is proposed. The heat-exchange modes involving EWs and the surrounding materials (concrete, soil, air) are investigated via extensive three-dimensional hydrothermal finite-element simulations involving the non-isothermal flow in the heat exchangers (HEs), as well as all other heat-exchange modes. The results are first presented as charts related to the thermal behavior of the HEs under different hydrothermal environments. Finally, a methodology for early-stage thermal performance design is proposed, and a corresponding flowchart is presented. This study may be helpful for incorporating EGs into the engineering design.
Experimental and numerical studies on an energy piled wall: The effect of thermally activated pile spacing
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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.
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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.

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Research PaperA robust prediction model approach to energy geo-structure design

Abstract

Section snippets

Ground source heat pump systems and energy geo-structures

Energy geo-structures and current approaches

Prediction model evaluation methodology

Results and Discussion

Summary and Conclusions

Acknowledgements

Renew Energy

Energy

Procedia Eng

Energy Build

Appl Therm Eng

Appl Energy

Renew Energy

Energy

Energy

Energy Build

Tunn Undergr Sp Technol

Geomech Energy Environ

Renew Energy

Data Br

Geomech Energy Environ

Eng Geol

Nucl Eng Des

Emerging geothermal energy technologies

KSCE J Civ Eng

An introduction to thermogeology: ground source heating and cooling

Ground energy systems: from analysis to geotechnical design

Géotechnique

Ground heat exchanger design for direct goethermal energy systems

Energy foundations and other thermo-active ground structures

Géotechnique

Thermo-mechanical behaviour of energy piles

Géotechnique

Energy performance of diaphragm walls used as heat exchangers

Proc Inst Civ Eng – Geotech Eng

Research Paper
A robust prediction model approach to energy geo-structure design