The interaction factor method for energy pile groups

Prior to this study, no simplified yet rational methods were available for estimating the vertical displacements of energy pile groups subjected to thermal loads. Observing such a challenge, the goal of this study has been threefold: (i) to extend the interaction factor concept from the framework of conventional pile groups to that of energy pile groups, (ii) to present charts for the analysis of the displacement interaction between two identical energy piles over a broad range of design conditions, and (iii) to propose, apply and validate the interaction factor method for the displacement analysis of energy pile groups

Energy and geotechnical behaviour of energy piles for different design solutions

Energy piles are heat capacity systems that have been increasingly exploited to provide both supplies of energy and structural support to civil structures. The energy and geotechnical behaviours of such foundations, which are governed by their response to thermo-mechanical loads, is currently not fully understood, especially considering the different design solutions for ground-coupled heat exchangers. This paper summarises the results of numerical sensitivity analyses that were performed to investigate the thermo-mechanical response of a full-scale energy pile for different (i) pipe configurations, (ii) foundation aspect ratios, (iii) mass flow rates of the fluid circulating in the pipes and (iv) fluid mixture compositions. This study outlines the impacts of the different solutions on the energy and geotechnical behaviour of the energy piles along with important forethoughts that engineers might consider in the design of such foundations. It was observed that the pipe configuration strongly influenced both the energy and the geotechnical performance of the energy piles. The foundation aspect ratio also played an important role in this context. The mass flow rate of the fluid circulating in the pipes remarkably influenced only the energy performance of the foundation. Usual mixtures of a water-antifreeze liquid circulating in the pipes did not markedly affect both the energy and the geotechnical performance of the pile

Design and testing of ultrafast plasmonic lens nanoemitters

Nanoscale electron pulses are increasingly in demand, including as probes of nanoscale ultrafast dynamics and for emerging light source and lithography applications. Using electromagnetic simulations, we show that gold plasmonic lenses as multiphoton photoemitters provide unique advantages, including emission from an atomically at surface, nanoscale pulse diameter regardless of laser spot size, and femtosecond-scale response time. We then present fabrication of prototypes with sub-nm roughness via e-beam lithography, as well as electro-optical characterization using cathodoluminescence spectromicroscopy. Finally, we introduce a DC photogun at LBNL built for testing ultrafast photoemitters. We discuss measurement considerations for ultrafast nanoemitters and predict that we can extract tens of pA photocurrent from a single plasmonic lens using a Ti:Sa oscillator. Altogether, this lays the groundwork to develop and test a broad class of plasmon-enhanced ultrafast nanoemitters

Energy geostructures: Theory and application

The subsurface represents space and resource of ever-growing importance to meet human activity needs associated with the availability of built environments and energy. So-called energy geostructures represent a breakthrough technology in this context. By integrating the structural support role of earth-contact structures with the heating-cooling role of shallow geothermal heat exchangers, energy geostructures can sustain or enclose built environments while providing them with renewable thermal energy. Despite such promising features, the integrated roles of energy geostructures pose various challenges to understand their behavior and performance, and to address the related analysis and design. Appropriate formulation and application of scientific theory are crucial for the successful analysis and design of energy geostructures. This Bright Spark Lecture Paper presents selected theory for addressing the behavior and performance of energy geostructures, and discusses the application of this theory to analysis and design. In this context, the work focuses on energy piles and barrettes, energy tunnels, as well as energy walls and slabs. The ultimate goal of this paper is to provide competence for facilitating future research and development of energy geostructures across science and engineering

Group action effects caused by various operating energy piles

To date, no field data referring to various operating energy piles over time-scales of practical applications had been made available to investigate the thermally induced “group action”. This paper presents the results of a full-scale field test and a three-dimensional thermo-mechanical finite element analysis of four operating energy piles over 12 months. When the number of operating energy piles increases, greater thermally induced vertical strain and lower stress develop along the piles for the same average temperature change. Opposite stress variations to those that may be expected based on the type of applied thermal load can develop in piles

Thermally induced group effects among energy piles

The behaviour of conventional pile groups (e.g., closely spaced) that are subjected to mechanical loads has been shown to be different than the behaviour of single isolated piles. The so-called group effects are responsible for this behaviour and must be considered for an optimal design of pile foundations. In recent years, energy piles have shown potential to work as both structural supports and geothermal heat exchangers and thus are subjected to both mechanical and thermal loads. An increasing amount of research has investigated the previously unexplored impact of thermal loads on the thermo-mechanical behaviour of energy piles. However, no field data over typical time-scales of practical geothermal applications have been available to analyse the development and impact of thermally induced group effects between energy piles (e.g., closely spaced) on their thermo-mechanical behaviour. To investigate this problem, a full-scale in-situ test of a group of energy piles and coupled 3-D thermo-mechanical finite element analyses were performed and are presented in this paper. This work demonstrates that significant thermally induced group effects characterise closely spaced energy piles. Attention must be devoted to these effects throughout the design process (e.g., geotechnical, structural and energy) of energy piles because they play an important role in the serviceability performance of these foundations

The equivalent pier method for energy pile groups

This study presents a method for estimating the average vertical displacement of energy pile groups subjected to thermal loads. The method consists of replacing any regular energy pile group with a single equivalent pier of the same length and an equivalent diameter. This equivalent pier is described by material properties that are a homogenisation of those of the piles and the surrounding soil and by a load-displacement relationship of a characteristic energy pile in the group. The load-displacement relationship of the equivalent pier differs from that of a single isolated energy pile because it is modified to account for group effects. These effects include a greater vertical displacement of the piles subjected to loading in the group compared to the case in which they are isolated, thus involving a more pronounced average group displacement. Comparisons with results obtained through the interaction factor and finite element methods prove that the proposed approach can accurately estimate the average vertical displacement of energy pile groups. This novel formulation of the equivalent pier method may be used at both preliminary and successive stages of the analysis and design of energy pile groups to expediently assess the thermally induced displacement response of such foundations

Impact of thermally induced soil deformation on the serviceability of energy pile groups

This paper expands on the impact of the thermally induced deformation of the soil on the serviceability mechanical performance (i.e., deformation-related) of energy pile groups. The work is based on the results of a full-scale in-situ test that was performed on a group of energy piles at the Swiss Tech Convention Centre, Lausanne, Switzerland, and on a series of 3-D thermo-mechanical finite element analyses that were carried out to predict the considered experiment. This study proves that the serviceability mechanical performance of energy pile groups crucially depends on the relative thermally induced deformation of the soil to that of the energy piles. The relative thermally induced deformation of the soil to that of the energy piles is governed by (i) the thermal field characterising the energy pile group and (ii) the relative thermal expansion coefficient of soil to pile. Considering these aspects in the analysis and design of energy pile groups is key because they profoundly characterise the deformation of such foundations