Analysis of friction induced thermo-mechanical stresses on a heat exchanger pile in isothermal soil

Copyright © 2014 SpringerIn most analytical and numerical models of heat exchanger piles, strain incompatibilities between the soil and the pile are neglected, and axial stresses imposed by temperature changes within the pile are attributed to the thermal elongation and shortening of the pile. These models incorporate thermo-hydro-mechanical couplings in the soil and within the pile foundation, but usually neglect thermo-mechanical couplings between the two media. Previous studies assume that the stress changes imposed by temperature variations in a heat exchanger pile are mainly due to the constrained thermal elongation and shortening of the pile. Also, several recent approaches utilize spring models that focus only on the soil-pile interface in modeling temperature-induced stresses in a heat exchanger pile and implicitly ignore the effect of the full displacement field on soil-pile interaction. By contrast, in this paper, interface elements are introduced in a numerical model of a heat exchanger pile, analyzed in axisymmetric and stationary conditions. The pile is subjected to a uniform temperature increase, with free top and fixed top conditions in elastic and elasto-plastic soil profiles. Simulation results show that the constrained vertical elongation is the most detrimental factor for pile foundation performance. However it is worth noticing that while mechanical constraints (e.g., fixed top and/or fixed bottom) impose maximum stress increases at the ends of the pile , interface effects result in maximum stresses around the mid-length of the pile. This preliminary study indicates that soil-pile friction does not increase pile internal stresses to the point where it would be necessary to over-dimension the foundation pile for heat exchanger use. Furthermore, one cannot expect a significant gain in foundation performance due to the improvement of soil-pile frictional resistance as a result of increased lateral stresses at soil-pile contact. Additional numerical analyses are ongoing, in order to investigate the role of the degree of fixity induced by the building on the heat exchanger pile, and to extend these preliminary analyses to transient operational modes and cyclic thermo-mechanical loading of the heat exchanger pile

Thermo-mechanical radial expansion of heat exchanger piles and possible effects on contact pressures at pile-soil interface

Copyright © 2014 ICE PublishingDOI: http://dx.doi.org/10.1680/geolett.14.00018This letter shows that the increase of heat exchanger pile capacity in response to heating, observed in several small-scale laboratory studies, cannot be directly attributed to the increase of contact pressure at the soil–pile interface. The main thermo-hydro-mechanical processes that influence the capacity and behaviour of heat exchanger piles include thermal hardening of the soil, thermally induced water flow, excess pore pressure development and volume changes upon thermal consolidation. Due to the lack of understanding of the behaviour around the soil–pile interface, thermo-mechanical interactions between the heat exchanger pile and the ground are not taken into account appropriately in energy foundation design. However, in situ and reduced-scale experiments provide evidence about temperature-induced changes in pile capacity, presumably as a result of the altered stress state around the test pile. A finite-element analysis was conducted to quantitatively assess the radial stresses and strains undergone by a heated pile embedded in deformable soil. The study indicates that radial contact pressures typically increase less than 15 kPa, which cannot fully explain the increase in shaft resistance observed in heating tests. Further analyses are underway to characterise the mechanisms that govern pile load–displacement behaviour and the limit state

3D Numerical Modeling of Vertical Geothermal Heat Exchangers

This paper presents the development and validation of a 3D numerical model for simulating vertical U-tube geothermal heat exchangers (GHEs). For minimizing the computational effort, the proposed numerical model uses 1D linear elements for simulating the flow and heat transfer inside the pipes. These linear elements are coupled with the 3D domain using the temperature field along the exterior surface of the pipe and an optimized finite element mesh for reducing the number of elements. The discretization of geometry, finite element mesh generation and the specifics of the system physics and boundary condition assignments are explained in detail. The model is used to simulate two generic cases, a borehole with a single U-tube and an energy pile with double U-tubes. In each case, a constant heating followed by a recovery period (i.e., no heating) is simulated. A review of the theory of finite line source model is also presented, along with modifications to account for variable heat rate. Moreover, a method to estimate the steady state thermal resistances in the borehole/energy pile is presented in order to calculate the fluid temperatures analytically. The validation of the model is carried out by comparing the numerical results with the results obtained from the analytical model

Long-Term Performance of Heat Exchanger Boreholes at Different Climatic Conditions

Ground-source heat pump systems consist of heat exchanger boreholes, embedded with circulation pipes, buried in the ground and connected to a heat pump for heating and cooling of buildings. Due to the changes in seasonal energy demands of the building, sustainability of borehole heat exchangers depends on the seasonal load balance. As the soil can be gradually heated up or cooled down considering the unbalanced thermal loads, long-term performance of heat exchanger boreholes is closely related to maintaining a constant ground temperature as progressively changing temperatures indicate loss of heat exchange efficiency in the long term. In order to address the long-term thermal performance for different unbalanced climatic conditions, ground thermal loads were estimated and representative equivalent half-sine waves of thermal loads from a hypothetical four floor medium size office building are created for 100+ different locations. Total required heat exchanger lengths for each location are then estimated for the proposed building. Findings suggest that for different seasonal energy demands, amplitudes and durations of the sine waves change significantly, consequently suggesting different loop lengths. For unbalanced climates, loop lengths found are considerably higher than balanced cases. Ultimately, numerical analyses were simulated for 30 years of heat exchanger operation to investigate the thermal performance under different climatic conditions

Thermo-Mechanical Radial Expansion of Heat Exchanger Piles and Possible Effects on Contact Pressures at Pile-Soil Interface

This letter shows that the increase of heat exchanger pile capacity in response to heating, observed in several small-scale laboratory studies, cannot be directly attributed to the increase of contact pressure at the soil-pile interface. The main thermo-hydro-mechanical processes that influence the capacity and behaviour of heat exchanger piles include thermal hardening of the soil, thermally induced water flow, excess pore pressure development and volume changes upon thermal consolidation. Due to the lack of understanding of the behaviour around the soil-pile interface, thermo-mechanical interactions between the heat exchanger pile and the ground are not taken into account appropriately in energy foundation design. However, in situ and reduced-scale experiments provide evidence about temperature-induced changes in pile capacity, presumably as a result of the altered stress state around the test pile. A finite-element analysis was conducted to quantitatively assess the radial stresses and strains undergone by a heated pile embedded in deformable soil. The study indicates that radial contact pressures typically increase less than 15 kPa, which cannot fully explain the increase in shaft resistance observed in heating tests. Further analyses are underway to characterise the mechanisms that govern pile load-displacement behaviour and the limit state

Numerical Modeling of Vertical Geothermal Heat Exchangers using Finite Difference and Finite Element Techniques

This paper presents the development of a 2D finite difference modelling approach and a 3D finite element numerical model for simulating vertical geothermal heat exchangers (GHEs), explaining the theory governing the thermal processes, element discretization and the selection of the appropriate boundary conditions. Both of these models provide fully coupled solutions for the fluid flow in the circulation pipes and the thermal processes between the fluid and solid domains (pipes, grout and soil). The numerical models are verified with a field test and subsequently they are utilized to simulate the thermal performance of a borehole heat exchanger integrated with a single U-tube. Two different thermal operation cases are analyzed; a constant rate heat injection and a fluid injection at a constant temperature. A model validation study is also carried out for the constant rate heat injection case by comparing the numerical results with the available analytical solution for a finite line source. Furthermore, effective thermal conductivity of the ground back-calculated from the results of the numerical analyses is compared with the value used in the numerical models. Comparison of the results obtained from both numerical models and validating model predictions with the analytical solution confirms that both FE and FD models can accurately simulate the heat transfer mechanisms governing the thermal performance of GHE systems

Long-Term Performance of Heat Exchanger Piles

Heat exchanger piles utilize the constant temperature and the thermal storage capacity of the ground for heating and cooling of buildings. Sustainable use of the ground as a renewable energy source depends on the seasonal energy load balance. One of the critical factors for the sustainable operation of heat exchanger piles is that a constant temperature of the ground is maintained over seasons. The entire soil mass can be gradually heated up or cooled down if the energy demand is unbalanced. This paper presents the findings on the long-term performance of heat exchanger piles and their efficiency for areas where the demand is nonsymmetrical. Analyses have been performed to investigate the long-term performance of several pile arrangements ranging from single pile to numerous pile groups with a selection of 2 x 2, 3 x 3, 4 x 4 and 5 x 5 rectangular grids. The thermo-mechanical behavior of the single pile was also investigated. The analyses simulated 30 years of pile operation and resulted in significant findings for long-term performance of heat exchanger piles under different climatic conditions

Multilayer Finite Line Source Model for Vertical Heat Exchangers

This paper introduces a finite line source model for vertical heat exchangers considering a layered soil profile. The existing analytical models assume a homogeneous soil profile, where the thermal proper- ties of the ground along the entire length of the heat exchanger are uniform. This assumption can be unreliable since the typical length of heat exchangers is 60-100m (200-300 ft.) and stratified ground is expected over this length. In the approach presented herein, the heat exchanger is divided into a number of segments to represent various soil layers along its length. Heat exchange induced temperature change at a certain location within the soil formation is evaluated by summing up the individual contributions of all these segments. The effect of the heat exchanger segment within the soil layer around itself is estimated using the finite line source model. Furthermore, the finite line source model is utilized on transformed sections for estimating the contributions of heat exchanger segments at locations outside their layer domains. The proposed model also incorporates two adjustments; the first accounts for the different heat rates within different soil layers while the second adjustment considers the heat exchange along the vertical direction between soil layers. Estimated results using the proposed model agree well with the results obtained from a calibrated finite element analysis. The proposed procedure is promising and can also be adapted within the framework of cylindrical models