Heat Exchanger Pile Research Articles

R&D for Lower Cost and Higher Efficiency of Ground Source Heat Pump Systems with Pile Heat Exchangers

R&D for Lower Cost and Higher Efficiency of Ground Source Heat Pump Systems with Pile Heat Exchangers

Comparing heat flow models for interpretation of precast quadratic pile heat exchanger thermal response tests

This paper investigates the applicability of currently available analytical, empirical and numerical heat flow models for interpreting thermal response tests (TRT) of quadratic cross section precast pile heat exchangers. A 3D finite element model (FEM) is utilised for interpreting five TRTs by inverse modelling. The calibrated estimates of soil and concrete thermal conductivity are consistent with independent laboratory measurements. Due to the computational cost of inverting the 3D model, simpler models are utilised in additional calibrations. Interpretations based on semi-empirical pile G-functions yield soil thermal conductivity estimates statistically similar to those obtained from the 3D FEM inverse modelling, given minimum testing times of 60 h. Reliable estimates of pile thermal resistance can only be obtained from type curves computed with 3D FEM models. This study highlights the potential of applying TRTs for sizing quadratic, precast pile heat exchanger foundations.

Different types of energy piles with heating–cooling cycles

Energy piles are a new kind of foundation that can exchange heat with the surrounding soil. The long term thermal mechanical behavior of energy piles is important to the safety of the structure. Because different heat exchange piles may be used in practice, a comparative analysis under the same heat input is needed. This model test was conducted to assess the effects of cyclic heating and cooling on the thermo-mechanical behavior of semi-floating energy piles. Three types of energy piles with heat exchangers in U-shaped, spiral, W-shaped configurations were subjected to three heating and cooling cycles and a constant working mechanical load in dry sand. Results showed the pile temperature, horizontal earth pressure and thermal strain after heating and cooling cycles nearly recovered to the initial levels. The thermal stress increased gradually after each heating–cooling cycle. With a working load equal to 50 percent of the ultimate capacity, the settlement of each pile accumulated at the end of each cycle. In the end, the settlement of a W-shaped heat exchanger pile was larger than other two piles reaching 0·56% of the pile diameter. The displacement produced in each heating and cooling cycle decreased as the cycle increased.

ASSESSMENT OF STRESS-STRAIN BEHAVIOR OF ENERGY PILES INSTALLED IN SAND

The geothermal piles are dual functional geostructures which provide sub-structural support to thebuilding and also fulfill the energy demands in terms of its climate control. For efficient design of geothermalpiles, it is necessary to evaluate the thermo-mechanical behavior of geothermal piles. The purpose of this study isto evaluate the behavior of geothermal heat exchanger pile installed in sand, subjected to a coupled thermomechanical loading. Initially, in this study a full scale field test is simulated and results are compared. In thesecond phase, parametric study by varying mechanical and thermal loading on the geothermal pile placed indense sand, is performed. In this study, the sand and geothermal pile are simulated as axis-symmetrical models,where pile is considered to be thermo-elastic in nature and sand is considered as Mohr-Coulomb elastic-plasticmaterial. Coupled Temperature-Displacement analysis is applied in order to simulate the observed experimentalresults. It is shown that the simulated model is able to reproduce the major thermo-mechanical effects for theselected full scale model approximately. Stress and strain behavior along with the depth of energy pile inducedby thermo-mechanical loading has been reported. It was concluded that when mechanical load value iscomparatively low, the thermal variation influences the induced axial stress while at higher mechanical load thestress behaviour is nearly constant at different temperature variation.

Effects of soil anisotropy on a soil structure interaction in a heat exchanger pile

A coupled thermal-hydro-mechanical computational modeling of actual field tests, which had been conducted on an end bearing heat exchanger pile, has been carried out. Results of a conventional triaxial test program, which was performed during the site investigation, indicated that two soil layers were anisotropic. A simplified form of a transverse isotropy was used to model the response of these layers. Additional analyses were carried out for completely isotropic soil layers. Comparisons between the measurements obtained during the field testing and predictions of the computational models showed a very good to excellent agreement. Overall the anisotropic model performed better than the isotropic model.

Research Progress of Pile Heat Exchangers in Ground Source Heat Pump System

Pile heat exchanger has been developing rapidly in recent years and is an important research direction of ground source heat pump technology. This paper, from the aspects of principle structure, heat transfer model, numerical simulation, experimental study and engineering application and so on, sums up the domestic and overseas research status of pile heat exchangers in the construction industry and describes the laws and characteristics of heat transfer, underground soil temperature field and the factors impacting energy pile heat transfer. Pile heat exchanger has a variety of pipe laying methods, in which the spiral pipe-laying method can effectively increase the heat exchange area on the premise of security and reliability and has the best heat transfer efficiency. The paper also concludes the next step of research problems and development direction of pile heat exchanger technologies.

Empirical study on shallow depth ground source heat pump system with steel pipe pile heat exchanger wells

Empirical study on shallow depth ground source heat pump system with steel pipe pile heat exchanger wells

Feasibility of geothermal heat exchanger pile-based bridge deck snow melting system: A simulation based analysis

Snow melting system based on geothermal heat exchanger pile is an innovative technology that combines geothermal energy with structural foundation. It overcomes the problems of conventional chemical based snow melting in mitigating infrastructure corrosion and negative environmental effects. By integrating the underground heat exchanger into pile foundation that support the bridge structure, it effectively reduces the installation cost of geothermal system. This paper analyses the applicability and performance of such snow melting system for different regions. Energy demand for snow removal is firstly determined with ASHRAE criteria. A holistic 3D simulation model is developed to predict the energy extraction rate under different operation conditions. A hypothetical bridge deck (200 m length by 14.8 m (4 lanes) width) is analyzed to assess the feasibility of geothermal heat exchanger pile based snow melting system for 10 cities representing a variety of climatic regions of the United States. The number of pile foundation required for snow melting is used as indication of the technical feasibility. The results show that its feasibility and performance in bridge deck snow removal is dependent upon the geological and snow conditions of a particular region, as well as the design snow removal criteria.

浅部地中熱利用向け低コスト地中熱交換器の開発

Thermal performance evaluation of a pile heat exchanger utilizing shallow depth ground source heat was investigated numerically. The whole pile heat exchanger system including water natural convection was evaluated using CFD. The singularity of the water density around 4 degrees C is included as a phenomenon. Therefore the utilization of CFD is effective for development of a low-cost pile heat exchanger. Each phenomenon in the heat exchanger system was explained numerically.

浅部地中熱利用システムの評価

The GSHP system can be cooling, heating and hot-water supply with the ground source heat energy, but it was expensive in general residence. A GSHP with pile heat exchangers (PHEs) is expected to reduce the initial cost compared with the conventional GSHP systems. However, the thermal performance of GSHP with PHEs and influence of ground temperature with GSHP with PHEs are unknown. This paper described what results and subjects of thermal evaluation of the GSHP (Ground Source Heat Pump) system with pile heat exchangers for the residence.

一般住宅用地中熱利用システムの動的熱特性の研究

一般住宅用地中熱利用システムの動的熱特性の研究

地中熱リファレンスマップの開発

This paper describes a common view platform of the Reference-Map for designing GSHP System with pile heat exchangers. This platform planning to be designed for GSHP designers, is able to be used when the boring data near the GSHP system installation area. Heat performance of a double-cylindrical ground heat exchanger at cooling and heating, is shown in the platform, according to parameters set by designers. 2-dimensional unsteady-state heat transfer in the under-ground is estimated due to numerical simulations, and calculated results are set to have the designing margin.

浅部地中熱利用向け低コスト地中熱交換器の開発

浅部地中熱利用向け低コスト地中熱交換器の開発

Finite cylinder-source model for energy pile heat exchangers: Effect of buried depth and heat load cyclic variations

A solution to the finite cylinder-source model for the ground heat exchangers at a different buried depths that takes into account the heat capacity inside them and allows arbitrary heat rate changes is presented. Analytical expressions for the average ground temperature are derived by integrating the exact solutions over the cylinder-source depth for vertical and time-dependent changes of heat rate. The influence of buried depth variations on thermal response to a constant and uniform mean heat rate of the cylinder-source is examined by the integral mean temperature method in a self-consistent approach. In addition, approximate expressions for steady-state average temperature are derived for periodic heat rate in the long-time limit.

Determining Soil Thermal Conductivity Through Numerical Simulation of a Heating Test on a Heat Exchanger Pile

Heat exchanger pile foundations have a great potential of providing space heating and cooling to built structures. This technology is a variant of vertical borehole heat exchangers. A heat exchanger pile has heat absorber pipes firmly attached to its reinforcement cage. Heat carrier fluid circulates inside the pipes to transfer heat energy between the piles and the surrounding ground. Borehole heat exchangers technology is well established but the heat exchanger pile technology is relatively new and requires further investigation of its heat transfer process. The heat transfer process that affects the thermal performance of a heat exchanger pile system is highly dependent on the thermal conductivity of the surrounding ground. This paper presents a numerical prediction of a thermal conductivity ground profile based on a field heating test conducted on a heat exchanger pile. The thermal conductivity determined from the numerical simulation was compared with the ones evaluated from field and laboratory experiments. It was found that the thermal conductivity quantified numerically was in close agreement with the laboratory test results, whereas it differed from the field experimental value.

Finite cylinder-source model for energy pile heat exchangers: Effects of thermal storage and vertical temperature variations

A mean integral solution to the FCS (finite cylindrical source) model for ground heat exchangers that takes into account the heat capacity inside the borehole or foundation and allows estimation of temperature field for an arbitrary borehole configuration is presented as a single integral. Approximate expressions for the average temperature over a wide range of time values are derived analytically in the vicinity of both sides of the finite cylindrical surface of heat source. Exact and approximate results for the mean temperature are compared to those calculated at the mid-depth from the exact solution of the same FCS model.

Analysis of Friction Induced Thermo-Mechanical Stresses on a Heat Exchanger Pile in Isothermal Soil

In 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.

Numerical Simulations of a Ground Source Heat Pump System With Pile Heat Exchangers

Feasibility of a ground source heat pump (GSHP) system with pile heat exchangers for use in houses is evaluated through a numerical simulation. This GSHP system differs from ordinary borehole-type GSHP systems because short foundation piles installed at close intervals are used as heat exchangers. It is shown that the annual heat supply provided by this GSHP system is able to satisfy the demand of a house due to the air-source exchange at ground surface.

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.

Laboratory Thermal Performance Tests on a Model Heat Exchanger Pile in Sand

Pile-anchored geothermal systems are increasingly being implemented as an innovative and sustainable method of harvesting shallow geothermal energy. With a view of characterizing heat exchange through geothermal piles in sand, this paper describes and analyzes results from a series of laboratory thermal performance tests on a model geothermal pile in sand. Heat carrier fluid was circulated through a U-shaped circulation tube embedded within the model concrete pile. The effects of fluid circulation velocity and initial temperature difference between soil and circulation fluid (at the inlet point) on heat transfer performance of the model pile is investigated. Temperature measurements were obtained at several locations within the test setup and data collected during the thermal tests are further used to characterize time-dependent heat exchange behavior of the model pile. Moreover, soil thermal conductivity values obtained from element thermal conductivity tests are compared with equivalent soil thermal conductivity values derived from thermal performance tests on the model pile.