Closed-loop Vertical Ground Heat Exchanger Research Articles

Applicability evaluation of cast-in-place energy piles based on two-year heating and cooling operation

An energy pile is a type of ground heat exchanger (GHEX) that can reduce the initial construction cost by being installed in the existing foundation structure (i.e., pile). However, despite economic benefits, energy piles have not been globally employed owing to a lack of construction cases and appropriate design methods. Therefore, in this study, a design method applicable to large-diameter cast-in-place energy piles was developed and verified by conducting long-term monitoring on the heating and cooling operation. In addition, to evaluate the applicability of the cast-in-place energy piles, their thermo-mechanical behaviors and economic feasibility were analyzed using the long-term monitoring results. Although degradation of thermal performance and significant changes in the ground temperature were observed in the heating operation, it was concluded that the proposed design method could provide reliable design outcomes. The maximum thermal stress was estimated to be 2.38 MPa during the long-term monitoring, which is approximately 8.5% of the design strength criterion for cast-in-place concrete. Finally, compared with a conventional closed-loop vertical GHEX, the cast-in-place energy piles could reduce the initial investment cost by 25.31%, and the investment payback period was evaluated to be 1.61 years.

Performance evaluation of coaxial-type GHEX in GSHP system installed in Korean residential building

A coaxial-type ground heat exchanger (GHEX) is a closed-loop GHEX with heat exchange pipes in a borehole, arranged in a concentric tube-in-tube configuration. A coaxial-type GHEX enlarges the heat exchange area and induces turbulent flow more actively than conventional closed-loop GHEXs owing to the annular space formed by outer and inner pipes. Therefore, it improves the thermal performance of conventional GHEXs. Herein, the thermal performance of the coaxial-type GHEX was evaluated, and then compared with that of the closed-loop vertical GHEX. The Ground source heat pump (GSHP) system connected with the coaxial-type GHEXs was designed with the ground loop heat exchanger design program (i.e., GLHE-Pro). In accordance with the design, the GSHP system was constructed in a building basement, with four coaxial-type GHEXs and a closed-loop vertical GHEX for comparison purpose. Then, a series of field experiments (i.e., thermal response tests (TRTs), thermal performance tests (TPTs), short-term heating COP tests, and energy saving tests) were conducted to compare the thermal performance of the coaxial-type GHEXs and the closed-loop vertical GHEX with or without the GSHP system. As a result, the coaxial-type GHEX had 20% and 6.7% better thermal performance in TRT and TPT, respectively, than the closed-loop vertical GHEX. In addition, geothermal energy showed more stable performance with the coaxial-type GHEX in the GSHP system, than with the closed-loop vertical GHEX.

Effect of thermal interference on energy piles considering various configurations of heat exchangers

In general, the design of closed-loop vertical ground heat exchangers (GHEXs) is to determine the required borehole length for providing the heating and cooling load of a target building. However, the same design procedure cannot be applied to the cast-in-place energy pile since the configuration of the heat exchange pipe inside the pile is too varied and the size of the borehole is relatively large. In this paper, the effect of configuration and denseness of heat exchange pipe inside the energy pile was experimentally and numerically evaluated comparing with conventional GHEXs. First, field experiments were performed for six cast-in-place energy piles constructed in a test bed. The results showed that the cast-in-place energy piles could provide higher thermal performance per unit borehole length than the other conventional types of GHEXs by increasing the heat exchange area along with much longer heat exchange pipes. However, the over-tight layout diminished the improvement of thermal performance due to thermal interference between each pipe loop. In addition, the long-term thermal behavior of energy piles and changes in ground temperature were estimated with numerical simulations. With the simulation results, a design chart was provided for evaluating the effect of thermal interference on energy pile performance.

Field Experiments to Evaluate Thermal Performance of Energy Slabs with Different Installation Conditions

The energy slab is a novel type of horizontal Ground Heat Exchanger (GHEX), where heat exchange pipes are encased in building slab structures. The thermal performance of energy slabs is usually inferior to the conventional closed-loop vertical GHEX because its installation depth is relatively shallow and therefore affected by ambient air temperature. In this paper, heat exchange pipes were made of not only conventional high-density polyethylene (HDPE), but also stainless steel (STS), which is expected to enhance the thermal performance of the energy slabs. In addition to a floor slab, a side wall slab was also used as a component of energy slabs to maximize the use of geothermal energy that can be generated from the underground space. Moreover, a thermal insulation layer in the energy slabs was considered in order to reduce thermal interference induced by ambient air temperature. Consequently, two different field-scale energy slabs (i.e., floor-type and wall-type energy slabs) were constructed in a test bed, and two types of heat exchange pipes (i.e., STS pipe and HDPE pipes) were installed in each energy slab. A series of thermal response tests (TRTs) and thermal performance tests (TPTs) were conducted to evaluate the heat exchange performance of the constructed energy slabs. Use of the STS heat exchange pipe enhanced the thermal performance of energy slabs. Additionally, the wall-type energy slab had a similar thermal performance to the floor-type energy slab, which infers the applicability of the additional use of the wall-type energy slab. Note that if an energy slab is not thermally cut off from the building’s interior space with the aid of thermal insulation layers, heat exchange within the energy slabs should be significantly influenced by fluctuations in ambient temperature.

Field experiment on heat exchange performance of various coaxial-type ground heat exchangers considering construction conditions

The coaxial-type Ground Heat Exchanger (GHEX) possesses a concentric tube-in-tube configuration, which can provide a sufficient heat exchange area, and induces turbulent flow conditions. Therefore, the coaxial-type GHEX is expected to outperform the conventional U-type GHEX in terms of thermal performance. However, it is very important to design an optimal configuration (i.e., pipe length, the roughness of pipe wall and the shape of cross section) for the coaxial-type GHEX to generate turbulent flow inside the pipe and to achieve sufficient heat exchange area. In this paper, GHEXs of various construction conditions were considered, and the factors governing the thermal performance of coaxial-type GHEX were identified through field tests. Four 50-m-deep coaxial-type GHEXs were constructed in a test bed with different pipe materials, pipe diameters and grouting materials. In addition, a 50-m-deep closed-loop vertical GHEX was separately constructed to compare the thermal performance with the coaxial-type GHEXs. A series of in-situ thermal response test (TRT) and in-situ thermal performance test (TPT) was performed in the constructed coaxial-type GHEXs to investigate the effect of various construction conditions on thermal performance. As a result, the thermal performance of coaxial-type GHEXs is directly influenced by the thermal conductivity of the pipe and grouting material. The pipe diameter also influences the thermal performance of coaxial-type GHEX. Especially, it is noted that an optimal flow rate exists, which maximizes the thermal performance of the coaxial-type GHEX.

Engineering chart for thermal performance of cast-in-place energy pile considering thermal resistance

An energy pile is one of the novel types of ground heat exchanger (GHEX), which has been introduced as an economical alternative to conventional closed-loop vertical GHEXs. The energy pile contains heat exchange pipes inside a pile foundation and circulates a working fluid through the pipe inducing heat exchange with the surrounding ground formation. Using the existing foundation structure, the energy pile can reduce the initial construction cost such as drilling and grouting cost, and eliminate the need for additional construction space. However, most commercial design programs and analytical models for estimating the thermal performance of GHEXs are mainly applicable to the closed-loop vertical GHEXs. In this paper, an engineering chart is proposed for evaluating the thermal performance of cast-in-place energy piles considering various layouts of heat exchange pipe. First, parametric studies on different ground and operation conditions were performed with the aid of a series of computational fluid dynamic (CFD) analyses to determine design factors of the cast-in-place energy pile. Based on the design factors, the engineering chart for the thermal performance of cast-in-place energy piles was developed. Since the CFD simulations were exclusively carried out for the typical 5-pair-parallel U-type energy pile, the concept of thermal resistance was adopted for estimating the thermal performance of other types of cast-in-place energy piles. The accuracy of the developed engineering chart was verified by comparing with the results of CFD simulations, and the maximum error was measured to be about 9%.

Influence of coil pitch on thermal performance of coil-type cast-in-place energy piles

Recently, the energy pile has been introduced as a viable alternative to the conventional closed-loop vertical ground heat exchanger (GHEX). The energy pile contains a heat exchange pipe inside the pile foundation and allows a circulation of working fluid through the pipe inducing heat exchange with the ground formation. In this paper, a series of thermal performance tests was conducted on two coil-type cast-in-place energy piles with different coil pitches, 200mm and 500mm, to evaluate the effect of coil pitch on the heat exchange performance of energy piles. Thermal performance tests were carried out by applying intermittent (8h operating–16h pause) artificial cooling and heating loads with the aid of a constant-temperature water bath. The experimental result indicated that the heat exchange rate per pile length was not directly proportional to the installed pipe length because the tight coil pitch caused thermal interference between each pipe loop. In addition, a parametric study was conducted by computational fluid-dynamic (CFD) simulations to assess the effect of coil pitch and thermal interference on the thermal performance. The CFD model was verified by comparing the numerical simulation results with the field test data. Finally, an optimum coil pitch in the test bed condition was proposed considering the thermal performance and the economic feasibility of coil-type cast-in-place energy piles.

Constructability and heat exchange efficiency of large diameter cast-in-place energy piles with various configurations of heat exchange pipe

A new ground heat exchanger (GHEX) assembled in a ground source heat pump (GSHP) system has been devised, which is equipped in a structural pile, and thus called an “energy pile”. In this study, six large-diameter cast-in-place energy piles that encased various types of heat exchange pipes (i.e., three parallel U-types with 5 pairs, 8 pairs and 10 pairs; two coil-types with 200 mm coil pitch and 500 mm coil pitch; and S-type) were constructed in a test bed, and the constructability of each type of the energy pile was compared. Additionally, a 30 m-deep closed-loop vertical GHEX was separately constructed to compare the thermal performance and economic feasibility with the energy piles. A series of in-situ thermal response tests (TRTs) was performed to evaluate the relative heat exchange efficiency of each energy pile according to the various heat exchange pipe configurations. The relative heat exchange efficiency obtained in the cast-in-place energy piles and the down-sized 30 m-deep closed-loop vertical GHEX was normalized by the pile length (or borehole length) and the total heat exchange pipe length. The result from the in-situ tests indicates that the longer the heat exchange pipe installed, the higher heat exchange efficiency per pile length. However, in the case of tight layouts of the heat exchange pipe, thermal interference occurs between adjacent pipe loops, which decreases heat exchange efficiency. Economic feasibility of the energy piles was evaluated, considering the material and installation costs for assembling the heat exchange pipe with consideration of the heat exchange efficiency. It is concluded that the coil-type cast-in-place energy pile can achieve sufficient capacity for heat exchange, compared with the closed-loop vertical GHEX.

Viscosity and salinity effect on thermal performance of bentonite-based grouts for ground heat exchanger

Due to its high swelling potential and low hydraulic conductivity, the bentonite-based grout has been popularly used to backfill a borehole constructed for a closed-loop vertical ground heat exchanger in a geothermal heat pump system. In this study, three types of bentonites were compared in terms of viscosity and thermal conductivity. With the consideration of the groundwater inflow, the viscosity and thermal conductivity of the bentonite grouts having bentonite contents of 5%, 10%, 15%, 20% and 25% by weight were also examined. To evaluate the effect of salinity in the coastal area on the swelling potential of the bentonite, volume reduction tests were performed. And in order to reproduce the particle segregation of bentonite–sand mixture when the bentonite-based grout has relatively low viscosity, the segregation was observed using a ring-type separable test apparatus.

현장 열응답 시험(TRT)과 CFD 역해석을 통한 지반의 열전도도 평가

본 연구에서는 일련의 현장 열응답 시험결과를 동일한 지중열교환기와 지반 조건에 대한 CFD(Computational Fluid Dynamics) 수치해석 결과와 비교하고 역해석을 통해 지반의 열전도도를 평가하였다. 총 6개의 보어홀을 원주에 소재하고 있는 시험시공 현장에 설치하였으며 순환 파이프의 형상과 그라우트 재료에 대한 수직 밀폐형 지중열교환기의 성능을 비교하기 위해 일반적인 U형 순환 파이프와 새롭게 개발된 3공형 순환 파이프를 보어홀 내 시공하였다. 수치해석은 CFD 해석 프로그램인 FLUENT를 적용하여 3차원 열전달 거동 해석을 수행하였으며 각각의 보어홀에 대해 시간에 따른 순환수의 유입, 유출 온도 차이와 지반의 깊이별 온도변화를 User Define Function (UDF)을 이용하여 실제 조건을 모사하였다. 주어진 보어홀 조건과 실내시험을 통해 시험시공 현장의 열 물성을 입력치로 적용하여 수치 해석을 수행하였으며, 현장 열응답 시험에서 측정된 시간에 따른 유입, 유출 순환수의 온도 변화를 모사하였다. 수치해석 결과, 지반의 열전도도를 3W/mK로 적용하였을 때 보다 4W/mK일 때 현장 열응답 시험과 유사한 결과를 얻었다. In this study, a series of CFD (Computational Fluid Dynamics) numerical analyses were performed in order to evaluate the thermal performance of six full-scale closed-loop vertical ground heat exchangers constructed in a test bed located in Wonju. The circulation HDPE pipe, borehole and surrounding ground formation were modeled using FLUENT, a finite-volume method (FVM) program, for analyzing the heat transfer process of the system. Two user-defined functions (UDFs) accounting for the difference in the temperatures of the circulating inflow and outflow fluid and the variation of the surrounding ground temperature with depth were adopted in the FLUENT model. The relevant thermal properties of materials measured in laboratory were used in the numerical analyses to compare the thermal efficiency of various types of the heat exchangers installed in the test bed. The simulation results provide a verification for the in-situ thermal response test (TRT) data. The CFD numerical back-analysis with the ground thermal conductivity of 4 W/mK yielded better agreement with the in-situ thermal response tests than with the ground thermal conductivity of 3 W/mK.

Back-analyses of in-situ thermal response test (TRT) for evaluating ground thermal conductivity

In this study, a series of computational fluid dynamics (CFD) numerical analyses was performed in order to evaluate the performance of six full-scale closed-loop vertical ground heat exchangers constructed in a test bed located in Wonju, South Korea. The high-density polyethylene pipe, borehole grouting and surrounding ground formation were modeled using FLUENT, a finite-volume method program, for analyzing the heat transfer process of the system. Two user-defined functions accounting for the difference in the temperatures of the circulating inflow and outflow fluid and the variation of the surrounding ground temperature with depth were adopted in the FLUENT model. The relevant thermal properties of materials measured in laboratory were used in the numerical analyses to compare the thermal efficiency of various types of the heat exchangers installed in the test bed. The numerical simulations provide verification for the in-situ thermal response test (TRT) results. The numerical analysis with the ground thermal conductivity of 4.0 W/m⋅K yielded by the back-analysis was in better agreement with the in-situ TRT result than with the ground thermal conductivity of 3.0 W/m⋅K. From the results of CFD back-analyses, the effective thermal conductivities estimated from both the in-situ TRT and numerical analysis are smaller than the ground thermal conductivity (=4.0 W/m∙K) that is input in the numerical model because of the intrinsic limitation of the line source model that simplifies a borehole assemblage as an infinitely long line source in the homogeneous material. However, the discrepancy between the ground thermal conductivity and the effective thermal conductivity from the in-situ TRT decreases when borehole resistance decreases with a new three pipe-type heat exchanger leads to less thermal interference between the inlet and outlet pipes than the conventional U-loop type heat exchanger. Copyright © 2012 John Wiley & Sons, Ltd.

Performance evaluation of closed-loop vertical ground heat exchangers by conducting in-situ thermal response tests

The effective thermal conductivity of six vertical closed-loop ground heat exchangers (GHEXs), which were installed in a test bed located in Wonju, South Korea, has been experimentally evaluated by performing in-situ thermal response tests (TRTs). To compare the thermal efficiency of the GHEXs in field, various installation conditions are considered such as different grouting materials (cement vs. bentonite), different additives (silica sand vs. graphite) and shapes of the circulating pipe-section (conventional U-loop type vs. 3-pipe type). From the test results, it can be concluded that the cement grout has higher effective thermal conductivity than the bentonite grout by 7.4–10.1%, and the graphite outperforms the silica sand by 6.7–9.1% as a thermally-enhancing additive. In addition, the new 3-pipe type heat exchange pipe that yields less thermal interference between the inlet and outlet pipes shows better thermal performance over the conventional U-loop type heat exchange pipe by 14.1–14.5%. Based on the results from the in-situ thermal response tests, a series of cost analyses has been carried out to show the applicability of the cement grouting, the graphite additive, and the new 3-pipe type of heat exchange pipe section. For the same condition, the cement grouting can reduce the construction cost of GHEXs by around 40% in the given cost analysis scenario. In addition, an addition of graphite and use the new 3-pipe heat exchange pipe lead to about 8% and 6% cost reduction, respectively.

Numerical simulation for thermal response test performance in closed-loop vertical ground heat exchanger

In this study, a series of numerical analyses was performed in order to evaluate the performance of full-scale closed-loop vertical ground heat exchangers constructed in Wonju, South Korea. The circulating HDPE pipe, borehole and surrounding ground formation were modeled using FLUENT, a finite-volume method (FVM) program, for analyzing the heat transfer process of the ground heat exchanger system. Two user-defined functions (UDFs) accounting for the difference in the temperature of the circulating inflow and outflow fluid and the variation of ground temperature with depth were adopted in the FLUENT modeling. The thermal conductivities of grouts (cement vs. bentonite) measured in laboratory were used as input values in the numerical analyses to compare the thermal efficiency of the cement and bentonite grouts used for installing the closed-loop vertical ground heat exchanger. A series of numerical analyses was carried out to simulate in-situ thermal response tests performed in the construction site. From the comparison between the in-situ thermal response test results and numerical simulations, the average thermal conductivity of the ground formation in the construction site is back-calculated as approximately 4 W/mK. This value can be used in evaluating the long-term performance of the closed-loop vertical ground heat exchanger.

Applicability of cement-based grout for ground heat exchanger considering heating-cooling cycles

The applicability of cement grout (or cement-based grout) has been considered as an alternative to bentonite grout commonly used to backfill closed-loop vertical ground heat exchangers. In a geothermal heat pump system, repeated heating-cooling cycles may cause adverse effects on the integrity of cement grout in the ground heat exchanger. To account for the temperature cycling effect, the strength degradation of cement grout due to temperature cycling has been examined by measuring the unconfined compression strength of cured specimens in a humidity-temperature controlling chamber with applying temperature cycles between −5°C and 50°C. There is a tendency that the unconfined compression strength decreases with an increase in the number of temperature cycles. On the other hand, an equivalent hydraulic conductivity of a pipe-embedded cement grout specimen was evaluated by carrying out a modified flexible wall permeameter test equipped with a water circulating system to control temperature inside the pipe section. The applied operating temperature range was from 5 to 35°C. After three cycles of heating-cooling circulation, the equivalent hydraulic conductivity becomes asymptotic to a constant value, which implies there is no severe detachment of the pipe from the cement grout.

Comparison of effective thermal conductivity in closed-loop vertical ground heat exchangers

Performing a series of in-situ thermal response tests, the effective thermal conductivity of six vertical closed-loop ground heat exchangers was experimentally evaluated and compared to each other, which were constructed in a test bed in Wonju, South Korea. To compare thermal efficiency of the ground heat exchangers in field, the six boreholes were constructed with different construction conditions: i.e., different grouting materials (cement vs. bentonite), different shape of heat exchange pipe-sections (conventional U-loop type vs. new 3 pipe-type), and different additives (silica sand vs. graphite). One observation borehole was installed in the middle of the test site to measure a subsurface temperature change during performing the in-situ thermal response test. From the test results, it can be shown that cement grouting has a higher effective thermal conductivity than that of bentonite grouting, and graphite better performs over silica sand as a thermally enhancing addictive. In addition, a new 3 pipe-type heat exchanger yields less thermal interference between the inlet and outlet pipe than the conventional U-loop type heat exchanger, which results in superior thermal performance.

Numerical characterization of heat transfer in closed-loop vertical ground heat exchanger

A series of numerical analyses has been performed on the characteristics of heat transfer in a closed-loop vertical ground heat exchanger (U-loop). A 2-D finite element analysis was conducted to evaluate the temperature distribution over the cross section of the U-loop system involving high-density polyethylene (HDPE) pipe/grout/soil to compare the sectional efficiency between the conventional U-loop and a new latticed HDPE pipe system, which is equipped with a thermally insulating lattice in order to reduce thermal interference between the inlet and outlet pipes. In addition, a 3-D finite volume analysis (FLUENT) was adopted to simulate the operation of the closed-loop vertical ground heat exchanger with the consideration of the effect of a distance between the inlet and outlet pipes, grout’s thermal properties, the effectiveness of the latticed HDPE pipe system, and the rate of circulation pump. It was observed that the thermal interference between the two strands of U-loop is of importance in enhancing efficiency of the ground heat exchanger. Consequently, it is recommended to modify the configuration of the conventional U-loop system by equipping the thermally insulating lattice between the two pipe strands.