Coil Type Ground Heat Exchangers Research Articles

Suggestion of a design method for horizontal spiral coil type ground heat exchangers

The need of geothermal energy is constantly increasing for economical and environmental benefits. Horizontal ground heat exchangers (GHEs) can reduce the installation cost and they can also compromise between increasing in efficiency and costs. There are many kinds of GHEs, and it is known that slinky and spiral coil type GHEs show high thermal performance in the horizontal system. However, there is no adequate guideline to design the spiral coil type GHEs even though there is a design program for the slinky type GHEs. Therefore, this paper presents a design module for the horizontal spiral coil type GHEs. Dried Joomunjin standard sand was filled in a steel box including a horizontal spiral coil type GHE of which the size was 5m x 1m x 1m and thermal response tests (TRTs) were conducted for 30 hours to evaluate heat exchange rates. Based on the experimental results, a modified design module, that includes a scheme to properly evaluate design parameters such as ground thermal resistance and borehole thermal resistance used in the Kavanaugh theory for designing a spiral coil type GHE, was suggested.

Thermal performance evaluation and parametric study of a horizontal ground heat exchanger

A ground-source heat pump (GSHP) system uses a relatively constant ground temperature to emit heat in summer and to obtain heat during winter for heating and cooling energy. Among diverse GSHP systems, in designing a closed horizontal system, it is important to accurately estimate the thermal performance of the horizontal ground heat exchanger (GHE). Therefore, in this study, we investigated the performance of a horizontal GHE via experiments and numerical analyses. Thermal response tests (TRTs) were conducted to evaluate heat exchange rates by using horizontal slinky- and spiral-coil-type GHEs installed in a 5m×1m×1m steel box filled with dried Joomunjin sand. Numerical analyses were conducted for verification with experimental results and a parametric study of affecting parameters on horizontal GHE. The results of the TRTs and the numerical analyses were well matched, and coil-type GHEs were found to perform better than the horizontal slinky-type GHEs. Our results show that, among horizontal GHE design parameters, GHE type and soil thermal conductivity are the main factors to determine the heat exchange rate of a GHE, whereas the pipe diameter does not have any effect on the GHE performance.

Evaluation of the thermal efficiency and a cost analysis of different types of ground heat exchangers in energy piles

This paper presents an experimental and numerical study of the results of a thermal performance test using precast high-strength concrete (PHC) energy piles with W and coil-type ground heat exchangers (GHEs). In-situ thermal performance tests (TPTs) were conducted for four days under an intermittent operation condition (8h on; 16h off) on W and coil-type PHC energy piles installed in a partially saturated weathered granite soil deposit. In addition, three-dimensional finite element analyses were conducted and the results were compared with the four-day experimental results. The heat exchange rates were also predicted for three months using the numerical analysis. The heat exchange rate of the coil-type GHE showed 10–15% higher efficiency compared to the W-type GHE in the energy pile. However, in considering the cost for the installation of the heat exchanger and cement grouting the additional cost of W-type GHE in energy pile was 200–250% cheaper than coil-type GHE under the condition providing equivalent thermal performance. Furthermore, the required lengths of the W, 3U and coil-type GHEs in the energy piles were calculated based on the design process of Kavanaugh and Rafferty. The additional cost for the W and 3U types of GHEs were also 200–250% lower than that of the coil-type GHE. However, the required number of piles was much less with the coil-type GHE as compared to the W and 3U types of GHEs. They are advantageous in terms of the construction period, and further, selecting the coil-type GHE could be a viable option when there is a limitation in the number of piles in consideration of the scale of the building.

An experimental and numerical approach to derive ground thermal conductivity in spiral coil type ground heat exchanger

This paper presents an experimental and numerical study on the evaluation of thermal response test (TRT) results observed in a precast high-strength concrete (PHC) pile and a general conventional vertical type borehole with spiral coil type ground heat exchangers (GHEs). Field TRTs were carried out on a PHC energy pile and a general vertical type borehole, and an equivalent ground thermal conductivity was estimated using the spiral coil source model. The PHC energy pile and conventional vertical type borehole were numerically modeled using a three dimensional finite element method for the estimation of borehole thermal resistance and the comparison with the TRT results. Based on the results, this paper suggested a method to evaluate the ground thermal conductivity using the infinite line source model and the results were compared with those of the spiral coil source model.

Relative constructability and thermal performance of cast-in-place concrete energy pile: Coil-type GHEX (ground heat exchanger)

Energy piles encase heat exchange pipes in a pile foundation to use geothermal energy. This paper evaluates the constructability and thermal performance of energy piles equipped in a large-diameter drilled shaft. Two energy piles were constructed by fabricating coil-type heat exchange pipes in cast-in-place concrete piles. The coil pitch was designed to be 200 mm and 500 mm, respectively. The constructability of each energy pile was evaluated in terms of time and ease for installing. In-situ TRTs were performed to compare the thermal performance of energy piles. The relative heat exchange efficiency for the coil pitch 200 mm is only 1.2 times greater than that of the coil pitch 500 mm energy pile, even though the coil pitch 200 mm energy pile encases the heat exchange pipe 2.4 times as longer as the coil pitch 500 mm. In addition, the result of TRTs was compared with two well-known analytical to estimate thermal properties of ground formation. A thermal performance test was carried out by applying artificial cooling operation to the energy piles, and indicates the heat exchange rate is not directly proportional to the pipe length because the tighter coil pitch configuration may cause thermal interference between the coil loops.

에너지 파일의 현장 열응답 시험에 관한 연구

최근 들어 경제적인 지열에너지 활용을 위하여 에너지 파일의 적용이 확대되고 있다. 특히 더 높은 열 교환 효율을 확보하고자 에너지 파일의 경우 통상적인 U자형 또는 W자형과 같은 라인형 지중 열교환기가 아닌 코일형 지중 열교환기를 매입하는 경우가 늘어나고 있다. 본 연구에서는 매립지 부지에 PHC 코일형 열교환기 형태의 에너지 파일을 설치하고 240시간 동안 현장 열응답 시험(thermal response test)을 실시하였다. 또한 현장에서 지층별로 시료를 채취하여 실내에서 현장 지층 물성으로 시료를 재조성한 후 비정상 탐침법(non-steady state probe method)을 이용하여 지층별 열전도도와 열확산계수를 측정하였고 등가의 열물성으로 환산하였다. 실험 결과 현장 열응답 시험에 의한 지반의 열전도도와 실내 탐침법으로 측정된 지반의 열전도도는 5%내에서 일치하였으며 아울러 지반의 또 하나의 중요한 설계인자인 등가 열확산계수의 측정방법도 제시하였다. Use of geothermal energy has been increased for its economical application and environmentally friendly utilization. Particularly, for energy piles, a spiral coil type ground heat exchanger (GHE) is more preferred than line type GHEs such as U and W shaped GHEs. A PHC energy pile with spiral coil type GHE was installed in an area of partially saturated dredged soil deposit, and a thermal response test (TRT) was conducted for 240 hours under a continuous operation condition. Besides, remolded soil samples from different layers were collected in the field, and soil specimens were reconstructed according to the field ground condition. Non-steady state probe methods were conducted in the lab, and ground thermal conductivity and thermal diffusivity were measured for the different soil layers. An equivalent ground thermal conductivity was calculated from the lab test results and it was compared with the field TRT result. The difference was less than 5%, which advocates the use of an equivalent ground thermal conductivity for the multi-layered ground. Furthermore, this paper also represents an equivalent ground thermal diffusivity evaluation method which is another very important design parameter.

Experimental Verification to Estimate Ground Thermal Conductivity Using Spiral Coil Type Ground Heat Exchangers

Experimental Verification to Estimate Ground Thermal Conductivity Using Spiral Coil Type Ground Heat Exchangers

지중 열교환기 종류에 따른 성능 및 시공비 분석

This paper presents a preliminary experimental and computational study on the evaluation of thermal performance and initial cost of U, W and coil type ground heat exchangers (GHEs). Heat exchange rate of the individual GHE was evaluated from the thermal resperformance test (TPT) results, and the construction cost was also calculated. For more information, GLD (ground loop design) simulations of various piping size are carried out. From simulation results, the optimized GHE was suggested based on the thermal performance and construction cost as well. Besides, the required borehole length of U and W type GHEs was calculated considering a real construction condition using GLD program.

Evaluation of Heat Exchange Rate of Different Types of Ground Heat Exchangers

This research presents an experimental study of heat exchange rate of U, W, 2U and coil type ground heat exchangers (GHEs) measured by thermal performance tests (TPTs). The four types of GHEs were installed in a partially saturated dredged soil deposit of Incheon International Airport area. Thermal response tests (TRTs) were conducted for U, W and 2U type GHEs to deduce the ground thermal conductivity. Besides, TPTs were also conducted for U, W, 2U and coil type GHEs to evaluate heat exchanger rates under 100-hr continuous and 8-hr intermittent operation conditions for five days. Coil shaped GHE showed about twice higher thermal performance than the others GHEs. Furthermore, intermittent operation condition showed 30~40% higher heat exchange rates than continuous operation condition.

Characteristics of an analytical solution for a spiral coil type ground heat exchanger

This paper presents an efficient spiral coil source model and its analytical solution, developed to consider 3-dimensional shape effects and radial dimension effects of a spiral coil type ground heat exchanger (GHE) using Green’s function method. To avoid singular integrals, the solution is expressed using an error function by which the computational limitation is avoided. To analyze the characteristics of the analytical model, predicted results of the model were compared with test measurements by a thermal response test conducted in a model chamber, and numerical analysis results implemented in ABAQUS/Standard.