Thermal performance analysis of borehole size effect on geothermal heat exchanger
来源期刊：中南大学学报(英文版)2012年第12期
论文作者：CHOI Hoon-ki YOO Geun-jong LIM Kyung-bin LEE Sang-hoon LEE Chang-hee
文章页码：3524 - 3529
Key words：large size borehole heat exchanger; thermal response test; effective thermal conductivity; thermal resistance; borehole
Abstract: Thermal performance was the most important factor in the development of borehole heat exchanger utilizing geothermal energy. The thermal performance was affected by many different design parameters, such as configuration type and borehole size of geothermal heat exchanger. These eventually determined the operation and cost efficiency of the geothermal heat exchanger system. The main purpose of this work was to assess the thermal performance of geothermal heat exchanger with variation of borehole sizes and numbers of U-tubes inside a borehole. For this, a thermal response test rig was established with line-source theory. The thermal response test was performed with in-line variable input heat source. Effective thermal conductivity and thermal resistance were obtained from the measured data. From the measurement, the effective thermal conductivity is found to have similar values for two- pair type (4 U-tubes) and three-pair type (6 U-tubes) borehole heat exchanger systems indicating similar heat transfer ability. Meanwhile, the thermal resistance shows lower value for the three-pair type compared to the two-pair type. Measured data based resistance have lower value compared to computed result from design programs. Overall comparison finds better thermal performance for the three-pair type, however, fluctuating temperature variation indicates complex flow behavior inside the borehole and requires further study on flow characteristics.
CHOI Hoon-ki1, YOO Geun-jong1, LIM Kyung-bin2, LEE Sang-hoon3, LEE Chang-hee4
(1. Department of Mechanical Design and Manufacturing Engineering, Changwon National University,
Changwon 641-773, Korea;
2. Department of Mechanical Design Engineering, Hanbat National University, Daejon 305-719, Korea;
3. Department of Energy Engineering, Jeonju University, Jeonju 560-759, Korea;
4. STX Institute of Technology, Changwon 642-845, Korea)
Abstract:Thermal performance was the most important factor in the development of borehole heat exchanger utilizing geothermal energy. The thermal performance was affected by many different design parameters, such as configuration type and borehole size of geothermal heat exchanger. These eventually determined the operation and cost efficiency of the geothermal heat exchanger system. The main purpose of this work was to assess the thermal performance of geothermal heat exchanger with variation of borehole sizes and numbers of U-tubes inside a borehole. For this, a thermal response test rig was established with line-source theory. The thermal response test was performed with in-line variable input heat source. Effective thermal conductivity and thermal resistance were obtained from the measured data. From the measurement, the effective thermal conductivity is found to have similar values for two- pair type (4 U-tubes) and three-pair type (6 U-tubes) borehole heat exchanger systems indicating similar heat transfer ability. Meanwhile, the thermal resistance shows lower value for the three-pair type compared to the two-pair type. Measured data based resistance have lower value compared to computed result from design programs. Overall comparison finds better thermal performance for the three-pair type, however, fluctuating temperature variation indicates complex flow behavior inside the borehole and requires further study on flow characteristics.
Key words:large size borehole heat exchanger; thermal response test; effective thermal conductivity; thermal resistance; borehole
