Journal of the Korea Academia-Industrial cooperation Society (한국산학기술학회논문지)

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A Study on the Effects of Design Parameters of Vertical Ground Heat Exchanger on the Borehole Thermal Resistance

수직밀패형 지중열교환기의 설계인자가 보어홀 전열저항에 미치는 영향에 관한 연구

  • Chang, Keun Sun (Department of Mechanical Engineering, Sunmoon University) ;
  • Kim, Min-Jun (Korea Refrigeration and Air-conditioning Assessment Center)
  • Received : 2018.07.18
  • Accepted : 2018.10.05
  • Published : 2018.10.31
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Abstract

Currently, vertical closed ground heat exchangers are the most widely utilized geothermal heat pump systems and the major influencing parameters on the performance of ground heat exchangers are the ground thermal conductivity(k) and borehole thermal resistance($R_b$). In this study, the borehole thermal resistance was calculated from the in-situ thermal response test data and the individual effects of design parameters (flow rate, number of pipe, grout composition) on the borehole thermal resistance were analyzed. The grout thermal resistance was also compared with the correlations in the literatures. The borehole thermal resistance of the investigated ground heat exchanger results in 0.1303 W/m.K and the grout thermal resistance (66.6% of borehole thermal resistance) is the most influencing parameter on borehole heat transfer compared to the other design parameters (pipe thermal resistance, 31.5% and convective thermal resistance, 1.9%). In addition, increasing the thermal conductivity of grout by adding silica sand to Bentonite is more effective than the other design improvements, such as an increase in circulating flowrate or number of tubes on enhancing borehole heat transfer.

현재 지열 열펌프 시스템에 수직밀폐형 지중열교환기가 가장 많이 적용되고 있으며, 수직밀폐형 지중열교환기의 성능에 영향을 미치는 주요 인자로는 지중 열전도율(k)과 보어홀 전열저항($R_b$)이 있다. 본 연구에서는 현장에서 측정된 열응답시험 데이터를 이용하여 보어홀 전열저항을 계산하였으며 지중열교환기 개별 설계인자들(순환수유량, 파이프 수, 그라우팅재)이 보어홀 전열저항에 미치는 영향을 분석하였다. 또한 도출된 그라우팅 열저항은 문헌에 제시된 다양한 상관식과 비교 분석하였다. 시험데이터를 통해 본 시험에서의 지중열교환기 보어홀 전열저항은 0.1303 W/m.K로 나타났으며, 보어홀 전열저항에서 그라우트 열저항이 66.6 %, 파이프 열저항이 31.5 %, 순환수 대류열저항이 1.9 %를 차지하여 그라우트가 보어홀 열전달에 가장 큰 영향을 미치는 인자임을 확인하였다. 또한 각 설계인자의 설계변수가 보어홀 전열저항에 미치는 영향을 분석한 결과 실리카샌드를 혼합하여 그라우트 열전도율를 높이는 방법이 파이프 수 증가나 순환수 유량증가보다 열전달 증진에 더 효과적임을 알 수 있었다.

Keywords

References

  1. Kim, M. J., A study on the thermal performance evaluation of a borehole ground heat exchanger using in-situ thermal response test, Ph.D thesis, Sunmoon University, KOREA, 2017.
  2. Ministry of Trade, Industry and Energy Announcement 2015-263, Standards of Support, Installation and Management for New and Renewable Energy System, (2015).
  3. K. S. Chang, M. J. Kim, "Thermal performance evaluation of vertical U-loop ground heat exchanger using in-situ thermal response test", Renewable Energy, Vol.87, Part 1, pp.585-591, 2016. DOI: https://dx.doi.org/10.1016/j.renene.2015.10.059
  4. Incropera, F. P. and Dewitt, Fundamentals of Heat and Mass Transfer, 4th Edition, 1996, John Wiley and Sons, Inc.
  5. Remund, C.P., Borehole thermal resistance: laboratory and field studies, 1999, ASHRAE Transactions, 105(1): pp. 439-445.
  6. L. Lamarche, S. Kajl, B. Beauchamp, "A review of methods to evaluate borehole thermal resistances in geothermal heat pump systems", Geothermics, Vol.39, No.2, pp.187-200, 2010. DOI: https://dx.doi.org/10.1016/j.geothermics.2010.03.003
  7. M. H. Sharqawy, E. M. Mokheimer, H. M. Badr, "Effective pipe-to-borehole thermal resistance for vertical ground heat exchanger", Geothermics, Vol.38, No.2, pp.271-277, 2009. DOI: https://dx.doi.org/10.1016/j.geothermics.2009.02.001
  8. Shonder, J.A. and Beck, J.V., Determining effective soil formation thermal properties from field data using a parameter estimation technique, 1999, ASHRAE Transactions Vol. 105, part 1, pp 458-466.
  9. Gu, Y., O'Neal, D., Development of an equivalent diameter expression for vertical U-tubes used in ground-coupled heat pumps. ASHRAE Transactions, (1998) 104(2): pp. 1-9.
  10. Ground Loop Design User's Manual, Gaia Geothermal, USA, 2007, pp. 1-136.
  11. K. S. Chang, M. J. Kim, "Analysis and thermal response test for vertical ground heat exchanger with two U-loop configuration", International Journal of Energy Research, Vol.40, No.2, pp.189-197, 2016. DOI: https://dx.doi.org/10.1002/er.3448