DOI QR코드

DOI QR Code

Evaluation of Ground Thermal Conductivity by Performing In-Situ Thermal Response test (TRT) and CFD Back-Analysis

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

  • Park, Moonseo (School of Civil, Environmental and Architectural Engineering, Korea Univ.) ;
  • Lee, Chulho (BK21, Korea Univ.) ;
  • Park, Sangwoo (School of Civil, Environmental and Architectural Engineering, Korea Univ.) ;
  • Sohn, Byonghu (Green Building Research Division, Korea Institute of Construction Technology) ;
  • Choi, Hangseok (School of Civil, Environmental and Architectural Engineering, Korea Univ.)
  • 박문서 (고려대학교 건축사회환경공학부) ;
  • 이철호 (고려대학교 BK21) ;
  • 박상우 (고려대학교 건축사회환경공학부) ;
  • 손병후 (한국건설기술연구원 그린빌딩연구실) ;
  • 최항석 (고려대학교 건축사회환경공학부)
  • Received : 2011.07.06
  • Accepted : 2012.12.06
  • Published : 2012.12.31

Abstract

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.

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.

Keywords

Acknowledgement

Supported by : 한국건설교통기술평가원, 한국연구재단

References

  1. Austin, W. A. (1995) Development of an in situ system for measuring ground thermal properties, M.S. Thesis, Oklahoma State University, US.
  2. Beier, R. A. and Smith, M. D. (2002) "Borehole thermal resistance from line-source model of in-situ tests", ASHRAE Transactions, Vol. 108, pp.212-219.
  3. Carslaw, H. S. and Jaeger, J. C. (1959) "Conduction of heat in Solid", 2nd edition, Oxford Science Publications, pp.261-262.
  4. Choi, H-S, Lee, C-H, Choi, H-P, and Woo, S-B (2008), "Study on physical characteristics of grout material for backfilling ground heat exchanger, Journal of Korean Geotechnical Society (KGS), Vol.24, No.1, pp.37-49.
  5. Engineering Toolbox : www.Engineeringtoolbox.com
  6. Gehlin, S. E. A., Hellstrom, G., and Nordell B. (2003), "The influence of the thermosiphon effect on the thermal response test", Renewable Energy, Vol.28, pp.2239-2254. https://doi.org/10.1016/S0960-1481(03)00129-0
  7. Gil, H., Lee, K., Lee, C., and Choi, H. (2009), "Numerical evaluation on thermal performance and sectional efficiency of closed-loop vertical ground heat exchanger", Journal of Korean Geotechnical Society (KGS), Vol.25, No.3, pp.57-64.
  8. IGSHPA (2000), "Closed-Loop/Ground-Source Heat Pump System : Design and Installation Standards 2000", Oklahoma state University, stillwater Oklahoma.
  9. Kim E-J, Roux, J-J, Rusaouen, G., and Kuznik, F. (2010) "Numerical modelling of geothermal vertical heat exchangers for the short time analysis using the state model size reduction technique", Applied Thermal Engineering, Vol.30, pp.706-714. https://doi.org/10.1016/j.applthermaleng.2009.11.019
  10. Korea Expressway Corporation (2007), Study on applicability of cast-in-place PSC with design strength above 60Mpa, Final Report, Figure 3.109-3.118.
  11. Lamarche, L., Kajl, S., and Beauchamp, B. (2010), "A review of methods to evaluate borehole thermal resistances in geothermal heat-pump systems", Geothermics, Vol.39, pp.187-200. https://doi.org/10.1016/j.geothermics.2010.03.003
  12. Lee, C., Park, M., Min, S., Choi, H., and Sohn, B. (2010), "Evaluation of performance of grouts and pipe sections for closed-loop vertical ground heat exchanger by in-situ thermal response test", Journal of Korean Geotechnical Society (KGS), Vol.26, No.7, pp.93-106.
  13. Lee, S., Woo, J., and Kim, D. (2008), "A study of determining initial ignoring time of line source model used in estimating the effective soil formation thermal conductivities", Journal of Energy Engineering, Vol.17, No.3, pp.167-174.
  14. Li, X., Chen, Z., and Zhao, J. (2006) "Simulation and experiment on the thermal performance of U-vertical ground coupled heat exchanger" Applied Thermal Engineering, Vol.26, pp.1564-1571. https://doi.org/10.1016/j.applthermaleng.2005.12.007
  15. Li, X., Chen, Z., and Zhao, J. (2006) "Simulation and experiment on the thermal performance of U-vertical ground coupled heat exchanger" Applied Thermal Engineering, Vol.26, pp.1564-1571. https://doi.org/10.1016/j.applthermaleng.2005.12.007
  16. Martin, C. A. and Kavanaugh, S. P. (2002) "Ground thermal conductivity testing control site analysis", ASHRAE Transactions, Vol.108, pp.945-952.
  17. Mogensen, P. (1983) "Fluid to duct wall heat transfer in duct system storages", Proceedings of the International Conference on Subsurface Heat Storage in Theory and Practice, Swedish Council for Building Research, June 6-8.
  18. Niekamp, A., Unklesbay, K., Unklesbay, N., and Ellersieck, M. (1984) "Thermal properties of bentonite-water dispersions used for modeling foods", Journal of Food Science, Vol.49, pp.28-31. https://doi.org/10.1111/j.1365-2621.1984.tb13661.x
  19. Pahud, D. and Hubbuch, M. (2007) "Measured thermal performance of the energy pile system of the Dock Midfield at Zurich Airport", Proceedings of European Geothermal Congress 2007, Unterhaching, Germany, pp.1-7.
  20. Raymond, J., Therrien, R., Gosselin, L., and Lefebvre, R. (2011) "Numerical analysis of thermal response tests with a groundwater flow and heat transfer model", Renewable Energy, Vol.36, pp. 315-324. https://doi.org/10.1016/j.renene.2010.06.044
  21. Shonder, J. A. and Beck, J. V. (2000) "Field test of a new method for determining soil formation thermal conductivity and borehole resistance", ASHRAE Transactions, Vol.106, pp.843-850.
  22. Signorelli, S., Bassetti, S., Pahud, D., and Kohl, T. (2007) "Numerical evaluation of thermal response tests", Geothermics, Vol.36, pp.141-166. https://doi.org/10.1016/j.geothermics.2006.10.006
  23. Smith, M. D. and Perry, R. L. (1999) "Borehole grouting: field studies and thermal performance testing", ASHRAE Transactions, Vol.105, pp.451-457.
  24. Sohn, B. (2007), "Evaluation of ground effective thermal conductivity and borehole effective thermal resistance from simple line-source model", Korean Journal of Air-Conditioning and Refrigeration Engineering, Vol.19, No.7, pp.512-520.
  25. Sohn, B. H., Shin, H. J., and Park, S. K. (2005), "Evaluation of effective thermal conductivity and thermal resistance in ground heat exchanger boreholes", Korean Journal of Air-Conditioning and Refrigeration Engineering, Vol.17, No.8, pp.695-703.
  26. Spilker, E. H. (1998) "Ground-coupled heat pump loop design using thermal conductivity testing and the effect of different backfill materials on verticl borehole length", ASHRAE Transactions, Vol.104, pp.775-779.
  27. Woo, S., Kim, J., Shin, S., and Whang, K. (2007), "The Comparison of the EWT&LWT between Field Measurement and CFD of Vertical-type Geothermal Heat Exchanger" Korean Journal of Geothermal Energy, Vol.3, pp.11-16.
  28. Zanchini, E. and Terlizzeses, T. (2008) "Finite-element evaluation of thermal response tests performed on U-tube borehole heat exchangers", Proceedings of the COMSOL Confernece, Hannover.
  29. Zhang, Q. and Murphy, W. E. (2000) "Measurement of thermal conductivity for three borehole fill materials used for GSHP", ASHRAE Transactions, Vol.106, pp.434-441.