Analysis of Effective Soil Thermal Conductivities and Borehole Thermal Resistances with a Line Source Method

선형열원법에 의한 지중유효열전도도와 보어홀 전열저항 해석

  • Received : 2010.07.05
  • Accepted : 2010.08.12
  • Published : 2010.08.30

Abstract

Investigation of the effective soil thermal conductivity(k) is the first step in designing the ground loop heat exchanger(borehole) of a geothermal heat pump system. The line source method is required by New and Renewable Energy Center of Korea Energy Management Corporation in analyzing data obtained from thermal response tests. Another important factor in designing the ground loop heat exchanger is the borehole thermal resistance($R_b$). There are two methods to evaluate $R_b$ : one is to use a line source method, and the other is to use a shape factor of the borehole. In this study, we demonstrated that the line source method produces better results than the shape factor method in evaluating $R_b$. This is because the borehole thermal resistance evaluated with the line source method characteristically reduces the temperature differences between an actual and a theoretical thermal behaviors of the borehole. Evaluation of $R_b$ requires soil volumetric heat capacity. However, the effect of the soil volumetric heat capacity on the borehole thermal resistance is very small. Therefore, it is possible to use a generally accepted average value of soil volumetric heat capacity($=2MJ/m^3{\cdot}K$) in the analysis. In this work, it is also shown that an acceptable range of the initial ignoring time should be in the range of 8~16hrs. Thus, a mean value of 12 hrs is recommended.

Keywords

References

  1. Ingersoll, L.R. and Plass, H.J., Theory of the Ground Pipe Heat Source for the Heat Pump, ASHVE Transactions, Vol. 47, pp. 119-122. 1948.
  2. Mogensen, P. Fluid to Duct Wall Heat Transfer in Duct System Heat Storage, Proceedings of International Conference on Subsurface Heat Storage in Theory and Practice, Swedish Council for Building Research, June 6-8, 1983.
  3. Eskilson, P., Thermal Analysis of Heat Extraction Boreholes, Dept. of Math. Physics, University of Lund, Sweden, 1987.
  4. Smith, Marvin, "Comments on in-situ borehole thermal conductivity testing", The Source 1-2/99, Stillwater, OK, 1999.
  5. 이세균, 우정선, 김대기, 지중유효열전도율 해석에 사용되는 선형열원모델의 초기제외시간 결정에 관한 연구, 에너지공학, 제17권, 제3호, pp.167-174, 2008.
  6. 한국에너지기술연구원, 현지 지중열전도도 측정기술, 지식경제부, 2008.
  7. Carslaw, H. S. and Jaeger, J. C., Conduction of Heat in Solids, 2nd ed., Clarendon Press, Oxford, 1959.
  8. Kavanaugh, S.P., Investigation of Methods for Determining Soil and Rock Formation Thermal Properties from Short-Term Fields Tests, ASHRAE 1118-TRP. 2000.
  9. Remund, C. P., Borehole Thermal Resistance :Laboratory and Field Studies, ASHRAE Transactions : Symposia, CH-99-2-1, pp. 43-445, 1999.
  10. Hellstrom G. and Sanner B., PC-programs for Borehole Heat Exchanger Design, Asterweg 2, D-35633, Lahnau, Germany, 2000.