18 May 2017

O.2.8.4 Dynamic Determination of Ground Thermal Conductivity and Minimum Thermal Response Test Duration Using the Line Source Model


Ground thermal properties, such as the undisturbed ground temperature and the effective thermal conductivity along the depth of a vertical borehole, play an important role in designing a proper ground heat exchanger (GHX) for ground source heat pump (GSHP) applications. The effective ground thermal conductivity (GTC) is usually determined with a combined approach of thermal response test (TRT) an inverse modeling. Among existing models for analyzing the TRT data, the line source model is most widely used in practice because it is easy, fast and straight-forward to implement. However, the current practice requires extremely stable power supply for 48 hours during a TRT. In order to reduce the test duration, and the associated cost, while retaining a reasonable accuracy, an algorithm is developed to dynamically analyze the TRT data during the test and determine the minimum test duration. It is found that, for TRTs with stable power supply, the test duration could be shortened by 20 to 40% with this algorithm while retaining the GTC value within ±5% uncertainty of that determined with 48 hours or longer TRT data.