Abstract
The thermal resistance of a borehole can be reduced by employing thermally enhanced grout, increasing the surface area of the loop and locating the legs proximal to the bore wall. Thermal models that are used to predict borehole heat exchange are characterized by either simplified formulations that are restrictive in their application, but utilitarian, or complex multi-dimensional analyses that are cumbersome to implement. The borehole resistance methodology presented here offers a straightforward solution that is built on single loop conduction shape factor analysis with thermal shunt accounting and pipe-pipe configuration analysis, to extend to multi-loop borehole configurations and custom kidney extrusions. The borehole resistance predictions are compared to published data and information listed by manufacturers of multi-loop products in third party thermal tests against standard loops. The results are found to agree within the constraints posed by the model assumptions. The methodology offers a straightforward solution that can be incorporated into popular geothermal loop sizing software such as GLD, GLHEPRO and other system software. The advantages and challenges of these advanced loop designs are discussed and conclusions drawn. By reducing the bore resistance, one can take advantage of less drilling and proportionally less capital cost for the bore field, while achieving the same loop temperatures.
Highlights
The thermal resistance of a borehole can be reduced by employing thermally enhanced grout, increasing the surface area of the loop and locating the legs proximal to the bore wall
This is because the heat transfer surface area of an individual loop for exchange with the bore wall is occulted by the presence of additional loops
Using the method outlined in this paper, a compilation of multi-loop borehole resistance is provided in Fig. 9 for specific loop designs and values of the pipe spacing ratio, d/D
Summary
The thermal resistance of a borehole can be reduced by employing thermally enhanced grout, increasing the surface area of the loop and locating the legs proximal to the bore wall. The high-density polyethylene (HDPE) pipe loop contacts the bore wall via a thermal grout. A 7–8 °C temperature difference between the water circulating in the loop and the bore wall reduces the heat pump operating performance. The use of a thermally enhanced grout, while advantageous to heat transfer between the loop and the bore wall, presents its own challenges. The addition of sand to enhance conduction increases thermal shunting between the legs of the loop and presents challenges in uniform mixing and deployment through a tremie pipe
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