Thermal resistance of borehole heat exchangers composed of multiple loops and custom shapes

Abstract

Background: 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. Methods: 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. Results: 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. Conclusions: 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.