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.

Accuracy of borehole thermal resistance calculation methods for grouted single U-tube ground heat exchangers

“…The expression is based on the well-known two-dimensional heat conduction shape factor [52] for a circular isothermal cylinder eccentrically placed in another cylinder of same length. The same shape factor has also been used by other researchers, including Koenig and Helmke [53], Beier [15] and Koenig [54], to determine the grout thermal resistance. However, the expression proposed by Bauer et al [35] has been corrected for U-tube geometry using numerical simulations.…”

Development of Analytical Model for a Vertical Single U-Tube Ground-Coupled Heat Pump System

“…When the obstruction factor is dismissed, then (σ) is equal to unity, and the same expression will be obtained as that of Koenig [16].…”

“…Koenig [16] has analyzed the heat transfer problem in a borehole with single and multi-vertical Utube loops. He has arrived at an analytical solution to the borehole thermal resistance for different U-tube geometry configuration and presented a validation for the model with acceptable accuracy limits.…”

“…The tube spacing (Sp) was selected according to the relation given by Koenig [16] for practical applications of the ground U-tube heat exchanger as follows:…”

Energy storage in the geological subsurface: dimensioning, risk analysis and spatial planning: the ANGUS+ project

“…Therefore, a resource efficient and environmentally sustainable thermal use of the subsurface requires the parameterization of geological and hydrological conditions connected with the site exploration as well as cost-efficient methods for longterm monitoring of the thermal application . In particular, low-temperature heat storage in the shallow subsurface and high-temperature heat storage in depths below about 500 m are considered, as here installation costs may still be reasonable in connection with residential or office buildings (Koenig 2015;Raymond et al 2015). High-temperature heat storage would allow an increased storage efficiency compared to the lower temperatures usually applied at present.…”