Heat Transport Phenomena in Shallow Geothermal Boreholes

Development of a Numerical Model and a Novel Extension for the Thermal Response Test Method by Applying Oscillating Excitations

by Phillip Oberdorfer
Doctoral thesis
Date of Examination:2014-02-21
Date of issue:2014-02-26
Advisor:Prof. Dr. Martin Sauter
Referee:Prof. Dr. Martin Sauter
Referee:PD Dr. Ekkehard Holzbecher
Persistent Address: http://dx.doi.org/10.53846/goediss-4387

 

 

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Abstract

English

The branch of shallow geothermics is part of renewable energies with a potential that has not been fully exploited. Although various analytical and numerical approaches to determine the processes in vertical borehole heat exchangers (BHEs) have recently been developed, plenty of open questions about this topic remain. Also, the state-of-the-art testing routines and evaluation methods may be improved for academic purposes. A 3-D-numerical model is developed to solve the partial di erential equation systems that determine the heat-transfer phenomena in BHEs and their ambient subsurface parameters. This model is based on the nite element method and takes into account the temperature dependencies of the accompanying materials. Furthermore, it provides a simpli cation method to calculate the heat transfer within the BHE heat pipes, which is based on correlation solutions. The most common BHE design types (single-U, double- U, and coaxial) may be implemented easily. The single-U and double-U approaches are comprehensively validated with experimental data. The advantages of the full 3-D model are exploited by performing parametric studies including material and geometric properties as well as groundwater ow to examine the in uence of di erent conditions on the performance of BHEs. The validated numerical model is used to study a novel extension approach of the thermal response test, the constant heat injection step is overlapped by an oscillatory injection rate and the system response, in terms of the pipe uid temperature, is evaluated. The results of experimental in situ test series are evaluated and numerical parametric studies are performed, to interpret the thermal reaction of BHEs on oscillating excitations. The oscillation data is found to provide further information about the borehole quality and the subsurface ambient parameters.
Keywords: Geothermics; Thermal Response Test; Numerical Simulation; Oscillating Excitations; Borehole Heat Exchanger; Finite Element Method
 
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