TY - THES

T1 - CFD Analysis of Ground Source Heat Exchangers

AU - Dörr, Constantin Julian

N1 - embargoed until null

PY - 2020

Y1 - 2020

N2 - This master’s thesis was conducted in collaboration with the Austrian Institute of Technology (AIT) as part of the GEOFIT research project’s framework, funded by the European Union’s Horizon 2020 programme. The objective of this thesis is to model heat flow in the vicinity or “near-field” of non-standard compact ground source heat exchangers in varying substrates. Earth baskets and horizontal slinky heat exchangers have been chosen as the most suitable geometric configurations to be examined and corresponding models with the CFD software ANSYS Fluent were developed. The influence of the heat conducting medium’s material properties affecting the heat transport, namely the thermal conductivity and thermal diffusivity, was investigated for sands and soils with varying moisture content. As the variation of the heat conducting substrate in the AIT’s large-scale earth basket experiment requires significant time and man-power, a small-scale experiment named the “Thermo-Pipe” was developed at AIT. The objective of the Thermo-Pipe is to model and test the thermal response of varying heat conducting substrates before they qualify to be used in the large-scale experiment. The corresponding model has been developed as part of the framework of this thesis as well and its matching results were compared to the experimentally measured data. Additionally, to further substantiate the models’ results, the governing physical concepts of heat transport incorporated in the numeric ANSYS Fluent solver were investigated. This was accomplished through creating a geometrically simplified model, to reduce the calculation effort and thus make an analytical solution possible. The analytically calculated solution was then compared to the solver’s numerically calculated solution and showed highly accurate matches for varying time-independent heat inputs. The analytical solution has shown that the thermal conductivity is the sole material property appearing in the steady-state solution and thus directly influences the maximum temperature reached at the system’s thermal equilibrium. The thermal diffusivity on the other hand appears in the transient solution and influences the time it takes until the thermal equilibrium is reached. This could be observed in the CFD solutions for varying substrates as well. To validate the results of the heat exchangers’ models, they were compared to a pre-existing model and corresponding experiment, developed at AIT. The solutions of these near-field models will serve as input parameters for further far-field modelling, conducted by the AIT’s GEOFIT partners. This thesis aims to contribute to the groundwork of the GEOFIT project’s greater objective, to develop an engineering design tool through accurate heat flow modelling for compact ground source heat exchangers applicable in large-scale geothermal retrofitting of pre-existing European housing.

AB - This master’s thesis was conducted in collaboration with the Austrian Institute of Technology (AIT) as part of the GEOFIT research project’s framework, funded by the European Union’s Horizon 2020 programme. The objective of this thesis is to model heat flow in the vicinity or “near-field” of non-standard compact ground source heat exchangers in varying substrates. Earth baskets and horizontal slinky heat exchangers have been chosen as the most suitable geometric configurations to be examined and corresponding models with the CFD software ANSYS Fluent were developed. The influence of the heat conducting medium’s material properties affecting the heat transport, namely the thermal conductivity and thermal diffusivity, was investigated for sands and soils with varying moisture content. As the variation of the heat conducting substrate in the AIT’s large-scale earth basket experiment requires significant time and man-power, a small-scale experiment named the “Thermo-Pipe” was developed at AIT. The objective of the Thermo-Pipe is to model and test the thermal response of varying heat conducting substrates before they qualify to be used in the large-scale experiment. The corresponding model has been developed as part of the framework of this thesis as well and its matching results were compared to the experimentally measured data. Additionally, to further substantiate the models’ results, the governing physical concepts of heat transport incorporated in the numeric ANSYS Fluent solver were investigated. This was accomplished through creating a geometrically simplified model, to reduce the calculation effort and thus make an analytical solution possible. The analytically calculated solution was then compared to the solver’s numerically calculated solution and showed highly accurate matches for varying time-independent heat inputs. The analytical solution has shown that the thermal conductivity is the sole material property appearing in the steady-state solution and thus directly influences the maximum temperature reached at the system’s thermal equilibrium. The thermal diffusivity on the other hand appears in the transient solution and influences the time it takes until the thermal equilibrium is reached. This could be observed in the CFD solutions for varying substrates as well. To validate the results of the heat exchangers’ models, they were compared to a pre-existing model and corresponding experiment, developed at AIT. The solutions of these near-field models will serve as input parameters for further far-field modelling, conducted by the AIT’s GEOFIT partners. This thesis aims to contribute to the groundwork of the GEOFIT project’s greater objective, to develop an engineering design tool through accurate heat flow modelling for compact ground source heat exchangers applicable in large-scale geothermal retrofitting of pre-existing European housing.

KW - GEOFIT

KW - earth baskets

KW - horizontal slinky

KW - shallow geothermal

KW - heat conduction equation

KW - CFD

KW - analytical

KW - numerical

KW - modelling

KW - GEOFIT

KW - Erdwärmekörbe

KW - Ringgrabenkollektor

KW - oberflächennahe Geothermie

KW - Wärmeleitungsgleichung

KW - CFD

KW - analytische

KW - numerische

KW - Modellierung

M3 - Master's Thesis

ER -